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An alternative to landfills for mixed plastic waste.

An Alternative to Landfills for Mixed Plastic Waste

The opening paragraph of the Council for Solid Waste Solutions' 12-page brochure "The Urgent Need to Recycle" - published in Time, July 17, 1989 - vividly describes one of the advantages of a plastic dock: A child scrapes his bare foot on the surface and remains uninjured. One such dock, at Lido Beach, Long Island, New York, made from unsorted, contaminated, post-consumer plastic waste, daily demonstrates another advantage: Mixed plastic waste need not be landfilled.

As of October 1989, more than 40 mixed plastic processing systems were installed worldwide, with an annual capacity approaching 100 million lbs of post-consumer plastic. By the end of 1989, mixed plastic molding systems with an additional 30-million-lb capacity were in place or were being installed. Other systems that reprocess a specific recycled plastic, such as HDPE, EPS, PVC, or PET, account for another estimated 400 million lbs/yr. Systems that recycle a specific post-consumer plastic utilize technology that is traditional to the plastics processing industry or is a slight modification of it. This article is an update on a system for recycling mixed, contaminated plastic waste - and thereby keeping it out of landfills.

Plastic Waste Streams

The distinction between mixed plastic and specific plastic wastes is often vague, so definitions of types of plastic waste are in order.

Post-consumer plastic waste (PCPW) is any plastic that has been used by the consumer and discarded. It applies to an individual plastic or to a mixture of plastics. It could be one type of plastic, such as HDPE milk jugs; two types of plastic, such as PET beverage bottles with HDPE base cups; or a mixture of a large variety of plastics.

Commingled plastic waste may be a mixture of two plastics or a variety of plastics. PET and PE, the focus of most PCPW collection programs, are said to be commingled. "Commingled" also is used to describe PCPW that includes a mixture of all types of resins that are in multilayered, printed, laminated, plated, pigmented, painted, or modified forms.

Industrial plastic waste is nonconsumer plastic, which includes waste from molders, converters, packers, resin manufacturers, and so on. Normally this plastic waste is easy to identify, comes from a single source, can be processed on traditional molding equipment, and may be commingled. Some cleaning may be required.

Contaminated plastic waste may have nonplastic material enclosed, such as paper foil, wood chips, floor sweepings, lunch bags, product residue, aluminum closures, wire reclaim, fiber waste, magnetic strips, and plating.

Currently, most curbside and voluntary programs concentrate on PET beverage bottles and HDPE milk jugs because markets exist for these products and they are easily identified by the consumer. Wellman/BFI, Dow/Domtar, and Du Pont/Waste Management have begun joint ventures to accept commingled PCPW. They focus on recycling PET and HDPE. However, 20% of other mixed plastic is received along with their PET and HDPE. If they are equipped with a multi-mixed plastic processing system, the mixture can be used - otherwise it is discarded.

Separation and Resale

Approximately 60 billion lbs of plastics were produced in 1989. The plastics most subject to legislative pressure and consumer recognition are those used in packaging, accounting for 22%, or 13 billion lbs/yr - some 7.5 billion for rigid packaging, primarily blowmolded containers, and 5.5 billion for flexible packaging. These plastics are currently the focus of post-consumer consumer recycling efforts. As these programs mature, all consumer-used plastics will be included. Plastics most used for packaging are PE, 60%; PET, 10%; PVC, 5%; PP, 3%; and others, 22%.

Separating these products, including multilayer structures, into each type of plastic is the problem. Further, is it economical, based on end-use or resale markets? The resale market for plastic is not dependable, similar to the market for paper waste. In the second quarter of 1988, for example, HDPE clean regrind sold for 40[cents] to 50[cents]/lb and virgin HDPE for 50[cents]/lb. Today the shortage is over, 1 billion lbs of new PE capacity is coming onstream, and the clean regrind price has fallen to 20[cents] to 25[cents]/lb. At the municipal level, PET is worth 4[cents] to 9[cents]/lb, and HDPE is 5[cents] to 12[cents]/lb, and the market is soft. To the municipality that is collecting, sorting, and baling HDPE and PET, these figures represent a "breakeven" at best.

Each person in the U.S. contributes 100 lbs/yr of mixed plastic to the waste stream. Approximately 50% will be recoverable, with perhaps 30% collected by a recycling program. In reality, each community's percentage varies, based on local support and promotion, but the amount of plastics available for recycling can be estimated.

