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Recycling of Plastic Materials PDF

194 Pages·1993·6.866 MB·English
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Recycling of Plastic Materials Francesco Paolo La Mantia Editor 3 CP ChemTec Publishing Copyright © 1993 by ChemTec Publishing ISBN 1-895198-03-8 All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book. Printed in Canada ChemTec Publishing 38 Earswick Drive Toronto-Scarborough Ontario M1E 1C6 Canada Canadian Cataloguing in Publication Data Main entry under title: Recycling of plastic materials Includes bibliographical references and index ISBN 1-895198-03-8 1. Plastics - Recycling. I. La Mantia, F. P. (Francesco Paolo) TP1122.R43 1993 668.4 C93-093134-3 W.DeWinter 1 Poly(ethylene terephthalate) Film Recycling W. De Winter Agfa-Gevaert N.V., Research & Development, Septestraat, B-2640 Mortsel, Belgium INTRODUCTION Theimpactofman-madepolymersontheenvironmentisaproblemofhighpri- ority in most industrialised countries. Mainly due to a build-up of disposed wasteinlandfills,andduetocampaignsinthepressaboutmistakesmadeinthe managementofwastetreatment,publicopinionisfocusingonthisproblem.The factthatthecorrespondingpercentagebyvolumeishigher,duetothelowpack- ingdensityofwastes,makestheproblemmorevisible. Although“plastics”con- stitute not even 10 wt% of the total amount of wastes, both residential and industrial,foundinlandfills(seeFigure1),publicattentiontothemisincreas- ing. A possible explanation1 of such a reaction suggests that there is a lack of compatibilityofplasticswiththeenvironment,despitethefactthatthemajority ofproductsusedinpresentdailylifearemadeofmaterialswhichhavealsobeen manufactured by a chemical process. The plastic waste in landfills consists of about two-thirds polyolefines, and onlyca.15% of styrene polymers, ca.10% of polyvinyl chloride,and less than 10% of all other polymers, including poly(ethylene terephthalate) (PET). ThelargestuseofPETisinthefibersector.PETfilmandPETbottlesrepre- sentsonlyabout10%eachofthetotalPETvolumeproducedannually.2 Itisalso generallyknownthatthetotalECO-balance,consideringenergyconsumption, atmosphericandwaterpollution,aswellassolidwastecontent,isbyafactor2 to5morefavorableforPETfilmthanforitsgreatestcompetitorsinthepackag- ing sector, namely glass and aluminium.3 Inaddition,PETisoneofthelargestrecycledpolymersbyvolume,4becauseit issuitableforpracticallyallrecyclingmethods.1 PETrecyclingbythefollowing technological processes is discussed below: 2 PETFilmRecycling • direct re-use • re-use after modification • monomer recovery • incineration • and re-use in a modified way. In addition, attention will be given to some other attempts for recycling which have not been thoroughly evaluated so far, like biodegradability and photodegradation. ThispaperislimitedtothediscussionofPET-filmrecycling.Aglobalreviewof PET-recyclinginthesectorsoffibres,films,andbottleswaspublishedearlier.2 Figure1.Compositionoflandfill-waste(domesticandindustrial). W.DeWinter 3 DIRECT RE-USE Over 50 % of the PET film produced in the world is used as a photographic filmbase. The manufacturers of these materials, mainly Agfa-Gevaert, East- man Kodak, du Pont de Nemours, Fuji, Minnesota Mining & Manufacturing, andKonishirokuhavelongbeeninterestedinPETfilmrecovery.Animportant motivationfortheeffortsmadebythesecompaniesisthefactthatphotographic films are usually coated with one or more layers containing some amount of rather expensive silver derivatives, which have been recovered since the early 20thcentury,whencellulosicswereusedasafilmbase.Silverrecoverymakes PET-base recovery more economical.5,6 In a typical way of operation, PET film recyclingiscoupledwiththesimultaneousrecoveryofsilver,asrepresentedin Figure 2. Figure2.CombinedrecoveryofsilverandPET. 4 PETFilmRecycling Inthefirststepoftheprocess,photographicemulsionlayerscontainingsilver are washed with, for example, NaOH, and after separation, silver is recovered ononeside,andcleanedPET-wasteontheotherside.2Importantinthisprocess is that the washed PET-film scrap is clean enough to be recovered by direct re-extrusion, although careful analysis remains necessary. Direct recycling of PET-waste in the molten state, before re-extrusion to PET-film, is of course the most economical process thinkable, as recovered PET-scrap can be substituted for virgin PET-granulate without requiring any additionalsteps.Itiswell-knownthatPETinthemoltenstategivesrisesimul- taneouslytopolymerbuild-upandtopolymerdegradation,sothatreactioncon- ditionsforthisprocesshavetobecontrolledverycarefullyinordertoobtainan end-product with desired physical, chemical and mechanical properties, like color, molecular weight, and molecular weight distribution. Alargenumberofreactionparametershavetobekeptunderpermanentcon- trol (temperature, environmental atmosphere, holding time in a melt state, amount of impurities, type of used catalysts and stabilizers, etc.). The order of addition of the PET flakes is very important. A typical flowsheet of a batch-PET-process7isrepresentedinFigure3.Insuchaprocess,thePET-flakes can be added after polymerization, before the melt enters the film extruder screw(Figure3,indication1).Suchaprocedure,however,hastwomaindraw- backs: • a highly viscous melt is difficult to filter (to eliminate possible gels or microgels) • resulting low-boiling or volatile side-products cannot be discarded any- more. In order to eliminate these disadvantages, several alternative operation modeshavebeenworkedoutinthepast.AmethodtoaddrecycledPETduring W.DeWinter 5 Figure3.Batchprocessflowsheet. theesterificationstep(Figure3,indication2)hasbeendescribedbyduPont.8In suchawayfiltrationcantakeplaceinthelow-viscosityphase,andvolatilescan still be eliminated during the prepolymerisation phase. AlthoughPET-recyclingbydirectre-useisbyfarthemosteconomicalprocess, it is only useful in practice for well characterized PET-wastes, having exactly knownchemicalcomposition(catalysts,stabilizers,impurities).Therefore,this process is the most suited for the recovery of in-production wastes, but it may notbeidealforcustomer-recollectedPET-film.Anindustrialprocessfor X-ray film-recyclingwasworkedoutbytheIPR-company9andintroducedtothemar- ket under the name REPET on the basis of a triple motivation: • availability of the waste chips on a repetitive basis • suitable purity • very competitive price. 6 PETFilmRecycling RE-USE AFTER MODIFICATION Similartothemethoddescribedunderdirectre-use,inwhichPET-flakesare added during the esterification process, PET-polymer is broken down into low-molecular,low-viscousfractions.Suchmethodcouldalreadybeviewedasa methodofre-useaftermodification.Becausetheintermediateproductsarenot separatedatanymomentoftheprocess,thedegreeofpurityofPET-scrapmust be high. ForPET-wasteshavingahigherdegreeofcontamination,othertechnological processes are applied, including further degradation by either glycolysis, methanolysis,orhydrolysis,10yieldingproductswhichcanbeisolated.Theprin- ciplesofchemicalprocessesonwhichthesemethodsarebasedareschematically represented in Figure 4. Figure4.PETdegradationbyglycolysis,methanolysis,andhydrolysis. W.DeWinter 7 Glycolysis can be considered as a method for direct re-use, whereas methanolysisandhydrolysisaremainlytakenintoconsiderationformonomer recovery, as discussed below. TheduPontCompanypublished11manydetailsconcerningtheglycolyticrecy- cling of PET. Less costly ingredients than those required for hydrolysis or methanolysis, and more versatility than direct remelt recycling are quoted as thereasonsforglycolysischoice.GoodyearhasalsodevelopedthePETrecycling process based on glycolysis which is called REPETE.12 Glycolytic recycling of PET, which can be done in a continuous or in a batch process,ispreferentiallyperformedbyadditionofaPETwastetoaboilingeth- ylene glycol, which leads to the formation of low-molecular weight intermedi- ates and eventually to crystallizable diglycol terephthalate (DGT). The rate of the degradation reactions is primarily controlled by varying the holding time andtemperature,whichdependsonachoiceofsuitablecatalysts(e.g.,titanium derivatives),12,13 and by adjusting the PET/glycol ratio. It is also necessary to avoid side reactions which might occur, e.g., by adding “buffers” or by keeping down reaction time and temperature. The low-molecular weight depolymerizates can be introduced directly into a polymerizationsystem,14preferentiallyafterfiltration.Inthismethod,particu- larcarehastobetakeninordertoavoidglycoletherformation,whichmaylead to PET of inferior properties. The glycolytic degradation can also be pushed to further completion, leading to DGT-recovery, rather than to direct re-use. InadditiontotheglycolyticrecoveryofPETforproductionofanewPET-film, granulate,ormonomer(EGandDGT),alternativemethodshavebeendescribed forthepreparationofso-calledPETGs(i.e.,glycol-modifiedPET),whichcanbe usedfordifferentpurposes.15,10Dependingonthetypeofglycol(orpolyol)used fordepolymerization,andonthenatureofdicarboxylicacidusedforsubsequent polycondensation,theobtainedpolyestermaybeusedasasaturatedpolyester resin(e.g.forfilms,fibresorengineeringplastics),unsaturatedpolyesterresin, mixedwithvinyl-typemonomers,oralkydresin,wherepolycondensationisper- formed in the presence of tri- or poly-functional organic acids. Althoughthismethodforproducingunsaturatedresin,e.g.,foruseinregular castings or in fiber-reinforced laminates, has been thoroughly studied by PET-filmmanufacturers,itisbelievedthatthemethodisnotcurrentlyusedin production.16

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