Processing Characteristics Blowing Agents

Function Creating a cellular structure during processing, usually by formation of an inert gas at the processing temperature used for foams and expanded materials, injection moulded structural foam. 

Properties affected Cellular structure, stiffness/rigidity, lightness in weight reduced shrinkage (especially at thick sections) reduction in moulding pressure. 

Materials/characteristics Physical blowing agents low-boil hydrocarbons. Chemical blowing agents fluorocarbons (for polyurethane and PS foams) sodium bicarbonate/citric acid or azo compounds, etc., for moulded structural foams. 

Disadvantages Structural foam blowing agents may produce swirl eflect on surface of mouldings, requiring treatment before painting. 

New developments Replacement of CFC blowing agents (in polyurethane and polystyrene foams). 

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The effects of compound formulations and processing conditions on the structure and properties of extruded EPDM and NR/EPDM blends of tube foams were studied. The characteristics of blends of blowing agents (azodicarbonamide(ADC)/dinilrosopentamethylenelriamine... 

Carbon dioxide gas was used as a physical blowing agent to produce medium density polypropylene foam sheets using a single screw extruder. The mechanical properties of the foam were similar in the machine direction and in the transverse direction. Abetter surface finish and a lower density was produced by using a commercial wrapping film as a cap layer. The process conditions and the die design data are presented and an attempt made to relate them to the product characteristics. 4 refs. 

Usually the polymerization process is carried out in two or more stages. During the first stage, the final particles are formed, and in the second stage blowing agent is added and this penetrates into the beads. The duration of the second step is preferably determined by the residual monomer concentration that needs to be achieved (normally <1000 ppm). For both stages of the process, initiators with different decomposition characteristics have to be applied to initiate the polymerization. 

In the low-pressure process, a resin containing a blowing agent is forced into the mold, where it expands to fill the mold under pressures of 690-4100 kPa (100-600 psi). This produces structural foam products with a characteristic surface-swirl pattern caused by the collapse of cells on the surface of molded articles. 

The process of PET glycolysis with DEG has several disadvantages the reaction products are viscous liquids with a tendency to solidification or even to be solid at room temperature, the reproducibility of the characteristics of the resulting polyester polyols are difficult to realise (poor consistency) and the products of transesterification are not compatible with the blowing agents (pentanes or hydrofluorocarbons) [4, 6]. 

The bulk of the flexible polymethane foam is now being manufactured by the one-shot process using polyether-type polyols because they generally produce foams of better cushioning characteristics. The main components of a one-shot formulation are polyol, isocyanate, catalyst, surfactant, and blowing agent. 

On the contrary, the gas dissolution foaming process, and in particular the high-pressure or supercritical CO2 gas dissolution foaming, allows obtaining micro-and nanoporous polymers. In this technique CO2 is used as a physical blowing agent [39,57-59] this gas is one of the best options for this kind of process because of its excellent characteristics of diffusion in the supercritical state and the mild conditions to reach this state (31°C and 7.3 MPa). Last but not the least, carbon dioxide is a green solvent that can be removed without residue or production of any pollutant compound [60,61]. 

Yun and Lee (2004) investigated pultrusion of phenolic foam composites. The effect of process variables on the foaming characteristics of phenolic resin during pultrusion was studied experimentally with the consideration of variables such as the heating temperature, the pulling speed and the mass fraction of the blowing agent. 

TABLE 1 Some characteristics of the chemical blowing agents used in polyolefin processing... 

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