Loss factor measurements

The time-temperature superpositioning principle was applied f to the maximum in dielectric loss factors measured on poly(vinyl acetate). Data collected at different temperatures were shifted to match at Tg = 28 C. The shift factors for the frequency (in hertz) at the maximum were found to obey the WLF equation in the following form log co + 6.9 = [ 19.6(T -28)]/[42 (T - 28)]. Estimate the fractional free volume at Tg and a. for the free volume from these data. Recalling from Chap. 3 that the loss factor for the mechanical properties occurs at cor = 1, estimate the relaxation time for poly(vinyl acetate) at 40 and 28.5 C. 

Perhaps the best comparison is that of Alford and Dole (1955) who measured the specific heat of a sample of polyvinyl chloride from the same source as the polyvinyl chloride used by Fuoss (1941) in his extensive dielectric loss studies. The comparison between c and e", the dielectric loss factor measured at 60 cps, is shown in Fig. 15, and covers the glass transition range. Note that the peak of the dielectric loss curve is at 100° C, about 20 degrees higher than the inflection point of the c — T curve. 

Though a thorough study of the modulus and loss factor measurement ranges has not been conducted, current experience indicates the range of the real part of the shear modulus, G, is 10 to 10" dyn/cm the range of loss factor is from 0.05 to 1.2. 

Figure 1. RTV silicone cure study (A) Permittivity measurement, (B) Loss factor measurement with time at ambient temperature.

Figure 4. RTV silicone loss factor measurement at complete ambient cure.

The extent to which a material is heated when subjected to microwave radiation depends on two parameters the dielectric constant e and the dielectric loss factor e". The dielectric constant describes the ease with which a material is polarized by an electric field, while fhe loss factor measures fhe efficiency wifh which the electromagnetic radiation is converted into heat. The ratio of these properties gives the dielectric loss tangent or dissipation factor ... 

Figure 5 - Micro-dielectric (loss factor) Measurement of Silicone Gel Cure Study.

Even with silicone gel formulation with excess vinyl functional group vs. hydride functional group, this microdielectric loss factor measurement provides a fast, real-time measurement of the degree of cure of silicone gel. 

Another approach to reduce the loss factor was to heat treat (set) the oriented film under tension after it had been elongated or shrunk by less than 5%. For example, an oriented PVDF film was heat set at a temperature of 140°C-160°C and elongated 5%. Results of loss factor measurements can be seen in Fig. 6.26. Loss factor, in general, decreased with an increase in the time of treatment to less than 1%. 

The dielectric loss factor measured is the sum of a dielectric contribution and a dc conductance contribution, see 5.1.3. 

By comparing log ionic conductivity profiles with loss factor measurements, the operator can identify the window in the process during which the material is fluid (and therefore workable), develops molecular weight and crosslinks (which is critical to product performance and appearance), and is completely cured (which identifies the proper time to demould the product or remove jigs). The dielectric analyser can also record the dielectric properties of the resin during cooling. 

In order to use the microdielectrometiy technique to study the ECN-PN resin, very thick coatings of the resin had to be avoided because of errors induced in the loss factor measurements when such coatings were used. An example of the noise, which is... 

A typical temperature curve for shear modulus and mechanical loss factor, measured using torsion pendulum, for different grades of PP is shown in Figure 8. It can be seen from the figure that a copolymerised grades of PP has two peaks in the mechanical loss factor curve while PP homopolymer has only one peak. The first peak, above 0 °C, denotes the glass transition temperature, similar to that of homopolymer PP. The secondary transition peak at -45 °C is due to the presence of comonomer which provides some mobility to polymer chains above -45 °C, thereby, giving enhanced impact properties. 

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