Blowing agents, chemical

All chemical blowing agents may be divided into the following groups carbonates and bicarbonates, nitrites, hydrides, peroxides, oxygen-containing acid derivatives, azo compounds, urea derivatives, hydrazines, semicarba-zides, azides, N-nitroso compounds, and triazols. 

Most of them (except for azo compounds, hydrazines, semicarbazides, N-nitroso compounds, and triazols) are of negligible importance. They are inexpensive but they are insoluble in plasticizers and organic solvents. They are not easily dispersed in polymer mixtures and cannot be stocked for a long time because of their unstability [1]. However, barium carbonate is sometimes used for plastics with a high foaming temperature such as ABS terpolymers, PP, and rigid PVC. At 245°C under atmospheric pressure, 1 g of barium carbonate will develop 177 mL of CO2 [2]. 

High stability against decomposition during storage. 

Good affinity with the polymer to obtain a good dispersion in the polymer bulk. 

Maximum production of gases in a minimum time at the polymer foaming temperature. 

Typical activators for azo blowing agents include acids, bases, metal organic salts, oxides, amines and urea, etc. 

The decomposition temperature range of a mixture of citric acid and sodium bicarbonate is generally at 150-210°C. The sole gaseous 

Endothermic and inorganic chemical blowing agents have included materials that degrade into carbon dioxide when heated, such as sodium bicarbonate. Sodium bicarbonate s low decomposition temperature (100°C-140°C) suits its use with POs, though it can decompose rapidly, forming open-cell stmctures. Other endothermic agents include citric acid, sodium borohydride, and polycarbonic 

Microcapsule foams are a currently more expensive alternative to chemical and physical blowing agents. These small thermoplastic bubbles added into the melt do not release gas, but rather expand to an optimum size as they are heated, through the expansion of the hydrocarbon gas inside them [13-12]. 

2 PRODUCT SECTORS REQUIRING CHEMICAL BLOWING AGENTS 

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Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

A most widely used decomposable chemical blowing agent is azodicarbonamide. Its decomposition temperature and rate of evolution of gaseous components are greatly influenced by the stabilizers containing zinc. Lead and cadmium are considered moderate activators for, -oxybis benzenesulfonyl hydrazide (OBSH). OBSH can also be used as a blowing agent for PVC foams. 

By fai the largest (ca 85% of the total) volume chemical blowing agent is azodicaibonamide (44), made by the oxidation of hydiazodicaiboxamide [110-21 -4] (51) using chlorine or sodium chlorate. The hydrazo precursor is made by refluxing an aqueous solution of urea and hydrazine (172) ... 

Incorporation of a chemical compound which decomposes at some stage of the processing operation to yield volatile reaction products. These are known as chemical blowing agents. 

A number of general eomments may be made about chemical blowing agents. 

The foam effect is achieved by the dispersion of inert gas throughout the molten resin directly before moulding. Introduction of the gas is usually carried out by pre-blending the resin with a chemical blowing agent which releases gas when heated, or by direct injection of the gas (usually nitrogen). When the compressed gas/resin mixture is rapidly injected into the mould cavity, the gas expands explosively and forces the material into all parts of the mould. An internal cellular structure is thus formed within a solid skin. 

I. S. Abou-Sayed and R. D. Hazlett. Removing fracture fluid via chemical blowing agents. Patent US 4832123,1989. 

CBA Chemical blowing agent DEVP Diethyl vinyl phosphonate... 

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