Reactive Polymers Fundamentals and Applications || Reactive Extrusion

15 Reactive Extrusion

Reactive extrusion is an attractive route for poly-mer processing in order to carry out various reac-tions including polymerization, grafting, branching,and functionalization. There are monographs on reac-tive extrusion [1,2].

In this chapter, we deal mainly with the formation ofpolymers by reactive extrusion, i.e., reactive extrusionpolymerization. Aspects of reactive extrusion are cov-ered in other chapters: This includes grafting, compat-ibilization, and controlled rheology. Reactive extru-sion polymerization involves polymerizing a liquidor solid monomer or a prepolymer during the resi-dence time in the extruder to form a high-molecular-weight melt.

Low-cost production and processing methods forbiodegradable plastics are of great importance, sincethey enhance the commercial viability and cost-competitiveness of these materials. Reactive extrusionis an attractive route for the polymerization of cyclicester monomers, without solvents, to produce high-molecular-weight biodegradable plastics.

Extruders can be used for bulk polymerization ofmonomers, like methyl methacrylate, styrene, lactam,and lactide. From a mechanistic perspective, nearly allkinds of polymerization have been performed in anextruder. These include radical polymerization, ionicpolymerization, metathesis polymerization [3], andring opening polymerization. The techniques of char-acterization and experimental setup for reactive extru-sion can be found in the literature [4,5]. The techniqueis also attractive for melt spinning [6,7].

The economics of using an extruder as a bulk poly-merization reactor are favorable when high through-put and control of molecular weight are realized. Thelimitation arises due to the residence time required tocomplete the polymerization, which ideally should beless than 5 min.

There are significant kinetic, heat transfer, anddiffusion-related issues in a bulk polymerization pro-cess that make it difficult to develop and design pro-cessing methods that result in high-molecular-weightpolymer at high throughput with a high conversion ofthe monomer. However, extruders are ideal process

vehicles for this purpose as they can be tailored togive various flow patterns, residence time distribu-tions, and shear effects, each of which affects the poly-merization and polymer quality.

15.1 Extruder

Reactive extrusion is a complex process, and numer-ical simulation is an important method in optimiz-ing operational parameters. Two different simula-tion methods, the one-dimensional model and thethree-dimensional model, have been used to predictthe polymerization of ε-caprolactone in fully filledscrew elements. The predicted results of polymer-ization progression under different simulation con-ditions based on these two methods have been com-pared. The simulation results show that the simplifica-tions and assumptions in the one-dimensional modelmake it difficult to capture the complex mixing mech-anism, heat generation, and heat loss in reactive extru-sion. The one-dimensional model is feasible onlyunder particular conditions, such as low screw rotat-ing speed, small heat from reaction, and small screwdiameter, whereas the three-dimensional model is amore powerful simulation tool for much wider pro-cessing conditions [8,9].

In reactive extrusion, the extruder is used as asolvent-free continuous chemical reactor able to pro-cess highly viscous materials. Process modeling andsimulation constitute useful tools for process under-standing, development, optimization, and scale-up.Reactive extrusion modeling is still a challengebecause of the complex geometry and the strong cou-pling between operating parameters, flow conditions,material properties, and reaction kinetics [10].

In this section the reactive extruder depicted inFigure 15.1 is modeled mathematically in a very sim-ple way. There is an input of monomer on the left sidewith a volume rate of V . The average residence timetr is then

tr = V

V, (15.1)

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340 REACTIVE POLYMERS FUNDAMENTALS AND APPLICATIONS

V

H

R

ΔT

ΔTH

.Figure 15.1 Balance of an extruder.

when the volume of the extruder is V. The reaction ratein the extruder is R. Let us assume for simplicity thatthe rate of reaction is not dependent on the conversion.The conversion C, as a fraction, is then

C = tr R. (15.2)

To obain full conversion, i.e., C = 1, the residencetime should be tr > 1/R. The rate of reaction heatgeneration is calculated by means of Eq. 15.3:

H = RH0V , (15.3)

H , rate of heat released in the whole extruder;H0, heat released for full conversion

in the unit volume;R, rate of reaction;V, volume of extruder.

The heat released in the extruder must be conductedthrough the walls. Here we neglect that some of theheat is transported away with the melt. We also neglectthat additional heat is generated by friction forcesthrough kneading. The heat that can be transportedthough the walls of the extruder is given by the heatflow equation, Eq. (15.4):

H = k A�T . (15.4)

Here k is the overall heat transfer coefficient(J s−1 K−1 m−2; different from the conductivitycoefficient). The area A relates to the volume of theextruder with a geometry factor g:

V = g A. (15.5)

In the case of a cylinder, V = r2πh = g2rπh. Let usassume that the heat is transferred through the enve-lope of the cylinder. Combining Eqs. (15.3) and (15.4)yields Eq. (15.6):

�T = V

V

H0g

k. (15.6)

We have previously implied the condition of full con-version. Equation (15.6) states that a temperature gra-dient will be created by the reaction in the extruder.We are restricted by the temperature difference in thecooling facilities. For example, the outer temperatureis usually not set below the room temperature for eco-nomic reasons. On the other hand, the temperatureinside the extruder cannot get too high. Otherwisethe material will pyrolyze. The temperature gradi-ent is limited. Now the temperature difference willbe affected by the throughput. The throughput willbe pushed to a maximum for economic reasons. Theheat of reaction for a given process cannot be changed.However, if there are alternative processes found thatachieve a material with identical properties, the pro-cess with a low heat of conversion should be selected.The geometry factor can be influenced by the designof the extruder. Clearly, a smaller diameter is advanta-geous. This will lead to a design of a longer machine,if a large volume is desired. The length of a machineis restricted by the mechanical properties of a screw.The situation is simpler in a chemical plant. There arebent loop reactors in which the material can freelyflow.

The model described is very simple, because thetemperature gradient in the melt, the residence timedistribution, and many other parameters have not beentaken into account. It does provide a basic insight intoimportant parameters of the device.

More sophisticated models are available in the lit-erature. The reactive extrusion process in a single-screw extruder has been assessed by power-law flu-ids undergoing isothermal homogeneous and hetero-geneous reactions. The reaction was reported to befirst order. The equation of conservation of componentspecies was transformed into an eigenvalue problem.Analytical solutions were developed for the concen-tration distribution in the extruder. Expressions for theconversion of the reactant and Sherwood number weregiven [11].

A mathematical model for the reactive extrusion ofmethyl methacrylate has been described [12]. The keyvariables, such as pressure, temperature, residencetime, filling ratio, and molecular weight along theextruder length, can be calculated using this model.The flow in the extruder is modeled by a simplifiedapproach. However, this approach is versatile enoughto include any screw profile, such as right-handed andleft-handed elements, and kneading disks. In addition,a kinetic model that considers a mixture of initiators

15: REACTIVE EXTRUSION 341

is coupled to the flow equations. Further, an effectivemodel for the auto-acceleration effect has been intro-duced. The model can be easily implemented on apersonal computer, and a wide range of process con-ditions can be modeled, because of its flexibility.

Analysis of reactive extrusion and devolatilizationin a modular co-rotating twin-screw extruder and theappropriate software has been presented [13].

Spatially averaged low-dimensional models havebeen developed to study the mixing effects and degra-dation in the peroxide-induced reactive extrusion ofpoly(propylene) (PP) [14]. These models are basedon the Liapunov-Schmidt technique which is used toaverage the convective-diffusion equation in the trans-verse direction and obtain low-dimensional two-modemodels that describe the mixing effects in laminarflow tubular reactors [15].

A model to predict the residence time distributionin fully intermeshing co-rotating twin-screw extrud-ers has been developed, using screw speed and flowrate as control parameters and screw profile and diedesign as geometrical parameters [16]. A simplifiedsolution of the Navier-Stokes equations is used forthe description of the fluid flow at steady state in afully intermeshing co-rotating twin-screw extruder.The Navier-Stokes equations are solved under sev-eral simplifying assumptions concerning the extrudergeometry, fluid properties, and the flow type. Severalexamples have been presented to demonstrate the val-idation of the model.

A steady-state mathematical model for the oxida-tion of a biopolymer by reactive extrusion has beendeveloped. The model is based on a hybrid approachcombining chemical engineering methods and sim-plified continuum mechanics laws. The combinationof these two approaches enables simplification of thecalculations. The model has been validated by a semi-pilot co-rotating twin-screw extruder [10].

A three-dimensional model of closely intermeshingco-rotating twin-screw extruders was established. Anumerical computation of the activated anionic poly-merization of styrene has been presented [10]. Theresults were compared with those of other classicalmodels [17].

The free-radical grafting of poly(ethylene) (PE)with vinyl monomers by reactive extrusion was stud-ied by numerical computation [18,19]. The evo-lutions of the relevant variables, such as initiatorand monomer concentration, viscosity, etc., werepredicted by an uncoupled semi-implicit iterative

Table 15.1 Parameters for Modeling the Grafting ofPoly(ethylene) [18]

Parameter Numerical Value

Density of poly(ethylene) 968 kg m−3

Density of monomer 970 kg m−3

Density of initiator 871.2 kg m−3

Molecular weight ofpoly(ethylene)

27.6 kg mol−1

Molecular weight of monomer 148 g mol−1

Molecular weight of initiator 290 g mol−1

Initial concentration ofpoly(ethylene)

34.7 mol m−3

Initial concentration of monomer 64.7 mol m−3

Initial concentration of initiator 1.08 × 10−5 mol m−3

Initiator efficiency 1.0Frequency factor for initiator

decomposition 2.3 × 1016 s−1

Activation energy for initiatordecomposition 1.61 × 105 J mol−1

Apparent activation energy 3.99 × 104 J mol−1

Molecular weight distributionof the base polymer 7.69

Flow speed on the wall of thegeometrical model

(throughput = 20 kg h−1) 0.0696 m s−1

Flow speed at the entrance of thegeometrical model(throughput = 20 kg h−1) 0.0485 m s−1

algorithm. The parameters used in the model are sum-marized in Table 15.1.

The monomer conversion monotonically increaseswith decreasing throughput or increasing initial initia-tor concentration. The simulated results are in quitegood agreement with the experimental results [18].

Sometimes severe fluctuations in product qualityhave been observed. These fluctuations can be causedby thermal, hydrodynamic, or chemical instabilities[20]. Some of these instabilities are dependent onthe scale of the equipment. The experimental designis thus important when a reactive extrusion processis developed in the laboratory for scale up to largermachines.

15.1.1 Heat of PolymerizationThe performance of a reactive extrusion polymer-ization depends on the heat of polymerization itself.Table 15.2 summarizes heat and entropy of polymer-ization for selected compounds.

