A TEN YEAR REVIEW OF PLASTICS RECYCLING

by: S. Garry Howell Risk Reduction Engineering Laboratory Cincinnati, Ohio 45268

ABSTRACT

A short history o f the practice cf plastics recycling as practiced i n the United States and Europe for the past ten years indicates that much progress has been made in educating the public sector about the environmental damage done by indiscriminating disposal of plastic items. Recent legislation has made the collection of some discarded plastic items more efficient, and has provided economic incentives t o recover and reuse waste plastics. The methods of collection, separation, cleaning, and fabrication of plastic wastes into useful and saleable items are discussed. A l s ~ discussed will be examples of products made from recycled plastic.

35

INTRODUCTION

Considering the enormous quant i t i e s o f p l a s t i c ma te r ia l produced t h i s country, as shown i n Figure 1, i t i s l i t t l e wonder t h a t d iscarded p l a s t i c s have become a s i g n i f i c a n t p o r t i o n o f municipal waste streams Figure 2 shows the compostion of municipal waste i n the Uni ted States;

n

the q u a n t i t i e s g iven are percent by weight , fo r w h i l e p l a s t i c s c o n s t i t u t e on l y 6.5% ( t h e General Accounting O f f i c e estimates 7.2%) by weight, t h e inherent low d e n s i t i e s o f t he r e s i n s and low b u l k d e n s i t i e s o f p l a s t i c products such as b o t t l e s are sa id by some t o compose 32% o f the volume o f l a n d f i l l e d waste. A l a r g e p a r t o f the waste i s discarded packaging o f one type o r another. The l o w p r i c e o f p l a s t i c s h r i n k wrap, b l i s t e r packs, b o t t l e s , ad i n f i n i t u m , has r e s u l t e d i n a throwaway men ta l i t y ; we d i sca rd i tems which appear t o have an almost i n f i n i t e l i f e if l a n d f i l l e d , and sometimes present a hazard t o marine l i f e . which w i t h the except ion o f some o f t he halocarbons, have a h igh f u e l va l ue; i n c i ne ra t ma te r ia l s s ince p l a s t i c ma te r ia l and recyc l i n g p l saved, a l a r g e p a r t o f

A common pe rcep t io i nven t ion . I n r e a l i t y , developed over 120 years 1909. The volumes o f t h such as c e l l u l o s smal 1, and s ince equipment o r i n s considered a nuisance when d i considered as beginning i n the 1 o f low d e n s i t y po lyethy lene (LDP o f the i n d u s t r y i s i l l u s t r a t e d i n F igure 1, whic (once formed o r molded they cann (which w i t h i n reason can be recy d i f f e r e n t ob jec ts ) . Product ion vo i s shown i n F igure 3, along w i t h the recyc led i n 1988.

I n c i n e r a t i o n i s an op t i on f o r most p l a s t i c s ,

1 ume product ion

This review w i l l p r i m a r i l y be p a r t i c u l a r l y those most o f t e n f a b r i very sho r t usable l i f e and a r e d i s sources o f wastes, means o f recover conver t ing the waste ma te r ia l , and p l a s t i c s w i l l be discussed.

ms which have a

TYPES OF THERMOPLASTICS

Thermopl as t i c s are general 1 y group, o f t e n c a l l e d commodity p l a ude t h e polyethylenes (PE), polypropylene (PP), p o l y ( v i n y t h e i r copolymers, and po ly ( e t h y l enetereph t e ) (PET). These products are low p r i c e d and produced i n huge q u a n t i t i e s ; p r i c e s range from $0.50 t o

) , polystyrene (PS) ,

36

U.S. LASTICS PRO UCTION BILLIONS OF POUNDS

0 20 40 60 80

Source: Modem Plastics

37

N

38 L .

Low Density PE

High Density PE

Poly(ethy1enetere- 2.007

recycled i 8.1 19

phthalate)

I f

!

r 4 6 8 10 12 t

OILLION P6UNDB

Source: Modem Plastics blt3UiWI 3

39

I I I I I i I I I I

I I I I I I I i i

i I

$0.70 per pound. The combination of low price and physical properties explains their widespread use in disposable packaging, in one-time use agricultural and architectural coverings, and in some engineering applications where their physical properties are acceptable.

higher strength, resistance to heat, and impact resistance, they have become widely used to replace metals in many automotive, appliance, aerospace, and industrial products. Prices of these range from $0.85 to more than $10.00 per pound. Their applications to more durable goods and lower production volumes result in less o f a pollution problem and will not be discussed at length in this review. excellent clarity, mechanical properties, and heat resistance. PET could be said to straddle the line between commodity and engineering plastics, as it is the plastic used in the ubiquitous clear plastic soft drink bottles, some hot filled food products, and in many mechanical applications as well. PET can also be made into fiber (Dacron, Kodel, etc.) and films (Mylar, Cronar) e

The second group is called engineering plastics. Characterized by

An exception i s PET, which has

RESIN MANUFACTURE

The great majority of modern synthetic plastics are derived from petroleum or natural gas. Polyethylenes and polypropylene belong to a class called polyolefins, since their starting materials (monomers) are olefins made by thermally cracking the larger hydrocarbon molecules in natural gas 1 iquids or petroleum fractions. Poly(vinylch1oride) (PVC)

t also be described as an olefinic material, since it differs from ethylene only by having every second hydrogen on the chain replaced by tom of chlorine. General i ze tructures of the polyolefins and PVC are

shown bel ow.

