1、 A REVIEW OF A REVIEW OF RUBBER RUBBER OXIDATIONOXIDATION 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 rub
2、bers.The mechanisms relative to The mechanisms relative to oxidation of rubberoxidation of rubber Methods of protectionMethods of protectionThe mechanisms relative to oxidation of rubber Polymer degradation is caused by chemical bondscission reactions in macromolecules.The initiationmodes of chemica
3、l degradation include thermal,mechanical,photochemical,radiation chemical,biological and chemical degradation of polymetricmaterials.In addition,we ofen find the aging phenomenon of white,cracked surface,the deterioration of physical and mechanical properties in the processof rubber products used.Th
4、e aging of rubber is not a simple process The mechanisms relative to oxidation of 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 rubberthe loss of unsaturation of,-unsaturated oxi
5、dized products through crosslinking:x-CH=CH-CH2-x-CH-CH-CH2-?-CH2-CH=CH-CH2-CH2-CH-CH-CH2-with:x=-C-,-C-OH,-C-,-C-H?OH O O Ooxidative crosslinking reaction The mechanisms relative to oxidation of rubberthe formation of an ether linkage through photooxidation:CH3 CH3?C=C-CH-CH=C-CH2-CH-CH=C-CH2-?+H O
6、OH O CH3?-CH-CH=C-CH2-?O?-C-CH-?oxidative crosslinking reaction Methods of protectionOzone 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
7、.These reactions lead to chain scission and the formation of various decomposition products.There are several methods of protection rubber from oxidation.How to prevent the aging of rubberBlock MethodReaction MethodPPDMethods of protectionMethods of protectionBlock MethodThe block method (protective
8、 film)relies on the stabilizer film formed at the surface of rubber that prevents ozone attack on the rubber.Methods of protectionBlock MethodThe 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 imp
9、ervious to ozone.Protection is achieved only when the film is thick enough to provide a barrier 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,sin
10、ce higher temperatures increase their mobility and hence their rate of migration to the surface.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 MethodThe reaction
11、method(scavenger mode)states that the stabilizers migrate to the surface and preferentially reacts with ozone so that the rubber is not attacked until the antioxidant is exhausted.Methods of protectionReaction MethodStudies have shown that the following features of p-phenylenediamines contribute:1)a
12、 higher rate of reaction with ozone than that shown 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 o
13、f the p-phenylenediamine.Methods of protectionPPDN,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
14、of protectionPPDSUMMERAY 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 conditions.Using the proper experimental and theoretical tools,and understanding the oxidation mechanism can lead to improve methods of stabilization and longer useful engineering lifetime of the elastomeric material.
