1、,A REVIEW OF RUBBER OXIDATION,The aging of rubber caused by oxidative degradation leads to the deterioration in the physical and mechanical properties of vulcanized rubbers. The critical review will cover the major develop-ments in our understanding of the oxidation of rubbers.,The mechanisms relati

2、ve to oxidation of rubber Methods of protection,The mechanisms relative to oxidation of rubber,Polymer degradation is caused by chemical bond scission reactions in macromolecules.The initiation modes of chemical degradation include thermal, mechanical,photochemical,radiation chemical, biological and

3、 chemical degradation of polymetric materials. In addition,we ofen find the aging phenomenon of white, cracked surface , the deterioration of physical and mechanical properties in the process of rubber products used.,The aging of rubber is not a simple process,The mechanisms relative to oxidation of

4、 rubber,R+O2RO2 (1) RO2+RHROOH+R (2) ROOHRO+OH (3) RO+RHROH+R (4) RH+OHH2O+R (5) RO2+RO2ROOR(stable)+O2 (6),auto-accelerated oxidation,The mechanisms relative to oxidation of rubber,the loss of unsaturation of ,-unsaturated oxidized products through crosslinking: x-CH=CH-CH2- x-CH-CH-CH2- -CH2-CH=CH

5、-CH2- -CH2-CH-CH-CH2- with:x=-C-, -C-OH, -C-, -C-H OH O O O,oxidative crosslinking reaction,The mechanisms relative to oxidation of rubber,the formation of an ether linkage through photooxidation: CH3 CH3 C=C -CH-CH=C-CH2- -CH-CH=C-CH2- - +H OOH O CH3 -CH-CH=C-CH2- O -C-CH- ,oxidative crosslinking r

6、eaction,Methods of protection,Ozone attack on rubber compounds causes characteristic cracking perpendicular to the direction of applied stress. This type of degradation is caused by reactions of ozone with the double bonds in the rubber molecules.These reactions lead to chain scission and the format

7、ion of various decomposition products. There are several methods of protection rubber from oxidation.,How to prevent the aging of rubber,Methods of protectionBlock Method,The block method (protective film)relies on the stabilizer film formed at the surface of rubber that prevents ozone attack on the

8、 rubber.,wax,Methods of protectionBlock Method,The key effects of a wax in rubber have been summarized as the following: A wax protects by blooming to the surface to form a film of hydrocarbons which is impervious to ozone. Protection is achieved only when the film is thick enough to provide a barri

9、er to ozone. Thicker films ensure longer protection. Thickness of bloomed film depends on both the solubility and diffusion rate of wax in rubber. Microcrystalline waxes perform better at higher temperature,since higher temperatures increase their mobility and hence their rate of migration to the su

10、rface. It is common practice to blend paraffin and microcrystalline waxes together to achieve an all season or wide range of temperature protection against ozone.,Methods of protectionReaction Method,The reaction method(scavenger mode)states that the stabilizers migrate to the surface and preferenti

11、ally reacts with ozone so that the rubber is not attacked until the antioxidant is exhausted.,substituted p-phenylenediamines,Methods of protectionReaction Method,Studies have shown that the following features of p-phenylenediamines contribute: 1)a higher rate of reaction with ozone than that shown

12、by the rubber; 2)diffusion of fresh antiozonants to the exposed surface once a concentration gradlent has been established; 3)formation of a surface film,which recent spectroscopic analyses have confirmed consists of the ozonation reaction products of the p-phenylenediamine.,Methods of protectionPPD

13、,N,N-substituted p-phenylenediamines(PPD) are widely used antioxidant agents for diene rubbers.It has been shown that long unsubstituted and substituded polyene chains generate radical cations as one of the first elementary reactions with ozone.,Methods of protectionPPD,SUMMERAY,Exposure of engineering elastomers to air during aging(radiation,thermal,UV)often results in the heterogeneously oxidized samples,a complication which challenges attempts to understand the oxidation process and to extrapolate accelerated exposures to long-term c