For those from a technical and chemistry background you might find the following article an interesting insight to the chemistry of polyurethanes. It is intended to give a brief technical introduction to the chemistry of polyurethanes and to satisfy the frequent student requests we get to provide some details on the chemistry of polyurethane foam.

Polyurethane plastics are produced by the condensation reaction of a polyol and a ‘di-isocyanate.’ Note the 'iso' within diisocyanate which is often used as a root word in the naming of polyurethane technology companies. These chemicals are often referred to as 'A' and 'B' when industrially mixed. The chemical reaction has the following formula:

When polyol and diisocyanate are reacted the reaction is quick and can be violent if not controlled by specific quantities. The commercial production of spray foam polyurethane typically involves reacting the reactants, polyol and diisocyanate, in 50:50 proportions and mixing small amounts under pressure at the focus of a spray gun to give the resultant polyurethane foam. Heat and pressure are also used to and it is typical to produce industrial spray foams at 120 degrees temperature under 600 psi pressure to optimise the volume of foam produced and provide a quick cure time within minutes.

No by-product is formed from the reaction of a polyol and a diisocyanate. Toluene diisocyanate is a widely used monomer. Diols and triols produced from the reaction of glycerol and ethylene oxide or propylene oxide and are considered very suitable for producing polyurethanes. Depending on the mix and type of the polyol, polyurethane polymers are either rigid or flexible. For example, triols derived from glycerol and propylene oxide are used for producing block slab foams. These polyols have moderate reactivity because the hydroxy groups are predominantly secondary. More reactive polyols (used to produce injected moulding polyurethane foams) are formed by the reaction of polyglycols with ethylene oxide to give the more reactive primary group:

Other polyhydric compounds with higher functionality than glycerol (three-OH) are commonly used. Examples are sorbitol (six-OH) and sucrose (eight-OH). Triethanolamine, with three OH groups, is also used. Diisocyanates generally employed with polyols to produce polyurethanes are 2,4-and 2,6-toluene diisocyanates prepared from dinitrotoluenes.

Synthetic Petroleum-Based Polymers
A different diisocyanate used in polyurethane synthesis is methylene diisocyanate (MDI), which is prepared from aniline and formaldehyde. The diamine product is reacted with phosgene to get MDI. The physical properties of polyurethanes vary with the ratio of the polyol to the diisocyanate. For example, tensile strength can be adjusted within a range of 1,200–600 psi; elongation, between 150–800%.

Improved polyurethane can be produced by copolymerization. Block copolymers of polyurethanes connected with segments of isobutylenes exhibit high-temperature properties, hydrolytic stability, and barrier characteristics. The hard segments of polyurethane block polymers consist of (–RNHCOO)–n, where R usually contains an aromatic moiety.

Properties and Uses of Polyurethanes
The major use of polyurethanes is to produce foam although non foam type polyurethane plastics are also made. The density as well as the mechanical strength of the rigid and the flexible type foams can vary widely depending on the polyol type and reaction conditions. The polyol is where the chemistry research money is spent as all the 'magic' of the subsequently produced foam is largely in the polyol. For example, polyurethanes could have densities ranging between 10 - 100 Kg/M3 for the flexible types and 50 - 1000 Kg/M3 for the rigid and more high density foams. Densities can be achieved such that large trucks can be driven over the foam without causing any damage. Polyurethane foams typically have good abrasion resistance, very low thermal conductance (hence their wide use as thermal insulators) and good load bearing characteristics. However, they have moderate resistance to organic solvents and are attacked by strong acids. Uncured foam can be ‘dissolved’ by using acetone which is quite effective at removing uncured foam but cannot remove foam that has already cured. Foam can also be cut and shaped by carpenter tools, saws and chisels as it behaves like very soft timber. Foams can be flammable unless specific flame retardants are added into the mix on the polyol side. Flame retardency of polyurethanes could be improved by using special additives such has boron compounds, spraying a coating material such as magnesium oxychloride on top of the foam, or by grafting a halogen phosphorous moiety to the polyol. Trichlorobutylene oxide is sometimes copolymerized with ethylene and propylene oxides to produce the polyol.

Major markets for polyurethanes are furniture, packaging, transportation, and building and construction. Other uses include carpet underlay, textural laminates and coatings, footwear, packaging, toys, and fibres. The largest use for rigid polyurethane is in construction and industrial insulation due to its high insulating property. The foam can be installed in sheets or sprayed in place where it will provide the best available thermal insulation per depth of material at an acceptable price versus performance. No other commercially available product has a better price performance point and polyurethane foams are increasingly being specified to meet the needs of more exacting building regulations for thermal insulation.

The diagram below compares the degree of insulation of some common insulating materials used in construction. These figures are calculated under the assumption of 'still' air but under real world conditions polyurethane sprayed in place foam is more than 5 times more effective per depth over products such as glass or mineral wool insulation. This is because no air currents can pass through the polyurethane due to the tight cell structure of the foam and it acts as an air barrier preventing air from either side from mixing.

Comparison of building materials and relative insulation values under 'still' air conditions:

Under real world conditions with thermal air currents, ventilation and wind air movements the difference between rigid, high density polyurethane foam  and mineral wool or glass fibre insulation is 5 to 6:1 depth factor in favaour of polyurethane foam since polyurethane is an air barrier as well as an insulator and needs less depth to achieve a specified U value. This fact is not bourne out by laboratory calulations for lamda and 'U' values which adopt still air conditions for testing and this unduly favours mineral wool and glass fibre insulation when considering price versus performance.

Moulded urethanes are used in the automotive industry for items such as bumpers, steering wheels, instrument panels, and body panels. Elastomers from polyurethanes are characterized by toughness and resistance to oils, oxidation, and abrasion. They are produced using short-chain polyols such as polytetramethylene glycol from 1, 4-butanediol. Polyurethanes are also used to produce fibres. For example, ‘Spandex’ (trade name) is a copolymer of polyurethane (85%) and polyesters.

Polyurethane foam typically has a low melting point and so cannot be used as a fire barrier. However, polyurethane networks based on triisocyante and diisocyanate connected by segments consisting of polyisobutylene are fairly rubbery and exhibit high temperature properties, hydrolyic stability, and barrier characteristics. This makes these types of polyurethanes suitable as water proof coatings.