Silicone vulcanisation

Silicone elastomers are used to produce numerous molded products which become parts of artificial organs (artificial hearts and assist circulation systems, joint prostheses, etc.). The use of silicone vulcanisates for coating the walls of the artificial ventricle allows it to be used for prolonged assist circulation and prevents the formation of clots on its surface. 

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). 

Room temperature vulcanising silicone rubbers (r. t. v. rubbers) have proved of considerable value where elaborate processing equipment is not available. These rubbers are low molecular weight silicones with reactive end-groups and loaded with reinforcing fillers. The RTV silicone rubbers may be classified into two types ... 

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. 

If polypropylene is too hard for the purpose envisaged, then the user should consider, progressively, polyethylene, ethylene-vinyl acetate and plasticised PVC. If more rubberiness is required, then a vulcanising rubber such as natural rubber or SBR or a thermoplastic polyolefin elastomer may be considered. If the material requires to be rubbery and oil and/or heat resistant, vulcanising rubbers such as the polychloroprenes, nitrile rubbers, acrylic rubbers or hydrin rubbers or a thermoplastic elastomer such as a thermoplastic polyester elastomer, thermoplastic polyurethane elastomer or thermoplastic polyamide elastomer may be considered. Where it is important that the elastomer remain rubbery at very low temperatures, then NR, SBR, BR or TPO rubbers may be considered where oil resistance is not a consideration. If, however, oil resistance is important, a polypropylene oxide or hydrin rubber may be preferred. Where a wide temperature service range is paramount, a silicone rubber may be indicated. The selection of rubbery materials has been dealt with by the author elsewhere. ... 

A vulcanising agent particularly for silicone rubber and fluoroelastomers it has been used as a non-sulphur vulcanising agent for natural rubber. It is also a catalyst in emulsion polymerisation. Beta Rays... 

A method of manufacturing rubber articles by pouring a compounded latex into an absorbent hollow mould the skin of rubber thus formed is removed, dried and vulcanised. The term is also apphed to the pouring into moulds of liquid polymer systems based on silicone or polyurethane elastomers. 

S2C12, a by-product in the manufacture of carbon tetrachloride from carbon disulphide. Was used, dissolved in solvent naphtha, in the vulcanising of mbber by the cold cure process and the vapour cure process. The process was fraught with health and safety problems and has been superseded by low temperature accelerators and room temperature vulcanising (RTV) systems for silicone and polyurethane. 

It is possible to replace a few of the methyl groups (<0.5%) with a vinyl group, and the resultant vinyl methyl silicones (ISO designation VMQ) exhibit improved vulcanisation characteristics and... 

Being saturated, the resistance of silicones to oxygen, ozone and UV light is excellent, but means that peroxides have to be used for vulcanisation. 

Silicones are readily available in liquid form (LR or LSR), room temperature vulcanising (RTV) and high temperature vulcanising (HTV). 

The degree of moisture present affects the properties of the silicone rubber vulcanisate. Moisture levels also determine the ease with which the filler is incorporated into the silicone rubber. Low moisture levels improve the final physical properties but definitely detract from the incorporation speed of the silica filler. 

A silicone oil plasticiser is desirable to facilitate dispersion and to prevent undesirable polymer-filler interaction prior to vulcanisation. 

Many rubber compounds have a tendency to stick in the mould cavity after vulcanisation and require some type of mould release agent. The substances used are surface-active materials such as detergents, soaps, wetting agents, silicone emulsions, aqueous dispersions of talc, mica and fatty acids, applied by spray or brush. Alternatively, dry types based on polytetrafluoroethylene or polyethylene, usually carried in a solvent, can be aerosol applied. An alternative is the addition of an incompatible material to the rubber compound which will bleed to the rubber surface during vulcanisation. 

Hot air tunnels are often used for vulcanisation of cable covered with rubbers such as silicone. These systems may also incorporate infrared radiation as a means of boosting heat transfer to the product. 

Conveyor systems are applied in a number of areas in the rubber industry. The types used can range from simple canvas belt conveyors used for haul-off from conventional extruders, to systems used for transport and cooling of profile products, both in and emerging from continuous vulcanisation units. The latter types have to be resistant to the temperatures used in such systems and are variously constructed from glass fibre-reinforced polytetrafluoroethylene or a silicone rubber covered belt. 

