c© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a05 419.pub2

Cellulose Esters 1

Cellulose Esters

Klaus Balser, Wolff Walsrode AG, Walsrode, Federal Republic of Germany (Chap. 1)

Lutz Hoppe, Wolff Walsrode AG, Walsrode, Federal Republic of Germany (Chap. 1)

Theo Eicher, Stuttgart, Federal Republic of Germany (Chaps. 2.1 and 2.2)

Martin Wandel, Bayer AG, Leverkusen, Federal Republic of Germany (Chaps. 2.2 and 2.4)

Hans-Joachim Astheimer, Rhodia AG, Freiburg, Federal Republic of Germany (Chap. 2.3)

Hans Steinmeier, Rhodia Acetow AG, Freiburg, Federal Republic of Germany (Chap. 2.3)

1. Inorganic Cellulose Esters . . . . . 21.1. Esterification . . . . . . . . . . . . . 31.2. Cellulose Nitrate . . . . . . . . . . . 41.2.1. Physical Properties . . . . . . . . . . 41.2.2. Chemical Properties . . . . . . . . . . 51.2.3. Raw Materials . . . . . . . . . . . . . 71.2.4. Production . . . . . . . . . . . . . . . . 81.2.4.1. Cellulose Preparation . . . . . . . . . 81.2.4.2. Nitration . . . . . . . . . . . . . . . . . 101.2.4.3. Stabilization and Viscosity Adjust-

ment . . . . . . . . . . . . . . . . . . . 111.2.4.4. Displacement and Gelatinization . . 121.2.4.5. Acid Disposal and Environmental

Problems . . . . . . . . . . . . . . . . . 121.2.4.6. Other Nitrating Systems . . . . . . . 131.2.5. Commercial Types and Grades . . . 141.2.6. Analysis and Quality Control . . . . 151.2.7. Uses . . . . . . . . . . . . . . . . . . . 161.2.8. Legal Provisions . . . . . . . . . . . . 171.3. Other Inorganic Cellulose Esters 191.3.1. Cellulose Sulfates . . . . . . . . . . . 191.3.2. Cellulose Phosphate and Cellulose

Phosphite . . . . . . . . . . . . . . . . 191.3.3. Cellulose Halogenides . . . . . . . . 201.3.4. Cellulose Borates . . . . . . . . . . . 201.3.5. Cellulose Titanate . . . . . . . . . . . 201.3.6. Cellulose Nitrite . . . . . . . . . . . . 201.3.7. Cellulose Xanthate . . . . . . . . . . 212. Organic Esters . . . . . . . . . . . . 212.1. Cellulose Acetate . . . . . . . . . . . 222.1.1. Chemistry of Cellulose Esterifica-

tion . . . . . . . . . . . . . . . . . . . . 222.1.2. Raw Materials . . . . . . . . . . . . . 232.1.3. Industrial Processes . . . . . . . . . . 232.1.3.1. Pretreatment . . . . . . . . . . . . . . 23

2.1.3.2. Esterification . . . . . . . . . . . . . . 242.1.3.3. Hydrolysis . . . . . . . . . . . . . . . . 262.1.3.4. Precipitation and Processing . . . . 262.1.4. Recovery of Reactants . . . . . . . . 272.1.5. Properties of Cellulose Acetate . . . 272.1.6. Analysis and Quality Control . . . . 282.2. Cellulose Mixed Esters . . . . . . . 292.2.1. Production . . . . . . . . . . . . . . . . 292.2.2. Composition . . . . . . . . . . . . . . 292.2.3. Properties . . . . . . . . . . . . . . . . 292.2.4. Other Organic Cellulose Mixed Es-

ters . . . . . . . . . . . . . . . . . . . . 302.2.5. Uses . . . . . . . . . . . . . . . . . . . 302.3. Cellulose Acetate Fibers . . . . . . 302.3.1. Properties . . . . . . . . . . . . . . . . 302.3.2. Raw Materials . . . . . . . . . . . . . 312.3.3. Production . . . . . . . . . . . . . . . . 322.3.4. Uses . . . . . . . . . . . . . . . . . . . 322.3.5. Economic Aspects . . . . . . . . . . . 322.4. Plastic Molding Compounds from

Cellulose Esters . . . . . . . . . . . . 332.4.1. Physical Properties . . . . . . . . . . 332.4.2. Polymer-Modified Cellulose Mixed

Esters . . . . . . . . . . . . . . . . . . . 372.4.3. Chemical Properties . . . . . . . . . . 382.4.4. Raw Materials . . . . . . . . . . . . . 392.4.5. Production . . . . . . . . . . . . . . . . 402.4.6. Trade Names . . . . . . . . . . . . . . 402.4.7. Quality Requirements and Quality

Testing . . . . . . . . . . . . . . . . . . 402.4.8. Storage and Transportation . . . . . 402.4.9. Uses . . . . . . . . . . . . . . . . . . . 412.4.10. Toxicology and Occupational Health 423. References . . . . . . . . . . . . . . . 42

2 Cellulose Esters

1. Inorganic Cellulose Esters

Definition. Cellulose esters are cellulosederivatives which result by the esterification ofthe free hydroxyl groups of the cellulose withone or more acids, whereby cellulose reactsas a trivalent polymeric alcohol. Esterificationcan be carried out by using mineral acids aswell as organic acids or their anhydrides withthe aid of dehydrating substances. Cellulose ni-trate [9004-70-0] is the most important and onlyindustrially produced inorganic cellulose ester(abbreviation CN, according to DIN 7728, T 1,1978). A comprehensive bibliography on inor-ganic cellulose esters may be found in [12–19].

Historical Aspects [20]. The nitric acid es-ter of cellulose is the oldest known cellulosederivative and is still the most important inor-ganic cellulose ester. The term “nitrocellulose”is still used, but it is not the precise scientificterm for cellulose nitrate. Cellulose esters werefirst described and industrially used at a timewhen the structure of esters was unknown andinformation on the polymeric primary materialcellulose was not yet available.

The nitration of polysaccharides with con-centrated nitric acid had already been describedin 1832. H.Braconnot obtained a white andeasily inflammable powderwhenhe transformedstarchwith nitric acid. The product obtainedwasxyloidine. Th.-J. Pelouze treated paper withnitric acid and obtained an insoluble productcontaining ca. 6 % nitrogen which he called py-roxiline and which he provided for military use.

C. F. Schonbein andR.Bottger are consid-ered to be the inventors of so-called gun cotton(1845). They transformed cotton with a mixtureof nitric and sulfuric acid into a highly nitratedproduct that could serve as a substitute for blackpowder. Production on an industrial scale wasstopped in 1847 because its extremely rapid cat-alytic decomposition was the cause of numerousplant explosions. Productionwas legally prohib-ited in 1865.

The use of cellulose nitrate as an explosivebrought new momentum to its further industrialand scientific development, as well as to its eco-nomic significance. F.Abelmade a basic break-through in 1865 when he succeeded in develop-ing a safe method of handling. He was able toachieve a better washing of the adhering nitrat-

ing acid and a hydrolytic decomposition of theunstable sulfuric acid ester by grinding the ni-trated fibers in water. This process allowed thisproduct to attain military importance for its useas gunpowder.

In 1875, A.Nobel phlegmatized nitroglyc-erine by mixing with cellulose nitrate and dis-covered blasting gelatin. In the 1880s, smoke-less gunpowders were developed. Vieille de-veloped Poudre B (blanche) and Nobel devel-oped Ballistit, the first dibasic gunpowder fromcellulose nitrate and nitrogycerol. Abel andDewar developed a similar gunpowder calledCordit.

The discovery that fibrous products could bemodified by, for example, dissolution in an al-cohol/ether mixture (film for wound protection)or by gelatinization with softeners brought addi-tional uses. Filmsmade fromcamphor and castoroil were used in collodium photography as carri-ers for light-sensitivematerials (Archer, 1851).Nitrofilms found increasing use in photographyand cinematography until they were replaced bynonflammable cellulose acetate films.

The year 1869 is considered to be the begin-ning of the age of plastics. J.W.Hyatt discov-ered celluloid, the first thermoplastic syntheticmaterial. It was originally used as a substitutefor ivory in the production of billiard balls.

The practical use of cellulose nitrate as araw material for lacquers began in 1882, whenStevens suggested amyl acetate as a highlyvolatile solvent (Zaponlack). The nitro lacquersachieved importance at first after World War I(Flaherty, 1921), when new applications werebeing sought as a result of the sharply decliningdemand for gunpowder. Only after the possibil-ity of depolymerization of cellulose nitrate bypressure boiling during production had becomeknown during the 1930s it was possible to usecellulose nitrate in protective and pigmented lac-quers. Thus, it became possible to use nitro lac-quers for painting automobiles on the assemblyline.

Cellulose xanthate [9032-37-5] is a celluloseester obtained with the inorganic acid dithiocar-bonic acid. Ch.F. Cross discovered this impor-tant alkali-soluble cellulose ester in 1891 whilehe was reacting cellulose with alkali and carbondisulfide. It represents the base of the viscoseprocess introduced in 1894 by Bevan and Bea-

Cellulose Esters 3

dle for producing man-made cellulosic fibers(rayon, rayon staple) and cellophane.

Other cellulose esterswith inorganic acids arepresently only of theoretical interest and havenot attained any industrial or economic impor-tance.

Present Significance. Cellulose nitrate isstill important, 150 years after its discovery. It isindustrially produced in large quantities for di-versified applications. The reasons for this arethe relatively simple production process withhigh yields, its solubility in organic solvent sys-tems and its excellent film-forming propertiesfrom such solutions (collodion cotton as a rawmaterial for lacquers), compatibility and gelatin-ability with softeners and other polymers (ther-moplastics), as well as inflammability (gun cot-ton for explosives). Cellulose nitrate has main-tained its importance as a raw material for themanufacture of protective and coating lacquersas well as blasting agents and explosives.

Densified products, colored or pigmentedchips kneaded with softeners, as well as aque-ous dispersion systems with a low solvent con-tent, are available today. They facilitate trans-port and processing, secure existing applicationforms, open up new ones, and are becoming in-creasingly nonpolluting.

The viscose process (see →Cellulose,Chap. 3.1.1.) with its essential intermediate cel-lulose xanthatewill remain, because of the avail-ability of a constantly regrowing raw materialsupply, an important source of textile fibers foryears to come. Alternative processes are in-tensively being sought to reduce pollution bysulfurous decomposition gases resulting duringmanufacturing. Cellulose nitrite, the celluloseester of nitrous acid, is the most prominent ex-ample.

Cellulose esters with other inorganic acidshave been frequently described and investigated.Cellulose sulfate [9032-43-3] was of some in-terest because of its solubility in water, butnever achieved any practical importance. Cellu-lose phosphate [9015-14-9], borate, and titanateshow interesting properties such as fire retarda-tion, but are not yet of any industrial signifi-cance.

1.1. Esterification

Mechanism. The alcoholic hydroxyl groupsof cellulose are polar and can be substitutedby nucleophilic groups in strongly acid solu-tions. The mechanism of esterification assumesthe formation of a cellulose oxonium ion fol-lowed by the nucleophilic substitution of an acidresidue and the splitting off of water. Esterifica-tion is in equilibrium with the reverse reaction;saponification can be inhibited largely by bind-ing the resulting water.

Course of Reaction. The three functionalhydroxyl groups on each anhydroglucose unitof cellulose are blocked by intermolecular andintramolecular hydrogen bonds and, therefore,are not freely accessible for the reaction part-ners. The supermolecular arrangement and mi-crostructure within the cellulose fiber, whose in-tensity depends on the origin and previous his-tory of the cellulose material, is determined bythese hydrogen bonds. The accessibility to thereaction partners and the reactivity of the alco-hol groups also depend on this structure.

Due to the fact that cellulose is insoluble inall common solvents, reactions to form deriva-tives are usually carried out in heterogeneoussystems. As the reaction proceeds, new reactivecenters are created so that ultimately almost allparts of the cellulose fibers are included and inspecial cases yield soluble derivatives which re-act to completion in a homogeneous phase.

Little information is available on the esterifi-cation process. The following two reaction typesare under discussion:

– An intermicellar reaction, which initiallyconsists of the penetration of the reactionpartner into the so-called amorphous regionsbetween the highly organized cellulose mi-celles and proceeds during the course of es-terification from the surface to the innermostregions of the micelles. The reaction speedis determined by diffusion.

4 Cellulose Esters

– An intramicellar or permutoid reaction, inwhich the reagent penetrates all regions in-cluding the micelles so that practically allcellulose molecules react almost simultane-ously. The reaction speed is specified by ad-justment of the esterification equilibrium.

Arguments for bothmechanisms are based onX-ray analyses. The possibility exists that bothreaction types occur and ultimately merge. Thisdepends on the reaction conditions, especiallythe esterification mixture and the temperature.

The hydrogen bonds between the cellulosemolecules are almost completely broken downduring esterification. The introduction of estergroups separates the cellulose chains so com-pletely that the fiber structure is either alteredor completely destroyed. Whether the celluloseester is soluble in a solvent or in water dependson the types of substituents added.

Substitution. The esterification reactions donot necessarily proceed stoichiometrically be-cause of equilibrium adjustment. The maximalattainable substitution with a mean degree ofsubstitution (DS) of 3 is generally not reached.Atriester can only be obtainedunder carefully con-trolled conditions. The primary hydroxyl groupon the C-6 atom reacts most readily, while theneighboring hydroxyl groups on theC-2 andC-3atoms of the anhydroglucose ring react consid-erably slower due to steric hindrance.

Basically, esterification is possiblewith all in-organic acids. Limiting factors are the type andthe size of the acid residue as well as the vary-ing degree of acid-catalyzed hydrolysis, whichcan lead to a complete cleavage of the cellulosemolecule as the result of statistical chain split-ting.

1.2. Cellulose Nitrate

Summarymonographs on cellulose nitrate in ad-dition to those in the Reference list can be foundin [21] and [22].

1.2.1. Physical Properties

Cellulose nitrate (CN) is a white, odorless, andtasteless substance. Its characteristics are depen-dent on the degree of substitution.

Density. Thedensity of cellulose nitrate is de-pendent on its nitrogen content and, therefore, onthe degree of substitution (Table 1).

Table 1. The density of cellulose nitrate in relation to its nitrogencontent (degree of substitution)

Nitrogen content, % Degree of Density atsubstitution (DS) 20 C, g/cm3

11.5 2.1 1.5412.6 2.45 1.6513.3 2.7 1.71

The bulk density of commercially availableCN types is between 0.25 and 0.60 kg/L formoistened CN cotton, 0.15 – 0.40 kg/L whenconverted to dry mass.

Cellulose nitrate chips, which contain at least18 % dibutyl phthalate in addition to cellulosenitrate, have a density of 1.45 g/cm3 (measuredat 20 C in an air-comparison pycnometer). Thebulk density is 0.3 – 0.65 kg/L.Specific Surface. The laboratory apparatus

described by S. Rossin [23] is best suited for thedetermination of the specific surface of cellulosenitrate, which is 1 850 – 4 700 cm2/g, dependingon the fineness of the cellulose nitrate.

The determination of the inner surface ac-cording to the BET method showed dependenceon the molar mass (i.e., an inner surface area of1.44m2/g would correspond to a molar mass of180 000 g and a surface area of 2.41m2/g wouldcorrespond to a molar mass of 400 000 g).

It must, however, be noted that the degassingtemperature was lowered from the usual 200 Cto 60 C due to the fact that cellulose nitrate de-flagrates at 180 C. It is possible that completedesorption did not take place under these condi-tions.Thermodynamic Properties , see [24,

pp. 137 –154]. The most important thermody-namic properties are listed in Table 2.

Table 2. Thermodynamic properties of some cellulose nitrates

Heat offormation

trinitrate − 2.19 kJ/g

dinitrate − 2.99 kJ/gcellulose − 5.95 kJ/g

Heat ofcombustion

trinitrate − 9.13 kJ/g

dinitrate − 10.91 kJ/gcellulose − 17.43 kJ/g

Cellulose Esters 5

Table 2. continued

Specific heat celluloid film(70 % CN and 30 %camphor)

1.26 – 1.76 J g−1 K−1

Thermalconductivity

celluloid film(70 % CN and 30 %camphor)

0.84 kJm−1 h−1 K−1

Heat of solutionin acetone

CN with 11.5 % Ncontent

− 73.25 J/g

CN with 14.0 % Ncontent

− 81.64 J/g

Electrical Properties [24, p. 136]. The fol-lowing electrical properties were measured oncellulose nitrate containing 30wt % camphor(celluloid):

Dielectric constantat 50 – 60Hz7.0 – 7.5

106 Hz6.0 – 6.5Dissipation factor (tan δ)

at 50 – 60Hz0.09 – 0.12106 Hz0.06 – 0.09

Specific resistance 1011 – 1012 Ω · cm

Mechanical Properties [25].The stress – straindiagramof cellulose nitratefilms shows the elon-gation and tensile strength to be dependent onthe size of the molecule (expressed as a term ofviscosity).

