Recycling of Polypropylene - American Chemical Society

Chapter 7

Recycling of Polypropylene Robert M . Prioleau

Downloaded by UNIV OF OKLAHOMA on March 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch007

Exxon Chemical Company, 6011 Deerwood, Houston, TX 77057

In 1993 about 206 million pounds of post-consumer polypropylene were reclaimed, making this the third largest volume recycled plastic in the U.S. Over 90% of the polypropylene was reclaimed from lead-acid battery cases which are collected for recovery of the lead. However, other sources of discarded polypropylene include certain fabrics, containers, and components of automobiles and appliances. There are ten major reclaimers with over 320 million pounds per year of capacity. The technology for reclaiming polypropylene has developed by using equipment designed to grind, dry, extrude, and melt-filter other resins and adapting it to meet the unique requirements of polypropylene. The reclaimed resin is used in a variety of molding compounds and applications such as battery cases, automotive parts, cases, trays, and other rigid parts.

Polypropylene, which is the fourth largest volume plastic sold in the U.S., is also the third largest volume post-consumer recycled plastic. In 1993, the volume of polypropylene reclaimed in the U.S. was about 206 million pounds. Almost 93% of this came from the reclaiming of discarded lead-acid battery cases, and the balance was from reclaiming of post-consumer bottles, tubs and other rigid containers, film and various types of fabric. The recovery of polypropylene from battery cases developed during the eighties when laws were enacted to ban the disposal of used lead-acid batteries in landfills or incinerators. As the infrastructure to collect batteries and recover the lead developed, reclaiming of the polypropylene battery case became economical. About 75% of the resin recovered is used in producing new battery cases, and new

0097-6156/95/0609-0080$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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batteries typically contain about 50% recycled polypropylene. This is a large economically sustainable recycling business, which is expected to grow at a moderate pace. Recycling of other polypropylene (PP) products has grown slowly due to the low prices for reclaimed PP, the challenges of collecting and sorting commercial quantities of PP containers, and the high costs of the reclaiming process itself. Polypropylene is only about 2% of the mix of blow-molded bottles and jugs, and about 11% of the entire group of rigid containers and closures. It is difficult to differentiate from HDPE containers in many cases, and frequently reclaimed PP is comingled with reclaimed HDPE where it can be present in concentrations up to 67% without significantly altering the properties of the HDPE. Recycling of other PP products such as industrial fabrics, automotive parts, appliance parts, and other durables have been demonstrated but are not as well established as the reclaiming of battery cases. Some of these sources of postconsumer PP are discussed in the following section. Markets for Polypropylene The challenge of collecting commercially significant volumes of post-consumer polypropylene (PP) products is illustrated in Table I which shows the approximate markets for PP in the U.S.A. in 1993. Polypropylene is used in a very wide variety of durable and non-durable products, only a few of which can be readily recovered, sorted, and collected after they've served their intended purpose. Products formed by extrusion processing includefibersand filaments which go into textiles, films which are used in packaging, and sheet-stock which is further processed into thermoformed or stamped thin-wall products. Polypropylene fibers and filaments are used extensively in producing carpeting as a woven backing fabric and as face yarn where it finds applications in commercial carpeting, wear-resistant residential carpets, automotive uses, and in outdoor uses. Recovery of plastics in used carpeting is complicated by the variety of resins, fibers, and adhesives used in producing carpets. Polypropylene fiber is used in certain clothing and sports apparel primarily because of its tendency to promote transmission of moisture away from the body. It is frequently blended with other fibers in these uses. Fabrics for agricultural use and construction are produced from woven filaments, split-film, and non-woven polypropylene. Geotextiles are used in preparing substrates for road stabilization or soil erosion control. Non-woven polypropylene fabrics are widely used for medical drapes, gowns, and disposable products. Diapers generally have a PP inner-liner which draws moisture away from the skin.Polypropylene fabrics are used in a variety of industrial applications as bags for grains or dry products. Heavy duty fabric is also used to produce bulk-bags for transporting industrial materials. Polypropylene filaments are used to produce synthetic rope and cordage, and they have some uses as monofilament bristles.