A TG-DTA study of the thermochemical processes occurring at vulcanisation temperatures with N-oxydiethylene-2-benzthiazyl sulphenamide and N-cyclohexyl-2-benzthiazyl sulphenamide and their mixtures with sulphur showed the formation of high molecular weight polysulphides [73]. The influence of metallic oxides (Fe203, Sn02) on hot air ageing of one-pack room temperature vulcanised fluorosilicone rubber has been studied by means of TG-DTA [74, 75]. TG-DTA and TG were both applied to study the thermal characteristics of room temperature vulcanised silicone rubber [76]. 

As a result of its saturated polymer backbone, EPDM is more resistant to oxygen, ozone, UV and heat than the low-cost commodity polydiene rubbers, such as natural rubber (NR), polybutadiene rubber (BR) and styrene-butadiene rubber (SBR). Therefore, the main use of EPD(M) is in outdoor applications, such as automotive sealing systems, window seals and roof sheeting, and in under-the-hood applications, such as coolant hoses. The main drawback of EPDM is its poor resistance to swelling in apolar fluids such as oil, making it inferior to high-performance elastomers, such as fluoro, acrylate and silicone elastomers in that respect. Over the last decade thermoplastic vulcanisates, produced via dynamic vulcanisation of blends of polypropylene (PP) and EPDM, have been commercialised, combining thermoplastic processability with rubber elasticity [8, 9]. 

Methylphenyldimethoxysilane is used as a stabiliser (antistructuring additive) in the production of rubber compounds based on silicone elastomers and highly active fillers. Introducing up to 10% (weight) of methylphenyldimethoxysilane into a rubber mixture improves the physicochemical properties of vulcanised rubbers and helps to preserve the technological characteristics of the compounds in storage. 

Silicone elastomer-based rubber compounds are prepared in conventional apparatuses (closed agitators, roll mills, etc.) and consist of the following ingredients elastomer, active fillers, vulcanising agent, stabiliser, pigment additives. 

Silicone rubber compounds can be molded, extruded and calendered. The vulcanisation of rubber compounds occurs in two stages 1) in a press or steam boiler at high pressure and 120-150 °C 2) by thermostatic control at atmospheric pressure and 200-250 °C. 

The main properties of some rubber compounds and vulcanised rubbers, as well as their applications, are given in Table 20. As seen from the table, silicone elastomer-based rubbers are designed for prolonged use in a wide range of temperatures from -50 to +250 °C, some from -70 to + 350 °C (for a short period of time). These rubbers are efficient in air, ozone and in an electric field rubbers based on IRP-1339 and IRP-1401 compounds are also efficient in case of limited air supply. They function well in high humidity and under the influence of oxidants, hot water, vapour and low pressure. They are stable in weak-acid and weak-alkali media and are nontoxic. 

Dethyltin dicaprylate is used as a stabiliser in polymers and as a catalyst in the process of their production. Besides, the tetraethoxysilane solution of diethyl tin dicaprylate is used as a vulcaniser for elastomers and for the production of silicone compounds and sealants. 

Today, silicones have become virtually irreplaceable materials worldwide and have an extremely wide range of applications. The use of silicone room-temperature vulcanising (RTV) elastomers in the construction industry originated in the early 1960s. The value of silicone RTV elastomers is based on important properties such as their thermal stability, unusual surface properties, water repellency, high permeability, oxidative stability and ultraviolet (UV) resistance (Cash, 1970). 

Silicone coatings used in the construction industry are one-component silicone rubber dispersions. They cure at room temperature, vulcanisation being effected by the influence of atmospheric humidity. The process evolves various primary aliphatic amines and the solvent evaporates. 

Silicones are well known for their versatility, which makes them ideally suitable for a variety of applications. The fluids can be used as solvents, as foam-control systems, or as release agents (20% of the total volume). High-molecular-weight silicones are mainly used in mbber applications such as High Temperature Vulcanisable (HTV) and Room Temperature Vulcanisable (RTV) (43%), resins (4%), or specialties (15%). Other applications for silicones are masonry protection (8%), textiles (7%), and paper coatings (3%). Silicones can be uniquely tailored for each application area by substitution by reactive groups, allowing them to be cured by different mechanisms. 

In parallel with these developments, organotin compounds have found a variety of applications in industry, agriculture, and medicine, though in recent years these have been circumscribed by environmental considerations. In industry they are used for the stabilization of poly(vinyl chloride), the catalysis of the formation of the polyurethanes, and the cold vulcanisation of silicone polymers, and also as transesterification catalysts. 

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