Polyolefins peroxidation products

Field experience has demonstrated the reality of bioassimilation in practice. Plastor S-G has been in continuous use on the same fields for up to 15 years with no accumulation of plastic film or degradation products. The fact that leaching media do not remove degradation products from the surface of polyethylene film in the presence of soil microflora provides assurance that polyolefins peroxidation products present no threat to the environment. 

Peroxidic groups in oxidized polyolefins have frequently been employed as sources of free radicals to allow grafting of vinyl monomers to polyolefins (2f[). Some of the products from the gas reactions also have interesting potential as reactive sites. For example, chloroformate groups are well known to react with alcohols, and amines 2J[). Thus chloroformate groups could be useful for example in coupling highly oriented polyolefin fibres to resins such as epoxy based systems. 

Common to these photopolymer products was the use of coinitiators, such as 2-MBO, diacrylate and triacrylate monomers, binders, and other additives as required. In general, the film products were coated on a polyester support and a polyolefin sheet laminated to the coated material, to permit exposure of the films in an oxygen-free environment, as was required to obviate the chain-stopping effect of peroxide formation. 

The technique involves the uses of rotatable sampling ports that enable the polymer to be removed and quenched as it passes particular zones (corresponding to different values of LID) as shown in Figure 1.37 (Van Duin et al, 2001). In this way 2g of polymer can be collected for off-line analysis in about 3-5 s. For the above polymers, the most common profile for product formation along the axis of the extruder was one that followed the profile for the decomposition of the peroxide with time (and thus distance along the extruder). Reaction of MA was very rapid, and there was an exponential growth in the amount of maleated polyolefin as the MA was consumed. 

The same technique can be used to dye a material that is otherwise difficult to dye. An ethylene-propylene copolymer rubber was reacted first with maleic anhydride, then with an aromatic amine dye in an extruder to produce a dyed rubber.81 Dye sites can also be inserted into polyolefins by grafting them with dimethylaminoethyl methacrylate, using azo or peroxide catalysts in an extruder.82 jV-Vinylimidazole has been grafted to polyethylene in an extruder with the help of dicumylperoxide.83 The product was mixed with an acrylic acid-modified polypropylene and used to compatibilize polyethylene and polypropylene. This could be helpful in the recycling of mixed polyolefins from municipal solid waste. Recycling of cross-linked (thermoset) polymers is more of a problem because they cannot be remelted in an extruder. However, they can be if... 

A number of volatile compounds are obtained apart from the crosslinked products in the final oxidation process. With polyolefines, water, formaldehyde, acetaldehyde, acetone, methanol, hydrogen peroxide, carbon monoxide, and carbon dioxide were identified [Refs. 75, 76, 122, 292, 424, 425, 449, 487, 560, 608]. Table 5 shows the percentage content of particular volatile products formed during the photo-oxidative degradation of polypropylene [122]. 

During heating to 150-220° C polyolefins isolate thermooxidative products of macromolecules with pronounced toxic properties, namely organic acids, ethers, unsaturated hydrocarbons, peroxide and carbonyl compounds (formaldehyde and acetaldehyde), carbon oxide and dioxide, etc. A mixture of such products of the thermooxidative destruction of polyolefins may result in acute chronic poisoning on inhalation. 

Cross-linking can also be achieved by compounding a polymer with a peroxide such as dicumyl peroxide or di-/-butyl praoxide, followed by heating. This technique, although economically disadvantageous compared to sulfur vulcanization, is particularly useful for saturated polymers that cannot be easily cross-hnked, particularly polyethylene and other polyolefins. Similar results can be obtained through radiation cross-linking, and this method is employed commercially in the production of electrical wires and cable insulation made of polyolefins and poly(vinyl chloride). 

Nitroxide oxidative regeneration (BClemchuk-Gande mechanism) from >NOP based on its interaction with oxygen-centered radical oxidation products of polyolefins (polymeric alkylperoxyls and acylpero ls) was proposed (7) where symbol CHR R represents P in >NOP [18]. According to this scheme no peroxidic by-products with pro-oxidative character are formed within the resulting products. 

As a result of cross-linking the polyolefins with silanes the Si-O-Si bonds are produced, which are more elastic than the rigid bonds C-C created as a result of cross-linking of polymers induced by radiation and peroxides (Fig. 8.2). Use of silanes gives more elastic products and the cross-linking process is more cost-effective. 

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