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

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PLASTICS, RUBBER, AND PAPER RECYCLING

Table I U.S. Markets For Polypropylene (1993) millions of pounds


Extrusion

Fabrics & Filaments Multifilament Non-Wovens Slit-Film Other Film Oriented Non-Oriented Sheet Thick Parts (10 Mil or more) Thin-Wall Other Sub-Total

860 750 640 190 660 140 140 120 130 3630

Injection Molding

Rigid Packaging Closures Containers Other Consumer Products Housewares Medical Other Transportation Automotive Battery Cases Appliances Major Small Other Sub-Total

530 270 60 350 220 300 250 70 160 70 200 2480

Blow Molding

Consumer Medical

90 60 Sub-Total

Compounding & Resellers

Total

150 2040

8300



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Extrusion of polypropylene into film can be sub-divided into oriented (OPP) and cast film. The OPP film has grown rapidly as a high-strength clear packaging film which is often coated or laminated with other resins to improve its barrier properties. Cast film is used for packaging clothing, consumer goods, and food produce. Polypropylene sheet can be transformed into large parts such as automotive panels or liners by thermoforming. Thin sheeting can be formed into food containers, disposable trays, and other types of packaging. In general, PP has good moisture barrier and attractive clarity when oriented. Because it is frequently combined with other resins in laminated structures recovery of pure polypropylene can be very difficult. Injection molded PP parts include caps and closures for many bottles and rigid containers. Polypropylene is used for some blow-molded bottles where its properties are important (eg. in hot-fill applications) and it is an economical resin for thin-wall molded cups and tubs. It is widely used in injection molded housewares, cases, luggage, and other durable goods. In major appliances, PP is found in washing machines and dish washers where its resistance to hot water and detergents is important. Recovery of components (eg. agitators, pumps, etc.) is being evaluated. Applications in smaller appliances such as coffee-makers, blenders, mixers, etc. also take advantage of the temperature resistance, imperviousness to water and chemicals, and good mechanical properties of PP. However, recovery of PP components from small appliances and housewares is not practical. Automotive applications include interior trim, fender liners, ducts, housings, and some under-hood uses. The development of PP dashboard assemblies and bumper components could result in improved prospects for recovery during disassembly of a scrapped car. As mentioned previously, the PP copolymer used to produce battery cases is being successfully reclaimed in conjunction with the collection and recovery of lead from discarded batteries. About 190 million pounds of PP was reclaimed from this source in 1993, much of which is reused again as a blend with virgin copolymer to make new battery casings. Polypropylene Reclaiming Capacity Polypropylene reclaiming first became significant in the early eighties when recovery of lead-acid batteries was mandated. Discarded batteries were drained and after being disassembled the lead was collected for secondary recovery. The battery cases which were mostly copolymer polypropylene were chopped into "chips" and sent to reclaimers where they would be further ground, washed, dried, and extruded into pellets. All heavier materials (eg. other polymers, adhesives, metal residue, etc.) were removed in sink-float tanks, and the extrusion step frequently included continuous melt filtration to remove any wood, paper, or other



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non-polymer contaminants. Reclaimed PP is generally pigmented black, and may contain some added stabilizer to minimize breakdown of high the molecular weight components. About 75% of the PP reclaimed from battery cases is used to mold new cases, in many cases by blending with virgin PP resin. Loss of physical properties is negligible since the total battery population has about 40% original equipment which are generally made from 100% virgin polymer. As Table 2 shows, the largest PP reclaimer in the U.S. is KW Plastics Recycling Division which has two plants with a combined capacity of about 200 Mlbs per year of reclaimed PP resin. They have also added facilities to reclaim PP fabric such as bale wrap, but this capacity is believed to be dedicated to reclaiming HOPE film at the present. The second largest PP reclaimer is Exide Corporation which operates a battery recovery plant at Reading, PA. producing 35 Mlbs per year of PP. Other battery reclaimers in the top ten include: Richardson Battery Parts in Indianapolis, IN with 12 Mlbs per year capacity, Resource Plastics (a division of Gopher Smelting) in Eagan, MI with 10 Mlbs per year, and Tulip Corporation with about 8 Mlbs per year of capacity. There are other smaller battery recyclers who also either sell the PP chip or reprocess it into PP pellets. Aside from reclaiming battery cases, PP is also recovered from discarded cotton bale wrap or other PP fabric used in the textile business. Washington Penn Plastic operates a plant in Summerville, SC which produces about 17 Mlbs per year of homopolymer PP which is recovered principally from sources such as woven PP bale wrap, non-woven PP wrap, and other types of PP industrial fabrics. This plant can also handle PP film and filament plant scrap as feed stocks. Other similar plants include Tech Polymer in Naples, FL. with 12 Mlbs per year of capacity, Cycletex in Dalton, GA. with 12 Mlbs per year capacity, and Kelley (a division of Thermofil) in Tunnel Hill, GA with about 10 Mlbs per year of capacity. These plants are all designed to handle waste woven and non-woven fabrics as a feedstock, and can produce high-quality extrusion grade PP resin. Reclaiming of PP bottles and rigid containers has not developed as a large output since the concentration of PP in mixed recovered bottles is very low, and reclaimed HDPE can accommodate up to 6% wt. PP with very little affect on the properties of the reclaimed HDPE. However, the Polymer Resource Group (which is jointly owned by Himont and ITC) operates a 5 Mlbs per year facility in Elkton, MD to produce reclaimed PP from rigid bottles. The overall capacity for reclaiming PP in the U.S. in '94 is believed to be somewhat ver 350 Mlbs per year. Production of reclaimed PP in '94 is expected to be about 50 Mlbs indicating a 71% utilization of capacity. However, there are some indications that not all this capacity is demonstrated, and effective capacity at yearend 1994 may be closer to 280 Mlbs, which would mean an operating rate of about 90%.


