**3.1 Methods of foaming**

Foaming techniques can be divided into several groups. Similar to the plastics processing technology, they can be divided on continuous (extrusion foaming) and periodic (injection molding or press foaming) processes. Periodic technology requires a long time and excluding manufacturing of expanded polystyrene (EPS), they are rarely applied.

Other method of rather seldom use is manufacturing of polymer composites filled with easily soluble compounds, like salt or sugar. After the filler is eluted with an appropriate solvent, the empty holes form cells of the resulted foam.

Cellular structure can be also formed by sintering of polymer powders at high temperature. Soft surfaces of neighboring spheres stick each other, whereas the free volumes between them create foam cells.

Akzo Nobel offers a foaming method based on mixing of a matrix polymer with thermoplastic spheres filled with volatile hydrocarbons (Expancel). At heating the polymer becomes soft, while the hydrocarbon evaporates, expanding the material. Initial sphere diameter is 12 μm, which after expansion increases to 40 μm. The material of spheres should be compatible with the matrix polymer, whereas a hydrocarbon is selected depending on a required decomposition temperature. The spheres of Expancel are added to a polymer in an

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Foaming with gases results mostly in foams of large cell size, however using supercritical liquids bring about manufacturing of microfoams (Cooper, 2000). At critical conditions (temperature and pressure) the density of a liquid and a gas equals. Above the critical temperature condensation of a gas is impossible, independing on a pressure applied. From that reason carbon dioxide is most appropriate for transportation, storage and dosing conditions, since its critical temperature is +31.1, whereas that for nitrogen is –146,9ºC and

Chemical blowing agents decompose within a specific temperature range, emiting a stechiometric amount of gases (usually nitrogen or carbon dioxide). Chemical blowing agents are classified as egzo- or endothermic, depending on the effect of a decomposition process. Due to a vigorous character of the decompositin reaction, egzothermic CBAs produce large size cells (>100 μm) of a non-uniform size distribution and cause a high

Fig. 10. Stucture of polypropylene foam produced with egzothermic CBA

water in a two-stage reaction: first at 130-140ºC, second at 180-200ºC (Fig. 11).

(24%), carbon dioxide (5%) and ammonia (5%).

The most popular egzothermic chemical blowing agent is azodikarbonamide (ADC) H2N-NH-(CO)-N=N-(CO)-NH2. ADC decomposes at 200-220ºC with emission of gases in the amount of 220 cm3/g. The mixture of gases comprises of nitrogen (65%), carbon monoxide

Endothermic chemical blowing agents need heat to continue decomposition, therefore it is easier to control the process just by changing its temperature. For that reason one can produce with endothermic CBA foams of lower cell size. Most popular endothermic blowing agent is a mixture of sodium hydrogen carbonate and citric acid. It decomposes to carbon dioxide and

argone –122,3ºC.

overall expansion of the material (Fig. 10).

amount of 2 - 8% and such mixture is processed by extrusion or injection molding technology. Decrease in a density for 30% was reported after addition of 3% microspheres, however the cells were of diverse size (Fig. 9).

Undoubtedly the principal polymer foaming technology is that involving a gas delivered to a polymer by means of the chemical (CFA) or physical foaming agent (PFA). Low density foams (2 - 500 kg/m3) are manufactured with physical blowing agents, whereas chemical blowing agents produce foams density of 500 - 750 kg/m3.
