Chemistry radiation cross-linking

Radiation cross-linking of PVC in the absence of additives has been reported by several authors [20,21,267,268]. PVC by itself is not readily cross-linkable by EB radiation. In the study of radiation chemistry of PVC, it is known that the oxidation takes place in the presence of atmospheric oxygen during irradiation [269] and hydroperoxide groups are produced on oxidation. The formation of carbonyl group along with the elimination of HCl from PVC on irradiation in air has also been reported [270,271]. 

The detailed chemistry of radiation grafting has, in most cases, not been rigorously established. Process characterization is complicated by the fact that often only surface layers are involved and, in other cases, by the substrates being cross-linked or intractable. 

Feller, R. L., "Cross-Linking of Methacrylate Polymers by Ultraviolet Radiation", Preprints of papers presented at the New York Meeting, Division of Paint, Plastics and Printing Ink Chemistry, American Chemical Society, Sept., 1957, 17, No. 2, 465-470. 

Controlling the Crosslinking Density. The selective, controlled manipulation of polymers using radiation chemistry can be achieved by preferential irradiation shielding. By using a shield made of selected materials, of a certain shape, the overall properties of the irradiated polymer may be controlled and tailored. Thus, the cross-linking density can be locally controlled in a direction perpendicular to the direction of irradiation by (34) ... 

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

It is important to recognize that polypropylene, which is the major constituent of TPO, is a typical degrading-type polymer in the radiation chemistry of polymers, i.e., once a free radical is formed on a polymer chain, the free radical unzips the chain rather than cross-links. CASING effect was first found with polyethylene [24], which is a typical cross-linking-type polymer. The same CASING effect, however, could not be anticipated with the treatment of the degrading-type polymers because the degradation of substrate polymer enhances the extent of weak boundary layer. 

Early results on the radiation chemistry of polydimethylsiloxanes have been reviewed by Chapiro [285]. The yield of gas production varies between 1.2 and 2, according to the various workers. The gas consists of about 30—40% H2, 50% CH4 and 15—25% C2H6. The value for cross-linking lies between 2 and 3.4 depending on the workers and the methods used. Main-chain scission was found to be negligible. 

The history of radiation chemistry of polymers started in the early 1950s (Chapiro 1962 Charlesby and Alexander 1955). Poly(Ai-vinyl pyrrolidone) (PVP) was and still is an often applied polymer, also as hydrogel, in medicine and pharmacy. In 1955 Charlesby and Alexander first reported on cross-linking of PVP (Charlesby and Alexander 1955). Since then various other water soluble polymers have been radiochemically cross-linked, even for creating new biomaterials (Hoffman 1981). Hydrogels can be synthesized by radiation techniques in different ways ... 

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