Silicone rubber blend properties

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. 

Blending of polymers provides a convenient way of combining the different properties of individual polymers. Hydrophilization of the silicone mbber can be obtained by blending silicone rubber with hydrogels. These kinds of composites combine the good mechanical properties with the hydrophilicity. 

Ghosh, A. and De, S.K. Dependence of Physical Properties and Processing Behavior of Blends of Silicone Rubber and Fluorombber on Blend Morphology. Rubber Chem. Technol. 77(5), 856-872, November/December 2004. 

Kole S, Roy S, Bhowmick AK (1995) Influence of chemical interaction on the properties of silicone-EPDM rubber blend. Polymer 36(17) 3273-3277... 

Many components in medical devices are made of rubber and rubber blends, consisting of latex, polyisoprene, silicone rubber, fluoroelastomers, ethylene propylene, etc. Some examples of conunercial products refer to seals for medical appliances, O-rings for dialyzers, medical pump seals, intravenous components, feeding devices, etc [http //www.applerubber.com/products/medical-seals.cfm]. However, if the concerned product interacts with tissues or biological fluids, it must fulfill some requirements referring to the biocompatibility, temperature resistance, chemical stability, mechanical properties, and electrical properties. Silicone rubber assures aU of these demands, being the most encountered type of rubber used in the fabrication of medical device parts. It is commonly used for the fabrication of different tubes and catheters because of its hemocompatibility and inert character. 

The thermal stability was also evaluated for blends made of SBR-recycled NBR, which were stabilized using electron beam irradiation [171]. The application of this material is given by the recycling of rubber gloves. The use of silicone rubber in different industrial applications is given by its properties, such as (1) thermal stability, (2) dielectric properties, and (3) ozone and corona resistance. 

Wang, Y Hu, Y Gong, X. Jiang, W. Zhang, P. Chen, Z., Preparation and Properties of Magnetorheological Elastomers Based on Silicone Rubber/ Polystyrene Blend Matrix. J. Appl. Polym. Sci. 2007,103, 3143-3149. 

Polymer alloys are a commercial polymer blend with improvement in property balance with the use of compatibilizers. They exhibit an interface and show varied physical characteristics. Sometimes they have excellent physical properties in one area but possess poor physical properties in others. For example, silicone rubber has poor oil and abrasion resistance but possess excellent heat resistance. A product solution in this regard would be to obtain a polymer blend with constituents possessing physical properties that complement each other such that the resultant polymer blend would exhibit superior physical properties compared with the components of the blend. 

The miscibility of natural rubber (NR) blends is one of the most important factors when designing NR products. For instance, when the NR is miscible with a dissimilar polymer on a molecular level, we may improve the properties of NR as a function of the composition of the polymer. This is significantly different from the design for immiscible NR blends, whose properties are greatly dependent upon the morphology of the blend but less so on the composition. In most cases, NR is immiscible with non-polar synthetic rubbers, i.e. NR/butadiene rubber (BR) with high c -1,4-butadiene units, NR/styrene-butadiene rubber (SBR), NR/butyl rubber (IIR), NR/silicone rubber (q)13,i4 NR/ethylene-propylene-diene rubber (EPDM). This means it is important to find miscible NR blends and to control the morphology of the immiscible NR blends in a rational way. In this chapter, properties of NR blends are described from the viewpoint of miscibility, i.e. the miscible blend of NR/BR and the immiscible blend of NR/SBR. 

In addition to the diene rubbers, it is also possible to use DMA to identify blends of the other commercially available rubbers, provided that there is a sufficient difference in their glass transition temperature, and to characterise their properties. For example, Carlberg, Colombini and Maurer [44] mixed ethylene-propylene rubber and silicone rubber in a number of blend ratios and studied their morphology and viscoelastic properties. The results obtained experimentally by DMA and DSC were compared to theoretical data produced from self-consistent models, both indicating that the silicone rubber was the dispersed phase in a continuum of ethylene-propylene rubber. 

The effect of thermally conductive particles on the thermal properties of silicone rubber was studied. Different sized aluminum oxide was blended with addition cured silicone resin at various crosslink densities and filler loading levels. Thermal impedance of each sample was measured. Statistical analysis of the experimental data showed that hardness was not affected by filler type/size or filler amount however, the amount of crosslinker was statistically significant with respect to hardness. 

Several other resins can be blended with alkyd resins to introduce desired improvement in properties, e. g. cellulose nitrate, chlorinated rubber, phenolics, amino resins or silicons oils. Vinyl monomers like styrene can be added to alkyd resins along with initiator to get a tougher resin with shorter drying times and lighter colour. 

---