Entrapped air

Thickness. Because two fabrics that have identical weight per unit area values may have widely varying bulks, the specification of thickness is essential for properly characterizing a fabric. Fabric thickness has been shown to be direcdy proportional to thermal insulation, or warmth (121). Fabric warmth is the result of the entrapment of air between fibers and yams. A thicker fabric in general allows an increased amount of entrapped air and thus is warmer. 

A variation of the preceding process is used to produce oriented vinyUdene chloride copolymer films. The plastic is extmded into tube form and then is supercooled and subsequently biaxiaHy oriented in a continuous bubble process. The supercooled tube is flattened and passed through two sets of pinch roUs, which are arranged so that the second set of roUs travels faster than the first set. Between the two sets, air is injected into the tube to create a bubble that is entrapped by the pinch roUs. The entrapped air bubble remains stationary while the extmded tube is oriented as it passes around the bubble. Orientation is produced in the transverse and the longitudinal directions, creating excellent tensile strength, elongation, and flexibiUty in the film. The commercial procedure has been described (157). 

Industrial, centrifugal elevators usually operate at speeds of about 75 m /min, and handle free-flowing, fine and loose materials having lump sizes of <50 mm. Sticky material can be a problem. Fine fluidizing materials often require perforations in the bottom of the buckets to vent entrapped air. Centrifugal elevator capacities range up to 370 m /h for a single row of buckets, and up to 1400 m /h for multiple rows of buckets. The buckets can be mounted on a belt or chain. 

A serrated piece of aluminum used to work a plastic laminate. Purpose of device is to compact a laminate and to break up large air pockets to permit release of entrapped air. 

Opposite flow fronts produce a weld line that could also contain entrapped air. 

Corrosion has been encountered infrequently to date and has been a surface type, as opposed to pitting corrosion that can result in perforations. Entrapped air in the beverage or in the cans headspace increases the corrosive action of the product according to Koehler et at (21). As with beer and other canned foods, aluminum ends provide electrochemical protection when combined with tinplate or tin-free-steel can bodies. The level of iron pickup is reduced while the amount of aluminum dissolved in soft drinks increases without detrimental effect. Aluminum containers with vinyl epoxy and vinyl organosol coatings are compatible with carbonated soft drinks. 

For example, TNT shrinks from 11 to 12% on solidification, so that if the melt is too viscous during pouring, the entrapped air may not escape prior to solidification. These bubbles are further enlarged by the volume shrinkage of TNT, resulting in a porous cast of low density... 

The microstmcture appeared well mixed although co-continuity of the phases was not obvious. The blends appeared to have a continuous PP phase containing extended, yet isolated, SBR components as shown in Figure 11.17. It appeared to be similar to the microstmcture of the TPV-based on nylon and EPDM. The presence of entrapped air or mumal dissolution was not observed. As the fraction of PP increased, the microstmctures became clustered into larger PP and SBR single phases, with lower SBR-PP interface area. Both the materials were shear thinning. There is a large decrease in the viscosity of the composites at small shear rate. The viscosity values of the phases followed the equation... 

Secondly, a stable icing foam requires a low surface tension. Consider the case of egg whites in a beater. With slight whipping, entrapped air bubbles are large and the whites appear foamy, yet transparent and runny. With longer whipping the whites become less transparent, white, and more solid. Thus, because of the low surface tension inherent in the egg protein in solution, more and more air may be incorporated and held in place by the colloidal protein which forms a film around each air cell. 

The porosity of refractory bricks has a direct bearing on the thermal conductivity. The densest and the least porous bricks have the highest thermal conductivity owing to the absence of air voids. On the other hand, in porous bricks the entrapped air in the pores acts as a nonconducting material. 

Enzymes. Where tablets are not naturally very cohesive and have thus been manufactured by a wet granulation process involving one of the binders listed in Table 8, addition of small quantities of appropriate enzyme may be sufficient to produce rapid disintegration. It has also been proposed that disintegration action might result from expansion of the entrapped air due to generation of heat of wetting when the tablet is placed in a fluid. This concept has received little attention. 

