Electrical insulation properties

Ethylene. Under the influence of pressure and a catalyst, ethylene yields a white, tough but flexible waxy sohd, known as Polythene. Polyethylene possesses excellent electric insulation properties and high water resistance it has a low specific gravity and a low softening point (about 110°). The chemical inertness oi Polythene has found application in the manufacture of many items of apparatus for the laboratory. It is a useful lubricant for ground glass connexions, particularly at relatively high temperatures. 

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene, catalysed by oxygen, yields polytetrafluoroethylene (Tejlon) as a very tough horn-hke material of high melting point. It possesses excellent electrical insulation properties and a remarkable inertness towards all chemical reagents, including aqua regia. 

Electrical Properties. Like unfluorinated siHcone counterparts, fluorosihcone elastomers have inherently good electrical insulating properties. The dielectric properties remain relatively unchanged when the elastomer is exposed to severe environments. 

Poly(phenylquinoxaline—arnide—imides) are thermally stable up to 430°C and are soluble in polar organic solvents (17). Transparent films of these materials exhibit electrical insulating properties. Quinoxaline—imide copolymer films prepared by polycondensation of 6,6 -meth5lene bis(2-methyl-3,l-benzoxazine-4-one) and 3,3, 4,4 -benzophenone tetracarboxyUc dianhydride and 4,4 -oxydianiline exhibit good chemical etching properties (18). The polymers are soluble, but stable only up to 200—300°C. 

Cases can be classified as either hermetic or nonhermetic, based on their permeabiUty to moisture. Ceramics and metals are usually used for hermetic cases, whereas plastic materials are used for nonhermetic appHcations. Cases should have good electrical insulation properties. The coefficient of thermal expansion of a particular case should closely match those of the substrate, die, and sealing materials to avoid excessive residual stresses and fatigue damage under thermal cycling loads. Moreover, since cases must provide a path for heat dissipation, high thermal conductivity is also desirable. 

The maximum recommended film thickness is 25 p.m. At greater thicknesses, volatiles from the curing reaction, mainly water and some formaldehyde and phenol, can cause defects. These coatings have excellent electrical insulation properties, ie, up to 20 V/p.m, because of low moisture absorption and low conductance. The coatings are hard with low flexibiUty, depending on curing conditions and film thickness. 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

Highly desirable properties of PPS include excellent chemical resistance, high temperature thermal stabiUty, inherent flame resistance, good inherent electrical insulating properties, and good mechanical properties. 

The insulation around the central electrode is an example of a non-metallic material - in this case, alumina, a ceramic. This is chosen because of its electrical insulating properties and because it also has good thermal fatigue resistance and resistance to corrosion and oxidation (it is an oxide already). 

The final polymerised product is formed in particles much smaller (50-500 nm) than produced with suspension polymerisation. Emulsion polymerisation can lead to rapid production of high molecular weight polymers but the unavoidable occlusion of large quantities of soap adversely affects the electrical insulation properties and the clarity of the polymer. 

Finally mention may be made about the influence of humidity on the electrical insulating properties of plastics. Once again the polymers may be classified into two groups, those which do not absorb water and those which do. The nonabsorbent materials are little affected by humidity whereas the insulation characteristics of the absorbent materials deteriorate seriously. These latter materials are generally certain polar materials which all appear capable of forming some sort of bond, probably a hydrogen bond, with water. Three reasons may be given for the deleterious effects of the water. 

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. 

Development of polar groups such as carbonyl groups in polyolefins causing a deterioration in electrical insulation properties and also changes in chemical activity. 

