Effects of Temperature and Cure

This Section addresses the effects of temperature and cure on the dominant dipolar relaxation, i.e., the a-transition between the unrelaxed and relaxed permittivity, with its associated loss-factor peak. As illustrated in Figs. 20 and 21, this dipolar relaxation is observed as the temperature increases through Tg, or during a cure, as Tg increases toward and even through the cure temperature. 

Microdielectrometry was introduced as a research method in 1981 14 and became commercially available in 1983 20). The microdielectrometry instrumentation combines the pair of field-effect transistors on the sensor chip (see Sect. 2.2.3) with external electronics to measure the transfer function H(co) of Eq. (2-18). Because the transistors on the sensor chip function as the input amplifier to the meter, cable admittance and shielding problems are greatly reduced. In addition, the use of a charge measurement rather than the admittance measurement allows the measurements to be made at arbitrarily low frequencies. As a matter of practice, reaction rates in cure studies limits the lowest useful frequency to about 0.1 Hz however, pre-cure or post-cure studies can be made to as low as 0.005 Hz. Finally, the differential connection used for the two transistors provides first-order cancellation of the effects of temperature and pressure on the transistor operation. The devices can be used for cure measurements to 300 °C, and at pressures to 200 psi. 

Figure 4.6 shows that all wall-slip techniques can be used successfully to determine the wall slip of a material over a wide range of shear stresses. Once again, for full characterization of wall slip in reactive systems the effects of temperature and extent of cure on wall slip should be determined. 

The effects of temperature and time on the chemoviscosity can also be described explicitly in terms of the extent of cure (a) from knowledge of the kinetics of the cure (i.e. (T, t)) and temperature by the following equation ... 

The effect of temperature and humidity on the insulation resistance of photopolymerized and thermally cured solder masks was determined. Dry film and screen ink solder masks were applied to a test pattern. The insulation resistance of the coated test pattern decreased with exposure to elevated temperature and humidity. Elevated temperature alone, however, had only a small negative affect on the insulation resistance. 

Levels of alkaloids and their derivatives in air-cured and fire-cured KY171 dark tobacco during prolonged storage Effects of temperature and moisture Tob. Sci. 34 (1990) 50-56. 

Two methods have been discussed in the literature for relating the effect of temperature to cure rate temperature coefficient and activation energy. The tanperature coefficient method was usually used at that time, but most workers suggested that the activation energy approach is more fundamental [2]. 

Zia Ziaee, S., Palmese, G. R. Effects of temperature on cure kinetics and mechanical properties of vinyl-ester resins. J. Polym. Sci. B Polym. Phys. 37 (1999) 725-744. 

Rate of strength development. The effect of temperature on curing rate will vary for different adhesives. In general, low temperatures increase the curing period considerably and many epoxy resin formulations stop curing altogether below 5 °C. A rule of thumb often quoted is that the curing period doubles for every 10 °C fall in temperature below ambient but halves for every 10 °C rise in temperature above ambient. 

Fig. 2.11. Effect of formulation and cure temperature on flexural strength development of a two-part epoxy (Ref. 19). (a) Normal type, (b) Rapid type.

Fig. 6. Effect of temperature on cure rate and cure state. Base compound NR, 100 N347 carbon black, 50 naphthenic oil, 8 stearic acid, 1 ZnO, 3 antioxidant, 0.75 sulfur, 3 TBBS, 0.75. To convert dNm to in lb, multiply by 0.885.

The equivalent age maturity method was developed to account for the nonlinear effects of temperature and time on the rate of concrete property development (Carino and Lew 2001). In this method, the concrete equivalent age (in hours) describes the amount of time that a concrete mixture would need to be cured at a constant reference temperature in order to achieve the same degree of hydration as the same concrete mixture cured at a different temperature. The equivalent age method, based on the Arrhenius equation that describes the temperature dependence of a chemical reaction, is given as follows (ASTM C1074 2011) ... 

Copper naphthenate added to the resin at levels between 100—200 ppm effectively extends gel and cure characteristics, resulting in a reduction in exothermic heat (Eig. 7). Copper additives are used widely in commercial laminating resins to modify process exothermic effects. a-Methylstyrene [98-83-9] substituted for styrene at levels of 5—8% has also been used effectively in resins cured at above ambient temperatures. The inhibitor 2,5-di-/-butyIhydroquinone exerts significant exotherm suppression at levels of 200—400 ppm and is useful in high temperature mol ding processes. 

Solvent absorption measurement has been shown to be a sensitive and useful test method in the manufacture of epoxy powder coatings. A test method was defined and the effects of time and temperature of immersion described. It was shown that solvent absorption is a measure of raw material properties (EEW of the epoxy resin, and CTBN elastomer type and concentration), the homogeneity of the extrudate, as well as the state of cure. The information obtained from solvent absorption measurements has proven to be extremely important not only in quality control analysis but also in providing an insight into the structure function relationships in epoxy resin chemistry. 

In the 177-300 °C temperature range studied, epoxide isomerization, oxidation and homopolymerization can occur followed by complex degradation reactions. There have been numerous studies on the homopolymerization of epoxides including the effects of catalysts, alcohols, cure temperature and epoxide-amine ratio on the... 

Series of runs were made to explore the effects of temperature, catalyst, and initiator on the cure rate (Figures 1 through 3). Taking a Barcol hardness of 63 as a reasonable level for comparison, standard formulations reached this level in 48 hours at 75°C, 24 hours at 85°C,... 

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