Reactive Polymers Fundamentals and Applications || Furan Resins

7

Furan Resins

Furan resins are condensation products of furfuryl alcohol (FA). The resinsare derived from vegetable cellulose, a renewable resource.1 Furans asconstituents of polymers have been reviewed.2

7.1 HISTORY

In Latin, furfur means bran. Furfural was first isolated in 1832 (or 1821)by Döbereiner∗, as a by-product of the synthesis of formic acid. In 1840the ability of furfural to form resins was discovered by Stenhous.3 Theindustrial production of furfural started in 1922, and one year later the firstfuran-based resins emerged. Early patents on furan resins include that ofClaessen4 and one for synthetic resins, (actually mixed phenol furan resins)suitable for use in molding gramophone records.5

7.2 MONOMERS

Monomers suitable for furan resins are listed in Table 7.1. One of the chiefadvantages in furan resins stems from the fact that they are derived fromvegetable cellulose. Suitable sources of vegetable cellulose are corn cobs,sugar cane bagasse, oat hulls, paper mill by-products, biomass refinery

∗Johann Wolfgang Döbereiner, born in Hof an der Saale 1780, in Germany, died in Jena1849

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308 Reactive Polymers Fundamentals and Applications

Table 7.1: Monomers for Furan Resins6

Furan Compound Remarks or Reference

FuranFurfuralFurfuryl alcohol5-Hydroxymethylfurfural (HMF)5-Methylfurfural2-Furfurylmethacrylate 7

Bis-2,5-hydroxymethylfuran Glass fiber binder2,5-Furandicarboxylic acid

OH

OH

OH

OH

OHO

OH

OH

OH

OHHO

OC

H

O

Figure 7.1: Mechanism of the Formation of Furfural

eluents, cottonseed hulls, rice hulls, and food stuffs such as saccharidesand starch.6

Pentoses hydrolyze to furfural and hexoses give 5-hydroxymethyl-furfural on acid digestion.8

7.2.1 Furfural

Furfuraldehyde is a by-product from the sugar cane bagasse which pro-duces resins with an excellent chemical stability and low swelling. 2-Furanformaldehyde or furfural is made from agricultural materials by means ofhydrolysis.

The mechanism of formation of furfural is shown in Figure 7.1. Itis a light yellow to amber colored transparent liquid. Its color graduallydeepens to brown during storage. It tastes like apricot kernel. It is mainly

Furan Resins 309

used in lubricant refinement, furfuryl alcohol production, and pharmaceu-tical production.

Furfural is the chief reagent used to produce materials such as fur-furyl alcohol, 5-hydroxymethylfurfural (HMF), bis(hydroxymethyl)furan(BHMF), and 2,5-dicarboxyaldehyde-furan. The furan-containing mono-mers in turn can undergo reactions to produce various furan-containingmonomers with a wide variety of substituents as shown in Table 7.1.

7.2.2 Furfuryl Alcohol

Furfuryl alcohol is made from furfural by reduction with hydrogen. It is acolorless transparent liquid and becomes brown, light yellow, or deep red,when exposed in the air. It can be mixed with water and many organic sol-vents such as alcohol, ether, acetone, etc., but not in hydrocarbon products.

7.2.3 Specialities

7.2.3.1 Furan-based Polyimides

Polyimides based on poly(2-furanmethanol-formaldehyde) can be preparedby a Diels-Alder reaction (DA) of the respective furan resin with bismale-imides.9 The Diels-Alder reaction proceeds in tetrahydrofuran (THF) orin bulk. The tetrahydrophthalimide intermediates aromatize in the pres-ence of acetic anhydride. Polyimides based on the furan resin exhibit goodthermal stability.

7.2.4 Synthesis

Furan-based monomers can polymerize through two well-known mecha-nisms. The first involves chain or polyaddition polymerization, which isinitiated by free radical, cationic or anionic promoters. Polymerizationproduces macromolecules with furan rings pendant on the main chain.

