High-density polyethylene processing stability

A number of ortho hindered alkyl-substituted phenyl phosphites and phosphonites were found to be effective process stabilizers for polypropylene and high density polyethylene combining more effective stabilization activity at high temperatures with good storage stability at relatively elevated humidity and ambient temperature, as well as resistance to discoloration. 

An additive system was developed for poly(vinyl chloride) for medical applications. The additives include primary stabilisers (Ca-Zn stearate and Zn stearate), secondary stabilisers (epoxides) and lubricants (ethylene bisamide and high density polyethylene), to improve melt processing and heat stability. The use of the stabilisers resulted in reduced equipment down-time, increased the level of recycled material which could be incorporated, and enhanced the product characteristics, including colour, clarity, blush, aqueous extractables and particle generation. 5 refs. 

Processing Stability for High Density Polyethylene (HDPE). The specific polymer used in the test described below was a high molecular weight HDPE made by BASF Lupolen 5260 Z. The following test runs were carried out by the test laboratories of CIBA-GEIGY Corporation Basle, Switzerland. 

Several other common industrial polymers are also used in biomedical applications [51]. Because of its low cost and easy processibility, polyethylene is frequently used in the production of catheters. High-density polyethylene is used to produce hip prostheses, where durability of the polymer is critical. Polypropylene, which has a low density and high chemical resistance, is frequently employed in syringe bodies, external prostheses, and other non-implanted medical applications. Polystyrene is used routinely in the production of tissue culture dishes, where dimensional stability and transparency are important. Styrene-butadiene copolymers or acrylonitrile-butadiene-styrene copolymers are used to produce opaque, molded items for perfusion, dialysis, syringe connections, and catheters. 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

The eombination of high moleeular mass phenolie antioxidants with phosphites or phosphonites confer adequate proteetion. They are now the state of the art for PP, HDPE, and LLDPE stabilization. Phenolic antioxidants can confer sufficient processing stability to Ziegler-type high density polyethylene (HDPE, Ti-based catalysts) used for tape manufacturing. However, with HDPE, as with PP fibers, the combinations of phenolic antioxidants with phosphites or phosphonites yield improved processing stability. These combinations are characterized by good extraction resistance, low volatility, and positive influence on substrate color [6, 7]. 

For high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE), and PP combinations of phenolic antioxidants and phosphites lead to a synergistic protection of the polymer. For HDPE and LLDPE an increased amount of phosphite provided increased processing stability at constant phenol concentration. A 4 1 ratio of phosphite to phenol exhibited the best performance, while the most efficient ratio for protecting melt flow of the PP was found when a 2 1 ratio of phosphite to phenol was added [167], According to Parrondo molecular weight changes can best be reduced with phenolic antioxidants, while phosphites prevent discoloration [168]. 

Epacher E, Tolv6th J, Krdhnke C, Pukanszky B. Processing stability of high density polyethylene effect of adsorbed and dissolved oxygen. Polymer 2000 41 8401-8. 

Parrondo A, Allen NS, Edge M, Liauw CM, Fontan E. Optimization of additive packages for processing and long-term thermal stabilization of film grade high-density polyethylene. J Vinyl Addit Technol 2002 8(2) 103-17. 

Naugard 10 is a versatile antioxidant capable of giving excellent processing stability and long term heat aging characteristics. It is used in a variety of polymers where elevated processing temperatures are encountered. It is particularly effective in high density polyethylene and polypropylene. Product Features ... 

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