Autoclave gradients

For thick epoxy laminates processed in the autoclave, voids once formed and stabilized can only be removed by dissolution or by resin flow. Furthermore, resin gradients are deleterious to structural laminates. These two key phenomena make an understanding of resin transport vital to the development of any processing model. 

To investigate the potential pressure gradients that exist within a laminate during autoclave processing, miniature pressure transducers (Fig. 10.5), which are capable of measuring the hydrostatic resin pressure, were embedded at multiple locations within several laminates to study the effects of vertical and horizontal pressure gradients [9]. 

The pressure curves also illustrate the horizontal flow process. The resin pressure initially approaches the applied autoclave pressure and then decreases as bleeding occurs. The opposite occurs in the bleeder. The applied vacuum is measured initially, and the pressure increases as resin begins to fill the bleeder. Note that the horizontal pressure gradient is very small for a majority of the laminate but becomes large near the edges. 

The autoclave is mounted in a two-zone tube furnace. The axis of the furnace should be at an angle of about 15° to the horizontal, to cause high convection when a temperature gradient is applied. 

If a temperature gradient is applied, the lower part (charge zone) is at the higher temperature. Temperatures are measured by thermocouples which are placed at the ends of the autoclave. 

Although some benefits of high temperature can be gained with traditional autoclaves as an alternative to microwave heating, the energy is usually applied to the reaction mixture conductively, via the vessel. Thus, the rate of temperature increase is usually low, thermal gradients develop and even with stirred batch reactions, not all of the sample will be at the temperature of the applied heat. With microwaves, the whole sample can be bulk-heated and the energy input readily adjusted to match that required. 

For the experiments with increased water content or suppressed water removal, a 5 cm-long piece of coated monolith was mounted in a 500-mL autoclave. All liquid concentrations, operation conditions and catalyst hold-up were the same as in the pilot-scale plant. To maintain a gradient-less operation, a turbine-type stirrer recirculated the liquid very rapidly through the monolith channels. During the experiments, liquid samples were taken from the reactor and analyzed as described above. 

The most convenient gradient coil is an opposite-wound Helmholtz coil fixed on to the outside of the autoclave body. Extensive tests have been carried out " which have shown such a design is superior—in terms of its reproducibility of pulsed field gradients—to both quadrupole coils and Helmholtz coils wound on the inside of the autoclave. In order to generate these higher pressures, commercial 1 GPa equipment may still be used. However, a different pressurizing medium from that described previously is employed. Either... 

Nutrient (high purity sand or natural crystal quartz) is contained at the bottom and dissolves as the autoclave is heated. A temperature gradient is most often used so that the nutrient dissolves and is transported to the cooler area where it is redeposited as single crystal material. Note the temperature gradient at the baffle. 

Figure 4.1-5 Modified Bridgeman seal autoclave showing the temperature gradient crystal growth onto seed crystals [17]. The nutrient is held at a higher temperature than the seed crystals, thus leading to supersaturation conditions in the seed region. The transport is ill a...

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

Figure 11-2 shows the kinetics of crystallization in the presence of a P40 in an Al-free reaction mixture. To follow the kinetics, crystallization was carried out in a 5 liter-autoclave that could be probed directly. It is seen that crystallization is accelerated with increasing temperature, when compared to syntheses listed in Tables 11-2 to 11-4. Furthermore, this experiment emphasized the metastable character of the new layer silicates. Quartz instead of cristobalite is observed as the final product of the crystallization sequences in the Al-free cases. Crystallization starts at 4 hours and is completed after 9-10 hours. Quartz appears as a product component after 11-12 hours. The growth of quartz ceases after 40-44 hours. From the gradient of the crystallization curve, a growth rate of 15% per hour is determined for this novel silicate under these conditions. 

---