Lactate stabilizers rescue virgin & regrind polyolefins.

The Patco Polymer Additives Div. of American Ingredients Co., Kansas City, Mo., has shown the possibilities of using lactic acid-based products for stabilizing both virgin and reprocessed polyolefins. Patco says lactic-acid salts, though still relatively new to plastics, have proven successful as acid neutralizers in post-reactor compounding, helping maintain color and melt-flow stability. More recently it has become evident that calcium lactates can provide provide similar benefits in restabilizing recycled polyolefins.

According to Patco technical director Dale Dieckmann, lactic-acid derivatives provide better properties in recycled polyolefins than can be achieved with calcium stearate, zinc stearate, zinc oxide or synthetic hydrotalcites, including better regrind stability, better pellet and part consistency and greater processing latitude.

LACTATE SALTS TO THE RESCUE

Patco says that in postreactor compounding of virgin resins, calcium lactate and calcium stearoyl lactate form harmless complexes with the titanium and aluminum catalyst residues left over from polymerization, preventing these Lewis-acid residues from reacting with the resin's primary and secondary antioxidants and thereby leaving the polymer vulnerable to degradation. (Dieckmann delivered a paper on this topic last fall at the SPO '92 Conference in Houston sponsored by Schotland Business Research, Inc., Princeton, N.J. Proceedings may be obtained from the sponsor.)

With recycling a high priority today, Patco has applied its lactic acid chemistry to restabilizing regrind. Because the processing of recycled polyolefins requires more extreme heat, pressure and shear than virgin resins, strong stabilization is critical. At a recent SPE RETEC in Atlanta on improving the marketability of recycled plastics, Yatco showed how lactates can reduce yellowing and increase color stability in HDPE, LLDPE and PP. About 470 ppm of calcium lactate/stearoyl 2-lactylate with an overbased neutralizer proved to be the most effective.

In all the formulations, Patco compared this stabilizer with other calcium lactates, calcium stearates, synthetic hydrotalcite (SHT), zinc oxide and zinc stearate. The least sensitive regrind was PP, where little change in melt flow was observed from stabilizer to stabilizer but yellowness index decreased sharply with the lactate compared with all others.

In HDPE regrind, an overbased calcium lactate/stearoyl 2-lactylate showed better color than any other stabilizer, with calcium stearate and zinc oxide second best. Regrind melt flow was virtually unchanged from the virgin value with overbased calcium lactate/stearoyl 2-lactylate, but flow decreased dramatically with the use of other lactate stabilizers and with a phosphite-based antioxidant. In HDPE regrind, yellowness index increased regardless of the stabilizer used. However, plain calcium lactate and overbased calcium lactate both showed the least amount of darkening, Patco reported. Melt flow was consistently lower after reprocessing.

Both the overbased calcium lactate/stearoyl 2-lactylate and a phenolic antioxidant (Ciba-Geig's Irganox 1076 promoted the least change; while formulations with calcium stearate, zinc oxide, SHT, zinc stearate and a phosphite antioxidant suffered the biggest losses in flowability. The results were almost the same for ground-up HDPE milk jugs, where the phenolic antioxidant also provided good color retention.

Patco says these additives cost "slightly less" than most primary and secondary antioxidants. (CIRCLE 58).