reactive exturision

POLYMER ENGINEERING AND SCIENCE, AUGUST 2004, Vol. 44, No. 8 1579

INTRODUCTION

Post-consumer PET undergoes a reduction in intrin-sic viscosity, [�], when recycled in a normal extru-

sion system. The occurrence of thermal and hydrolyticdegradation reactions during recycled PET melt pro-cessing is responsible for the reduction in [�] or molarmass of the PET. The presence of water and polyvinylchloride (PVC) in the recycled PET flakes produces PETchain scission during normal extrusion. At processingtemperature (280°C), hydrolysis reactions occur be-tween water and PET, resulting in shorter chains withcarboxyl and hydroxyl end groups. The thermal cleavage

of the PET ester bond results in PET chains with car-boxyl and vinyl ester end groups. Recently, intensivedrying to remove moisture, and vacuum degassingprocessing, were introduced by Erema in their plastic-recycling systems to minimize the effect of these reac-tions, resulting in higher PET [�] in comparison withnormal extruded PET (1). Solid state processing hasalso been reported to achieve higher PET [�] (2, 3). How-ever, this process is considered to be slow and expen-sive. In this study, chain extension by reactive extru-sion was chosen to overcome the reduction of [�] forseveral reasons. It is less expensive than solid stateprocessing and easier to apply in an existing normal ex-trusion system, and because of the proven success ofthe chain extension process with virgin and recycledPET (3).

In order to raise the [�] of the PET to a higher desiredlevel, reactive blending of virgin PET with chain exten-ders was investigated by Inata and Matsumura (4�9).

Recycled Poly(ethylene terephthalate)Chain Extension by a Reactive Extrusion Process

FIRAS AWAJA and FUGEN DAVER*

RMIT UniversitySchool of Aerospace, Mechanical and Manufacturing Engineering

PO Box 71, Bundoora, Victoria 3083, Australia

EDWARD KOSIOR

Visy R&D13 Reo Crescent

Campbellfield, Victoria 3061, Australia

A commercial-scale reactive extrusion processing system for recycled poly(ethyleneterephthalate) (PET) flakes with an added chain extender, pyromellitic dianhydride(PMDA), was investigated. The PMDA concentration was varied with the intention ofreaching a higher recycled PET intrinsic viscosity ([�]). The effect of changing the ex-truder residence time on the system’s stability and the recycled PET [�] was also in-vestigated. Reactive extruded PET with a PMDA concentration up to 0.3 wt% wasfound to have a higher [�] and lower carboxyl content than recycled PET processedin a normal extrusion system. A shift in [�] of about 0.18 dl/g was obtained with a0.3 wt% PMDA concentration. A PMDA concentration above 0.3 wt% produced chem-ical, thermal and hydrodynamic instability in the system, causing crosslinking reac-tions and gel formation. The reactive extrusion system was stable at low residencetime (45 s) and moderate (0.15 wt%) PMDA concentration; however, using 0.2 wt%PMDA produced higher reactive extruded recycled PET [�] with lower carboxyl con-tent than other PMDA concentration levels examined. Residence times higher than45 s produced higher reactive extruded recycled PET [�]. Reactive extruded recycledPET was also tested for mechanical properties. Polym. Eng. Sci. 44:1579–1587, 2004.© 2004 Society of Plastics Engineers.

*To whom correspondence should be addressed.E-mail: [email protected]© 2004 Society of Plastics EngineersPublished online in Wiley InterScience (www.interscience.wiley.com).DOI: 10.1002/pen.20155

Their efforts concentrated on evaluating potential chainextenders. They showed that additive type di- or poly-functional chain extenders were the preferred chain ex-tenders because of their high reaction rate without gen-erating by-products.

