Foaming Blowing Agents

Foaming agents are either liquids or solids. In the case of liquid foaming agents, there is in general no decomposition of the type mentioned previously. Instead boiling occnrs, the melt foams spontaneously and this structure is captured as the melt freezes or cures finally to a solid structure. 

There are a number of options available to select the blowing agent. Eight basic materials are used worldwide  

Usually, the decomposition of any of these is incomplete, although certain catalyst activators can be used to accelerate their decomposition, some residue usually remains in the system (e.g., for ADC, decomposition is 32% by weight, which is mostly nitrogen and carbon monoxide, while the solid residue is mostly urazole and cyanuric acid. One use of ADC is in the production of blown vinyl wall-coverings. As another example, OBSH, which is mostly used in the rubber industry and in the foamed insulation of cables, decomposes with a rather low yield, leaving mostly non-polar oligomers in the residue [12]. 

Among these agents, DNPT seems to pose the most serious issues of toxicity and also carries the intense smell of the residue, so that although it is the lowest cost blowing agent available, its use is declining. 

For poljmrethane (PU) systems, water (or traces of hmnidity) can be used as the foaming agent. Directly injected liquid carbon dioxide is also used as a foamer, as well as a plasticiser. After release of the pressure constrainer of the system, the carbon dioxide boils, causing the polymer to foam [13]. 

Nitrogen, air, carbon dioxide, a mixture of air and helium, are examples of gaseous foaming agents used in the production of polymer foams. Nitrogen and air are preferred since they are inert, nontoxic, non-flammable and have a low diffusivity with respect to the majority of polymers. An air/helium mixture permits an easy control of the foam density. 

Organic chemicals able to release nitrogen as the main component of the liberated gas are the most important, their decomposition usually occurs in narrow temperature ranges, and it is an irreversible exothermic reaction independent of external pressure. 

Organic foaming agents have some important advantages such as  

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The use of CFCs as foam blowing agents has decreased 35% from 1986 levels. Polyurethanes, phenoHcs, extmded polystyrenes, and polyolefins are blown with CFCs, and in 1990 the building and appHance insulation markets represented about 88% of the 174,000 t of CFCs used in foams (see Foamed plastics). 

In the paste coating method, a PVC paste, which contains emulsion-polymerized PVC and additives, is appHed onto a substrate and heated to gelation before fusion to produce a coating layer. This method is employed for products with a thin layer, ie, of 0.007—0.05 mm thickness. For foamed vinyl-coated fabrics, a substrate is laminated onto a transfer paper on which a PVC paste containing a foam-blowing agent has been appHed and geUed. After removal of the transfer paper, the paste is blown. 

Other Uses. Other appHcations for sodium nitrite include the syntheses of saccharin [81-07-2] (see Sweeteners), synthetic caffeine [58-08-2] (22), fluoroaromatics (23), and other pharmaceuticals (qv), pesticides (qv), and organic substances as an inhibitor of polymerization (24) in the production of foam blowing agents (25) in removing H2S from natural gas (26) in textile dyeing (see Textiles) as an analytical reagent and as an antidote for cyanide poisoning (see Cyanides). 

Ozone depletion polential (ODP) and global warming potential (GWP) ol various foam blowing agents. 

The term CFCs is a general abbreviation for ChloroFluoroCarbons. They have been extensively used since their discovery in the thirties, mainly as refrigerant, foam blowing agent, or solvent because of their unique properties (non toxic, non flammable, cheap). However, after the first warning of Rowland and Molina [1] in 1974 that CFCs could destroy the protective ozone layer, the world has moved rapidly towards a phase-out of CFCs. Because the destruction of stratospheric ozone would lead to an increase of harmful UV-B radiation reaching the earth s surface, the production and use of CFCs is prohibited (since January 1, 1995 in the European Union and since January 1, 1996 worldwide). 

In 1974, F. Sherwood Rowland and Mario Molina, who shared the 1995 Nobel Prize in Chemistry with Crutzen, showed that chlorine from photolyzed chlorofluorocarbons (CFCs) such as CF2C12 and CFCI3, which were used as supposedly inert refrigerants, solvents for cleaning electronic components, plastic foam blowing agents, and aerosol spray propellants, can also catalyze ozone loss. Subsequently, the chlorine monoxide molecule CIO, which is involved in the chlorine-catalyzed ozone destruction cycle, has been shown to be present in the holes in the ozone layer and to correlate inversely with... 

Nonfluorine CFC substitutes have been considered, but few are fully satisfactory. For example, we could go back 50 years to the use of anhydrous ammonia as a refrigerant, but NH3 is as toxic now as it ever was. Cyclopentane could be used as a foam-blowing agent, but it is less effective than HCFC-141b and besides would contribute to the volatile organic compound load in the troposphere, which is the root cause of ozone pollution (Section 8.3.2). On the other hand, supercritical CO2 is emerging as an alternative to CFCs in various steps in the preparation of fluorocarbon polymers (Section 8.1.3). 

A polymer composition and a polymer film, which protects thermoplastics from chemical attacks by various polymer foam blowing agents is made of a blend or mixture of polymers and copolymers comprising (29) ... 

Foam blowing agents Foam-bonded nonwovens Foam controllers Foamed lonomers Foamed matenals Foamed plastic Foamed plastics... 

Refrigerants, Propellants, and Foam Blowing Agents R L Powell Fire Suppression Agents Mark L Robin Organic Fluoropolymers IV IV Schmiegel... 

Methylene chloride is used in refrigeration, aerosol propellants, paint stripping, urethane foam-blowing agents, adhesive, and food extractants. It has low toxicity compared with other chlorinated hydrocarbons and has been show n lo be neither mutagenic nor carcinogenic toward humans. 

Common uses for the fluorocarbons are as refrigerants, foam-blowing agents, solvents, and fluoropolymers. Recent environmental legislation has restricted or banned the use of chlorofluorocarbons. 

The main uses of methylene chloride are in paint remover, aerosols, chemical processing, urethane foam blowing agents, metal degreasing, and electronics. 

Methylene chloride is used primarily as a solvent for degreasing and paint removal, and it is also used in aerosols and foam-blowing agents. Since 1985, new environmental regulation has had a major impact on this chemical. The consumption of methylene chloride has been reduced by 60 percent due to recycling and product substitutions. 

Chlorofluorocarbons such as CFC13 and CF2a2, used as foam-blowing agents, aerosol propellants, and refrigerants, are photochemically decomposed to give Cl atoms, and these catalyze ozone decomposition22 via the mechanism... 

Refrigerants, Propellants, and Foam Blowing Agents R L Powell... 

CFCs (chlorofluorocarbons) - As applied to polyurethane foams, blowing agents having chlorine and fluorine in their chemical structure. CFCs are gradually being replaced by other blowing agents because of concern about their adverse effect on the atmospheric ozone layer. 

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