Cell Vinyl Foams

The second step consists of reheating the molded part at approximately 200-250 F (93-120 C), at which time the resin softens and the gas expands to form a closed-cell foam. With this technique it is possible to produce foams with densities as low as 2 Ib/ft, although the usual range is 50 to 10 Ib/ft . Because of this two-step procedure the process is much slower than the foaming procedure for open-cell materials. For this reason it is used mainly for specialty items such as 

Low-density unicellular foam made from this pressure process is available in a range of densities and hardnesses in various sizes ready for fabrication into the final product. Slab stock is used to produce shockabsorbing, flotation, and insulating foams. Typical densities range from 7 Ib/ft for shock-absorbing foams, 4 Ib/ft for flotation foams, and 6 Ib/ft for insulation foams. It is also possible to produce pressure-molded foams in cylindrical and ring-shaped molds (24) (3). 

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The open-cell vinyl foams produced by mechanical frothing, is used to produce sheets, such as flooring underlay, wall coverings, and other applications requiring relatively close thickness tolerances. Plastisol is mixed with a given amount of air in a high-shear, temperature-... 

Nitrosan ). This blowing agent is unique in that its low decomposition temperature permits the expansion of vinyl plastisol prior to gelation (93°C). Subsequent fusion at 177°C produces open-cell vinyl foam. Closed-cell foam can be produced by heating to fusion in a closed mold, releasing the pressure and subsequently heating in an oven at 100°C. 

Soft, very flexible vinyl foams used for garment insulation, upholstery and similar applications are made by this CBA process. The more rigid foams used as underlays for rugs and flooring can also be made by this method, but require different plastics and lower plasticizer contents. Open-cell chemically blown vinyl foams generally have densities in the range of 5 to 30 lb/ft3. 

AMS 3635C-84 Plastic Sheet and Strip, Modified Vinyl, Foamed, Closed Cell, 7 pp (DOD Adopted) (FSC 9330) (AS)... 

Keywords PVC, vinyl foam, acrylic, blowing agents, torque, melt strength, melt extensibility, melt density, cell morphology. 

Molded Vinyl Foam With other materials such as urethanes and elastomers, most foam products are molded. They are either allowed to expand under low pressure to fill the mold, yielding an open cell stmcture, often with a skin (as in, e.g., crutch pads) or molded and thermoset under pressure and subsequently expanded in an oven, yielding a closed cell structure (as with foam rubber shoe soles or mattress components). The products of oven expansion, referred to as buns, can be skived and die cut to gaskets or seals. The latter process is also carried out with vinyl. Molds are filled with plastisol, fused and cooled under pressure, and then expanded in an oven at 100-120 °C, softening the polymer and enabling the trapped gas to expand. For highly defined shapes, this step can be carried out in a second mold. 

The Dynamit-Nobel extmsion process (252) utilizes a volatile plasticizer such as acetone which is injected into the decompression section of a two-stage screw and is uniformly dispersed in the vinyl resin containing a stabilizer. The resulting PVC foam has low density and closed cells. 

The foamed, non-crosslinked resin particles, which have a bulk density of not greater than 0.045 g/cu.cm. and an average cell diameter of at least 200 pm, include PP to which a vinyl comonomer is graft polymerised in such an amount that the weight ratio of the PP to the graft polymer of the vinyl comonomer is in the range of 97 3 to 65 35. They are produced by graft polymerisation at a... 

Cellular Polymers 111. Conference proceedings. Coventry, 27th-28th April 1995, paper 13. 6124 CELL STRUCTURE DEVELOPMENT IN COMPRESSION MOULDED CROSSLINKED POLYETHYLENE AND ETHYLENE-VINYL ACETATE FOAM Sims G L A Khunniteekool C UMIST... 

BXL Plastics ERP Division is to introduce new grades of antistatic and conductive closed-cell crosslinked PE and ethylene copolymer foam at the Internepcon Exhibition to meet a growing demand from the electronics industry, for ways of minimising the effect of static electricity on circuits embodying static-sensitive devices, particularly using metal oxide/silicone technology. Very brief details are noted of Evazote C conductive closedcell, crosslinked ethylene-vinyl acetate copolymer foam. 

EVA is a copolymer of ethylene with minor amounts (ca. 10 0%) of vinyl acetate. EVA has many uses as a foam rubber in everyday goods (like shoes etc.), as cable insulation and as encapsulation material in photovoltaic cells. 

By crosslinking, the viscosity of EVA at high temperatures is increased and the individual cells are kept in a stable condition without rupture or agglomeration. A low density microcellular foam can be thus obtained. By selecting the vinyl acetate content, the EVA foam is flexible and highly resilient with easy coloring and adherent to other materials. The application is used widely in shoe soles, sandals and cushion materials. 

The newer open-cell foams, based on polyimides (qv), polybenzimidazoles, polypyrones, polyureas, polyphenylquinoxalines, and phenolic resins (qv), produce less smoke, are more fire resistant and can be used at higher temperatures. These materials are more expensive and used only for special applications including aircraft and marine vessels. Rigid poly (vinyl chloride) (PVC) foams are available in small quantities mainly for use in composite panels and piping applications (see Flame retardants Hrat-rrststantpot.ymf.rs). 

The moisture resistance, low cost, and low-density closed-cell structure of many cellular polymers resulted in their acceptance for buoyancy in boats, floating docks, and buoys. Because each cell is a separate flotation unit, these materials cannot be destroyed by a single puncture. Foamed-in-place polyurethane between thin skins of high tensile strength is used in pleasure craft [98]. Other cellular polymers that have been used where buoyancy is needed are produced from polystyrene, polyethylene, poly(vinyl chloride), and certain types of rubber. Foams made from styrene-acrylonitrile copolymers are resistant to petroleum products [99,100]. 

According to Thomas elastic polymers often produce open-cell foamed plastics, whereas rigid polymers generally form closed-cell materials. However, there are many exceptions to this rule, owing to the variety of blowing techniques. Closed-cell structures are more likely to be produced from polyurethanes, epoxy resins, silicones, poly(vinyl chloride), polystyrene, etc., whereas open-cell materials mainly result from phenolic and carbamide foamed plastics. 

Fig. 27. Cell diameter D distribution (cell number N in relative units) of foamed polymer based on poly-(vinyl formal) at compression deformation e (%) (1) 0.2, (2) 0.257, (3) 0.457...

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