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Table II U.S. Polypropylene Reclaiming Capacity as of Year-End 1994

Location

Capacity Mlbs/yr.

Batteries

Troy, AL Bakersfield, CA

160 40

Exide Corporation

Batteries

Reading, PA

35

Washington Penn Plastic

Fabric

Summerville, SC

17

Tech Polymer

Fabric

Naples, FL

12

Richardson Battery Parts (Div. ofWitco)

Batteries

Indianapolis, IN

12

Cycletex

Fabrics

Dalton, GA

12

Kelley & Associates (Div. ofThermofil)

Fabrics

Tunnel Hill, GA

10

Batteries Resource Plastics (Div. of Gopher Smelting)

Eagan, MN

10

Tulip Corporation

Batteries

NY State

8

Polymer Resource Group

Containers

Elkton, MD

5

Reclaimer

Primary Feedstock

K W Plastics Recycling Division

Subtotal

321

29

Others Total

350


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Trends In Technology For Recycling Reclaiming of PP uses many plastics processing steps such as grinding, washing, drying, extrusion and melt filtration which have been well developed over the years. However, the diversity of the feeds and the special requirements for reclaiming post-consumer plastics has challenged engineers and equipment suppliers to provide technology and machines to meet the needs of commercial recycling. Size Reduction. Whether feeding battery cases, rigid containers, woven fabrics, or film the first step in reclaiming is size reduction. Recovered battery cases are frequentiy converted to chips of resin at the recovery site, and then shipped in this form to a reclaiming plant. Bottles or rigid containers are generally baled for shipment, and must go through a bale-breaker to provide lose containers for feed to a granulator. Recovered woven fabric or film is generally shipped as a bale, and must pass through a chopper or shredder to reduce it to a suitable size for feed to a granulator. Granulators for rigid cases or containers are typically rotary grinders with a large cavity to draw material into the cutting chamber and chop it quickly into small granules which can pass through a screen with holes of about 1/2 inch diameter. Grinders for woven or non-woven fabrics and films, on the other hand, may have a semi-closed design with more blades and staggered placement to provide a less aggressive cutting action. Close tolerances are essential for good shearing of film or fabric. There are incentives to run at reduced speed to avoid excessive heat build-up and reduce noise. Cooling is provided by drawing ambient air through the cutting chamber by a blower on the discharge line. There have been many improvements in knife blade metallurgy and knife mounting techniques to extend the blade life and minimize the time required to change blades. Extruded PP filament and/or fabric is difficult to grind, and machines in this service must have ample power and adequate cooling. Knife blade changes may be required every three-five days depending on the type of feed, any mineral fillers, and non-plastic impurities in the feed. Careful consideration should be given to protection of personnel and equipment in this size reduction process. The feed conveyors should be equipped with emergency shut-offs, and all moving parts should be adequately shielded. Grinders should be designed for acceptable noise levels, or acoustic insulation may be required. Generally a metal detector is installed on the feed conveyor to the grinder to indicate the presence of any tramp metal before it reaches the cutting chamber. In some cases, a combination of a shredder and a smaller grinder provides better throughput and performance than a single larger machine. Drying. Unless the ground PP is exceptionally clean and free of paper, adhesives, labels, tags, and other contaminants it is generally necessary to wash and rinse the reclaimed PP. During washing of the ground PP the water is removed between stages. This can be done with a shaker table which leaves about 20-30% moisture. However, after the last stage of washing the polymer must be dried to 0.5%



