Air Entrapment

Air entrapment is a rather common problem in extrusion. It is caused by air being dragged in with particulate material from the feed hopper. Under normal conditions, the compression of the solid particulate material in the feed section will force the air out of the solid bed. However, under some circumstances the air cannot escape back to the feed hopper and travels with the polymer until it exits from the die. As the air pockets exit from the extruder, the sudden exposure to a much lower ambient pressure may cause the compressed air bubbles to burst in an explosive manner. However, even without the bursting of the air bubbles, the extrudate is generally rendered unacceptable because of the air inclusions. 

There are a number of possible solutions to air entrapment. The first approach should be to change the temperature in the solids conveying zone to achieve a more positive compacting of the solid bed. Often, a temperature increase of the first barrel section reduces the air entrapment however, in some cases, a lower temperature causes an improvement. In any case, the temperatures in the solids conveying zone are important parameters in the air entrapment process. It should be realized that both the barrel and screw temperatures are important. Thus, if a screw temperature adjustment capability is available, it should definitely be used to reduce the air entrapment problem. 

The next step is an increase in the diehead pressure to alter the pressure profile along the extruder and to achieve a more rapid compacting of the solid bed. The diehead pressure can be increased by simply adding screens in front of the breaker plate. Another possible solution is to starve feed the extruder however, this may reduce extruder output and requires additional hardware, i.e., an accurate feeding device. 

It should be noted that bubbles in the extrudate are not only a sign of air entrapment, but it may also be an indication of moisture, surface agents, volatile species in the polymer itself, or degradation as shown in the fishbone diagram shown in Fig. 11.36. 

Vent flow Piugged vent port Vacuum too iow 

Air can be entrapped in the melt during processing. This can happen when plastic (pellets, flakes, etc.) is melted in a normal air environment (as in a plasticating extrusion process or in an injection barrel, compression mold, casting form, spray system, etc.), and the air cannot escape. Generally the melt is subject to a compression load, or even a vacuum, which causes release of air but in some cases the air is trapped. If air entrapment is acceptable, no further action is required. However, it is usually unacceptable, for reasons of both performance and aesthetics. 

Changing the initial melt temperature in either direction may solve the problem. With a barrel and screw, it is important to study the effects of temperature changes. Another approach is to increase the pressure in processes that use process controls. Particle size, melt shape, and the melt delivery system may have to be changed or better controlled. A vacuum hopper feed system may be useful. With screw plasticators, changes in screw design may be helpful. Usually, a vented barrel will solve the problem. 

The presence of bubbles could be due to air alone or moisture, plastic surface agents or volatiles, degradation, or the use of contaminated regrind. With molds such as those used for injection, compression, casting, or reaction injection, air or moisture in the mold cavity will be the culprit. So 

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Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Direct Compression. This process is relatively simple and time saving. AH the ingredients are blended and then compressed into the final tablet. This is an excellent method, but encumbered by a number of problems. Not all substances can be compressed directly, necessitating a granulation step. Likewise, the flow properties of many blends of fine, particle-sized powders are not such as to ensure even filling of the die cavities of tablet presses. In addition, air entrapment can occur. 

The term steam quaUty refers to the amount of dry steam present relative to Hquid water in the form of droplets. The steam deUvered from the boiler usually contains some water. Excessive amounts can result in air entrapment, drying problems following exposure, and unacceptable steam levels (>3% water or <97% quaUty steam). Excessive amounts of water deposits dissolve boiler chemicals onto the load to be sterilized. Boiler chemicals are used to prevent corrosion in the lines. Inappropriate boiler chemicals, also called boiler amines, may introduce toxicity problems (see CORROSION AND CORROSION control). 

The placing of jigging points is particularly important to ensure adequate drainage, to allow gas to escape and, to avoid air entrapment. 

These are the primary process interactions that the designer must be aware of in order to determine process interference in product performance and design. Specific materials may introduce other problem areas as, for example, air entrapment, differential expansion, and the problem of a level of crystallinity in a crystalline plastic that exceeds the allowed level for stability of a product. 

Nonfills Air entrapment Gel and/or resin timet too short Additional air vents and/or vacuum required Adjust resin mix to lengthen time cycle... 

