Resistance, polymer electrical properties moisture

PCTFE exhibits very good electrical properties ia terms of high iasulation resistance, minimal trackiag, corona formation, and surface flashover due to the polymer s nonwettable surface and ultralow moisture absorption (Table 3). 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Sihcones (qv) have an advantage over organic resias ia their superior thermal stabiUty and low dielectric constants. Polyurethanes, when cured, are tough and possess outstanding abrasion and thermal shock resistance. They also have favorable electrical properties and good adhesion to most surfaces. However, polyurethanes are extremely sensitive to and can degrade after prolonged contact with moisture as a result, they are not as commonly used as epoxies and sihcones (see Urethane polymers). 

It is having excellent electrical properties and the chemical inertness and moisture resistance typical of hydrocarbon polymers. 

Moisture has, in itself, usually not much effect on polymer properties, though the amount of moisture which can be absorbed by polymers varies within wide limits (between zero and a few %). Logically, the electric properties such as resistivity and dielectric losses are the most sensitive to water. As to mechanical properties, nylons show the strongest dependence on water absorption. PA-6 is able to take up a... 

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

Acetal This translucent crystalline polymer is one of the stiffest thermoplastics available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with... 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

PET and PBT are characterized by high strength, rigidity, and toughness low creep at elevated temperatures excellent dimensional stability low coefficient of friction good chemical, grease, oil, and solvent resistance minimal moisture absorption and excellent electrical properties. Reinforcement of these polymers with glass fibers enhances many of their properties (Table 15.16). 

Another commercially produced polyolefin is isotactic poly(4-methyl pentene-1). The polymer carries a trade name of TPX. This material is known for high transparency, good electrical properties, and heat resistance. PoIy(4-methyI pentene-1) has a density of 0.83 g/cm. This polyolefin exhibits poor load-bearing properties and is susceptible to UV degradation. It is also a poor barrier to moisture and gases and scratches readily. This limits its use in many applications. 

Table 3 compares properties of PTT, PET, and PBT (40). They were measured on injection-molded samples using ASTM test methods. Tensile strength, flexural modulus, and notched Izod impact of PTT fall in between those of PET and PBT. All three polymers have similar electrical properties except for PET which has lower volume resistivity. The lower moisture absorptions of PTT and PBT are due to the high crystallinities from fast crystallization. 

Electrical properties of PBI films are shown in TABLE XIV. As would be expected, films with higher moisture contents have lower resistivities and higher dielectric constants. PBI plasticized with phosphoric acid, which should be a polycationic polymer with an unbound phosphate anion, has a very low resistivity and a high dielectic constant which is even higher at elevated temperatures. In general, dielectric constants are higher at 100 Hz and lower at 1 MHz. 

A polymer nanocomposite is a multiphase material in which a polymer matrix is combined with nanometer-sized materials having one, two, or three dimensions smaller than 100 nm, or structures having nanoscale repeat distances between the different phases that make up the bulk polymer material (Thostenson et al. 2005 Lin et al. 2011a, b). In a polymer nanocomposite, the polymer matrix functions as a concentrated component that enables a phase continuum, while nanomaterials are deliberately added to induce an enhanced performance in the functional properties such as mechanical, barrier, electrical, triggered biodegradabilily, moisture resistance, and thermal properties of the polymer (Thakur et al. 2012, 2014c, d, e). 

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