Silicone rubbers extrusion

Extrusion Coating - The use of liquid silicone rubber in coating application requires less energy than conventional dispersion coating processes. Solventless material, rapid cure time, absence of by-products and the elimination of multiple passes all contribute to the reduced energy demand of the liquid polymer system. 

Hot vulcanizable silicone rubbers can be processed in two different ways either by extrusion to tubing or cables or by molding to so-called molded articles, such as, for example, crankshaft seals and membranes. High viscosity hot vulcanizable silicone rubber is exclusively used for extrusion articles, extruders typically used in the rubber industry being employed in their processing. 

Another important reinforcement application is in silicone rubber. Historically, fumed silicas have played the major role here, but recently precipitated silicas have been developed that possess the characteristics required for this application (6). Compared to conventional precipitated silicas, a product designed for this end use must have higher purity (to impart acceptable electrical properties, because silicone rubbers are often used as insulating materials) and lower water adsorption (to prevent bubbles from forming during extrusion and to impart resistance against moisture pickup). Good dispersibility is also important. 

SE-. [GE Silicones] Silicone rubber compds. for conq)r., transfer, and inj. molding, extrusion, calendering, o-tings, cylinder lino seals, hydraulic seals, valve gaskets, tubing, belting, shock mounts, tolls, closed cell qxxige, coated fabrics. 

Silastic . [Dow Coming Dow Coming France SA] Silicone rubber conqxl. for conq>r., transfer, or inj. molding, calendering, extrusion, and blending. 

The equipment used for extrusion coating is similar to that used for the extrusion of flat film. Figure 2.30 shows a typical extrusion coating setup. The thin molten film from the extruder is pulled down into the nip between a chill roll and a pressure roll situated directly below the die. The pressure between these two rolls forces the film on to the substrate while the substrate, moving at a speed faster than the extruded film, draws the film to the required thickness. The molten film is cooled by the water-cooled, chromium-plated chill roll. The pressure roll is also metallic but is covered with a rubber sleeve, usually neoprene or silicone rubber. After trimming, the coated material is wound up on conventional windup equipment. 

Again, because silicone rubber will hold up to 20 percent or more of solids, has a high capacity to imbibe drugs, and is compatible with tissue, it has been preferred for most matrix devices (77, 79). Kalkwaif, et al. (20), however, found that ample release rates (14 (ig/day) could be achieved for progesterone by dry mixing this drug with low density polyethylene followed by melt extrusion. Initial release rates, however, were in excess of 2(X) (ig/day and dropped to less than 14 pg/day in six days. 

Various micronized polytetrafluoroethylene powders were compounded with silicone rubber (MQ) and the mechanical properties of the composites were evaluated. At a PTFE level of only 5 wt%, the fractured surface of the composites showed layered structure morphology. This stracture effectively improved the tear strength of the MQ but it also lowered the tensile properties of the composites. The addition of fluorosUicone rubber (FMQ) as a compatibilizer, improved considerably the tensile and tear strength of the composites. Extrusion of the MQ/PTFE/FMQ composites on an electric wire indicated that the spherical PTFE powder was suitable for the extrusion process [55]. 

In this method, the nanofillers are mixed with the polymer matrix in the molten state. This method has great advantages over either in-situ intercalative polymerization or solution intercalation. First, this method is environmentally benign due to the absence of organic solvents. Second, it is compatible with current industrial processes such as extrusion and injection molding. The melt intercalation allows the use of polymers, which are not suitable for in-situ polymerization or the solution intercalation method. A wide range of silicone rubber nanocomposites have been prepared by this method [143,144]. 

The use of hydrosilylation-curing technology also allowed silicone rubber to attain a better surface cure, particularly when hot-air vulcanised. The combination of a polished extruder die and such materials gave an outstandingly smooth, glossy finish to extrusions. Figure 2 shows a selection of parts made from this type of silicone rubber. 

Silicone rubbers have Mooney viscosity values typically in the 35-50 range when measured at room temperature. This is why it is unnecessary to heat silicone rubber in order to obtain good flow characteristics during shaping processes. However, for processors more used to handling organic rubbers, silicone is softer and usually more prone to collapse—for example, in thin section extrusions. 

The use of heat curable liquid silicone rubbers to manufacture a range of supported extrusions and moulded parts is a recent innovation in the development of silicone elastomer processing. For some years, addition cured RTV silicones have been available as heat curable liquid materials, but have never been widely used in this way for a number of reasons ... 

Techniques of supported extrusion are basically similar to those of the cross-head extrusion of rubber and plastic materials used by the wire and cable industries. The main difference is the absence of an extruder Liquid silicone rubber is pumped directly to a cross-head from the meter-mix unit or pressure pot. Curing is often carried out vertically using infra-red heaters or circulated hot air, but horizontal HAV units can also be used. A typical layout is shown in Fig. 7. The technology has been applied successfully to wire coating, optical fibres, ignition core, various tapes and to braided glass... 

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. ... 

Quality control testing of silicones utilizes a combination of physical and chemical measurements to ensure satisfactory product performance and processi-bility. For example, in addition to the usual physical properties of cured elastomers, the plasticity of heat-cured rubber and the extrusion rate of RTV 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. 

Wire and cable insulation has a significant volume of rubber consumption. Rubber compounds based on such elastomers as CPE, EPDM, silicone, and thermoplastic elastomers are used for this purpose. In a continuous extrusion process, these compounds are applied to the wire as insulation. 

The extrusion of silicone elastomers is similar in technique to organic rubbers, but there are certain important differences ... 

Silicones generally have less green strength (see Section 2.5) than organic rubbers and should be cured immediately after extrusion on a continuous basis. 

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