Hot, demineralized water is used in the preparation of the dipping solution. Initially, a 30 40% w/w solution of gelatin is prepared in large stainless steel tanks. Vacuum may be applied to assist in the removal of entrapped air from this viscous preparation. Portions of this stock solution are removed and mixed with any other ingredients, as required, to prepare the dipping solution. At this point, the viscosity of the dipping solution is measured and adjusted. The viscosity of this solution is critical to the control of the thickness of the capsule walls. 

Formulations should have a moderate bulk density. It was suggested that low bulk density materials or those that contain entrapped air may not consolidate well and that capping similar to that which occurs in tableting may result. 

Because of the high quality of surface smoothness of films, wind-up onto rollers causes adjacent film layers to adhere together, either by static electricity or cold-flow (creep). This is overcome by adding finely divided fillers (less than 1 pm diameter) of, for example, chalk, silica, crosslinked polyacrylates, which roughen the surface allowing entrapped air to keep the successive layers apart. 

Underground mains should be tested before joints are covered. Water leakage through the joints should be measured by pumping at the specified test pressure from a calibrated container into the section of pipe being tested. Care must be taken to expel all entrapped air and ensure the main is completely full of water. Permissible leakage per 100 joints per hour is 2 quarts (1.9 I) (NFPA 24). 

Source plates may be prepared in advance and stored frozen, depending upon fhe probe ink stability. In any case, source plates should be brought to ambient temperature to assure that all components have redissolved. We recommend that source plates be centrifuged to remove any entrapped air bubbles from fhe bottoms of the wells. While manual preparation of source plafes is possible, the use of a robotic dispensing sysfem is highly recommended in order to avoid mistakes in placement of fhe probe inks in fhe wells and to avoid cross-contamination. The use of such a device will also allow more xmiform preparation of subsequent source plates. 

To size the two species however, it is necessary to separate them. A sink-float technique has been developed to separate the two. A carbon tetrachloride - tetrabromoethane mixture having an SG about 2.5 was used. The dry crystal mixture is added to the dense organic liquid, then put under vacuum to remove entrapped air bubbles. The suspension is then centrifuged for several hours to separate out the gypsum which floats and the hemihydrate which sinks. Tests on synthetic mixtures showed that a very clean separation is possible. 

Another example of a food emulsion is the ice cream, in which the colloidal dispersion of ice particles is achieved together with tiny entrapped air bubbles in an emulsion consisting of fats, sugar, and thickening agents (polysaccharides). 

The air-entraining admixtures are organic materials, usually in solution form, which when added to the gauging water of a concrete mix, entrain a controlled quantity of air in uniformly dispersed microscopic bubbles. This type of air should not be confused with entrapped air which is often present in concrete in the form of irregularly shaped cavities and which can be due to inadequate compaction or flaky aggregates. 

The long-term performance of synthetic liners is not known, and even in the short term can be affected by the presence of subsoil settlement, rock punctures, seam splitting, or entrapped air bubbles (Gee et al. 1990). The possible failure of synthetic liners creates an additional problem if they are the only barrier used in the tailings impoundment instead, they should be seen as a short-lived barrier that is only one part of a multilayer system. 

The standard process cycle for polymer matrix composites is a two-step cure cycle, as seen in Figure 8.1. In such cycles the temperature of the material is increased from room temperature to the first dwell temperature and this temperature is held constant for the first dwell period ( 1 hour). Afterward, the temperature is increased again to the second dwell temperature and held constant for the second dwell period (2-8 hours). After the second dwell, the part is cooled down to room temperature at a constant rate. Because there are two dwell periods, this type of cure cycle is referred to as a two-step cure cycle. The purpose of the first dwell is to allow gases (e.g., entrapped air, water vapor, or volatiles) to escape and to allow the matrix to flow, which leads to compaction of the part. Thus, the viscosity of the matrix must be low during the first dwell. Typical viscosity versus temperature profiles of polymer matrices show that as the temperature is increased, the viscosity of the polymer decreases until a minimum viscosity is reached. As the temperature is increased further, the polymer begins to cure rapidly and the viscosity increases dramatically. The first dwell temperature must be chosen judiciously so that the viscosity of the resin is low while the cure is kept to a minimum. 

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