A review of the data in Table 15.2 reveals that vinylpolymers, such as propene, styrene, and acrylics,


Table 15.2 Heats and Entropies of Polymerization [21]

Compound State −�H −�S TemperaturekJ mol−1J mol−1 K−1 (◦ C)

Propene lc 84 116 25Acrylic acid lc 67 75Acrylonitrile lc 77 109 75Methyl lc 56 117 130

methacrylateStyrene lc 70 149 25Maleic anhydride ls 59 – 751,3-Dioxolane lc 24 76 100Tetrahydrofuran lc 19 16 25�-Butyrolactone lc −5 30 25Caprolactone lc 17 4 25D,L-Lactide lc 27 13 127

lc from liquid to crystallinels from liquid to solid

have a high enthalpy of polymerization. Polymersthat are formed by ring opening polymerization havea relatively lower enthalpy of polymerization. Fromthe point of view of heat transfer it is desirable touse monomers that have a lower polymerizationheat, because the heat must be removed through thewalls of the reactor, which has a limited surface area.Only a low polymerization heat guarantees a highthroughput.

15.1.2 Ceiling TemperatureOn the other hand, low polymerization heat implieslow thermal stability from the viewpoint of thermody-namics. The free enthalpy of polymerization is givenby Eq. (15.7):

�G = �H − T �S. (15.7)

�G, free enthalpy of polymerization;�H, enthalpy of polymerization;�S, entropy of polymerization.

If �G is negative, then the polymer is no longer sta-ble with respect to the monomer. Assuming an equi-librium is established, then the ceiling temperature Tccan be calculated by equating Eq. (15.7) to zero:

Tc = �H

T �S. (15.8)

The ceiling temperature yields reasonable resultsfor vinyl monomers, but in the case of polymers

formed by ring opening polymerization, unreasonablevalues are obtained.

15.1.3 Strategy of ReactiveExtrusion

As pointed out above, it is desirable to use materialswith a low polymerization heat in reactive extrusion.Only then can a high throughput be obtained. On theother hand, it is possible to use a mixture of a poly-mer and monomer. The latter is then polymerized inthe extruder. This concept can reduce the amount ofheat to be transferred. In compatibilization, a modifiedpolymer is used, with only one chemically reactivegroup in the chain. In this case the heat of polymer-ization with respect to volume is reduced drastically.

On the other hand, in injection molding of smallarticles with a high surface-to-volume ratio, the vis-cous melt often must be driven though small chan-nels before the melt is placed in the form. In the caseof small articles only small amounts of material areneeded. Therefore, the cost of the material used is lessinfluential in the choice of the process.

The cycle time can be reduced if the form filling canbe reduced. This can be done by selecting a materialthat is less viscous. In the case of small articles, theheat of polymerization is reduced. Therefore, reactiveextrusion is possibly attractive in the manufacture ofsmall articles.

15.2 Compositions of IndustrialPolymers

Before going into detail, we summarize the polymersthat have been obtained by reactive extrusion accord-ing to the mechanism of reaction in Table 15.3.

Table 15.3 Polymers Obtained by Reactive Extrusion

Polymer Reference

Radical PolymerizationPoly(styrene) [22]Poly(butyl methacrylate) [23]Ring Opening PolymerizationPoly(lactide) [24]Anionic PolymerizationPoly(styrene)Styrene butadiene copolymer [25]Polyamide 12 [26]Metathesis PolymerizationPoly(octenylene) [3]


15.2.1 PolyolefinsThe fabrication of polyolefin nanocomposites by reac-tive extrusion has been reviewed [27]. Special atten-tion has been devoted to the mechanism of in situgrafting reactions and the hydrogen bonding effectin the reactive blend processing and the formation ofnanostructures.

15.2.1.1 Poly(ethylene)The kinetics of melt grafting of acrylic acid (AA) ontolinear low-density PE in the course of reactive extru-sion has been investigated [28]. Polymeric peroxidesare used that are generated by electron beam irradia-tion in order to initiate the grafting.

Samples were taken out from the barrel at five portsalong the screw axis and were analyzed by infraredspectroscopy (IR). The spectra show that both the graftcopolymerization and homopolymerization proceedin two stages [28]:

1. the degree of grafting increases linearly with thereaction time in the initial stage and then

2. gradually in the second stage.

The rate of graft copolymerization is always fasterthan that of homopolymerization [28].

An irradiation of PE with UV light facilitatesthe subsequent production of hydroperoxide. In thisway, a reaction with maleic anhydride (MA) runseasier [29].

The graft copolymerization of itaconic anhydrideonto pre-irradiated linear low-density PE was carriedout in a twin-screw extruder [30]. No obvious changescan be found for the tensile strength, elongation atbreak, and Young’s modulus of the graft copolymercompared to those of the neat polymer.

The contact angle of water on the film surfacedecreases with increasing content of itaconic anhy-dride and an outstanding peel strength can be obtainedby the introduction of polar groups onto the linearlow-density PE.

Formaldehyde-free binding systems for wood com-posite products are highly desirable for environmen-tal reasons [32]. Wood particles can be modified ina reactive extrusion process with maleated PE andmaleated PP [33]. The efficiency of the modificationwas assessed using IR and XPS surface analysis tech-niques, along with a titrimetric analysis, to verify theesterification reaction between the wood particles and

maleated polyolefins. In summary, reactive extrusionwas found to be a suitable technique for the modifi-cation of wood particles, using maleated polyolefins.

Poly(ether pentaerythritol monomaleate) is a reac-tive nonionic surfactant. This compound was usedto functionalize linear low-density PE by reactiveextrusion [34].

The crystallization rates of the grafted linear low-density PE are faster than those of plain linear low-density PE at a certain temperature. The tensile prop-erties and light transmission of blown films from thiscompound in comparison with an unmodified linearlow-density PE film did not change significantly, aswell as other mechanical properties. The water con-tact angle decreases with increasing percentage ofpoly(ether pentaerythritol monomaleate) [34].

The grafting of trans-ethylene-1,2-dicarboxylicacid onto metallocene modified linear low-densitypoly(ethylene) (LDPE) and neat LDPE by reactiveextrusion was assessed [35]. As initiator 1,3-bis-(tert-butyl-peroxyisopropyl)benzene was used. The effi-ciency of grafting of trans-ethylene-1,2-dicarboxylicacid increases with increasing initiator concentrationfor both PE types. In the grafted product some car-boxyl groups are transformed into anhydride moieties.The process ensures that the concentration of terminaldouble bonds is reduced, but intramolecular unsatu-ration in both polyethylenes increases both PE types.

The molecular weight of ethylene-propylene blockcopolymers was adjusted by reactive extrusion usingdicumyl peroxide [31]. In Table 15.4, the influenceof the amount of dicumyl peroxide on the molecularproperties of the copolymers is summarized.

With increasing content of dicumyl peroxide, themolecular weight and the polydispersity decrease.

Table 15.4 Modification of the Molecular Properties bythe Initiator [31]

DCP MFR Mn Mw Mw /Mn(%) (g/10 min) (kg mol−1) (kg mol−1)

0 1.44 122.5 532.5 4.350.02 2.50 117.0 371.9 3.180.04 4.29 100.2 309.7 3.090.06 8.34 72.9 255.1 3.500.08 8.40 86.6 272.9 3.15

DCB Dicumyl peroxideMFR Melt flow rate


Also, the number of spherulites with obscure bound-aries increases as does the size of the spherulites.

Reactive blending of functionalized PE types andpolyamides (PAs) can be performed [36]. Dependenton the molecular properties of the two polymers, twotypes of stable morphologies can be obtained. The firsttype has a co-continuous morphology and the secondtype is a dispersion of sub-micron droplets of the PAphase in the PE matrix.

In composites from low-density PE, cork powderand suberin may act as coupling agents to promotethe interfacial adhesion [37]. Suberin is a hydropho-bic waxy biopolymer. Suberin polymers are extractedfrom cork and birch outer bark. The compounding isperformed using reactive extrusion. The mechanicalproperties of the composites suggest that the additionof suberin acts as a coupling agent, as the strain andthe modulus are improved.

PE/SiO2 nanocomposites have been prepared byreactive extrusion from vinyl functionalized nano-SiO2 particles [38]. Dicumyl peroxide was used asa radical initiator.

Silane-water crosslinked PE-octene is obtainedthrough a reactive extrusion process and used for theproduction of fibers by melt spinning [39]. In thefirst step, PE-octene is silane-grafted by extrusion.Eventually a spinning process follows. At the end,the grafted monofilaments are introduced in water-based solution to effect the desired crosslinking. Themost important parameters in this process are extru-sion temperature and time. The thermal stability of thefilaments increases with the degree of crosslinking upto 170 ◦C for crosslinking degrees higher than 5%.The crosslinked fibers exhibit higher elastic proper-ties than neat PE-octene fibers.

15.2.1.2 Poly(propylene)The free-radical grafting of MA to isotactic PP canbe carried out by reactive extrusion in the presence ofrare earth oxides, such as neodymium oxide [40].

The addition of Nd2O3 into the compound leads toan enhancement of the degree of grafting of MA, alongwith an elevated degradation of the PP. Mostly tertiarymacroradicals that are initially formed undergo a β-scission, then grafting with MA takes place.

Nd2O3 enhances the initiating efficiency of theinitiator, dicumyl peroxide. The synergistic effect is

maximal when the molar ratio of dicumyl peroxide toNd2O3 is approximately 1–6 [40].

PP is used in a wide range of applications dueto its low cost, superior chemical resistance, andproper mechanical properties. However, PP suffersfrom hydrophobicity and chemical inertness. Theseproperties are counterproductive for the applicationin textiles, where dyeing and surface modificationare desirable. These drawbacks have been reduced bymodification using reactive extrusion techniques. Car-danol has been grafted onto PP by reactive extrusion[41]. Cardanol is a main component of the cashew nutshell liquid, which is extracted from natural cashewshell.

A melt blend material was prepared by extrud-ing a mixture of 3-aminopropyltriethoxysilane, MA-grafted PP, and PP powder [42]. An extremely highmelt strength of resultant blend materials can beobtained. This superior property is assumed to becaused by the synergy between the present melt reac-tion and the higher molecular weight portion of thePP powder. Since trace amounts of water definitelywill produce active silyltriethoxy groups during thereactive extrusion, such polymers should be formedby the condensation between the hydrolyzed graftedpolymer chains.

The flame retardancy of PP can be improved bya modification with an intumescent flame retardant[43]. Solid acid catalysis technologies were adoptedto introduce melamine salt pentaerythritol phosphateinto PP. Silicotungstic acid is used as a catalyst, whichcan maintain a satisfactory conversion even with a lowextrusion temperature and a short residence time. Thesolid acid effectively suppresses any foaming in theprocess of the reaction. Further, silicotungstic acid isnot removed after the processing and effects a syner-gism for flame retardancy.

AA can be grafted onto PP by reactive extrusionusing a pre-irradiated PP as such as a radical initiator[44]. A relatively high degree of grafting and a slightdegradation of the modified PP were obtained when20% of pre-irradiated PP was used. Compared withthe neat PP, the modified PP shows a high-notchedimpact strength and an improved adhesion to polarsubstrates.

MA-grafted PP can be reacted with aliphaticdiamines in a twin-screw extruder to get aminiatedPP types [45].


0

20

40

60

80

100

0 0.1 0.2 0.3 0.4 0.5

Mel

t flo

w ra

te/[g

/10

min

]

Benzophenone/[%]

170°C200°C230°C

Figure 15.2 Melt flow rate plotted against initiator con-centration at different temperatures [47].