PMDA has previously been reported as an efficientchain extender or branching agent (10, 11). It is ther-mally stable, produces no side products on reactionwith PET, and is tetra-functional, commercially avail-able, and economical. Khemani (10) showed that when0.2 to 0.3 wt% PMDA was used with virgin PET, a sig-nificant increase in melt strength was obtained. Be-cause of these results, PMDA was the selected chainextender in this study; its concentration was chosen tobe one of the process variables. The influence of varia-bles such as the chain extender concentration and thereaction time on the virgin and recycled PET chain ex-tension process has been investigated (4�17).

Chain extender concentration has been the mostimportant parameter examined in reactive blendingprocess research in recent years (14, 15, 18, 19). Thetheoretical amount of chain extender required was cal-culated according to the chain extension reaction stoi-chiometry. Chain extender in amounts higher thantheoretically needed leads to crosslinking reactions, re-sulting in gel formation.

Akkapeddi (20) investigated a reactive extrusion sys-tem of virgin PET with added chain extenders. He ob-tained lower carboxyl content using different types ofchain extenders.

Different models have been introduced to describethe reactive extrusion system. Janssen (21) introducedan interaction chart that presented the parameters af-fecting reactive extrusion. These parameters and con-ditions influencing the reactive extrusion process andits stability have been described by many researchers(21�24). Vergnes et al. (25) developed a model andsoftware to characterize the flow in a co-rotating twin-screw extruder. Thermal, hydrodynamic and chemicalreactions were found to be the main cause of instabil-ity in the reactive extrusion process. Controlling thecauses of these instabilities and then minimizing themis the key to successful reactive extrusion.

It is well known that in general, an increase in reac-tion conversion can be expected as a result of an in-crease in residence time (22). In this study, the resi-dence time was considered as the process variable, andits effect on the process stability and product propertieswas examined.

So far, there have been no reported studies describinga recycled PET reactive extrusion system implementedon a commercial-size extruder. This study investigatesthe reactive extrusion of recycled PET with a PMDAchain extender, in order to produce reactive extrudedrecycled PET material with high intrinsic viscosity.

EXPERIMENTAL PROCEDURE

Reactive extrusion experiments were performed usinga commercial PET co-rotating twin-screw extruder at

Visy Plastics, Melbourne facilities. Table 1 shows thenormal extrusion operating conditions. PMDA was ob-tained from Nippon Shokubai Co. Ltd. with a purity of99.7%, and recycled PET having an [�] of 0.75 dl/g wasobtained from Visy Plastics. Flakes of recycled PETwere subjected to intensive drying at 170°C beforebeing fed to the extruder. The flakes were dried byblowing desiccated air (dew point � �20°C) at 170°Cthrough them for 4 hours. The average moisture con-tent after drying was about 500 ppm. The reason forthe relatively high moisture level of the recycled flakesis the limited drying time. Less moisture content couldbe achieved using a drying time longer than 4 hours;however, the commercial PET recycling industry limitsthe drying time of the flakes for economic reasons. Therelatively high moisture content in the flakes is com-pensated by the efficient vacuum system attached tothe extruder. The PET (flakes) used in the experimentshad an average content of 400 ppm of PVC.

In order to determine the amount of chain extenderto be used in the reactive extrusion process, the theo-retical amount of chain extender required was calcu-lated using the chain extension stoichiometry equa-tion. Equation 1 shows the theoretical amount of chainextender to be added in the recycled PET (5).

(1)

where W is the weight percentage of chain extender, Nmis number of moles of recycled PET reacted, and G isthe molecular weight of chain extender, equal to 218g/mole in this study. CVh is the hydroxyl content of re-cycled PET, which is calculated from the Berkowitzequation, Mn � 2,000,000/(CV � CVh ) where CV is thecarboxyl content; in this study it was measured to beequal to 52 eq./ton, Mn for Post Consumer PET flakeswas calculated as 33,630 g/mole according to theMark-Houwink equation.

The possible combinations of PMDA-PET reactionsare shown in Table 2.