A Solution for Mixed

Plastic Waste

The ET-1, by Advanced Recycling Technology, Ltd., is a commercially proven system for turning mixed plastic waste, previously considered only for landfilling, into a valuable, heavy-wall plastic profile (see PE, June 1988, p. 39, and Feb. 1990, p. 33). This system eliminates the need for sorting plastic by type. However, if the community sees value in sorting, the remaining mixed plastic can be processed into Syntal (synthetic alternative) profiles. Syntal is durable, long lasting, and maintenance free.

The four main components of the fully automated ET-1 system are the extruder, molding unit, part extractor, and control panel.

The extruder is specifically designed to maximize melting and minimize degradation, so it can be used to process a wide range of plastics. A short, adiabatic screw rotating at high RPM is used for melting. The short melt history prevents degradation of sensitive resins and the high RPM permits thorough mixing of recycled materials. The extruder stops after filling each mold and starts only when another empty mold is positioned for filling.

Melt temperatures of 360 [degrees] F to 400 [degrees] F are regulated by a cooling fan, water circulation lines, RPM adjustment, or by adjustment of the tolerance between the extruder screw and the barrel. Higher melt plastics such as PET and polycarbonate, and "unmelts" such as copper and aluminum, become encapsulated in the melted plastic and, if used in the proper ratio, act as fillers that enhance specific properties.

The molding unit consists of linear molds mounted on a turret that rotates through a water cooling tank. The molds are interchangeable, so they can be used to meet specific production requirements. Inexpensive custom molds can be built using standard seamless steel shapes.

The extraction unit. Shrinkage of the part as it cools in the mold permits air ejection from the open end of the mold. The pneumatics of the ET-1 system require a 140-psi compressor with a 100-gal tank. Following ejection, the part is delivered into a covered receiver and then discharged onto an open shelf for manual removal. The part must be completely cooled on a rack made of continuous slotted angle iron to prevent warpage or deformation.

The ET-1 is designed to run 100% PCPW. Rigid plastic must be ground into 0.25-in chip or flake. Flexible plastic, film or thin sheet, must be densified into small granules to maintain friction in the extruder. The regrind and granules are blended to ensure an even dry mixture of feedstock. During this blending, plastic modifiers, additives, colorants, or feedstock may be added. The blended feedstock is automatically conveyed as needed by an auger to the extruder hopper.

Plastics Sources

Plastics from the municipal solid waste stream is the material of choice because of its zero or negative cost. A municipality supplies the plastics at no charge to the collector and eliminates paying tipping fees, landfill depletion costs, and sortating charges. However, its availability depends on local recycling practices. Where the ET-1 is installed, local authorities adopt mixed curbside or voluntary recycling in place of landfilling or incineration. Also, the natural combination of mixed plastics in household waste produces a Syntal profile that is suitable for most popular end-uses.

In addition to PCPW, a good source of material is the scrap produced by the packaging industry. Much of this material is coextruded and cannot be processed on conventional equipment. Similarly, plastic laminated on paper or aluminum can be reprocessed only on machines like the ET-1.

Another good source is the automotive industry. Chromium-plated ABS and polypropylene, which cannot be handled by the conventional recycler, are entirely suitable for the ET-1 process. Car batteries are now made largely from polypropylene; scrap batteries go to the lead industry for metal recovery, and after acid neutralization, the plastic is also suitable for ET-1 processing.

The medical and surgical industry is another good source for material. In the interest of public health and safety, in-house recycling of this very high-grade scrap is usually not allowed. Sterilized medical and surgical waste is also suitable after removal of sharp metal.

Companies recycling PET and HDPE or buying film for repelletization are frequently obliged to take a percentage of scrap that is too highly contaminated for their use or that contains polymers that they cannot handle. Such material is passed on. Electrical cable scrap is another worthy source of raw material.

What the ET-1 Can


The ET-1 Recycler's patented short adiabatic extruder screw was designed to handle as wide a variety of materials as possible. In principle, the machine will handle any thermoplastic resin, but there are technical limitations imposed by the processing and physical properties of the available plastics. The following guidelines for the most popular plastics may prove useful.