The higher the molecular mass of a CN, themore elastic is the film made from it. Films be-come more brittle and their tensile strength de-clines with decreasing molecular mass (see Ta-ble 3).

Table 3.Mechanical properties of CN lacquer films

Type∗ Elongation, Tensile strength,% N/mm2

E 4 24 – 30 98 – 103E 6 23 – 28 98 – 103E 9 23 – 28 88 – 98E 13 20 – 25 88 – 98E 15 18 – 23 78 – 98E 21 12 – 18 78 – 88E 22 10 – 15 74 – 84E 24 8 – 12 69 – 78E 27 5 – 10 59 – 69E 32 <5 39 – 49E 34 <3 29 – 49

∗ According to DIN 53179: The E-type designation specifies theCN concentration (% in dry condition) in acetone which gives aviscosity of 400± 25mPa · s.

Optical Properties. Cellulose nitrate filmsare optically anisotropic because of their micro-crystalline structure. The colors change in po-

larized light in relation to the nitrogen contentof the CN:

11.4 % N weakly red11.5 – 11.8 % N yellow12.0 – 12.6 % N blue to green

The index of refraction is 1.51, and the max-imal light transmission is achieved at 313 nm.Light Stability. Exposure to sunlight, and es-

pecially to ultraviolet light, has a detrimental ef-fect on cellulose nitrate film by causing it to be-come yellowish and brittle. Solvents, softeners,and resins can either promote or hinder yellow-ing.

1.2.2. Chemical Properties

The three hydroxyl groups of cellulose can becompletely or partially esterified by nitratingacid. The varying degrees of nitration can be re-lated to the following theoretical nitrogen con-tents:

Cellulose mononitrate, C6H7O2(OH)2(ONO2):6.75 % N

Cellulose dinitrate, C6H7O2(OH)(ONO2)2:11.11 % N

Cellulose trinitrate, C6H7O2(ONO2)3:14.14 % N

Cellulose nitrate with a nitrogen content bet-ween 10.8 and 12.6 % is a suitable raw materialfor lacquers, and CNwith> 12.3%N is suitablefor explosives exclusively.Degree of Substitution – Nitrogen Content –

Solubility. The degree of substitution can be cal-culated from the nitrogen content of the variousCN types (Fig. 1). The degree of substitution de-termines the solubility of cellulose nitrate in or-ganic solvents. CN for lacquers can be classifiedaccording to its solubility in organic solvents asfollows:

alcohol-soluble CN (A types)nitrogen content: approx. 10.9 – 11.3 %readily soluble in alcohols, esters, and ke-tones

moderately soluble CN (AM types)nitrogen content: approx. 11.4 – 11.7 %soluble in esters, ketones, and glycolethers with excellent blendability or com-patibility with alcohol

CN soluble in esters (E types)

6 Cellulose Esters

nitrogen content: 11.8 – 12.2%for lacquercotton, up to 13.7 % for guncottonreadily soluble in esters, ketones, and gly-col ethers

Figure 1. Variation of the degree of substitution with thenitrogen content of cellulose nitrate

Intrinsic Viscosity – Degree of Polymer-ization [24, pp. 85 – 121]. The mean numberof anhydroglucose units in cellulose nitratemolecules is designated as the mean degree ofpolymerization (DP). The viscosity of the so-lution (at the same concentration in the samesolvent) is generally considered to be a rel-ative measure of the molecular mass. Themolecular mass can be mathematically ex-pressed as a function of the intrinsic vis-cosity (Staudinger –Mark –Houwink equation).For further information →Plastics, Analysis.Distribution of the Molecular Mass. The

starting material of cellulose nitrate is natu-ral cellulose, the quality of which is subjectedto annual growth cycles. It is, therefore, of greatimportance to have polymolecular data, suchas the mean degree of polymerization and thedistribution of the molecular mass, available inaddition to viscosity, solubility behavior, andnitrogen values. These values are important, forexample, in assessing the mechanical propertiesand aging processes of polymer products.

The isolation of the polymers according totheir molecular mass can be achieved elegantlyby gel permeation chromatography (GPC).Chemical Compatibility. An everyday use of

cellulose nitrate is in nitro lacquers, where it isdissolved in organic solvents. In this solution,cellulose nitrate is extremely compatible withessential substances in the lacquer formulation

such as alkyd resins,maleic resins, ketone resins,urea resins, and polyacrylates. A large numberof softeners, such as adipates, phthalates, phos-phates, and raw and saturated vegetable oils arecompatible with cellulose nitrate.Chemical and Thermal Stability. Cellulose

nitrate, as a solid or in solution, should not bebrought into contact with strong acids (degra-dation), bases (denitration), or organic amines(decomposition) since they all induce a destruc-tion of cellulose nitrate. This may proceed veryrapidly and lead to deflagration of the cellulosenitrate.

The ester bonds of cellulose nitrate, whichcan be broken by saponifying agents or by catal-ysis, are responsible for its physicochemicalinstability. This substance-specific property isdependent on the temperature, the specimen,and whether catalytically active decompositionproducts remain or are removed from the sam-ple.

Another basic instability of cellulose ni-trate is observed during the production process.Mixed sulfuric acid esters transmit a chemicalinstability to the nitrocellulose molecule. Thesemixed esters are destroyed in weakly acid waterduring the stabilization phase of production. Thelong reaction time required by this procedurecan be considerably shortened by increasing thereaction temperature. The time required can bereduced to only a few hours by raising the tem-perature to 60 – 110 C. Under these conditionsthe nitrate ester remains stable; the glucosidicbond of cellulose nitrate, on the other hand, isattacked. This property is used to advantage tospecifically reduce the degree of polymerizationof the cellulose nitrate.

Thermogravimetry, IR spectroscopy, andelectron spectroscopy (ESCA) [26], [27] havebeen used to determine the extent of thermallyinduced and light-induced decomposition of cel-lulose nitrate. The reaction proceeds as follows:

Cell –O –NO2 −→ Cell –O ·+ ·NO2

It is proceeded by a series of extremely exother-mic oxidation reactions triggered by the NO2radical, which often leads to spontaneous defla-gration. NO2 is reduced to NO and in the pres-ence of air NO2 is reformed, thus initiating anautocatalytic chain reaction, at the end of whichthe gaseous reaction products COx, NOx, N2,H2O, and HCHO are found.

Cellulose Esters 7

By adding stabilizers such as weak organicbases (diphenylamine) or acids (phosphoricacid, citric acid, or tartaric acid), intermediarynitric oxides can be bound and the autocatalyticdecomposition prevented.

Thermal decomposition does not occur attemperatures below 100 C. The temperature(according to [28]) at which cellulose nitratespontaneously deflagrates is used as a mea-sure of its thermal stability. A well-stabilizedlacquer cotton has a deflagration tempera-ture of ≥ 180 C. The deflagration temperatureof plasticized cellulose nitrate chips with atleast 18wt % softener (i.e., dibutyl or dioctylphthalate) is ≥ 170 C.

The Bergmann – Junk test [29] and the warmstorage test are additionalmethods for determin-ing the stability of cellulose nitrate.

1.2.3. Raw Materials

Cellulose. Until the beginning of World WarI, the only raw material available for nitrationwas cellulose obtained from cotton in the formof bleached linters (as flakes or crape). This wasdue to the high degree of purity (α-cellulose> 98 %), which allowed a high yield and prod-ucts with good clarity and little yellowing.

Especially in times when linters were scarce,it was possible to produce gunpowder fromwood celluloses, even unbleached, other cellu-lose fibers (annual plants), and even from woodif attention was given to the adequate disinte-gration of the raw materials. Lacquer types ob-tained from wood celluloses, especially fromhardwood, gave dull and mat films and lacquerswith inferior mechanical properties. This is dueto the high content of pentosans,which is also ni-trated but is easily split by hydrolysis in conven-tional nitrating acid systems and thus becomesinsoluble.

The development of highly purifiedchemical-grade wood pulp by refinement withhot and cold alkali having R18 values of92 – 95 % (see Table 16) allows this type ofraw material to be used in the same manner aswere linters, which currently are used only forthe production of special and highly viscous CNtypes. The highly refined prehydrolyzed sulfatepulps with R18 values of above 96 % are es-pecially well-suited for nitration. The viscosity

range of CN products can be adjusted in advanceby choosing an initial cellulose with an adequateDP. A low ash content, and above all a low cal-cium content, of the cellulose is important inpreventing calcium sulfate precipitation duringindustrial nitration.

A comparative study on the nitrating behav-ior of linters and wood pulps [30] shows themorphological factors of the fibers (fiber lengthand distribution, cross-section form and thick-ness of the secondary wall, and fine structureincluding packing density, degree of crystalliza-tion, and lateral arrangement of the fibrils), thechemical composition of the cellulose (DP andpolydispersibility) as well as the type, quantity,and topographic distribution of the accompany-ing hemicelluloses and lignin to be responsiblefor the nitrating capability of celluloses. Thesefactors determine the swelling properties andthereby the uniformity of nitration, as well asthe compressibility and the relaxation capacityof the fibers, which in turn influence the reten-tion capacity of the fiber mass. Linters with alower acid retention capacity of 110 – 130 % aredefinitely superior to wood pulp (acid retentioncapacity of prehydrolyzed sulfate pulps up to230 %, of sulfite pulps up to 300 %) in this re-spect. The suitability of a raw material for ni-tration can be tested by a specially developedmachine that measures the compression and re-laxation characteristics of a cellulose fiber pile.

Approximately 150 000 t (3.4 %) of the an-nual worldwide production of 4.4×106 t ofchemical-grade pulps are used for the produc-tion of cellulose nitrate.

Industrial Nitrating Agents. The so-callednitrating acid as developed by Schonbein, thenitric acid/sulfuric acid/water system, is still thenitrating agent of choice for industrial purposes.The highest attainable degree of substitution us-

ing this system is at DS 2.7∧= 13.4 % N. This is

achieved only when the nitric acid used is nothydrated and the molar ratio of nitric acid tosulfuric acid monohydrate is 1 : 2. The optimalnitrating mixture is as follows:

HNO3 H2SO4 H2O

molar ratio 1 2 2wt % 21.36 66.44 12.20

8 Cellulose Esters

Water plays a special role as far as the at-tainable degree of nitration is concerned. Below12 % water there is no increase in substitution,but a higher water content results in a drasticdecline in the degree of nitration (Fig. 2).

Figure 2. Dependence of the degree of esterification (DS)on the water content of the optimal nitrating mixture(HNO3 : H2SO4= 1 : 2)

It is assumed that increasingly hydrated nitricacid causes increased swelling and gelatiniza-tion of the cellulose so that the nitrating acid isno longer able to penetrate into the inner struc-tures of the micelles.

The desired degrees of esterification can beadjusted by varying the nitrating acid mixtureaccording to the CN types (Table 4), wherebyin industrial processes the nitric acid content iskept nearly constant at 25 – 26 %.

The ternary system HNO3/H2SO4/H2O hasbeen extensively investigated. The results aresummarized in Figure 3.

The curves of the same degrees of substitu-tion (% N) in relation to the nitrating acid com-position are presented here. The cross-hatchedband identifies those areas inwhich the cellulosematerial is extremely swollen and gelatinized.Three zones can therefore be differentiated inthe phase diagram:

1) Area of technical nitration:nitric acid 15 – 100 %sulfuric acid 0 – 80 %water 0 – 20 %In this range, nitric acid is present in a non-hydrated form and induces true nitration (Ncontent 10 %). The industrially used rangewith 20 – 30 % HNO3, 55 – 65 % H2SO4,

and 8 – 20 % water is also included in thisarea.

2) Area of solution:nitric acid 0 – 10 %sulfuric acid 60 – 100 %water 0 – 40 %Little or no nitration takes place in thisrange. Cellulose is degraded to the point ofcomplete dissolution in concentrated sulfu-ric acid.

3) Area of swelling:Nitric acid is increasingly hydrated in thisrange of increasing water content. Nitrationdecreases rapidly.

A process developed in the United States,but less important, uses magnesium nitrate in-stead of sulfuric acid as a dehydrating agent[31]. Magnesium nitrate can bind water as itshexahydrate. The nitrating mixture consists of45 – 94 % nitric acid, 3.3 – 34 % magnesium ni-trate, and 2.7 – 21 % water; the ratio of magne-sium nitrate : water is 1.2 – 2.2 : 1. A cellulosenitrate with an N content of 11.9 % was ob-tained, for example, with 64.5 % HNO3, 19.5 %Mg(NO3)2, and 16 % H2O. This nitrating sys-tem is appropriate for a continuous process, inwhich waste and washing acid are reprocessedin ion exchangers and the magnesium nitrate isrecycled. Thus, acid and sulfate no longer posea waste disposal problem.

1.2.4. Production

The flow diagram (Fig. 4) shows the industrialproduction of CN according to the mixed acidprocess. The viscosity of the end product is de-termined by the choice of the initial cellulose,and the degree of nitration is determined by thecomposition of the mixing acid. The final vis-cosity adjustment follows during the pressureboiling step (see Section 1.2.4.3).

1.2.4.1. Cellulose Preparation

Cotton linters with a moisture content of upto 7 % are mechanically disintegrated homoge-neously. Pressed pulp sheets must be appropri-ately shredded to obtain rapid and uniform ni-tration. Spruce or beech celluloses, preactivatedwith 20 % sodium hydroxide (mercerization),

Cellulose Esters 9

Figure 3. Composition of the nitrating mixtures and attainable N contents of cellulose nitrates

Figure 4. Flow diagram of cellulose nitrate production

10 Cellulose Esters

Table 4. Industrially used nitrating acid solutions

CN type Nitrating acid N content, DS

% HNO3 % H2SO4 % H2O %

Lacquer cotton A 25 55.7 19.3 10.75 1.90Celluloid cotton 25 55.8 19.2 10.90 1.95Lacquer cotton AM 25 56.6 18.4 11.30 2.05Dynamite cotton 25 59.0 16.0 12.10 2.30Lacquer cotton E 25 59.5 15.5 12.30 2.35Powder cotton 25 59.8 15.2 12.60 2.45Gun cotton 25 66.5 8.5 13.40 2.70

were formerly used for this purpose in the formof crape papers with a mass per unit area ofca. 25 g/m3. To avoid the costly transformationof the cellulose to paper sheets, a process wasattempted to obtain a loose product resemblinglinters by direct disintegration of pulps to fibers.A moisture content of 50 % proved to be opti-mal for nitration and washing out the acid. Therequired drying of the cellulose flakes before ni-tration proved to be disadvantageous.

The Stern shredder [32], in which the pulpsheets are torn rather than being cut into smallelongated shreds to avoid compression at theedges, was a definite improvement. Currently,cellulose for nitration is used in the form of fluff,shreds, or chips. The packing density and com-pression behavior of the cellulose fibers in thefiber pile are decisive factors for the swellingand nitration kinetics, as well as the acid reten-tion capability [30].

1.2.4.2. Nitration

Nitration on an industrial scale is still frequentlycarried out according to a batch process that wasdeveloped from a process described by DuPontin 1922. The equipment is constructed of stain-less steel. The adjusted and preheated nitratingacid reaches the stirring reactor that is chargedwith cellulose by means of a measuring system;a large excess of acid (1 : 20 to 1 : 50) is addedto retain the ability of the reaction mixture to bestirred and to ensure that heat is carried off. Thenitrating temperature is between 10 C (dyna-mite type) and 36 C (celluloid type). The totalheat of reaction is estimated to be over 200 kJ perkg of CN, of which the enthalpy of formation ofCN is about one-third.

Even though the reaction is nearly completeafter ca. 5min, the mixture remains in the reac-

tor for about 30min. The temperature must re-main constant (cooling), since hydrolytic degra-dation processes that lead to considerable lossesin yield begin at temperatures as low as 40 C.