POLYETHYLENE

H H C-C H H h0.t. P r O l l l U r O ~ Oat.lY@t*

ethylene pol yet hyleno

* Low density polyethylene ( L D P E ) is produced by contacting ethylene monomer and an organic peroxide catalyst (or more properly, initiator) at pressures of 15,000 to 50,000 psi and temperatures of 300' to 575OF.

40

POLYPROPYLENE

H H H C x C - C H

H M hOQt, pr@88Mi+O~ %at88y$%+*

polypropyl.nlB propylon.

** Polypropylene i s polymerized with Ziegler-Natta type catalysts similar to those used for high density polyethylene but at lower temperatures and pressures.

POLY(VINYLCHLOR1DE)

P o l y ~ v i n y l c h l o r i d e ~ r i H H H

c1 - C = C > i-c-c-jn H C I H

vinyl chloride h e a t t pressure, catalyst*** polytvinylchloridol

*** Poly(vinylch1oride) polymerizations are carried out with peroxides, persul fates, or redox catalysts at relatively low temperatures and pressures. The type o f catalyst and method of polymerization vary according to the intended end use for the product.

(C = carbon, H = hydrogen, Cl = chlorine)

lower molecular weight polymers, or under severe conditions, to oily materials or to carbon and hydrogen. method of recycling but will not be discussed here. PVC can also be thermally cracked, with the evolution of hydrogen chloride gas (HCl), which i s poisonous and corrosive. In common with other organic polymers, they can also be depolymerized or otherwise affected by the ultraviolet portion of sunlight and oxygen in the air. even slower by special inhibitors (antioxidants and ultraviolet stabilizers) added before fabrication into the final product. Without these inhibitors many uses for plastics (such as telephone cable jacketing exposed to sunlight and heat) would be limited.

The polyethylenes can be thermally cracked or depolymerized back t o

This process has been proposed as a

This is a slow process, and is made

High density polyethylene i s catalyzed by heterogeneous catalysts which are organometals such as triethyl aluminum combined with titanium tetrachloride at 18Oo-250*F and 150-2OOpsi.

41

POLY ( ETHYL EN ETERE PHTHALATE)

PET is also a petroleum derived product; a copolymer of terephthalic acid, (which is made by oxidizing para xylene), and ethylene glycol, or

such as diethylene glycol or cyclohexane diethanol. A ction sequence showing the direct esterification reaction

e actually three different processes all arrjving at the same end product) is shown here:

QH + ‘*--

ti 2

CH i: H

2

1 I

n t o r e ~ b t h a l l o ro ld othylono g lyoo l poly(othylonetorepht halato

The fibers Dacron, Kodel, etc., made from PET are chemically identical Some attempts t o incorporate them into the to the resin made into bottles.

recycling chain will be covered in a later section.

POLYSTYRENE

General purpose polystyrene i s a c lea r , somewhat brittle resin. Most disposable polystyrene items are of resins made by the following reaction

n rt y r m a p d y a t y r a n o

**** A peroxide such as benzoyl peroxide.

The familiar foamed beverage cups, insulated containers, and home insulation are chemically identical to the clear drinking cups used on airlines. such as n-pentane during the polymerization process.

The foamed materials are made by incorporating a hydrocarbon

SOURCES OF PLASTICS WASTE

DOMEST IC

As indicated in Figure 2 , in 1984 the percentage of plastics in municipal trash was about 7.2% by weight. o r about 9 million tons - almost double that present in 1976. according to Ramani Narayan of Purdue University (1).

By volume, plastics. amount to about 32%, No assay of the

42

volumes o f i n d i v i d u a l polymers (PET, PE, PVC, e tc . ) appears t o be e n t i r e l y accurate, b u t i t seems f a i r t o assume t h a t o v e r a l l , the r a t i o s o f the p l a s t i c s made i n t o d isposable i tems such as b o t t l e s , packaging f i l m s and Containers, e t c . would approximate the r a t i o s o f res ins s o l d f o r these purposes. packaging. Th is excludes t h e 935 m i l l i o n pounds o f PET (polyethy lene te reph tha la te ) s o l d f o r s o f t d r i n k and s p e c i a l t y b o t t l e s (29.

I n 1988, over 16 b i l l i o n pounds o f p l a s t i c s were used i n

AGRICULTURAL

The main uses o f p l a s t i c s i n a g r i c u l t u r e are i n mulch f i l m s , feed bags, f e r t i l i z e r bags, and i n temporary t a r p a u l i n - l i k e uses such as covers f o r hay, s i lage , e tc . A growing use i s temporary cover ing f o r f i e l d s t h a t are be ing fumigated. This f i l m and t h e var ious bags would appear t o be pr ime candidates f o r r e c y l i n g , as they are reasonably c lean and e a s i l y accumulated. Mulch and cover ing f i l m a re u s u a l l y d iscarded a f t e r they have

un t o de te r io ra te , genera l l y as the r e s u l t o f be ing exposed t o sun l i gh t . rocessed r e s i n s produced w i t h a h igh p ropor t i on o f these f i l m s w i l l have r phys ica l p roper t i es .

