Separation or compatibilization? Alternative recycling solutions emerge.

For recycling mixed or contaminated plastics waste, you can either separate the different ingredients or find some way to compatibilize the mixture so that it results in useful properties. New ways to accomplish both were presented at the SPE's Fourth Annual Recycling Conference, held recently in Chicago. Another paper at the conference showed that in the case of paint removal from automotive plastics, a certain amount of contamination may be allowed without sacrificing valuable material properties.

GIVE SEPARATION A WHIRL

Delamination of hard-to-separate composite or multi-component materials without use of heat or volatile solvents is said to be possible with a new process developed by Result Technology AG of Mammern, Switzerland. The company's newly designed accelerator separates plastics from other plastics, metals, or paint, says general manager Christoph Muther. The material is first crushed or shredded, then transported pneumatically to the bottom-fed vertical accelerator. The accelerator has a spiral rotor and hardened inner paneling, which subjects the material to repeated high-speed acceleration. As Muther explains it, the different materials in a laminate or composite accelerate differently, creating high shearing forces between them that literally tear the laminates apart at the adhesive boundaries between layers.

After this delamination, the accelerator causes metals to clump together in globules, while plastics form flakes, chips, or cubes. When the delaminated materials reach the top of the accelerator, they are air-conveyed to a cyclone for dedusting and then to sieving and fluidized-bed separation units. An electrostatic separator can be used to carry out final separation if needed, Muther says.

The first U.S. plant using this technology is being built through a joint venture between Aerodynamic Recycling Technology (ART) Corp., Spokane, Wash. and M.R. Capital, Chicago, and is scheduled to come on line by midyear. The 4-ton/hr plant will initially recycle automotive bumpers and perhaps also cable scrap.

MIXED MARRIAGES CAN WORK

Dissimilar or incompatible materials that appear together in a waste stream can be difficult to separate but usually result in lower mechanical properties if reprocessed together. However, the use of compatibilizers and impact modifiers can strengthen the mechanical properties of mixed plastics waste, giving some virgin-resin-like qualities, say Darilyn H. Roberts and Robert C. Constable of Uniroyal Chemical Co. Inc., Middlebury, Conn. The authors mixed virgin resins in combinations typically found in post-consumer or post-industrial waste streams, such as nylon and PP from carpet waste, PE and PET from bottles, PC and PMMA from automotive scrap, and PP and wood flour from construction waste, pallets, and tree trimmings.

These mixtures were blended with three of Uniroyal's commercial modified polyolefins: Polybond 3150, a maleated homopolymer PP; Royaltuf 372P20, a SAN-grafted EPDM; and Royaltuf 485, a maleated EPDM. Use of 5% Polybond in three nylon 66/PP blends (80/20, 50/50, and 20/80) improved the flexural and tensile strengths, as well as the notched Izod impact (the latter was better for the 80/20 and 50/50 blends than for nylon alone). Maleated PP also improved processing by reducing die swell and increasing melt strength.

Similar results were obtained when comparing nylon 66 toughened with 20% maleated EPDM against an 80/20 blend of the toughened nylon and PP, as well as 5% maleated PP. Toughness of the nylon and the blend were similar.

PET and PE are typically found together in bottles but are incompatible and result in poor mechanical properties if reprocessed together. Adding a combination of SAN-grafted EPDM plus polycarbonate could eliminate the need to segregate PE from PET, the authors claim. The modifier blend greatly enhances room-temperature impact strength of PET/PE - much more so than does grafted EPDM or PC alone.

PC/PMMA blends are reasonably compatible, although they exhibit poor impact properties. Addition of up to 30% SAN-grafted EPDM improves the notched Izod value though it decreases the tensile and flexural properties.

Wood flour imparts low density to PP, along with high specific strength and modulus, low abrasiveness, and improved dimensional stability. But the two materials are not chemically reactive, so problems of poor surface wetting, poor dispersion, and poor premix uniformity can arise. Wood flour also has high water absorption and poor stability at temperatures above 392 F. Use of 5% maleated PP reportedly alleviates some of these problems while improving the blend's tensile and flexural strengths and flex modulus. Notched Izod, however, is reduced by the modifier.

IS PAINT REMOVAL NECESSARY?

The growing use of TPOs in automotive bumper fascias has Ford Motor Co. thinking about potential obstacles to post-consumer recycling of these large components. In particular, Ford addressed the question of how much paint can be allowed to remain on recycled TPO bumpers without sacrificing desired mechanical properties. Previous studies have shown that residual paint can degrade impact strength, but it may not always be economically sensible to achieve 100% paint removal.

Amos Golovoy and John Sullivan, technical specialists at Ford's Scientific Research Laboratories in Dearborn, Mich., reprocessed mixtures of painted and unpainted TPOs to mimic different degrees of paint removal. Typically 3% of the weight of a painted TPO fascia is the paint itself. Hence, no paint removal results in 3% paint contamination in reground TPOs. (However, some paint particles would be removed if a screen pack were used to repelletize the scrap.) Ford's test samples contained 0-3% paint by weight. Ford's stated goal of using 25% post-consumer recycled material would result in 0.75% paint if 25% reground fascias with no attempt at paint removal were combined with 75% virgin resin.

Paint contamination had no effect on elongation at yield, but it did reduce the notched Izod impact strength at room temperature and 0 [degrees] C, though all samples remained ductile. At -40 C, all samples became so brittle that paint had no effect. At the two higher temperatures, the Ford researchers noted, very little toughness was lost at paint contamination levels up to 0.3% by weight. They concluded that if 25% painted regrind were used with virgin, then as much as 10-20% of the paint could be allowed to remain on the post-consumer material. If a repelletizing process were used (rather than merely dryblending virgin and regrind), enough paint might be removed in grinding and extrusion to perhaps require only 25-40% paint removal from the recycled fascias instead of 80-90%.

The researchers also noted that tensile and flexural strengths and moduli were unaffected by paint. "Recycled material from painted TPO bumpers could possibly be used in automotive applications with lesser performance demands than bumper fascias," they concluded.