A reactive extrusion process for the synthe-sis of controlled-rheology PP has been developed.This process uses benzophenone as photoinitiator andUV light [46,47]. After mixing of the photoinitiatorwith PP, the melt stream is irradiated for several sec-onds over two open barrel sections. In Figure 15.2 thechanges in the melt flow rate with temperature andinitiator concentration are shown.

Reactive extrusion of PP in the presence of a perox-ide has been simulated by means of a one-dimensionalmodel [48]. The results show that a good agreement isobtained between the molecular weight and polymerdispersing index from a scale-up procedure at constantthermal time.

The reactive extrusion of MA-grafted PP withethylenediamine as coupling agent results in long-chain branched PP. These polymers exhibit excellentmechanical and rheological properties [49].

Long-chain branched PP has been added to linearPP by reactive extrusion in the presence of diben-zoyl peroxide and acrylics [50,51]. It has been foundthat 1,4-butanediol diacrylate could not produce long-chain branched structures. However, 1,6-hexanedioldiacrylate favored the branching reaction. This isbelieved to be dependent on the boiling point ofthe monomers, which should be above the high-est extrusion temperature to remain active. Perox-ides with lower temperatures of decomposition andmore stable radicals promote the branching reac-tion [52]. Also, trimethylol propane triacrylate anddicumyl peroxide were used to modify PP by reactiveextrusion [53].

When the reactive extrusion process is performedin the presence of supercritical carbon dioxide, long-chain branched polyolefins with a lower melt flowrate, higher complex viscosity, and increased tensilestrength and modulus are produced [49]. Such a super-critical carbon dioxide-assisted reactive extrusion cantake place in the presence of cumene hydroperoxideand 1,6-hexanediol diacrylate [54].

For isotactic PP the grafting of MA initiated bydicumyl peroxide in the presence of supercriticalcarbon dioxide can be done at 160 ◦C, instead ofat 190 ◦C without supercritical carbon dioxide. Thisresults in an effective suppression of main chaindegradation reactions. Products with higher molec-ular weights and a narrower molecular weight distri-bution can be obtained. Also, the efficiency of graftingis increased [55].

The foaming behavior of both linear and long-chain branched polyolefins has been studied. Theresults show that cellular materials produced from thelong-chain branched polyolefins have a higher weightreduction, smaller cell size, and better mechanicalproperties than those produced from the linear poly-mers [56].

Nanocomposites based on PP and unmodifiedmontmorillonite have been prepared by a water-assisted extrusion process [58]. Aqueous suspensionscontaining cationic or anionic surfactants and an MA-grafted PP as a compatibilizer were injected duringthe extrusion to promote the dispersion of the clay.The cationic suspensions facilitate the dispersion ofthe clay platelets in PP.

PP/titanium dioxide nanocomposites can be pre-pared by creating the titanium dioxide in situ duringthe extrusion process. This occurs by a sol-gel methodusing titanium n-butoxide as a precursor. This methodwas originally developed for coating applications. Themain advantage is that the reaction is conducted in themelt without any solvent. There is no need to manip-ulate the nanoparticles as such [59].

Likewise, a montmorillonite clay which is surfacemodified by dodecylamine can be used for the manu-facture of nanocomposites [60].

A high-melt-strength PP can be prepared using atwin-screw reactive extruder by grafting with MA andthen by reacting with epoxy moieties to extend thebranched chains [61]. The long-chain branches act asa nucleating agent in the crystallization of the poly-mer, resulting in a high crystallization temperatureand high crystallinity.


0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5

Mel

t stre

ngth

/[K P

a s)

]

Divinylbenzene/[%]

Figure 15.3 Dependence of melt strength on the con-tent of divinylbenzene [57].

High-melt-strength PP types can also be preparedby reactive extrusion of PP with various amountsof divinylbenzene using dicumyl peroxide as cat-alyst and pentaerythrite tetra(β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) pentaerythritol ester as anantioxidant [57]. The dependence of the melt strengthon the content of divinylbenzene is shown in Figure15.3.

Using 2% dicumyl peroxide and low amounts ofcrosslinking agent, i.e., divinylbenzene, some prod-ucts of degradation resulting from PP were observed.The crystallinity of modified PP is somewhat reduced,but the number of spherulites increases and theirindividual size decreases with increasing addition ofdivinylbenzene.

In situ compatibilization of PP and PS can beachieved by the addition of di-tert-butyl peroxide asthe radical initiator and tetraethyl thiuram disulfideas an inhibitor for degradation by reactive extrusion.The peroxide-induced degradation of PP can be effec-tively depressed by the addition of tetraethyl thiuramdisulfide [62].

Durable and regenerable biocidal textiles andpolymers can be fabricated by incorporating bioci-dal structures such as N-halamines into polymers.N-Halamines are compounds in which one halogenatom is attached to nitrogen in the form of a cyclicor acyclic imide, amide, or amine. Several cyclicand acyclic halamine precursors have been graftedonto a PP backbone by a melt free-radical graftcopolymerization [64].

In particular, N-tert-butyl acrylamide [65], acryl-amide, and methacrylamide can serve as acyclic hala-

O

HF

FF

F

F

F

F

F

F

F

F

F

F

F

F

FH

H

O

Figure 15.4 Acrylic acid 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl ester.

mine precursors. 2,4-Diamino-6-diallylamino-1,3,5-triazine, 3-(4′-vinylbenzyl)-5,5-dimethylhydantoin,and 3-allyl-5,5-dimethylhydantoin halamine arecyclic precursors [66].

The initiators 2,5-dimethyl-2,5-(tert-butylperoxy)-hexyne and dicumyl peroxide were compared ingrafting efficiency of acyclic halamine precursorsto PP [67].

After grafting by reactive extrusion, the samplesare halogenated by immersion in diluted chlorinebleach. The halogenated products of the correspond-ing grafted samples exhibit potent antimicrobial prop-erties against Escherichia coli. The antimicrobialproperties are durable and regenerable [66].

A fluorinated PP type was prepared by reactiveextrusion [63]. In this process, PP was grafted with afluorinated acrylate, i.e., acrylic acid 2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9-hexadecafluorononyl ester. Thestructure is shown in Figure 15.4.

The surface tension of the fluorinated PP is less thanthat of neat PP. Further, improved impact strength andthermal stability were reported, but slightly decreasedtensile strength. The dependence on the surface ten-sion on the content of fluorinated monomer is shownin Figure 15.5.

18

20

22

24

26

28

30

0 5 10 15 20

Surfa

ce te

nsio

n /[m

N m

-1]

Fluorinated monomer /[%]

Figure 15.5 Surface tension and content of fluori-nated monomer [63].


15.2.2 Poly(styrene)Styrene was polymerized in a twin-screw extruder.The polymerization reaction mainly occurred in thezone between 400 and 1000 mm along the screw axisin the extruder, corresponding to the residence time ofthe reactants ranging from 1 to 4 min in the extruder.Based on dimensionless analysis, a model of the res-idence time was established. A kinetic model of thepolymerization was set up under the assumption thatthe screw extruder can be treated as an ideal plug flowreactor [22].

A styrene-butadiene multiblock copolymer wassynthesized by anionic polymerization in a twin-screw extruder. The polymerized materials exhibita nanometer-sized styrene and butadiene multiblockstructure. Further, they show an ultrahigh elongationat break, which differs considerably from conven-tional polymers made by traditional solution polymer-ization methods [25].

The reactive extrusion for the anionic copolymer-ization of styrene and butadiene is a new synthesismethod and its mechanism is different from that ofthe anionic copolymerization in solution [68].

When the anionic copolymerization of styrene andbutadiene is conducted in a conventional tank reac-tor, butadiene tends to polymerize first because thereactivity of butadiene is much higher than that ofstyrene, and the styrene monomer cannot polymerizeuntil most of the butadiene monomer has been con-sumed.

In contrast, in an extruder as reactor, the barreltemperature is much higher than the boiling point ofthe butadiene monomer, which is −4 ◦C. Therefore,most of the butadiene monomer is vaporized imme-diately after feeding. Then, the butadiene monomeris in the gas phase and occupies the unfilled partof the extruder, but only small amounts are in theliquid phase and thus can copolymerize with thestyrene monomer. So, the styrene starts essentially ahomopolymerization.

Only when the melt viscosity increases to a certainextent due to the styrene polymer will the gaseousbutadiene monomer gradually diffuse into the poly-mer melt and polymerize. According to this qualita-tive description, a kinetic model has been formulated[68]. The kinetic model of block copolymerizationconstructed in this way is simple, and it shows thetrend of the complex reaction, but it is not very pre-

cise. It has been suggested to include terms dealingwith diffusion into the kinetic model.

Styrene-isoprene copolymers were synthesized bya bulk copolymerization in a co-rotating closely inter-meshing twin-screw extruder. NMR studies indicateda multiblock structure of these copolymers [69]. Anincrease of the isoprene content results in lowerstrength and higher elongation at break of the copoly-mer.

Poly(styrene) can be modified by reactive extrusionwith trimethylol propane triacrylate (TMPTA) anddicumyl peroxide (DCP) [70]. The TMPTA increasedthe molecular weight of PS by a coupling reaction.The coupling was enhanced in the presence of DCPat a high ratio of TMPTA to DCP.

The imidization of poly(styrene-co-maleic anhy-dride) with aniline by reactive extrusion has beenachieved [71]. During this kind of reactive extrusion,the process temperature is much higher than the boil-ing point of aniline. Therefore, most of the anilineshould be vaporized immediately after being intro-duced into the extruder and it occupies the unfilledpart of the extruder. However, it is transferred into themelt phase where it is consumed through the reaction.Actually this process is a vapor-melt heterogeneousprocess.

A kinetic model for this process has been devel-oped. The residence time and the mass transport of theaniline from vapor to melt phase play significant rolesin this heterogeneous reactive extrusion process [71].

A styrene/butadiene block copolymer was synthe-sized by anionic bulk polymerization in a twin-screwextruder [72,73]. Butyl lithium was used as initiator.The copolymer contains a long PS block, several shortPS and poly(butadiene) blocks. To explain this struc-ture, a bubble theory was proposed.

15.2.3 Poly(tetramethylene ether)and Poly(caprolactam)

A polyetheramide, composed of poly(tetramethyleneether) (PTMEG) as soft segment andpoly(caprolactam) as the hard segment, is syn-thesized in a one-step, solvent-free process. Novolatile by-product is formed during the process.An isocyanate-terminated telechelic PTMEG waspremixed with caprolactam, and this mixture wasallowed to react in the twin-screw extruder to formthe polyetheramide triblock copolymer [74].


15.2.4 PolyamidesPolyamide (PA)-based polymers are used as engineer-ing plastics because of their excellent properties.

15.2.4.1 Polyamide 6To analyze the rather complicated relationshipsamong the variables during the reactive extrusion pro-cess of polyamide 6 (PA6), the process of contin-uous polymerization of caprolactam into PA6 in aclosely intermeshing co-rotating twin-screw extruderwas simulated by means of the finite volume method,and the influences of some key processing parameters,such as flow rate, temperature, and catalysts, on thereactive extrusion process were discussed [75]. Thesimulated results of an example were in good agree-ment with the experimental results.

