Blowing agents function

Foam density is largely a function of the concentration of blowing agents. There has been a strong development towards the use of less expanded, i.e. higher density rigid cellular polyurethanes. This includes not only the so-called structural foams for simulated wood but also unexpanded solid materials used for brush handles and gun stocks. This range is clearly indicated in Table 27.4. ... 

Figure 3.6 Sound velocity in polystyrene/HCFC 142b before phase separation as a function of temperature. The blowing agent concentrations (wt%) are indicated. After Sahnoune el al. [189]. Reproduced by permission of the Society of Plastic...

Whilst sodium bicarbonate is the primary blowing agent, it is common compounding practice to use it in conjunction with a proportion of a weak acid, such as stearic or oleic acid, whose function is to trigger the reaction and assist in the uniform decomposition of the bicarbonate. The higher than normal fatty acid level will also act as a process aid, facilitating the bubble expansion process. 

PE is suitable for foam manufacture. Azodicarbonamide added in various concentrations, up to 20%, was successfully employed as the blowing agent. The crosslinking of commercial grade material was carried out by exposure to radiation. The expanding temperature was 205C. This study is focused on the functional properties of foams gel content correlated with expansion ratio, and cell density. 11 refs. 

The foams were prepared by first mixing the bark and the isocyanate, the other components (catalyst, surfactant, and blowing agent) were pre-mixed and added to this bark - isocyanate mix, and the mixing was continued to yield a homogeneous blend which began to rise and yield a foam product. The rate of foaming was a function of the catalyst used. 

Use of unmodified starches in combination with mild acids and carbonates has been claimed.214 Starch moisture content should be no greater than 25%. Acids such as tartaric, citric or malic acids are added at levels between 0.2% and 7%, while the carbonate level is 0.1% to 2%, both based on total starch composition. The acid is stated to serve a two-fold function it depolymerizes the starch during extrusion via hydrolysis, improving expansion at the same time, it liberates carbon dioxide, which acts as a blowing agent, from the carbonate. A typical formula extruded in a twin-screw extruder at 170-195°C yielded a product with a density of approximately 1 lb/ft3 (16kg/m3) and resilience of 60-85%. The foam had a continuous skin with a closed cell structure. 

Ingredients used in intumescent systems usually fulfill more than one function. Most compounds release some gas on heating, therefore they can be considered to be blowing agents. Many compounds produce some char. 

The topics to be covered in some detaU in this chapter are antistats, blowing agents, catalysts, fire retardants, mold-release agents, nucleating agents, reinforcements, stabilizers, and surfactants. These topics are presently in alphabetical order as a matter of convenience. The reader should be aware that there are a number of additives used in plastic foams that serve dual functions. These will be noted in the following text. 

A high-temperature cyclic peroxyketal peroxide cross-linking agent for polyethylene has been found to function as a blowing agent as well. This is another example of dual-function additives. Activated by thiodipropionate antioxidants, it evolves COj and should be useful in making crosslinked polyethylene foams. 

Flexible polyurethane foams are blown with water, methylene chloride, and chlorofluorocarbons (CFCs). Carbon dioxide from the water/isocyanate reaction functions as the blowing agent. The methylene chloride and CFCs assist in the blowing and contribute properties such as added softness and resilience. The CFCs also contribute to the insulation properties of rigid urethane foams. 

Expanded polystyrene is also produced by extrusion of the melt containing a volatile solvent, or more recently carbon dioxide, through a slit. The pressure drop caused by the emergence of the molten polymer from the slit of the extruder allows the hot solvent to rapidly vaporize, and in the process expands the extruded shape to an extent related to the temperature and proportion of solvent used. When carbon dioxide is used as the blowing agent, its expansion on emergence of the melt from the slit can perform the same function. This product is referred to as extruded polystyrene foam. Some manufacturers incorporate a flame retardant into grades intended for construction. 

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