The ratio of the apparent volume of a material in powder form to the volume of the material in solid form, i.e., after removal of the air entrapped between the particles. In moulding (plastics or rubber) using powdered material, the bulk factor is thus the ratio of the volume of the powder to the volume of the moulding made from it. Bumping... 

In order to minimise air entrapment and blistering, the thickness of each sheet is generally limited. To build up the required thickness of the final sheet, two or more plies of calendered sheet are usually laminated on the bottom bowl of a three-bowl calender. 

Presses with vacuum extraction fitted to extract gases from moulding cavities eliminate moulding faults arising from air entrapment, or gases generated during vulcanisation. These presses are... 

The various tyre components are built together on the building drum of a tyre-making machine. The builder applies strips at appropriate points to provide a gradual transition from one major component to the next. Many of the processes have been automated, but operator skill is still important. Air entrapment between layers must be avoided to prevent ply separation in service. 

Common defects that are observed in this coating technique are voids, pin holes, thickness variations, and wavy surfaces. Voids and pinholes are caused by air entrapment in the film, surface contamination, or dust particles. 

Low compression rate and low compression ratio Splay and air entrapment 11.12.4... 

Of the two hot precompacted laminates (Laminates 3 and 4), the wet exposed laminate was of significantly poorer quality. The attenuation ranges for Laminate 3 was from 30 to 62 dB, whereas that for Laminate 4 was 10 to 42 dB. Because the hot precompaction temperature was only 66°C (150°F), the additional porosity should not be due to voids caused by the vaporization of absorbed moisture, but is more likely due to voids caused by air trapped during collation. The wet exposed prepreg exhibited a noticeable increase in tack that could contribute to increased air entrapment during collation. 

The impact of hydrodynamic stress on animal cells has been reviewed extensively (29,43). Most of the work reported in the literature on cell damage in agitated bioreactors has been done at bench-scale. Kunas and Papoutsakis (44) reported that in 1-2 L bioreactors equipped with a 7 cm diameter pitched-blade impeller, cell damage was not observed until the impeller rate was raised to above 700 rpm (tip speed 513cm/s), as long as air entrapment did not occur. However, it is not clear how these bench-scale observations translate into damaging impeller rates at manufacturing scale. 

Typical pharmaceutical blends separate from each other by three common mechanisms sifting/percolation, air entrapment (fluidization), and particle entrapment (dusting). 

Goodness-of-fit analysis applied to release data showed that the release mechanism was described by the Higuchi diffusion-controlled model. Confirmation of the diffusion process is provided by the logarithmic form of an empirical equation (Mt/ M=ktn) given by Peppas. Positive deviations from the Higuchi equation might be due to air entrapped in the matrix and for hydrophilic matrices due to the erosion of the gel layer. Analysis of in vitro release indicated that the most suitable matrices were methylcellulose and glycerol palmitostearate. 

As the concentration of polyvinyl chloride and glycerol palmitostearate was increased between 15% and 20%, a non-significant decrease in released amount was obtained, while a slight difference in the amount released was observed beween 10% and 15% of polymer concentration (Table 4). Deviations from the Higuchi equation were observed (Table 3). These positive deviations might be due to the air entrapped in the matrix. Similar results were also obtained with polyvinyl chloride by Desai et al. [14] and Korsmeyer etal. [15]. 

Add item 4 and balance of item 2, cool to room temperature, apply vacuum to remove air entrapped. 

Add 40.0 g of item 8 (70°C) to the stainless steel container while mixing to make a clear mucilage mix for 15 minutes. Avoid air entrapment. 

After pumping Miglyol 829, set the propeller with optimum circulation and revolution to ensure no air entrapment. 

J.J. Minnick, USP 3338165(1967) CA 67, 118754k (1967) [Expls prepd from NMe (Nitromethane) sensitized to initiation by incorporation of spherical resin balloons of finely divided air-entrapped materials. The balloons of diam 2 to 360 microns (such as described in CA 59, 9731a) and bulk d 0.3 g/cc were made from a polymerized thermosetting resin, either urea-formaldehyde or phenol-formaldehyde. Suitable amts of balloons to sensitize NMe were in the range 1.5 to 10%. Fuels, oxidizers and thickeners could be incorporated]... 

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