Dielectric constant of plastics

Figure 10.47. Thermostability number A from Figures 10.45 and 10.46 for 0.06 M plasticizer vs. dielectric constants of plasticizers.

The dielectric constant of plastic materials is measured as specified in ASTM D150. It is a dimensionless factor derived by dividing the parallel capacitance of the system with a plastic material by that of an equivalent system with a vacuum as a dielectric. The lower the number, the better the performance of the material as an insulator. 

Dielectric constant of plastics materials generally decreases vdth frequency.This is also observed in our case. The dielectric constant of a cured polymer is less than that of its uncured state. This is diae to the restricted motion of the polar grovps present in the polymer sanple in the crosslinked state. 

For these applications, one is looking for materials with very low dielectric constant. Table 4-2 lists typical dielectric constant values of various plastics. The dielectric constant of air (or vacuum) is 1 at all frequencies. The dielectric constant of plastics varies from 2 to 20. 

Dielectric Constant The dielectric constant of material represents its ability to reduce the electric force between two charges separated in space. This propei ty is useful in process control for polymers, ceramic materials, and semiconduc tors. Dielectric constants are measured with respect to vacuum (1.0) typical values range from 2 (benzene) to 33 (methanol) to 80 (water). TEe value for water is higher than for most plastics. A measuring cell is made of glass or some other insulating material and is usually doughnut-shaped, with the cylinders coated with metal, which constitute the plates of the capacitor. 

A correlation between the dielectric constant of the plasticizer used in a polymer and its water uptake might be expected. However, according to the table below17 (Table 19), no predictions can be made. 

In contrast to metals and semiconductors, the valence electrons in polymers are localized in covalent bonds.The small current that flows through polymers upon the application of an electric field arises mainly from structural defects and impurities. Additives, such as fillers, antioxidants, plasticizers, and processing aids of flame retardants, cause an increase of charge carriers, which results in a decrease of their volume resistivity. In radiation cross-linking electrons may produce radiation defects in the material the higher the absorbed dose, the greater the number of defects. As a result, the resistivity of a radiation cross-linked polymer may decrease. Volume resistivities and dielectric constants of some polymers used as insulations are in Table 8.3. It can be seen that the values of dielectric constants of cross-linked polymers are slightly lower than those of polymers not cross-linked. 

Triboelectrification. In triboelectrification, plastics are brought into repeated contact resulting in a loss or gain electrons depending on the relative dielectric constant of the materials (125). In the case of ABS and HIPS, ABS exhibits the higher dielectric constant. Therefore ABS should become positively charged. Once materials have been... 

Omitted from this elementary theory are effects of plasticizer and ion pairing. Ion pair formation constants in the organic phase increase with decreasing dielectric constants of the plasticizer, in the absence of specific bonding effects. In the more general theory the single ion partition coefficients are replaced by the product of partition coefficient and ion pair formation constant. 

Some degree of solvency of the plasticizer for the host polymer is essential for plasticization. Not surprisingly, a match of solubility parameters of the plasticizer and polymer is often a necessary but not a sufficient condition for compatibility. In the case of PVC, the dielectric constants of the plasticizer should also be near that of the polymer. 

The dielectric constants of polar polymers are also extremely sensitive to moisture. Most polar polymers, and especially polymers containing hydrogen-bonding moieties, are susceptible to moisture. Water molecules can become incorporated into them upon exposure to humidity. Water has =80.4 at room temperature. The incorporation of even a small amount of moisture can therefore drastically increase . In addition, is often sensitive to the presence of even small quantities of many common plasticizers, stabilizers and other additives. Very different values are therefore often quoted for the of the same polar polymer. These values may all be "correct", in the sense that, even though they may not describe the "intrinsic properties" of the polymer, they describe the properties of the specimen being tested. 

Electrical properties of polyphenylene sulfide compounds are summarized in Table III. The dielectric constant of 3.1 is low in comparison with many other plastic materials. Similarly, the dissipation factor is very low. Dielectric strength ranges from about 500-600 volts per mil for the various compounds these values are quite high. Thus, both... 

A compatibility stability idiosyncrasy of diisodecyl adipate (DIDA) could pose a troublesome problem. A PVC plastisol formulated with DIDA and fused in an oven for 10 min at 175 C will exude but if the fusion period is lengthened to 0.5 h, it will not. One explanation is the dielectric constant of DIDA is below A it should be incompatible and therefore exude. However, upon extensive heating in air this oxidation-susceptible plasticizer builds up peroxides, undergoes chain scission and weight loss, and changes in compatibility. As shown in Table I, if the oxidation phenomenon is negated in per se DIDA by the presence of an antioxidant, bisphenol A, the plasticizer remains unchanged in dielectric constant and predictably would be incompatible (20). 

Electrical properties of polymers that are subject to low electric field strengths can be described by their electrical conductivity, dielectric constant, dissipation factor, and triboelectric behavior. Materials can be classified as a function of their conductivity (k) in (Q/cm)- as follows conductors, O-IO" dissipatives, and insulators, lO or lower. Plastics are considered nonconductive materials (if the newly developed conducting plastics are not included). The relative dielectric constant of insulating materials (s) is the ratio of the capacities of a parallel plate condenser with and without the material between the plates. A correlation between the dielectric constant and the solubility parameter (6) is given by 6 7.0s. There is also a relation between resistivity R (the inverse of conductivity) and the dielectric constant at 298 K log R = 23 - 2s. 

For the development of edible and biodegradable bioplastics, it is required solvents and a pH regulating agent, when necessary, in addition to the plasticizer and polymer. The pH adjustment in the case of proteins is necessary to control the solubility of the polymer. Some regulators of pH found in the literature [13] acetic acid and sodium hydroxide. The solvents commonly used to prepare these bioplastics are water, ethanol or a combination of both [14]. A crucial aspect in the preparation of films is the solubility of proteins and the ability to interact with the same solvent used, since the total solubility of the protein is required for films formation [15]. The dispersion of the protein molecule in water is possible due to the large number of amino acid residues that interact with the polar solvents. These interactions can be improved depending on the dielectric constant of the solvent, since this constant is inversely proportional to the strength of intermolecular attraction. Films can be simple, made with one type of macromolecule or composed by two or more types of macromolecules, and can be formed with two or... 

Higher initial flux values for the extracted species are usually obtained when PIMs incorporate high-polarity and low-viscosity plasticizers such as NPOE and NPPE, and this has led to the conclusion that the initial flux values increase with an increasing dielecfiic constant and decreasing viscosity of the plasticizer. However, it should be noted that most of the plasticizers used in PIMs have similar viscosity values (Table 27.4), and moreover, the dielectric constant of the membrane liquid phase is also dependent on the dielectric constants of the carrier and the base polymer [32]. 

Figure 10.33. The effect of the number of methylene groups on the dielectric constant of phthalate plasticizer.

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