Recycling Conference Shows Off New Ways To Enhance Recycled Plastics

News from the recent SPE Global Plastics Environmental Conference in Detroit includes lots of technological “firsts.” Among them: electronic “noses” to sniff out chemical contaminants in reclaim; long-fiber molded parts made with auto shredder waste and recycled carpets; the U.S. debut of a waterless recycling system commercialized by Germany’s Green Dot program; and new additives to enhance recycled PET bottles. 

News from the recent SPE Global Plastics Environmental Conference in Detroit includes lots of technological “firsts.” Among them: electronic “noses” to sniff out chemical contaminants in reclaim; long-fiber molded parts made with auto shredder waste and recycled carpets; the U.S. debut of a waterless recycling system commercialized by Germany’s Green Dot program; and new additives to enhance recycled PET bottles.


The ‘e-nose’ knows

Electronic sniffers that can identify and quantify airborne molecules have been used since the 1980s but not in recycling—until now. While not as sensitive as a canine or even human nose, “e-nose” devices are consistent, objective, and immune to any harmful chemicals they may sniff.

Their potential in plastics recycling was presented for the first time by Eric Koester, a consultant for MEGO Consulting, who presented a list of suppliers, web sites, and basic technologies. Some two dozen suppliers globally have e-nose technology, though some are university research departments and others do not offer the instruments commercially E-noses are effective at screening for one or two specific chemicals, not for every chemical contaminant. They can check whether reclaimed containers have been used to hold gasoline or other specific chemicals that would be undesirable contaminants in recycled plastics.

Most e-nose devices are used in the security industry to detect explosives, drugs, or anthrax. Hospitals use them to diagnose pneumonia from breath samples. And the food and cosmetics industries use them to check freshness and fragrances. Resin producers use e-noses to check pellets for polymer blend ratios and possible contamination. Other plastics applications include testing the effects of barrier packages on flavors and fragrances.

At least three e-nose makers have experience with plastics applications. Units range from $5000 for hand-held devices measuring gases in parts per thousand up to $100,000 for industrial-size lab units capable of measuring parts per million.

French-based Alpha M.O.S. makes large lab systems, modular systems for on-line analysis, and hand-held units. Pellet or flake samples are heated in a jar, and the sensor “sniffs” the air in the head space. Alpha M.O.S. has over a dozen plastics applications in the U.S., including uses in QC by resin producers and recyclers of plastics for packaging.

Cyrano Sciences has made hand-held e-nose units commercially for two years. Cyrano has four plastics applications, including QC testing of virgin PET and HDPE for bottles.

Illumina Inc. is developing a fiber-optic “bead array” to detect chemicals in plastics. Illumina is working with Dow Chemical Co. and Chevron Phillips, but doesn’t have a commercial product for plastics yet.


Long fibers toughen ASR

Two companies reported that long-glass fibers can boost properties of recycled polymer blends. Salyp NV, a Belgian engineering firm (see PT, May ’01, p. 54) with a proprietary recycling process for auto shredder residue (ASR), presented data on the first U.S. molding trials using its ASR-PP. Mayco Plastics Inc. of Sterling Heights, Mich., a Tier One supplier to DaimlerChrysler, used the reclaimed PP to mold a 3-lb automotive battery-support bracket. Mayco used its own patented long-fiber compounding and molding process to incorporate half-inch glass fibers at 40% by weight.

Salyp’s U.S. sales rep, 21st Century Polymers & Associates, expects that PP recovered from ASR will cost 15¢ to 20¢/lb, depending on the scale of production. That’s a discount of 50% or more from the current price of virgin PP. 21st Century will market ASR resins produced by any user of Salyp equipment worldwide. 

Even without process optimization, Mayco’s initial molding trial made parts that reportedly had more than 75% of the structural properties of fiber-reinforced virgin PP. ASR-PP is an amalgamation of many PP grades found in cars, appliances, and other durables. The resulting blend has an MFR of 10.4 g/10 min, density of 0.94 g/cc, tensile strength of 2350 psi, flexural modulus of 80,578 psi, and Gardner impact strength of 160 in.-lb. After adding long glass, Mayco’s test parts had a density of 1.15 g/cc, tensile strength of 20,300 psi, flex modulus of 1.087 million psi, and notched Izod impact strength of 3.4 ft-lb/in.

Salyp’s PP recovery system starts with size classification of raw ASR via two large trommels, or screeners, one on top of the other. These units sift ASR at 19,800 lb/hr into four size fractions: under 6 mm, 6 to 16 mm, 16 to 38 mm, and over 38 mm. The two smallest fractions contain ferrous fines, glass, textiles, and organic matter. After extracting the metal, the rest can be compressed into fuel briquettes for steel or cement production.

The largest size fraction consists of plastic chunks and urethane foam. The two are separated by a compressing roller developed by Central Materials Handling in Peoria, Ill. It compresses the foam but not solid plastic. When the compressed foam is released after passing under the roller, it springs back, jumping high enough off the conveyor that it can be mechanically separated from the solid plastic at about 3300 lb/hr.

Solid plastics, together with the 16-to-38 mm fraction, go together into an integrated three-step separation process. The first step separates plastics, metals, and wood from fibers and textiles. The second step shreds the plastics, metal, and wood to 16 to 25 mm at about 4400 lb/hr while removing ferrous metals and light materials. The third step optically identifies nonferrous metals and wood and removes them with air jets from the plastic at 6600 lb/hr. The plastic is then washed using bacteriological cleaners. Sludge is extracted from the water, which is recirculated.

