Epoxy resins dielectric losses

Grade G-10, glass fabric with epoxy resin binder, has extremely high mechanical strength (flexural, impact, and bonding) at room temperature and good dielectric loss and electric strength properties under both dry and humid conditions. 

Until the 1970s the chemical used as the impregnating and dielectric medium for capacitor units was PCB (polychlorinated biphenyl) liquid. It was found to be toxic and unsafe for humans as well as contamination of the environment. For this reason, it is no longer used. The latest trend is to use a non-PCB, non-toxic, phenyl xylyl ethane (PXE-oil), which is a synthetic dielectric liquid of extremely low loss for insulation and impregnation of the capacitor elements or to use mixed polypropylene or allpolypropylene (PP) liquids as the dielectric. A non-oil dielectric, such as epoxy resin, is also used. 

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

Parts from PVDF can be machined, sawed, coined, metallized, and fusion bonded more easily than most other thermoplastics. Fusion bonding usually yields a weld line that is as strong as the part. Adhesive bonding of PVDF parts can be done epoxy resins produce good bonds.24 Because of a high dielectric constant and loss... 

Figure 8 shows the temperature dependencies of e" at four frequencies for Epikote 1001 (M w=1396) in comparison with that of a/E0 calculated from the data by the DC conduction measurements [10]. A broad peak is observed for each of the four frequencies at low temperatures on the plot of e", which is due to the rotational diffusion of the dipole moments. A good agreement is observed between e" (plots) and o/E0 (a solid curve) at higher temperatures and at lower frequencies in Fig. 8. The dielectric loss e" can be used as an indicator of the ionic conduction in the DGEBA oligomer at a fixed frequency at the temperatures where the dipole component is negligible. The ionic conduction from the dielectric loss can be measured in a short period of time and is widely used for the cure analysis of epoxy resin systems [62,79-82].

Although some competitor resins (e.g., polyimides and cyanate esters) are replacing epoxy resins in some more demanding applications, in which the superior glass transition temperatures or lower dielectric permittivity/low dielectric loss are preferred, brominated epoxies are still widely used. 

Major results. Figure 14.7 shows that the resistivity of aluminum-filled PMMA changes abruptly. Smaller volumes of filler contribute a little to resistivity but, after certain threshold value of filler concentration, further additions have little contribution. A similar relationship was obtained for nickel powder the only difference is in the final value of resistivity, which was lower for nickel due to its higher conductivity. The same conclusions can be obtained from conductivity deteiminations of epoxy resins filled with copper and nickel. Figure 14.8 shows the effect of temperature on the electric conductivity of butyl rubber filled with different grades of carbon black. In both cases, conductivity decreases with temperature, but lamp black is substantially more sensitive to temperature changes. Even more pronounced changes with temperature were detected for the dielectric loss factor and dissipation factor for mineral filled epoxy." ... 

The dielectric loss/frequency relation of a liquid epoxy/polyamide resin system... 

At very low temperatures, most degrees of freedom are frozen. The detailed chemical structure of the polymer chains does not remarkably influence most of the elastic and thermal properties at these temperatures. (Properties, such as mechanical strength or dielectric loss, may be influenced by the chemical structure because of factors such as steric hindrance and dielectric polarization.) Cross-linking is one structural feature of epoxy resins which might influence low-temperature properties. 

Fig. 10. Mechanical and dielectric loss vs. temperature for various epoxy resins and polyethylene.

Amorphous epoxy resins are polar polymers and have comparably higher losses. Some loss measurements on epoxy resins are plotted versus temperature in Fig. 11. It is interesting to note that the mechanical and dielectric losses are only different by a factor of roughly 2. Also, the dependence on temperature is rather similar. The dielectric and the mechanical parameters of Fig. 11 were determined at different frequencies. At low temperatures, this is a minor error. Mechanical measurements performed at a higher frequency, namely 50 Hz, would, at most yield lower values, more similar to the dielectric ones. Thus, for epoxy resins, the electrical and mechanical dipole forces are similar. 

The mechanical losses of partly crystalline epoxy resins were found to be lower than those for amorphous ones. First measurements also show a similar situation for dielectric low-frequency losses. The value plotted in Fig. 11 is an upper limit and the true value is probably lower. Further measurements are in preparation. However, despite the fact that plastic flow has not yet been found, because of their good dielectric properties, crystalline epoxy resins seem to be of advantage. 

Fig. 19. Dielectric loss factor as a function of time for isothermal curing of an epoxy resin. From Ref. 82.

The volume resistivity, permittivity, and dielectric loss factor of nanostructured interpenetrating polymer networks based on natural rubber/polystyrene have been found to increase as a function of blend composition, reaching a maximum of 10 -10 Hz dielectric loss factor [27]. Measurements of volume resistivity have also been reported on epoxy resin-polyaniline blends resulting in the establishment of a correlation between a shoulder on the 1583 cm band with the degree of volume resistivity [31]. 

BMI resins were modified with a wide range of other polymers in order to achieve improved properties. Toughened BMIs may be obtained by using polyetherketones (Han et al. 2009), while enhanced processing characteristics and low dielectric losses may be achieved for BMI by modification with allyl phenyl compounds, allyl epoxy resins, and epoxy acrylate resins (Liang et al. 2007). Flame retardancy was accomplished by modifying BMIs with fully end-capped hyperbranched polysilox-ane (Zhuo et al. 2011a). 

Blends of epoxy resins with other types of resins have also been developed. These are used when performance demands exceed the capabilities of even the high-Tg/Td epoxies, but where the costs of the highest performance materials cannot be justified. In many cases, the driving force behind these materials is the need for improved electrical properties versus the standard epoxy offerings. Specifically, improvements in the dielectric constant (permittivity) and dissipation factor (loss tangent) are the properties of interest. Materials with lower values for these properties are needed for circuits that operate at high frequencies. 

The particles are hexagonal platelets which tend to stack together producing larger particles. Surface treatment is needed to facilitate dispersion in a resin. The surface treatment can have side-effects, inhibiting reaction with epoxy and vinyl polymers. The particles are rendered hydrophobic by suitable treatment, for achieving low dielectric loss. Very fine kaolin particles can increase rather than decrease the strength of certain thermoplastics. 

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