Silicone elastomers have been employed for many years in medical, aerospace, electrical, construction, and industrial applications. Good resistance to compression set, flexibility over wide temperature ranges, a wide range of durometers, inert and stable compounds are among the reasons for their popularity today.

CHEMICAL STRUCTURE

Silicone rubbers are synthetic polymers with molecular structure consisting of a giant backbone with alternating silicon and oxygen atoms.

VULCANISATION

There are two popular catalyst systems used to cross-link silicone polymers: peroxide (free-radical) systems and platinum (addition-cure) systems.

First generation silicones used peroxide as the catalyst to initiate curing of the silicone. However, the peroxide reaction leaves a residue in the rubber that can deposit a powder on the part surface if not removed through a postcure process.

Though peroxide is still used, addition-cured, platinum-catalyzed silicones are used nowadays on a large scale because of their faster cure rates, lack of peroxide, and availability in an injectable, liquid form.

PROPERTIES

The strong silicon-oxygen chemical structure of silicone gives the elastomer its unique performance properties, including biocompatibility, superior temperature and chemical resistance, good mechanical and electrical properties, and natural clarity or translucence.

Mechanical Properties. Silicone rubbers have high tear and tensile strength, good elongation ( up to 1300%) and flexibility, low compression set, and a durometer range of 5 to 80 Shore A.

Electrical Properties. Silicones exceed all comparable materials in their insulating properties as well as in their versatility for electrical applications. They are nonconductive and can maintain dielectric strength in temperature extremes far higher or lower than those in which conventional insulating materials are able to perform.

Chemical Resistance. Silicones resist water and many chemicals, including some acids, oxidizing chemicals, ammonia, and isopropyl alcohol. Concentrated acids, alkalines, and solvents should not be used with silicones. However some grades have poor hydrocarbon, oil and solvent resistance.

Heat Resistance. Silicones can withstand a wider range of temperature extremes than nearly all other elastomers, remaining stable through temperature variations from –50° to 250°C. They can be sterilized via steam autoclaving, gamma or E-beam irradiation, and other methods.

Gas permeability. Silicone rubbers exhibit high gas permeability, allowing for this material to be used in medical applicattions such as oxigen permeable membranes.

Biocompatibility. In extensive testing, silicone rubbers have exhibited superior compatibility with human tissue and body fluids and an extremely low tissue response when implanted, compared with other elastomers. Odorless and tasteless, silicones do not support bacteria growth and will not stain or corrode other materials.

PROCESSING

Silicone elastomers are typically molded by three main methods: liquid injection molding (LIM), transfer molding, or compression molding. In designing for the molding process, designers should take into account the material shrinkage rate, which can range from 2 to 4% depending on the type of silicone.

During molding, the three variables that must be controlled are temperature, pressure, and time. The temperature must be high enough to minimize cure times, yet low enough to prevent scorching of the elastomer. The pressure selected must allow for complete mold filling while permitting the venting of all the air, and must be optimized to prevent voids and flash. As in most molding, precise timing of all functions is critical for the production of consistently high-quality, fully cured parts.

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