INDUSTRIAL

I n d u s t r i a l and cons t ruc t i on use o f d isposable p l a s t i c s i s d i f f i c u l t t o quant i f y as some p l a s t i c f i l m i s used f o r temporary enclosures and then l e f t i n p lace as a vapor b a r r i e r , wh i l e some i s removed and discarded. Polystyrene foam i s l i k e w i s e w ide ly used as i n s u l a t i o n , w i t h an almost i n d e f i n i t e l i f e t i m e , b u t many i n d u s t r i a l products are shipped w i t h foamed "popcorn" o r molded packing, almost a31 o f which i s discarded. d e n s i t i e s o f these foams (1 -2 l b s / f t vs. s o i l a t 70) make them e s p e c i a l l y undesi rab le i n l a n d f i l l s , w h i l e a t the same t ime making hau l i ng cos ts t o recyc le rs p r o h i b i t i v e l y high. Much machinery, lumber, plywood, bagged goods such as cement and mortar are shipped w i t h po lyethy lene f i l m covers. The m a j o r i t y o f t h i s cou ld be e a s i l y recycled. Shr ink wrapped cases are rep lac ing corrugated paper car tons i n many grocery i tems. be pr ime candidates t o i n i t i a t e r e c y c l i n g programs, s ince they could serve as c o l l e c t i o n centers as we11 as be ing generators themselves.

'I FAST FOOD" CONTAINERS

The low

Grocer ies should

Foamed po lys ty rene i s w ide ly used i n food packaging (meat and produce t r a y s ) a t t h e r e t a i l grocery l e v e l , bu t the l a r g e s t p a r t o f t h i s 725 m i l l i o n l b / y r market i s i n " f a s t food" conta iners. generated from t h i s 725 m i l l i o n pounds is tremendous, f o r t h e dens i t y o f t h e foamed mate r ia l i s on l y 2 t o 3 pounds per cubic foo t , which would i n d i c a t e an uncompressed b u l k o f 13 t o 20 m i l l i o n cub ic vards occupvinq sDace i n our l a n d f i l l s . A consort ium o f chemical and o i l companies has announced t h e format ion o f the N a t i o n a l Polystyrene Recycl ing Company t o b u i l d f i v e reg iona l p l a n t s t o recyc le po lystyrene ( 3 ) . The f i v e p lan ts , s a i d t o cos t $14 m i l l i o n , p lan t o c o l l e c t the used ma te r ia l from restaurants , hosp i ta l s , schoolss and other b i g users i n 30 s ta tes . The consort ium i s o n l y p lann ing t o recyc le 25% of the scrap ava i lab le ; t h e r e i s

The volume o f waste

the rs t o en te r the market, though f inding la rge , easy f scrap may be d i f f i c u l t .

43

MI L ITARY

No reliable estimate-of the amounts of recyclable plastics used by Contact is now domestic military installations is presently available.

being made with the Department of Defense to quantify their usage. Because ncentration of these facilities, recycling should be easier than

BILE WRECKING The amount of plastics used in automobile manufacturing has been

growing at approximately a 9% rate over the past ten years. (the approximate date of production of autos now being scrapped) the amount of plastics used in automobiles was 787,000 tons ( 4 ) . It would thus appear

a large part o f this could be recovered. An investigation of this was in 1975 by the Bureau of Mines ( 5 ) , but only a crude sink/float

technique was used to separate the plastics fraction, and little interest was generated since good homogenous materi a1 s could not be produced. Interest is growing in identifying the various plastics components in automobiles with a molded or stamped-in label so that salvagers could more easily segregate the types as they strip the vehicle prior to crushing and grinding. An evaluation of an innovative process to separate the various kinds of resins from finely ground waste, such as from automobile grindings is planned as a joint project between EPA and a large plastic producer.

Even in 1979

A large source of recyclable plastic should be the electrical and s from demolished buildings. Until recently, these were er the copper or aluminum; now that strict emissions

controls are required on these incinerators, physical separation o f the plastic insulation would appear to be a viable option. One site where wire and cable was processed to recover the metal conductors is reputed to have over 100,000 tons of mixed@plastic "fluff" in a pile covering over 7 acres. The amount of plastic recoverable from wire and cable is not known, but 433 million lbs. of PVC, 386 million lbs. of LDPE, and 125 million lbs. of HOPE were sold for insulation in 1988.

COLLECTION/RECOVERY

Segregation and collection of waste plastics from agricultural

These operations operations, industrial, and construction sites should not be difficult if it were economically attractive to the generator. typically generate relatively large quantities of discarded plastics in small areas, so segregation would appear to be entirely feasible.