PA6 nanocomposites from montmorillonite (MMT)can be prepared in a twin-screw reactive extruder [76].The crystallization and the thermal behavior of PA6are influenced by the addition of MMT. The materialscan be spun into fibers.

Nano-attapulgite with a high aspect ratio canimprove the comprehensive performance of PA6 andhas little effect on the processing parameters [77]. Thesynthesis of PA6, its modification and processing canbe combined into one step using reactive extrusion.Fe3O4 and attapulgite have been compounded to get asuper-paramagnetic attapulgite. A magnetic field wasadded in the die of the extruder and the changes of themagnetic particles in the polymer were investigated.

15.2.4.2 Polyamide 12PA12 was prepared in a reactive extrusion processby the anionic polymerization of lauryllactam [26].Sodium hydride was used as initiator and N,N′-ethylene-bis-stearamide was used as activator. Thereaction was complete to 99.5% in less than 2 minat 270 ◦C and could be performed in an internalmixer/extruder mixer and a twin-screw extruder withco-rotating intermeshing screws. Rubber-toughenedPA12 blends were obtained when poly(ethylene-co-butyl acrylate) was dissolved in lauryllactam. 2,2′-(1,3-Phenylene)bis(2-oxazoline) is a suitable chainextender [78]. It reacts with the terminal carboxylgroups of the PA. During the extrusion process, theresidence time distribution has been measured by theaddition of ultraviolet and ultrasonic detectable trac-ers [79,80].

In another study, maleated low-density PE wasadded to improve mechanical properties of PA12[81]. With increasing content of maleated low-densityPE the tensile strength and the flexural strengthdecrease, whereas the blend exhibits an improvedimpact strength and reaches super-toughness with30% maleated low-density PE.

15.2.5 Poly(butyl methacrylate)Telomers of butyl methacrylate were obtained by reac-tive extrusion with 1-octadecanethiol as chain transferagent. The transfer constant to 1-octadecanethiol wasmeasured. It was shown that the use of relatively highratio of chain transfer agent to monomer had no per-ceptible effect on the kinetics of telomerization [23].

Optical fibers can be manufactured by the reactiveextrusion of butyl methacrylate. The reactive systemhas been adapted to the reduced reaction time in theextruder combining concepts based on the free vol-ume theory and a kinetic model for the mass polymer-ization of butyl methacrylate [82]. A kinetic modelhas been proposed and the reaction evolution can besimulated at different temperatures and initiator con-centrations. The residence time distribution has beenmeasured by a UV fluorescence method.

15.2.6 Poly(carbonate)Poly(carbonate)s, such as bisphenol A poly(carbon-ate), are typically prepared either by interfacial or meltpolymerization methods.

The reaction of a bisphenol such as bisphenol Awith phosgene in the presence of water, a solventsuch as methylene chloride, an acid acceptor such assodium hydroxide, and a phase transfer catalyst suchas triethylamine is typical of the interfacial method-ology.

The interfacial method for making poly(carbonate)has several inherent disadvantages. The processrequires phosgene, which is highly poisonous. Fur-ther, the process requires large amounts of organicsolvent.

The reaction of bisphenol A with diphenyl carbon-ate at high temperature in the presence of sodiumhydroxide as a catalyst is typical for the melt polymer-ization method. The melt method, although obviatingthe need for phosgene or a solvent, such as methy-lene chloride, requires high temperatures and rela-tively long reaction times. As a result, by-productsmay be formed at high temperature, such as the


products arising by Fries rearrangement of carbonateunits along the growing polymer chains. Fries rear-rangement gives rise to undesired and uncontrolledpolymer branching, which may negatively impact thepolymer’s flow properties and performance. The meltmethod further requires the use of complex processingequipment capable of operation at high temperatureand low pressure, and capable of efficient agitation ofthe highly viscous polymer melt during the relativelylong reaction times required to achieve high molecularweights.

On the other hand, poly(carbonate) can be formedunder relatively mild conditions by reacting a bisphe-nol A with a diaryl carbonate formed by the reactionof phosgene with methyl salicylate. Early proceduresused relatively high levels of transesterification cata-lysts such as lithium stearate in order to achieve thedesired high-molecular-weight poly(carbonate).

The effects of the transesterification reactions on thethermal properties of a PC/poly(ethylene terephtha-late) (PET) copolymer formed by reactive extrusionhave been investigated [83]. As transesterification cat-alyst tetrabutyltitanate was used and as inhibitor tri-phenyl phosphite was used.

Differential scanning calorimetry (DSC) measure-ments indicate the occurrence of transesterificationreactions [83].

The weight fractions (1 = wPC + wPET) of PC andPET can be calculated from the variation of the glasstransition temperature Tg and the glass transitiontemperatures of the pure compounds according toWood [84]:

Tg = wPCTg,PC + wPETTg,PET. (15.9)

Wood’s equation shows a strong dependence on theconcentration of the catalyst and on the initial ratioof the homopolymers in the system.

15.2.6.1 Linear Poly(carbonate)Poly(carbonate) is prepared by introducing an ester-substituted diaryl carbonate, such as bis(methyl sali-cyl)carbonate, a bisphenol A, and a transesterificationcatalyst, e.g., tetrabutylphosphonium acetate (TBPA),into an extruder [85]. Within the extruder, a moltenmixture is formed in which the reaction between car-bonate groups and hydroxyl groups occurs, giving riseto a poly(carbonate) product and an ester-substitutedphenol by-product.

O C R

O

HO RC

O

Figure 15.6 Fries rearrangement.

The extruder may be equipped with vacuum ventswhich serve to remove the ester-substituted phenolby-product and thus drive the polymerization reac-tion toward completion. The molecular weight of thepoly(carbonate) may be controlled by controlling,among other factors, the feed rate of the reactants,the type of extruder, the extruder screw design andconfiguration, the residence time in the extruder, thereaction temperature, the number of vents presentin the extruder, and the (vacuum) pressure. Thepoly(carbonate) reaches a weight-average molecularweight of greater than 20,000 Da.

In a special experimental design, the extruderincluded 14 segmented barrels, each barrel having aratio of length to diameter of about four, and six ventports for the removal of the by-product methyl salicy-late. Two vents were configured for the operation atatmospheric pressure and four vents were configuredfor operation under vacuum. The methyl salicylateformed as the polymerization reaction took place wascollected by means of two condensers.

The poly(carbonate)s have extremely low levels ofFries rearrangement products and possess a high levelof end capping. Contrary to this is a bisphenol Apoly(carbonate) prepared by a melt reaction method inwhich the Fries reaction occurs. The Fries rearrange-ment is shown in Figure 15.6.

15.2.6.2 Branched Poly(carbonate)sBranched poly(carbonate) resins differ from mostthermoplastic polymers used for molding in theirmelt rheology behavior. Most thermoplastic poly-mers exhibit non-Newtonian flow characteristics overessentially all melt processing conditions. However,in contrast to most thermoplastic polymers, certainbranched poly(carbonate)s prepared from dihydricphenols exhibit Newtonian flow at normal processingtemperatures and shear rates below 300 s−1.

Copolyester-carbonate resins are prepared analo-gously to the preparation of poly(carbonate), but adifunctional carboxylic acid is added. Usually thecarboxylic acid is aromatic and used as halide, i.e.,isophthaloyl dichloride and terephthaloyl dichloride.


Aliphatic diacid components yield soft segment co-poly(carbonate)s.

Poly(carbonate) and copolyester-carbonate resinscan be branched by reaction with tetraphenoliccompounds during synthesis. On the other hand, apoly(carbonate) resin possessing a certain degree ofbranching and molecular weight can be produced viareactive extrusion. This is achieved by melt extrud-ing a linear poly(carbonate) resin with a specificbranching agent and an appropriate catalyst system[86]. The resulting molecular weight increases withbranching, but can also decrease if conditions are cho-sen that favor degradation.

Branching agents useful to branch linear poly(carbonate)s are polyacrylates and polymethacry-lates, in particular pentaerythritol triacrylate.Organic peroxides include 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and 2,5-dimethyl-2,5-di(tert-butylperoxy)-3-hexyne.

Upon melt extrusion, branching and crosslinkingoccurs in the poly(carbonate) resin melt. The materialis compounded on a melt extruder, a co-rotating twin-screw extruder under reduced pressure of 0.5 atm, ata temperature profile of 200–300 ◦C.

The assumed mechanism of branching consists ofthermal decomposition of a radical initiator whichattacks the methyl groups of the BPA units in orderto create poly(carbonate) macroradicals. The macro-radicals can be recombined by a radical branch-ing agent (compound containing at least two dou-ble bonds) to generate a branched structure. The keyto the process will be the lifetime of the radicalsand the sensitivity of the poly(carbonate) backboneversus radicals. A copolyester-poly(carbonate) con-taining long-chain aliphatic diacid moieties, such asdodecyl diacid, is more sensitive to radical attack.Branched poly(carbonate) resins produced by reactiveextrusion are useful blow-moldable resins exhibit-ing enhanced melt strength and melt elasticity. Thebranched poly(carbonate) products are useful in appli-cations such as [86]:

• profile extrusion: of wire and cable insulation,extruded bars, pipes, fiber optic buffer tubes, andsheets;

• blowmolding: of containers and cans, gas tanks,automotive exterior applications such as bumpers,aerodams, spoilers, and ground effects packages;and

• thermoforming: of automotive exterior applica-tions and food packaging.

15.2.7 PolyestersPET is one of the most widely used engineeringplastics with high performance. However, the poorimpact strength limits its applications because of itsnotch sensitivity. A toughened PET composition wasprepared by blending recycled PET with PC andMDI [87].

Rheological measurements indicated an increase ofthe molecular weight. IR studies proved the existenceof copolymers. Also, the compatibility of PET phaseand PC phase was improved. The reaction induced byMDI affects the crystallization behavior of PET, asproved by DSC. The crystallinity of PET decreaseswith an increase of the MDI content. The notched-impact strength can be greatly improved from 17.3 to70.5 kJ m−2 [87].

Hybrid materials from poly(butylene adipate-co-terephthalate) and talc have been preparedthrough reactive extrusion [88]. Before extrusion,the polyester was free-radically grafted with MA toimprove the interfacial adhesion between the compo-nents.

Then, the grafted polyester was reactively meltblended with talc in the presence of tin octoate anddimethylaminopyridine as catalysts. There the silanolmoieties from the talc react by an esterification withthe pending MA units. The tensile properties of thesecomposites are improved due to an improved interfa-cial adhesion between both components.

Poly(butylene adipate-co-terephthalate) acts as atoughener for poly-(lactic acid) (PLA) [89]. PLAis high in strength and modulus but brittle, whilepoly(butylene adipate-co-terephthalate) is flexibleand tough.

Ternary nanocomposites composed of PET, org-anoclay, and an ethylene/methyl acrylate/glycidylmethacrylate terpolymer have been fabricated [90].The terpolymer acts as an impact modifier for PET.The composition was optimized with respect to itsmechanical properties. The sequence of componentaddition plays a role. Best results are obtained whenPET is first compounded with the terpolymer. After-wards this mixture is compounded with the organo-clay. X-ray diffraction studies revealed that exfoliatedstructures are formed.