The theoretical yield of commonly used in-

dustrial types with a DS of 1.8 – 2.7 (∧= 10.4 –

13.4%N) is between150 and176%with respectto cellulose. The practical yield, however, is upto 15 % lower and depends on the type and pu-rity of the cellulose, aswell as on the temperatureand duration of nitration. Losses arise from theinevitable complete decomposition of celluloseto oxalic acid by way of oligo- and monosac-charides, whereby the nitric acid is reduced tonitrogen oxides, NOx. In addition, mechanicallosses during the subsequent separation process,due particularly to short fibers (cellulose fromhard wood), must also be taken into considera-tion.

The reaction mixture is drained from the re-actor into the centrifuge,where the excess acid isseparated and removed at high speed and repro-cessed for recycling. The mixture must remainmoist so that it does not ignite and deflagrate.

The degree with which the product retainsacid after separation is of economic importancebecause of the acid loss and the expense of theensuing washings. Linters with an acid retentionof 100 – 130 % clearly surpass wood cellulosesin this respect which, depending on the woodand cellulose type as well as its processing, canretain up to 3 times more adhering acid relativeto CN [30].

The still acid-moist product is immediatelyplaced into a great excess of water (consistency1 %) so that the adhering acid is displaced asrapidly as possible and the saponification of theCN is prevented.

Continuous nitrating processes, which aremore economical, were developed in the 1960s

Cellulose Esters 11

[33], [34]; they ensure a more uniform productquality and are safer to handle. The nitratingsystem consists of two or more consecutivelyarranged straight-run vats or tube systems con-taining conveyers (screw conveyer or turbulencestirring apparatus) which forward the reactionmixture. The prepared cellulose is directed intothis cascading equipment from storage bunkersover automatic weighing scales and continu-ously mixed with the added nitrating acid. It isimportant that the cellulose is rapidly added andimmediately covered with acid. There it remainsfor 30 – 55min. A newer process using a contin-uous loop-formed pressure reactor [35] requiresthe cellulose to remain only for 6 – 12min. Thereactant is then sent into a continuously oper-ating special centrifuge, where the excess acidis separated and simultaneously taken up withwater. The fact that the reactant remains onlya few seconds reduces the risk of spontaneousdeflagration and saponification.

Figure 5 shows schematically the continuousprocess according to Hercules [31].

The broken-up and preconditioned cellulose(a) is brought by way of the automatic scales(b) to the continuous reactor (c). The reactionproduct is centrifuged in a washing zone (d) andsimultaneously washed by zones with water ina countercurrent. The product leaves the cen-trifuge almost free of acid, and the washed outacid can be recycled and reused almost withoutloss [36].

1.2.4.3. Stabilization and ViscosityAdjustment

The prestabilization step following the prewash-ing further purifies the product by means of re-peated washing and boiling with water that con-tains 0.5 – 1 % acid residue. The batch methodrequires large amounts of space, water, and en-ergy; the required boiling time varies between6 (celluloid type) and 40 h (guncotton). Auto-matic continuousprocesses havebeendevelopedin this case as well [37].

Most of the remaining sulfuric acid is re-moved during prestabilization, since it wouldpromote the catalytic self-decomposition of CN.The sulfuric acid is bound by adsorption andesterified. A total sulfate content of 1 – 3 %was found in weakly nitrated CN, of which

70 – 85% is in the formof the acidic sulfuric acidsemiester, while highly nitrated CN containsonly 0.2 –0.5 % total sulfate, of which 15 – 40%is thought to exist as an ester. Semiesters can beeasily saponified andwashed out by boilingwithwater. It is not yet certain whether the so-calledresistant sulfate content exists in the form of theneutral sulfuric acid ester or the physically ad-sorbed sulfuric acid.

The desired final viscosity of the CN is ad-justed in the following process step, which ispressure boiling (digestion under pressure) in aconsistency of 6 – 8%at 130 – 150 C, bymeansof specific degradation of the degree of polymer-ization. The remaining extremely low sulfuricacid content induces hydrolytic decompositionat this temperature and under pressure. The vis-cosity can, for example, be reduced to 1/10 ofthe initial viscosity within 3 h at 132 C by us-ing this process. This process made the develop-ment of high solid coating and protective nitrolacquers possible. The stabilization process ofguncottons is accelerated by pressure boiling;dynamite wools are usually not pressure boiled.

Further product losses are due to chain degra-dation ranging fromsoluble cleavage products tooxalic acid. Nitrous gases (NOx) are released bythe reduction of nitric acid, which must be con-tinuously drawn off to avoid decomposition ofCN.

Pressure boiling can be achieved batchwisein autoclaves, as well as continuously in a tubereactor of 1 500m in length and100mmindiam-eter, e.g., with direct steam. A one-pot processin which prestabilization, pressure boiling, andpoststabilization are carried out in one operationis described in [38].

During the stabilization process, the remain-ing sulfuric acid is almost completely removedby additional washing and boiling. While cel-luloid and lacquer types are finished in flaky,fibrous form, guncotton must be ground. This isdone in grinding hollander engines at 12 – 15 %consistency or continuously in a series of conerefiners, whereby the material is gradually con-centrated from 3 to 10 % between the vari-ous grinding steps. Sorting steps are insertedby hydrocyclones during the final washing pro-cesses. The last acid remnants in the fiber cap-illaries are removed during the grinding processbymeans of diffusion againstwater.Weak bases,sodium carbonate, or chalk are used to main-

12 Cellulose Esters

Figure 5. Continuous cellulose nitrate production according to Herculesa) Preconditioning; b) Auto-matic scale; c) Reactor; d) Washing zone; e) Centrifuge

tain a pH of 7. Stabilizers (organic acids) maybe added during this step.

1.2.4.4. Displacement and Gelatinization

A water-wet CN cotton with a water content of25 – 35 % remains in the centrifuge after the fi-nal separation and is then packed into drums orPE sacks.

Water contained in celluloid and lacquertypes is displaced by alcohols specified bythe processors (ethanol, 2-propanol, n-butanol)in displacement presses or displacement cen-trifuges. Continuous processes prevail here also[39]. The resulting aqueous alcoholsmust bedis-tilled to remove the water.

The water-wet CN cotton can be gelatinizedwith softeners such as phthalates in kneadingaggregates and dried on drum or band driers forthe production of CN chips [40], [41]. Coloredchips are obtained by adding carbon black orpigments from which colored enamels can beproduced without the use of ball mills or rollermills.

1.2.4.5. Acid Disposal and EnvironmentalProblems

The nitrous gases formed during the nitratingand stabilizing side reactions are drawn off andwashed out in trickling towers. The lower ni-trogen oxides are regained after oxidation as50 – 60 % nitric acid.

The waste acids resulting from the first sep-aration contain 2 – 3 % more water and 3 – 4 %less nitric acid than the initial mixture. They arecirculated in a closed system and constantly re-generated with nitric acid and oleum. The acidthat adheres to the productmust also be replaced.

The proportion of adhering acid depends onthe initial cellulose and the CN type. It rangesbetween 80% (guncotton) and up to 200 – 300%(lacquer types)with regard toCNand is removedwith the water used for washing and boiling.

Aside from the economic aspects of acid lossin wastewater, environmental considerations arebeginning to play an increasingly important role.While older manufacturing facilities using sim-ple centrifuging to remove waste acid produced300m3 of water per ton of CN, containing 0.5%

Cellulose Esters 13

acid and with a pH of 1, it was possible to re-duce the volume of wastewaters to a fraction ofits previous volume by almost completely clos-ing the cycles.

Before proceeding into the draining ditch, thewastewater must be separated from the hardlydecomposable sludge consisting of cellulose andCN, and then be neutralized. The sulfate pro-portion can be reduced by calcium sulfate pre-cipitation, while the nitrate proportion remainscompletely in the wastewater. Organic matter ofcommunal sewage, for example, can be biolog-ically decomposed without additional oxygen,whereby nitrates disappear almost completelyas a result of biological denitrification.

The salt/acid process with magnesium nitrate(see Section 1.2.4.2) is more favorable with re-gard to the wastewater problem. Sulfates arecompletely absent, and magnesium nitrate is re-cycled and, therefore, causes no water pollutionproblems. The amount of wastewater can be re-duced by 80 % and the nitric acid requirementby 83 % in comparison to the formerly used dis-continuous processes.

1.2.4.6. Other Nitrating Systems

Numerous attempts have been made to improvenitration by the introduction of other nitratingsystems, or at least to increase the degree of sub-stitution. Further details may be obtained from[12], [14], [17], and [21]. Table 5 gives a sum-mary of alternative nitrating systems, none ofwhich was able to displace the ternary systemHNO3/H2SO4/H2O for industrial nitration.

Nitration with pure nitric acid is possible inprinciple. Esterification is not possible with acidconcentrations below 75%.Acid concentrationsless than 75 % cause the formation of the un-stable so-called Knecht compound, which hasbeen described as either a molecular complexor an oxonium salt of the nitric acid. Cellulosenitrates with 5 – 8 % N, which dissolve in ex-cess acid, are formed at acid concentrations of78 – 85 %. Nitrogen contents of 8 – 10 % are at-tained at concentrations between 85 and 90 %HNO3; these products have a strong tendencyto gelatinize. Heterogeneous nitration withoutapparent swelling takes place at a HNO3 con-centration above 89 %, and 13.3 % N can beachieved with 100 % HNO3. Nitration can be

increased to 13.9 % N with 100 % HNO3 byaddition of inorganic salts such as sulfates, acidphosphates, and particularly nitrates, preferablyin a 15 % concentration.

Table 5. Nitrating systems

Nitrating system Max. N Commentscontent,%

HNO3/H2SO4/H2O 13.4 Industrial nitration

HNO3 < 75 % “Knecht compound,” unstable78 – 85 % 8 Dissolution in the nitrating acid85 – 89 % 10 Gelatinization90 – 100 % 13.3 No swelling

HNO3 + nitrates,sulfates, phosphates

13.9

HNO3 vapor 13.75 Slow reaction, stable nitrate

HNO3 vapor + nitrogenoxides

13.8

N2O5 14.12

N2O5 in CCl4 14.14 Trinitrate

HNO3 in CH2Cl2 14.0

HNO3 in nitromethane 14.0 Homogeneous reaction

HNO3 +H3PO4/P2O5 14.04 Rapid reaction withoutdecomposition (polymeranalogue)

14.12 After extraction with methanol

HNO3 + aceticacid/acetic anhydride

14.08 Great stability

14.14 After extraction with ethanol

HNO3 + propionicacid/butyric acid

14.0

The nitric acid/phosphoric acid system is ofspecial interest in a 1 : 1 ratio with 2.5 % phos-phorus pentoxide added, with which an almostcompletely nitrated product of great stabilitywas achieved. The nitric acid/acetic acid/aceticanhydride system in a ratio of 2 : 1 : 1 giveshighly nitrated and highly stable products inwhich the fiber structure remains intact largely.After extraction of these nitrates with water oralcohol, the theoretical degree of substitution ofthe trinitrate may be attained.

Nitrating systems which achieve a high de-gree of nitration without degradation of the cel-lulose chain are of special scientific interest. Thisprocess is known as polymer analogous nitra-tion.After a critical examination of all knownni-tratingmixtures, the nitric acid/acetic acid/acetic

14 Cellulose Esters

anhydride system in a ratio of 43 : 32 : 25 at0 C [42] was recommended for determining themolecular mass of native celluloses of such so-lutions by using absolute methods and the in-trinsic viscosity number [43]. The system anhy-drous nitric acid in dichloromethane also allowsthe application of such polymer analogous re-actions at temperatures between 0 and −30 C[44]. Other authors [45] prefer the system nitricacid/phosphoric acid/phosphorus pentoxide.Nitration in the Laboratory. Preparative

cellulose nitration with HNO3/H2SO4 nitratingacid to products with whatever N content upto 13.65 % is desired, stabilization and stabi-lization tests, nitration with the nitric acid/phos-phoric acid (< 13.9 % N) and nitric acid aceticanhydride systems up to the trinitrate, denitra-tion with hydrogen sulfide to cellulose II, theanalytic determination of the N and sulfate con-tent, and the solution of the CN and the viscositydetermination of the solution are extensively de-scribed in [46].

1.2.5. Commercial Types and Grades

Cellulose nitrates receive, because of their fluffystructure and cottonlike appearance, the addi-tional designation “cotton.”

Two parameters are decisive for the industrialuse of cellulose nitrate:

Nitrogen content (including the resulting sol-ubility properties)Viscosity

As seen in Table 6, cellulose nitrates withdiffering nitrogen contents have various appli-cations. Cellulose nitrates for lacquers are avail-able in numerous viscosities. It is possible tocategorize all stages of viscosity according tothe European norm (DIN 53179), but the viscos-ity of cellulose nitrates is primarily categorizedby using the Cochius method and the British orAmerican ball dropmethod (ASTMD1343-69).

In addition to the so-called cotton types den-sified CN types are available. These may be ob-tained by either nitrating compressed celluloseor by subsequently compressing the fluffy cel-lulose nitrate. It is possible to almost double thebulk density by compression.

For safety reasons, the commercially avail-ableCNcotton typesmust bewettedwith at least

25wt % water or aliphatic alcohols. In additiontowater, ethanol, n-butanol, and 2-propanolmayalso be used as wetting agent.

Table 6. Cellulose nitrate types

Type N content, Degree of% substitu-

tion (DS)

Celluloid cotton 10.5 – 11.0 1.82 – 1.97Alcohol-soluble >lacquer cotton 10.9 – 11.3 1.94 – 2.06Lacquer cotton moderately 11.4 – 11.7 2.08 – 2.17soluble in alcohol

Ester-soluble >lacquer cotton 11.8 – 12.2 2.20 – 2.32Powder cotton 12.3 – 12.9 2.55 – 2.57Gun cotton 13.0 – 13.6 2.58 – 2.76

The largest manufacturers of cellulose ni-trates are the following:

Hercules Inc. USAWolff Walsrode AG FR GermanyHagedorn FR GermanyWNC Nitrochemie GmbH FR GermanySociete Nationale des Poudreset Explosifs (SNPE) France

Imperial Chemical Industries Great Britain(ICI)

S.I.P.E. Nobel S.p.A. ItalyUnion de Explosivos RıoTinto S.A. Spain

Bofors SwedenAsahi JapanDaicel Chemical Industries, Ltd. Japan

Many countries in South America, Asia, andEastern Europe maintain small CN productionfacilities. The total world capacity may be esti-mated to 150 000 t/a of dry cellulose nitrate.

Other Commercial Types. Also available,in addition to cellulose nitrate cotton types, areso-called cellulose nitrate chips, made from cel-lulose nitrate plasticized by gelatinizing soften-ers. For safety reasons, the softener content hasbeen established at aminimumof 18wt%.Chipsare preferred in processes where alcohols inter-fere in the formulation of lacquers.

The dispersions of cellulose nitrate with soft-eners or resins in water manufactured by theWolff Walsrode AG are other available forms.The solvent-free or low-solvent dispersions arenot polluting and may be used in all areas inwhich cellulose nitrate lacquers also are used[55].

Cellulose Esters 15

1.2.6. Analysis and Quality Control

Themost important analytical characteristics re-late to the determination of the N content and,thereby, the average degree of substitution (DS),as well as the viscosity of the solution as a mea-sure of the average molecular mass or chain-length.

Analytic Tests. The most commonly usedanalytic procedures are summarized in [25],[46], [47], and [48].Dry content is determined by careful drying

of a small, thinly layered alcohol or water-wetsample at room temperature for 12 – 16 h, in aweighing glass at 100 – 105 C for 1 h, or withcompressed warm air at 60 – 65 C for 0.5 – 1 h.Ash content is determined by decomposing

a dried sample with HNO3 and incinerating theresidue. Specifications require that the ash con-tent should not be above 0.3 %.N-content is determined by reducing nitrates

according to the following reaction (Schulze-Thiemann):

NO−3 + 3 FeCl2 + 4HCl −→ 3 FeCl3 +Cl− + 2H2O+NO

or by the following reaction:

2NO−3 + 4H2SO4 + 3Hg −→ 3HgSO4 +SO

2−4 + 4H2O

+2NO

The resulting NO is collected in a Du Pont ni-trometer.