DOMESTIC WASTE

A successful plastics recycling operation must have a dependable source of waste which is not overly contaminated with wet garbage, which makes sorting and washing difficult. far from the collection points, since the cost of hauling the bulky

The recycling plant should not be too

44

materials such as empty bottles or polystyrene foam would greatly increase operating costs. An efficient collection system is therefore mandatory, and should operate over a large enough area (preferably urban) to supply an adequate amount of feedstock. plastics from urban waste has proven to be the most difficult and costly aspect of polymer recycling. In the United States, early attempts to hand sort ~ u ~ ~ c ~ ~ a l garbage in bulk by spreading it over a moving conveyer belt proved t o be unsuccessful, mainly due to the difficulty of finding employees willing to perform the sorting for low wages, The present trend i s for states and municipalities to enact legislation requiring homeowners, and in some instances businesses, to presort trash. Under this system, separate containers are furnished for wet garbage, newspapers, glass, metals, and plastics. Several states, Michigan among them, have enacted "bottle laws" which require purchasers to pay a deposit on a41 beverage containers, wnether or not they were meant to be returnable. This has resulted in greatly increased recycling of PET bottles; but other types of containers such as PE milk and detergent containers, bags, film, etc., have been re1 atively unaffected a1 though these items constitute about 80% of a l l plastics trash. The City of Cincinnati recently started a voluntary

ing system, and has found that 65% of households are participating; ad only predicted 35%. It is apparent that the public is aware of

the need to recycle. In Cincinnati, the only plastics that the recycler, BFI, will take are PET bottles, (easily identifiable, being clear, usually with an opaque base cemented t o them), and polyethylene milk bottles, which are mostly translucent white. In mixed wastes, these would be worth the extra effort to separate, as they can be easily hand sorted Prom a conveyer belt. In Europe, PVC bottles are widely used for wine and edible oils, and some attempts have been made to separate them for recycling. are usually clear, and differentiating them from PET might be difficult for an untrained

As mentioned previously, a special case which is well worth mentioning are the foamed polystyrene fast food trays and cups which have become a 1 itter problem,.as well as occupying a disproportionate amount of landfill volume. Resin producers have formed at least two joint ventures to recycle these discards into usable products ( 3 ) .

Thus the collection of reprocessible

PVC bottles

In contrast to American practice, the separation and collection of recyclables in Europe is much more efficient and has a long history. Kunststoffe German Plastics, a technical magazine which covers the production, testing, and fabrication of plastics, devoted a large part of its May 1978, issue to plastics recycling ( 7 ) . Since Kunststoffe is aimed primarily collect and separate plastics are not covered in detail. covered in this issue discussed other aspects which will be covered later in this review.

at the production and fabrication of plastics, the means used to The articles

SEPARATION TECHNOLOGIES

MUNICIPAL WASTE

Municipal waste from which the "wet garbage" has been separated i s the feed to several recycling operations in European countries. A flow chart

45

o f a Dutch process w h i c h had been operating for a year i n 1978, i s shown i n Figure 4 (8) . per hour, and i s designed t o recover metals, plast ics , and papers from a mixed waste stream us ing a number of different operations. Figure 4 i s a Sankey diagram of the TNO process, indicating the percent recovery of the various input materials. t o mechanical means t o separate the var ious fractions I s very d i f f i cu l t ; one can count fourteen major pieces of capital equipment, exclusive of the conveyers, e tc . required t o carry material from one operation t o the next. The final product i s merely called "bales of plast ic" , apparently a mixture of a l l the overhead from the zigzag a i r c lass i f ie r .

A process developed by SINTEF in Norway (9 ) i s shown i n Figure 6. Note the s imilar i t ies t o the TNO process as fa r as the types of equipment employed in the f i r s t stages. Both preshred or grind the refuse, use trommels t o screen i t , then zigzag a i r c lass i f ie rs t o separate the l ight from the heavy fractions. The SINTEF process takes the overhead from the a i r c lass i f ie r and passes i t t o a simple wash t a n k holding 250 l i t e r s (66 ga l . ) of water where i f i s washed and separated i n t o sink/float fractions. In rea l i ty , t h i s separation i s relatively easy; a f te r the ground particles of plast ic are freed o f d i r t , o i l , grease and adhering paper, the polyolefins (density <1) will f loa t , while the PVC, other plast ics , and wet paper, e tc . whose densities are greater t h a n water, will sink. While the polefins a l l have a density less than one, and will be separated as a mixture of HDPE, LDPE and PP, using the mixture t o make even as prosaic as a garbage bag i s d i f f i cu l t . section.

The TNO system operates a t 15-20 tonnes (33,000-44,000 lbs)

I t i s apparent from the flow chart t h a t resorting

This problem will be covered in a l a t e r

Even i f plast ic waste has been presorted a t the curb, some final classification i s usually done t o segregate the more valuable plastics from the main stream. One such system i s described i n a paper recently presented a t Recyclingplas IV (10). The AKW plastics recycling plant which has been operational in Blumenrod, West Germany since July 1988, is said t o have i t s ent i re o u t p u t sold. The Blumenrod plant, i n the.Coburg d i s t r i c t , collects waste from a population center of about 280,000 people. Three collection systems are employed in the d i s t r i c t . One, (Figure 7-1) has a number of larger receptacles located in heavily used areas such as near parks, c i ty centers, etc. The receptacles are marked for glass and for paper, with the non recyclable materi a1 segregated. Further separation must be done in a "Material Recovery Facility" (MRF) . material collected from these points, while appreciable, i s n o t nearly enough t o jus t i fy the construction of a recycling f ac i l i t y , so a second system of curbside collection i s also practiced (Figure 7 - 2 ) . Here the recyclables are a l l placed in a single b i n which i s taken t o the M R F . system, which i s presently used in several U.S. c i t i e s and proposed for others, requires the householder t o separate the waste paper, metal, and plast ic into a "green bin", which greatly f ac i l i t a t e s the sorting a t the recovery f ac i l i t y . This system does n o t require the householder t o make the decision as t o w h a t constitutes the most desirable recyclables, b u t does greatly decrease the amount of final sorting required. The third, and preferred system, requires some education of the householder, as a three bin system i s in place, i s shown a t the bottom of Figure 7. cases a final sorting i s performed a t the recycling p l a n t .

