Properties of Silicone Rubber

The hardness of silicone rubbers remains almost unchanged down to ca. -50°C and theiefore they are usable in the unusually wide temperature range of -50 to -i-180°C (for short periods up to 300°C). Silicone rubbers exhibit good stability to chemicals except for strong acids, strong bases and chlorine. Under normal environmental conditions they are stable for decades. 

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Silicone rubber-hydrogel composite is a two-phase system that is capable of swelling in water. The hydrogels prepared have different chemical compositions, size and shape of particles, and correspondingly different specific surfaces. It was found that the mechanical properties of silicone rubber-hydrogel composites depend mostly on the magnitude of the contact surface of both phases. ... 

Ghosh, A.K., Rajeev, R.S., Bhattacharya, A.K., Bhowmick, A.K., and De, S.K., Recyebng of silicone rubber waste Effect of ground silicone rubber vulcanizate powder on the properties of silicone rubber, Polym. Eng. Sci., 43, 279, 2003. 

Silicone rubbers have one more very significant advantage in comparison with rubbers based on organic elastomers, and that is high dielectric characteristics. E.g., rubbers based on silicone elastomers do not conduct electric current even at 250-300 °C, whereas rubbers based on organic elastomers become conductive already at 120-150 °C. Insulating properties of silicone rubbers are preserved even at contact with water. 

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. 

Considerable effort has been spent to explain the effect of reinforcement of elastomers by active fillers. Apparently, several factors contribute to the property improvements for filled elastomers such as, e.g., elastomer-filler and filler-filler interactions, aggregation of filler particles, network structure composed of different types of junctions, an increase of the intrinsic chain deformation in the elastomer matrix compared with that of macroscopic strain and some others factors [39-44]. The author does not pretend to provide a comprehensive explanation of the effect of reinforcement. One way of looking at the reinforcement phenomenon is given below. An attempt is made to find qualitative relations between some mechanical properties of filled PDMS on the one hand and properties of the host matrix, i.e., chain dynamics in the adsorption layer and network structure in the elastomer phase outside the adsorption layer, on the other hand. The influence of filler-filler interactions is also of importance for the improvement of mechanical properties of silicon rubbers (especially at low deformation), but is not included in the present paper. 

Razzaghi-Kashani M, Gharavi N, Javadi S (2008) The effects of organo-clay on the dielectric properties of silicone rubber. Smart Mater Struct 17 065035... 

Properties of Silicone Rubbers. Gum vulcanizates have essentially no tensile strength. Fillers are therefore essential. Reinforcing silicas are frequently used. Even then, the tensile properties and abrasion resistance of silicone rubber vulcanizates are poor in comparison with of other types of elastomer. However, the properties change very little with increasing temperatures. These polymers excel in high-temperature applications. 

The physical bonding mechanism relies on van der Waals and London Dispersion forces. This omnipresent mechanism is favoured by the low surface energy and high degree of mobility of the polymeric chains. It is the basis of the Release property of silicone rubbers and coatings. 

Dai, L. Zhang, Z. Zhao, Y. Liu, H. Xie, Z., Effects of Polymeric Curing Agent Modified with Silazanes on the Mechanical Properties of Silicone Rubber. J. Appl. Polym. Sci. 2009, 111, 1057-1062. 

Swanson, J. W. Lebeau, J. E. (1974) The effect of implantation on physical properties of silicone rubber. Journal of Biomedical Materials Research, 8, 357-367. 

The characteristics of silicone rubber vary with both the kind and amount of additives used and the mixing and vulcanization conditions. Various thermal properties of silicone rubber can be inspected by means of thermal analysis. 

Vulcanized silicone rubber is characterized by its wide temperature use range (-50 to >200°C), excellent electrical properties, and resistance to air oxidation and weathering conditions (484). Improvements in properties such as heat aging can be achieved by in situ treatment of the filler during rubber manufacture (480). Silicone rubber is also extremely permeable to gases and water vapor. The mechanical properties of silicone rubber are generally inferior to most organic (butyl)... 

Another, useful property of silicone rubbers is their unique electrical behaviour. Silicone rubbers, if processed correctly exhibit extremely high volume resistivity, high dielectric strength and excellent tracking and arcing resistance. This is also the reason why substantial quantities of silicone rubbers are used by the cable, cable accessory and insulator and electronics industries. 

A significant advance in the technique of controlled release will be given by the development of polymer systems which combine the release properties of silicone rubbers with biodegradability. It is immediately obvious that the rate of biodegradation has to be coordinated with the projected life span of the device, i.e., during the release period the release properties of the device should remain essentially unchanged. In the ideal case substantial degradation should commence after depletion of the device. 

Table 3. VIechanical properties of silicone rubber vu canizates...

S.M. Gubanski, Properties of silicone rubber housings and coatings, IEEE Transactions on Electrical Insulation, Vol. 27, Issue 2, p. 374-382, Apr. 1992. 

W. Shaowu, L. Xidong, G. Zhicheng, W. Xun, Hydrophobicity transfer properties of silicone rubber contaminated by different kinds of pollutants, in 2000 Conference on Electrical Insulation and Dielectric Phenomena, Canada, 2000, p. 373-376. 

Wang, S., et al.. Synthesis and properties of silicone rubber/organomontmorillonite hybrid nanocomposites. Journal of Applied Polymer Science, 1998. 69(8) 1557-1561. 

A very high proportion of the usage of silicone rubbers is associated with electrical applications, particularly wire and cable for the electrodomestic appliance and automotive industries. The inherent properties of silicone rubber make it a natural choice in areas such as safety (flame retardant) cables and food contact applications. Examples of the latter uses are given. In addition reference is made to automotive hose, electrically conductive applications, dynamic seals and medical implants. 

The trend towards the use of silicone rubber in consumer products (cars, domestic appliances, etc.) seems certain to expand its use. This has been made possible by the unique properties of silicone rubber combined with a new commercial and technical flexibility for the fabricator offinished parts. 

Table 1. Effect of irradiation dose before foaming on physical and mechanical properties of silicone rubber foam... 

Electron beam irradiation can be used to precisely tailor properties of silicone rubber and it is a valuable tool to define the response of these materials in medical applications where irradiation sterilization is used. E-beam is an effective technology for crosslinking to predict property changes to materials used in critical applications. E-BEAM Services has performed other studies to predict life cycle estimates. Two examples are wire and cable materials used in nuclear facilities and polymeric materials designed for use in space applications. 

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. 

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