Silicone rubbers processing conditions

The main types of rubber used in the field of anti-corrosion are natural rubber, polyisoprene, polybutadiene, polyurethane, butyl rubber, styrene butadiene, nitrile rubber, ethylene propylene rubber, polychloroprene, silicone rubber, and vinylidene rubber. The wide ranges of available natural and synthetic rubbers offer a versatility of properties to suit almost every corrosive condition encountered in the process industries. 

The inhibitors act as ligands, blocking the hydrosilylation reaction by coordination to the metal center, but vmder conditions employed for cure, they release the active catalyst. The numerous vmsaturated organic compounds, such as esters, alcohols, ketones, sulfoxides, phosphines, phosphites, nitriles, azodicarbonyl compounds, triazoline, dienes, amine N-oxides, hydroperoxides, and others, are known and have been reported as platinum catalyst inhibitors during the cure of silicon rubber through addition processes (3,4). The unsaturated diesters (e.g., maleates and fumarates) are the most important and commonly used platinum inhibitors in industrial curing processes and have been described in many articles and patents. 

Figure 10.7 (8) illustrates the stress relaxation of a poly(dimethyl siloxane) network, silicone rubber, in the presence of dry nitrogen. The reduced stress, o(t)/(T(0),is plotted,so that under the initial conditions its value is always unity. Since the theory of rubber elasticity holds (Chapter 9), what is really measured is the fractional decrease in effective network chain segments. The bond interchange reaction of equation (10.2) provides the chemical basis of the process. While the rate of the relaxation increases with temperature, the lines remain straight, suggesting that equation (10.2) can be treated as the sole reaction of importance. 

Liquid silicone rubbers have a fluid or paste-like consistency and cure in the cavity by heating and thus cross-linking. It must therefore be ensured that the material to be processed is transported in the cold runner under defined conditions to prevent premature curing before reaching the mold cavities. 

Example 6.3.12 Consider selective pervaporative transport of a VOC species i through a silicone rubber membrane in preference to water. The separation conditions are such that Ei is 1, of the order of 1000 further, the permeation rates are quite low with respect to the feed side mass-transfer coefficient (i.e. Vz ku). In this pervaporation process, it is known that the membrane resistance is quite low but the boundary layer resistance is quite high for the VOCs. On the other hand, for water, it may be safely assumed that the membrane resistance controls the water transport. Determine an expression for the separation factor, and find the selectivity of the membrane for VOC over water when the permeate pressure is negligible. Assume that the VOC concentration in water is low for this pervaporation process. 

Although natural adhesives (animal glue, casein, starch, and rosin) are still used for many applications, a host of new adhesive materials based on synthetic polymers have been developed these include polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and rubber materials. Adhesives may be used to join a large variety of materials—metals, ceramics, polymers, composites, skin, and so on—and the choice of which adhesive to use will depend on such factors as (1) the materials to be bonded and their porosities (2) the required adhesive properties (i.e., whether the bond is to be temporary or permanent) (3) maximum/minimum exposme temperatmes and (4) processing conditions. 

It is silicone rubber, hot vulcanisation, liquid rubber LSR-2K ciosslinking compound, which is discussed in the following sections on LSR processing. All other silicone rubber types require different machine equipment and process conditions for LSR which are beyond the scope of this book. 

Nevertheless, by exploiting silicone technology, and especially the vulcanization process of silicone rubber, it is also possible to develop improved ceramic insulators, and this solution is Room Temperature Vulcanized Sihcone Rubber (RTV SIR) coatings. These coatings can be appHed on the surface of a ceramic insulator and ascribe a behavior similar to silicone rubber insulators, and therefore ensure an improved performance in the case of poUution [9,10]. The coating properties, capabilities and efficiency are correlated with the formulation and the fillers incorporated, to the application conditions and procedures, and certainly with the service conditions experienced [11-16]. 

The need to maintain elasticity of rubber is of paramount importance under any serious and severe environmental conditions. The most stable rubbers in radiation environments are polyurethanes and phenyl silaxanes which are usable at well above 108 rads (106 Gy). Butyl rubber liquefies and neoprene evolves hydrochloric acid at similar dose levels. Most polyurethane rubber foams can be used at a dose level of 109 rads (107Gy) in vacuum at temperature levels of between -85°C to +250°C. Silicone and polysulphide sealants are probably less tolerant to ionizing radiation in a nuclear plant where chemical processes are being carried out. A schematic graphical representation of the tolerance of rubbers to ionizing radiation in nuclear plant is shown below in figure 7.4. 

Differences in results can occur between tests in a liquid and a gaseous medium. This is often because different times are required to reach equilibrium temperature, and if crystallisation is occurring, for example, the stiffness will be dependent on time of conditioning. It is also essential that if a liquid medium is used the liquid does not affect the rubber by swelling it or removing extractables, as either process can have a considerable effect on low temperature behaviour. Ethanol is most widely used but acetone, methanol, butanol, silicone fluid and n-hexane are all suggested in ISO 2921. Not all of these will be suitable for all rubbers and the suitability of any proposed liquid must be checked by preliminary swelling tests. 

In the rubber industry, moisture absorbed on the surface of silicate, impacts the rate and extent of cure and results in sponge-like textures. In moisture cured systems such as polyurethanes, polysulfides and silicones, moisture causes a premature reduction in shelf-life. In extrusion and injection molding the moisture absorbed on fillers contributes to various defects and a strict regime must be followed regarding the drying time and the conditions prior to processing. Lacing, a less well known phenomenon, is caused by the absorption of moisture on the surface of titanium dioxide. ... 

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