Stability Tests [25]. Deflagration Tempera-tures:Well-stabilized CN deflagrates at temper-atures above 180 C.Bergmann – Junk Test [29]: A quantity of 2 g

of dried CN is kept at a temperature of 32 Cfor 2 h in a special apparatus for the eliminationreaction, after which time the amount of the de-veloped nitrous gases (after reduction to NO) isdetermined. CN is stable according to this testif no more than 2.5 cm3 of NO per gram is mea-sured.Warm Storage Test: A quantity of 5 g of

dried CN is stored in a glass-stoppered tube at75 C. Note is then made when the first nitricoxide (red-brown gas) becomes visible. Well-stabilized CN can be stored at 75 C for at least10 days.ASTM Stability Test [48]: After storage at

134.5± 0.5 C the time is noted in which thenitrous gases discolor methyl violet test paper.

Viscosity. Viscosity according to DIN53179: If CN is dissolved in acetone in theappropriate concentration, CN solutions meet-ing this requirement show a apparent dynamicviscosity of 400± 25mPa · s in the ball dropviscometer according to Hoppler (ball no. 4) at20 C (Table 7).Cochius Viscosity [25]: The viscosity of the

various cellulose types is measured in com-monly used solvent mixtures:

A and AM types: butanol/ethyleneglycol/toluene/ethanol in the following propor-tions 1 : 2 : 3 : 4

E types: butanol/butyl acetate/toluene in thefollowing proportions 3 : 4 : 5

Dried CN is dissolved in varying concentra-tions depending on the type and the time whichan air bubble requires to rise 500mm betweentwo calibrations in a 7mmCochius tube at 18 Cismeasured in seconds. TheCochius seconds areconverted to absolute viscosity units mPa · s bymultiplying with the factor 3.64mPa.

Figure 6.Degree of polymerization (DP) and technical vis-cosity (“ball drop” in seconds of a 12.2 % CN solution with12 % N in acetone). DP = 170×viscosity

Ball drop method according to ASTM [49]:Dried CN is dissolved according to its vis-cosity stage in 12.5, 20.0, or 25.0 % etha-nol/toluene/ethyl acetate according to [48]. The

16 Cellulose Esters

Table 7. Characterization of cellulose nitrates according to DIN 53179

A types CN concentration, %(absol. dry)

AM types CN concentration, %(absol. dry)

E types CN concentration, %(absol. dry)

E 1440 4E 1160 7E 950 9E 840 12

AM 760 14AM 750 15

E 730 15AM 700 17

E 620 21E 560 22E 510 24

A 500 27 AM 500 27A 400 30 E 400 27

E 375 32AM 330 36 E 330 34

drop time of the balls with a diameter between1/4 and 1/16 in. at 25 C is given in seconds orconverted into Pa · s. Figure 6 shows the rela-tionship between the degree of polymerizationand the technical viscosity (fall velocity of theballs in a 17.2 % CN solution in acetone).

Comparative viscosity charts for convertingthe various viscosities and comparing the vari-ous types are found in [25].

Solubility and Color. The color and cloudi-ness of solutions produced according to [48]are tested visually. Consistency, appearance, anddepth of color can be controlled according to[50].Dilution with Toluene. Toluene is added to

a 12.2 % CN solution in butyl acetate at 25 Cuntil CN continuously precipitates. The dilutionfactor is noted. The dilution ratio of CN solu-tions with other solvents and blending agents isdetermined according to [51].

Film Test. The solutions made according to[48] are dilutedwith an equal volumeof butyl ac-etate and poured as a film onto a glass plate. Thedried films are examined for undissolved parti-cles, surface structure, transparency, and gloss.

1.2.7. Uses

Explosives. Explosives may be categorizedaccording to their use:

blasting agentspropellants and shooting agents

detonating agentsigniting agentspyrotechnical agents

Cellulose nitrates are used primarily as pro-pellants and gun powder, whereby the follow-ing distinctions can be made: monobasic pow-der, which is based solely on cellulose nitrate;dibasic powder, which contains further energycarriers such as, for example, nitroglycerin ordiglycol dinitrate in addition to cellulose nitrate;tribasic powder, which contains in addition tothe components of the dibasic powder a thirdagent such as nitroguanidine.

The selection of the cellulose nitrate is of spe-cial importance. The types of cellulose nitratesthat differ in the degree of nitration were stan-dardized as follows:

CP I (Collodium powder) also known as gun-cotton, nitrogen content: 13.3 – 13.5 %

CP II (Collodium) nitrogen content: 12.0 –12.7 %, mostly 12.6 %

PE (Powder standard) nitrogen content:11.5 – 12.0 %, mostly 11.5 %

Aromatic amines, such as diphenylamine, areadded to gunpowder as stabilizers. They are ca-pable of binding the nitrous gases generated dur-ing the decomposition of the nitric acid ester. Amixture of ca. 80 % highly nitrated gunpowder(13.4 %N) and ca. 20 % less-nitrated collodiumcotton (12.5 % N) is used for the production ofthe monobasic propellant powder. Since cellu-lose nitrate granules are easy to charge electro-statically, they are made conductive with a finegraphite coating.

Cellulose Esters 17

The multibasic powders usually contain cel-lulose nitrate CP II. Mixtures of 40 % PE cottonand 60 % CP I are also used because they havethe same energy content as CP II with 12.6 %N.

The introduction of a third component to trib-asic powder results in a lower heat of combus-tion in comparison to dibasic powder, therebylengthening the life of the gun barrel.

Gunpowder is used in small-arms ammuni-tion as well as large-caliber guns and tanks. (Forfurther details →Explosives.)

Lacquers. Cellulose nitrate lacquers arecharacterized by the outstanding film-formingproperties of the physically drying cellulose ni-trate. Moreover, cellulose nitrate is compatiblewith many other raw materials used in lacquersand can be used advantageously in combinationwith resins, softeners, pigments, and additives.

In addition to the nonvolatile lacquer compo-nents, the composition of the solvent mixture isdecisive for the formation of a film.

The most important uses for nitro lacquersare as follows: wood lacquers (especially furni-ture lacquers), metal lacquers, paper lacquers,foil lacquers (also as hot sealing lacquers, e.g.,cellophane, plastic, and metal foils), leather lac-quers, adhesive cements, putties, and printingink (for flexo and gravure printing).

The processes used for applying cellulose ni-trate lacquers to substrates are as follows: spray-ing (compressed-air, airless, and electrostaticspraying), casting (for example, with a curtaincoater), rolling (especially for the application ofsmall amounts of lacquer), doctor knife coating,and dipping.

The casting and rolling processes are used forlacquering large, even areas. Irregularly shapedobjects are sprayed. The choice of a suitable typeof cellulose nitrate (e.g., completely or moder-ately soluble in alcohol, soluble in esters, degreeof viscosity) is dependent on the lacquer type.A highly viscous cellulose nitrate type is usedif elastic and thin applications are desired (e.g.,leather). However, if hard and thick layers aredesired, low-viscosity types are preferred.

The concentration or the degree of viscosityof the cellulose nitrate determine the viscosity ofthe lacquer solution. However, the formulationof the lacquer must be taken into considerationwhen the mode of application is chosen. For ex-

ample, a highly viscous dipping lacquer cannotbe sprayed or casted.

Furthermore, the striking differences bet-ween ester-soluble and alcohol-soluble typesshould be taken into consideration when nitrolacquers are formulated (Table 8).

For further information on the formulation ofcellulose nitrate lacquers, see [52], [53], and also→Paints and Coatings.

Dispersions. Conventional cellulose nitratelacquers contain between 60 and 90 % organicsolvents, which are released during drying. Foreconomic and environmental reasons, it is de-sirable to substitute organic solvents by water.Aqueous cellulose nitrate/softener dispersions(e.g., Isoderm, Bayer AG; Coreal, BASF; Walo-ranN,WolffWalsrodeAG) are available for suchabsorbing substrates as leather [54]. Other aque-ous cellulose nitrate dispersions for use onwood,foil, and metal have also been developed (Walo-ran N, special-types, Wolff Walsrode AG) [55].The film forming process of water-insoluble cel-lulose nitrate requires a small amount of coales-cents in the dispersion systems.

Celluloid. A special use of cellulose nitrateis in the production of celluloid [56]. Cellulosenitrate with a nitrogen content of 10.5 – 11.0 %ismixed in a kneaderwith softeners, particularlycamphor, and solvents (alcohols).

Normal celluloid contains ca. 25 – 30% cam-phor and 70 – 75 % cellulose nitrate. Celluloidthat contains 10 – 15 % solvent can be formedinto the desired articles in heated piston or screwpresses (e.g., tubes and round and profile rods).

In the past decades, celluloid has beenwidelyreplaced by synthetic materials and thermoplas-tics. Celluloid is still of economic importance inthe following areas: combs and hair ornaments,toilet articles, office supplies (drafting and mea-suring instruments), ping-pong balls, and vari-ous special uses.

1.2.8. Legal Provisions

Toxicology and Industrial Safety. Concen-trated sulfuric acid, nitric acid, and nitrous gasesformed during the production of cellulose nitrateare considered hazardous chemical products[57]:

18 Cellulose Esters

Table 8. CN lacquer cottons

Ester-soluble type Alcohol-soluble type

Possible use of alcohol in the lacquer formulation Use of alcohol, especially ethanol, in any desired amount as asolvent

Good dilutability with aliphatic and aromatic hydrocarbons Good dilutability with aromatic hydrocarbons

Very rapid solvent release Rapid solvent release

Formation of hard films Formation of films with thermoplastic properties

Attainment of good mechanical properties as far as the cold-check test,stretch, hardness, and tensile strength are concerned

Attainment of good mechanical properties; some specialproblems of lacquer production may be solved such as:Lacquers which can be diluted with ethanol in any desiredmanner (wood polishes)Odorless lacquers (printing inks)Gel dipping lacquersHot sealing waxes (cellophane lacquers and aluminum foillacquers)

1) Sulfuric acid5 – 15 % EC-No. 016-020-01-5above 15 % EC-No. 016-020-00-8

2) Nitric acid20 – 70 % EC-No. 007-004-01-9above 70 % EC-No. 007-004-00-1

3) Nitrous gasesEC-No. 007-002-00-0

They are subjected to the Arbeitsstoff-verordnung (working substance regulation) [58]and must, therefore, be adequately labeled.

Concentrated nitric acid and mixed nitratingacids are oxidizing when brought into contactwith organic materials [59]. The MAK values(maximum working place concentration) are asfollows:

nitric acid vapors: 10mL/m3 (ppm);∧=

25mg/m3

nitrogen oxides (NO2): 5mL/m3 (ppm);∧=

9mg/m3

Employees should be examined regularly forobstructive respiratory tract illnesses.

Cellulose nitrate is neither toxic nor haz-ardous to health [60]. Damping agents in CNand nitrous gases which may be formed duringcombustion and smoldering processes are poten-tially hazardous to health if inhaled.

Commercially available phlegmatized cellu-lose nitrate for the production of lacquer withless than 12.6 % N contains at least 18 % of agelatinizing softener. According to the first para-graph in [58], cellulose nitrate is a hazardous

substance and must be packaged and labeled ac-cordingly. EEC regulations (1982) are similar.

Damping agents such as ethanol and 2-propanol are not subjected to these regulations;butanol belongs to category II d, but is not con-sidered to be hazardous to health in a dampedmixture of a maximum 35 % concentration.

Storage and Shipping. Cellulose nitrate,especially guncotton, burns in air with a yel-low flame and deflagrates if present in largerquantities, especially after rapid heating. Anexplosion can be caused by friction or a sharpimpact. Dry CN has electrostatic charge. Fric-tion, particularly on metals but also on plastics,can cause sparks which lead to a deflagration.Therefore, cellulose nitrate should be stored ina moist and cool place [60], [61]. Rooms inwhich cellulose nitrate is processed must be ad-equately protected according to the guidelinesfor protection from explosions.

Cellulose nitrate is subjected to the regula-tions governing explosives [62]. The transporta-tion of phlegmatized cellulose nitrates proceedsaccording to the most recent versions of thehazardous materials regulation; see [25]. Wet-ted cellulose nitrate is shipped in thick-walled,galvanized, tightly closing iron or fiber drumswhich are adequately labeled.

Dried cellulose nitrate may not be shippedunder any circumstances.

For further information on the properties,handling, storage, and transportation of haz-ardous goods, see also [63].

Cellulose Esters 19

1.3. Other Inorganic Cellulose Esters

Summaries on the esterification products of cel-lulose with other inorganic acids may be foundin [12–19]. For publications on the modificationof cellulose, including esterification, see [64].

1.3.1. Cellulose Sulfates

Cellulose sulfates [9032-43-3] are the most fre-quently investigated of all other inorganic cellu-lose esters. The ability of concentrated sulfuricacid to dissolve cellulose, particularly in concen-trations between 70 and 75 %, has been knownsince 1819. After precipitation immediately fol-lowing dissolution, the cellulose contains littleor no bound sulfate. An almost homogeneous es-terification takes place only if the cellulose is leftin a sulfuric acid solution over a longer period oftime. However, the ester yield is very poor. Themajor portions of the reaction products consistof hydrolytically split decomposition productswith a maximum degree of substitution of 1.5.

In their free acidic form, cellulose sulfatesare fairly unstable and easily saponified. Asemiester was developed in 1953 in the UnitedStates [65] in an esterification mixture consist-ing of 1mol of cellulose with 20 – 30 % water,3.5 – 15mol of sulfuric acid, 0.3 – 1.0mol of aprimary or secondary C3–C5 alcohol, and aninert volatile organic solvent such as toluene orcarbon tetrachloride (reaction temperatures bet-ween − 5 and − 10 C). The product was solu-ble in hot or cold water, yielded relatively sta-ble, clear, and highly viscous solutions, and wasrecommended for use as a thickener for aque-ous systems (emulsion paints and printing inks,printing pastes for textiles, and food products),as well as for fat- and oil-proof finish, and aspaper glue. This product, however, is of no eco-nomic importance.

Numerous attempts have been made tofind improved preparation methods for water-soluble cellulose sulfates stable to saponifica-tion.Known reaction systems are summarized ina tabular overview [65]. The reaction of cellulosewith sulfuric acid in organic solvents, especiallyin lower-mass aliphatic alcohols, gives by wayof a heterogeneous reaction fibrous and water-soluble cellulose sulfates with a maximum DSvalue of 1. More highly substituted products are

obtained by reaction with sulfuric acid/aceticacid anhydride (up to a DS value of 2.8) oresterification with chlorosulfuric acid in pyri-dine or formamide. The reaction with SO3 onlyor in various organic systems yields trisubsti-tuted products. The reaction mechanism may bedescribed as the addition of the strongly elec-trophilic SO3 to the hydroxyl groups with thesucceeding disintegration of the intermediatelyformed oxonium ion.

Completely water-soluble, highly viscoussodium cellulose sulfate semiesters are obtainedin homogeneous systems by the reaction ofcellulose nitrite [67]. The intermediate that isformed and dissolved, cellulose nitrite, is ob-tained in the N2O4/dimethylformamide systemand is at the same time transesterified by theSO3/DMF complex. Uniformly substituted cel-lulose sulfate with a range of DS values bet-ween 0.3 and 2.0 and solution viscosities up to7 000mPa · s (in 1 % solution) can be obtainedby using this process [68]. Such transesterifiedproducts can be cross-linked by metal ions toformhighly effective thickening agents for aque-ous media [69].

Such processes have been further developed[66] and make interesting novel fields of appli-cation accessible as a result of the rheologicaland gel-forming properties of the Na cellulosesulfate semiester.

Mixed esters such as cellulose acetate sul-fates, cellulose acetate butyrate sulfates [70],cellulose acetate propionate sulfate [71], andethyl cellulose sulfates [70], [72] are describedin the patent literature.

Being polyelectrolytes, cellulose sulfatesform salts and have ion-exchanging properties;thus, they have been recommended for use ascation exchangers [64, p. 65], [73], [74].

1.3.2. Cellulose Phosphate and CellulosePhosphite

Reaction of cellulose with aqueous phosphoricacid gives the following unstable addition com-pound: 3C6H10O5 ·H3PO4, fromwhich the cel-lulose can be regenerated unchanged by reaction

20 Cellulose Esters

with water. Cellulose phosphates [9015-14-9]with a low phosphorus content are obtained byreacting cellulose or linterswith phosphoric acidin an ureamelt [75].Higher phosphorus contentsand a lower degradation rate of the cellulosemaybe obtained with excess urea at reduced reac-tion time (ca. 15min) and at high temperature(ca. 140 C). Water-soluble cellulose phosphatewith a high degree of substitution may be ob-tained from a mixture of phosphoric acid andphosphorus pentoxide in an alcoholic medium[76].

