Polymer Scrap

PET producers have had to deal with recovery of scrap polymer since the 1950s, particularly when raw materials were rather expensive. Glycolysis has been and is practiced within the production setting on material of known composition and acceptable purity. Methanolysis was practiced in the United States and Europe until about 1980 as most PET production was then based on DMT and EG. Methanolysis is still practiced on post-consumer and pre-consumer X-ray film by the Eastman Kodak Company which uses the DMT and EG to make more film. Other than an occasional use of glycolysis, no other large-scale, on-going commercial use of post-consumer PET is made in developed countries. 

In the case of condensation polymers such as aromatic polyesters, there are several ways of recovering starting materials from scrap polymer (post-industrial and post-consumer) rather than the polymer itself. It is therefore possible to isolate purer materials than would be the case with attempting to recover a single polyester, and even contaminated and degraded scrap polymer can be utilised. 

Yarahmadi N, Jakubowicz I, Gevert T. Effects of repeated extension on the properties and durability of rigid PVC scrap. Polym Degrad Stab 2001 73 (l) 93-99. 

The flexible foam market is the largest global pol5mrethane consumption. In the United States and Canada a total of 1114 kt/a was used in flexible foam applications in 1998. Flexible slab foam is used predominantly in furniture, carpet underlay, and bedding molded foam is used extensively in transportation. Carpet underlay is manufactured from either virgin or scrap polym-ethane foam, which is combined with a binder adhesive. 

When the forward and reverse reactions have equal rates, namely when polymerization-depolymerization propagation rates are equal, the concept of ceiling and floor temperatures arises. Most polymerizations have ceiling temperatures, temperatures above which the monomer cannot be polymerized, but the polymer will spontaneously depolymerize back to the monomer. Commercially this fact leads to an important method of polymer recycling whereby scrapped polymer is heated under anaerobic conditions to allow distilling off the resultant monomers. 

A further outstanding property of poly(methyl methacrylate) is its good outdoor weathering, in which respect the material is markedly superior to most other thermoplastics. After several years under tropical conditions the colour change is extremely small. When poly(methyl methacrylate) is heated above the glass transition temperature (105 C) it becomes rubbery and sheet material is easily manipulated at 150—160 C. Above about 200 C decomposition becomes appreciable and at 350—450 C a nearly quantitative yield of monomer is readily obtained. Thus the recovery of monomer from scrap polymer is a feasible proposition. 

With respect to current technology, there are significant technical barriers associated with every process link in the commercial infrastructure, fi om the source of scrap polymer to utilisation of the recovered material. Most importantly, the economic ving force today is insufficient (or not adequately defined) in relation to potentially large arisings of scrap from brown, white or automotive products, to attract entrepreneurs or to encourage those in related businesses to extend their operations to include used plastics. 

An important newer use of fluorine is in the preparation of a polymer surface for adhesives (qv) or coatings (qv). In this apphcation the surfaces of a variety of polymers, eg, EPDM mbber, polyethylene—vinyl acetate foams, and mbber tine scrap, that are difficult or impossible to prepare by other methods are easily and quickly treated. Fluorine surface preparation, unlike wet-chemical surface treatment, does not generate large amounts of hazardous wastes and has been demonstrated to be much more effective than plasma or corona surface treatments. Figure 5 details the commercially available equipment for surface treating plastic components. Equipment to continuously treat fabrics, films, sheet foams, and other web materials is also available. 

The chemical resistance and excellent light stabiUty of poly(methyl methacrylate) compared to two other transparent plastics is illustrated in Table 5 (25). Methacrylates readily depolymerize with high conversion, ie, 95%, at >300° C (1,26). Methyl methacrylate monomer can be obtained in high yield from mixed polymer materials, ie, scrap. 

Most off-quahty or scrap polypropylene fibers may be repeUetized and blended in small percentages with virgin polymer to produce first-grade spunbonded fabrics. The economics are of great importance in a process where high yields are required in order to be competitive. Some manufacturing equipment direcdy recycles edge-trim back into the extmder where it is blended back into the polymer melt (see Fibers, olefin). 

Polymers based on trimellitic anhydride are widely used in premium electromagnetic wire enamels requiring high temperature performance. Several types of trimellitic anhydride-derived polymers are used as wire enamels poly(amide—imide)s (133), poly(ester—imide)s (134), and poly(amide—imide— ester)s (135). Excellent performance characteristics are imparted by trimellitic anhydride-based polymers for wire enamel requirements of flexibiUty, snap, burnout, scrap resistance, heat shock, and dielectric strength. 

As with all thermoplastic elastomers, the copolyesterethers can be processed as thermoplastics. They are linear polymers and contain no chemical cross-links, thus the vulcanisation step needed for thermosetting elastomers is eliminated and scrap elastomer can be re-used in the same process as virgin material (176—180). 

Nippon Zeon estimated that the break-even cost of its tire pyrolysis pilot plant was 0.25 per tire (29,30). One study indicates that pyrolysis of tires and other polymers should be considered as a means for disposing of scrap within environmental constraints. A plant processing 81,000 t/yr of scrap could be profitable, based on sales of reclaimed products (31). 

In the depolymeri2ed scrap mbber (DSR) experimental process, ground scrap mbber tines produce a carbon black dispersion in ok (35). Initially, aromatic oks are blended with the tine cmmb, and the mixture is heated at 250—275°C in an autoclave for 12—24 h. The ok acts as a heat-transfer medium and swelling agent, and the heat and ok cause the mbber to depolymeri2e. As more DSR is produced and mbber is added, less aromatic ok is needed, and eventually virtually 100% of the ok is replaced by DSR. The DSR reduces thermal oxidation of polymers and increases the tack of uncured mbber (36,37). Depolymeri2ed scrap mbber has a heat value of 40 MJ/kg (17,200 Btu/lb) and is blended with No. 2 fuel ok as fuel extender (38). 

Tile is manufactured ia several ways. la each method, a coatiauous sheet is formed gauge refinement and planishing are carried out ia subsequeat caleaderiag steps. Stresses that could lead to poor dimensional stabiHty are avoided. The efforts to preveat stresses are governed by formulatioa, stock and roU temperatures, conveyor speeds, etc. After the final calendeting, a resia—polymer—wax finish is appHed to the surface of the sheet which is thea buffed before it moves to the puach press. Frame scrap and tile rejected because of defects are returned to the mixers and recycled. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Copolymer technology is progressing along two "fronts." First, new appHcations for copolymers are being found to increase the volume of materials that are already commercially available. One example of this is the rapid growth of styrenic block copolymers sold as asphalt (qv) and polymer modifiers over the past 10 years (Fig. 7). Another is the increased interest in graft and block copolymers as compatihilizers for polymer blends and alloys. Of particular interest are compatihilizers for recycled polymer scrap. 

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