The amount of

This

I n a l l three

46

c c 7

c

c c c c

- 4

4 - * c C a

C c

47

Solid household refuse input 1 1.00

I

r

Sased on the average tompasibon of the refuse. the plant gives tne following results:

magnel

paper

plasuc folio

sleel-tins

mill 1% mill relep!

36% zig-zag d a s i f i e r heavy fraaion

70 . 8096

70 - 90%

9596

sieve 25?1, line lradion

mastening 41 rig-zag eiassfier 0 Yo

plasoe frardon

'"+.;- :>2% rest fracuon

. 15% after seuaratw

degree of cleanliness

approx. 5 % DIasuc ana sirav 0.9% sand

1 O X , mainty paper

5 06

- I - line fraciion I

F I G U R E 5 48

ACCEPT

G R I N D E R R

ACCEPT

H U V Y FRACTION

D R Y W r-i CYCLCSE

WASHING TANK

MECHANIUll DPNATER ING

P R O C E S S F O R M E C H A N I C A L L Y S E P A R A T I N G P L A S T I C S

F R O M D O M E S T I C W A S T E ( S I N T ' E F I

F I G U R E 6 49

AKW Apparate t Verfahren GmbH I

1 ! I

Central Coliection Points f

Direct to Processing Facitity 40-25% < Remaining bvasce V o l ~ m e 98-75 96

4 9

!

‘Green Bin’ System with MRF I

(MATERIAL RECOVERY FACILITY), Througn

1ttds:er Recovery System up to 35%

Remain inff 7 Waste Volume up to 55?6

o Processing

4

Multiple Bin System without M R F

Rernai n in 5 Waste Volume

Througn After-Sofling up io 45% To Processing

. 50

Diagrams o f some o f t he e a r l y r e c y c l i n g operat ions i n d i c a t e t h a t l i t t l e o r no p r e s o r t i n g was done. Even i f the s i n k / f l o a t separat ion mentioned i n the prev ious paragraph were used on a g ranu la r waste p l a s t i c stream con ta in ing say, polyethylenes, polypropylene, foamed polystyrene, so l i d polystyrene, PVC, and PET, t he polyethylenes and polypropylene ( p o l y o l e f i n s ) p l u s a l a r g e p o r t i o n of t he foamed polystyrene would f l o a t . Attempts t o f a b r i c a t e use fu l i tems fom t h i s m ix tu re would probably be f u t i l e , s ince t h e p o l y o l e f i n s and polystyrene are incompat ib le. The bottoms would have a m ix tu re o f a l l t h e others, and attempts t o f a b r i c a t e useable products from them would a l so y i e l d a r t i c l e s o f both poor s t reng th and appearance.

var ious types o f p l a s t i c s e i t h e r by hand s o r t i n g before g r ind ing , o r by some mechanical means. F igure 7 i s a p i c t o r i a l p resen ta t i on o f the AKW s o r t i n g p l a n t where mixed waste i s hand sor ted on moving conveyers before g r ind ing . c o l l e c t i o n po in ts , metals, cardboard, paper, wood, and p l a s t i c s are hand sor ted again. I n t h i s i l l u s t r a t i o n , on l y two classes o f p l a s t i c s are def ined, r i g i d and f i l m , al though the t e x t o f Reference 10 uses the terms p l a s t i c f i l m and p l a s t i c b o t t l e m a t e r i a l . Since 99t% o f p l a s t i c f i l m s are p o l y o l e f i n , no problem would l i k e l y r e s u l t f r o m compounding them i n t o i t e m s f o r r e s a l e i f an i n t e n s i v e mixer were used. F igure 7 does no t i n d i c a t e i t , bu t t h e p l a s t i c b o t t l e s should be f u r t h e r separated i n t o th ree groups, s ince n o t on l y are t h e th ree major types incompatible, b u t they represent products o f h ighe r economic value i f they a r e recovered i n a r e l a t i v e l y pure form.

It i s apparent t h a t an e f f i c i e n t recovery operat ion must segregate the

Although some attempt has apparent ly been made t o p r e s o r t a t t he

O f p a r t i c u l a r i n t e r e s t t o most American r e c y c l e r s are PEP b o t t l e s , as they are e a s i l y i d e n t i f i a b l e , being c l e a r , u s u a l l y w i t h an opaque base cemented t o them, w h i l e po lyethy lene b o t t l e s are most ly t rans lucen t wh i te . Both o f these are worth t h e e x t r a e f f o r t t o separate, as they can be e a s i l y hand sor ted from a conveyer b e l t . f o r wine and e d i b l e o i l s , and some attempts have been made t o separate them f o r r e c y c l i n g . PVC b o t t l e s a r e u s u a l l y c l e a r , and d i f f e r e n t i a t i n g them from PET might be d i f f i c u l t .

