Polyurethane rubbers cellular

The cellular materials discussed in Section 27.5 may also be considered as polyurethane rubbers, but because of their importance are treated separately. 

The low hardness has led to uses in printers rollers and stereos. It is, however, to be noted that when the material has been used to replace cellular rubbers or flexible polyurethane foams in sealing applications, problems have arisen where it has not been appreciated that although the rubber is very soft it is for practical purposes incompressible. 

A form of cellular rubber in which the cells are non-intercommunicating, self-contained units. It has low thermal conductivity. Expanded rubber is buoyant and does not absorb water and was therefore initially used in both the soft rubber and ebonite forms in the construction of lifebuoys and other marine buoyancy equipment. The most commonly used polymer is now polyurethane for both flexible and rigid systems. 

While unaffected by water, styrofoam is dissolved by many organic solvents and is unsuitable for high-temperature applications because its heat-distortion temperature is around 77°C. Molded styrofoam objects are produced commercially from expandable polystyrene beads, but this process does not appear attractive for laboratory applications because polyurethane foams are much easier to foam in place. However, extruded polystyrene foam is available in slabs and boards which may be sawed, carved, or sanded into desired shapes and may be cemented. It is generally undesirable to join expanded polystyrene parts with cements that contain solvents which will dissolve the plastic and thus cause collapse of the cellular structure. This excludes from use a large number of cements which contain volatile aromatic hydrocarbons, ketones, or esters. Some suitable cements are room-temperature-vulcanizing silicone rubber (see below) and solvent-free epoxy cements. When a strong bond is not necessary, polyvinyl-acetate emulsion (Elmer s Glue-All) will work. 

Cellular polymers, especially polystyrene and polyurethane, are also widely used for pipe and vessel insulation. The use of cellular rubber and cellular poly(vinyl chloride) in insulation for small pipes is attributed to their ease of application, combustion properties, and low thermal conductivity. 

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]. 

Cellular Cellulose Acetate Conventional adhesives recommended include polyurethanes, synthetic resins, thermoplastics, resorcinol-formaldehyde, nitrile-phenolic, and rubber-based materials (8). 

Thermal insulators comprise an equally broad range of materials. Such inorganics as mineral fibers, magnesia, aluminum silicate, cellulose, and glass fibers are widely used for steam and hot-water pipes, furnaces, and blown-in home insulation. Organic products that are effective include plastic foams (polyurethane, polyvinyl chloride, polystyrene) and cellular rubber. There are a number of materials that may be called double insulators, since they have both electrical and thermal insulating properties,... 

A rigid foam is defined as one in which the polymer matrix exists in the crystalline state or, if amorphous, is below its Tg. Following from this, a flexible cellular polymer is a system in which the matrix polymer is above its Tg. According to this classification, most polyolefins, polystyrene, phenolic, polyycarbonate, polyphenylene oxide, and some polyurethane foams are rigid, whereas rubber foams, elastomeric polyurethanes, certain polyolefins, and plasticized PVC are flexible. Intermediate between these two extremes is a class of polymer foams known as semirigid. Their stress-strain behavior is, however, closer to that of flexible systems than to that exhibited by rigid cellular polymers. 

Because of the relatively low temperature of decomposition, DTA can be used to make foams with a uniform cellular structure without deterioration of the polymer. The disadvantages of DTA are, however, poor dispersive abihty in mixtures and sensitivity to moisture. Nevertheless, DTA is used in foaming PVC (especially for thin walled articles), polyurethane, polystyrene, polyamides, and siloxane rubbers. 

Several elastomers including Neoprene, polyurethane, silicone and ethylene-propylene-diene monomer rubber were tested according to the UL 94 VO procedure. Each material was exposed to flame for two, one second intervals. To pass the test, each individual specimen had to cease burning 10 seconds after flame application. Of these, only a cellular silicone called Bisco passed while another manufacturer s silicone severely charred and supported a flame. 

The first cellular synthetic plastic was an unwanted cellular phenol-formaldehyde resin produced by early workers in this field. The elimination of cell formation in these resins, as given by Baekeland in his 1909 heat and pressure patent (2), is generally considered the birth of the plastics industry. The first commercial cellular polymer was sponge rubber, introduced between 1910 and 1920 (3). Most plastic polymers can be foamed. However, a relative few have commercial significance, such as polystyrene, polyolefins, poly(vinyl chloride), polyimides, and polyurethanes. 

The combination of structural strength and flotation has stimulated the design of pleasure boats using a foamed-in-place polyurethane between thin skins of high tensile strength (242). Other cellular poljmiers that have been used in considerable quantities for buoyancy applications are those produced from polyethylene, polystyrene, poly(vinyl chloride), and certain types of rubber. The susceptibility of polystyrene foams to attack by certain petroleum products that are likely to come in contact with boats led to the development of foams from copolymers of styrene and acrylonitrile that are resistant to these materials (243,244). 

Sims G L A and Sombatsompop N (1996) Pulverised flexible polyurethane foam particles as a filler in natural rubber vulkanisates. Cellular Polymers 15 90-104. 

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