Emerging Technologies in Plastics Recycling - American Chemical

Chapter 7

Stabilization of Recycled Plastics Peter P. Klemchuk and Tom Thompson

Downloaded by GEORGETOWN UNIV on September 8, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch007

Additives Division, Ciba-Geigy Corporation, Ardsley, NY 10502

As concerns about the disposal of increasing quantities of municipal solid waste (MSW) have grown, many approaches to solid waste disposal are being explored: source reduction, recycling, composting, incineration, degradable plastics and sanitary landfills. Recycling is experiencing a "grass roots" upsurge of interest and, although much remains to be accomplished, especially to increase effectiveness of collection and reduce costs of recycled materials, recycling is likely to be a major means of resource recovery and MSW reduction. The recycling of paper, metals, and glass has been an ongoing activity for some time. The recycling of plastics is relatively new but showing signs of becoming an important means for reducing the amounts of plastics in MSW. Plastics are unique among recoverable materials in that they require stabilizers for processing and fabrication and to withstand end-use conditions. This paper presents some information regarding the needs of plastics for stabilization and proposes some strategies for dealing with the stabilization needs of recovered plastics. Findingsfroma limited study of the stabilization of recycled high density polyethylene are presented. All-in-all, published results from earlier studies and the results from this study indicate that recycled plastics can be stabilized adequately to meet performance requirements of new applications. However, it should be pointed out that much more must be learned about the stabilization of recovered plastics before routine practices for the purpose can be established. Solid waste disposal is an important issue in the United States because of the increasing volume of MSW and the decreasing landfill capacity for disposing of it. Source reduction, recycling, composting, and incineration are receiving attention as alternatives to landfills. Heightened awareness of the solid waste problem has led to much activity in the political arena and to multiple approaches in research to solve the problem. The main political activities have shifted from the federal government in Washington to

0097-6156/92/0513-0074$06.00/0 © 1992 American Chemical Society

In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


7. KLEMCHUK & THOMPSON

Stabilization of Recycled Plastics

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individual states and cities. The problem is receiving much "grass roots" attention. This has had the effect of bringing about changes in the way municipal trash is handled at the local level. Many recycling programs have been started which recycle paper, metal, plastics and other materials. This recycling renaissance is expected to gain momentum and become a major feature of solid waste disposal programs. Recycling of plastics is expected to be an important part of that activity. Among recyclable materials - paper, metals, glass, plastics - plastics are unique in being prone to thermo-oxidative and photo-oxidative degradation. They require small amounts of stabilizing additives to retard the loss of properties and to find practical uses. In addition, they are dissimilar chemically and morphologically, as are metals, and are most useful and valuable when homogeneous and not mixed. Sorting plastics economically, developing strategies for reusing recovered materials, especially regarding stabilization needs and applications, and finding markets for recovered plastics are three big needs for a successful plastics recycling program. This paper reviews the status of plastics recovery from M S W , the future prospects for plastics recovery, some problems associated with recycling plastics, some perspectives on the need of plastics for stabilization, and some proposed strategies for the stabilization of recovered plastics. Also included are some limited results from our own work and that of others on the degradation and stabilization of recycled high density polyethylene (HDPE) and polyethylene terephthalate (PET). The need for moving quickly on solid waste problems has resulted in recycling running ahead of investigations of the stabilization of recovered plastics. However, information from stabilization practices with virgin plastics can be used in the meantime, until studies with recovered materials are available, to develop meaningful strategies for the stabilization of recovered plastics. Not until studies with recovered materials are completed will it be possible to be comfortable with current practices for stabilizing recovered plastics. Even then, there may be instances when unexpected situations may be experienced due to unusual materials being recycled, especially materials of unknown history which may have included exposure to severe conditions.