Reactive Extrusion Process

A commercial-size twin-screw co-rotating extruderwas used for the reactive extrusion process. The ex-truder was attached to a pelletizer and a crystallizer,

W � 11>Nm 2 * G *CVh *10�4

Firas Awaja, Fugen Daver, and Edward Kosior

1580 POLYMER ENGINEERING AND SCIENCE, AUGUST 2004, Vol. 44, No. 8

Table 1. Normal Extrusion Conditions.

Flow rate 500 kg/hFlakes moisture 500 ppmMelt temperature 290°CBackpressure 1.7 MPaTemperatures Adapter 2 � 279°C

Adapter 1 � 278°CZone 1 to 8 � 279°CBerringer � 279°C

Melt pressure 0.1 MPaFeeder screw rotation speed 90 rpmExtruder screw rotation speed 210 rpmPressure Vacuum 1 � 10.3 kPa

Vacuum 2 � 9.6 kPa

and was operated under a vacuum so that all light com-ponents resulting from the recycled PET degradationreactions were removed continuously.

Experiments were conducted at 0.05, 0.08, 0.1, 0.15,0.2, 0.25 and 0.3 wt% PMDA concentrations at a con-stant residence time of 112 seconds, corresponding tothe normal production speed of 500 kg/h. Another setof experiments were conducted at a constant PMDAconcentration of 0.15 wt% and at residence times of112, 90, 64, 56, 50 and 45 seconds corresponding to aextruder production speed of 200, 250, 350, 400, 450and 500 kg/h respectively.

During the experiments, the extruder screw rotationspeed was maintained at a constant speed for each testrun and PMDA was fed to the extruder as a powder viaa volumetric feeder.

Characterization of Reactive Extruded Samples

Reactive extruded samples were characterized andcompared with samples taken after a normal extrusionprocess. Several tests were conducted as describedbelow.

The change in molar mass of the reactive extrudedsamples, which could be indicated by the change in [�],is one of the important outcomes of reactive extrusion.[�] tests have been performed with a solution viscome-ter (PAAR AMV200). A mixture of 60/40 (w/w) of phe-nol-tetrachloroethane at 120°C was used to dissolvethe PET. Solutions of PET were tested at a PET concen-tration of 0.5 wt% at 25°C. All reactive extruded recy-cled PET samples were tested for [�]. Virgin and recy-cled PET were also tested for comparative purposes.

The degree of crosslinking (the percentage of insolu-ble gel) test was performed in order to obtain an under-standing of PMDA-PET reaction mechanism. All reactiveextrusion experimental samples were tested; recycledand virgin PET were tested also for comparison. Ap-proximately 250 mg of PET pellets were dissolved in amixture of phenol and tetrachloroethane (60/40 w/w)at 120°C for one hour. The solution was then filteredand the insoluble gel was separated and washed withacetone. The gel material was then dried under vacuumuntil a constant weight was reached (15).

Carboxyl end-group analysis is important to illus-trate the conversion rate of the chain extension reac-tion in the reactive extrusion process. Determination ofcarboxyl end groups in PET was done according to thePohl method (26).

The PET samples were finely ground in a mill to about20 mesh screen. A small amount—0.1 to 0.2 g—of

ground PET was transferred to a small analyzing flask,to which 5 ml of benzyl alcohol was added using a 5 mlpipette. The flask was then transferred to a hot oil bath(210 � 1°C) for 1.8 minutes and stirred using a mag-netic stirrer.

