Silicone rubbers curing

Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown (Isayev et al., 1996b) that the revulcanization process of devulcanized SBR was essentially different from those of the virgin SBR. The induction period is shorter or absent for revulcanization of the devulcanized SBR. This is also true for other unfilled and carbon black-filled mbbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules resulting in crosslinking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber (Levin et al., 1997a). 

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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). 

Silicone rubbers are normally cured with peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide and t-butyl perbenzoate being used for the dimethyl-silicones in quantities of 0.5-3%. These materials are stable in the compounds for several months at room temperature but will start to cure at about 70°C. 

Table 29.5 Physical properties of general purpose silicone rubbers (Values determined at 20°C after curing for 24 hours at 250 C) ...

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. 

Uses. There are about forty to fifty organic peroxides commercially available in more than seventy formulations designed for specific applications which include (1) initiators for vinyl monomer polymerizations, and copolymerizations of monomers such as vinyl chloride, ethylene, styrene, vinyl acetate, acrylics, fluoroolefms and buta-dienestyrene (2) curing agents for thermoset polyesters, styrenated alkyds and oils, silicone rubbers and poly allyl diglycol carbonates ... 

Cold cure silicone rubbers and available as pastes. These pastes are mixed with an organometallic catalyst and silicate and cured at room temperature. These are used as adhesives, and as encapsulating materials for electronic components. They are also used for textile coating and in moulds. 

On the other hand, silicones are used in organic composites. The application of alloys composed of the fully cured silicone rubber particles in a continuous thermoplastic phase, called thermoplastic silicone vulcanizate, which offers high-performance materials, has been reported.506,507 Functionalized POSS was not only co-polymerized, but also blended with poly(methylmethacrylate) to study the effect of silsesquioxanes on the thermal stability of thermoplastics.508... 

Hydrosilylation finds practical application as the curing (crosslinking) process in the manufacture of silicone rubbers.407... 

Silicone Rubber A G G A G A P A A Applied as the liquid monomer to magnesium powder and cured after incorporatiori of the other ingredients... 

Systems using either gamma radiation from cobalt 60 or electron beams have been used for vulcanization. The electron beam method has been used for curing silicone rubbers. 

B13. — The curing of silicone rubber with benzoyl peroxide. J. Polymer Sci. 15, 105 (1955). 

Room-temperature-vulcanizing silicone rubber (General Electric and Dow Corning) is available at hardware stores and is very useful as an adhesive and sealant. Atmospheric moisture is necessary to effect the cure, so broad areas of impermeable materials should not be cemented with these preparations. The uncured material evolves acetic acid, and the cured material appears to lose some weight in high vacuum but if used with moderation, it can be considered a satisfactory vacuum sealant for most chemical vacuum systems. 

Hydrosilylation (the addition of R3SiH to a double bond) is an important reaction in the silicone polymer industry. It is used for curing silicone rubber, by cross-linking polymer chains. The reaction is catalyzed by Pt and Rh complexes, following the cycle shown in Figure 3.45. 

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. 

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