Chain scission oxidative

On the other hand, rubbers are exposed to repeated mechanical flexing and deformation in service. Previous studies [55] have shown that flex cracking of rubber materials is accelerated by the presence of oxygen. Environmental flex cracking is caused by oxidative chain scission of the rubber polymer at mechanically induced cracks. 

Creep, stress relaxation and set are all methods of investigating the result of an applied stress or strain as a function of time. Creep is the measurement of the increase of strain with time under constant force stress relaxation is the measurement of change of stress with time under constant strain and set is the measurement of recovery after the removal of an applied stress or strain. It is important to appreciate that there are two distinct causes for the phenomena of creep, relaxation and set, the first physical and the second chemical. The physical effect is due to rubbers being viscoelastic, as discussed in Chapter 9, and the response to a stress or strain is not instantaneous but develops with time. The chemical effect is due to ageing of the rubber by oxidative chain scission, further crosslinking or other reaction. 

The most important mode of degradation for parylenes is oxidative chain scission. Oxidative degradation limits the use of parylenes at elevated temperatures in many common applications. Conventional antioxidants, incorporated during or after VDP, can extend the life of the parylenes at elevated temperatures. 

The second step (6.23) involves the breaking of a carbon-hydrogen bond and has a higher activation energy. The rate of this reaction is therefore much slower, and the rate of this step in the chain reaction generally determines the rate of oxidation, once it has started. Oxidative chain scission also occurs subsequently, according as the following reactions for polyethylene ... 

It is well known that thermal oxidation in polyethylene (PE) in the presence of oxygen leads to sudden and deep embrittlement, in which oxidative chain scission plays a key, but perhaps indirect, role. We have developed a kinetic model, derived from a branched radical chain mechanistic scheme, able to predict accurately molecular structural changes (7). 

Silicone materials are known to be fairly resistant to thermal degradation under relatively harsh conditions (less than a few hundred degrees centigrade). However, they are known to be subject to post-curing reaction, oxidative chain scissioning and crosslinking reactions, hydrolysis and unzipping reactions 23-... 

The oxidative chain scission of PVC is better illustrated in Figure 3 depicting the decrease of the number average molecular weight (Mn) of PVC samples as a function of reaction time. As shown in this Figure, significant decrease of M takes place in a short period of time followed by slower Mn... 

Oxidative chain scission processes in polyethylene have been described in detail (6). During the course of exhaustive oxidation, the reaction rate subsides as accessible... 

Degradation of polyacetals may also occur by oxidative attack at random along the chain leading to chain scission and subsequent depolymerization (unzipping). Oxidative chain scission is reduced by the use of antioxidants (see Chapter 1), hindered phenols being preferred. For example, 2,2 -methylene-bis(4-methyl-6-r-butylphenol) is used in Celcon (Celanese) and 4,4 -butylidene bis(3-methyl-6-t-butylphenol) in Delrin (Du Pont). 

Table 1 Major reactions Polymer Oxidation, Chain Scission, and Cross-Linking... 

Clearly oxidative chain scission has an effect on mechanical performance similar to reduction of molecular weight ah initio. This experiment provided the strongest mechanical evidence of oxidation to date, in that a mechanistic relationship between oxidation and molecular weight was displayed. 

Oxygen in the surrounding atmosphere is found to increase crack growth, presumably by an oxidative chain scission reaction catalyzed by mechanical rupture. The minimum energy Go is found to be somewhat larger for experiments carried out in vacuo [55,59,60]. When antioxidants are included in the elastomer formulation, then the results in an oxygen-containing atmosphere approach those obtained in vacuo. 

Polymethacrylate a) Decrease in average molecular weight Oxidative chain scission. Reduction in average molecular weight (depending on residual monomer concentration), on exposure to a filtered xenon source. [148]... 

EPDM rubber Carbonyl absorption Oxidative chain scission Effect of crosslinking [158]... 

Equation 19.2 Oxidative chain scission reaction in polybutadiene phase of ABS... 

Because of the above oxidative chain scission reaction in PBD phase, ABS gradually loses its impact strength upon exposure to sunlight. The ASA polymers, on the other hand, retain their impact strength and toughness even after prolonged outdoor/UV exposure, significantly better than ABS, due to the presence of the weatherable acrylic elastomer phase, which replaces the unstable PBD phase. The outdoor/UV resistance of ASA and ASA-PC blend vs. ABS is schematically illustrated in Fig. 19.9a. 

Products of degradation methylene group oxidation, chain scission Bera, M Rivaton, A Gandon, C Gardette, Eur. Polym. J., 36,1753-64, 2000. 

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