Silicones heat-cured silicone rubbers

Silicone elastomers are either room-temperature vulcanization (RTV) or heat-cured silicone rubbers, depending on whether cross-linking is accomplished at ambient or elevated temperature. [The term vulcanization (see Chapter 1 and Chapter 2) is a synonym for cross-linking. While curing is also a synonym for cross-linking, it often refers to a combination of additional polymerization plus cross-linking.] RTV and heat-cured silicone rubbers typically involve polysiloxanes with degrees of polymerizations of 200-1500 and 2500-11,000, respectively. 

TABLE 3.39 Properties of Heat-Cured Silicone Rubbers... 

Heat-cured silicone rubber is commercially available in several forms as gum stock, reinforced gum, or partially filled gum. These compoimds can be im-catalyzed, catalyzed, or can contain catalyst dispersions. Catalyzed compounds are ready for use without additional processing. Silicone rubber is often freshened, ie, the compound is freshly worked on a rubber mill until it is a smooth continuous sheet. The freshening process eliminates the structuring problems associated with pol5uner-filler interactions. Rubber can be extruded on wire and cured in place (479). 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of Cr Uses Catalyst, crosslinking agent used for curing heat-vulcanized silicone rubbers, food-contact rubber articles Manuf./Distrib. Gelest http //www.geiest.com Trade Name Synonyms Aztec DCLBP-50-PSI [Aztec Peroxides] Perkadox PD t[Flexsys http //www. fiexsys. com]... 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

In order to obtain the degree of cure and rate of curing, we must first measure the reaction. This is typically done using a differential scanning calorimeter (DSC) as explained in Chapter 2. Typically, several dynamic tests are performed, where the temperature is increased at a constant rate and heat release rate (Q) is measured until the conversion is finished. To obtain Qt we must calculate the area under the curve Q versus t. Figure 7.17 shows four dynamic tests for a liquid silicone rubber at heating rates of 10, 5, 2.5 and 1 K/min. The trapezoidal rule was used to integrate the four curves. As expected, the total heat Qt is the same (more or less) for all four tests. This is to be expected, since each curve was represented with approximately 400 data points. 

Rubber Plug Molding. Liquid silicone rubber is cast into a female mold cavity and cured. The plug is removed, and layers of mat or fabric are built onto the plug. This assembly is then inserted into the mold cavity, pressed, and heated until cured. 

Silicone rubbers, such as General Electric RTV, represent a useful type of flexible adhesive. They can be clear or opaque, are fairly inert chemically, and some are usable up to 300°C or above. These silicones are just poured or extruded from a tube, and no heat cure is required since they cure by air drying. The resulting seals are not mechanically strong but will provide protection against water and oil. 

Casting The silicone-rubber molds and premixed resin were heated to 150°C prior to casting. After the Eporal was totally dissolved, the resin was deaerated in a vacuum bell jar, reheated to 150 C, and poured into the preheated molds. The samples were cured for 1 h at 150 C, followed by 5 h at 177°C then the oven was turned off and the samples were allowed to cool slowly to room temperature. 

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