Some materials are able to withstand quite lengthy thermal histories , a term loosely used to describe both the intensity (temperature) and the duration of heating. Polyethylene and polystyrene may often be reprocessed a number of times with little more than a slight discoloration and in the case of polyethylene some deterioration in electrical insulation properties. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

The excellent electrical insulation properties of polyethylene have led to extensive use in cable and other wire-covering applications. Spectacular early uses included undersea cables and airborne radar and the materials continue to be used in substantial quantities. One particular trend is the increasing use of cross-linked polyethylene for this area of use. Such materials have improved heat resistance and in addition have given generally better resistance to stress cracking. Cellular polyethylene is used as the insulator for television downlead aerials. 

As saturated hydrocarbon copolymers the ESIs will have many of the characteristics of such materials including good electrical insulation properties. 

In the early days of the commercial development of PVC, emulsion polymers were preferred for general purpose applications. This was because these materials exist in the form of the fine primary particles of diameter of the order of 0.1-1.0 p,m, which in the case of some commercial grades aggregate into hollow secondary particles or cenospheres with diameters of 30-100 p,m. These emulsion polymer particles have a high surface/volume ratio and fluxing and gelation with plasticisers is rapid. The use of such polymers was, however, restricted because of the presence of large quantities of soaps and other additives necessary to emulsion polymerisation which adversely affect clarity and electrical insulation properties. 

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. 

There has been some recent interest in polymers containing very small proportions (<2000 ppm) of a second comonomer. These can interfere with crystallisation and the resulting products are claimed to have improved compression strength, electrical insulation properties, weldability and transparency compared with the unmodified homopolymers. 

Its excellent electrical insulation properties lead to its use in wire insulation, in valve holders, in insulated transformers, in hermetic seals for condensers, in laminates for printed ciruitry and for many other miscellaneous electrical applications. 

The commercial polymers are mechanically similar to PTFE but with a somewhat greater impact strength. They also have the same excellent electrical insulation properties and chemical inertness. Weathering tests in Florida showed no change in properties after four years. The material also shows exceptional non-adhesiveness. The coefficient of friction of the resin is low but somewhat higher than that of PTFE. Films up to 0.010 in thick show good transparency. 

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. 

A 50 50 mol/mol copolymer of hexafluoroisobutylene (CH2 = C(CF3)2) and vinylidene fluoride was made available by Allied Chemical in the mid-1970s as CM-1 Fluoropolymer. The polymer has the same crystalline melting point as PTFE (327°C) but a mueh lower density (1.88g/cm ). It has excellent chemical resistance, electrical insulation properties and non-stiek characteristics and, unlike PTFE, may be injeetion moulded (at 380°C). It is less tough than PTFE. 

Because the polymer is polar it does not have electrical insulation properties comparable with polyethylene. Since the polar groups are found in a side chain these are not frozen in at the Tg and so the polymer has a rather high dielectric constant and power factor at temperatures well below the Tg (see also Chapter 6). This side chain, however, appears to become relatively immobile at about 20°C, giving a secondary transition point below which electrical insulation properties are significantly improved. The increase in ductility above 40°C has also been associated with this transition, often referred to as the 3-transition. 

The use of ABS has in recent years met considerable competition on two fronts, particularly in automotive applications. For lower cost applications, where demands of finish and heat resistance are not too severe, blends of polypropylene and ethylene-propylene rubbers have found application (see Chapters 11 and 31). On the other hand, where enhanced heat resistance and surface hardness are required in conjunction with excellent impact properties, polycarbonate-ABS alloys (see Section 20.8) have found many applications. These materials have also replaced ABS in a number of electrical fittings and housings for business and domestic applications. Where improved heat distortion temperature and good electrical insulation properties (including tracking resistance) are important, then ABS may be replaced by poly(butylene terephthalate). 

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 electrical insulation properties are quite good at room temperature in dry conditions and at low frequencies. Because of the polar structure they are not good insulators for high-frequency work and since they absorb water they are... 

Compared with aliphatic nylons it also shows greater rigidity and hardness, lower water absorption, low temperature coefficient of expansion, good resistance to heat and moisture, better electrical insulation properties, particularly under hot and damp condition, and of course transparency. 

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