When the flask was in the bath for the required lengthof time, it was removed and dipped immediately into abeaker of cool water (20°C) to quench the sample for 6to 7 seconds. After quenching, the sample was imme-diately poured into a 50 ml beaker containing 10 ml ofchloroform. The chloroform acts as a dispersant andprevents thick gel formation. A rinse of 5 ml of benzylalcohol was then added to wash the walls of the flask.The flask containing the rinse was put into a 210°Cbath for 80 � 5 seconds, and then its contents werecarefully added back to the rest of the sample. Twodrops of an indicator solution (phenolic red) were addedto the mixture and the mixture was titrated with 0.1 Nsodium hydroxide dissolved in benzyl alcohol. A 10 �linjection needle capable of 2 �l drops was used for thetitration. The titration was carried to the first recog-nized overall pink color that remained for 10 seconds.The results were expressed by eq./106g. Calibrationwas made for PET degradation during solution.

The reactive extruded samples along with virgin andrecycled PET were injection molded to produce speci-mens for impact and tensile strength tests. The reactiveextruded, recycled, and virgin PET samples, in the formof pellets, were dried for 3 hours at 170°C and 0 mbarin a vacuum oven before injection molding. The injectionmolding machine was a Battenfeld BA 350/75 PLUS,and the operating conditions are shown in Table 3.

PET specimens produced after injection moldingwere conditioned for at least a week at 20°C and werethen subjected to an impact strength test performedaccording to ASTM D256. The machine used was aCeast (RESIL); the test parameters and specimen di-mensions are shown in Table 4.

The tensile properties test was conducted accordingto Australian Standard AS 1145. All samples were con-ditioned for at least a week before testing. The instru-ment used was a Zwick 2010. A test speed was 50 mm/min. A type 1 test specimen was used with an overalllength of 150 mm, width at end of 20 mm, length ofnarrow parallel portion of 60 mm, width of narrowparallel portion of 10 mm, radius of 60 mm and initialdistance between grips of 115 mm.

Recycled PET Chain Extension


Table 2. The Amount of Chain Extender Needed for 1 Moleof PMDA To React With 2, 3 or 4 Moles of PET.

ChainPMDA (Moles) PET (Moles) Extender wt%

1 2 0.311 3 0.201 4 0.15

Table 3. Injection Molding Operating Conditions.

Operating Temperatures

Mold temperature 40°CNozzle temperature 300°CBarrel temperature (zone 1) 295°CBarrel temperature (zone 2) 295°CBarrel temperature (zone 3) 295°C

Injection Section

Injection time 7.22 sShot size 53.5 mm

RESULTS AND DISCUSSION

Reactive Extruded Recycled–PET Processing

Reactive extrusion is a process with the potential forchemical, thermal and hydrodynamic instabilities tooccur. The system has many parameters, all of whichcan affect the reactive extrusion process. Performing astable operation was required in order to achieve a suc-cessful reactive extrusion. To accomplish this, chemi-cal, thermal and hydrodynamic reactions need to becontrolled. During tests, the thermal and hydrody-namic instabilities were controlled by keeping the bar-rel temperature between 282°C and 285°C and thescrew rotation speeds at 75, 92, 154, 174, 188 and 216rpm, corresponding to residence times of 112, 90, 64,56, 50 and 45 s respectively.

At a PMDA concentration between 0.25 and 0.3 wt%,an increase in the extruder die pressure was observed.A hazy thicker material coming from the edge of the ex-truder die was indicative of chemical instabilities. Thechain extension reaction conversion increased at stag-nant regions because of extended residence times. Itwas noticed that the process was stable when the screwrotation speed was increased by 10 rpm even thoughthe die pressure was still high. The higher die pressuredid not appear to further disturb any other parameters.

At 0.35 wt% PMDA concentration, chemical and hy-drodynamic instabilities were detected; the extruderstruggled to overcome the increase in melt viscosity asindicated by the increase in die pressure. Thick, cross-linked PET was obtained from the extruder. The viscos-ity was dramatically increased in the fully filled extrudersection that intensifies crosslinking reactions. As theviscosity increased, it led to an increase in the die re-sistance, which in turn increased the residence time ofthe reactive extruded PET, leading to process instabili-ties. During these instabilities, the extruder sufferedfew blockages in the flow. The screw rotation speed wasincreased by 40 rpm in an attempt to overcome theblockage. In a few minutes after feeding PMDA to theextrusion system, it was not possible to overcome theblockage and the material obtained remained gel-like.

