Electrical insulation shrinkage

The polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications. Their value in casting and encapsulation lies mainly with their low curing shrinkage and flexibility in the cured state. Their tendency to corrode copper and the somewhat inferior electric insulation properties of the blends does lead to certain limitations. 

Wood flour, a fine sawdust preferably from soft woods, is the most commonly used filler. Good adhesion occurs between the resin and the wood flour, and some chemical bonding may also occur. Wood flour reduces exotherm and shrinkage, improves the impact strength of the moldings, and is cheap. For better impact strength cotton fabric or chopped fabric may be incorporated. Asbestos may be used for improved heat and chemical resistance, and iron-free mica powder maybe used for superior electrical insulation resistance characteristics. 

The main attributes of properly cured epoxy systems are outstanding adhesion to a wide variety of substrates, including metals and concrete ability to cure over a wide temperature range very low shrinkage on cure excellent resistance to chemicals and corrosion excellent electrical insulation properties and high tensile, compressive, and flexural strengths. 

Fillers may be divided into particulate and fibrous types. Particulates include calcium carbonate, china clay, talc and barium sulphate. Fillers affect shrinkage on moulding and the dimensional stability of the finished plastic, increase tensile strength and hardness, enhance electrical insulation properties and reduce tackiness. They also impart opacity and colour (Figure 3.16). Carbon black is now the most widely used filler for polymers usually in the form of furnace carbon black, which has a particle diameter of 0.08 mm. Fibrous fillers reinforce polymers and greatly increase their tensile strengths. They include fibres of glass, textile and carbon. Plastics filled with fibrous fillers are known as composites. 

Epoxy adhesives are also easy to modify for special purposes. For example, they can be filled with carbon, silver, or gold to provide electrical conductivity. Other additives can enhance thermal conductivity, while maintaining electrical insulation. Additional performance properties of epoxy-based adhesives that can be modified include impact resistance, shrinkage, glass transition temperature, high-temperature strength, surface specific adhesion characteristics, and chemical or moisture resistance. 

Allylic ester resins possess excellent clarity, hardness, and color stability and thus are used to cast into optical parts. These castings can be either homopolymers or copolymers. The free radical addition polymerization of the allylic ester presents some casting difficulties such as exotherm control, monomer shrinkage during curing, the interaction between the exotherm and the free radical source, and the environmental heat required to decompose the peroxide and initiate the reaction. Cast allylics are noted for their hardness, heat resistance, electrical properties, and chemical resistance. They lack strength, and so their usage is confined to optical parts and some small electrical insulators. (See also allyl.)... 

Epoxy vinyl esters Low viscosity, high cure shrinkage, wets fibers easily, fast cures Excellent corrosion resistance, high impact resistance, excellent electrical insulation properties Absorption towers, process vessels, storage tanks, piping, hood scrubbers, ducts and exhaust stacks... 

---