15.2.8 ThermoplasticPoly(urethane)

The reactive extrusion of thermoplastic poly(ure-thane) in a co-rotating twin-screw extruder wasinvestigated. The monomers were the MDI isomers,methylpropane diol, and a polyester polyol. A math-ematical model was designed for comparison withthe experimental results. This model captured theextrusion behavior fairly well. The operation of theextruder is greatly affected by the depolymerizationreaction. The depolymerization reaction limits themaximal conversion, stabilizes the extruder operation,and causes an undesired post-extrusion curing of thepoly(urethane) [91].

Similarly, a kinetic model and an axial dispersionmodel, dealing with non-segregated and totally segre-gated fluid structures, have been discussed. The pro-cess model is based on linear chain growth theory.The numerical simulations showed that the effect ofthe segregation phenomenon appears to be significant,particularly in the evaluation of the polydispersity ofthe final product [92].

A thermoplastic poly(urethane) can be fabricatedby the twin-screw extrusion from feeds contain-ing a poly(butylene adipate) polyol and a mixtureof 1,4-butanediol and dicyclohexylmethane diiso-cyanate [93].

Poly(urethane)/montmorillonite nanocompositeshave been synthesized using a one-step directpolymerization-intercalation technique in a twin-screw extruder [94]. Poly(propylene oxide) glycol(POP), 4,4′-diphenylmethane diisocyanate (MDI),and 1,4-butanediol are used as organic precursors.SEM studies confirmed that the silicate layer is welldispersed in poly(urethane) matrix.

The layered silicate acts as a high aspect ratio rein-forcement and enhances the tensile strength of thecomposite. The addition of montmorillonite leads toa remarkable decrease of the heat release rate duringthermal stress, thus contributing to an improvementof flammability performance [94].

A reactive branched thermoplastic polyether-ester elastomer precursor was synthesized bythe esterification reaction of dimethyl terephtha-late with poly(tetramethylene glycol), 1,4-butadiene,and glycerol as soft segment, hard segment,and branching agent, respectively [95]. Further,a branched thermoplastic polyether-ester elas-tomer and 4,4′-diphenylmethane diisocyanate as

Table 15.5 Biodegradable Compositions

Compounds Reference

Poly(lactide)s [102]Poly(ε-caprolactone)Poly(ε-caprolactone)-grafted starch [103,104]Poly(propylene) wood flour composites [105]Poly(ε-caprolactone), wood flour or lignin [106]Starch and poly(acrylamide) [107]Protein and polyester [108]Poly(styrene)-grafted starch [109]

the diisocyanate compound were melt extruded toenhance the melt viscosity for the blow moldingprocess [96].

The chain extended thermoplastic polyether-esterelastomer showed an enhancement of molecularweight and a slightly crosslinked structure. Tensilestrength and tear strength increased significantly.Thermoplastic poly(urethane)s based on methyl-2,6-diisocyantocaproate, i.e., L-lysine diisocyanate andpoly(ε-caprolactone), have been synthesized by reac-tive extrusion [97].

L-Lysine diisocyanate is particularly desirable froma toxicity standpoint since it is a highly degradableproduct [98].

15.3 Biodegradable Compositions

Many biodegradable compositions have been synthe-sized and investigated. Composites from renewableresources including the technology of reactive extru-sion have been reviewed [99–101]. These compoundsare summarized in Table 15.5.

Poly(β-hydroxybutyrate-co-valerate), poly(buty-lene succinate), poly(ethylene succinate), and poly(ε-caprolactone) are biodegradable polymers which arethermally processable. Poly(β-hydroxybutyrate-co-β-hydroxyvalerate) (PHBV) can be made by both thefermentation process of carbohydrate and an organicacid by a microorganism, e.g., Alcaligenes eutrophus,and by the use of transgenic plants.

Polyalkylene succinate (PAS) is produced bythe reaction between aliphatic dicarboxylic acidsand ethylene glycol or butylene glycol. Poly(ε-caprolactone) (PCL) is produced by the ring openingpolymerization of ε-caprolactone.

By grafting polar monomers onto poly(β-hydroxy-butyrate-co-valerate), poly(butylene succinate), or


poly(ε-caprolactone), the resulting modified poly-mer is more compatible with polar polymers andother polar substrates. Useful polar monomers,oligomers, or polymers include ethylenically unsat-urated monomers containing a polar functionalgroup, such as 2-hydroxyethyl methacrylate andpoly(ethylene glycol) methacrylate.

The grafted biodegradable polymer may con-tain 1.5–20% of grafted polar monomers. Otherreactive ingredients which may be added to thecompositions include free-radical initiators, such asLupersolTM 101.

The amount of free-radical initiator ranges from 0.1to 1.5%. A low dosage of free-radical initiator cannotinitiate the grafting reaction. On the other hand, if theamount of free-radical initiator is too high, it will cre-ate undesirable crosslinking of the polymer composi-tion. Crosslinked polymers are undesirable, becausethey cannot be processed into films, fibers, or otherproducts.

The grafting reaction can be performed by a reactiveextrusion process [110]. A particularly useful reactiondevice is a co-rotating twin-screw extruder having oneor more ports. Such an extruder allows multiple feed-ing and venting ports and provides high intensity dis-tributive and dispersive mixing. The grafting may beachieved in several ways:

1. All of the ingredients, including a biodegradablepolymer, a free-radical initiator, and the polarmonomer, are added simultaneously to a meltmixing device or an extruder.

2. The biodegradable polymer may be fed to a feed-ing section of a twin-screw extruder and subse-quently melted, and a mixture of a free-radical ini-tiator and the polar monomer is injected into thebiodegradable polymer melt under pressure. Theresulting melt mixture is then allowed to react.

3. The biodegradable polymer is fed to the feedingsection of a twin-screw extruder, then the free-radical initiator and the polar monomer are fedseparately into the twin-screw extruder at differ-ent points along the length of the extruder. Theheated extrusion is performed under high shearand intensive dispersive and distributive mixing,resulting in a grafted biodegradable polymer ofhigh uniformity.

Dodecanedioic acid

H OOC (CH2)10 C OOH

Aspartic acid

H OOC CH2 CH

NH2

C OOH

Adipic acid

H OOC (CH2)4 C OOH

Azelaic acid

H OOC (CH2)7 C OOH

Sebacic acid

H OOC (CH2)8 C OOH

Figure 15.7 Diacids for reactive extrusion.

The modified polymer compositions have a greatercompatibility with water-soluble polymers, such aspoly(vinyl alcohol) and poly(ethylene oxide), than theunmodified biodegradable polymers.

The compatibility of modified polymer composi-tions with a polar material can be controlled by theselection of the monomer, the level of grafting, andthe blending process conditions. Tailoring the compat-ibility of blends with modified polymer compositionsleads to better processability and improved physicalproperties of the resulting blend.

The compositions are biodegradable so that the arti-cles made from them can be degraded in aeration tanksby aerobic degradation, and by anaerobic degradationin wastewater treatment plants.

PHBV allows only a low cooling rate, such thatcommercial use of this material is impractical. On theother hand, PLA is brittle. However, a blend of PLAand PHBV allows the PHBV to cool at an acceptablerate and also makes PLA more flexible such that thesematerials can be used.

Low-molecular-weight poly(aspartic acid)s areconventionally prepared by heating aspartic acid upto more than 220 ◦C for several hours [111]. In amore rapid, continuous, melt polymerization proce-dure, aspartic acid was copolymerized with adipicacid, azelaic acid, sebacic acid, and dodecanedioicacid in a vented twin-screw extruder. The chemicalstructures of these acids are shown in Figure 15.7.

Copolymers with molecular weights up to 9 kDacan be prepared at 240–260 ◦C with residence timesof only around 5 min. The molecular weight increasedwith the ratio of aspartic acid to diacid, but melt vis-cosities become very high and the processing becomes


difficult at ratios of aspartic acid to diacid greaterthan 16.

Most of the copolymers exhibit an inhibition of theprecipitation of calcium carbonate similar to that ofpure poly(aspartic acid) and thus may be used as anti-scalants [111].

15.3.1 Poly(lactide)sIt is generally known that lactide polymers are unsta-ble. The concept of instability has both advantagesand disadvantages. The advantage is the biodegrada-tion or other forms of degradation that occur whenlactide polymers or articles manufactured from lactidepolymers are discarded or composted after completingtheir useful life. A negative aspect of such instabilityis the degradation of lactide polymers during process-ing at elevated temperatures as, for example, duringmelt processing by end users.

Thus, the same properties that make lac-tide polymers desirable as replacements for non-degradable petrochemical polymers also create unde-sirable effects during production of lactide polymerresins and processing of these resins. In general,poly(lactide) (PLA) is a relatively brittle polymer withlow impact resistance. Articles made of PLA may bebrittle and prone to shatter under use conditions.

For example, if PLA is made into articles such asrazor holders, shampoo bottles, and plastic caps, thesearticles may be prone to undesirable shatter in use[102]. However, compositions with modified physicalproperties can avoid these drawbacks.

15.3.1.1 Ring Opening Polymerizationof Lactide

The ring opening polymerization of lactones, lactides,and glycolide has been reviewed. The continuous ringopening polymerization in twin-screw and extrudersis described, as well as the ring opening polymeriza-tion in supercritical carbon dioxide [112].

The ring opening polymerization of a lactide usingan equimolar complex of 2-ethylhexanoic acid tin(II)salt Sn(Oct)2 and triphenylphosphine P�3 as catalystexhibits a high reactivity to polymerization that is toohigh to allow a continuous single-step reactive extru-sion process for bulk polymerization. The catalyst alsodelays the occurrence of undesirable backbiting reac-tions. The ring opening polymerization is shown inFigure 15.8. A sophisticated screw design is requiredto ensure further enhancement of the polymerization

O

OOO

CH3

H3C

HO CH

CH3

C

O

O

n

Figure 15.8 Ring opening polymerization of a lactide.

reaction by using mixing elements and by the intro-duction of shear into the melt. It is possible to design asingle-stage process using reactive extrusion to poly-merize the lactide into a PLA that can be fabricatedby most known polymer processing techniques [24].Possible uses of such polymers include food packag-ing for meat and soft drinks, films for agro-industry,and non-wovens in hygienic products [113].

PLA is produced by the polymerization of renew-able products, i.e., lactic acid or lactide. The synthesisof PLA has been performed by reactive extrusion viaring opening polymerization of L,L-lactide using acontinuous single-stage process [114]. The resultingpolymer has been characterized by NMR. It has beendemonstrated that the thus produced polymer exhibitsproperties which are similar to those of a PLA syn-thesized by traditional methods.

The crosslinking of PLA with a wide variety of per-oxides has been examined in the course of reactiveextrusion [115]. The peroxides were classified intothree groups according to their decomposition rates,i.e., fast, moderate, and slow. Comparisons were madewithin each group. The mechanisms of decompositionand the main reactions were identified.

In the case of fast decomposing peroxides themolecular weight and the gel fraction were foundto be higher than for the other groups despite thelower hydrogen abstraction ability of the groupof fast decomposing peroxides. It has been con-cluded that the decomposition of the peroxide insolid PLA causes a partial crosslinking, because ofthe smaller ratio of peroxide lifetime to extrusiontime.