The above experience shows that the extruder die re-sistance indicated by die pressure is another importantparameter in the reactive extrusion process, as was alsoshown in a previous study (23). According to Janssen’sinteraction diagram for reactive extrusion (21), the dieresistance is affected by flow rate and viscosity. It wasexpected that when the feeding of PMDA commenced, a

sudden increase in melt viscosity would take place as aresult of the fast reaction between PMDA and PET. Pre-liminary trials showed that stable die pressure in thefirst few minutes of the reactive extrusion process wasa very good indication of a stable operation, as the ma-jority of the die pressure fluctuations happen when thefeeding of the chain extender is initiated. Steady diepressure, in other words, a uniform distribution of themelt viscosity in the extruder, is preferred.

Figures 1 and 2 show the extruder die pressurereadings during the first few minutes of selected testswith respect to variable PMDA concentrations and res-idence times, respectively. The extruder die pressureinitially increases with the start of PMDA feeding. Thisincrease, which corresponds with the commencementof chain extension reaction, immediately introduceshigher-viscosity material, resulting in an instant dieresistance. The die pressure increased by a value of 21bar when the 0.3 wt% PMDA was added to the extru-sion system, as shown in Fig. 1. Figure 2 illustrates



Table 4. Impact Strength Test Operating Conditionsand Specimen Dimensions.

Specimen type 3 63.5 � 12.7 � 3.17 mmNotch type BDimensions A 10.16 � 0.05 mmRadius 1.00 � 0. 05 mmAngle 45° � 1Hammer 2.8 JImpact velocity 3.46 m/sDissipation energy 0.005 J

Fig. 1. The extruder die pressure response readings to thechain extension process at different PMDA concentrations.

Fig. 2. The extruder die pressure response readings to thechain extension process at a constant PMDA concentration of0.15 wt% for 112, 64 and 45 s residence times and at a PMDAconcentration of 0 and 0.15 wt% for 45 s residence time.

the effect of 0.15 wt% PMDA on die pressure at a resi-dence time of 45 s, in comparison to no PMDA addition.It also compares the effect of residence time on the diepressure at a constant PMDA concentration of 0.15wt%. At constant PMDA concentration of 0.15 wt%, thedie pressure increased by 7 bar when the residencetime decreased from 112 to 45 s, owing to increased ex-truder production speed.

Another important aspect of the die pressure is theprocess safety limitation. It is considered hazardous tohave die pressure readings above 40 bar. Many operat-ing problems occurred when the die pressure was atthis level and above. At that level of die resistance, it isbelieved that the reacted recycled PET became so vis-cous near the die that it was impossible for the incom-ing PET to force that material out of the extruder. As aresult, the material is forced out through the vacuumlines, causing a disturbance in the extruder as well asbeing deleterious to the vacuum system.

It is seen from Fig. 1 that operating with a PMDA con-centration higher than 0.3 wt% would be likely to pro-duce a die pressure that is above the safety limit.

With 0.15 wt% PMDA concentration, there were nonoticeable pressure fluctuations when operating at dif-ferent residence times. As shown in Fig. 2, the reactiveextruder was always kept within the safety limits for allthe residence times studied.

The Reaction MechanismBetween PMDA and Recycled PET

The reaction between PMDA and recycled PET can becategorized into three possible types: (a) blocking reac-tion, which means a PMDA molecule joins one PETmolecule, (b) coupling reaction, which comes after theblocking reaction when a PMDA molecule joins twomolecules of PET linearly, and (c) branching reactions,in which PMDA molecules join more than 2 PET mole-cules and/or more than one PMDA molecule joins morethan two PET molecules, which leads to crosslinkingand later to gel formation. Blocking and coupling reac-tion rates are important determinants of increasingmolar mass or [�] of the recycled PET inside the ex-truder. The results of the crosslinking test showed thatthe amount of gel formation was less than 4%. Branch-ing reactions at such low PMDA concentrations did notreach the point of extensive cross-linking.

