Carbon dioxide blowing agent

Halocarbons have the further advantage of reducing the viscosity of the reaction mixture and, where used as the main blowing agent instead of the carbon dioxide produced by the isocyanate-water reaction, cheaper foams are obtained since less isocyanate is used. The reader should, however, note the comments made about the use of chlorofluoroearbons and their effect on the ozone layer made in Section 27.5.4. 

Polyurethanes (PURs) are usually described as being prepared by the reaction of diols with diisocyanates. However, this is an oversimplification because often water is deliberately added in the production of flexible polyurethane foams. Unreacted isocyanate groups react with water to form carbon dioxide and urea groups in the polymer chain. The carbon dioxide acts as a blowing agent in the production of PUR foams. Also, polyurethanes can be formed by the reaction of bischloroformates with diamines. 

We use variants of profile extrusion to produce tubing -with diameters of less then 1 mm and pipes with diameters exceeding 1 m, Wall thicknesses can vary from a few tens of micrometers up to several centimeters. Extruded window and door frames are more complex than pipes. Such profiles are largely hollow with internal ribs and fins that reinforce and divide the interior into two or more channels. We use solid rubber profiles in applications such as door seals and windshield wipers. We can produce foamed extrudates by incorporating a blowing agent, such as butane or carbon dioxide, into the polymer in the molten state. As the polymer exits the die, its internal pressure drops and the dissolved gas expands to form bubbles within the product. Examples of foamed extrudates include pipe insulation and automobile door gaskets. 

Polyether-based foams account for more than 90% of all flexible polyurethane foams. The properties of foams are controlled by the molecular structure of the precursors and the reaction conditions. In general, density decreases as the amount of water increases, which increases the evolution of carbon dioxide. However, the level of water that can be used is limited by the highly exothermic nature of its reaction with isocyanate, which carries with it the risk of self-ignition of the foamed product. If very low density foams are desired, additional blowing agents, such as butane, are incorporated within the mixing head. 

Chemical blowing agents undergo decomposition at the vulcanisation temperature to form a gaseous species such as nitrogen or carbon dioxide, e.g., azo compounds, nitroso compounds, sulphonyl hydrazide compounds, ammonium carbonate, ammonium bicarbonate and sodium bicarbonate. 

Other inorganic blowing agents used are ammonium carbonate and ammonium bicarbonate which produce mostly carbon dioxide with a small amount of ammonia. 

Blends of poly (ethylene terephthalate) (PETP) and polypropylene (PP) with different rheological properties were dry blended or compounded, and extrusion foamed using both physical blowing and chemical agents, and the foam properties compared with those of foam produced from the individual components in the absence of compatibilisers and rheology modifiers. The foams were characterised by measurement of density, cell size and thermal properties. Low density foam with a fine cell size was obtained by addition of a compatibiliser and a co-agent, and foamed using carbon dioxide. The presence of PP or a polyolefin-based compatibiliser did not effect... 

PE extmdates, are combined with physical atmospheric blowing agents, snch as nitrogen, argon, carbon dioxide, water and blends thereof, to prodnce nncrossUnked foams having densities as low as 10 kg/cn.cm. 

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. 

Thermoformable foamed PP sheet that processes with the same drawdown, sag and cycle times as PS has been manufactured by Omam using new PP blends and Safoam carbon dioxide blowing agents from Reedy International. Oman has already produced monolayer PP sheet with a 40% density reduction and three layer sheet with a 50% density reduction. The trials have been carried out using blends of conventional PP and a new high melt strength PP grade developed by Montell. The Omam line used to produce the foamed PP sheet is conventional apart from... 

---