Elastomers Electrically insulating materials

Do you think that daily life would have been easier and colourful without the discovery and varied applications of polymers The use of polymers in the manufacture of plastic buckets, cups and saucers, children s toys, packaging bags, synthetic clothing materials, automobile tyres, gears and seals, electrical Insulating materials and machine parts has completely revolutionised the daily life as well as the industrial scenario. Indeed, the polymers are the backbone of four major Industries viz. plastics, elastomers, fibres and paints and varnishes. 

Wire and cable Thermoplastics and elastomers are widely used as electrical insulating material due to their physical properties and processability. Cross linking is an effective means for improving e.g. the thermal resistance and tensile strength. EBA-irradiation ( 50 kGy) affords a rapid, well controlled cross linking and is used by several major producers of thin wires and cables. 

Elastomers are electrically insulating materials, usually in the form of silicone rubber pads, ranging in thickness from 0.001 to 0.20 in. and filled with high-thermal-conductivity materials such as alumina and boron nitride. They require a mechanical pressure to fill the voids. Figure 3.13 shows the variation of thermal impedance vs. pressure of an elastomeric pad for a TO-220 package [19]. 

Thermoplastic elastomers based on polyolefins (TPO) are blends of PE or PP wifh EPDM elastomers wherein the elastomer is often cross-linked using thermochemical systems. TPOs more suitable for medical producfs with no chemical residuals can be made using EB processing to cross-link the elastomer portion in such an elastomer-plastic blend. The thermoplastic governs the melt transition, and thus the extrusion properties of TPOs. The radiahon response of these materials is also governed by the choice of fhe thermoplastic. An example of an EB cured blend of EPDM and polyefhylene used is for fluid transmission tubing and electrical insulation. ... 

Through copolymerization there can be made materials with different properties than those of either homopolymer, and thus another dimension is added to the technology. Consider, for example, styrene. Polymerized alone, it gives a good electric insulator that is molded into parts for radios, television sets, and automobiles. Copolymerization with butadiene (30%) adds toughness with acrylonitrile (20-30%) increases resistance to impact and to hydrocarbons with maleic anhydride yields a material that, on hydrolysis, is water-soluble, and is used as a dispersant and sizing agent. The copolymer in which butadiene predominates (75% butadiene, 25% styrene) is an elastomer, and since World War II has been the principal rubber substitute manufactured in the United States. 

Due to the free-radical character of both the y-radiation induced transformation and oxidation in the post-irradiation phase, free-radical scavenging stabilizers - phenols and amines - effectively protect PO [11,226,227]. This was confirmed by excellent antioxidant/antiradiant effects of DP A, PNA and PD in particular in the post-y-irradiation phase of PO and elastomers [11]. Oligomeric DHQ 21 or its combination with 9b efficiently stabilized EPDM or X-LPE used as insulation materials for electric cables exposed to irradiation doses up to 2MGy at a dose rate 300 Gy h 1 [242]. 

Dielectric measurements, as a function of temperature, have been widely used to study the properties of semiconductors, insulation materials, plastics, elastomers, oil shales, inorganic substances, and others. The dielectric constant technique provides more insight into the segmental motions of the molecule and relaxation phenomena (1161 than does other electrical measurements such as electrical conductivity, resistivity, and so on. 

Another important reinforcement application is in silicone mbber elastomers. Historically, fumed silicas have played the major role here, but recently precipitated silicas have been developed that possess the characteristics required for this application [13]. Compared to conventional precipitated silicas, a product designed for this end use must have higher purity (to impart acceptable electrical properties, because silicone mbbers are often used as insulating materials) and lower water adsorption (to prevent bubbles from forming during extmsion and to impart resistance against moisture pickup). Good dispersibility is also important. 

The electrical industry also makes wide use of the elastomer in the manufacture of electrical equipment. It is used as an insulating material and in molded terminal covers, plugs, transformer connectors, splices, and insulating and semiconductor tape. 

VOLUME resistivity The electrical resistance between opposite faces of a 1-cm cube of insulating materials, commonly expressed in ohm-centimeters. The recommended test is ASTMD 256 51T. Also called the specific insulation. vulcanization A chemical reaction in which the physical properties of an elastomer are changed by causing the elastomer to react with sulfur or some other cross-linking agent. WATER absorption The ratio of the weight of water absorbed by a material to the weight of the dry material. 

Rubber is an organic macromolecule with high elasticity, also known as an elastomer. It has excellent elasticity in a wide temperature range (-50°C to 150°C). In addition to the unique high elasticity, rubber also has good mechanical, chemical, and wear resistance, as well as good electrical insulation, so it has become an indispensable material in the national economy. 

Direct Dielectric Breakdown. All insulating materials, including dielectric elastomers, present trade-offs between maximum applied field strength and application lifetime. In a dielectric transducer, electric field breakdown will occur as the field across the dielectric elastomer is increased. Material choices and geometry can also influence electric breakdown phenomena. For example, sharp metal edges can cause higher electric fleld concentrations and exacerbate electric breakdown. However, in dielectric elastomers, electric breakdown is often a symptom of some other type of failme rather than the cause. 

Silicones polymers which can take the form of fluids for lubrication, rubbers or plastics. Silicone resins are employed in electrical insulation applications, including component encapsulation. Silicone rubber is unique among synthetic elastomers being the only one with useful operating temperature, and resistant to weather, ozone, chemicals, with a non-stick surface. Silicones are used as release agents, because of incompatability with most materials, and as lubricants and additives. 

The basic chemical technology of silicones and silicone elastomers is covered in Chapter 18 so the discussion here is limited to silicone resins. These materials have many applications, including use in paints and varnishes, molding compounds, encapsulants, electrical insulation, pressure-sensitive adhesives, laminates, and release coatings. Their heat stability, water repellency, and resistance to solvents and weathering favor their use in these applications. 

The applications mentioned are the more well known but newer ones in the automotive sector, building, electrical insulation, domestic appliances, agriculture, etc. are also mentioned. Information is also given on uses of ethylene-propylene elastomers where the products do not undergo the traditional processing of the rubber industry, but are used as additives to various materials such as plastics, lubricating oils, bitumen and waxes, in order to modify or improve certain properties. 

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