Rubber thermal oxidative

Pyrolysis of synthetic polyisoprene in the presence of oxygen also is expected to be identical to that of natural rubber. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. The effect of oxygen is to increase the reaction rate of scission and therefore to decrease the temperature where the scission starts. The oxidation may take place after the initial formation of a free radical that reacts with oxygen ... 

Solid SBR is often prefened to natural rubber because of its better thermal oxidative stability, higher abrasion resistance and easier processability. Solid SBRs are generally grouped into three families according to the production method. 

Imaging chemiluminescence technique. Chemiluminescence analysis is suitable for studying the early stages of the thermal oxidation of rubbers. A weak emission of light formed by chemical reactions appears during the oxidative degradation of hydrocarbons. This technique can be used to depth profile the oxidation of rubbers. MR... 

Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction. 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

Processing Stability. As with elastomers or other rubber modified polymers, the presence of double bonds in the elastomeric phase increases sensitivity to thermal oxidation either during processing or end use. Antioxidants are generally added at the compounding step to ensure retention of physical properties. Physical effects can also have marked effects on mechanical properties due to orientation, molded-in stress, and the agglomeration of dispersed rubber particles under very severe conditions. Proper drying conditions are essential to prevent... 

As in purely thermal degradation, thermal oxidation of rubber is accompanied by formation of low-molecular-weight products in yields too high to be accounted for by random attack on the... 

The overwhelming majority of foams are TPs, but TSs are also foamed with CBAs, although some of them do create problems. Popular TS foams are made from polyurethane, polyester, phenolic, epoxy, and rubber. Thermal decomposition of the blowing agent with certain plastics such as TS polyesters cannot be applied in this system because the heat of polymerization is not high enough to induce decomposition. But chemical reactions simultaneously produce gas and free radicals they typically involve oxidation and reduction of a hydrazine derivative and peroxide. The reactions are catalyzed by metals, which can be used repeatedly. 

The above discussion indicates that synthesized CLSP with phenyl framing groups are characterized by high thermal oxidative stability and easy processing to articles. That is why these substances we-re suggested for production of specific rubbers [12],... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the rubber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene —acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Antioxidants have been shown to improve oxidative stability substantially (36,37). The use of rubber-bound stabilizers to permit concentration of the additive in the rubber phase has been reported (38—40). The partitioning behavior of various conventional stabilizers between the rubber and thermoplastic phases in model ABS systems has been described and shown to correlate with solubility parameter values (41). Pigments can adversely affect oxidative stability (32). Test methods for assessing thermal oxidative stability include oxygen absorption (31,32,42), thermal analysis (43,44), oven aging (34,45,46), and chemiluminescence (47,48). 

Commercially available antioxidants include phenols and amine derivatives the latter, though generally more effective, have the drawback of alteriiig the coloration of dyed products. These additives are necessary to prevent, to some extent, the process of thermal oxidation of rubbers, though it has to be borne in mind that the stability of rubbers is primarily determined by the chemical nature of the chains as well as by the cross-links that define network structure. 

Natural rubber was also studied regarding pyrolysis in the presence of oxygen. Thermal oxidation of natural rubber is assumed always to be associated with scission, although photo-oxidation at low temperature may involve peroxide formation without scission. 

Bevllacqua (7, 8, 9, 10)studied the change in molecular weight and the volatile products In the thermal oxidation of synthetic and natural polylsoprene. At 90°C It appeared that every mole of chain scission was accompanied by one mole of a volatile carbonyl compound. Bevllacqua suspected this compound was levullnaldehyde, and this has been substantiated by Percy (11) and Houseman (12) In the oxidation of purified sol rubber at 75 C. Levullnaldehyde could result from the chain scission of a cyclic peroxide structure similar to the Bolland structure ... 

It is now well established that wear of rubber is not a purely mechanical failure process in that it contains a contribution due to thermal-oxidative breakdown of the polymer (226—228). Still, under severe conditions, which apply to all accelerated laboratory tests as well as to many situations encountered in tire wear, abrasion is dominated by tearing processes. 

Thermal oxidation occurs when heat and oxygen are exposed to the seal. The term auto-oxidation is used for temperatures between ambient and 200°C. Shelton proposed the kinetic scheme for rubber shown in Figure 8.19. Autotermination by coupling or disproportionation of radicals will occur [18]. Hydrogen peroxide is assumed to be a source of radicals that will initiate oxidation in polymers [18]. 

HEXACHLOROBUTADIENE or HEXACHLORO-l,3-BUTADIENE (87-68-3) C4C1 CliC=CClCCl=CCIj Combustible liquid (flash point 195°F/90°C autoignition temp 1125°F/607°C Fire Rating 1). Unless inhibited, may form unstable peroxides in storage. Reacts strongly with oxidizers, aliuninum powder may cause fire and explosion. Mixtures with bromine perchlorate forms heat-, friction-, and shock-sensitive explosive compound. Attacks aluminum, and some plastics and coatings decomposes rubber. Thermal decomposition releases toxic and irritating chloride fumes. On small fires, use dry... 

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