Strength silicone rubber

Table 4.12 Adhesive shear strengths (silicone rubber)...

In order to overcome this shortcoming high green strength silicone rubber is now available. It was first developed in the USA in order to allow hose to be made with silicone rubber by a continuous process originally designed for organic rubbers. Since then it has by virtue of its excellent... 

Silicone rubbers find use because of their excellent thermal and electrical properties, their physiological inertness and their low compression set. Use is, however, restricted because of their poor hydrocarbon oil and solvent resistance (excepting the fluorosilicones), the low vulcanisate strength and the somewhat high cost. 

Similar types of lamellar morphologies were observed for triblock copolymers of diphenylsiloxane and dimethylsiloxane having 40 wt% polydiphenylsiloxane, using electron microscopy, 47-148>. The lamellae thickness was approximately equal to the chain length of the rigid polydiphenylsiloxane blocks. These copolymers showed elastomeric properties comparable to those of conventional silica-reinforced, chemically crosslinked silicone rubbers. Tensile tests yielded an initial modulus of 0.5-1 MPa, tensile strength of 6-7 MPa and ultimate elongation between 400 and 800 %. 

Silicone rubbers exhibit good resistance to heat ageing, and are considered to be usable up to temperatures of 200 °C. Although silicones do not exhibit high strength at room temperature, they do retain their properties at high temperatures to a much greater extent than other rubbers. 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

Fig. 7.4. Fracture toughness (O) and flexural strength ( ) of silicone rubber coated carbon fiber-epoxy matrix composites as a function of coating thickness. After Hancox and Wells (1977).

Coagulation is not the only problem with materials intended for implantation, however. Cardiac pacemakers are intended to correct arrhythmias. Insulating materials for a pacemaker lead must be tough and long lasting. The first leads were insulated with polyethylene or silicone rubber. Neither material was considered ideal because of endocardial reactions (polyethylene) and limited durability (silicone rubber). The strength and flexibility of polyurethanes led to their introduction in 1978 as lead insulators. 

Silicone rubber has both excellent low temperature and high temperature properties. It can withstand temperatures up to 315°C and workable at -65°C. Poor performance with low tear strength and abrasion resistance limit their use in most applications. Liquid silicone compounds LTV which are room temperature vulcanizable are useful for small repairs and sealing application and have been used for poured-in-place gaskets. 

Curves for typical polymeric materials are shown in Figure 2.47. The first is for a soft and weak material, such as an unfilled silicone rubber. Soft refers to the fact that the initial slope is small which means a low value of the modulus. Weak refers to the low value of the ultimate strength. One does not have to be very strong to pull it apart. 

Figure 9.6 shows the separation factors measured by Nijhuis et al. [28] for various membranes with dilute toluene and trichloroethylene solutions. The separation factor of silicone rubber is in the 4000-5000 range, but other materials have separation factors as high as 40000. However, in practice, an increase in membrane separation factor beyond about 1000 provides very little additional benefit. Once a separation factor of this magnitude is obtained, other factors, such as ease of manufacture, mechanical strength, chemical stability, and control of concentration polarization become more important. This is why silicone rubber remains prevalent, even though polymers with higher selectivities are known. 

As in carbon-black-filled EPDM and NR rubbers, the physical network in silica-filled PDMS has a bimodal structure [61]. A loosely bound PDMS fraction has a high density of adsorption junctions and topological constraints. Extractable or free rubber does virtually not interact with the silica particles. It was found that the density of adsorption junctions and the strength of the adsorption interaction, which depends largely on the temperature and the type of silica surface, largely determine the modulus of elasticity and ultimate stress-strain properties of filled silicon rubbers [113]. 

An elastomer filled with Aerosil, technical carbon (lamp or acetylene black), iron and titanium oxides and other ingredients including a vulcan-iser is raw rubber used to manufacture various products. The elasticity and resilience of silicone rubbers depend on the number of siloxane links in the chain and on the number of cross links. The higher the molecular weight of the elastomer and its elasticity the more the quantity of cross links (to a certain extent), the greater its mechanical strength. 

Of considerable interest is the use of silicone rubbers for insulation in electrotechnical equipment. This is accounted for by superior heat resistance of elastomers and their good dielectric properties. E.g., the dielectric permeability of polyorganosiloxane elastomers at 500 V and 60 Hz is 3.5-5.5, their electric strength at 60 Hz is 15-20 KV/mm, and the dielectric loss tangent, which characterises the losses of electric energy in insulation, at 500 V and 60 Hz amounts only to 0.001. It is very important that these characteristics are preserved in a much wider temperature range than in the case of natural and synthetic organic elastomers. 

Prior to the introduction of the LPS process, the low consistency liquid silicone rubber was not considered for use in fabricated parts because of the inadequate physical properties. Recent advancements in the low consistency silicone elastomer technology, however, have led to the development of high strength material. 

As with silicone oil, the properties of silicone rubber change slowly with temperature the elasticity persists down to —55° C. Although the mechanical properties require improvement before the material can be recommended for usage under severe stress or abrasion, it is well suited to other applications where thermal stability and resistance to chemical reagents are more important than tensile strength or tear resistance. 

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