Polystyrene Recycling: An Overview of the Industry in North America

Chapter 8

Polystyrene Recycling: An Overview of the Industry in North America Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch008

David A. Thomson Canadian Polystyrene Recycling Association, 7595 Tranmere Drive, Mississauga, Ontario L5S 1L4, Canada

Polystyrene (PS) is a versatile thermoplastic material used in a wide range of durable and single use applications. Post consumer PS is being commercially recycled throughout the United States and Canada, largely through the efforts of corporations involved in the synthesis, conversion and distribution of PS products. Markets for recycled polystyrene that can provide a basis for a sustainable industry are being developed. Polystyrene, a product that has been closely associated with a litany of environment issues ranging from overburdening of solid waste disposal facilities to global ozone depletion can, and is, being recycled throughout the United States and Canada. Recycled polystyrene materials are now commercially utilized in the electronics, horticultural, houseware, and construction industries. In recent developments, post consumer recyclate has been approved for use in the manufacture of food packaging materials. Synthesis Polystyrene is an amorphous thermoplastic produced by either the batch suspension, or mass-continuous polymerization of styrene monomer. In the batch suspension process, styrene monomer is dispersed in water in a stirred reactor. Free radical initiators are used to control the polymerization parameters. A colourless polystyrene bead, similar in appearance to table salt, is generated from the reaction. The batch suspension process is commercially used to produce expandable polystyrene (EPS) moulding beads through the addition of 5% - 8% (weight) of a hydrocarbon blowing agent, such as one of the isomeric pentanes, to the monomers prior to suspension in water. Dense EPS beads prepared in this manner are shipped to other locations, where they are expanded and formed into the desired product. Direct foam moulding of intricate shapes can be readily achieved near to the point of use, minimizing the cost of transport of the low density foam.

0097-6156/95/0609-0089$12.00/0 © 1995 American Chemical Society Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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PLASTICS, RUBBER, AND PAPER RECYCLING

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 22, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch008

EPS possesses good insulating value, light weight, low water pick-up, high rigidity, and excellent cushioning properties. These properties, combined with the relatively low costs associated with the production of EPS products, have resulted in the large scale commercial use of expandable polystyrene. The mass-continuous (or solution) process, made after technological advances in the 1950's made control of the exothermic heat of reaction and the handling of the highly viscous materials possible, has developed as the process of choice for all grades other than EPS. Styrene monomer, solvent and occasionally an initiator, are passed through a series of heat exchangers until the desired degree of polymerization is achieved. Residual monomers and solvents are stripped from the polystyrene and recirculated to the front end of the reactor. Careful manipulation of residence time, reactor temperature, solvent content, and initiators produce polystyrene of high purity and low residual monomer content over a broad range of molecular weights and molecular weight distributions. The homopolymer general purpose polystyrene (GPPS), produced from this process exhibits excellent clarity and is widely used in applications where transparency is required. GPPS extruded sheet can be foamed with the introduction of a suitable blowing agent during the extrusion process or formed into tough, clear sheets by biaxial orientation of the sheet in an operation downstream of the extruder. Impact resistant polystyrene (HIPS) can be produced in the mass-continuous process by the addition of an elastomer, usually polybutadiene rubber, during the polymerization reaction. Alternatively, specialty impact grades can be produced by post reactor compounding of styrene butadiene block copolymers into a base resin. HIPS derived by the former, more prevalent method, are translucent to opaque while the latter technique, which is considerably more costly is capable of yielding tough resins with good clarity. Mass-continuous polymers are converted to finished goods in well known plastics processing operations such as injection moulding (solid and structural foam), solid extrusion, foam extrusion and blow moulding. Typical products of these various fabrication techniques are shown in Table 1. Additives and Copolymers GPPS and HIPS have attained significant commercial success as a result of their versatility, ease of fabrication, thermal stability, low specific gravity, and low cost. Additional benefits can be achieved through the use of additives to modify performance or enhance processing. The range of additives includes flame retardants, lubricants, antioxidants, ultraviolet light stabilizers, nucleating agents, and colorants. The use of additives and copolymers, while extremely desirable in the initial fabrication process, can create substantial challenges for subsequent recycling operations. The range of properties of styrene homopolymer can be greatly broadened by copolymerization. Common commercial copolymers include Styrene-Acrylonitrile (SAN), Acrylonitrile-Butadiene-Styrene(ABS), Poly(styrene-Co-Methyl Methacrylate) (SMMA), and Poly (sty rene-Co-Maleic Anhydride)(SMA). ^PPS is often referred to as crystal polystyrene (XTAL), although this term refers only to the clarity of the resin, and not its other properties.

Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Polystyrene Recycling


Table 1 Typical Single Use Application Fabrication Technique

Single Use Products

GPPS

HIPS

Injection Moulding (Solid)

Beverage Containers Cutlery Garment Hangers Dairy Containers Packaging (personal care products, medical supplies etc)

X

X X X X

X

X

Extruded Sheet (Solid)

Cup Lids Salad Boxes Dairy Containers Plates Baked Goods Containers Vending Cups Closures Trays: - Retail Display - Medical Disposable - Horticultural

EPS

X X X X X X X X X X

Extruded Sheet (Foam)

Meat (Poultry Trays) Produce Trays Clam Shells Hinged Lid Containers Egg Cartons Cafeteria Trays Foam Cups

X X X X X X X

Blow Moulding

Bottles (Vitamin, etc)

X

Foam Moulding

Cups and Containers Loose Fill Packaging Cushion Packaging

X X X

X X X

Commercial Use Polystyrene is the fourth largest volume commodity thermoplastic, ranking behind polyethylene, polypropylene, and PVC. The demand and nameplate production capability for PS and EPS in Canada and the United States are illustrated in Table 2 . m


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Table 2 Polystyrene Capacity Country

1993 Demand Metric Tonnes x 1000

United States


Canada Total NOTE:

1993 Capacity Metric Tonnes x 1000

2,330

2,910

170

182

2,500

3,092

Some data from ref. 1.

Polystyrene, in its various forms, is employed, almost equally in the production of both durable and disposable (single use) goods. Table 3 shows the consumption of polystyrene for each of the common fabrication processes . l2]

Table 3 Polystyrene Consumption Patterns Consumption Metric Tonnes x 1000

% of Total Consumption


615

25.6

Extrusion (Solid)

202

8.4

Extrusion (Foam)

99

4.1

EPS

128

5.5

1,044

43.5

Durable

Total Durable

Consumption Metric Tonnes x 1000

% of Total Consumption


294

12.3

Extrusion (Solid)

557

23.2

Extrusion (Foam)

297

12.4

EPS

206

8.6

Total Disposable

1,354

56.5

Disposable

NOTE:

Some data from ref. 1.

Recycling Polystyrene Scrap polystyrene can be readily refabricated through direct introduction as regrind flakes back to the original moulding or extrusion process, or into alternate application after an extrusion and pelletizing step. The pelletizing process is particularly beneficial in cases where the ground flake material may contain contaminants, or consists of different grades



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or colours. This practice has been common for many years as a method to recover defective parts, sprues, runners, sheet extrusion trim scrap, and web trim from vacuum forming operations. These pre-consumer scrap sources are generally of high quality, contain minimal contamination, and are available in quantities that support economic transportation and processing. An industry aimed at the recycling of post consumer, or post use, polystyrene began to emerge in the late 1980's in response to demands for an alternative to landfilling or incinerating of single use polystyrene packages. Despite accounting for only some 12% of total polystyrene consumption, much of the early effort has focused on management of the foam single use packaging wastes. Various alternatives to traditional disposal methods have been proposed and researched. Mechanical Recycling Polystyrene materials sorted or segregated from the solid waste stream can be subjected to a process that may include some or all of the following elements; densification (foam), granulation, washing, drying, extrusion, and pelletizing. The resulting product is a pellet that possesses properties similar to the virgin resin and may be converted to finished goods in the same processes used in the initial fabrication operations. In certain applications, post-use polystyrene may be left as a component of a mixed plastic stream where it may be useful in improving properties of "lumber" manufactured from commingled plastic. Applications have been developed for the inclusion of granulated polystyrene foam directly into lightweight concrete mixtures or as a soil amendment to retain moisture and prevent compaction. Fabricators of moulded EPS cushion packaging and insulation board have successfully incorporated granulated post consumer EPS directly into the production of new foam products. Energy From Waste (EFW) Incineration plants that convert the stored energy in solid waste into electrical energy are available throughout many industrialized nations. Polystyrene constitutes a significant source of energy at 46,000KJ/Kg. For comparison, the calorific value of heating oil is 44,000 KJ/Kg In areas that EFW facilities are permitted, it represents a viable alternative when polystyrene volumes do not support economic mechanical recycling operations, or that energy required to collect, sort, and transport the product exceed the energy demands of virgin production. I3]

