Stabilizers help process reclaim.

Stabilizers Help Process Reclaim

Recent research by GE Specialty Chemicals, Parkersburg, W. Va., demonstrates that adding phosphite antioxidants to a variety of resins preserves some of the physical and mechanical properties of those materials through several reprocessings.

With the spiraling price of oil directly affecting the cost of petrochemicals, and mounting public pressure for recycling of post-consumer plastics waste, forecasters see the creation of higher-quality regrind as a vital need in plastics' future. The hunt for better quality second-and third-generation materials has set additive manufacturers scurrying to find ingredients they hope will provide recycled plastic with mechanical and physical properties equal to those of virgin resin. Among the work that has been done toward this end is GE's extensive tests on its Ultranox 626 pentaerythritol diphosphite antioxidant.

WORKS WELL WITH PET

In the past, GE has touted Ultranox 626 for its ability to aid in recycling of PET, showing how it improves color stability and maintains constant viscosity and melt flow. This summer, the company released evidence that addition of the stabilizer to both the initial formulation and subsequent reformulations is beneficial to the recycling of other polymers as well. In a paper presented at an international recycling conference in Davos, Switzerland, last summer, GE advanced chemist Suzanne Dietz outlined some of the benefits of Ultranox 626.

"Upgrading recycled resins through proper stabilization may be the key to higher value products and better economic return for recyclers," Dietz said. "Use of these and other stabilizers should allow valuable products to be made from plastic materials that now end up in landfills after their first use."

HELPS IN HDPE RECYCLING

Tests GE performed on HDPE involved numerous extruder passes of both virgin and recycled resins. According to Dietz' findings, the use of 500 ppm Ultranox 626 in virgin resin ensured a consistent melt index of about 0.85 through five passes. After those same five passes, the yellowness index of the phosphite-stabilized virgin HDPE was -0.46 versus +9.04 for HDPE containing 500 ppm of a hindered phenolic antioxidant.

"Post-consumer HDPE also responds favorably to the addition of stabilizers," Dietz said. GE data show that through five passes, a consistent M.I. of around 4 was achieved with a blend of Ultranox 626 and a high-molecular-weight-hindered phenolic, GE's Ultranox 276. Unstabilized HDPE regrind had M.I.'s ranging from slightly less than 3 on the first extruder pass, but dipped to less than 1 by the third pass.

Another experiment exposed recycled HDPE flake to heat and shear in a torque rheometer at 410 F. The result, Dietz said, was a gradual loss in torque over 27 minutes after the fluxing point. "Torque then begins to increase and continues increasing for 30 min," she explains. "At this point, the HDPE is crosslinked to an extent that would make it difficult to process and the experiment was stopped." With 0.1% by weight of Ultranox 626, torque remained constant from flux to 60 minutes, at which point the experiment was terminated, Dietz said.

GOOD FOR PP, TOO

Polypropylene usually contains a comprehensive stabilization package in its original formulation, and therefore responded well to the stabilization in GE's tests, Dietz said. Comparing a compound of virgin PP with 500 ppm hindered phenolic antioxidant and 1000 ppm calcium stearate to virgin PP plus 500 ppm Ultranox 626, Dietz found that the virgin/phenolic compound had a melt flow rate of 5 g/10 min after the initial extruder pass, and more than 12 MFR by the fifth pass. The PP/Ultranox blend had a more consistent melt flow, starting at a little more than 2 MFR on the first pass and rising to approximately 3 MFR by the fifth pass.

Mixed with post-consumer PP from filled battery cases, Ultranox 626 was found to reduce torque loss and provide a melt flow ranging from about 8.75 to 10 MFR through five extruder passes, Dietz said.

MINOR HELP FOR PC

Of all resins Dietz tested, Ultranox 626 had the least (although still positive) affect on polycarbonate. "Polycarbonate holds its properties well through processing," Dietz said. Still, GE found that its phosphite stabilizer helps maintain color and viscosity in PC exposed to heat and shear in a torque rheometer. A blen of recycled PC and 500 ppm Ultranox 626 showed about 6% higher torque than unstabilized PC recyle. Yellowness index of the PC/Ultranox blend was about 31, compared with 37 for untreated PC. (CIRCLE 38)