Heat-cured silicone elastomers

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. 

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). 

Summary. Microwaves have been employed successfully to heat and subsequently cure silicone compositions to give elastomers and foams. Microwaves were found to be responsible for heating the silicone polymers and the heat, thus generated, was responsible for the curing. In order for a silicone polymer to be heated efficiently, it should contain some substituents [e.g.,... 

Quality control testing of silicones utilizes a combination of physical and chemical measurements to ensure satisfactory product performance and processibility. For example, in addition to the usual physical properties of cured elastomers, the plasticity of heat-cured mbber and the extrusion rate of TVR elastomers under standard conditions are important to the customer. Where the silicone application involves surface activity, a use test is frequently the only reliable indicator of performance. For example, the performance of an antifoaming agent can be tested by measuring the foam reduction when the silicone emulsion is added to an agitated standard detergent solution. The product data sheets and technical bulletins from commercial silicone producers can be consulted for more information. 

For example, in heat-cured mbber systems a common loading with synthetic silica is around 30%. For economic reasons, precipitated silicas are used in increasing volumes in silicone elastomers such as high-consistency mbber (HCR), liquid silicone mbber (LSR), and two-component room temperature vulcanized elastomers (RTV2). The reinforcement provided by precipitated silicas is only slightly lower than that provided by fumed silicas. However, fumed silicas are necessary in silicone sealants and other high-end applications because of their desirable properties like low moisture content. 

Poly(dimethylsiloxane) (PDMS) is the simplest of organosilicone elastomers. PDMS was commercially developed by Dow Coming in the 1960 s. It possesses inertness and usefiil mechanical properties, and can also be readily made into various desired shapes for medical items including prostheses of different bone and soft tissue elements (in surgery), and ancillary components such as tubes, catheters, shunts and dmg carriers. In addition, PDMS has shown good compatibility in clinical research.(7) So it has been a very usefiil synthetic polymer in medicine. In fact, silicone polymers are often employed in checking the biocompatibility of new polymers. Furthermore, organosilicones comprise an important class of compounds used in resins, and room temperature and heat-cured robber industrial and consumer products. 

Practical application of the vulcanization of silicone rubbers using UV light is opening up new areas of application for silicone elastomers, since UV vulcanization enables extremely rapid curing to take place at low temperatures. This process makes it possible to combine silicone elastomers with heat-sensitive plastics, components, chemicals or medicinal products, which would previously have led to the destruction of the material or product because of the high curing temperatures [128]. 

Silicone elastomers have good electrical insulating properties, especially if the decomposition products of the vulcanizing agent are removed by heating the vulcanizate after cure. 

Room temperatur vulcanizing (RTV) silicone elastomers [32] account for about 7097o of the market, and heat-cured types (HCR) account for the remainder 30%. RTV silicone elastomers are based on polymers of intermediate molecular weight, which are comprised of either one component (moistureactivated) or two component systems (contain a catalyst for curing). 

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