Degradation of rubber

Oxidation of elastomers is accelerated by a number of factors including heat, heavy metal contamination, sulfur, light, moisture, swelling in oil and solvents, dynamic fatigue, oxygen, and ozone. Three variables in the compound formulation can be optimized to resist degradation polymer type, cure system, 

Thermo-oxidative stability is primarily a function of the vulcanization system. Peroxide vulcanization or cure systems tend to perform best for reversion resistance as a result of the absence of sulfur and use of carbon-carbon crosslinks. Efficient vulcanization (EV) systems that feature a low sulfur level (0.0-0.3 phr), a high acceleration level, and a sulfur donor similarly show good heat stability and oxidation resistance. Such systems do, however, have poor resistance to fatigue because of the presence of predominantly monosulfidic crosslinks. Conventional cure systems that feature a high sulfur level and low accelerator concentration show poor heat and oxidation resistance because the polysulfidic crosslinks are thermally unstable and readily oxidized. Such vulcanization systems do, however, have better fatigue resistance. Semi-EV cure systems, which are intermediate between EV and conventional systems, are a compromise between resistance to oxidation and required product fatigue performance. 

The degradation of unsaturated elastomers is an autocatalytic, free radical chain reaction, which can be broken into three steps  

Like any chemical process, the rate of reaction will increase with temperature. Increase in service temperature will thus accelerate the degradation of rubber, the rate of reaction with oxygen being governed by the Arrhenius equation. 

Ultraviolet light initiates free radical oxidation at the exposed surface of an elastomeric product to generate a layer of oxidized rubber. Heat, moisture, or high humidity can then initiate crazing of the surface, which subsequently can be abraded off. Such degradation of the surface is more severe with nonblack stocks than with black compounds. Nonblack compounds such as white tire sidewalls thus require higher levels of nonstaining antioxidants than carbon black-loaded formulations. 

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Degradation of rubbers and resins can also be produced by ozone attack. Ozone directly reacts with, and cleaves, the carbon-carbon double bonds of rubbers and resins. Thus only polymers with backbone unsaturation will be cracked by ozone. Unlike oxidation, ozone attack cannot be accelerated by increasing the... 

Harries showed that the degradation of rubber by ozone yielded chiefly levulinic acid and aldehyde (38). This fact, he concluded, indicated that rubber was made up of the repeating unit ... 

Higher alcohol content may cause the degradation of rubber gaskets and should be limited to under 1%. The acidity number reflects the presence of free fatty acids, which can contribute to accelerated engine aging. For the same reasons the water content should be kept quite low, at max. 300ppm. Finally, in order to ensure proper storage the oxidation stability of the fuel should be at least 5h at 110°C. 

It must be borne in mind that the presence of traces of copper cause accelerated degradation of rubber on storage. Copper after-treated dyes are therefore unsuitable for yarns to cover, or fabrics containing, rubber threads. 

The role of ozone is also very important in the degradation of rubber. It was found that on exposing a stressed rubber sample to ozone, small cracks formed on the surface. These cracks are perpendicular to the direction of applied stress [450]. The velocity of cracking depends on the chemical structure of the polymer, time of exposure, magnitude of applied stress, content of plasticizers, and so on [13, 14, 111, 186, 239, 360, 361, 553, 554]. Several workers [13, 184] have indicated that the ozonization of diene elastomers is accompanied by an autocatalytic... 

Thermal processes are mainly used for the feedstock recycling of addition polymers whereas, as stated in Chapter 2, condensation polymers are preferably depolymerized by reaction with certain chemical agents. The present chapter will deal with the thermal decomposition of polyethylene, polypropylene, polystyrene and polyvinyl chloride, which are the main components of the plastic waste stream (see Chapter 1). Nevertheless, the thermal degradation of some condensation polymers will also be mentioned, because they can appear mixed with polyolefins and other addition polymers in the plastic waste stream. Both the thermal decomposition of individual plastics and of plastic mixtures will be discussed. Likewise, the thermal coprocessing of plastic wastes with other materials (e.g. coal and biomass) will be considered in this chapter. Finally, the thermal degradation of rubber wastes will also be reviewed because in recent years much research effort has been devoted to the recovery of valuable products by the pyrolysis of used tyres. 

This section describes the different processes that have been patented for the catalytic conversion of plastic mixtures without any previous thermal treatment. In many cases, the authors claim that the process is also successful in the degradation of rubber wastes or plastic and rubber mixtures. 

Ecological study of microbial degradation of rubber is still at a primitive stage. The NR vulcanizate has been more or less degraded by microorganisms with both in vivo and in vitro conditions. However, the rate and the extent of microbial degradation nray be greatly influenced by the rubber formulations and by environmental circumstances. 

Aging, rubbers n. The process of oxidation and other degradations of rubber or elastomeric materials. 

The PPD class of antidegradants provides by far the best antiozonant protection (for a review of antiozonants, see Reference 82). PPDs inhibit ozone degradation of rubber by multiple mechanisms (76,83-85) ... 

Usually, elastomers are processed through extrusion, forming in matrices, mixing. The first observations on the cold degradation of rubber by two roll-mixing, in the last century, refers to the fact that, under prolonged processing, natural rubber becomes sticky [703-705],... 

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