A f t e r separat ion o r s o r t i n g , t he p l a s t i c f r a c t i o n s are f i n e l y ground o r shredded ( i n t h e e a r l i e r TNO and SINTEF processes the t o t a l stream was ground then separated i n an a i r c l a s s i f i e r ) t h i s would appear t o be a r e l a t i v e l y s t r a i g h t f o r w a r d operat ion, bu t i n p r a c t i c e can be very d i f f i c u l t . P l a s t i c s are tough, and tend t o t e a r r a t h e r than break c l e a n l y . Also, when g r i n d i n g a waste s t ream, there are pieces va ry ing i n s i z e f r o m one inch (25.4 mm) t o pieces several y a r d s o r m e t e r s long; thicknesses may be as l i t t l e as 0.001 i n . (0.0254 mm) o r up t o 0.5 i n . (12.7 mm) t h i s v a r i a b i l i t y r e q u i r e s g r i n d i n g equipment w i t h extremely c lose tolerances, and the presence o f d i r t , g r i t , o r o t h e r contaminants causes these t o change q u i c k l y . Gr ind ing a l so r e s u l t s i n heat bu i l dup which can change k n i f e clearances, and make the p l a s t i c more r e s i l i e n t and harder t o c u t c lean ly . Some g r i n d e r s operate w i t h a gas stream f l ow ing through the g r i n d i n g chamber (some spec ia l i zed operat ions use l i q u i d n i t rogen) which serves t h e dual purpose o f coo1 i n g and conveying the m a t e r i a l . The AKW system g r i n d s t h e waste under water, which cools more e f f i c i e n t l y and a ids i n washing t h e p l a s t i c p a r t i c l e s .

I n Europe, PVC b o t t l e s are widely used

51

MECHANICAL SEPARATION/CLASS I F ICATION

Zig zag or cyclone air separators used in the earl ier European processes are not very efficient for classifying particles of low density with high surface to volume ratios such as granulated plastics. This, coupled with the small density differences between the several types of plastics (approximately 0.9 to 1.5gm./ml) yield an overhead product that is a mixture of the feed material. The AKW process uses hydroclones, which are cyclones operating with liquids as the conveying media, as shown in Figure 8 . The hydroclones complete the washing, and separate the solids into a heavy fraction which in their operation i s primarily PVC and PS (polystyrene), and a light fraction composed of the polyolefins {low and high density PE and PP). centrifuge and dried, both operations being carried out in conventional commercially available equipment. in Figure 9.

These fractions are then dewatered in a

An overall view of the AKW MRF is shown

PET BOTTLES

Another special case in the municipal waste classification i s the ubiquitous PET soft drink bottle. In 1979, 1 . 5 billion of these containers were produced, consuming about 150 million Ibs. of resin (11). PET i s now used in many other types of containers, from salad oil to peanut butter; the 1989 usage is expected to be 1.02 billion lbs. These bottles are easily identified in waste (and often as roadside litter), and the recycled resin has a high resale value. the most profitable of plastics recycling efforts, aided by mandatory recycling laws such as the one in Michigan. by machines which return cash for each bottle inserted; the machines grind the bottles for periodic collection by the recycler.

These facts have resulted in PET becoming

Another incentive is offered

Most PET bottles have two piece bodies, the clear or slightly tinted bottle proper, and a base molded o f high density polyethylene. arid seal rings (which stay on the body after opening) are aluminum with an ethylene-vinyl acetate gasket or liner. paper label, which are now being superseded by shrunk on polyethylene film labels. Several recyclers have been in operation since 1980. Most of them grind the bottles, air separate the paper and fines,then use a sink/float process in water t o separate the PE base from the PET and aluminum. The Center for Plastics Recvclinq Research (CPRR), has a pilot plant in Piscataway, NJ which uses an electrostatic separator t o remove aluminum after the above separation steps ( 1 2 ) . CPRR is offering the process for license at a nominal cost.

The caps

Earlier bottles had a glued on

PRODUCT FINISHING

A1 though some plastics fabricators have equipment capable of extruding or molding articles from the fluffy granulates, the majority are used to handling pelletized resins. granulate to an extruder to melt blend or homogenize the mixed plastics and to form small pellets which are much more easily fed to the final

It i s therefore preferable to pass the dried

52

- - --

A K W A p p a r a t e + V e r f a h r e n G m b H

H Y D R O C L O N E

cd n deri I ow

F I G U R E 8 53

v s

ion step. Plastics extruders are deceptively simple in appearance ation, consisting of a heated cylinder enclosing a long auger like

screw is specially configured for each type of plastic, and as special mixing flights, interruptions for the removal of rom the plastic melt,etc, The design of extruders and their quipment is a complex mixture of art and science, and they must ith great care.