The second method is a polycondensation, also referred to as poly-merization condensation. Polymers and copolymers resulting from acidcatalyzed condensation reactions result in macromolecules with furan ringsin the main chain.6

As a general rule, the furan resins formed by polycondensation re-actions have stiffer chains and higher glass transition temperatures. Thesereactions may involve self-condensation of the furan monomers described


OCH2 OH

H+

OCH2

+

OCH2

OCH2 OH

OCH2

+

OCH2 OH

Figure 7.2: Acid Catalyzed Self-condensation of Furfuryl Alcohol

above, as well as condensation reactions of such monomers with amino-plast resins, organic anhydrides, and aldehydes such as formaldehyde, ke-tones, urea, phenol, and other suitable reagents. Most common, furan re-sins are produced by acid catalyzed condensation reactions.

The condensation results in linear oligomers, the furan rings beinglinked with methylene and methylene-ether bridges, c.f. Figure 7.2.

The synthesis of furan resins proceeds in a pH range of 3 to 5, ata temperature range of 80 to 100°C. The condensation is stopped, when adesired viscosity value is reached, by neutralizing the liquid resin.

Furfuryl alcohol can also be condensed with formaldehyde to ob-tain furan-formaldehyde resins. The content of free formaldehyde can belowered by the addition of urea at the late stages of synthesis.

7.3 SPECIAL ADDITIVES

7.3.1 Reinforcing Materials

Aramid fibers were used as reinforcing material for a phenol resin and afuran resin. A comparative study of the mechanical performance of thematerials showed that the furan resin is more suitable as a matrix than thephenol resin.10

Furan Resins 311

Table 7.2: Global Production Data of Furan Resins Related Components11

Monomer Mill. Metric tons Year ReferenceFormaldehyde 24 2003 12

Furfural 0.225 1999 13

7.3.2 Production Data of Important Monomers

The most important industrial furan resins are based on 2-furfuryl alcohol.The largest producers of furfural are China and the Dominican Republic.Production data are shown in Table 7.2. Around 30% of the furfural con-sumption is used to produce furfuryl alcohol, which is mainly consumedby the production of furan resins.

7.4 CURING

Materials known to be suitable for curing furan resins include inorganicand organic acids. Examples of suitable organic and inorganic acids in-clude hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, tartaricacid, and maleic acid. Friedel-Crafts catalysts include aluminum trichlor-ide, zinc chloride, aluminum bromide, and boron fluoride.

Resins with improved fire resistance are cured with a mixture oftrimethylborate, boric anhydride, and p-toluenesulfonic acid.14

Salts of both inorganic and organic acids may also be used. Ammon-ium sulfate is preferred. Ammonium sulfate is a latent catalyst which maybecome active at approximately 110 to 150°C. Suitable organic salts are theurea salt of toluenesulfonic acid, the polyammonium salts of polycarbox-ylic acids such as the diammonium salts of citric acid, and the ammoniumsalts of maleic acid. Cyclic anhydrides such as maleic anhydride are alsosuitable for use as catalysts.

It is believed that polyester co-polymers are formed between the an-hydride and the free hydroxylated species present in the resin. Maleicacid promotes the polymerization reaction. Furthermore, it is believed thatmaleic acid may preferentially reduce the emission of bis(hydroxymeth-yl)furan monomer during the curing process. A significant reduction ofvolatile organic compounds (VOCs), will use a catalyst system comprisedof maleic acid and ammonium sulfate.6


OCH

OCH2

CH2

CH2

OCH

O

Figure 7.3: Crosslinked Methylene Bridges

7.4.1 Acidic Curing

The resin can be crosslinked by an acidic catalyst. The reaction is notsensitive to air. The main route of curing is an additional condensation re-action at the free α-hydrogen of furan rings. These positions are connectedby methylene bridges.

7.4.2 Oxidative Curing

The oxidative crosslinking of furfuryl alcohol (FA) polycondensates pro-ceeds at temperatures of 100 to 200°C. Structures with tertiary carbonatoms, as shown in Figure 7.3, could be identified.

