5 mixed cell foams

The mixed cell foams exhibited mean densities that fluctuated considerably depending on the position of the specimen. The skin and upper parts of the foams had higher densities than the core and bottom parts. Again, this local... 

Figure 3. Mean densities (pf) of the LDPE foams with MI=2.3 as a function of process conditions (a) Mixed cell foams at = llO C and r = 2J...

Figure 4. DCS thermograms and crystallinities of (a) as-received resins and (b) the DDC mixed cell foams, with a commercial foam (C-foam).

Figure 5. SEM micrographs of the DDC mixed cell foams (a)(b)(c) Closed cell walls, (d) Open cell walls.

Urea.—Forma.IdehydeResins. Cellular urea—formaldehyde resins can be prepared in the following manner an aqueous solution containing surfactant and catalyst is made into a low density, fine-celled foam by dispersing air into it mechanically. A second aqueous solution consisting of partially cured urea—formaldehyde resin is then mixed into the foam by mechanical agitation. The catalyst in the initial foam causes the dispersed resin to cure in the cellular state. The resultant hardened foam is dried at elevated temperatures. Densities as low as 8 kg/m can be obtained by this method (117). 

Rigid foams are based primarily on polyfunctional, low molecular weight alcohols and amines. Most global applications conventionally use polymeric isocyanates, TDI, or an undistilled grade of mixed TDI isomers. TDI prepolymers which have hydroxyl and isocyanate groups have been marketed as a low vapor pressure alternative to undistilled TDI. Density reduction is effected via the addition of chlorofluorocarbons, low molecular weight alkanes, or via the in situ generation of carbon dioxide. The resultant closed cell foams find applications as insulators in construction, appliance, transportation, pipeline, and tank end uses. 

Water-in-Monomer Polymerizations for Open-Celled Foams. Porous polymers or mixed polymers of styrene are prepared by polymerizing the... 

On the contrary, in the second binodal region (b) in Fig. 20.1-7, where the local mixing point finds the polymer-rich solution as the continuous phase, dispersed spharaids of nesrly polymer-free fluid are nucleated. These do meins then coalesce to p red nee a foam structure whose walls are composed of the solidified dispersed polymer phase. To obtain an open-cell foam with low resists ace to flow, defects clearly must occur in the waits of (be cells.39 Soch a structure is shown in Fig. 20.1-66. Tbe dense film on the surface can be promoted by a brief exposure of the cast or apan nascent membrane to air to obtain a more concentrated region at the surfhee prior to immersion in the nonsolvent precipitation hath, which then sets the dease layer in place and proceeds to nucleate the subetracture as described above, This evaporation step, however, is not required in all cases to produce acceptuble skins.56,65,66... 

Bimodal PE s (LCB = 0.01- 3 > 7) was lightly crosslinked. The PE-1 was prepared in the hrst reactor, and PE-2 was prepared in the presence of 15 - 65 wt% of PE-1 in the second reactor. The reaction could be carried out in the slurry, solution, or gas phase. blends (p < 885 kg/m ) were used for wrre/cable coating, weather-stripping, seals, foamed articles with closed, open or mixed cells, containers, medical appliances, drapes and coverings, hbers, tapes, tubes, pipes and hoses, bellows, boots, gaiters, footwear, etc. Cree etal, 1998... 

Hard, scanihard and soft elastic foams Closed-cell, open-cell and mixed-cell... 

There are several types of foamed plastics including syntactic foam, closed-cell foam, and open-cell foam. Syntactic foam is a special type of plastic foam in which the cellular structure is formed by the use of smaU, hollow spheres made of resin, glass, or other hollow particles. The spheres are dispersed throughout a matrix of resin, usually thermosetting, which when cured resanbles a conventional foam plastic. Once mixed, syntactic foam may be cast or formed in place by a pouring or trowling action. 

The elastomer process is very similar to the Dennis process involving a number of steps in which a gas, formerly carbon dioxide and now fluorocarbon, is mixed with a plastisol under pressure. When released to atmospheric pressure, the gas expands the vinyl compounds into a low density, open-cell foam that is then fused with heat. 

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