Phosphorylated cellulose fibers show in-creased swelling after partial hydrolytic degra-dation and transfer into the alkali salt form andwere, therefore, suggested for use as adsorbents[77].

Cellulose phosphateswith a 17%phosphoruscontent (this represents about 3/4 of themaximalpossible substitution of triphosphate with 23 %phosphorus) were already produced in 1933 byreacting cellulose with a mixture consisting ofconcentrated sulfuric acid and phosphoric acidin the presence of a weakly acidic catalyst [78].

Cellulose phosphites [37264-91-8] and cellu-lose phosphonates may be prepared by transes-terification with alkyl phosphites. All celluloseesters containing phosphorus have fire-retardingproperties [78] and have attracted some interestdue to their ion-exchanging effect [74], [79], butare not yet industrially used.

1.3.3. Cellulose Halogenides

Various preparative methods are suitable for thesynthesis of halogenated cellulose derivatives[64, p. 64]. Halogenation can be carried out bytransesterification of such cellulose esters as to-sylate, nitrate, and sulfate with hydrohalic acids[80]. Nucleophilic substitution proceeds consid-erably faster in homogeneous systems than inheterogeneous aqueous systems.

The Finkelstein transesterification processof cellulose nitrate with sodium iodide in an-hydrous acetone leads to deoxyiodo cellulose.The reaction of cellulose with thionyl chloride,SOCl2, in the presence of pyridine produced amonosubstituted, but strongly decomposed andunstable, hydrogen chloride ester.

Halogenation of cellulose improves its water-resistant and flame-resistant properties. Slight

fluorination increases oil resistancy and lowersthe soiling potential of cellulose textiles [64].Commercial applications are not yet known.

1.3.4. Cellulose Borates

Thepreparationof cellulose borate succeededbymeans of transesterification ofmethyl andn-pro-pyl borate with cellulose [64, p. 7]. The productswith a maximumDS value of 2.88 are, however,extremely sensitive to hydrolysis and alcoholy-sis.

1.3.5. Cellulose Titanate

Cellulose can be reacted to cellulose titanatesin a heterogeneous reaction system by reactingit with titanium tetrachloride in DMF or withchlorinated anhydrides, chlorinated ester anhy-drides, and esters of the hypothetical orthotitanicacid Ti(OH)4 [81]. Ethyl trichlorotitanate hasbeen shown to be the most reactive. Esters with16 % titanium content are possible.

Cellulose esters with a titanium content bet-ween 3 and 5 % do not burn or smolder. Theypossess considerable hydrolytic stability in neu-tral and weakly alkaline media, but are easilyhydrolyzed at a low pH.

1.3.6. Cellulose Nitrite

The nitrite of cellulose came of scientific andpossibly practical interest as a cellulose deriva-tive in 1974 [67]. It is obtained by reacting cel-lulose with nitrosyl compounds such as dini-trogen tetroxide, N2O4 (corresponding to ni-trosyl nitrate), or nitrosyl chloride, NOCl, indimethylformamide or dimethylacetamide as aproton acceptor and solvent for the resulting es-ter. The reaction proceeds in a homogeneousphase to the trinitrite.

Cellulose nitrite is extremely sensitive to hy-drolysis. Chain degradation to a DP of 200

Cellulose Esters 21

(level-off DP) was observed to take place within3 h in the presence of water. The scientific andpreparative importance of cellulose nitrite isbasedon its high reactivity,whichmaybeused toproduce many other cellulose esters, also mixedesters, by transesterification in a homogeneousphase [82]. Transesterification to stable cellu-lose sulfates has already been mentioned [67].In this manner, water-soluble cellulose nitrateswith aDSvalue of 0.5 – 0.6may also be obtained[83].

Cellulose solutions produced under cold con-ditions (up to ca. 5 C) in a N2O4/DMF systemare relatively stable to degradation and can beproduced, depending on the DP of the cellulose,up to a concentration of 14 %. The cellulose canbe regenerated in an unaltered form to celluloseII, with the result that this process has alreadybeen considered as an alternative to the environ-mentally detrimental viscose process [84]. Notonly an attempt was made to achieve good me-chanical textile properties from the regeneratedfibers, but also to recycle the expensive solvent.An economic solution to the competition withthe viscose process has not yet been found.

1.3.7. Cellulose Xanthate

Cellulose xanthate [9032-37-5], an important in-termediarymolecule for the production of regen-erated cellulose according to the viscose process(see Cellulose, Chap. 3.1.1.), must also be con-sidered as an ester of an inorganic acid, namelythe nonexistent thiol – thion carbonic acid.

The O ester of this compound with organicresidues is the xanthic acid and the appropri-ate salts. Sodium cellulose xanthate is obtainedby reacting alkali cellulose with carbon disul-fide, which dissolves in dilute sodium hydroxideto an orange-yellowish, highly viscous solution,the so-called viscose.

The regenerated cellulose is precipitated inthe form of fibers (rayon, cord, and rayon sta-

ple), foils (cellophane), or tubes in precipita-tion baths containing sulfuric acid and salts (see→Cellulose, Chap. 3.1.3.). About 4×106 t of re-generated cellulose is presently producedworld-wide by using the viscose process.

For further information, see →Cellulose,Chap. 3.

2. Organic Esters

Cellulose can theoretically form an unlimitednumber of organic acid esters because of its an-hydroglucose units with three reactive hydroxylgroups each. Industrial possibilities are, how-ever, drastically limited by the complex natureof the cellulose molecule. Highly esterified or-ganic esters are, therefore, only produced froma few aliphatic fatty acids with up to four carbonatoms.

Cellulose acetate [9004-35-7], cellulose ac-etate propionate [9004-39-1], and cellulose ac-etate butyrate [9004-36-8], which have beenknown for quite some time and are in large-scale production, are especially important esters.Formic acid esters are, because of their instabil-ity, of no industrial importance.

Only cellulose acetate phthalate [9004-38-0]has found limited use as a tablet coating; all theother described cellulose esters such as cellu-lose palmitate, cellulose stearate, esters of un-saturated acids such as crotonic acid, or estersof dicarboxylic acids are not used industrially.Historical Aspects. Cellulose acetate was

first synthesized by P. Schutzenbergerin 1865 by heating cellulose and acetic acid un-der pressure, whereby a product of very lowmolecularity was obtained. In 1879, A. P. N.Franchimont added sulfuric acid to the es-terification process, which remains to this daythe most frequently used catalyst. The limitedsolubility of cellulose acetate in less-expensivesolvents and poor compatibility with the then-known softeners was a considerable obstaclefor its industrial use. The problem was solvedin 1904 when F.D.Miles and A. Eichengrunsimultaneously succeeded in synthesizing anacetone-soluble secondary acetate by partiallyhydrolyzing a primary triacetate.

During World War I, the less-flammable air-plane paints based on cellulose secondary ac-etate reached considerable importance as a re-

22 Cellulose Esters

placement for nitrocellulose. At almost the sametime, the manufacture of foils, films, syntheticsilk, and plastic masses developed.

An especially high number of publicationsand patents were achieved between 1920 and1935. Ultimately, only a few processes provedto be industrially useful, most of which are stillused today.

Even though the technology of cellulose es-ter production is considered for the most partto be complete, research continues in the fieldsof rationalization and improvement of produc-tion methods, leading to products with greateruniformity and improved properties, as well asin the development of new fields of application,especially with the introduction of cellulose ac-etate propionate and cellulose acetate butyrate.

2.1. Cellulose Acetate

2.1.1. Chemistry of Cellulose Esterification

The esterification reaction of the primary andsecondary hydroxyl groups of cellulose does notbasically differ from that of other alcohols. Thepeculiarities lie in the macromolecular structureof the cellulose molecule. The splitting of themolecule chain competes with the catalyzed es-terification, but can be fairly well controlled un-der appropriate conditions. The speed and com-pleteness of the reaction is dependent on thequality of the cellulose, whereas the different re-activities of the primary and secondary hydroxylgroups [83], [86] have little influence on indus-trial processes.

Acids, acid chlorides, and acid anhydrides arepossible esterification reagents for the three hy-droxyl groups in each glucose unit.

Esterification with free acids, with the ex-ception of formic acid [87], whose esters are

not stable, requires such high temperatures andcatalyst concentrations that only low molecularmass products are obtained. Acid chlorides inpyridine, however, were suggested for use in theproduction of esters from higher fatty acids (lau-ric acid, stearic acid, and palmitic acid), withoutever having attained any industrial significance.Attempt was made to manufacture cellulose ac-etate industrially with acetyl chloride and cata-lysts, but the process proved to be useless.

All industrial processes in current use, there-fore, are based on acetic acid anhydride as a re-actant, whereby theoretically 3mol of anhydrideper unit of glucose are used and 3mol of aceticacid are formed.

Attempts to use ketene, which could bedirectly accumulated without incurring aceticacid, which must again be processed, did notlead to any results [88].

Numerous catalysts were suggested to ac-celerate the reaction. Only sulfuric acid andperchloric acid are of any practical impor-tance. Zinc chloride, which is required in largeamounts of 0.5 – 1 part per part of cellulose,is no longer used today because of the highreprocessing costs. Other mineral acids, how-ever, are not sufficiently acidic in the water-freeacetic acid – acetic acid anhydride system. Thecatalytic effect of sulfuric acid is primarily inthe rapid and quantitative formation of acidiccellulose – sulfuric acid esters (sulfoesters) [89],which are substituted by acetyl groups as the re-action progresses and the temperature rises.

Due to the topochemical character of the re-action, soluble cellulose esters with low aceticacid content can be obtained only from the tri-ester stage by way of hydrolysis. Incompletelyesterized and insoluble derivatives are found inaddition to the triester before completion of thereaction.

Cellulose esters with low acetic acid contentare produced by subsequent hydrolysis in a ho-mogeneous system by adding water or diluteacetic acid to destroy the excess anhydride andpossibly by adding sulfuric acid for accelerationand then again split off under controlled condi-tions (temperature, water content, and time) acertain number of acetyl groups without furtherbreakdown of the cellulose chain.

Cellulose Esters 23

2.1.2. Raw Materials

Cellulose. The production of high-qualitycellulose esters requires that special attention bepaid to the selection of the starting materials.The cellulose bases generally consist of highlypurified cotton linters with an α-cellulose con-tent of over 99% and celluloses fromwood pulpwhich contain between 90 and 97%α-cellulose.

After the long layered spinnable cotton hasbeen freed of the cotton seed by ginning, the re-maining shorter fibers on the seed pod are usu-ally removed with two cuts before the seeds goto the oil presses for further processing. The firstcut gives about 4 % longer linters relative tothe entire cotton flower, which are preferentiallyprocessed to medicinal cotton, felt, paper, etc.The second cut gives about 8 % shorter layeredlinters,which are best suited for further chemicalprocessing.

The raw linters undergo mechanical clean-ing by means of screening, pressure boiling in a3 – 5 % sodium hydroxide solution, and finallyacid – alkaline bleaching. Special care should betaken during drying, since local overdrying ofcellulose, the water content of which should liebetween3 and8%, impairs the reactivity consid-erably. Table 9 shows analytical values of goodlinters [90].

Table 9.Analytical values obtained from bleached linters accordingto [90]

α-Cellulose 99.7 %β-Cellulose 0.2 %γ-Cellulose 0.1 %Carboxyl groups <0.02 %Total ash 0.02 %Degree of polymerization 1000 – 7000

For a long time, cellulose from wood pulpcould only be used for the manufacture oflower-quality cellulose esters because of the90 – 95 % α-cellulose content. Celluloses withan α-cellulose content of 96 % have been avail-able since about the 1950s. Due to special pro-cessing techniques, they give cellulose esterscomparable to those produced from linters as faras tensile strength, color, clarity of the solutions,and light stability as well as thermal stability areconcerned.

Acetic Acid Anhydride. Most manufactur-ers of cellulose acetate convert the resultingacetic acid to anhydride directly on the premises

and adjust the concentrations as required fortheir process, generally between 90 and 95 %.

2.1.3. Industrial Processes

Only a few of the proposed industrial processesfor the manufacture of cellulose esters have at-tained industrial significance. Even when thefact that no twomanufacturers use identical pro-cesses is taken into consideration, the followingcategories can be distinguished:

1) Acetylation in a homogeneous system (solu-tion acetate)Use of glacial acetic acid as a solvent(glacial acetic acid process)Use of methylene chloride as a solvent(methylene chloride process)

2) Acetylation in a heterogeneous system (fiberacetate)

Whereas the triester that is formed during es-terification according to the solution acetate pro-cess goes into solution and can subsequently behydrolyzed to secondary acetate, fiber acetate isformed in the presence of nonsolvents, similarto the nitration of cellulose. This method doesnot permit hydrolysis.

A flow diagram of the entire process is shownin Figure 7.

2.1.3.1. Pretreatment

The cellulose is first dried to obtain a moisturecontent of 4 – 7 %. Too little moisture wouldlower the reactivity of the cellulose, and toomuchmoisturewould lead to a higher acetic acidconsumption and an extremely violent reactionstart.

Acetic acid is generally used as a pretreat-ment reagent,which for someprocesses can con-tain small amounts of sulfuric acid to further im-prove the diffusion of the acetylating reagents. Itis believed that the pretreatment process causesa partial swelling of the cellulose by splitting thehydrogen bonds so that the acetylating reagentscan enter the fibers more rapidly.

A ratio of 30 – 100 parts of glacial acetic acidper 100 parts of cellulose, which is sprayed intothe thoroughly mixed cellulose at temperatures

24 Cellulose Esters

Figure 7. Flow chart for the production of cellulose esters according to [91]a) Acid reconditioning; b) Acidanhydride; c) Esterification; d) Hydrolysis; e) Precipitation; f)Washing; g) Centrifuge; h) Drier;i) Evaporator; k) Azeotropic distillation; l) Cooler; m) Decanter

of up to 50 C in appropriate equipment suchas mixing vats, is usually sufficient for the pre-treatment process. Pretreatment, depending onthe process and temperatures, takes from one toseveral hours.

2.1.3.2. Esterification

Acetic Acid Process. The acetylation mix-ture consists of glacial acetic acid as a solvent fortriacetate, an excess of 10 – 40 % glacial aceticacid anhydride and, depending on the process,2 – 15 % sulfuric acid with respect to the cellu-lose.

Esterification begins after the initial sponta-neous reaction of the water contained in the cel-lulose with part of the anhydride; the semiliquidmass uniformly saturated with acetylation mix-ture forms a fiber pulp while developing tem-peratures of up to 50 C and ultimately a highlyviscous solution.

A decomposition of the cellulose chain takesplace parallel to the strongly exothermic esterifi-cation. The chain degradation can be controlledby controlling the reaction temperature, e.g., byadding cooled solution portionwise and coolingthe reaction vessel down to the desired temper-ature range. The use of glacial acetic acid as asolvent, in which the highly esterified triacetateis poorly dissolved, results in gel formation and

precipitation of the triacetate. This is due to theformation of sulfate acetate, which is formed asan intermediate during the reaction. It is reesteri-fied to pure acetic acid esters especially at highertemperatures.

After the reaction solution is free of fibers, thedegradation of the cellulose ester molecule cancontinue until the desired viscosity is attained.The reaction is then stopped by adding wateror dilute acetic acid, which destroys the excessanhydride.

Cooled kneaders are suitable reaction vesselsin that they allow a rapid and uniform mixtureand catalyst distribution through intensive mix-ing. This is important for controlling the reaction(Fig. 8).

Methylene Chloride Process. Usingmethylene chloride (bp 41 C) as a solventpresents several advantages over acetic acid.Due to the fact that methylene chloride is suchan excellent solvent for primary triacetate, lowercatalyst concentrations (1 % sulfuric acid) arerequired at higher esterification temperatures.Furthermore, due to its low boiling point, theheat of reaction can be removed by means ofvaporization and return of the cooled methylenechloride. The reaction of highly viscous solu-tions can, thus, be better controlled. Finally, onlya half to a third as much dilute acetic acid must

Cellulose Esters 25

Figure 8. Cellulose acetate production by the kneader method according to [92]a)Weighing scale; b) Sprinkling vat; c) Kneader; d)Mill; e) Rinsing vat; f) Stabilizing vat; g) Bleaching vat; h) Floater; i) Stockpan; k) Centrifuge; l) Dust chamber; m) Drier

Figure 9. Cellulose acetate production according to the methylene chloride process [93]a) Weighing scale; b) Bale opener; c) Sprinkling vat; d) Acetylator; e) Precipitating vat; f) Prebreaker; g) Vacuum vessel;h) Pipe cooler; i) Pump for viscous substances; k) Filter bath; l) Mill; m) Floater; n) Sprinkling line; o) Centrifuge; p) Vacuumshovel drier; q) To reprocessing of methylene chloride

be recycled compared with the glacial aceticacid process.

