Cooling closed-cell

Under pressure in a closed mould during gelation. After cooling and demoulding, the expanded parts are re-heated (post-expansion) to obtain closed-cell foams such as life jackets. 

In one of the few studies of plastic foam behavior at cryogenic temperatures Hingst discovered that upon cooling of closed-cell PUR foam in the 30-70 K range, an abrupt change in the linear temperature dependence of heat conduction occurs (Fig. 15). This phenomenon is closely related to the problem which we are... 

CO was purified by passing through the trap with liquid nitrogen. Normally, the first spectrum was run when about 5 torr of a gas was let into the cell, then the excess of gas was removed by adsorption on zeolite cooled with liquid nitrogen, without pumping the helium, so that the temperature of the sample would be constant. Then, if necessary, the sample was evacuated at the desired temperature or heated in a closed cell, in order to observe the changes in the spectra of surface species without CO removal. Temperature was measured by a thermocouple inserted in the volume for coolant of the cell. 

For the IR (Perkin-Elmer 1730 FTIR) spectroscopic studies self-supported wafers were pressed from the zeolite powder with a thickness of 15 mg/cm . The wafers were placed into a sample holder inside a pyrex glass cell with KBr windows, which allowed pretreatment (773 K, 1 hr, vacuum <10 Torr), introduction of probes, after cooling to the desired temperature, and registration of the spectra at 295 K. Three types of IR experiments were run (i) stepwise loading (0.1-6 Torr) the wafers with adsorbates at 295 K, (ii) generation of surface species with increasing temperature in the presence of adsorbate (closed cell), and (iii) loading adsorbate at 453 K followed by detection of surface species formed with time (1-240 min) at the same temperature. After each set of IR measurement the same sample was analyzed by UV-VIS spectroscopy. 

By comparing the weight of the aerogel at 0 and 30 % relative humidity the amount of water that each RF aerogel adsorbs is determined. After storing the samples at 90 °C for 48 h and cooling down to room temperature in dry N2-atmosphere the weight at 0 % relative humidity is measured. To obtain the values at 30 % relative humidity the samples are placed in a closed cell with a specific salt-water solution. By vapor pressure depression above saturated solutions this specific solution keeps the atmosphere constant at 30 % relative humidity. 

Closed-cell foams result when the decomposition and gelation are carried out in a closed mold almost filled with plastisol. After the heating cycle, the material is cooled in the mold under pressure until it is dimensionally stable. The mold is then opened, and the free article is again subjected to heat (below the previous molding temperature) for final expansion. Protective padding, life jackets, buoys, and floats are some items made by this process. 

Similarly, cells can be sealed temporarily with covers whenever they are opened. Cooling a cell to room temperature before opening also reduces the initial surge of vapor. A more elaborate measure was the replacement or modification of mercury pumps to provide sealless canned pumps. Along with the use of properly annealed piping and caulking with a urethane polymer to close any pinholes, this reduced emissions from the mercury recycle system. 

It appears at this time that there is disagreement over the librational frequency assignment in a-Na- From a purely experimental point of view, the discrepancy between the observations of Brith, Ron, and Schnepp (1969) and Cahill and Leroi (1969) on the one hand and those of Anderson, Sun, and Donkersloot (1970) on the other cannot be resolved. This is due to the fact that the method of sample preparation was different, which could have led to small differences in the observations. Anderson investigated a solid layer prepared by vapor deposition on a cold metal surface, whereas the other workers used solids prepared by cooling liquid in closed cells. Strains in solids prepared by nonequilibrium means could cause the appearance of weak Raman lines and the line in dispute is... 

The Kleber-Colombes rigid PVC foam (265,266) is produced by compression molding vinyl plastisol to react and gel the compound, followed by steam expansion. The process involves mixing, molding, and expansion. The formulation consists of PVC, isocyanate, vinyl monomers such as styrene, anhydrides such as maleic anhydride, polymerization initiators, FC-11, and nucleators. The ingredients are mixed in a Werner-Pfleiderer or a Baker Perkins type of mixer, and the resulting plastisol is molded under pressure. The initial temperature of the molds is 100-110°C, which increases to 180-200°C because of exothermic polymerization of the vinyl monomers and anhydride. The mold is cooled and the partially expanded PVC is removed and then further expanded by steam. After the water treatment, the foam is thermoset with a closed-celled structure and a relatively low thermal conductivity. 

Foaming by discharging the melt into the atmosphere, on the other hand, produced closed cells with a mean cell diameter of ca. 500 LLm. The foam cells were polyhedral in shape and quite uniform in size, which is comparable to a commercial foam with a mean cell size of ca. 1.0 mm. But, the DDC foams contained slightly collapsed cellular structures. Seemingly, this foaming process created a more rapid and uniform cooling of the melt, causing a rapid build-up of viscosity, which substantially stabilized bubbles. 

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