Recycling of Plastic Materials

194 Pages·1993·6.866 MB·English

by  la Mantia F.P.

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
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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