Table 10. Acetylating preparations according to the glacial aceticacid process and the methylene chloride process [92]

Acetic acid process Methylene chlorideprocess

Cellulose 700 kg 3 500 kgPretreatment 700 kg glacial 1 200 kg glacial

acetic acid acetic acidAcetylation 1 900 kg anhydride 10 500 kg anhydride

4800 kg glacial 14000 kg methyleneacetic acid chloride

50 kg sulfuric acid 35 kg sulfuric acid

Table 10 shows typical examples for acety-lation according to the glacial acetic acid andmethylene chloride processes. Figure 9 showsa sceme for the production of cellulose acetateaccording to the methylene chloride process.

Acetylation according to the methylene chlo-ride process is carried out largely in rotatingdrums (roll vats) or in horizontal containers withshovel-like stirrers on both sides. The problemof corrosion, which arises during esterificationand especially during hydrolysis, has only been

26 Cellulose Esters

partially solved by using equipment constructedof bronze, high-alloy steels, or plates containingmetals such as silver, titanium, or tantalum.

Fiber Acetate. Cellulose can be esterifiedmaintaining its fiber structure by adding suffi-cient amounts of nonsolvents to the triacetateduring acetylation. Carbon tetrachloride, ben-zene, or toluene can be used as nonsolvents [94],[95].

Figure 10. Acetylation equipment for cellulose triacetatefibers [92]a) Perforated drum; b)Reaction solution; c)Cellulose fibers;d) Cooler for acetylating liquid; e) Acetylating liquid circu-lation

Temperature and catalyst concentrations aresimilar to those required for the solvation pro-cess. A large amount of liquid is required to keepthe loose voluminous cellulose in suspension.Perchloric acid is preferred as a catalyst becauseof the great difficulty with which the sulfuricacid ester is split in this system.

The hydrolysis of fiber acetate to acetone-soluble esters is not possible in a heterogeneoussystem. Its utilization is limited to special ap-plications, such as the manufacture of foils andfilms from triacetate.

As shown in Figure 10, fiber acetate is pro-duced by rotation in various directions and atvarious speeds in a perforated drum enclosed ina metal casing. The shaft of the drum is hollowso that liquid may be added during rotation [96].

Continuous Processes. In spite of numerousattempts, continuous acetylation and hydrolysisprocesses have achieved only limited industrialapplications and are not used in the productionof high-quality esters for, among others, plas-tic masses. Due to the varying reactivities of the

different celluloses, considerable variations bet-ween batches are possible so that the develop-ment of a continuous process is seriously im-peded.

2.1.3.3. Hydrolysis

After the esterification is completed, the processis interrupted by adding water or dilute aceticacid. Sufficient water must be added to decom-pose the excess anhydride and adjust the wa-ter content of the solution to 5 – 10 % so that,aside from hydrolysis, no further decomposi-tion of the molecular mass occurs, but at thesame time the bound sulfuric acid is almost com-pletely split off. The speedof hydrolysis dependson the temperature, which – depending on theprocess – ranges between 40 and 80 C, and onthe amounts of sulfuric acid and water.

The course of hydrolysis is constantly moni-tored by checking the solubility of the secondaryacetate. The reaction is terminated at the desireddegree of substitution by neutralizing the cata-lyst, preferably with sodium acetate or magne-sium acetate. The proportion of free hydroxylgroups in the ester should be maintained as re-producibly precise as possible because it deter-mines the properties and the utilization of theester.

2.1.3.4. Precipitation and Processing

The product precipitates in the form of flakesor powder during intense stirring after wateror dilute acetic acid has been gradually added.Themethylene chloride process requires that themethylene chloride be completely distilled be-fore precipitation occurs. A part of the precipi-tating acid is added to the solution to just beforethe point of precipitation when the main por-tion of the acid is added. The process must becarefully monitored to achieve a product with anopen structure which can be easily rinsed out.The precipitate can be broken down and thor-oughly rinsed,whereby the dilute acid is broughtback into the cycle. Rinsing is done primarily byusing continuous methods based on the counter-current principle (Fig. 9).

High-quality products for plastic masses arestabilized and bleached. Bound sulfuric acid

Cellulose Esters 27

remnants are removed by either boiling underpressure or heating in 1 % mineral acids duringstabilization.

After further rinsing and removal of excesswater by suctioning or by thrust extraction, theproduct is carefully dried, preferably in a vac-uum shovel drier, to awater content of< 1 – 3%.

The cellulose acetate yield from a good pro-cess is at least 95 % of the theoretical yield.

Major manufacturers of cellulose acetateare the following (1971) [103]: Bayer AG,Leverkusen; British Celanese Ltd., London;Celanese Corp. of America, New York; Cour-taulds Ltd., Manchester; Daicel Ltd., Osaka,Japan; E. I. DuPont de Nemours & Co., Inc.,Wilmington;EastmanKodakComp.,Kingsport;UCB Fabelta, Tubize, Belgium; Gevaert-Agfa,Antwerp; Hercules Powder Comp. Ltd., Lon-don; Rhodiaceta, Lyon; Soc. Rhodiatoce SpA,Milan; Rhone-Poulenc, Paris; VEB Orbitaplast,Eilenburg, GDR.

A number of products are further processedto fibers, films, or injection-molding compoundsin the manufacturers’ own facilities and neverreach the market as a raw material.

2.1.4. Recovery of Reactants

The recovery of the incurred large amounts ofacetic acid is a decisive factor for the profitabil-ity of a process.

Recovery of Acetic Acid. Depending on theprocess, 2 – 6parts of 15 – 25%dilute acetic acidper part of cellulose accumulate. They must bereprocessed to glacial acetic acid and acetic acidanhydride.

Only continuous processes consisting of acombination of extraction and azeotropic dis-tillation are of practical importance. The diluteacid is, for example, extracted with ethyl acetatein a countercurrent and is subsequently distilled,so that 99.8 % pure glacial acetic acid can be re-moved from the bottom of the column while theazeotropic ethyl acetate water is removed fromthe top.

Recovery of Acetic Acid Anhydride.Since only a portion of the accumulated glacialacetic acid is required for the acetylation pro-cess, the remainder must be converted to glacialacetic acid anhydride.

The ketene process developed by the WackerCo. (→Acetic Anhydride, Chap. 1.3.1.1.) is ingeneral use [97]: Pure, almost anhydrous glacialacetic acid is continuously vaporized under avacuum and is split to ketene in the presenceof small amounts of the catalyst triethyl phos-phate; ketene then reacts with glacial acetic acidto form the anhydride (→Acetic Anhydride,Chap. 1.3.1.2.).

Recovery of Methylene Chloride. Methyl-ene chloride can be recovered inexpensivelybecause of its insolubility in water, which all-lows its recovery from the raw solution withoutfurther processing steps in almost pure form.

2.1.5. Properties of Cellulose Acetate

Table 11.Physical characteristics of cellulose acetate [90], [92], [98]

Characteristic Triacetate Secondaryacetate

Density, g/cm3 1.27 – 1.29 1.28 – 1.32

Thermal stability, C >240 ca. 230

Tensile strength of fibers, 14 – 25 16 – 18kg/mm2

Tensile strength of foilslongitudinal, kg/mm2 12 – 14 8.5 – 10transverse, kg/mm2 10 – 12 8.5 – 10

Refractive index of fiberstoward the fiber axislongitudinal 1.469 1.478transverse 1.472 1.473

Double refraction −0.003 +0.005

Dielectric constant ε50 – 60Hz 3.0 – 4.5 4.5 – 6.5106 Hz 4.0 – 5.5

Dielectric loss factor tan δ50 – 60Hz 0.01 – 0.02 0.007106 Hz 0.026

Specific resistance, Ω · cm 1013 – 1015 1011 – 1013

Specific heat, J g−1 K−1 1.46 – 1.88

Thermal conduction, Jm−1 h−1 K−1 0.63 – 1.25

Cellulose acetate and the other fatty acidesters are white, amorphous products that arecommercially available as a powder or flakes.They are nontoxic, odorless, tasteless, and less

28 Cellulose Esters

flammable thannitrocellulose. They are resistantto weak acids and are largely stable to mineraland fatty oils as well as petroleum.

Some physical characteristics are given inTa-ble 11.

Properties and applications of cellulose ac-etates are primarily determined by the follow-ing:

1) viscosity of their solution2) degree of esterification or the amount of

bound acetic acid

Viscosityas an indicator for the degree ofpolymerization influences to a great extent themechanical properties of the resulting fibers,films, or plastic masses, as well as their worka-bility.

The degree of esterification primarily deter-mines the solubility and compatibility with soft-eners, resin, varnish, etc., and ultimately alsoinfluences the mechanical properties.

Figure 11. Solubility of cellulose acetate in various solvents(abridged according to [99])∗ Technical grade

The wide span of solubility properties of hy-drolyzed cellulose acetate is shown in Table 12.

The compatibility of the plasticizer and thesolubility in polar solvents increase with de-creasing acetic acid content while the solubil-ity in nonpolar solvents decreases. Moreover,a correlation between the incompatibility with

nonsolvents such as water, alcohol, benzene, ortoluene and a decreasing degree of esterificationexists. Furthermore, a number of solvent com-binations are known which are able to dissolvethe cellulose acetate although each of the com-ponents is a nonsolvent.

Figure 11 shows a selection of solvents forthe industrially interesting range of esterification(52 – 62%bound acetic acid). Detailed informa-tion on solubility and softener selection can befound in the literature and the manufacturers’information brochures.

2.1.6. Analysis and Quality Control

The viscosity is determined in practice by theusual methods. Along with the relative viscos-ity, the measurement of 15 – 20 % solutions ac-cording to the ball drop method correspondingto ASTM 871-56 has been generally accepted.

The acetic acid content is generally deter-mined by saponification of the ester with 0.5N-potassium hydroxide and back-titration ofthe excess. The free acids from mixed estersare separated from the residue after saponifi-cation by means of distillation or extractionand by extraction or azeotropic distillation iso-lated and titrated. Gas-chromatographic meth-ods have found increasing popularity. A com-prehensive presentation can be found in [100],[101].

Determination of the free hydroxyl groups inpure cellulose acetate is not necessary, since aprecise analysis of bound acetic acid is possible.It is primarily used to characterize the mixedesters and can be carried out according to vari-ousmethods. Complete esterificationwith aceticacid anhydride in pyridine and back-titration ofthe excess is a proven method [102]. The valueis mostly given as a percentage of the hydroxylor as the hydroxyl value (mg of KOH/g).

Additional quality control methods for un-processed cellulose esters are the following: de-termination of temperature stability by heatingto 220 – 240 C and evaluation of discolorationand melting behavior, the determination of freeacid as an indicator for the efficacy of the rins-ing process, and determination of the ash contentas well as clarity, color, and filterability of thesolution.

Cellulose Esters 29

Table 12. Solubility of cellulose acetate at various degrees of esterification [91]

Degree of esterification Bound acetic acid Chloroform Acetone 2-Methoxyethanol Water

2.8 – 3.0 60 – 62.5 % soluble2.2 – 2.7 51 – 59 % soluble1.2 – 1.8 31 – 45 % soluble0.6 – 0.9 18 – 26 % soluble

<0.6 <18 %

2.2. Cellulose Mixed Esters

Apart from the long-established cellulose ac-etate, only cellulose mixed esters of aceticand propionic acid, cellulose acetate propionate[9004-39-1], or acetic and butyric acid, celluloseacetate butyrate [9004-36-8], have attained anynotable importance.

Mention should also be made of cellulose ac-etate phthalate [9004-38-0],which is used in cer-tain special fields of application.

Pure cellulose propionates [9004-48-2] andcellulose butyrates [9015-12-7] are difficult toproduce and – like formates, palmitates andstearates, and esters of unsaturated acids and di-carboxylic acids – have attained no industrialimportance [4].

2.2.1. Production

As far as the chemistry of the esterification re-action and the subsequent partial saponificationare concerned, the basic description given inthe chapter on cellulose acetate is also valid formixed acids to a large extent.

The raw materials used are the same as forcellulose acetate, i.e., cotton linters or celluloseproduced by special processes.

Pretreatment of the cellulose raw materialsis similar to that used in the production of cel-lulose acetate. In practice, esterification takesplace only in a homogeneous system and not, asis sometimes the case with cellulose acetate, ina heterogeneous system. The esterification mixconsists of amixture of anhydrides of acetic acidand propionic acid or of acetic acid and butyricacid. The reactivity of the aliphatic fatty acidsdecreases very rapidly as the chain-length in-creases.

Mixed esters consisting of propionic acid andbutyric acid or of acetic acid, propionic acid, andbutyric acid are not produced on an industrialscale.

2.2.2. Composition

The properties of the mixed esters are deter-mined not only by their viscosity, but also inparticular by the ratio of the two bound acidsand by the content of free hydroxyl groups.

At present, cellulose acetate propionate andcellulose acetate butyrate flake is only producedby two manufacturers: Bayer AG, Leverkusen,in the Federal Republic of Germany, and theEastman Kodak Co., Kingsport, Tennessee, inthe United States.

Whereas pure cellulose acetates are clearlycharacterized by their viscosity and content ofbound acetic acid, with mixed esters, data on theindividual acids and possibly the free hydroxylgroups are also required.

2.2.3. Properties

As the degree of hydrolysis changes, the prop-erties of cellulose mixed esters vary over a widerange from pure acetates to pure butyrates, withthe propionates occupying a property-profile po-sition between the cellulose mixed esters andpure acetates.

In the case of cellulose acetate butyrate, forexample, if one considers the mixing range frompure cellulose acetate through the various mix-ing ratios to pure cellulose butyrate (the de-gree of esterification is adjusted in such a waythat the esters in pure cellulose acetate contain50 – 60 % acetic acid, and those in pure butyrate60 – 70 % butyric acid), then the density variesfrom ca. 1.32 (cellulose acetate) to 1.16 (cellu-lose butyrate). The melting point is between ca.300 C (cellulose acetate) and 160 C (cellulosebutyrate), while thewater absorption at 90% rel-ative humidity varies from ca. 12 % (celluloseacetate) to 1.5 % (cellulose butyrate).

30 Cellulose Esters

Solubility in various solvents (like acetone,methyl ethyl ketone,methyl isobutyl ketone, andvarious phthalate plasticizers) also varies over awide range [104].

Typical examples of cellulose acetate pro-pionates, whose acetic acid and propionic acidlevels are practically identical, are given in Ta-ble 13.

With cellulose acetate propionates, the aceticacid content varies fromca. 3 to 8%, and the pro-pionic acid content from ca. 55 to 62 %. Withcellulose acetate butyrates, acetic acid contentsof 19 – 23 % and butyric acid contents between43 and 47 % are normal.

There are many more grades of cellulose ac-etate butyrate available than of cellulose acetatepropionate.

Examples of cellulose butyrate grades aregiven in Table 14.

2.2.4. Other Organic Cellulose Mixed Esters

Cellulose acetate phthalates are cellulose mixedesters of minor industrial importance. They areproduced from hydrolyzed cellulose acetate byreaction with an excess of phthalic anhydride inacetone or dioxane [106].

This produces esters of phthalic acid with afree carboxyl group. The products are used ona small scale as water- or alkali-soluble textileauxiliaries, tablet coatings, and antistatic agentsin film coating.

2.2.5. Uses

The wide scope of variation for cellulose estershas led to the development of special grades fordifferent fields of application.

Mixed esters based on cellulose acetatebutyrate and cellulose acetate propionate arechiefly used in the production of molding plas-tics (seeSection 2.4). The applications describedin the following are of lesser importance.

Films. Triacetate is more widely used thancellulose acetate butyrate as an electrical insu-lating film, which is mainly produced by cast-ing. Cellulose acetate butyrates with an aceticacid/butyric acid ratio between 2 : 1 and 1 : 1are preferred.

The importance of cellulose mixed esters asfilm substrates in the photographic industry hasgreatly diminished as a result of the increasinguse of polyester film.