Thermal and Chemical Recycling Methods of thermally or chemically decomposing polystyrene and other plastics have been employed in commercial or pilot plant operations. The American Plastics Council (APC) in conjunction with Conrad Industries Inc., of Centralia, Washington currently operate a pilot plant in what is perhaps the most well known example of advanced thermal recycling in North America.


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Polystyrene industry initiatives such as The National Polystyrene Recycling Company (NPRC), The Canadian Polystyrene Recycling Association (CPRA), and The Association of Foam Packaging Recyclers (AFPR) have focused on mechanical recycling activities and the challenges faced in developing a viable industry based on post consumer recycled (PCR) materials.


Recycling Challenges Some of polystyrene's most valuable properties create some of the most challenging impediments to economical recycling of post consumer materials. Single service food packaging and cushion packaging are light weight. Beneficially, little plastic material is used in the manufacture of these products. Detrimentally, the quantity of material available at any given location is relatively small. Forty five kilograms per week of polystyrene packaging is produced by an average fast food restaurant or by a school cafeteria program. Light weight and low volumes require that materials be consolidated for transport, or that imaginative systems be employed to reduce the impact of transportation economics on the recycling industry. In one such initiative, the NPRC is able to make polystyrene available to all school cafeterias across the contiguous states through a venture with United Parcel Service. The consistency has resulted in FDS non-objection status for use of PCR at up to 50% in certain foam trays and containers . Students remove residual substances from the polystyrene foam service tray widely utilized in school lunch programs, and stack the trays back into the original shipping box. A pre-printed label is applied and the boxes of used trays are collected by UPS for delivery to one of the three NPRC facilities in Bridgeport, N.J., Lincolnshire II, or Corona, California. This system provides a feed source of uniform, high heat, general purpose polystyrene. The AFPR operates an extensive network across the United States. Over 100 EPS moulders provide backhaul of used packaging to their customers. Individual consumers may have access to drop off locations in some communities. The Canadian Polystyrene Recycling Association has developed a network of private waste management and recycling companies that provide collection of single service food service packaging across a broad base of industrial, commercial, and institutional generators. CPRA encourages the collection of solid polystyrene materials together with foam products to increase capture rates and to improve the weight to volume ratio. Specialized drying equipment in the CPRA Recycling plant in Mississauga Ontario is capable of separating foam from solid prior to extrusion. Large volume generators of EPS cushion package, or Material Recovery Facilities (MRF) handling this product, frequently utilize baling devices to raise the bulk density of cushion packaging above its loose value of 15Kg/m3. Typical downstroke balers will increase the bulk density to roughly 30 Kg/m3, although horizontal unit balers are available to produce bales of densities in the area of 250-400 Kg/m3. High density baling makes cleaning of the product in subsequent operations difficult. It is, therefore, imperative that all contaminants be excluded before baling. Baling at a point close to the source greatly improves transportation economics. Polystyrene is versatile, which enhances its appeal as the material of choice in a host of disposable and durable applications. The diversity of products, from foam cups to refrigerator liners, make recognition difficult for the uninitiated. Much of the recyclable [4]


8. THOMSON

95


polystyrene is not recovered simply because it does not resemble the foam cup or cushion packaging generally associated with the polymers. Data from Ontario, Canada curbside collection program show a recovery of foam that is highly disproportionate to resin consumption patterns (Table 4). Table 4 Distribution of PS Products in Curbside Collections