ruders used to blend and pelletize the granulate are usually fitted crammer feeder", which i s an auger to force the fluffy granulate

he feed section o f the extruder proper. As the mixed plastic d cylinder, they are heated both by y friction as they are compressed by ts. The frictional heat is of suck external heat for startup; while ater on the external barrel surface,

Feedstocks such as the granulated e water or other volatiles in them

g step, and may require devolatilization in the ccomplished by decompressing the melt near the middle

a fine screen, and exits the extruder through a o f the barrel i s applied. The melt is then further

cooled, solidified, and chopped into pellets.

alline polymers o f similar chemical homogenizing the molten plastic at

fended and molded or extruded into ality. PET is an excellent example; products or carpet fibers. Mixtures olyolefins are much more difficult to omponents has a different melting

10' C., high density 120°, and a mixture is melted by merely raising the

, the crystallites or spherulites of the result that a thin film or fiber polymer or dissimilar polymers such as e to the slaked eye. Such impurities

important that a good homogenous at the recycling plant. Many good ly available, and while not able to

sometimes ac ning the product, and at the least are

crap, will still make good recycled

pel 1 et i zi ng in preparat i on for other possibility for recycle exists,

ot be polymerizable. Polyolefins may 'IP yielding a mixture of liquid and ally consist o f the original monomer of the mixture will be composed of

ly severe conditions are used. Halogen 11 evolve hydrogen chloride (HC1) when

k t o their monomers and either r uses for the molecular

55

heated, become b r i t t l e , and f i n a l l y i nso lub le . Polyesters such a s PET a r e e a s i l y depolymerized by hyd ro l ys i s i n g l y c o l s o r even water a t h igh temperatures and appropr ia te pressures. Eastman Chemical Products Inc . has demonstrated t h a t they may be depolymerized i n propylene g l y c o l (16), then reacted w i t h acids such as maleic t o produce a thermoset po l yes te r r e s i n . Goodyear and duPont are a l so doing research us ing the depolymerizat ion approach

APPLICATIONS FOR RECYCLED PLASTICS

Pending development o f b e t t e r means o f s o r t i n g the var ious types o f p l a s t i c m a t e r i a l s i n t o r e l a t i v e l y pure f r a c t i o n s , i t appears t h a t a t the present on l y two types could be considered f o r making i n t o h igh q u a l i t y ob jects , PET and HDPE. High dens i t y polyethylene (HDPE), from such obJects as discarded m i l k containers, can be e a s i l y d i s t i ngu ished from o the r components o f a waste stream, o r sor ted a t the curb. A f t e r the b o t t l e s a r e reground and cleaned they may be recompounded i n t o p e l l e t s f o r remolding o r ext rus ion. v i r g i n r e s i n , and i f s t r i c t q u a l i t y c o n t r o l i s performed by the recompounder, phys ica l p roper t i es may be i n the same range as the v i r g i n polymer. be made from reprocessed mate r ia l , such as windshie ld washer f l u i d containers, fender and t r u n k l i n e r s , e t c . Procter and Gamble has pioneered among consumer goods companies i n the development o f recyc led p l a s t i c s packaging. ReSycled HDPE i s used t o make t e s t q u a n t i t i e s o f one g a l l o n jugs f o r Cheer detergent. These conta iners are made i n a th ree l a y e r ext rus ion; v i r g i n r e s i n i s used f o r the ou te r appearance laye r , r e c l a i m f o r the center, and again a l a y e r o f v i r g i n f o r the i n s i d e i n contact w i t h the product.

Polyethylene te reph tha la te (PET) i s a l ready being recovered i n such p u r i t y t h a t i t i s s u i t a b l e f o r some f i b e r uses such as p i l l o w o r cushion fi1l:rs. Span Pine cleaner. There a r e a number o f automotive o r i n d u s t r i a l p a r t s which could be made o f reprocessed PET, e i t h e r c l e a r ( r a d i a t o r over f low tanks) g lass f i l l e d ( f i l l e r caps, terminal b locks) o r compounded w i t h o the r r e s i n s such as low dens i t y polyethylene (LDPE) which improves p r o c e s s i b i l i t y , lowers moisture absorption, and also elevates some mechanical p roper t i es . (LDPE/PET blends have lower moisture absorpt ion than PET alone).

The cost o f such recompounded mate r ia l i s 25-35% l e s s than

I t would seem t h a t several non appearance automotive i t e m s could

Procter and Gamble has tes ted c l e a r PET b o t t l e s f o r Spic and

Most l ead -ac id automobile b a t t e r i e s a r e contained i n polypropylene (PP) cases. Cases from b a t t e r i e s being reclaimed f o r t h e i r metal value a r e ground t o 1/8" pieces, washed, mixed w i t h a foaming agent, and extruded i n t o I beams f o r bed r a i l s ( 1 7 ) . To quote f r o m t h i s a r t i c l e i n Modern P l a s t i c s "One r a t h e r cur ious f a c t o r emanating f r o m t h i s reclamat ion p r o j e c t i s a morphological change i n the scrap PP which i s thought t o s t e m from the res idua l l ead and s u l f u r imparted by the b a t t e r y environment. nature o f t he chemistry has no t been determined, bu t the r e s u l t i s d e f i n i t e improvements i n such p r o p e r t i e s as impact strength, f l e x u r a l modulus, and

The p rec i se

56

heat deflection." The above phenomena might also be related to results reported by Zamorsky and Muras (20) when PP was repeatedly extruded and the melt viscosity only decreased by 4.4%, while the carbonyl index during weathering was re1 atively unchanged.

