Elastomers, additives Antiozonants

FAB has been used to analyse additives in (un) vulcanised elastomer systems [92,94] and FAB matrices have been developed which permit the direct analysis of mixtures of elastomer additives without chromatographic separation. The T-156 triblend vulcanised elastomer additives poly-TMDQ (AO), CTP (retarder), HPPD (antiozonant), and TMTD, OBTS, MBT and A,lV-diisopropyl-2-benzothiazylsulfenamide (accelerators) were studied in three matrix solutions (glycerol, oleic acid, and NPOE) [94]. The thiuram class of accelerators were least successful. Mixture analysis of complex rubber vulcanisates without chromatographic separation was demonstrated. The differentiation of matrix ions from sample ions was enhanced by use of high-resolution acquisition. 

The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

There are two main approaches to avoidance of ozone damage. One is the addition of antiozonants. The more extensively used antiozonants are listed in Table 13-12. These antiozonants have limited solubility in elastomers and tend to bloom to the surface. The antiozonant action is not well understood chemically, but probably involves formation of a protective film. Crack initiation occurs at critical stress, which is strongly... 

Hypalon is one of the most weather-resistance elastomers available. Oxidation takes place at a very slow rate. Sunlight and ultraviolet light have little, if any, adverse effecfs on its physical properties. It is also inherently resistant to ozone attack without the need for fhe addition of special antioxidants or antiozonants to the formulation. 

The term antiozonant denotes any additive that protects rubber against ozone deterioration. Most frequently, the protective effect results from a reaction with ozone, in which case the term used is chemical antiozonant. Ozone is generated naturally by electrical discharge and also by solar radiation in the stratosphere. These sources produce ground-level ozone concentrations of 1-5 parts per hundred million (pphm). In urban environments, however, ozone reaches much higher levels, up to 25 (pphm) due to the ultraviolet photolysis of pollutants. Only a few parts per hundred million of ozone in air can cause rubber cracking, which may destroy the usefulness of elastomer products. Some desirable properties of an antiozonant additive are as follows ... 

Rubber is protected against ozone attack by the addition of physical and/or chemical antiozonants. Hydrocarbon waxes are the most common type of physical antiozonant, and p-phenylenediamine derivatives are the prevalent chemical antiozonants. Refer to Hydrocarbon Waxes and Chemical Antiozonants. (Source Handbook of Polymer Science and Technology Volume 2 Performance Properties of Plastics and Elastomers, N. P. Cheremisinoff - editor, Macrel Dekker Publishers, New York, 1989). 

In contrast to nitrile rubber, fully hydrogenated NBR is not attacked by ozone. Even under extreme conditions (e.g., 2 ppm ozone, 40 °C, 50% rel. humidity, 60% strain, 168 h) no cracks appear. Even under cyclic load these rubbers exhibit excellent ozone crack resistance. Elastomer parts based on HNBR are highly ozone-resistant without the addition of antiozonants. Partially-hydrogenated grades behave differently, depending on the vulcanization system. Ozone resistance quickly decreases with increasing double bond content. However, peroxide vulcanized rubbers behave more favorably than sulfur vulcanized rubbers [697]. 

EVM elastomers are largely resistant to ozone cracking, because they do not contain C-C double bonds in their molecule backbone. Addition of antiozonants is not required [697]. 

In addition to the information on the specialty elastomers, chapters on the more important ingredients used with them are included. These are plasticizers, vulcanization agents, antioxidants and antiozonants, and process aids. 

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