Stabilization of Polyethylene

Polyethylene is subjected to various environments during the fabrication process and during the service life of a wide variety of products. These environments include  

Attack by host materials (liquids and gases) Ultraviolet light and other forms of radiation Electricity 

The autocatalytic thermal oxidation of hydrocarbons at slightly elevated temperatures has been known for over 90 years [3], As an example of this process, the autoxidation of the saturated hydrocarbon octadecane at 105 C is shown in Figiue 6.3, where oxygen uptake was measured over 100 hours. 

Several historic publications that provided early evidence into the chemistry involved in this process were published between the 1920s and 1940s. Christiansen [4] and Backstrom [5] provided data in 1924 and 1927, respectively, that a chain reaction was involved. Next, in the early 1930s, Backstrom [6] and Ziegler and Ewald [7] demonstrated that the oxidation of hydrocarbons was initiated by free radicals. In 1942, Farmer and coworkers reported that an autocatalysis process involving free radicals and the formation of hydroperoxides was involved [8]. This 1942 paper identified the three steps responsible for the auto-oxidation of polyethylene that result in the degradation of polyethylene properties. These three steps were free-radical initiation, propagation and termination. 

Moureu and Dufraisse classified compounds that slowed the oxidation process on hydrocarbons as antioxygens [9]. 

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Another issue is the stability of polyethylene vapor barriers. Polyethylene is known to be harmed by ultraviolet (UY) exposure. One radon mitigator has found polyethylene under slabs in Florida that deteriorated in less than 15 years more frequently, polyethylene of comparable age is in mint condition. 

In view of the relative stability of polyethylene toward ozone, it is... 

Han S, Kim C, Kwon D (1997) Thermal/oxidative degradation and stabilization of polyethylene glycol. Polymer 38(2) 317-323... 

Table 2 Stability of polyethylene oxide (MW 1,000,000) in hot-melt extruded tablet formulations containing 6wt% chlorpheniramine maleate processed at different temperatures...

Non-discoloring, sulfur containing phenolic antioxidant and stabilizer that provides long-term heat stability by preventing thermo-oxidative degradation. Used for the process stabilization of polyethylene wire and cable resins for polyethylene during extruder compounding. Can also be applied in styrenic polymers, polypropylene, elastomers such as EPDM and SBR and for carboxylated SBR latex, polybutadiene rubber and polyisopropene rubber. 

Sulfur containing, high performance primary (phenolic) antioxidant and heat stabilizer used for the base stabilization of polyethylene wire and cable resins. 

Can be used for the stabilization of polyethylene for food and medical packaging applications. Can aid in melt flow and color control during processing. 

In actual use, the catalytic oxidative d radatlon of polyolefins Is controlled, to a large extent, by the additives and contaminants present In the polymer. Although the stability of polyethylene Is related Initially to the antioxidant concentration and type, upon aging the critical factor Is the rate of loss of the antioxidant. Figure 4 shows typical examples of effective stabilizer loss, due to migration on aging. The samples, shown... 

One additive that improves both long-term photo and thermal stability of polyethylene Is carbon black. The ability of carbon black to retard destructive thermal oxidation In polyethy-lenes at elevated temperatures Is well known(17) and, as was seen In the earlier section of this paper on photo oxidation. It Is effective at lower temperatures also. Our studies of the thermal oxidation of low-density polyethylene show black samples to be outstanding, even In the presence of copper. For example black low-density polyethylene wire Insulation Is still Intact after 7 years at 80°C while all other colors. Including unplg-mented, failed mechanically due to oxidation after only about 3 months. 

A comparison of the thermal stability of polyethylene oxide, polypropylene oxide, polyethylene and polypropylene has been presented by Madorsky and Straus [230]. More recently, studies on the thermal degradation of polytetramethylene oxide (I), polyhexamethylene oxide (II) and polydioxolane (III)... 

Tocker [143] has observed a better light stability of polyethylene films with copolymerized ultraviolet absorbers than with dispersed ultraviolet absorbers. This effect is probably due to the absence of diffusion out of... 

J.-R. Pauquet, Technological advances in the stabilization of polyethylene films, Plast. Rubber Composit. Process. Appl. 1998, 27, 19-24. 

The oxidative stability of polyethylene has been reviewed by Collins and Wendlandt (197), Charsley and Dunn (198,199), and others (200). One of the most common measurements of the oxidative stability of polyolefin resins is the oxidative induction time (OIT) utilizing DSC. There are several versions... 

An alternative approach to uaiproving radiation stability has been through coupling the desirable processing properties of polypropylene with the radiation stability of polyethylene to form copolymers (polyallomers). Block polymers in which the chains comprise polymerized segments of each of the monomers are as amenable to processing as polypropylene while having better clarity and radiation stability. 

Crowley, M.M. Zhang, F. Koleng, J.J. McGinity, J.W. Stability of polyethylene oxide in matrix tablets prepared by hot-melt extrusion. Biomaterials 2002, 25(21), 4241 248. 

All polyethylenes are relatively soft, and hardness increases as density increases. Generally, the higher the density, the better the dimensional stability and physical properties, particularly as a function of temperature. The thermal stability of polyethylenes ranges from 190T (SS C) for the low-density material up to 250°F/121°C for the high-density material. 

The new additive masterbatches are products that are free of nickel quenchers, improving the stability of polyethylene films and developed specifically for horticultural films likely fo be contaminated by pesticides. They will provide protection for up to four years, even in high-radiation areas such as the Mediterranean. 

Derivatives of monohydroxybenzophenone which possess R = H, lower alkyl, or halogen, and finding use for the stabilization of vinyl halide polymers [9-13] derivatives containing normal or branched alkyl radicals with six carbon atoms or more are finding use for the stabilization of polyethylene [5]. 

Substances of the indicated structure are finding use for the thermal stabilization of polyethylene [203], various rubber mixtures [202], etc. 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

No.21, Nov.2002, p.4241-8 STABILITY OF POLYETHYLENE OXIDE IN MATRIX TABLETS PREPARED BY HOT-MELT EXTRUSION... 

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