Editing Polyurethane (section)

===Isocyanates===
Isocyanates used to make polyurethane have two or more isocyanate groups on each molecule. The most commonly used isocyanates are the [[aromaticity|aromatic]] diisocyanates,  [[toluene diisocyanate]] (TDI) and [[methylene diphenyl diisocyanate]], (MDI). These aromatic isocyanates are more reactive than [[aliphatic]] isocyanates.

TDI and MDI are generally less expensive and more reactive than other isocyanates. Industrial grade TDI and MDI are mixtures of isomers and MDI often contains polymeric materials. They are used to make flexible foam (for example slabstock foam for mattresses or molded foams for car seats),<ref>{{cite web|url=http://dowglobal.beta.ides.com/DocSelect.aspx?DOC=DOWTDS&E=101414|title=Technical data sheet from Dow Chemical|access-date=2007-09-15|archive-date=2007-10-13|archive-url=https://web.archive.org/web/20071013154430/http://dowglobal.beta.ides.com/DocSelect.aspx?DOC=DOWTDS&E=101414|url-status=dead}}</ref> rigid foam (for example insulating foam in refrigerators) elastomers (shoe soles, for example), and so on. The isocyanates may be modified by partially reacting them with polyols or introducing some other materials to reduce volatility (and hence toxicity) of the isocyanates, decrease their freezing points to make handling easier or to improve the properties of the final polymers.

[[File:MDI isomers v2.svg|class=skin-invert-image|500px|center|MDI isomers and polymer]]

Aliphatic and cycloaliphatic isocyanates are used in smaller quantities, most often in coatings and other applications where color and transparency are important since polyurethanes made with aromatic isocyanates tend to darken on exposure to light.{{page needed|date=August 2023}}<ref>{{cite book | first1=David | last1=Randall |last2=Lee|first2= Steve | title=The Polyurethanes Book | publisher=Wiley | location=New York | year=2002 | isbn=978-0-470-85041-1}}</ref> The most important aliphatic and cycloaliphatic isocyanates are [[hexamethylene diisocyanate|1,6-hexamethylene diisocyanate]] (HDI), 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl-cyclohexane ([[isophorone diisocyanate]], IPDI), and [[hydrogenated MDI|4,4′-diisocyanato dicyclohexylmethane]] (H<sub>12</sub>MDI or hydrogenated MDI). Other more specialized isocyanates include [[Tetramethylxylylene diisocyanate]] (TMXDI).