Surface coatings. Cellulose ester lacquers,with their excellent lightfastness, gloss, lowcombustibility, and good thermal stability, cou-pled with their indifference to hydrocarbons,oils, and greases, became very quickly estab-lished in numerous fields of application.

One significant step forward was the intro-duction of cellulose acetate butyrates and cellu-lose acetate propionates. Both are particularlycharacterized by lower water absorption andgood compatibility with extenders and, in thecase of the low-viscosity grades, also allow theproduction of very high-solid lacquers. Cellu-lose acetate butyrate has becomeparticularly im-portant as an extender for metal effect finishes[105].

2.3. Cellulose Acetate Fibers

Cellulose acetate is the most important cellu-lose ester. It is primarily used for textile yarnand cigarette filter tow. The cellulose acetate isusually dissolved in a suitable organic solventand spun by dry spinning (→ Fibers, 3. Gen-eral Production Technology). Secondary (2.5)acetate with an acetic acid content of 54 – 56 %is normally used, whereas only a small amountof cellulose triacetate is normally produced.

2.3.1. Properties

The viscosity and the filterability of the spinningsolution (spinning dope) are particularly impor-tant in the production of cellulose acetate fibers.The spinning dope has a high viscosity, whichdepends on the degree of polymerization. Thestrength and stretch properties of the fibers alsodepend on the concentration and the degree ofpolymerization as well as on the distribution ofthe acetate groups along the cellulose chain.

Because the fibers are produced by extrudingthe spinning dope through minute spinneretteholes, insoluble particles must first be removed

Cellulose Esters 31

Table 13. Characteristic data of cellulose acetate propionate (Cellit PR) [105]

PR 900 PR 800 PR 500

Acetyl content, % 3.5 3.5 3.5Propionyl content, % 45 45 45Hydroxyl content, % 1.6 1.6 1.6Viscosity (DIN53 015), mPa · s 4700 – 7800 2200 – 3800 150 – 240

Melting range, C 200 – 220 190 – 210 180 – 200

Table 14. Characteristic data of cellulose acetate butyrate (Cellit BP) [105]

BP 300 BP 500 BP 700/25 BP 700/40 BP 900

Acetyl content, % 14 14 15 14 15Butyryl content, % 37 37 37 37 37Hydroxyl content, % 1.2 1.2 0.8 1.2 0.8Viscosity (DIN 53015), mPa · s 30 – 60 150 – 240 750 – 1500 750 – 1500 5000 – 8000Melting range, C 160 – 180 170 – 190 170 – 190 170 – 190 180 – 200

from the spinning dope by filtration. These par-ticles are primarily composed of very small, in-completely acetylated cellulose fibers or gels,which can obstruct the spinnerett holes.

Secondary acetate and triacetate fibers havesimilar physical properties (Table 15). Theirdensities are lower than that of viscose rayonfibers and equal to that of wool. For textile yarns,the fibers should be as free of color as possible.

Table 15. Physical properties of acetate fibers and tow [107]

Secondary Triacetateacetate

Strength, cN/dtex 1.0 – 1.5 1.0 – 1.5Stretch, % 25 – 30 25 – 30Density, g/cm3 1.33 1.30Moisture uptake, % (65 %relative humidity, 20 C) 6 – 6.5 4 – 4.5

Water retention capability, % 25 – 28 16 – 17Melting point, C 225 – 250 decom-

positionat 310 – 315

DP 300 300

The chemical reactions of cellulose acetateare similar to those of organic esters. Celluloseacetate is hydrolyzed by strong acids and alkali;it is sensitive to strong oxidizing agents but notaffected by hypochlorite or peroxide solutions.

Acetate fibers cannot be dyed under the sameconditions as viscose rayon fibers because theirswelling properties are different. Acetate fiberscan only be dyed with water-disperse dyes atthe boiling point of the medium usually in thepresence of carriers (→TextileAuxiliaries). Thecarriers promote fiber swelling and enhance dyeuptake by the fibers. The dyeing process coupled

with the textile spinning operation assures colorfastness. Triacetate fibers have better wash-and-wear properties than secondary acetate becauseof better dimensional stability and higher creaseresistance.

2.3.2. Raw Materials

Table 16. Typical properties of acetate wood pulps

Characteristic∗ Sulfite softwoodpulp (conifer)

Sulfatehard-woodpulp(deciduous)

R10, % 95 96R18, % 97 98Ash, % 0.08 0.08Silica, % 0.001 0.003Calcium, % 0.006 0.008Pentosans, % 1.2 1.2Moisture content, % 6.5 6.5Apparent density, g/cm3 0.45 0.5DP 2300 1700

∗ R10 and R18 are residues in 10 or 18 % sodium hydroxide at20 C [108]

Wood pulp produced from various softwood(conifer) or hardwood (deciduous) species is thecellulose source for the production of celluloseacetate fibers. The wood pulps are produced bythe sulfite pulping process with hot alkali ex-traction or by the prehyrolized sulfate (Kraft)process with cold caustic extraction (→ Paperand Pulp). The lignins and hemicelluloses areremoved from the wood to give wood pulps withan α-cellulose content of over 96 % (Table 16).High-purity cotton linters are no longer used in

32 Cellulose Esters

the production of cellulose acetate fibers for eco-nomic reasons.

For the production of high-quality celluloseacetate fibers the wood pulp must have goodswelling properties for uniform accessibility ofthe cellulose to the catalyst and the acetylationagent and a uniform reactivity. In addition, itmust produce a spinning solution without fibersand gels which can easily be filtered.

2.3.3. Production

The general points discussed in Section 2.3.1 forthe production of cellulose acetate also applyhere. The sulfuric acid catalyst initially formsthe cellulose sulfate ester. The sulfate groups arethen replaced by acetyl groups as the acetylationproceeds. The sulfate ester contents is furtherreduced in the hydrolysis stage. However, anysulfate ester groups remaining at the end of thehydrolysis stage must be neutralized with an ap-propriate stabilizer, e.g., magnesium salts [109],[110]. Any “ free ” sulfate ester groups will af-fect the stability of the acetate because under theinfluence of heat and humidity they splitt off assulfuric acid and degrade the fiber [111].

For secondary acetate spinning, acetone isused as the solvent. For triacetate, the solventis 90 % dichloromethane and 10 % methanol oracetic acid (wet-spinningprocess). Theviscosityof the spinning solution with a cellulose acetateconcentration of 20 – 30 % is between 300 and500 Pa s at 45 – 55 C. The spinning dope is fil-tered in one or more steps and is then deaeratedin large vessels.

Dry spinning is used almost exclusively; wetspinning is occasionally used for triacetate only.The spinneretts for textile filament have bet-ween 20 and 100 holes and those for tow upto 1 000. The fibers are formed by evaporat-ing the solvent with a countercurrent of air at80 – 100 C in a 4- to 6-m spinning column. Thefibers are then stretched while still plastic to in-crease their strength. Melt spinning of celluloseacetate or triacetate has no commercial impor-tance due to the limited heat stability at themelt-ing point.

A core-skin structure is formed in triacetatefibers. The acetyl groups are distributedvery reg-ularly in cellulose triacetate compared to sec-ondary acetate; therefore, crystallization occurs

when triacetate fibers are heated at 180 – 200 C(heat setting) [107], [111], [112]. This heat treat-ment, which enhances the wash-and-wear prop-erties of triacetate textiles, requires several min-utes at 180 C or several seconds at 220 C.Heating for shorter periods is not effective andlonger heating periods lead to deterioration ofthe mechanical properties of the textile. Heat-setting reduceswater retention to 10%andwaterabsorption to 2.5 %.

2.3.4. Uses

By blending and twisting of cellulose acetate ortriacetate fibers with nylon or polyester a combi-nation of properties is achieved that make themsuitable for different end uses in linings. In thisway the weaker physical properties of acetatefibers can be compensated for while maintain-ing the positive characteristics, for instance, thehigh moisture absorption and the silk-like soft-ness.

Due to the unique hydrophobic – hydrophilicproperties, semipermeable membranes madefrom cellulose acetate fibers have a remarkablepotential in desalination (reverse osmosis) ofwater.

Cellulose acetate hollow fibers are also suit-able for gas separation and hemodialysis [113].

For cellulose acetate is non-toxic, biodegrad-able and the raw material is a renewable naturalpolymer, it is expected to find application forother uses in the future.

2.3.5. Economic Aspects

Secondary acetate and triacetate fibers for tex-tiles and filter cigarette tow accout for 80%of allcellulose ester production. The balance is usedfor plastics and film. Secondary acetate and tri-acetate textile fibers have a small share (about1 %) of all textile fiber production.

In the late 1990s synthetic fiber productioncontinued to expand (Table 17), whereas the ac-etate production was stable at about 850 000 t/a.The acetate fiber production decreased slightlycompensated by a slight increase in filter towproduction. The five largest manufactures offilter tow are Hoechst-Celanese and EastmanChemicals in the United States, Rhodia Acetow

Cellulose Esters 33

in Germany, Daicel in Japan, and Courtaulds inthe United Kingdom.

Table 17.Worldwide production of textile fibers (1000 t) [114]

Fiber 1993 1995

Man-made fibers ∗ 19781 21741Synthetics 16652 18471Cellulosics 3129 3270

Cotton 18494 18602Wool 1687 1767Silk 68 92

∗Excl. polyolefin fibers, textile glass fibers, and acetate cigarettefilter tow.

2.4. Plastic Molding Compounds fromCellulose Esters

In the category “plastics made from natural ma-terials,” thermoplastics based on cellulose estersor cellulose mixed esters are still the most im-portant [115].

As early as 1920, A. Eichengrun devel-oped thermoplastic cellulose estermolding com-pounds as a spraying and molding powder. Cel-lulose acetate and mixed esters are used in in-jection molding and extrusion; mixed esters arealso used for fluidized-bed dip coating and rota-tional molding.

The use of inorganic cellulose esters (see Sec-tion 1.2.7) is continually decreasing in the plas-tics sector because of their high flammability.

2.4.1. Physical Properties

Like cellulose acetate, cellulosemixed esters canbe plasticized at elevated temperatures by usingplasticizers. This results in a large number ofgrades with property combinations found in noother type of thermoplastic.

Thermoplastic cellulose ester molding com-pounds are generally characterized by goodtransparency, high mechanical strength, andtoughness; one particularly noteworthy featureis that the material reacts to mechanical stressesby exhibiting cold flow, so that the other-wise problematic insert molding of metal partspresents no problems and there is no risk ofstress cracking. Light-stable, transparent mate-rial is available in a wide range of transparent,

translucent, and opaque color shades. High sur-face gloss coupledwith antistatic properties (i.e.,electrical charges disperse rapidly and no annoy-ing dust patterns form) ensures that moldings re-tain their attractive appearance for years. Highsurface elasticity ensures a good “natural feel”and imparts a “repolishing” effect to the mate-rial: this means that scratches disappear as theobject is being used. The relatively lowmodulusof elasticity gives excellent damping of vibra-tions, so that the acoustic behavior is not affectedby annoying resonance or ambient noise.

The individual cellulose esters generally dif-fer in their mechanical properties and in theircompatibility with plasticizers. As a rule, cellu-lose mixed esters contain higher-boiling plas-ticizers in amounts ranging from 3 to 25 %whereas cellulose acetate contains 15 – 35 %plasticizers. Heat distortion temperature in-creases as the plasticizer content is reduced.Mixed esters absorb considerably lesswater thancellulose acetates, with the result that parts pro-duced frommixed esters retain their dimensionalstability even in humid climates. Finally, cellu-lose acetate butyrates and (with certain restric-tions) cellulose acetate propionates can also betreated with UV inhibitors to ensure serviceabil-ity of the moldings even during years of out-door exposure [116]. In principle, it is possibleto reinforce thermoplastic cellulose ester mold-ing compounds with glass fibers [117].

Figures 12, 13, 14, 15, 16, 17, 18, 19, 20, 21,22 show the physical properties as a function ofthe plasticizer content; the property levels mayvary by as much as 15 – 20% in either direction,depending on the type of plasticizer and the rel-ative viscosity of the cellulose ester. In all of thediagrams and tables featured here, the abbrevi-ations used are as follows:

CA =Cellulose acetate molding compound(acetic acid content > 55 %)

CP =Cellulose acetate propionate moldingcompound

CP∗ =Polymer-modified cellulose acetatepropionate molding compound

CAB =Cellulose acetate butyrate moldingcompound

CAB∗ =Polymer-modified cellulose acetatebutyrate molding compound

34 Cellulose Esters

Figure 12. Density of cellulose acetate (CA), cellulose ac-etate propionate (CP), and cellulose acetate butyrate (CAB)as a function of the plasticizer content (determined in ac-cordance with DIN 53479 or ISO/R 1183)

Figure 13. Tensile strength at yield σs and elongation εsof cellulose acetate, cellulose acetate propionate, and cellu-lose acetate butyrate as a function of the plasticizer content(determined in accordance with DIN 53455 or ISO/R 527;specimen no. 3, rate of deformation 25mm/min)

Figure 14. Tensile strength at break σR and elongation εRof cellulose acetate, cellulose acetate propionate, and cellu-lose acetate butyrate as a function of the plasticizer content(determined in accordance with DIN 53455 or ISO/R 527;specimen no. 3, rate of deformation 25mm/min)

Figure 15. Tensile modulus of cellulose acetate, celluloseacetate propionate, and cellulose acetate butyrate as a func-tion of the plasticizer content (determined in accordancewith DIN 53455 or ISO/R 527)

Figure 16. Flexural stress at a given strain σbG of celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate as a function of the plasticizer content (determinedin accordance with DIN 53452 or ISO/R 178; test specimen4×10×80mm, rate of deformation 2mm/min)

Figure 17.Notched impact strength ak of cellulose acetate,cellulose acetate propionate, and cellulose acetate butyrateas a function of the plasticizer content (determined in ac-cordance with DIN 53453 or ISO/R 179; specimen no. 2)

Cellulose Esters 35

Figure 18. Izod notched impact strength of cellulose ac-etate, cellulose acetate propionate, and cellulose acetate bu-tyrate as a function of the plasticizer content (determined inaccordance with ASTM D 256, Method A, or ISO/R 180;test specimen 63.5×12.7×3.2mm)

Figure 19.Rockwell hardness (R scale) of cellulose acetate,cellulose acetate propionate, and cellulose acetate butyrateas a function of the plasticizer content (determined in ac-cordance with ASTM D 785)

Figure 20. Vicat softening temperature VST/B50 of cel-lulose acetate, cellulose acetate propionate, and celluloseacetate butyrate as a function of the plasticizer content (de-termined in accordance with DIN 53460/B or ISO/R 306;sheet 10×10×4mm)

Figure 21. Heat distortion temperature FISO of celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate as a function of the plasticizer content (determinedin accordance with ASTMD 648, ISO/R 75, or DIN 53461;test specimen 12.7×12.7×120mm)

Figure 22. Melt flow index of cellulose acetate, celluloseacetate propionate, and cellulose acetate butyrate as a func-tion of the plasticizer content (determined in accordancewith DIN 53735 or ISO/R 1133)

Figure23.ShearmodulusG′ anddamping tanσ of celluloseacetate22, cellulose acetate propionate10, and cellulose ac-etate butyrate10 (determined in accordancewithDIN 53445or ISO/R 537)

36 Cellulose Esters

Figure 24.Temperature of the dampingmaximaof celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate as a function of the plasticizer content

Figure 25. Tensile creep strength σB/t of cellulose acetate,cellulose acetate propionate, and cellulose acetate butyrate(determined in accordance with DIN 53444 or ISO/R 899;test specimen no. 3)

The indices (e.g., CAB10) give the plasticizercontent in percent by weight.

Table 18 shows the electrical properties ofmedium-hardness cellulose ester molding com-pounds; their shear moduli and damping proper-ties are given in Figure 23. Figure 24 shows theposition of the damping maxima as a functionof the plasticizer content.

Long-term properties derived from the ten-sile creep test are shown in Figures 25, 26, 27,28, 29. Time-to-failure curves, modulus of creepcurves and isochronous stress – strain curves ofslightly and highly plasticized grades of cellu-lose acetates, cellulose acetate propionates, andcellulose acetate butyrates are given here.