The expected chain extension reaction mechanism,where the PET hydroxyl end group attacks the anhy-dride functional group of PMDA, will lead to couplingbetween the PET and the PMDA, thus forming two car-boxyl groups. The resulting carboxyl groups will laterreact with PET and introduce more coupling and/orbranching reactions and produce a molecule of water ineach stage of the reaction, as presented in Fig. 3. PMDAis a tetra-functional material; one molecule of PMDAcan join four molecules of PET. All blocking reactions(joining one molecule of PMDA with one molecule ofPET) produce one free carboxyl group, Fig. 3a. In thecase of two PET molecules reacting with PMDA, asshown in Figs. 3b and 8c, there is a possibility of pro-

ducing two carboxyl end groups with each coupling, asin Fig. 3b, or a molecule of water, as in Fig. 3c. Branch-ing reactions take place when three PET molecules areconnected to PMDA; one carboxyl group and one watermolecule are generated as well, as revealed in Fig. 3d.The typical reaction would be a complete connectionof the four PMDA functional groups with four PET mol-ecules, generating one water molecule, as shown inFig. 3e. Other cases include heavy branching andcrosslinking reactions, where two or more PMDA mole-cules linked together to form a network, shown inFig. 4. These carboxyl groups resulting from the in-complete extension of the four functional groups of thePMDA molecules will lead to an increase of the carboxylend groups in the mixture. In addition, the water mol-ecules generated will initiate further hydrolytic reac-tions, generating even more free carboxyl ends. ThePMDA concentration and the reaction time are themain factors that determine the blocking, coupling andbranching reactions, and thus the carboxyl content.

Impurities in the recycled PET, such as PVC andmoisture, will catalyze the degradation reaction, and in-crease its rate (3, 13), thus increasing carboxyl content.The carboxyl content of recycled PET has a significantnegative effect on the material’s physical properties andhydrolytic stability. The increase of carboxyl groups,which are generated during extrusion, is very signifi-cant. Starting from 8.5 eq./ton carboxyl content of



Fig. 3. PMDA reactions with PET.

feeding flakes, 52 eq./ton resulted in the produced ma-terial after a normal extrusion operation. The introduc-tion of PMDA chain extender reduces both hydroxylgroups (by reacting directly) and carboxyl content bylimiting the degradation reactions effect. Figure 5shows the hydroxyl content in response to varying thePMDA concentration. The hydroxyl content of reactiveextruded recycled PET is calculated from the Berkowitzequation, where Mn refers to an ‘apparent number aver-age molecular weight ’ since the Mark-Houwink equa-tion is applicable for linear polymers. Branched PETshows considerable deviation from a linear relationshipbetween [�] and Mn based on the Mark-Houwink equa-tion (7, 27, 28). The calculation of the hydroxyl groupcontent in PET was based on two assumptions. The firstassumption is that PET chains with hydroxyl groupends are not produced by the chain extension reaction;the other assumption is that PMDA reacts only withthe PET chain’s hydroxyl groups. Hydroxyl contentdecreases rapidly with increasing PMDA concentra-tion, and this is indicative of the reaction progress ofPMDA with PC-PET. Figure 6 shows the calculated hy-droxyl content vs. residence time. The hydroxyl contentdecreases with increasing residence time at constant

PMDA concentration of 0.15 wt%. A higher residencetime for the PMDA/PET reaction leads to a higher num-ber of hydroxyl groups coupled with PMDA.

The chain extension process depends on PMDA con-centration more than the residence time. The reductionin hydroxyl content at different PMDA concentrationsand constant residence time is double the reduction inhydroxyl content at different extruder residence timesand constant PMDA concentration.

