Blowing agents volatile liquids

As surfactants silicones can be used. As blowing agents liquids with a low boiling point and a low heat of vaporization are used. The agent must volatilize at the moment when the viscosity increases mixture under exothermic crosslinking. As blowing agents, volatile fluorocarbons... 

Incorporation of low boiling liquids which volatilise during processing. Such volatile blowing agents are important with polystyrene and polyurethanes and will be dealt with in the appropriate chapters. 

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

PE/PP (25 %), rubbers (including EVA) (20 %) and other thermoplastics (5 %). Physical blowing agents (PBAs) liberate gases as a result of physical processes (evaporation, desorption) at elevated temperatures or reduced pressures. PBAs are mostly volatile liquids, freons, aliphatic hydrocarbons or solid blowing agents. 

Expandable SAN microspheres are prepared by polymerizing a mixture of styrene and AN with a volatile liquid blowing agent in an encapsulating system. Expandable microspheres of larger size, narrower size distribution and improved expansion characteristics are obtained using an alcohol in the polymerization system, i.e., methanol (7). 

Extruded polystyrene board was first introduced in ihe early 1040s by Dow Chemical Co. with the tradename Styrofoam. The Styrofoam process consists of the extrusion of a mixture of polystyrene and volatile liquid blowing agent expanded through a die to form boards in various sizes. The conlinuous hoards are then passed through Ihe finishing equipment for further sizing. 

Blowing (foaming) agent — A substance that alone or in combination with other substances can produce a cellular structure in a plastic or elastomeric mass. It can be a compressed gas, a volatile liquid, or a solid that decomposes into a gas upon heating. 

Crosslinked Polystyrene. Polystyrene lightly crosslinked during polymerization by 0.01-0.25% divinylbenzene provides foams, expanded by carbon dioxide or other gases, with greater expandability and resistance to collapse at elevated temperatures (102, 103). Other divinylbenzene copolymers are reported, employing as blowing agent saturated aliphatic or fluoroaliphatic volatile liquids, tetramethylsilane, and azobis-isobutyronitrile (99,106, 108). 

Volatilization of a low-boiling liquid, either by the heat liberated by an exothermic reaction, or by externally applied heat. Commonly used liquids are chlorofluoro-carbons (CFCs). This is the most widely used technique in the production of rigid polyurethane foams. However, due to the ozone depletion problem in the stratosphere, they must be phased out and industry is presently searching for alternative blowing agents. 

They may be chemical blowing agents that decompose at certain temperatures and release a gas, or they may be low boiling liquids that become volatile at the processing temperatures. Gases may be introduced into the polymer under pressure and expand when the polymer is depressurized. Mechanical whipping and the incorporation of hollow glass spheres can also be used to produce cellular materials. 

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