Polyurethane

• Polyurethanes are in the class of compounds called reaction polymers, which include epoxies, unsaturated polyesters, and phenolics. 
• Produced by reacting an isocyanate containing two or more isocyanate groups per molecule (R-(N=C=O)_{n ≥ 2}) with a polyol containing on average two or more hydroxy groups per molecule (R'-(OH)_{n ≥ 2}), in the presence of a catalyst.
• The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it.
•  Long, flexible segments, contributed by the polyol, give soft, elasticpolymer. High amounts of crosslinking give tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy, short chains with lots of crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. 
• The crosslinking present in polyurethanes means that the polymer consists of a three-dimensional network and molecular weight is very high. 
• Isocyanates are very reactive materials and making polymers requires special care in handling and use. 
• The aromatic isocyanates, diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI) are more reactive than aliphatic isocyanates, such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI). 
• Most of the isocyanates are difunctional, that is they have exactly two isocyanate groups per molecule. An important exception to this is polymeric diphenylmethane diisocyanate, which is a mixture of molecules with two-, three-, and four- or more isocyanate groups. In cases like this the material has an average functionality greater than two, commonly 2.7.
• Polyols are polymers in their own right and have on average two or more hydroxyl groups per molecule. 
• Polyether polyols are mostly made by co-polymerizing ethylene oxide andpropylene oxide with a suitable polyol precursor.
• Polyester polyols are made similarly to polyester polymers. The polyols used to make polyurethanes are not "pure" compounds since they are often mixtures of similar molecules with different molecular weights and mixtures of molecules that contain different numbers of hydroxyl groups, which is why the "average functionality" is often mentioned. 
• Polyols used to make rigid polyurethanes have molecular weights in the hundreds, while those used to make flexible polyurethanes have molecular weights up to ten thousand or more.

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