Void diameter

The difference in the void diameter predictions between Equations 6.13 and 6.17 is a factor of 2y/p. 

If zero initial void diameter is assumed, then no void growth occurs while Oat = Q,. The time increment from the start of the cure cycle to the moment when Qat — Coo is denoted as tBEGIN ar d is given by ... 

Figure 6.6 shows the effect of the processing cycle on the void diameter for pure water and air-water voids of 0.1 cm initial diameter under the specified cycle conditions. It was assumed that the air—water void initially consists of pure air, even though there is likely to be a small but finite water partial pressure. This plot can be divided into the various stages of void growth and dissolution and interpreted as follows. 

Figure 6.7 shows the final diameter of a pure water void at the end of the cure cycle after it has grown from voids of various initial diameters under the conditions specified. For an initial void diameter of zero, the final diameter is about 1.25 cm under the specified conditions of growth. On the other hand, relatively large initial void diameters (0.5 cm) only triple in size. 

Initial Pure Water Void Diameter = 0.1 cm 21 Initial Humidity Exposure - 75%... 

The data on which this relationship is based are shown in Table 70. The average void diameter was obtained from the equation log D% = mt + b, where = percent voids, and m and b are. constants. (See Eq 6-28.)... 

Traxler and Baum (1936) showed that the value for the slope m (m = 0.019) holds for a large number of mineral powders. For irregular particles it is necessary to determine the average void diameter experimentally. This subject was discussed in Chapter 6. Note that in Table 70 the value of C = v-mfDv is practically constant for a given mixture, regardless of the amount of solids present. 

An important conclusion from Fig. 6 is that for a small void, it is immaterial wheter or not the initial void contains pure water or an air/water mixture. The diameters at any particular time during the cure cycle are nearly identical when the initial void diameters under 0.1 atm are the same. An air/water void initially containing pure air has a very large driving force for diffusion of water vapor from the resin to the void during the first few minutes of the cycle. This results in diffusion into the void of a large amount of water vapor (relative to the original amount of dry air in the void). Consequently the mole fraction of water vapor in the air/water void quickly approaches unity, and thereafter the rate of diffusion of water vapor across the interface of the air/water void is nearly the same as that for a pure water void. 

Scanning electron microscopy of the anodic film formed in the dilute phosphoric acid electrolyte at 170 V reveals the typical porous film morphology, including well-defined barrier layer and pores of about 80 nm in diameter (Fig. la). Scrutiny of the cell material reveals bubble-shaped features located at the cell boundaries with an occasional disturbances to the parallel-sided pore walls. After further chemical dissolution in phosphoric acid, resulting in pore widening to 150 nm, numerous voids become evident in the cell material adjacent to the pore. The mouths of the voids appear circular, and the defects pass into the cell material in a tubular manner (Fig. lb). The defects are oriented approximately perpendicular to the pore walls and are separated from each other by about 200 nm. The void diameters are approximately 70 nm. 

P50(V2) is the pressure for which half of the interparticular void volume (V2) is filled. Using the Washburn equation one can calculate the corresponding pore access diameter, noted as dp50(V2), also defined as median interparticular void diameter. 

In the former, corona discharges occur in voids, causing hollow channels, lined with decomposed polymer, grow in the polymer. The field strength to cause corona discharge should be an inverse function of the void diameter, so breakdown should be avoidable if voids are smaller than 25 xm. 

Packed beds at lower capacities the exit drop diameter proportional to void diameter in the packing. 

However, the one-step-drawn P(3HB-co-3HV) fiber has many fine voids of cohesive elliptic shape in the drawing direction (Figs. 9b, 10). The results indicate that the streak scattering in the SAXS pattern shown in Fig. 9b is due to the presence of many fine voids throughout the fiber. The size and distribution of the voids in the one-step-drawn P(3HB-co-3HV) fibers after isothermal crystallization were analyzed by image analysis software. Most of the void diameters in cross sections perpendicular to the drawing direction were 1.0-1.6 [im and the average void diameter was 2.3 1.5 (un. 

To determine the influence of stress on the microstructure, TEM investigations are in progress. First results on the average void diameter and the void concentration seem to show that the stress-induced swelling is a result of the accelerated void nucleation rather than the accelerated void growth. But this result needs further confirmation. 

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