7.4.3 Ultrasonic Curing

Ultrasonic treatment, i.e., sonication during the curing process of a furanresin, showed changes of the curing performance. p-Toluenesulfonic acidwas added as curing catalyst in the proportion of 0.3%. Fine carbons werealso incorporated.

Using an ultrasonic homogenizer in the presence of carbonaceousfine particles showed an increased curing rate of the furan resin. This, inturn, increased the polymerization degree with an increase in ultrasoundintensity. The increase of curing rate was also observed by small additionsof carbonaceous fine particles. In this case, the curing accelerated with anincrease in the specific surface area of the additives.15

The increase of curing rate is believed to result from cavitation. Thecuring reaction proceeds slowly in the absence of cavitation and simplestirring fails to produce such a marked increase in the rate of reaction. Thecuring is accelerated by heat, oxygen, and the addition of phenol and urea.

Furan Resins 313

7.5 PROPERTIES

7.5.1 Recycling

Research has been conducted to introduce pendent furan groups into poly-mers such as poly(styrene) via copolymerization with a suitable comono-mer. The pendent furan moieties can be crosslinked with a bismaleimideto achieve polymers with better performance. In order to recycle thesecrosslinked materials, heating experiments with an excess of 2-methyl-furan were performed in order to induce the retro Diels-Alder reactionand break-up the network. The reaction proceeded in this manner and theoriginal copolymers could be recovered from the treatment. Therefore,the introduction of furan units is a potential path of recycling crosslinkedpolymers by thermal treatment with a diene in excess.16

The Diels-Alder reaction between styrene-furfuryl methacrylate co-polymer samples and bismaleimide can be monitored the ultraviolet ab-sorbance of the maleimide group at 320 nm or by 13C-NMR spectroscopy.7

7.6 APPLICATIONS AND USES

Furan resins are used mainly in the foundry industry, as sand binders forcasting molds and cores. Furan resins are often used in combination withother resins. Furan resins are highly corrosion resistant. Therefore, theyhave found use in mortars and in cements. Improved mechanical propertiesare implemented by reinforcing with glass fibers.

7.6.1 Carbons

Porous Carbon. Furan resins form a porous carbon by pyrolysis at 450°C.

Glass-like Carbon. Glass-like carbon is identified as an excellent carbonartifact due to its characteristics such as hardness and shape stability. Themicrostructure of glass-like carbon consists of a non-graphitic alignment ofhexagonal sheets. It has unique properties such as great hardness comparedwith other carbon materials and impermeability for gases.17

Glass-like carbon is of interest in the battery and semiconductor in-dustries. Glass-like carbon is prepared by heat-treatment on thermosettingresins in inert atmosphere.


Figure 7.4: SEM photographs of glass-like carbon derived from furan resin. (a)

300, (b) 600 °C.18 (Reprinted with permission from Elsevier)

During the heat-treatment of a furan resin, weight loss is very rapidup to 500°C, then continues gradually up to 1000°C, and then the weightstays almost constant above 1000°C. SEM photographs of heat treatedglass-like carbon reveal a large increase of micro-grain size in the rangeof 60 to 105 nm when treated at 2000°C. Up to 2500°C, the grain sizedecreases to 27 to 40 nm due to graphitization.18 There is a structuralcorrelation between the micro-texture of the furan resin and the glass-likecarbon formed from the particular resin. The pore structure in glass-likecarbon can be characterized by small-angle X-ray scattering (SAXS) tech-nique. The scattering intensities grow gradually with increasing heat-treat-ment temperature up to 1600 to 1800°C, and then the intensities increaseabruptly at a temperature higher than 1800°C.