Plastics in Municipal Solid Waste Tables 1 and 2 contain information on the breakdown of M S W in the United States in 1988 according to material type by weight and volume and by product type, respectively. Containers, packaging and nondurable goods were almost 60% of M S W in 1988 (Table 2). Of the nearly 180 million tons of M S W in 1988 (from 88 million tons in 1960), 73% was disposed by landfill, 14% by incineration, and 13% by recovery. Paper and paperboard have been the largest contributors to M S W from I960 to the present. Yard wastes, much of which could be degraded by composting, are in second place. Plastics, in fourth place after metals, were 8% of the M S W weight and almost 20% of the M S W volume. As shown in Table 3, most of the 13.1% by weight of M S W recovered in 1988 by recycling and composting were paper, metals and glass. Only 1.1% of the plastic component was recycled and that was mainly polyethylene terephthalate from soft drink bottles. A study by Franklin Associates Ltd.(2)predicts plastics will increase to 9.3% by weight of M S W by 1995, from 8.0% in 1988. The same study predicts by 1995


EMERGING TECHNOLOGIES IN PLASTICS RECYCLING

76

significant reductions in quantities of M S W discarded in landfills, significant increases in amounts incinerated and also increases in amounts recovered. The portion of plastics which is recovered from M S W is expected to be between 2.7 and 6.7% in 1995, up from 1.1% in 1988 (Table 3).(2) The amounts being recovered presently have increased significantly over 1988.

TAPÎS 1


MUNICIPAL SOLID WASTE IN THE UNITED STATES BY MATERIAL TYPE, 1988; WEIGHT AND VOLUME WEIGHT VOLUME Million Cu. Yds. 3L ibullion Tons % 71.8 40.0 136.2 34.1 31.6 17.6 41.3 10.3 48.3 12.1 8.5 15.3 79.7 19.9 14.4 8.0 13.2 7.4 13.2 3.3 7.9 2.0 12.5 7.0 20.8 73.4 18.4 11.6 400.0 Total: 179.6 * rubber, leather, textiles, wood, misc., inorganic wastes, other

Material Paper Yard Wastes Metals Plastics Food Wastes Glass Other*

SOURCE: Adapted from réf. 1.

TABLE 2 MUNICIPAL SOLID WASTE IN THE UNITED STATES BY PRODUCT TYPE 1988 Million Tons % 56.8 31.6 50.4 28.1 31.6 17.6 24.9 13.9 13.2 7.4 1.5 Zl Total: 179.6 Adapted from réf. 1.

Product Containers, Packaging Nondurable Goods Yard Wastes Durable Goods Food Wastes Other SOURCE:

TABLE 3 RECOVERY OF MSW IN THE UNITED STATES, 1988 AND 1995

Material Paper Yard Wastes Metals Plastics Food Wastes Glass Other Total:

Generated. Million Tons 1995 1988 71.8 85.5 33.0 31.6 16.2 15.3 14.4 18.6 13.2 13.2

% Recovered 1988 25.6 1.6 14.6

1995U 1995HÎ

30.8 20.0 21.0

1.1

2.7

Neg.

0.0

12.5

11.1

12.0

18.9

20,8 179.6

17,2 199.8

3.8 13.1

M 20.0

38.1 33.3 29.0 6.7 7.6 27.9 12.7 27.7

SOURCE: Adapted from réf. 1.




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The merits of recycling materials have been recognized and there is considerable activity in the United States in support of recycling. At the time of this writing, 32 states have passed laws to encourage and increase recycling. More than 1500 communities in 35 states have established M S W collection programs to support recycling.(J) Other states and communities are expected to do the same.