In the case of moderate and slow decomposing per-oxides a direct proportionality is observed betweenthe hydrogen abstraction ability of the peroxide andthe molecular weight of the crosslinked polymer. Thiscan be explained by the decomposition taking placemainly in the molten polymer and uniform crosslink-ing occurring [115].

The effect of the dicumyl peroxide content on thethermal and mechanical properties of PLA in the


course of reactive extrusion was investigated [116]. Itwas found that dicumyl peroxide caused crosslinkingof PLA, but also low-molecular-weight products fromdecomposition and degradation were formed. Theseproducts effect a plasticization of the PLA, leadingto a decrease of the glass transition temperature. Anincrease in the tensile strength and a decrease in theimpact strength are also observed.

PLA nanocomposites with multiwalled carbonnanotubes (MWCNTs) can be prepared by reactiveextrusion. The inclusion of MWCNTs shows a slightimprovement of the flame retardancy [114].

The kinetics of the ring opening polymerizationof L-lactide initiated by stannous octoate and triph-enylphosphine has been investigated in a batch appli-cation, a Haake Rheocord Mixer [117]. Based on thesedata, a kinetic model has been developed, relying on acoordination-insertion mechanism. Additional exper-iments using reactive extrusion were done for the samepolymerization process with a co-rotating twin-screwextruder. The model developed with the batch exper-iments can predict the experiential data from reactiveextrusion.

The blending of PLA and thermoplastic starchresults in brittle materials. However, if the blendsare properly compatibilized through reactive extru-sion and plasticized, the thermoplastic starch phasecan significantly increase the ductility of thematerial [118].

15.3.1.2 Functionalized Poly(lactide)sA functionalized PLA is a polymer which has beenmodified to contain groups capable of bonding to anelastomer or which have a preferential solubility inthe elastomer. Only a portion of the PLA needs to befunctionalized in order to gain the benefit of improvedimpact strength; however, uniform distribution of thefunctionalization throughout the PLA-based polymeris preferred. The functionalized PLA can be createdduring the lactide polymerization process, for exam-ple by copolymerizing a compound containing both anepoxide ring and an unsaturated bond. The function-alized PLA polymer, containing unsaturated bonds,can be blended and linked via free-radical reactionsto an elastomer which contains unsaturated bonds.The functionalized polymer can also be prepared sub-sequent to polymerization reaction, for example bygrafting a reactive group, such as MA, to the PLA-based polymer using peroxides [102]. Typically, the

resulting polymer compositions have an impact resis-tance of at least 0.7 ft-lb in−1 (120 kg s−2) and animpact resistance of at least about 1 ft-lb in−1 (180kg s−2).

A PLA can also be functionalized by radical graft-ing of maleic anhydride onto it [119,120]. A concen-tration of 2% MA in the presence of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane suffices to reach up to0.7% MA grafted onto the PLA. Increasing the initia-tor concentration results in an increase in the graftingof MA, but also in a decrease in the molecular weightof the polymer. Without initiator, extensive degrada-tion was observed.

The influence of the temperature during reac-tive extrusion of PLA on the molecular architec-ture and the crystallization behavior was investi-gated using o,o-(tert-butyl)-o-(2-ethylhexyl) peroxy-carbonate. This results in a slightly crosslinked PLA.An increased extrusion temperature induces differ-ent processes in the polymer: The lifetime of theperoxide decreases, but the the radical concentrationincreases due to an enhanced decomposition rate ofthe peroxide [121].

Low-molecular-weight plasticizers from citrateshave been investigated to improve the ductilityof PLA. Grafting between anhydride grafted PLAcopolymer with a hydroxyl functionalized citrate plas-ticizer, e.g., tributyl citrate, was done by reactiveextrusion [122].

Plasticizing is also effective by reactive blend-ing of an anhydride grafted PLA copolymer withpoly(ethylene glycol) (PEG). During the melt pro-cessing, a fraction of PEG is grafted into the anhydridefunctionalized PLA chains. In this way, the compati-bility between PLA and PEG is improved [123].

15.3.1.3 Poly(lactic acid)PLA is widely used in food packing materials andcontainers, and cases for electronics, thus replacingconventional plastics, with a worldwide market of150,000 t [124]. Accordingly, PLA resins have beenprimarily used in disposable products, e.g., in foodcontainers, wraps, and films, due to its biodegradablecharacteristics. PLA is a biodegradable polymer; nev-ertheless, applications of the neat material are limitedbecause of its brittleness and poor melt properties.

The neat material can be modified by functional-ization with epoxy materials in the course of reactiveextrusion [124–127]. By such a modification the melt


properties were improved by a chain extension reac-tion. The chain extension has an influence on the elon-gational melt properties as a strain hardening occurs[127]. Also, batch foaming of chain extended PLAwith supercritical carbon dioxide is possible [128].

Glycidol-modified PLA polymers have been pre-pared using reactive extrusion [129]. The influencesof the residence time and the concentration of glycidolon the conversion have been studied in PLA with dif-ferent molecular weights of 45–100 kDa. Structure-property relationships have been established by mea-suring the molecular and macroscopic properties.

Glycidol reacts with the end groups of PLA to ini-tiate a chain extension. PLA with a low molecularweight reacts faster than a medium-molecular-weightPLA, whereas a PLA type did not react significantly.Chain extended PLA shows a higher glass transitiontemperature and melting temperature than the unmod-ified samples. Chain extended PLAs were found toretain their viscoelastic properties for much longerthan the neat samples [129].

In PLA/ethylene glycidyl methacrylate copolymersboth the tensile and tear strength of blown-film spec-imens in the machine and transverse directions aresignificantly improved [130]. The melt shear viscos-ity values of such resins measured at varying shearrates are significantly higher than those of the neatPLA resin and increase consistently with the contentof ethylene glycidyl methacrylate.

The compatibility of thermoplastic dry starch/PLAblends can be improved with MA as compatibilizerand dicumyl peroxide as radical initiator in the courseof reactive extrusion [131]. Here, the plasticizationof starch in such blends can be improved and homo-geneous blends can be fabricated. Further, the blendbecomes more thermally stable as shown by thermo-gravimetric analysis. A novel peak of decompositionat 450 ◦C emerges in the compatibilized blend, whichwas higher than those for the individual components.

PLA injection-molded composites were modifiedwith small amounts of N ′-(o-phenylene)dimalemideand 2,2′-dithiobis(benzothiazole) using reactiveextrusion [132]. The modification effects an increasein crystallinity, heat deflection temperature, and inmechanical properties. IR studies suggest the forma-tion of hydrogen bonds and a thiol ester, respectively.The reactive extrusion of commercially available PLAin the presence of the above modifiers provides a low-cost and simple method for the enhancement of theproperties of PLA.

15.3.2 Starch and CelluloseDerivatives

The use of starch in tailored materials is limitedbecause of its high hydrophilicity. However, ahydrophobic modification of native, cationic, orthermoplastic starches by radical grafting is possible[133]. Suitable modifiers are bifunctional fatty acidoxazoline derivates. The oxazoline moiety offersthe opportunity for reactive blending with polymersthat contain carboxyl, amino, mercapto, or epoxymoieties.

15.3.2.1 Phosphorylated StarchTablet binder. Starch is one of the most commonlyused excipients in the manufacturing of tablets asfiller, a disintegrant, or a binder. Its availability andlow cost have allowed it to be integrated into awide variety of pharmaceutical formulations. How-ever, inferior characteristics of native starches such aspoor free-flowing properties, stability limitations, andnegligible cold-water swelling have limited its appli-cation in solid dosage forms as a sustained releaseagent [134]. There is growing interest in improvingfunctionality of starch for sustained release applica-tions because of its nontoxicity and biodegradability.

The effects of different shear and pH conditions onstarch phosphates prepared via reactive extrusion havebeen investigated [135]. Starches, including waxycorn, common corn, and potato, were used to preparestarch phosphates. Commercially available starcheswere used as summarized in Table 15.6.

The starch compounds were mixed with sodiumtripolyphosphate, sodium trimetaphosphate, andsodium sulfate by dry mixing. Reactive extrusion wasdone at a shear rate of 50 or 200 rpm and at pH of 9.0or 11.0. The reaction efficiency of phosphorylationwas improved when extruded at 200 rpm.

Unmodified starch extrudates exhibit more degra-dation at 200 rpm with a shorter residence timeover those at 50 rpm with a longer residence time.

Table 15.6 Starches for Phosphorylation [134]

Starch Type Amylose (%) Trade Name

Waxy corn 0 AMIOCACommon corn 27 MELOJELHigh amylose corn 50 Hylon VHigh amylose corn 70 Hylon VIIPotato starch 20


Table 15.7 Flavor Retention of Starches [136]

Starch Type Total Oil Retention (%)

Starch phosphate 55.7Starch acetate 61.31Starch succinate 94.75Commercially modified 89.1

starch (N-LOK)

The starch phosphates extruded at 200 rpm showedan increased proportion of high-molecular-weightcomponents.

The structural features of the hydrogel as modifiedby the phosphorylation reaction were found to alterthe kinetics of drug release from the swellable matri-ces. The unmodified extrudates formed weaker gels asevidenced by their rheological properties and showedfaster drug release [134].

The results of measurements of diffusion indicatethat the reaction efficiency of phosphorylation byreactive extrusion and subsequently drug release canbe affected by the shear rate and pH for different starchtypes [135].

Flavor retention. Flavors are volatile per defini-tion but can be retained in foods much more effec-tively by encapsulation. Hydrolyzed starches, modi-fied starches, and gum Arabic are important types forusage for flavor encapsulation [137].

Acetylated, n-octenylsuccinylated, and phospho-rylated waxy maize starches were prepared in asingle-screw extruder. The starches were hydrolyzedwith diluted hydrochloric acid before they wereesterified [136].

From the samples, microcapsules were producedby spray drying. The retention of orange peel oil dur-ing spray drying for the different modified starches isshown in Table 15.7.

15.3.2.2 Thermoplastic StarchThe principles and technologies in reactive extrusionand their application in starch modification to getstarch graft copolymers, glycosides, cationic starch,or oxidized starch have been reviewed [138]. Also,the market and the most promising chemistries avail-able for the reactive extrusion of starch-based polymerblends have been reviewed [139].

Dicumyl peroxide is suitable for the thermalplasticization of starch. The compatibilization and

modification with PE can be accomplished by a one-step reactive extrusion in a single-screw extruder atthe same time [140]. It was concluded that the thermalstability of the blends with MA is improved comparedwith blends without MA.

A biodegradable material for use in high mois-ture environments is made from thermoplastic potatostarch [141]. Using reactive extrusion, potato starchand sisal cellulose fibers were compounded togetherin the presence of sodium trimetaphosphate. Thefibers are included in order to increase the wet strengthof the composite.

A modified thermoplastic high amylose corn starchhas been synthesized by reactive extrusion with MAas an esterification agent, glycerol, and 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Luperox® 101)[143]. The modified starch can be pelletized andresults in pellets with more transparency than ther-moplastic starches that are not modified with MA.