Figure 26. Creep rupture strength σB103 of cellulose ac-etate butyrate at 23 C, 80 C, and 100 C as a function ofthe plasticizer content (determined in accordance with DIN53444 or ISO/R 899)

Figure 27. Creep modulus Ec/t of cellulose acetate, cellu-lose acetate propionate, and cellulose acetate butyrate (de-termined in accordance with DIN 53444 or ISO/R 899)

Figure 30 shows results of the dynamic fa-tigue test in the tensile pulsating range on amedium-plasticity cellulose acetate, a slightlyplasticized cellulose acetate propionate, and amedium-plasticity cellulose acetate butyrate.

Figure 31 shows results of the alternatingbending test on slightly and highly plasticizedcellulose acetate, cellulose acetate propionate,

Cellulose Esters 37

Table 18. Electrical properties of organic cellulose ester molding compounds

Type of test Unit Test specification Specimen Celluloseacetate [130]

Celluloseacetatepropionate[119]

Celluloseacetatebutyrate[119]

Dielectric strength Ed(50Hz, 0.5 kV/s)

dry VDE 0303 Circular 315 355 3504 days at 80 % rel. humidity kV/cm Pt. 2, discs 95mm Ø 290 330 33024 h water immersion DIN 53481 ×1mm 280 330 330

Surface resistance R0

dry VDE 0303 8×1013 2×1014 9×1013

4 days at 80 % rel. humidity Ω Pt. 3, 150×15×4mm 3×1012 1×1013 9×1012

24 h water immersion DIN 53482 4×1011 5×1012 9×1012

Insulation resistance Ra

dry VDE 0303 5×1015 5×1015 5×1015

4 days at 80 % rel. humidity Ω Pt. 3, 150×5×4mm 1×1013 6×1013 5×1013

24 h water immersion DIN 53482 7×1011 2×1013 2×1013

Volume resistivity D

dry Ω/cm VDE 0303 Circular 2×1015 1×1016 4×1015

4 days at 80 % rel. humidity Pt. 3, discs 95mm Ø 2×1012 5×1013 6×1013

24 h water immersion DIN 53482 ×1mm 2×1011 1×1013 2×1013

Relative permittivity εr, dry

at 50Hz VDE 0303 Circular 5.1 4.1 4.0at 800Hz Pt. 4, discs 95 and 4.0 3.9 3.8at 1MHz DIN 53483 80mm Ø×1mm 4.1 3.6 3.4

Dissipation factor tan δ, dry

at 50Hz VDE 0303 Circular 0.009 0.005 0.006at 800Hz Pt. 4 discs 95 and 0.019 0.011 0.012at 1MHz DIN 53483 80mm Ø×1mm 0.050 0.026 0.028

Tracking resistance VDE 030

KB method Pt. 1/9.64Test solution A DIN 53480/6 20×15mm >600 >600 >600

and cellulose acetate butyrate molding com-pounds.

2.4.2. Polymer-Modified Cellulose MixedEsters

In 1977, cellulose acetate butyratemolding com-pounds modified with ethylene – vinyl acetatewithout monomolecular plasticizers were intro-duced for the first time [118]. They have sincebecome firmly established, particularly in theextrusion sector (automotive decorative trim,etc.). As far as the property pattern was con-cerned, the decisive factor was the matching of

the ethylene –vinyl acetate component to the cel-lulose acetate butyrate used.

A polymer modification of cellulose acetatepropionate without monomolecular plasticizershas also been successfully carried out [119]. Inthis case, however, a modifier of complex struc-ture, based on an ethylene – vinyl acetate graftpolymer, is necessary [120].

The main advantages of these polymer-modified cellulose mixed ester molding com-pounds over the former plasticized systems arethat the existing combination of characteristicproperties is retained, while the values for heatdistortion temperature, creep behavior, and stiff-ness are considerably improved.

38 Cellulose Esters

Figure 28. Isochronous stress – strain curves of celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate for 1 h (determined in accordance with DIN 53444or ISO/R 899)

Figure 29. Isochronous stress – strain curves of celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate for 1000 h (determined in accordance with DIN53444 or ISO/R 899)

Another feature of thesemolding compoundsis their freedom from plasticizer migration.

The polymer modifiers that are incorporatedalso delay the occurrence of crazing during long-termoutdoor exposure, a factwhich significantlyincreases the service life of, for example, exter-nal automotive decorative trimmade from cellu-lose acetate butyrate molding compounds [118].

Some striking examples of the differentproperties of monomolecular-plasticized andpolymer-modified cellulose acetate propionateand cellulose acetate butyrate are shown in Fig-ures 27, 28, and 31.

Figure 30. Dynamic fatigue test in the range of pul-sating tensile stresses (number of load cycles) of cellu-lose acetate22, cellulose acetate propionate5, and cellu-lose acetate butyrate10 (determined in accordancewithDIN50100; stress amplitude ± σa (N = 1) means stress ampli-tude under initial loading)

Figure 31. Dynamic fatigue test in the range of alternat-ing flexural stresses (number of load cycles) of celluloseacetate, cellulose acetate propionate, and cellulose acetatebutyrate (determined in accordance with DIN 50100; stressamplitude± σa (N = 1)means stress amplitude under initialloading)

Polymer modification has a similar effect onthe tensile test, shear modulus, creep behavior,and hardness [119].

2.4.3. Chemical Properties

Thermoplastic cellulose ester molding com-pounds are resistant to white spirits, oil, andgrease. Table 19gives guide values for resistanceto a range of substances, but thorough practicaltests are recommended in each case.

Cellulose Esters 39

Table 19. Typical values∗ for the chemical resistance of organic cellulose ester molding compounds

Solvent Cellulose acetate(< 55 % acetic acid)

Cellulose acetate(> 55 % acetic acid)

Cellulose acetatepropionate

Celluloseacetatebutyrate

Water + + + +Alcohols − − − − − − − −Ethyl acetate − − 0 0 0Methylene chloride − − 0 0 0Acetone 0 0 0 0Carbon tetrachloride + + + − + −Trichloroethylene + + − − − − −Perchloroethylene + + + − + −Benzene + + − − − − −Xylene + + − − − −Petroleum spirit + + + +Motor fuel mixture (high octane) + + + − + −Mineral oil (paraffin) + + + +Linseed oil + + + +Turpentine oil + + + − + −Lavender oil + + − − − −Ether + + + − + −Formalin − − − − + − + −2-Chlorophenol 0 0 0 0Sulfuric acid, conc. − − − −Sulfuric acid, 10 % + − + − + − +Hydrochloric acid, conc. − − − −Hydrochloric acid, 10 % − − − −Nitric acid, conc. − − − −Nitric acid, 10 % − − − −Caustic potash solution, 50 % − − − −Caustic potash solution, 10 % − − +− +−

∗ Key to symbols: + = resistant; + − = resistant, but swells; − = not resistant; − − = not resistant, swells; 0 = soluble.

2.4.4. Raw Materials

The following cellulose esters are used forthe production of cellulose ester molding com-pounds:

Cellulose acetate propionate molding com-pounds,55 – 62 % propionic acid content3 – 8 % acetic acid content

Cellulose acetate butyrate molding com-pounds,43 – 47 % butyric acid content19 – 23 % acetic acid content

Cellulose acetate molding compounds,51.6 – 56.3 % acetic acid content51.5 – 53.5 % acetic acid content (forblock acetate only)

Of the large number of plasticizers that arecompatible with cellulose esters [6], the follow-ing have acquired industrial significance, eitheralone or in combination with one another:

For cellulose acetate propionates and cellu-lose acetate butyrates:

di-2-ethylhexyl phthalate, dibutyl adipate,di-2-ethylhexyl adipate, dibutyl azelate anddibutyl sebacate, dioctyl azelate, dioctyl se-bacate, palmitates, stearates, etc.For cellulose acetates:dimethyl, diethyl, dibutyl, di-2-ethylhexyl,and di-2-methoxyethyl phthalate; triphenyland trichloroethyl phosphate.

Ethylene – vinyl acetate copolymers haveproven to be particularly suitable as polymermodifiers for cellulose acetate butyrate moldingcompounds, while graft polymers based on eth-ylene – vinyl acetate are preferred for celluloseacetate propionate molding compounds.

The stabilizers and antioxidants used for cel-lulose ester molding compounds include: alkalisalts and alkaline-earth salts of sulfuric, acetic,and carbonic acid, tartaric acid, oxalic acid, cit-ric acid, higher molecular mass epoxides, andphenolic antioxidants. In special cases these sta-bilizers and antioxidants can be complementedby others [121].

From the range of ultraviolet absorbers avail-able, various benzophenones, benzotriazoles,

40 Cellulose Esters

salicylates, and benzoates are recommendedfor organic cellulose ester molding compounds[122].Processing auxiliaries for cellulose ester

molding compounds include zinc stearate, butylstearate, and paraffin oil.

Numerous combinations of dyes can be usedfor coloring cellulose ester molding compounds[123]. The following groups of dyes have provensuccessful in practice: alkaline, acid, and sub-stantive dyes (provided they are sufficiently sol-uble in the solvent); Zapon, Sudan, and Ceresdyes (provided they are sufficiently resistant tosublimation); and organic and inorganic pig-ments.

2.4.5. Production

Like cellulose acetates, cellulose acetate propi-onates and cellulose acetate butyrates are alsothoroughly mixed at room temperature withplasticizers, stabilizers, antioxidants, dyes, andsometimes ultraviolet absorbers and processingaids. Plastification and homogenization are car-ried out at higher temperatures (between 150 and210 C, depending on the type and the degree ofplasticization) in single- or twin-screw kneadersor roll mills. Depending on the type of equip-ment used, this results in granules in the formof pellets (bulk density 500 – 620 g/L) or cubes(bulk density 400 – 470 g/L).

There are no wastewater or waste gas prob-lems associated with the production of thermo-plastic cellulose ester molding compounds. Theinevitable plasticizer vapors that occur duringprocessing should be removed by exhaust ven-tilation.

2.4.6. Trade Names

The most important trade names of thermoplas-tic cellulosemixed estermolding compounds areas follows:Cellulose acetate propionates: Cellidor CP

(Bayer AG, Leverkusen, Federal Republic ofGermany), Tenite Propionate (Eastman Chem-ical Products, Inc., Kingsport, United States).Cellulose acetate butyrates: Cellidor B

(Bayer AG, Leverkusen, Federal Republic ofGermany), Tenite Butyrate (Eastman ChemicalProducts, Inc., Kingsport, United States).

Cellulose acetate: Acety (Daicel, Osaka,Japan), Cellidor S (Bayer AG, Leverkusen,FRG), Dexel (Courtaulds Chem. and Plastics,GB), Saxetat (VEB Eilenburg, GDR), Setilithe(Tubize Plastics, Belgium), Tenite Acetate(Eastman Chemical Products, Inc., Kingsport,United States).

2.4.7. Quality Requirements and QualityTesting

As has already been stated, themechanical prop-erties of thermoplastic cellulose ester moldingcompounds are dependent on the chain-lengthof the molecules, on the plasticizer or polymermodifier, the combination of plasticizers, and thecontent of plasticizer or polymer modifier.

With respect to cellulose acetate, determina-tion of the viscosity and the viscosity ratio in adilute solution [124], determination of the in-soluble constituents [125], viscosity loss dur-ing molding [126], light absorption before andafter heating [127], and the determination ofconstituents which are extractable with ethylether [128] are all standardized. The methodsdescribed for cellulose acetate are similarly ap-plicable to cellulose mixed esters.

The manufacturers also carry out numerousin-house tests during the course of their qualitycontrol programs. These include determinationof mechanical data, testing of purity, checkingthermal stability, colorimetry, determination offlow properties (melt index, Brabender, extru-siometer), etc.

In the Federal Republic of Germany, cellu-lose ester molding compounds (CA, CP, andCAB) are standardized in accordance withDIN 7742, Parts 1 and 2, and in theUnited Statesin accordance with ASTM D 706 (cellulose ac-etate), D 707 (cellulose acetate butyrate), and D1562 (cellulose acetate propionate).

2.4.8. Storage and Transportation

The transportation of thermoplastic celluloseester molding compounds is not governed bythe GGVS/ADR, GGVE/RID, GGVSee/IMDGcode or DGR/ICAO regulations for the trans-portation of hazardous goods. The storage of

Cellulose Esters 41

thermoplastic cellulose ester molding com-pounds presents no problems. Even after 10years in storage, no changes in compositionhavebeendetected. It is, however, recommendedthat thermoplastic cellulose ester molding com-pounds should be predried in accordance withthe particular manufacturer’s guidelines beforeprocessing.

2.4.9. Uses [129]

In the field of injection-molded eyeglass frames,cellulose acetate propionates have become in-creasingly important. This is due mainly to thefact that their dimensional stability is better thanthat of cellulose acetate, as a result of lowermoisture absorption and greater stiffness and di-mensional stability under heat [130].

Cellulose acetate propionate is also used forhigh-quality frames for sunglasses, protectivegoggles for industry, and sports goggles [130].

Cellulose acetate still dominates as sheet ma-terial for making ophthalmic frames. For thispurpose, uni- or multicolored plates are eithercast or made by extrusion.

Due to its high transparency, good impact re-sistance, and low level of light scattering, cel-lulose acetate propionate has become increas-ingly popular as a glazing for visors (for skiers,drivers, and workers in industry) and for sun-glasses and sports goggles. These applicationshave been made possible by surface saponifica-tion of the plastic, which ensures permanent an-tifogging properties (the surface has very goodwetting properties and excellent water absorp-tion) [130].

Cellulose acetate propionates with spe-cial infrared/ultraviolet-absorbing characteris-tics have become more important for weldinggoggles and certain types of sunglasses [130].

Where greater demands are placed on impactresistance, cellulose mixed esters as well as cel-lulose acetate are used (e.g., for the productionof tool handles, hammer heads, and covers forhandles of pliers, wrenches, etc.). Particularlyimportant factors in this field of application aretoughness, transparency, lightfastness, the “nat-ural feel,” the repolishing effect, and the absenceof stress cracking. No problems occur whenmetal parts are insertion molded (the blades can

even be driven “cold” into the handles), whichbrings obvious economic advantages.

With their minimal plasticizer migration, cel-lulose mixed esters are preferred over celluloseacetate for the packaging of toiletries. Other rea-sons for their use in this sector include the bril-liance and depth of color, as well as the abilityto produce special color effects.

Resistance to stress cracking and good tough-ness properties make cellulose mixed estersan excellent material for brushes, particularlytoothbrushes, for which new designs requireclose spacing of the drill holes and a high bristledensity. Cellulose mixed esters also satisfy therequirement for high tuft pull-out strength.

Cellulose acetate butyrate sheet is used forilluminated advertising signs, machine hoods,lamp covers, and dome lights. High light trans-mission, practically unrestricted choice of col-ors, antistatic characteristics, easy accurate pro-cessing, ease of joining (by simply gluing), goodprinting and coating properties, the absence ofstress cracking, and finally, excellent mechani-cal strength are the main factors influencing thechoice of this material. Cellulose acetate propi-onate is used in place of cellulose acetate fortransparent, large-size seat shells.

Transparent, thin-section, and large-areahoods, lids, and covers with excellent toughnessand good weathering resistance are injection-molded from specially modified cellulose ac-etate propionate.

Decorative trimmade of cellulose acetate bu-tyrate combined with aluminum foil [131] hasbeen firmly established in industry for years. Analuminum foil is coatedwith the cellulosemixedester and shaped in the crosshead die of an ex-truder [132].With its practically unlimited scopefor metal and wood effects, its elasticity, resis-tance to detergents, and simple fixing, this com-bination of materials has been used with greatsuccess in the automotive industry [133] in par-ticular, as well as in the electrical, audio, anddomestic appliance sectors [134].

Further applications for cellulose esters in-clude lamp covers, toilet seats,writing and draw-ing instruments, tap handles, casings for dispos-able hypodermic syringes, transparent mouth-wash spray attachments, handles, high-qualitytoys, instrument panel covers (glazing), andknife handle grips.

42 Cellulose Esters

Economic Facts. With an apparently guar-anteed supply of rawmaterials and a tremendousscope for variation of cellulose ester moldingcompounds and of their property combinations,this class of plastics should maintain its marketsignificance in special areas of application foryears to come.

2.4.10. Toxicology and Occupational Health

Cellulose acetate and cellulose propionatemold-ing compounds comply with RecommendationXXVI of the Federal Health Authorities of theFederal Republic of Germany [135].

There are also various cellulose ester mold-ing compounds on the market which satisfy therequirements of the U.S. Food andDrugAdmin-istration for plastics [136].

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