Recycling of Plastics in Municipal Solid Waste An estimated 61% of the top 15 plastic resins produced in the United States in 1988 was disposed in M S W (Table 4). The largest portion of the total, 59.7%, came from residential sources, 25% from commercial sources and 15.3% from institutions. Only about 1.1% of the plastics in M S W in 1988 was recycled and that was mainly recovery from recycling 28% of polyester soft drink bottles. Significant increases in the recycling of plastics are expected by 1995 (Table 3). To facilitate recycling, the Society of the Plastics Industry (SPI) has developed a voluntary code to identify the plastic used to make an article. The adoption and use of these codes by plastic fabricators should increase the amounts of sorted recycled plastics and increase the value of these materials. Recycling of polyester soft drink bottles has been the biggest success so far in the reuse of plastic materials from M S W . Major reasons for the success are the availability of a source of homogeneous plastic and of marketing outlets for the recovered material. In essence, recycling transforms polyester bottles into fiber for carpeting, outerwear insulation, etc. TABLE 4 S U M M A R Y O F 1988 RESIN PRODUCTION & DISPOSAL

MMJb

Quantity Disposed MSW Non-MSW M M lb MM lb

686.0 1,093.3 6,528.8 7,690.8 461.6 2,975.1

663.0 383.1 975.3 577.2 329.2 2,869.0

96.7 35.0 14.9 7.5 71.3 96.4

22.7 710.2 5,553.5 7,113.6 132.4 106.1

3.3 65.0 85.1 92.5 28.7 3.6

1,475.5 5,274.0 4,767.9 2,794.8 7,566.0 1,319.3

176.2 1,016.9 529.7 1,510.7 5,799.4 1,183.0

11.9 1,299.3 19.3 4,257.1 11.1 4,238.2 54.1 1,284.1 76.7 1,766.6 89.7 136.3 92.3 112.5 39.4 26,732.6

88.1 80.7 88.9 45.9 23.3 10.3

Production Acrylic Acrylonitrile-butadiene-styrene High-density polyethylene Low-density polyethylene Nylon Phenolic Polyethylene terephthalate & Polybutylene terephthalate Polypropylene Polystyrene Polyure thane Polyvinyl chloride Unsaturated polyester Urea & melamine Total:

1.459.2

1.346.7

44,092.3

17,359.7

11 60.6

SOURCE: Adapted from ref. 2. The Coca-Cola Company (Hoechst Celanese) and PepsiCo, Inc. (Goodyear) have announced plans to recycle the polyester used in their bottles. The former will use a



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process which recovers dimethyl terephthalate, the major starting material for polyethylene terephthalate, by methanolysis of the soft drink bottles. The recovered, purified monomer will be used in polymerizations to make virgin polymer for bottles. The latter will use a process which recovers bis(hydroxy-ethyl) terephthalate, also a starting material for PET, by glycolysis of the polymer. Coca-Cola/Hoechst Celanese have received approval from the F D A to use the polymer from their recovery process for food packaging. Major barriers exist to the large scale recycling of plastics. Among them are the infancy of the activity so that the infrastructure and procedures for efficient, large-scale recycling are not yet in place - getting waste plastic articles from sources to plastics recyclers is far from smooth and well organized. News reports of problems encountered in implementing state-wide recycling in Connecticut on January 1, 1991 emphasize the startup difficulties of new recycling programs - most communities had not been able to provide in time the necessary facilities for collection, sorting, and storage of materials to be recovered from MSW.(4) End-use applications for recycled plastics are far from optimum. With the exception of polyester bottles, sufficient means haven't existed until now for collecting and recycling sorted streams of plastics - i.e. all HDPE, all PP, all L D P E , etc. Such homogeneous streams would provide a higher value material for recycling into end products than the so-called commingled streams which are used for plastic "lumber" for picnic tables, park benches, marine facilities, etc. As recycling increases, as the necessary facilities and procedures are in place and working smoothly, these barriers will disappear. But, meanwhile, plastics recycling is far from a humming business activity. Economics and legislation will play important roles in determining the success of recycling. Legislation at the state and community levels has already led to the establishment of many municipal recycling programs. Legislation at the federal and state levels may be foreseen to mandate the use of recycled materials with virgin materials in a variety of items purchased by federal and state governments. However, recycling costs will continue to be a major factor. In a recent example, the federal government found the cost of tax forms printed on recycled paper would be as much as 42% more costly than forms printed on virgin paper, in spite of the cost of recycled newspapers having dropped by 80% to $1.30 a ton. 3