The next step is the core of Salyp’s process: thermal separation by resin type (PP, PE, ABS, PS, and PC). Chips are preheated on a vibrating table under infrared heaters and then spread one-layer thick on a conveyor. They pass under additional IR banks, which heat the chips to the Vicat softening point of the lowest-temperature resin in the mix. The chips then go under a roller with grooves that trap only the softened pellets, which are then scraped off by a brush.

Remaining chips are heated to the Vicat temperature of the next highest-temperature resin, removed by a grooved roll, and so on. The thermal process can separate up to 3300 lb/hr. Salyp’s full-scale pilot line in Belgium can recycle 66 million lb/yr of ASR, yielding about 20% of that weight in reclaimed resin. Metal contaminants are also recovered. Salyp hopes to sell its equipment to auto shredders in the U.S., Europe, Japan, and China.

Meanwhile, researchers at the Georgia Institute of Technology in Atlanta reported on test results with another mixed-polymer recyclate—obtained from post-consumer carpet. The mix is primarily nylon 6 from the face fiber, together with PP from backing fabrics, calcium carbonate-filled SBR latex from binders, and lots of contamination. Georgia Tech studied the use of styrenic block copolymers to compatibilize the different polymers in the blend and added long-glass fiber to boost strength and rigidity.

The test material came originally from Wellman Inc., Shrewsbury, N.J. Wellman sorts post-consumer carpet by face fiber, shreds the carpet so that much of the latex, calcium carbonate, and dirt drops out, and then bales the remaining fibers.

Georgia Tech palletized these fibers on a special vented single-screw extruder with a crammer feeder and in-line shredder. This model NGR A-Class Type 55 VSP was built by Next Generation Recyclingmaschinen GmbH in Austria (which has several U.S. distributors). Barrel temperatures are set from 392 F at the feed throat to 500 F at the downstream end in order to melt both the PP and nylon 6. After filtering with a 20-mesh screen, the pelletized compound contains 64% nylon 6, 11% PP, 2% nylon 66, and 23% calcium carbonate.

The pellets were dried and molded into test plaques, which had tensile strength of 5970 psi, flexural strength of 9460 psi, flex modulus of 324,000 psi, and elongation at break of 3.3%. Pellets were ground into powder and compression molded together with 30% long-glass mat into 1-ft-square plaques. These samples had flexural strength of 16,000 psi, flex modulus of 560,000 psi, and elongation at yield of 5.9%.


‘Dry cleaning’ plastics

Duales System Deutschland, the German government office that oversees the “Green Dot” system for recycling packaging waste, has set up a subsidiary to market its proprietary technology. Called Systec, the new commercial unit is introducing to the U.S. its Waterless Mechanical Purifier, first built for in-house use in 1997.

Systec has sold 23 units since then, mostly in Germany and Asia, for cleaning post-consumer bags and agricultural film. One is in Calgary, Alberta, removing labels and liquid residue from PET bottle flake. Subsequent water washing is still needed to remove glue.

Systec doesn’t disclose the inner mechanics of its waterless purifier, except to say that it uses a rapidly rotating centrifuge to combine impact and acceleration forces with a powerful pneumatic air stream. Throughput from one unit ranges from 2000 to 4500 lb/hr, depending on resin. Usable output is 10% to 20% less, depending on contamination level. The waterless purifier costs around $133,000. Systec also sells its NIR sorting technology to identify and separate containers by polymer. It has sold three units.


Upgrading RPET

The SPE conference also presented a novel additive to upgrade recycled PET bottles. Clariant Masterbatches launched a new family of reactive additives that impart long-chain branching, which raises the I.V. of RPET without the need for solid-stating in a reactor. The chain extender, called CESA-Extend (see PT, Apr. ’03, p. 15), restores molecular weight to the original level or higher without hurting mechanical or thermal properties or crystallinity.

Conventional chain extenders require predried PET to be heated under high vacuum in a solid-state reactor for several hours. They can produce gels from uncontrolled chain branching. CESA-Extend doesn’t require predried PET or vacuum and can be processed in a conventional twin-screw extruder. It is also said to produce fewer gels.

Clariant says its new additives have a further advantage in that they improve compatibility of RPET with other polyesters, PC, and nylon in subsequent compounding. The active ingredient is a proprietary functional acrylic oligomer from Johnson Polymer LLC in Sturtevant, Wis., which has joint patents on the technology with Clariant.

It reportedly works on any polycondensation resin, including PC, nylon, acetal, TPU, PBT, and blends of the above, but is more efficient in PET, less so in nylon. It reacts with end groups such as amines, isocyanates, carboxylates, and hydroxyls.

The additive improves melt strength so much that PET can be extrusion blow molded and even extruded into blown film. Extruding virgin undried PET (0.75 I.V.) plus 3% of CESA-Extend with no vacuum on the extruder reportedly yields PET with 0.780 I.V. The same PET extruded without chain extender drops to 0.543 I.V. Recycled PET with 0.558 I.V. plus 2% CESA-Extend yields 0.673 I.V.; 3% yields 0.720 I.V.

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