Compounding insulating properties

Particulate fillers are divided into two types, inert fillers and reinforcing fillers. The term inert filler is something of a misnomer as many properties may be affected by incorporation of such a filler. For example, in a plasticised PVC compound the addition of an inert filler will reduce die swell on extrusion, increase modulus and hardness, may provide a white base for colouring, improve electrical insulation properties and reduce tackiness. Inert fillers will also usually substantially reduce the cost of the compound. Amongst the fillers used are calcium carbonates, china clay, talc, and barium sulphate. For normal uses such fillers should be quite insoluble in any liquids with which the polymer compound is liable to come into contact. 

For this reason tribasic lead sulphate, a good heat stabiliser which gives polymer compounds with better electrical insulation properties than lead carbonate, has increased in popularity in recent years at the expense of white lead. Its weight cost is somewhat higher than that of lead carbonate but less than most other stabilisers. This material is used widely in rigid compounds, in electrical insulation compounds and in general purpose formulations. 

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Furthermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres. 

Being either brittle or soft, these resins do not have the properties for moulding or extrusion compounds. These are, however, a number of properties which lead to these resins being used in large quantities. The resins are chemically inert and have good electrical insulation properties. They are compatible with a wide range of other plastics, rubbers, waxes, drying oils and bitumens and are soluble in hydrocarbons, ketones and esters. 

The issues in these evaluations are safety as related to toxicity and flammability, environmental impact as related to the generation of volatile organic compounds and global warming, product performance as related to insulating properties, conformity to fire codes, and the like, cost and availability, and regulatory requirements. 

Teflon, a fluorocarbon polymer, is well known for chemical inertness, thermal stability at temperatures up to 290°C, and excellent electrical insulating properties. Most inorganic and many organic compounds are insoluble in it. Teflon also exhibits a relatively large hydrogen gas permeability. It, therefore, has potential as a selective osmotic membrane for hydrogen. 

From equation 46 it is possible to derive a necessary condition for semiconductor or insulator properties The Fermi level may lie within an energy gap if the number of electrons per atom (only atoms whose orbitals participate in band formation are counted) is nj i 2(21 + l)/v, where l is the angular momentum of the atomic orbitals participating in partially filled bands, v is the number of atoms per primitive unit cell, and n is an integer. (Perturbation mixtures from higher states do not change the number of states in the band.) Thus if s and p states are admixed (id and / electrons neglected), a compound may be a semiconductor or insulator if the number of electrons per atom is... 

To dissolve ionic compounds a solvent must also have a high dielectric con- stant, that is, have high insulating properties to lower the attraction between oppositely charged ions once they are solvated. 

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