Blowing agent
A blowing agent is a substance which is capable of producing a cellular structure
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 via a foaming  process in a variety of materials that undergo hardening or phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

, such as polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s, plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

s, and metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

s. They are typically applied when the blown material is in a liquid stage
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

. The cellular structure in a matrix reduces density, increasing thermal and acoustic insulation, while increasing relative stiffness of the original polymer.

Blowing agents (also known as 'pneumatogens')or related mechanisms to create holes in a matrix producing cellular materials, have been classified as follows:
Physical blowing agents e.g. CFC's (however, these are ozone depletents, banned by Montreal Protocol of 1987), HCFC's (replaced CFC's, but are still ozone depletents, therefore being phased out), hydrocarbons (e.g. pentane
Pentane is an organic compound with the formula C5H12 — that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer; the other two being called...

, isopentane
Isopentane, C5H12, also called methylbutane or 2-methylbutane, is a branched-chain alkane with five carbon atoms. Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pressure. The normal boiling point is just a few degrees above room temperature and...

, cyclopentane
Cyclopentane is a highly flammable alicyclic hydrocarbon with chemical formula 510 and CAS number 287-92-3, consisting of a ring of five carbon atoms each bonded with two hydrogen atoms above and below the plane. It occurs as a colorless liquid with a petrol-like odor. Its melting point is −94 °C...

), liquid CO2. The bubble/foam-making process is reversible and endothermic, i.e. it need heats (e.g. from a melt process or the chemical exotherm due to cross-linking), to volatile a liquid blowing agent. However, on cooling the blowing agent will condense, i.e. a reversible process.

Chemical blowing agents e.g. isocyanate
Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...

 and water (for PU's), azo-, hydrazine and other nitrogen-based materials (for thermoplastic and elastomeric foams), sodium bicarbonate
Sodium bicarbonate
Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...

 (aka baking powder, used in thermoplastic foams). Here gaseous products and other by-products are formed by a chemical reaction(s), promoted by process or a reacting polymer's exothermic heat. Since the blowing reaction occurs forming low molecular weight compounds acting as the blowing gas, additional exothermic heat is also released. Once formed the low molecular weight compounds will never revert back to the original blowing agent(s), i.e. the reaction is irreversible.

Mixed physical/chemical blowing agents — e.g. used to produce flexible PU foams with very low densities. Here both the chemical and physical blowing are used in tandem, to balance each other out with respect to thermal energy released/absorbed, so minimizing temperature rise. Otherwise excessive exothermic heat because of high loading of a physical blowing agent can cause thermal degradation of a developing thermoset or polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

 material. For instance, to avoid this in polyurethane systems isocyanate and water (which react to form carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

) are used in combination with liquid carbon dioxide (which boils to give gaseous form) in the production of very low density flexible PU foams for mattresses.

Mechanically made foams and froths, involves methods of introducing bubbles into liquid polymerisable matrices (e.g. an unvulcanised elastomer in the form of a liquid latex). Methods include whisking-in air or other gases or low boiling volatile liquids in low viscosity lattices, or the injection of a gas into an extruder barrel or a die, or into injection molding barrels or nozzles and allowing the shear/mix action of the screw to disperse the gas uniformly to form very fine bubbles or a solution of gas in the melt. When the melt is molded or extruded and the part is at atmospheric pressure, the gas comes out of solution expanding the polymer melt immediately before solidification. Frothing (akin to beating egg whites making a meringue), is also used to stabilize foamed liquid reactants, e.g. to prevent slumping occurring on vertical walls before cure – (i.e. avoiding foam collapse and sliding down a vertical face due to gravity).

Soluble fillers , e.g. solid NaCl crystals mixed into a liquid urethane system, which is then shaped into a solid polymer part, The NaCl is later washed out by immersin the solid molded part in water for some time, to leave small inter-connected holes in relatively high density polymer products, (e.g. Porvair synthetic leather materials for shoe uppers).

Hollow spheres and porous particles (e.g. glass shells/spheres, epoxide shells, PVDC shells, fly ash, vermiculite, other reticulated materials) are mixed and dispersed in the liquid reactants, which are then shaped into a solid polymer part containing a network of voids.
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