Prospects for The Stabilization of Recycled Plastics The major polymers to be considered for recycling are PET, H D P E , polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC). The data in Table 4 indicate more than 80% of all but P V C were disposed in M S W in 1988; 23% of P V C went into MSW. The data in Table 5 show only small quantities of these plastics sold in 1989 were recovered but that the potential for recycling into non-food applications is much greater and is very likely to increase with time as recycling becomes better established. Applications identified for recovered plastics disclose a mixture of low stress and high stress (weathering) uses.(5) The low stress applications will require minimal stabilization, usually low concentrations of processing stabilizers, often trivalent phosphorus compounds and phenolic antioxidants, to protect the plastics during fabrication and end-use. Weathering applications require the use of light stabilizers in addition to processing stabilizers and phenolic antioxidants. The stabilization




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TABLE 5 SALES, A C T U A L R E C O V E R Y , A N D POTENTIAL R E C O V E R Y O F MAJOR P O L Y M E R S IN MSW, 1989, MILLION LBS. Potential Non-Food Recovery i%"> Polvmer Recovered (%) Sales 529 (28) PET 190 (10) 1905 529 (78) 190 (28) PET* 680 440 (5) HDPE 145 (1.8) 8100 770 (11) 62 (0.9) PP 7200 490 (10) 5 (0.1) PVC 5000 480 (9) 20 (0.4) PS 5200 *soft drink bottles SOURCE: Adapted from ref. 5.

requirements of virgin plastics named in Table 5 are varied. P V C is usually stabilized with metal salts for thermal stability - those stabilizers are not used for the other polymers. PET is usually stabilized with small amounts of trivalent phosphorus compounds for stabilization during processing and to minimize discoloration caused by catalyst residues from polymerization. Virgin polyolefins and virgin PS are the biggest consumers of antioxidants used in polymers; polyolefins are the biggest consumers of light stabilizers used in polymers.(6) The stabilization requirements of recycled plastics in Table 5 are at least as varied as those of the virgin materials. Limited information is available on the stabilization requirements of recovered plastics. Until more is known about those needs and until users of recovered plastics have more information to guide them, it seems reasonable to suggest using stabilizers in amounts which would be used in virgin materials for the same applications. This suggestion deserves consideration especially since recycled materials come from a variety of sources with a variety of histories. The one exception to this suggestion may be the use of thermal stabilizers for P V C since those stabilizers are already used at levels usually greater than 1%, and lesser amounts may be sufficient for reuse of recovered PVC. Table 6 lists the concentrations and types of stabilizers used for the stabilization of virgin hydrocarbon polymers. The information can be used as a guide in selecting stabilizers and concentrations for all polyolefin and PS applications and for weathering applications of PVC. Published Information on The Stabilization of Recycled Plastics S. Dietz(?) in a paper, "The Use and Market Economics of Phosphite Stabilizers in Post Consumer Recycle" presented data which showed phosphite stabilizers were effective in providing color stability and melt stability in virgin PET during processing at 280°C. Phosphite stabilizers were effective also in providing color stability in post consumer recycled PET under the same processing conditions. Higher concentrations, from 0.2% to 0.5%, were used in the recycled PET whereas lower concentrations, 0.1 to 0.2%, were used in virgin polymer for color stability. The reuse of PET recovered from soft drink bottles in a number of new, non-food applications is already a success story. The new uses can be achieved with moderate amounts of stabilizers, partly because the quantities required for stabilization of virgin


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TABLE 6 STABILIZERS A N D CONCENTRATIONS U S E D A T VARIOUS STAGES IN H Y D R O C A R B O N P O L Y M E R S


Stage • Drying • Storage • Compounding

• Fabrication End-use: • Low stress • Thermal stress

> Weathering

Stabilizer antioxidant antioxidant antioxidant + phosphite

Usual Concentration

Emerging Technologies in Plastics Recycling - American Chemical

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