The dependence of the structural change of a glass-like carbon froma furan resin is almost the same as that of a phenolic resin. However, itwas found that the carbon prepared at 1200°C from furan resin shows thelargest interlayer spacing in the carbon matrix and at the same time thesmallest value of the gyration radius for the pores.17

7.6.2 Chromatography Support

Conventionally used packing materials for liquid chromatography are achemically bonded type of packing material based on silica gel and a pack-

Furan Resins 315

ing material based on synthetic resin. The silica gel-based packing materialhas relatively strong in mechanical properties and in its swelling or shrink-ing characteristics against various organic solvents. Therefore, it has a highresolving power and is superior in exchangeability of eluent for analysis.However, the silica gel-based packing material has problems in that the sil-ica gel dissolves under acidic or alkaline conditions and the solubility ofthe silica gel in an aqueous solution increases when warmed, resulting indurability problems.

The packing material of synthetic resin, on the other hand, is knownto be high in acid- and alkali-resistivity, and has a good chemical durabilityas a packing material. However, since the mechanical strength of the parti-cles is small, it has been difficult to convert them into finer particles. Rawmaterials which are highly chemically stable and exhibit high mechanicalstrength are graphitized carbon black.

A packing material for liquid chromatography is produced by mix-ing carbon black, a synthetic resin which can be graphitized, and pitches.Suitable synthetic resins are phenolic resins, furan resins, furfural resins,divinylbenzene resins, or urea resins.19 The pitches can be petroleumpitches, coal-tar pitches, and liquefied coal oil. The mixture is granulatedand heated up to 3000°C in an inert atmosphere.

7.6.3 Composite Carbon Fiber Materials

Impregnation of carbon fibers and subsequent pyrolysis at 1000°C im-proves strength of carbon fibers.20 A yarn is passed through a bath contain-ing a carbonizable resin precursor, such as a partially polymerized furfurylalcohol. It is advantageous to add a latent catalyst along with the precur-sor. Suitable catalysts are a complex of boron trifluoride and ethylamine ormaleic anhydride.

The use of a latent catalyst allows the application of a low-viscositysolution to the fiber with subsequent polymerization at the elevated temper-atures. If the precursor were to polymerize significantly prior to applica-tion, the treating bath would be so viscous that would allow only a coatingto be formed.

For high performance composite carbon fiber reinforced carbonace-ous material which is compositely reinforced with carbon fibers, prepregsof woven fabrics of carbon fibers are impregnated with a resin such asphenol resin, furan resin, epoxy resin, urea resin, etc. They then are lamin-


ated as a matrix and molded under heat and pressure, and after carboniza-tion, they are further graphitized by heating to a temperature of 3000°C.21

7.6.4 Foundry Binders

Furans are somewhat more expensive than other binders, but the possibil-ity of sand reclamation is advantageous. One of the most commerciallysuccessful no-bake binders is the phenolic-urethane no-bake binder. Thisbinder provides molds and cores with excellent strengths that are producedin a highly productive manner.

Furan-based binders have less VOC, free phenol level, low formal-dehyde, and produce less odor and smoke during core making and castings.However, the curing performance of furan binders is much slower than thecuring of phenolic urethane no-bake binders.

Furan binders can be modified to increase their reactivity, for in-stance by formulating with urea/formaldehyde resins, phenol/formalde-hyde resins, novolak resins, phenolic resol resins, and resorcinol. Never-theless, these modified furan binders do not provide the cure speed neededin foundries that require high productivity. Therefore, an activator, whichpromotes the polymerization of furfuryl alcohol, is added. Resorcinol pitchis used for this purpose.22 Further components in such a formulation arepolyester polyols or polyether polyols, and a silane, such as (3-aminoprop-yl)triethoxysilane.

The curing process of urea-modified furan resins in sands has beeninvestigated by infrared spectroscopy.23

7.6.5 Glass Fiber Binders

An alternative to phenol/formaldehyde-based fiberglass binders is furan-based binders. Furan binders provide many of the advantages of phenolicbinders while resulting in substantially reduced VOC emissions. Water asa significant component can be used. Formaldehyde is not a significantcuring or decomposition by-product, and the furan resins form very rigidthermosets.

