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Recycling polystyrene, add value to commingled products.

Recycle Polystyrene, Add Value to Commingled Products

A number of organizations throughout the U.S. are conducting research on several aspects of the solid waste problem. The Center for Plastics Recycling Research (CPRR) at Rutgers University is dedicated to advancing technically and economically sound plastics recycling. Its charge is to perform the research, process development, and systems engineering necessary to derive appropriate environmental benefits from the recycling of all plastics to their highest economic values. The CPRR also disseminates developing technologies relative to plastics recycling. The development of necessary and effective plastics recycling systems, although well under way, is far from complete.

Soon after its establishment in early 1985, the CPRR research staff knew that an understanding of both the makeup of the plastic waste stream and the economics of the plastics industry was essential to determining the most logical approach (or approaches) to plastics recycling. The economics of recycling indicate that mixed recyclable materials should be collected at their sources (residences). Various recyclables can be economically separated and prepared for resale at a material recovery facility (MRF). A strong and growing market exists for both polyethylene terephthalate (PET) soda bottles and unpigmented high-density polyethylene (HDPE) bottles, which are recycled in a resin recovery system. However, collection projects at the CPRR indicate that when these items are specified for recycling, a significant percentage of other types of plastic containers is also collected. These other types of containers, of mixed resin types and pigments, are known as commingled plastics and usually constitute a waste stream emanating from the MRF operation. One of CPRR's goals is to aid the development of technology needed to produce new and useful plastic products from this stream.

The CPRR has focused much of its technological development on producing, from a mixture of post-consumer waste plastics known as "New Jersey curbside tailings" (NJCT), molded linear profiles that have consistent mechanical properties. NJCT consists of unwashed and unseparated commingled plastics that have been collected from recycling programs in New Jersey and "mined" of the more valuable PET and unpigmented HDPE soda bottles. The mixture is composed largely of polyolefins; its extrusion into linear profiles results in products of a low-modulus, relatively flexible nature.

Investigations revealed consistent mechanical properties of products fabricated from such unwashed mixtures. Earlier studies had also shown that mixtures of virgin polystyrene (PS) and HDPE exhibited improved mechanical properties in comparison with those of PE. The CPRR staff hypothesized that the addition of PS to the NJCT mixture, which is largely PE, would enhance the mechanical properties of the post-consumer commingled plastics. They then devised a series of experiments to test this hypothesis.

In some of the experiments, the CPRR combined repelletized PS foam plant regrind with the NJCT mixtures. These experiments showed that PS could be readily incorporated into the NJCT mixtures and that the result of this incorporation is a material with improved mechanical properties. The results and implications of these experiments are the main focus of this article, which begins with a brief overview of the operation of a material recovery facility.

Collection and Sortation

It is the CPRR's position that the volume of collected recyclables is best maximized by regularly collecting mixed recyclables from households. Mixed, multimaterial recyclables include newspaper, glass, aluminum, tin cans, and plastic containers. However, only about 80% of the plastics mixture that people place curbside for collection comprises materials that recycling programs typically specify as recyclable plastics: unpigmented HDPE and PET soda bottles. The ideal collection method involves placing bundled newspapers at curbside, next to containers of trash and other mixed, multi-recyclable materials, all of which are picked up on the same day, but by different trucks. The newspapers and other mixed recyclables are loaded into two separate bins on a truck, which takes them to a material recovery facility (MRF) for separation and preparation for their distribution to reclaimers.

At a typical MRF, newspapers from the collection vehicles are emptied onto a belt and then taken directly to a baling machine and prepared for shipment. Mixed recyclables are then emptied onto a belt that leads them through a series of separation steps. A magnetic separation step removes tin-plated steel cans and other ferrous materials, and is followed by an eddy-current separation step that removes aluminum and other nonferrous metals. Glass and plastic containers are then separated according to density through the use of a chain curtain on an inclined plane. Glass is hand-separated according to three different colors, and plastics are hand-separated according to three categories: unpigmented HDPE; clear and green PET, which is sold directly to reclaimers; and the curbside tailing plastics, which are deposited in landfills.

Commingled Plastics

Plastic containers are manufactured from various types of polymers, including HDPE, PET, polyvinylchloride, polypropylene, PS, and multilayer and multimaterial configurations. Some industries have standardized the package material used for particular products. Plastic milk bottles, for example, are made from natural (unpigmented) HDPE; plastic carbonated beverage bottles, from PET (one-piece bottles) or PET/HDPE (two-piece bottles). Visual identification of these containers is easy, which makes them relatively easy to segregate. However, plastic containers used for household cleaners, cooking oils, foods, motor oils, and other packaged goods are not easily identifiable. Such containers are of different package designs, materials, and types, and may be manufactured from a variety of polymers; their colors, shapes, and materials are often specified by manufacturers for specific applications. It would be difficult and expensive to separate them by resin type: the value of the resulting plastic would be less than that of generic resin materials, because of the varied pigmentations and additives that are used in manufacturing.

Curbside tailings are the mixed plastic containers that remain after the removal of the PET and unpigmented HDPE beverage containers. (The technology of producing products from them is known as commingled plastics processing technology.) From a polymer science perspective, such a diverse combination of plastics is not considered to be readily capable of "blending" into a compatible product. However, the mixture can be easily processed into large cross-section items that have significant strength and utility. To study the properties and processing of commingled plastic waste, the CPRR obtained a low-pressure molding unit, the ET/1 extrusion molding machine, from Advanced Recycling Technology, Ltd., of Belgium.

