Editing Polyurethane (section)

==Chemistry==
Polyurethanes are produced by reacting di[[isocyanate]]s with [[polyol]]s,<ref name="ge2">''n''&nbsp;≥&nbsp;2</ref><ref name="Gum 1992">{{cite book | first1=Wilson | last1=Gum |last2=Riese|first2= Wolfram |last3=Ulrich|first3= Henri  | title=Reaction Polymers | publisher=Oxford University Press | location=New York | year=1992 | isbn=978-0-19-520933-4}}</ref><ref>{{cite book | first=Ron | last=Harrington |author2=Hock, Kathy | title= Flexible Polyurethane Foams | publisher=The Dow Chemical Company | location=Midland | year=1991 }}</ref><ref name="Oertel 1985">{{cite book | first=Gunter | last=Oertel | title=Polyurethane Handbook | publisher=Macmillen Publishing Co., Inc. | location=New York | year=1985 | isbn=978-0-02-948920-8}}{{page needed|date=June 2017}}</ref><ref>{{cite book | first=Henri | last=Ulrich | title=Chemistry and Technology of Isocyanates | publisher=John Wiley & Sons, Inc. | location=New York | year=1996 | isbn=978-0-471-96371-4}}{{page needed|date=June 2017}}</ref><ref>{{cite book | first=George | last=Woods | title= The ICI Polyurethanes Book | publisher=John Wiley & Sons, Inc. | location=New York | year=1990 | isbn=978-0-471-92658-0}}{{page needed|date=June 2017}}</ref> often in the presence of a [[catalyst]], or upon exposure to ultraviolet radiation.<ref name="Soto 2014">{{cite journal |doi=10.1021/jo5005789 |pmid=24820955 |title=Photochemical Activation of Extremely Weak Nucleophiles: Highly Fluorinated Urethanes and Polyurethanes from Polyfluoro Alcohols |journal=The Journal of Organic Chemistry |volume=79 |issue=11 |pages=5019–27 |year=2014 |last1=Soto |first1=Marc |last2=Sebastián |first2=Rosa María |last3=Marquet |first3=Jordi }}</ref> 
Common catalysts include tertiary [[amine]]s, such as [[DABCO]], [[DMDEE]], or [[metallic soap]]s, such as [[dibutyltin dilaurate]]. The [[stoichiometry]] of the starting materials must be carefully controlled as excess isocyanate can [[trimer (chemistry)|trimerise]], leading to the formation of rigid [[polyisocyanurate]]s. The polymer usually has a highly [[crosslink]]ed molecular structure, resulting in a  [[thermosetting]] material which does not melt on heating; although some [[thermoplastic polyurethane]]s are also produced.

[[File:Polyurethane synthesis.tif|class=skin-invert-image|800px|center]]

{{image frame

|caption=Carbon dioxide gas and urea links formed by reacting '''water''' and '''[[isocyanate]]'''

|content=
<math chem>
\begin{array}{l}
\ce{{R-N=C=O} + H2O ->[\ce{step}\ 1] R1-\underset{ | \atop \displaystyle H}{N}-\overset{\displaystyle O \atop \| }{C}-O-H ->[\ce{step}\ 2][\ce{decomposes}] R-NH2\ +\ CO2(g)} \\

\ce{{R-N=C=O} + R-NH2 ->[\ce{step}\ 3] -R-\underset{ | \atop \displaystyle H}{N}-\overset{\displaystyle O \atop \| }{C}-\underset{ | \atop \displaystyle H}{N}-R}{-}

\end{array}
</math>
}}
The most common application of polyurethane is as solid [[foam]]s, which requires the presence of a gas, or [[blowing agent]], during the polymerization step. This is commonly achieved by adding small amounts of water, which reacts with isocyanates to form CO<sub>2</sub> gas and an [[amine]], via an unstable [[carbamic acid]] group. The amine produced can also react with isocyanates to form [[urea]] groups, and as such the polymer will contain both these and urethane linkers. The urea is not very soluble in the reaction mixture and tends to form separate "hard segment" phases consisting mostly of [[polyurea]]. The concentration and organization of these polyurea phases can have a significant impact on the properties of the foam.<ref>{{Cite thesis
  | last=Kaushiva|first= Byran D.
  | title =Structure-Property Relationships of Flexible Polyurethane Foams
  | type=Ph.D.
  | publisher =Virginia Polytechnic Institute
  | date =August 15, 1999
  | url = http://theses.lib.vt.edu/theses/available/etd-083199-185156/
  }}</ref>

The type of foam produced can be controlled by regulating the amount of blowing agent and also by the addition of various [[surfactant]]s which change the [[rheology]] of the polymerising mixture. Foams can be either "closed-cell", where most of the original bubbles or cells remain intact, or "open-cell", where the bubbles have broken but the edges of the bubbles are stiff enough to retain their shape, in extreme cases [[reticulated foam]]s can be formed. Open-cell foams feel soft and allow air to flow through, so they are comfortable when used in seat cushions or [[mattresses]]. Closed-cell foams are used as rigid [[thermal insulation]]. High-density [[microcellular]] foams can be formed without the addition of blowing agents by mechanically frothing the polyol prior to use. These are tough elastomeric materials used in covering car [[steering wheel]]s or [[shoe sole]]s.

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, [[elasticity (solid mechanics)|elastic]] polymer. High amounts of [[cross-link|crosslinking]] give tough or rigid polymers. Long chains and low crosslinking give a polymer that is very stretchy, short chains with many crosslinks produce a hard polymer while long chains and intermediate crosslinking give a polymer useful for making foam. The choices available for the isocyanates and polyols, in addition to other additives and processing conditions allow polyurethanes to have the very wide range of properties that make them such widely used polymers.