Emulsified furan resins can be used. Emulsified furan-based glassfiber binding compositions are advantageous since they allow the use offuran resins that have high molecular weights or the addition of other ma-terials which would give rise to the formation of two-phase systems.6 Asuitable surfactant to be added to the furan binder compositions is sodium

Furan Resins 317

dodecyl benzene sulfonate. It may be added in an amount from 0.05 to1.0%.

7.6.6 Oil Field Applications

Wells in sandy, oil-bearing formations are frequently difficult to operatebecause the sand in the formation is poorly consolidated and tends to flowinto the well with the oil. Sand production is a serious problem because thesand causes erosion and premature wearing out of the pumping equipment.It is a nuisance to remove from the oil at a later point in the operation.Furan resin formulations can be used for in-situ chemical sand consolida-tion.24

7.6.7 Plant Growth Substrates

Conventional mineral wool plant growth substrates are based on a coherentmatrix of mineral wool of which the fibers are mutually connected by acured binder.

There is a need to reduce the phytotoxicity of the chemicals used.The phytotoxicity may result from the phenolic binder materials. If a phen-olic resin is used as binder, a wetting agent must be added in order to impartthe hydrophobic mineral wool matrix with hydrophilic properties. Howev-er, the use of a furan resin allows the abandonment of the use of a wettingagent.

A disadvantage of the use of a furan resin is its comparatively highprice. Therefore, the traditional phenol/formaldehyde resin substitutedonly partly by a furan resin, is sufficient to maintain or to achieve the de-sired properties.25, 26

7.6.8 Photosensitive Polymer Electrolytes

Both conjugated furan chromophores and polyethers can be grafted ontochitosan to result in a photosensitive polymer electrolyte. The furan chrom-ophore consists of conjugated furan chromophores of 5-[2-(5-Methyl furyl-ene vinylene)]furancarboxyaldehyde,27 c.f. Figure 7.5. The graft polymercan be photocrosslinked. The photochemical reaction consists of a π2+π2

cycloaddition reaction of the vinylene double bonds of the furan moiety sothat two pendent vinylene groups form a four membered ring. The cross-linking reaction is shown in Figure 7.6


OOH3C

C

O

H

Figure 7.5: 5-[2-(5-Methyl furylene vinylene)]furancarboxyaldehyde

O

OH

NH

CH2

O

O

O

CH3

O

OH

NH

CH2

O

O

O

CH3

O

OH

NH

CH2

O

O

O

OH

NH

CH2

O

O

CH3O

CH3O

Figure 7.6: Photo Crosslinking of the Furylene Vinylene Units Grafted on Chi-tosan

Furan Resins 319

REFERENCES

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5. J. S. Stokes. Improvements in and relating to synthetic resin composition.GB Patent 243 470, December 3 1925.

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11. R. Gubler, editor. Chemical Economics Handbook (CEH). SRI Consulting, aDivision of Access Intelligence, Menlo Park, CA, 1950–to present. (Internet:http://ceh.sric.sri.com/).

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(CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park, CA,January 2004. (Internet: http://ceh.sric.sri.com/).

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book (CEH). SRI Consulting, a Division of Access Intelligence, Menlo Park,CA, July 2001. (Internet: http://ceh.sric.sri.com/).


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16. C. Gousse, A. Gandini, and P. Hodge. Application of the Diels-Alder reactionto polymers bearing furan moieties. 2. Diels-Alder and retro-Diels-Alder re-actions involving furan rings in some styrene copolymers. Macromolecules,31(2):314–321, January 1998.

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18. Y. Korai, K. Sakamoto, I. Mochida, and O. Hirai. Structural correlation be-tween micro-texture of furan resin and its derived glass-like carbon. Carbon,42(1):221–223, 2004.

19. H. Ichikawa, A. Yokoyama, T. Kawai, H. Moriyama, K. Komiya, and Y. Kato.Packing material for liquid chromatography and method of manufacturingthereof. US Patent 5 270 280, assigned to Nippon Carbon Co., Ltd. (Tokyo,JP); Tosoh Corporation (Yamaguchi, JP), December 14 1993.

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Reactive Polymers Fundamentals and Applications || Furan Resins