The ET/1 adiabatic extruder is capable of processing most types of densified mixed thermoplastic materials, including many heavily contaminated plastics wastes. It can mold, at a rate of 400 lb/hr, objects of any continuous or tapered cross section up to 6 by 6 inches, and up to 12 ft long. The machine is capable of making posts, poles, stakes, planks, slats, and other products whose lengths greatly exceed their cross-section dimensions, as well as products whose relatively large (1 inch or more) cross sections are either constant or tapered in only one direction.

Mechanical Properties

One category of feedstock material used in the ET/1 extrusion molding machine's production of samples is 100% NJCT. The other is PS material, obtained from Mobil Chemical Co.'s expanded PS recycling (regrind) operation and densified prior to shipping to the CPRR. The composition of mixed post-consumer waste plastics is expected to vary slightly because of the nature of the plastic waste stream. The variation affects the composition of the feedstocks studied to date, but it has been found to have little effect on the physical properties of the resultant molded products.

The product profile used for the mechanical testing of commingled plastics wastes was 6.4 cm square in cross section and 2.41 meters in length. The actual sample size used in testing was 6.4 cm square by 12.8 cm in length.

Initial experiments exposed property variations along the extruded part length, indicating that the ET/1 process is not yet optimized. Thus, the CPRR decided that to directly compare feedstock variations only, it would use only samples obtained from the center regions of the profile for property measurements.

The CPRR performed compression tests on an MTS model 810 mechanical tester. The crosshead speed for all measurements was 0.254 cm/min; all samples were tested until a strain of 15% was reached or until failure occurred. All testing was performed in such a manner as to conform as closely as possible to ASTM standards.


The Table presents the mechanical property data concerning commingled samples produced from the ET/1 extrusion molding machine. The CPRR determined each sample's compressive modulus from the slope of the best-fit straight line through the first few points of the stress-strain diagrams. It determined the compressive yield stress in the conventional manner except in instances of no discernible yield point; in such cases it used the 2% offset method. The CPRR defined compressive strength, for all samples except the 100% NJCT, as the maximum compressive stress carried by a sample during a compression experiment. For the 100% NJCT sample, it defined compressive strength as the stress level at a strain of 10%, since no maximum was observable at smaller strains. Excessive "barreling" occurs at strains above 10%, and results in artificially [Tabular Data Omitted] high stress values.

The typical modulus of all 100% NJCT samples tested up to this point is 90,000 psi, with only a few percent variation in the other properties. To simplify comparison with results obtained from other PS mixtures, the values presented in the Table represent the average for all of the 100% NJCT that were tested; they are not the result of a single test. Figure 1 shows a typical 100% NJCT compressive stress-strain curve.

The CPRR found that the addition of PS to the NJCT mixture dramatically increased the mechanical properties of the commingled product. Inclusion of an amount of PS as small as 10% by weight increased the modulus by 60%, from 90,000 to 144,370 psi. The addition of 20% PS by weight increased the modulus by 82%, the yield stress by 43%, and the compressive strength by 22% (see the Table and Figs. 1 and 2).

It appears that the addition of up to 35% PS can further improve all of the compressive properties that were tested, while sacrificing ductility. At the level of 35% PS, the modulus increased by 166% over that of 100% NJCT (90,000 to 239,000 psi); there were corresponding increases in yield stress and compressive strength of 83% and 56%, respectively (see the Table and Fig. 3). Above 35% PS, the modulus decreased slightly and appeared to level off around 220,000 psi as the trends in yield stress and ultimate strength continued upward (see the Table and Fig. 4). The CPRR plans to examine the 25%-to-50% range of PS composition in future research.

Figures 5,6, and 7 clearly illustrate the observed variations in mechanical properties that occur with changes in composition. They show, respectively, modulus, yield stress, and compressive strength versus percent PS.

As indicated previously, NJCT is composed mainly of polyolefins and therefore displays mechanical properties of a ductile, flexible nature. The addition of PS to the NJCT mixture greatly increases the mechanical properties of the base material. A possible explanation for this phenomenon is that PS, which is a glossy polymer at room temperature, reinforces the NJCT matrix in a manner similar to that of fillers that are used in composite materials, even though PS and polyolefins are generally considered to be incompatible.

It is interesting to note that by performing a simple law of mixtures calculation for modulus, utilizing the high value for PS's modulus of 480,000, the experimenter will find that the measured values for the NJCT/PS mixtures are higher than the calculated values up to about 20 wt% PS.


Currently, most recycling systems, even those that collect plastic containers, do not collect PS. However, PS should be readily available to recyclers in fairly steady and significant quantities from such sources as quick-service food restaurants. It can be expected that such sources would rather comply with recycling than see a ban on one of their main packaging materials.

It is therefore recommended that recycling systems begin to collect PS and other commingled plastics. Incorporating these materials into products such as those produced by the ET/1 extrusion molding machine allows value-added materials to be produced for some applications. Because of the promising results of this research, the CPRR is studying the feasibility of a new project that would examine a resin recovery type of recycling for curbside tailings.

PHOTO : FIGURE 1. A typical 100% NJCT compressive stress-strain curve.

PHOTO : FIGURE 2. Compressive stress-strain curves that reflect increasing weight percentages of PS in NJCT.

PHOTO : FIGURE 3. Addition of up to 35% PS further improved all compressive properties tested.

PHOTO : FIGURE 4. Above 35% PS, modulus decreased, unlike yield stress and ultimate strength.

PHOTO : FIGURE 5. Compressive modulus versus weight percentage of PS.

PHOTO : FIGURE 6. Yield stress versus weight percent PS.

PHOTO : FIGURE 7. Compressive strength versus weight percent PS.
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Author:Nosker, Thomas J.; Renfree, Richard W.; Morrow, Darrell R.
Publication:Plastics Engineering
Date:Feb 1, 1990
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