1 Effect of molecular structure of raw gum
The intersection molecule structure is different, the location of oxidation is different, the difficulty of oxidation is different, the oxidation rate is different, the structure changes in the oxidation engineering is different, and the oxidation products are different.
In terms of carbon chain rubber
, saturated carbon chain rubber is more resistant to oxidation than unsaturated carbon chain rubber. In the saturated carbon chain rubber molecular chain, although the electron induction and hyperconjugation effect make the tertiary hydrogen atoms have greater activity, the oxidation reaction often occurs at this position, but the C-H bond cleavage to generate active free radicals, compared with the unsaturated carbon chain rubber molecule a-H bond cleavage, the fracture activation energy is much higher, so it is not easy to be oxidized. The a hydrogen atom in the unsaturated carbon chain rubber macromolecular structural unit has high reactivity, and can undergo oxidative dehydrogenation reaction at a low temperature, and generate stable macromolecular free radical R·. Then the oxidation chain reaction is carried out. The oxidation reaction has obvious automatic catalytic characteristics. Saturated carbon chain rubber has no obvious automatic catalytic characteristics in the oxidation process, because the oxidation of saturated carbon chain rubber must be carried out at a high temperature, and the generated hydrogen peroxide ROOH is quickly decomposed, and it is difficult to give full play to the automatic catalytic role.
Hybrid chain rubber such as silicone rubber, because the main chain is composed of inorganic silicon atoms and oxygen atoms alternately, it does not have the tendency to decompose according to free radicals, the initiation reaction of oxidation can only start from the side alkyl, the oxidation reaction temperature is much higher than the rubber above 280 degrees Celsius began to produce low molecular volatiles, the automatic catalytic characteristics of the reaction is not obvious. This shows that it is more resistant to oxidation than ordinary carbon chain rubber.
(2) The influence of side groups is the same unsaturated carbon chain rubber, but the oxidation resistance is different due to the double bond with or without substituents and the polarity of substituents (Chapter 1). Compared with ethylene propylene rubber, the substituted group of acrylic rubber in saturated carbon chain rubber is polar ester group, and the latter is methyl group, so the oxidation activity of the former is much lower than the latter. Fluorine rubber is also a saturated carbon chain rubber, but only a small number of hydrogen atoms are connected with carbon atoms, mainly fluorine atoms or fluoroalkyl, so it shows excellent oxidation resistance.
(3) The influence of the copolymer components The oxygen resistance activity of the copolymer rubber is related to the type and quantity of the second or third component in the copolymer. The oxidation resistance of styrene butadiene rubber is better than that of nitrile butadiene rubber, and the oxidation resistance is better with the increase of acrylonitrile content. This is related to the fact that cyanogroup is a strong polar group. On the one hand, it reduces the reactivity of A-H in adjacent butadiene structural units. On the other hand, NBR has a higher cohesive energy density and reduces the diffusion ability of oxygen.
Although the volume steric effect of benzene ring in styrene butadiene rubber can hinder the diffusion of oxygen, the effect of improving the oxidation resistance is very limited because the content of styrene is small and irregular distribution. Because its unsaturation is lower than that of natural rubber, its oxidation resistance is only better than that of natural rubber and is comparable to that of butadiene rubber. In addition, the oxidation activity of EPDM and butyl rubber increased with the increase of the content of the unsaturated third component.