Thermoplastic starch/silica/PVA compositefilms were fabricated by reactive extrusion [144].Tetraethoxysilane (TEOS) is used as a precursor forsilica. The efficiency of the reaction was measuredvia the silica content in the films. It is improvedwith increasing TEOS concentration. The mechan-ical properties of the starch composite films areenhanced by even small amounts of silica. The tensilestrength and Young’s modulus increase, while theelongation at break decreases with increasing silicacontent.

15.3.2.3 Starch NanomaterialsSince starch is a typical biodegradable natural poly-mer, it is of interest for nanocrystals and nanoparticles.Starch nanoparticles have been prepared by a reactiveextrusion method. Glyoxal was used as crosslinkingagent. Glyoxal is a dialdehyde and can react with thehydroxyl groups in starch molecules to form hemiac-etal bonds and full acetal bonds. Starch rods with adiameter of 2–3 mm were used.

The extruded starch rods were cut into small piecesand immersed in water. After a period of 30 min high-speed stirring, a well-dispersed starch particle sus-pension was obtained. The size and morphology ofthe starch particles were characterized with SEM, cf.Figure 15.9.

When the extrusion takes place without crosslink-ing agent, the starch particle sizes decrease with anincrease of the extrusion temperature. When the starch


Figure 15.9 (a) Extruded starch rods without glyoxalat 70 ◦C, (b) extruded starch rods with 2% glyoxalat 60 ◦C, (c) original starch granules, (d) extrudedstarch particles at 85 ◦C with 2% glyoxal after stirring.Reprinted from [142] with permission from Elsevier.

particles are extruded with crosslinking agent, at thesame extrusion temperature, the particle size becomesmuch smaller. During extrusion, the starch is sub-jected to a high temperature and a high shear force.Here, gelatinization, melting, and degradation occur.These reactions are responsible for the decrease in theparticle size [142].

Both thermal and mechanical energy may cause thescission of the covalent bonds and the hydrogen bondsbetween starch molecules [145].

15.3.2.4 Cellulose Acetate ButyrateA solvent-free graft copolymerization of maleic anhy-dride onto cellulose acetate butyrate by reactiveextrusion has been reported [146]. Maleic anhydridegrafted cellulose acetate butyrate is a compatibilizerfor short fiber-reinforced composites and can be usedin the fabrication of biocomposites. Grafting has beenachieved in a twin-screw extruder in the presenceof 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane as afree-radical initiator. This process does not need anysolvent for the grafting of MA. Moreover, no protec-tion of the hydroxyl groups is necessary.

15.3.2.5 Carboxymethyl CelluloseCarboxymethyl cellulose can be prepared using a con-tinuous, reduced solvent, reactive extrusion processwith a short reaction time [147].

Statistical analysis revealed a significant interactionbetween the effects of NaOH and H2O on the degree

of substitution. The degree of substitution decreasedwith increasing amounts of NaOH.

15.3.3 Biodegradable FibersOne of the most promising biodegradable poly-mers is PLA, in particular from the viewpoint ofenvironmental protection. PLA is of great interestdue to its mechanical property profile, its thermo-plastic processability, and its biodegradability. Fur-ther advantages of PLA compared to other biodegrad-able polymers are its renewable origin and low price.

PLA is synthesized by the polycondensation of lac-tic acid or by the ring opening polymerization ofthe lactide. In both cases, lactic acid is the startingmonomer. Lactic acid is commercially produced bymeans of bacterial fermentation. Fibers from PLAcan be obtained in a high-speed melt spinning andspin drawing process [148]. A copolymer of L-lactideand 8% meso-lactide is used that can be obtained byreactive extrusion polymerization.

The reactive extrusion with poly(3-hydroxybutyrate) and a peroxide is a comfortablepathway for the improvement of the crystalliza-tion behavior in a melt spinning process [149].As peroxide, dicumyl peroxide was chosen. Meltspinning experiments were carried out with poly(3-hydroxybutyrate) and 0%, 0.2%, 0.3%, and 0.5%dicumyl peroxide in the course of reactive extrusion.Because of the complex crystallization behavior,only a limited processing window is available for themelt spinning of each formulation.

15.3.4 Poly("-caprolactone)Bulk polymerization of ε-caprolactone in an extruderin the presence of starch to give a compatibilized blendof poly(ε-caprolactone), starch, and grafted starch-g-poly(ε-caprolactone) is described in the literature.A suitable catalyst is aluminum isopropoxide. Alu-minum isopropoxide can be generated in situ by usingtriethyl aluminum or diisobutyl aluminum hydride.

The lactone should contain less than 100 ppmwater and should have an acid value less than0.5 mg KOH/g. The presence of water and freeacid in the reactant mixture is especially signif-icant in the synthesis of high-molecular-weightpoly(ε-caprolactone) polymer by reactive extru-sion polymerization since it has a deleteri-ous effect on the kinetics and ultimately leadsto lower conversion of monomer to polymer.


The impurities interact with the polymerizationcatalyst or the propagating species and lower theoverall rate of polymerization. In cases where themonomer contains greater than 100 ppm water, thedesired water content may be achieved by drying itusing molecular sieves or calcium hydride (chemicalmethod).

The ring opening polymerization of ε-caprolactonein the presence of starch leads to a poly(ε-caprolactone)-grafted starch. The reactant mixture isextruded at a temperature of 80–240 ◦C with residencetimes up to 12 minutes [103].

15.3.4.1 Blends with StarchFilms produced from poly(ε-caprolactone) and itscopolymers, which have low melting points, are tacky,as extruded, and noisy to the touch and have a lowmelt strength over 130 ◦C. Due to the low crystalliza-tion rate of such polymers, the crystallization processproceeds for a long time after the production of thefinished articles, followed by an undesirable changeof properties with time.

However, the blending of pre-blended starch withother polymers, such as lactone polymers, improvestheir processability without impairing the mechani-cal properties and biodegradability properties [150].The improvement is particularly effective with poly-mers having low melting point temperatures from 40to 100 ◦C.

The pre-blends are obtainable by blending a starch-based component and a synthetic thermoplastic com-ponent, such as an ethylene-vinyl alcohol copolymer,in the presence of a plasticizer. Suitable plasticiz-ers are glycerol, sorbitol, and sorbitol monoethoxy-late. Urea as additive can destroy hydrogen bondsof the starch. The addition of urea is advantageousfor the production of blends for film-blowing. Bymeans of extrusion, thermoplastic blends are obtainedwherein the starch-based component and the syntheticthermoplastic component form an interpenetratingstructure.

In a first step, starch and an ethylene-vinyl alcoholcopolymer (1:1) with minor amounts of plasticizer,and other additives such as urea, are melt blended in atwin-screw extruder. This extrudate is pelletized. In asecond step the extrudate from the first step is blendedwith poly(ε-caprolactone).

The rheological behavior of reactively extrudedstarch-poly(ε-caprolactone) nanocomposite blendswas evaluated in a capillary rheometer [151]. Power-

law models for blends with different nanoclay volumefractions were developed using appropriate correc-tion factors. The nanocomposite blends showed shearthinning behavior with higher pseudoplasticity. Theviscosities of the nanocomposite blends were signifi-cantly lower than those of 10% poly(ε-caprolactone)and nonreactive starch-poly(ε-caprolactone) compos-ites synthesized from ordinary extrusion mixing.

15.3.4.2 Blends with Wood Flour andLignin

Poly(ε-caprolactone) was compounded in twin-screwextruder together with wood flour and lignin [106].Maleic anhydride grafted poly(ε-caprolactone) (PCL-g-MA) was used as a compatibilizer. The grafting ofmaleic anhydride onto PCL was achieved with 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. Low con-tents of grafted maleic anhydride and PCL-g-MAwere required to improve both mechanical proper-ties and interfacial adhesion. The addition of ligninretarded the biodegradation.

15.3.5 Cationically Modified StarchCationic wheat starch has been prepared by reactiveextrusion in a twin-screw extruder. The modifiersare 2,3-epoxypropyltrimethylammonium chlorideand 3-chloro-2-hydroxypropyltrimethylammoniumin aqueous sodium hydroxide (NaOH) [152]. A highreaction efficiency can be reached if a low degree ofsubstitution is adjusted.

15.3.6 Blends of Starch andPolyesters

Biodegradable starch-polyester polymer compositesare useful in applications such as packaging and tissueengineering. Because of the thermodynamic immis-cibility between these two polymers, the amount ofstarch is limited to typically less than 2% [154].

A reactive extrusion method has been developed inthat high amounts of starch up to 40% can be blendedwith a biodegradable polyester such as PCL. In thisway, tough nanocomposite blends are obtained withelongational properties close to those of 10% PCL.In an experiment hydrogen peroxide with iron andcopper catalysts (Fenton’s reagent) and a modifiedMMT organoclay were also added to the formulation.It is suspected that the starch is oxidized and thencrosslinked with PCL in the presence of an oxidizing


Table 15.8 Properties of Maleated Starch [153]

Starch Type MA (%) Yield (%) [ε] (dl g−1)

Corn 0 – 1.51Thermoplastic 0 85 1.19MA Modified 2.5 96 0.27MA Modified 5 92 0.24MA Modified 8 85 0.18

agent. This leads to a compatibilization of the twopolymers.

X-ray diffraction studies showed mainly an inter-calated flocculated structure of the organoclay. SEMmeasurements suggest an improved starch-PCL inter-facial adhesion in reactively extruded blends withcrosslinking than in starch-PCL composites withoutcrosslinking [154].

Also, starch-poly(tetramethylene adipate-co-terephthalate) blends prepared by reactiveextrusion showed the same trend of elongationalproperties [154].

Two different organically modified nanoclaysCloisite C20A and Cloisite C30B have been usedtogether with poly(butylene adipate-co-terephthalate)for the fabrication of nanocomposites [155]. The inter-facial region between the polyester matrix and theclays was modified by grafting with MA via a two-stage reactive extrusion process. Studies of the mor-phology of the nanocomposites indicated an intercala-tion and improved dispersion using Cloisite C30B. Atest for biodegradation confirmed a higher biodegrad-ability of the polyester in the presence of thermoplas-tic starch and Cloisite C30B [156].

Maleated thermoplastic starch can be usedin the reactive extrusion melt blending withpoly(butylene adipate-co-terephthalate) in blown filmapplications [157].

Maleated thermoplastic starch is prepared fromcorn starch with glycerol as plasticizer and MA foresterification [153]. The intrinsic viscosity [ε] and therecovery yield for the resulting maleated thermoplas-tic starch are summarized in Table 15.8.

In the case of high amounts of polyester addedto starch, graft copolymers can be obtained throughtransesterification reactions. At low polyester content,no significant reaction occurred, most likely due to aninversion in the phase morphology between both com-ponents.

The tensile strength of the graft copolymer with70% polyester is much higher than those of a meltblend of neat thermoplastic starch and the polyester

that is modified in situ with MA. This can be explainedby a finer morphology of the dispersed phase in thecontinuous matrix, and an increased interfacial areafor the grafting reaction [157].