The [�] versus the PMDA concentration relationshipcan be divided into three stages, as displayed in Fig. 7.The first stage lies between 0 and 0.1 wt% PMDA con-centration, where an increase of PMDA concentrationleads to a steep increase of [�]. At PMDA concentrationlower than the concentration needed theoretically forcomplete PET-PMDA reaction, 0.15 wt% in the case of4 moles of PET reacting with 1 mole of PMDA, PET shortchains with functional end groups are more available



Fig. 4. PMDA extensive branching reactions with PET.

Fig. 5. Carboxyl content vs. PMDA concentration at a residencetime of 45 s.

Fig. 6. Carboxyl content vs. residence time at PMDA concen-tration of 0.15 wt%.

Fig. 7. The effect of PMDA concentration on the [�] of the reac-tive extruded recycled PET at a residence time of 45 s.

for each individual PMDA molecule to react with. PMDAmolecules are more likely to react tri-functionally and/or tetra-functionally with PET short chains. Couplingand branching reactions are active; however, it is be-lieved that the branching reactions dominate over cou-pling reactions. Branching reactions will develop highermolecular weight, increasing the [�] of PET rapidly. Atthis stage, the chain extension reactions minimize theeffect of degradation reactions but do not overcome itcompletely. The second stage lies between 0.1 and 0.2wt% PMDA concentration. [�] is still increasing withincreasing PMDA concentration but at a steady rate. At this stage, where PMDA concentration is at a levelcloser to the theoretical amount needed for completePET-PMDA reaction, both branching and coupling areactive and in equilibrium with each other. Comparedwith the previous stage, a larger number of PMDA mol-ecules react with degraded PET chains, and branch-ing and coupling reactions increase PET viscosity at asteady rate.

The last stage lies between 0.2 and 0.3 wt% PMDAconcentration. PMDA-PET reactions start to form cross-linking, and the molecular weight starts to rise expo-nentially. Branching and coupling reactions coexist inthis stage; however, the branching of PET increaseswith increasing PMDA concentration. The chain exten-sion reactions overcome the degradation reaction, asindicated by the higher [�]’s obtained than that of theoriginal feedstock.

Reactive extruded PET with [�] above 0.73 dl/g wasobtained using a PMDA concentration at 0.2 wt% andresidence times above 45 s during reactive extrusion.In normal extrusion, using the same PET flakes ([�] of0.75 dl/g) produced a PET material with [�] of only 0.67dl/g.

Figure 8 shows that at different residence times with a constant PMDA concentration of 0.15 wt%, [�] in-creases linearly with increasing residence time. Thelonger residence time leads to more reaction conversionbetween PMDA and PET resulting in an increase inviscosity.

By implementing a reactive extrusion process con-sisting of PET flakes (having a [�] of 0.75 dl/g) andPMDA, a shift between 0.01 and 0.18 dl/g (dependingon the PMDA level) in reactive extruded recycled PET [�]was obtained.

As expected, the extruder die pressure increases withincreasing PMDA concentration when the residencetime is kept constant and the relationship is linear, aspresented in Fig. 5. Increasing the PMDA concentrationin the reactive extrusion process increases the meltviscosity. Flow becomes more difficult for the reactiveextruded recycled PET, and it applies more pressure to the extruder die. In response to this, the extruder dieresistance increases. By extrapolating the linear fit inFig. 9, it can be seen that the die pressure would exceedthe safety limit of 40 bar at about 0.35 wt% PMDA.

At a constant PMDA concentration of 0.15 wt%, therelationship between die pressure and residence time islinear, as shown in Fig. 10. The die pressure decreases

with increasing residence time because of lower extru-sion production speeds, and it is within the safety limiteven when operating at a very low residence times cor-responding to maximum extrusion production speed.