The example of the reclaimed PP given above is unusual, for reclaimed materials usually have physical properties inferior to virgin resin. Nearly a11 molders of products such as battery cases will use resins of about the same molecular weight and molecular structure. When recompounded, these polymers will retain essentially the same structure, so they will be physically compatible, and retain most of their original properties. other hand, will appear to be uniform, but in reality the many manufactured items will be made from a var ety of resins having vastly different molecular weights, copolymers (such as ethylene-vinyl acetate), pigments, and other additives. Despite thorough washing in systems such as the AKW process mentioned above, some of the products were used to package motor oil, vegetable shortening, or other products which absorb into the polymeric matrix and change its properties. This sensitivity to contamination i s much more evident in crystalline polymers, a class which includes the polyolefins (polyethylene, polypropylene, etc,).

plastics from the other constituents in a waste stream, and that further, these are separated by some mechanical method which yields two streams, m e hawing a density of less than 1.0, the other above 1.0, a number of marketable items could be produced. A few suck items and sources of raw materials are shown in Table 2.

Clear plastic ma erials picked from domestic trash on the

Assuming that the reclamation process i s efficient enough to separate

CONCLUSIONS

The continued buildup of plastic wastes in landfills has become a major source o f concern for mayn sectors of society, manufacturers and their trade organizations have begun to realize that they must help diminish the problem, and turn their csnsiderablg -.technical skills to this end. Cooperation between these organizations, academia, and EPA has been excellent, and together, the amount o f plastics recycled should be greatly increased.

Plastics

TABLE 2

PRODUCTS MADE FROM RECYCLED PLASTICS

Product S t a r t i n g M a t e r i a l s Reference

S t r u c t u r a l Concrete Foamed p ( w a l l s , e t c . ) p o r t l a n d cement. 8

Dun

Drainage Pipe ays , i r o n oxide ,

Reground foamed PS food t r a y s , ( l o o s e o r molded) 8

n u c l e a t o r f o r azo foaming agent . 12 "PVC b l i s t e r packs, b o t t l e s , c i r c u i t

board h o l d e r s +. v i r g i n PVC.

Recycled 1 ead-ac id b a t t e r y c a s e s , 0.5% foaming agent . 17

Bed R a i l s

Fence P o s t s o Polyolef in waste ( m i n 50%)) up t o

Bui 1 d ings .

F1 o o r Mats, Wheel Copper contaminated wire & cab1 e Chocks, Pads, Truck s c r a p . Bed Liners, Drainage P i pe Telephone Conduit 14

Floors f o r Fa 20% PVC, 20% paper +foaming agent . 13

Composite Board P1 a s t i cs a u t o s c r a p , sawdust, ( o r chopped g l a s s ) 5% MDI - i socyanate b inder . 15

P o l y e s t e r Pol yo1 s p o l y e s t e r f i b e r (raw maQ'eri al- f o r ing , chemica l ly pol ure thane foams) d i g e s t e d . 16

Garbage Bags P o l y o l e f i n waste from household and i n d u s t r i a l sources 18

Greenhouse F i 1 m l e a n rec la im f i l m blended w i t h l i n e a r low d e n s i t y PE 19

R E F E R E N C E S

1. Science News 135 282, May 6, 1989

2. Modern P l a s t i c s 66 5, p.42, 1989

3. Environment ReDortele June 16, 1989

4. Modern P l a s t i c s 56 1, p.69, 1979

5. Valdez, e t a1 Bureau of Mines ReDort. o f . I n v e s t i s a t i o n s RI 8QSf l9T5

6. Goldbaum, Ellen Chemical Week I9 p.9 1989

7. K u n s t s t o f f e 68 5, p.23 1978

8. Waste P l a s t i c Recvclinq Information Exchange Volume 11, C a n t r i b u t e d Papers9 NATO/CCMS Report 123, p. 3-15 1981

9. I b i d . p . 4-1

10. S t o l z e n b e r g , Andreas PaDer Dresented .-at' Recvcl i a l l

11. Modern P l a s t i c s 57 4, p.82 1980

1 i .

Washington, D.C. May 24, 1989

Ibid. . 64 2 , p.15 1987

13. Barnest* B.A. e t 1 Chemical Engineering 87 12, ~ ~ 5 0 - 5 1 1980

14. Modern P l a s t i c s 57 4, p. 82 1980

1%. I b i d . 57 5 p.64 1980

16. Car ls t rom, W.L. e t a1 5, p. 100 1985. Also U 668 and 4,4Ef,OMI

17. Hodern P l a s t i c s . 5 5 6, p. 61 1978

18. Brochure Pub1 i shed.. .bv AKW GmbH Hirschau, W . Germany

19. Ram, A. and Getz, S. L&RD 1 i ed Pol mer Science 29

20. Zamorsky, Z . and Muras, 3 , J Polymer Dearadation and S t a b i l i t y 14

2501-15 August, 1984

41-51 1986

59