Vulcanization thermal stability

The thermal stability of peroxides can be expressed in terms of their half-hfe (ti/a)- Half-life values can be estimated in solution utilizing the technique of differential thermal analysis. These values, or more precisely the temperatures at which their half-life is equivalent, provide an indication of practical vulcanization temperatures [49] (Table 14.29). 

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

The phenyl modified polymers possess the optimum combination of high temperature and elastomeric properties and were used in the study of formulation parameters These variables can have an important effect on the thermal stability and property profile of vulcanized systems For example, the use of reinforcing silicas, peroxide content, and oxidative stabilizers have been shown to be important ( 3, 10, in However, polymer-silica interactions had the most pronounced effect on retaining properties during high temperature aging studies ... 

These products find use in specialty applications requiring better thermal stability than available in the sulfur vulcanized elastomers. Other processes are also used to crosslink polydiene rubbers (Secs. 9-2c and 9-2d). 

By the process of vulcanization, rubber elasticity, impact resistance, flexibility, thermal stability and many other properties are either introduced or improved. In addition, the crosslinking of non-elastomeric polymers increases the toughness, abrasion resistance and, particularly, the maximum service temperatures of the material. 

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

Medium-molecular-weight PMTFPS with vinyl or hydroxyl end blocks are used for adhesives and sealants. They are cured either at ambient temperature (RTV-room temperature vulcanization) or at elevated temperature. One-part moisture-activated RTV sealants have been available commercially for many years. Because of then-very high resistance to jet engine fuels, excellent flexibility at very low temperatures, and high thermal stability, they have been used in both military and civilian aerospace applications.78 Two-part, heat-cured fluorosilicone sealants have been used in military aircraft applications and for sealing automotive fuel systems.79 Special class of fluorosilicone sealants are channel sealants or groove injection sealants, sticky, puttylike compounds, which do not cure. They are used to seal fuel tanks of military aircraft and missiles.75... 

The networks obtained through vulcanization by peroxides display high thermal stability. Nevertheless, the peroxide vulcanization process allows less control of the vulcanization than the sulfur process. Consequently it... 

This type of vulcanization-system design was reported by McCall (1969). He found that by judiciously balancing the levels of accelerator, sulfur, and DTDM, he could obtain good vulcanization characteristics, good thermal stability, good flex life, and superior retention of flex life. Others have reported on more recent work on the effects of crosslink type on reversion (Datta et al., 2007 Fan et al., 2001). 

Table 7.5 gives selected recipes for vulcanization by phenolic curatives, benzoquinone-dioxime, or m-phenylenebismaleimide. Vulcanizates based on these types of curatives are particularly useful in cases where thermal stability is required. 

The use of peroxide vulcanization systems has always been somewhat controversial among rubber compounders, largely based on its higher cost compared to sulfur. However, this needs to be examined on a compound by compound formulation basis. From the standpoint of thermal stability, the peroxide crosslink has a bond energy of about 82 kilocalories and is as stable as any of the carbon-carbon bonds in the polymer backbone. In contrast, the sulfur cross-link,... 

Recipes for Peroxide Vulcanization. Examples of starting-point recipes are given in Table 4.9. Outstanding characteristics of peroxide vulcanizates are low permanent set and thermal stability of the network. 

Cross-linking or vulcanizing a polymer to improve such properties as modulus, strength, thermal stability, etc. The 5-10 mm of resin in front of the screw tip that remains at the end of the injection cycle. It is used to maintain packing pressure on the melt until the cavity gate freezes. 

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