Custom screenchanger clears hurdles to recycling clear bottle-grade PVC.

Melt filtering vinyl reclaim may not be new, but Occidental Chemical Corp. of Dallas has a fresh twist on the technology. For processing a new recycled bottle-grade PVC, OxyChem relies on a specially customized screenchanger to overcome contamination hurdles that sorting and washing alone just couldn't clear.

The resulting clear bottle-grade resin, EcoVinyl Natural 25, consists of 25% post-consumer reclaim compounded with 75% virgin OxyClear 161J.

TWO-YEARS IN THE MAKING

Though fully commercial now, Natural 25 spent two years in development due to the difficulties in adapting existing screenchanger design to an unfamiliar application. In the past, notes OxyChem commercial development director Bill Carroll, most PVC melt filtration involved building products and used fixed screens. "They only provided a final check to make sure nothing really large got by," says Carroll.

With bottle-grade, by contrast, filtration has taken on a more essential role. More so than some other recycled materials, bottle-grade vinyl exhibits a sensitivity to contamination--mostly from PET bottles and various label materials in this particular case. Even with feedstock management capabilities that bring contamination levels below 500 ppm, the melt still needed a very thorough final removal of contaminants, according to Carroll.

As for the choice among screenchanger designs, Carroll says the feeling was that a rotary-wheel type would provide a shorter development path than either continuous-screen or slide-plate varieties. "We initially had some concern about a slide-plate's ability to move in and out of the melt as often as we knew it would have to," he says. Still OxyChem says these other designs have not been ruled out.

At the same time, a more intrinsic sort of difficulty arose because the material's flow characteristics had not been well understood in the context of a rotary wheel screenchanger. Carroll explains that bottle-grade PVC's flow characteristics don't let it "generate enough turbulence to wash the melt channel walls." As a result, the reclaimed vinyl had a tendency to hang up and degrade in an unmodified screenchanger. So, the primary design changes consisted of tailoring the melt flow channels and breaker-plate holes to the specific flow of the bottle grade. "We had to eliminate the possibility of any dead spots," says Carroll. To come up with the customized rotary-type filter, OxyChem teamed up with Gneuss Inc. of Langhorne, Pa.

ECOVINYL NATURAL 25 FOR BOTTLE BLOW MOLDING
Physical Properties
Appearance                           Clear Pellets
Bulk Density, lb/cu ft               50
Volatile Matter, %                   0.30 Max.
Specific Gravity                     1.31-1.33
Shore D Hardness                     79-81
Heat Deflection Temperature
Unannealed, F                        132.8-136.4
Tensile Strength, psi                6000-6500
Tensile Modulus, psi x |10.sup.5~    3.20-3.35
Flexural Strength, psi               10,000-12,000
Flexural Modulus, psi x |10.sup.5~   3.5-3.7
Light Transmission,
% 33.5 mils
@ 400 nm                 60-80
Light Transmission,
% 33.5  mils
@ 700 nm                80-85
Notched Izod Impact, ft lb/in.
Gate                                 18-22
Vent                                 18-28
Average                              18-26

The good news about all this development work is that Natural 25 recently became fully commercial. It costs 67|cents~/lb, a price level comparable with similar virgin vinyls. Its physical properties are also much like its virgin counterpart (see table). And Carroll reports that roughly 20 of OxyChem's customers have started trials of the material.

SCALE-UP PROBLEMS

Yet despite the recent transition from lab to marketplace, the PCR-rich vinyl still suffers from troubles on the filter end; what worked in the lab on a one-inch extruder apparently didn't translate to the full-scale production size filters supplied by Gneuss. In fact as of press time, only one of two outside compounding firms slated to supply OxyChem with the resin has been able to do so. Both of them agree that the screenchanger design that worked in Oxy's lab simply didn't function when scaled up to production size.

HiTech Polymers of Erlanger, Ky., currently serves as the sole compounder for Natural 25. According to company president Jerry Miller, extensive modifications of Gnuess's screenchanger still had to be made in-house before bottle-grade reclaim could be successfully processed. Plus, the company needed to develop other proprietary processing procedures before the reclaim would run successfully on an 80-mm twin-screw extruder dedicated to Natural 25 production. "Clear bottle-grade PVC requires more finesse than siding or pipe," Miller notes. "Getting the excess fines out is a major problem."

Meanwhile, the second compounding firm has been unable to make Natural 25 for more than the year since it took possession of its screenchanger. That compounder, Interplast II of Galax, Va., found out during its first test runs that the screenchanger didn't work as designed. According to company spokesman George Weaver, dead spots and clogged breaker plates stopped production from the beginning. Early last month, however, Gneuss completed and shipped a redesigned, production-sized screenchanger that does address the bottle grade's unique filtration requirements, according to company v.p. Joseph Altamari. "Recycled bottle-grade PVC had never been done before," he explains. "We were breaking new ground."