* Low-density polyethylene (LDPE or LLDPE): A material of choice for the process. However, LDPE is relatively soft, and products containing too much of it may be insufficiently rigid for some applications, particularly in thin sections. It should be mixed with stiffer materials such as HDPE or PP.

* High-density polyethylene (HDPE): A material of choice for the process. HDPE is stiff and its mixtures with LDPE give a range of stiffnesses that cover most product requirements. Much of the HDPE on the market is copolymer material - but this is of no consequence to the recycler because for recycling purposes, its performance is very similar to that of the homopolymer.

* Polypropylene (PP): A material of choice. It is relatively stiff and its mixtures with LDPE also cover most of the range of stiffness requirements. However, the use of more than 30% by weight of homopolymer PP is ill-advised because it is brittle at low temperatures and it is difficult to nail.

* Polyvinylchloride (PVC): PVC waste, finely ground and well homogenized, can be recycled on the ET-1. It can be mixed with other thermoplastics up to 50% by weight. PCPW normally contains less than 5% PVC.

* Polystyrene (PS): PS can be handled up to 40% by weight. Impact grades add toughness to the mix. Non-impact (crystal) material tends to cause surface finish problems. Expanded polystyrene (EPS) should be avoided as a foam because of its low bulk density. Tests at Rutgers University (see PE, Feb. 1990, p. 33) show considerable strength improvement at 10% to 40% levels of densified EPS.

* Acrylonitrile-butadiene-styrene (ABS): A material of choice for the process. The ABS family of resins combines rubbery and plastic properties, is extremely tough, and processes easily on the ET-1 system. The relatively high cost of the basic polymers limits their availability; an exception is chromium-plated ABS, which is very difficult to recycle in any other way.

* Nylon: A wide variety of nylons are on the market. The most common, 6 and 6/6, are both useful additives at up to 10% by weight as they can impart a desirable stiffness to an otherwise soft compound. Textile nylon scrap is usually 6 or 6/6. Nylon 6 castings are suitable. Nylons 11 and 12 are even more suitable, but they are difficult to find at an economical price.

* Polyethylene terephthalate (PET): Although its 500 [degrees] F flow temperature is considerably above the normal range of the ET-1, up to 15% PET can be mixed in if finely ground and carefully blended. PET beverage bottles, with HDPE base cups, labels, and aluminum closures, have been run at 100% in the ET-1. However, the product is brittle, which is attributable to crystallization caused by slow cooling of the thick sections and to degradation of the polymer caused by moisture content.

* Plastics fraction of city garbage: The plastics fraction can be removed from city garbage by separate collection or by any of a number of processes, of which the most widely used are air separation, mechanical separation, and float-sink. This fraction generally contains 50% to 60% PE; the remainder is an admix of plastic and nonplastic contaminants, mostly paper and aluminum. This mix is in fact a very suitable feedstock for the ET-1 and will always be available at a lower cost than any other raw material.

The most usual contaminants in plastic waste are paper, aluminum foil, and textiles. Paper, in limited proportion, copper, and aluminum foil, in some cases up to 30% by weight, present no problem. Textiles have a variable effect. Nylon and the much less common PP fiber are good raw materials. Polyacrylonitrile fiber and wool tend to decompose and generate acrid fumes. Cotton presents no problem except for scorching.

The most common contaminants in city waste, paper and adherent food particles, usually present no problem. The extruder operates at a sufficiently high temperature (360 [degrees] F to 400 [degrees] F) to obviate any bacterial problems; thus the product is sterile.


Color masterbatches, preferably granules, are mixed with the raw material in the auger before extrusion. Pigment quantities vary with the amount of color present in the raw material and with the depth of shade required in the finished product. Only if raw material is available with no pigment or printing should white or pastel colors be attempted. Products that are molded can be easily painted.

PHOTO : The ET-1 system for recycling post-consumer and industrial plastic waste into Syntal, "synthetic alternative."

PHOTO : Fence post and beams made of Syntal. Rot-proof Syntal can be placed directly into the soil.

PHOTO : Beams to support fencing. Normal carpentry techniques can be used on Syntal, and it can be painted.

PHOTO : Syntal planters have the appearance of wood, but they are weatherproof and will last indefinitely.
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Title Annotation:Recycling
Author:Maczko, John
Publication:Plastics Engineering
Date:Apr 1, 1990
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