The maleation of poly(butylene adipateterephthalate) has been performed by reactiveextrusion with MA and 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane as a free-radical initiator [158].The maleation of the polyester proved to be veryefficient in promoting strong interfacial adhesionbetween the polyester and high amylose corn starchin starch foams that can be prepared by melt blending.The foams exhibit improved hydrophobic propertiesand a high dimensional stability after sorption ofmoisture.

Films from starch and poly(butylene adipate-co-terephthalate) can be obtained by reactive extrusionusing MA and citric acid as compatibilizers [160].Blends containing citric acid show a better phase com-patibilization in the SEM images [159].

MA and citric acid promote esterification andtransesterification reactions, in which citric acid ismore efficient. With a greater proportion of com-patibilizer of 1.5% the films become more opaqueand have greater tensile strength. Also, the bar-rier properties with respect to water vapor areimproved [160].

Some of the properties of the formulations contain-ing polyester, glycerol, MA, and citric acid are sum-marized in Table 15.9.

15.3.7 Blends of Starch andPoly(acrylamide)

Starch-poly(acrylamide) copolymers have been pre-pared by reactive extrusion with ammonium persul-

Table 15.9 Properties of Starch Polyester Blends withPlasticizers [159]

GLY CA MA Tensile Young’s OpacityConcentration (%) Strength Modulus (%µ m−1)

(MPa) (MPa)

10 0 0 6.63 55.21 0.4488 2 0 9.82 143.58 0.5818 0 2 0.77 20.65 0.2489 0 1 4.44 49.88 0.3688 1 1 4.58 76.99 0.326

GLY GlycerolCA Citric acidMA Maleic anhydride


fate as initiator. The extrusion temperature had no sig-nificant impact on acrylamide conversion [107].

For the grafting of various substrates, ammo-nium persulfate, ceric ammonium nitrate, or ferrousammonium sulfate/hydrogen peroxide can be used asinitiator [161,162].

When the ratio of monomer to starch is high, moregrafting of higher molecular weight is obtained. Alow moisture content results in less grafting of highermolecular weight [163].

The effect of cationic starch modification wasexamined using unmodified and cationic dent starchwith 23% amylose and a waxy maize starch with 2%amylose [164]. Cationic starch graft copolymers ingeneral have a lower molecular weight in comparisonto unmodified starch, but the content of amylose hasno significant effect.

The cationic modification of the starch enhances theformation of grafting sites. In this way the propertiesof starch-modified poly(acrylamide) copolymers canbe tailored.

15.3.8 Blends of Chitosan andPoly(acrylic acid)

Chitosan is of great interest in the food and pharma-ceutical industries because of its biological compati-bility, biodegradability, and nontoxic properties [165].Such materials are used in drug delivery systems,food packaging, medical sutures, and wound healingfilms [166].

Chitosan and chitin are among the most com-mon natural polymers containing glucosamine andN-acetylglucosamine moieties in the backbone. Chi-tosan is obtained by the enzymatic N-deacetylation ofchitin, as shown in Figure 15.10.

The hot-melt reactive extrusion of blends of chi-tosan and poly(acrylic acid) can be carried out withoutany process additives such as an organic solvent or aplasticizer [166]. The maximum amount of chitosanin the blend was 40% because otherwise the melt vis-cosity increased too much.

During extrusion, the carboxylic groups of thepoly(acrylic acid) interact with the amine groups ofthe chitosan and a good melt flow was observed. IRdata indicate the formation of a complex between chi-tosan and poly(acrylic acid). SEM studies suggest thatthe chitosan is well dispersed in the blends up to 30%chitosan.

15.3.9 Blends of Protein andPolyester

Blends of soy protein and biodegradable polyester canbe prepared with glycerol as compatibilizer [108].Miscibility was only achieved when the soy pro-tein was processed with glycerol, applying highshear at elevated temperatures in an extruder. There,a partial denaturation of the soy protein occurred.Extruder screws with large kneading blocks werepreferred. Thermoplastic blends were obtained withhigh elongation and high tensile strength. Whenthe concentration of protein was increased, a lowerdegree of crystallinity and a lower melting point wereobtained. It is possible to use a soy protein concentrateinstead of a more expensive soy protein with higherpurity.

15.3.10 Modification of Proteinwith Monomers

The chemical modification of soy protein withmonomers, such as maleic anhydride, glycidylmethacrylate, and styrene, was accomplished by reac-tive extrusion [167].

The samples obtained were characterized with dif-ferential scanning calorimetry and with a dynamicmechanical analyzer. The denaturation of a protein

OOO

O

OOO

NH

NH

NH

O

O

O

CH3

CH3

CH3

OH

OH OH

OOO

O

OOO

N

N

NH

OCH3

OH

OH OH

H H

H H

Chitosan

Chitin

Figure 15.10 Chitosan by N-deacetylation of chitin.


Table 15.10 Denaturation Temperatures of ModifiedSoy Proteins [167]

Soy Protein Type DenaturationTemp. (◦C)

Soy protein, moisty 152Soy protein, dry 180Soy protein, MA modified 143Soy protein, glycidyl methacrylate modified 130Soy protein, styrene modified 167

is defined as any non-covalent change in the struc-ture of a protein. The denaturation temperature can befound as an endothermic peak in DSC. The denatura-tion temperatures of modified soy proteins are shownin Table 15.10.

A drop of the denaturation temperature is an indi-cation that a modifier acts as a plasticizer. As can beseen in Table 15.10, the denaturation temperature ofstyrene-modified soy protein is higher than those ofthe other artificially modified proteins. So styrene isnot plasticizing the protein [167].

15.4 Chain Extenders

15.4.1 Recycling of Poly(ethyleneterephthalate)

Recycled PET has a poor melt strength and viscosity.Therefore, the use of recycled PET in blow moldingapplications, where high melt strength is required, islimited [168].

Chain extenders are low-molecular-weight com-pounds that can be used to increase the molec-ular weight of polymers. Pyromellitic dianhydride(PMDA) is a suitable chain extender to increase themolecular weight of PET industrial scraps with lowintrinsic viscosity. Industrial scraps coming from PETprocessing plants are in many cases uncontaminated.However, their viscosity is lowered by the first extru-sion.

PMDA has a melting point (283 ◦C) close to thatof PET and it reacts within a few minutes under theprocessing conditions of PET. PMDA is a tetrafunc-tional compound; therefore, branching can occur. ThePET end groups consist of carboxyl and hydroxylgroups. The chain extension occurs by a polyaddi-tion between the hydroxyl groups and the pyromelliticdianhydride [169].

The crucial parameters of the process are the con-centration of the chain extender, the residence timeof the polymer in the extruder, and the working tem-peratures. Dry blends of PET chips and PMDA pow-der were prepared with different amounts of PMDA(0.25%, 0.50%, 0.75%, and 1.00% by weight). Thesewere vacuum dried for 12 h at 110 ◦C and extruded at280 ◦C. The average residence time is approximately150 s. An amount of PMDA from 0.50% to 0.75% issufficient to result in an increase of Mw, a broadeningof Mw/Mn , and branching phenomena. The recycledpolymer from PET scraps is then suitable for filmblowing and blow molding processes [169].

Shear and dynamic rheology studies in anotherstudy confirm an increase of the molecular weightwith an increase of the concentration of PMDA, andthe formation of branched structures at concentrationsabove 0.25% PMDA [168].

15.4.2 Modified Poly(ethyleneterephthalate)

Multifunctional epoxy-based modifiers, such as atetra-glycidyl-4,4′-diaminodiphenylmethane (TGD-DM) resin, can be used to increase the melt strength ofPET. The progress conversion with time can be mea-sured by the change of torque in an internal mixer.With a stoichiometric concentration of TGDDM, themolecular weight distribution of modified PET showsan eightfold increase of the z-average molecularweight (Mz) and the presence of branched moleculesof very large mass [170].

Further, a tetrafunctional epoxy-based additive canbe used to extrude PET in order to produce PETfoams. The molecular structure analysis and shear andelongation rheological characterization indicate thatbranched PET is obtained for small amounts, up to0.4% of a tetrafunctional epoxy additive. Gel perme-ation chromatography studies suggest that a randomlybranched structure is obtained, the structure beingindependent of the modifier concentration [171]. Anincrease in the degree of branching and the Mw and thebroadening of the molecular weight distribution causean increase in the Newtonian viscosity, the relaxationtime, flow activation energy, and the transient exten-sional viscosity. On the other hand, the shear thinningonset and the Hencky strain at the fiber break decreasemarkedly.


15.4.3 Poly(butylene terephthalate)The chain extension reaction in poly(butylene tere-phthalate) (PBT) can be achieved by a diglycidyl tetra-hydrophthalate with high reactivity [172]. The chainextender reacts with the hydroxyl and carboxyl endgroups of PBT very quickly and at a compara-tively high temperature. The chain extension reac-tion is complete within 2–3 min at temperatures above250 ◦C. The chain extended PBT is thermally morestable than the original polymer. In order to obtainPBT resins with a high molecular weight, the reac-tive extrusion process is simpler and cheaper than thepost-polycondensation method.

15.5 Related Applications

15.5.1 TransesterificationThe transesterification is a different concept frompolymerization. Transesterification of mixtures ofpolyesters and oligoesters enables the synthesis ofnew types of polymers. Block copolyesters havebeen synthesized from poly(neopentyl isophthalate)and poly(ethylene terephthalate) [173]. The ester-ification of poly(neopentyl isophthalate) is some-how resistant to transesterification. Therefore, blocksinstead of alternating polyesters will be obtained.Poly(neopentyl isophthalate) is expected to exhibithigh barrier properties. Therefore, such materials areof interest in the field of beverage containers.

Similarly, block copolyesters of PET and poly(ε-caprolactone) have been synthesized by reactiveextrusion. In the presence of stannous octoate, the ringopening polymerization of ε-caprolactone can be ini-tiated due to the hydroxyl end groups of molten PETto form poly(ε-caprolactone) blocks [174]. A blockcopolymer with a minimal degree of transesterifica-tion can be obtained under conditions of a fast dis-tributive mixing of the ε-caprolactone into the highviscous PET.

15.5.2 Hydrolysis and AlcoholysisThe continuous hydrolytic depolymerization of apoly(ethylene terephthalate) was carried out in a twin-screw extruder. The hydrolysis was achieved by theinjection of saturated steam at high pressure. Low-molecular-weight products were obtained even atlow residence times in the extruder. Therefore, high

depolymerization rates should occur under the condi-tions selected [175].

α,ω-Diols have been obtained by the alcoholysisof PET through reactive extrusion. The alcoholysisof PET with diols in the presence of dibutyltin oxidewas carried out in a twin-screw extruder with resi-dence times of ca. 1 min. Scissions of PET chainsare taking place and oligoester α,ω-diols are formedwith a number-average of around 1 kDa [176]. Thestudy revealed that oligoesters synthesized by reactiveextrusion are quite similar to oligoesters synthesizedby batch processes which last many hours.

15.5.3 Flame Retardant MasterBatch

A master batch of an intumescent flame retardant wasprepared by reactive extrusion of melamine phosphateand pentaerythritol with a poly(propylene) carrier ina twin-screw extruder [177].

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Reactive Polymers Fundamentals and Applications || Reactive Extrusion