% of PS Disposables

Recovered Ontario Curbside Collections Mississauga

Halton

Quinte

Solid Extrusion & Injection

62.8%

1.1%

5.9%

27.4%

Extruded Foam

21.9%

84.5%

37.0%

38.1%

EPS

15.3%

14.4%

57.1%

34.5%

Polystyrene often competes with other polymers for a share of the same market. "Lookalike" products enter the waste stream and must be removed from the polystyrene if a quality product is to be produced. Polyvinyl Chloride (PVC), and Polyethylene Terephalate (PET) are frequently confused with injection moulded GPPS or sheet extruded polystyrene single service food packaging and drink cups. Polypropylene and polystyrene are both extensively utilized in the production of disposable cutlery. The properties of polystyrene can be enhanced with additives. Additives that perform admirably in their original intended use frequently pose problems for recyclers. Primary among these are the flame retardant additives, typically a halogenated organic compound with or without an antimony trioxide synergist. Flame retardant EPS reacts rapidly under normal extrusion conditions, resulting in severe chain scission and evolution of halogen gases. If moisture is present, acid compounds will be generated and equipment corrosion problems can arise. Recycled flame retardant EPS will exhibit molecular weight loss to values in the order of 140,000 or less, at which point the properties of the material are seriously compromised. The presence of rubber from HIPS in a resin intended for a extruded foam process can inhibit the proper expansion of the foam sheet. Lubricants, such as metallic stearates commonly added to moulding resins can accelerate the decomposition of flame retardants, residual nucleating agents, or particulate contaminants that act as nucleators, can cause loss of control over foam cell structure. The recycler must take care to segregate polymer grades and match applications to feed stock properties in order to produce quality products for secondary applications. Durables and Semi Durables Durable goods manufactured from polystyrene may have a useful life ranging from several months to many years. These applications include housewares, toys, appliances, construction materials, and a gamut of other products, that eventually become part of a solid waste stream. Efforts have recently been directed at recovering and recycling polystyrene from these sources. Eastman Kodak Company's Fun Saver Camera return {5]


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program, and Pitney Bowes, Laidlaw Waste System, Purolator courier partnership aimed at office machines and components, are successful examples of the reuse and recovery of polystyrenefromdurables. Durable goods recyclers frequently encounter challenges with metallic components, and the presence of non-compatible polymers. Manual disassembly operations are frequently used to segregate polystyrene from the unwanted materials. Some applications, however, lend themselves to automation. CPRA and The Nova Group of Bocca Raton, Florida collect and recycle garment hangersfromthe retail industry. Both operations have developed technologies to separate the non-compatible polymers and metal hooks and clips common in these products. Outlook Polystyrene recycling is good business, and there are growing indications that a sustainable industry is emerging. Although polystyrene packaging is still viewed as environmentally unfriendly by a significant majority, a public opinion survey commissioned by CPRA showed that 80% of the population said their opinion would improve if they were better educated about the recyclability of polystyrene, and nearly half stated that their opinion would change "a lot". It is important to note that almost 20% indicated that they would "go our of their way" to visit a fast food restaurant that recycles its polystyrene In the relatively short time that post use polystyrene has been commercially recycled, substantial results have been observed. The Polystyrene Packaging Council (PSPC) reports that in the United States the recycling rate for post use EPS in 1993 reached 10,570 tonnes, or 10.9% of the total EPS cushion packaging market, making it the third most recycled plastic product, behind soda bottles and milk jugs. The total post use polystyrene recycled in the same period exceeded 19,000 tonnes . Consumer awareness of polystyrene recycling has risen dramatically as demonstrated in the results of CPRA's polling. In 1990, only 18% of consumers surveyed believed it was possible to recycle polystyrene products, a number that had risen to 46% in the March 1994 poll . Fuelled by growing public awareness, strong markets, generated in part by minimum PCR content regulations, and a solid industry commitment, the outlook for polystyrene recycling is encouraging, although the industry must convert increasing consumer awareness into increased consumer action, to generate the volumes required for sustainable development of facilities and markets. I6J

m

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[1] [2] [3] [4] [5] [6] [7] [8]

References Modern Plastics, 71(1) (1994) pp73-81 Modern Plastics, 71(1) (1994) pp73-81 The World of Plastics; Society of the Plastics Industry of Canada; pg71 Modern Plastics Int., 23(12) pg10 Eastman Kodak Company, SPE Recycling Conference, 14-16 June, 1993 Angus Reid, Canadian Polystyrene Recycling Association, internal unpublished poll Recycling Polystyrene - Polystyrene Packaging Council, 5(1) (1994) pg2 Angus Reid, Canadian Polystyrene Recycling Association, internal unpublished poll

RECEIVED July 7, 1995


Polystyrene Recycling: An Overview of the Industry in North America

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