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moisture or less for feed to an extruder. Special dryers have been developed to accomplish this by a combination of spin-drying and hot-air. In these dryers wet polymer slurry is introduced at the bottom of the cylindrical dryer where it contacts a rotor with paddles. The resin is thrown against a perforated screen which permits much of the water to pass through, and at the same time the polymer is moved up the dryer. Hot air is introduced at the top of the dryer and flows counter current to the polymer. Such a combination spin-dryer and hot-air dryer can dry flaked PP from rigid products (eg. battery cases, bottles, etc.) to about 0.3 - 0.6% moisture ready for feed to an extruder. However, if the PP is a finely shredded fluff from grinding fabric or film the moisture content is likely to still be 4-8% after the spin dryer, and a final drying step is needed in a moving-bed dryer supplied with a cross-flow of hot air to finish drying the polymer to 0.5% or less moisture. Careful control of the hot-air and polymer temperature is needed to avoid sticking or fusing of the polymer to the dryer bed. Extrusion. Conventional single screw extruders can handle flaked PP from battery cases or rigid containers quite readily. The feed hoppers can be provided with bridge-breakers, and the extruder normally has a vent section to take off any residual moisture or volatiles. However, feeding ground PP from fabric or film poses a challenge because of the extremely low bulk density of the dried fluff. In addition to providing the feed hopper with augers or bridge-breakers, the extruder can be fitted with a reciprocating ram stuffer to force the fluff into the feed throat. Development of a machine with a larger diameter screw in the feed section has also been used to solve the problem of feeding at an adequate rate to match the screw output capability. The extruder typically has a vented barrel to allow for removal of any residual moisture or volatiles. The polymer vent should be shielded to protect an operator in the vicinity from any sudden release of hot gas or polymer. The combination of feeding, venting, and achieving a discharge pressure adequate for melt-filtration often results in an screw design of 30-35 L/D ratio. An alternative to enhanced feed capability is to provide a pre-densifier which converts the fluff to free-flowing granules which are then in turn fed to the extruder in a conventional manner. Melt-Filtration. Although filtration of molten polymer through screen-packs is well established, handling reclaimed plastics introduced major challenges because of the greater contamination. As a result there has been a rapid development of continuous screen changes with automatic back-flushing to provide smoother continuous operation and reduce the cost of screens. With these systems a screen pack can be used for a hundred cycles or more before needing replacement. Continuous melt-filtration with backflushing is accomplished by designs which use slide blocks or rotating wheels to house the screen-packs. In the case of the rotating wheel filter, there are about ten cavities with large screens and the entire wheel is indexed slowly to constantly present fresh screen to the melt. A slipstream of filtered polymer is used to back-flush a small area of the screen that has just been rotated out of the melt flow. These machines make it possible to use 250


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mesh screen (55 micron) or even as fine as 325 mesh screens to achieve a high degree of filtration on the reclaimed PP. The filters operate at pressures up to 5000 psig and rely on very carefully machined surfaces and precise assembly to maintain a good polymer seal. Proper precautions should be taken to protect operators from contact with hot polymer or the metal surfaces of the melt-filtration unit.


Properties and Uses For Reclaimed Polyproylene The quality of reclaimed PP can be controlled by careful selection of feedstocks and close attention to operations. Identification of various types of PP (eg. homopolymer, copolymer, etc.) and eliminating the presence of any other polymers in the feedstock is essential to produce uniform materials and avoid contamination by other polymers. It is also desirable to separate the PP into consistent melt-flowranges (MFR) to provide grades of reclaimed resins which are suitable for various fabricating techniques (eg. extrusion, injection-molding, etc.). The physical properties of the resin are not altered significantly in reclaiming but there may be some increase in MFR of the melt. In some cases it may be advisable to add an anti-oxidant or processing stabilizer to minimize changes in melt flow characteristics. Feedstocks are often segregated by color to produce reclaimed resin that is nonpigmented, light-color, or black. In the case of black grades, some carbon-black masterbatch is generally added to improve the appearance. Most reclaiming facilities employ pellet blenders to improve lot-to-lot product uniformity. This also facilitates obtaining representative samples for quality control analysis before shipping. The product is normally tested for MFR, moisture content, and pellet size or count. Composition and density can be characterized by the type of feedstock used to produce the reclaimed resin. Special tests can be used to determine the cleanliness of the resin by running a sample through a laboratory extruder and noting the rate at which pressure builds up ahead of a finemesh screen. In addition to reusing reclaimed PP copolymer in battery cases, reclaimed PP is used in a variety of injection molding applications. It can be incorporated into blends or filled molding compounds, and is widely used as a general purpose molding resin for automotive parts, liners, components, industrial containers, commercial trays, and consumer housewares or cases. Reclaimed PP which has been gone through a fine mesh screen should be suitable for extrusion into filaments or industrial film. RECEIVED March 13,1995


Recycling of Polypropylene - American Chemical Society

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