Figures 7 to 10 summarize the various influences ofPMDA concentration, residence time and die pressureon [�], and from them Fig. 11 is derived. The linear re-lation between die pressure and [�] shown in Fig. 11 isvery useful for operating the reactive extrusion process.It makes it easy to predict the [�] of the PET in the re-active extruder using the die pressure reading at a con-stant residence time of 45 s.

Mechanical Properties of ReactiveExtruded Recycled PET

It was important to investigate the effect of the PMDAchain extension process on the mechanical propertiesof recycled PET.



Fig. 8. The effect of residence time on the [�] of the reactiveextruded recycled PET at constant PMDA concentration of 0.15 wt%.

Fig. 9. The effect of PMDA chain extender concentration on theextruder die pressure at a constant residence time of 45 s.

After injection molding, the virgin PET specimenswere transparent, while the majority of recycled andthe reactive extruded PET specimens were light yellowin color.

The impact strength of reactive extruded recycledPET materials is 311 j/m. No significant differenceswere noticed between the different reactive extruded re-cycled PET samples. In comparison with the impactstrength of the virgin PET sample (virgin PET had im-pact strength of 41.7 j/m), the recycled and reactiveextruded recycled PET samples were more brittle thanthe virgin PET samples. Reactive extrusion does notsignificantly change the elasticity modulus of PET.Figure 12 shows that for the investigated PMDA con-centrations, the reactive extruded recycled PET has aslightly higher elasticity modulus than the recycled PETsample.

Figure 13 shows that higher residence times producea lower value of elasticity modulus of the investigatedsamples. Increasing residence time increases the chain

extension reaction conversion, producing higher mo-lecular weight, heavily branched and entangled chains.Heavily branched and entangled chains have reducedflexibility, leading to a reduction in elasticity (29). A res-idence time of 45 s with a PMDA concentration of 0.15wt% resulted in reactive extruded PET with an elastic-ity modulus higher than that of virgin PET. The elastic-ity modulus of virgin PET was 1756 MPa, measured onthe same instrument.

Varying PMDA concentrations and residence timesdid not significantly change the upper yield stress ofthe samples. Reactive extruded recycled PET sampleshave an upper stress value of 58 � 1 MPa. Comparedwith the mechanical properties of virgin PET, reactiveextruded recycled PET has better tensile strength andslightly lower impact strength, while recycled PET showslower tensile and impact strength than virgin PET.



Fig. 10. The effect of residence time on the extruder die pres-sure at constant PMDA concentration of 0.15 wt%.

Fig. 11. The die pressure vs. [�] for a residence time of 45 sderived from different PMDA concentration.

Fig. 12. Elasticity modulus vs. PMDA concentration at resi-dence time of 45 s. Virgin PET has an elasticity modulus of1756 MPa.

Fig. 13. Elasticity-modulus vs. residence time at a constantPMDA concentration of 0.15 wt%.

CONCLUSION

PMDA has proved to be an effective chain extender inan industrial-scale reactive extrusion system for recy-cled PET. Reactive extruded recycled PET with higher[�] and lower carboxyl content than in non-chain ex-tended recycled PET was obtained. With appropriateadjustment to the extruder temperatures and the screwrotation speed, the reactive extrusion system was suc-cessfully implemented using an existing normal extru-sion system.

During this study, it was found that the chain exten-sion reactive extrusion process depends strongly onchain extender concentration. The range of residencetimes studied showed a lesser effect on the product [�],the product hydroxyl content and the process stability.Because of the dominating factor of PMDA concentrationin the reactive extrusion process, it is critical to deter-mine the most appropriate concentration in order toproduce reactive extruded recycled PET with a particu-lar viscosity.

ACKNOWLEDGMENTS

The first author acknowledges the Advanced Engi-neering Centre for Manufacturing (AECM) and VisyPlastics for a scholarship. The authors thank Prof. Dr. Werner Mormann of University of Siegen, Germany,for valuable discussions and Ms. Juliette Milbank forediting the paper.

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