Editing Polyurethane (section)

===Chain extenders and cross linkers===
[[Chain extender]]s (''f''&nbsp;=&nbsp;2) and [[cross-link|cross linkers]] (''f''&nbsp;≥&nbsp;3) are low molecular weight hydroxyl and amine terminated compounds that play an important role in the polymer morphology of polyurethane fibers, elastomers, adhesives, and certain integral skin and microcellular foams. 

The elastomeric properties of these materials are derived from the phase separation of the hard and soft copolymer segments of the polymer, such that the urethane hard segment domains serve as cross-links between the amorphous polyether (or polyester) soft segment domains. This phase separation occurs because the mainly nonpolar, low melting soft segments are incompatible with the polar, high melting hard segments. The soft segments, which are formed from high molecular weight polyols, are mobile and are normally present in coiled formation, while the hard segments, which are formed from the isocyanate and chain extenders, are stiff and immobile. As the hard segments are covalently coupled to the soft segments, they inhibit plastic flow of the polymer chains, thus creating elastomeric resiliency. Upon mechanical deformation, a portion of the soft segments are stressed by uncoiling, and the hard segments become aligned in the stress direction. This reorientation of the hard segments and consequent powerful hydrogen bonding contributes to high tensile strength, elongation, and tear resistance values.<ref name="Oertel 1985"/><ref>{{cite journal| first1=J. | last1=Blackwell |first2=M. R.|last2= Nagarajan |first3=T. B.|last3= Hoitink | title=The Structure of the Hard Segments in MDI/diol/PTMA Polyurethane Elastomers |journal=ACS Symposium Series| volume=172 | pages=179–196 | publisher=American Chemical Society | location=Washington, D.C. | year=1981 | issn=0097-6156| doi=10.1021/bk-1981-0172.ch014 | isbn=978-0-8412-0664-9 }}</ref><ref>{{cite journal | first1=John | last1=Blackwell |first2=Kenncorwin H.|last2= Gardner | title=Structure of the hard segments in polyurethane elastomers | journal=Polymer | year=1979 | issn=0032-3861 | doi = 10.1016/0032-3861(79)90035-1 | volume=20 | issue=1 | pages=13–17}}</ref><ref>{{cite conference | last1=Grillo | first1=D. J. |last2=Housel|first2= T. L. | title=Physical Properties of Polyurethanes from Polyesters and Other Polyols | book-title=Polyurethanes '92 Conference Proceedings  | publisher=The Society of the Plastics Industry, Inc. | year=1992 | location=New Orleans, LA }}</ref><ref>{{cite conference | last1=Musselman | first1=S. G. |last2=Santosusso|first2= T. M. |last3=Sperling|first3= L. H. | title=Structure Versus Performance Properties of Cast Elastomers | book-title=Polyurethanes '98 Conference Proceedings | publisher=The Society of the Plastics Industry, Inc. | year=1998 | location=Dallas, TX }}</ref>
The choice of chain extender also determines flexural, heat, and chemical resistance properties. 

The most important chain extenders are [[ethylene glycol]], [[1,4-butanediol]] (1,4-BDO or BDO), [[1,6-hexanediol]], [[cyclohexane dimethanol]] and hydroquinone bis(2-hydroxyethyl) ether (HQEE). All of these glycols form polyurethanes that phase separate well and form well defined hard segment domains, and are melt processable. They are all suitable for [[thermoplastic polyurethanes]] with the exception of ethylene glycol, since its derived bis-phenyl urethane undergoes unfavorable degradation at high hard segment levels.<ref name="Gum 1992"/> Diethanolamine and triethanolamine are used in flex molded foams to build firmness and add catalytic activity. Diethyltoluenediamine is used extensively in RIM, and in polyurethane and polyurea elastomer formulations.

{| class="wikitable plainrowheaders" style="text-align: center;"
|+Table of chain extenders and cross linkers<ref>
{{Cite book
  | title = A Guide to Glycols
  | id = Brochure 117-00991-92Hyc
  | location = Midland, Mich.
  | publisher = The Dow Chemical Co., Chemicals and Metals Department
  | year = 1992}}
</ref>
|-
!scope="col"|Compound type
!scope="col"|Molecule
! scope="col"|[[Molecular weight|Mol. <br />mass]]
! scope="col"| Density<br />(g/cm{{sup|3}})
! scope="col"| [[Melting point|Melting <br />pt]] (°C)
! scope="col"|[[Boiling point|Boiling <br />pt]] (°C)
|-
|rowspan="18"| Hydroxyl compounds – difunctional molecules
!scope="row"|[[Ethylene glycol]]
| 62.1 || 1.110 || −13.4 || 197.4
|-
!scope="row" |[[Diethylene glycol]]
| 106.1 || 1.111 || −8.7 || 245.5
|-
!scope="row" |[[Triethylene glycol]]
| 150.2 || 1.120 || −7.2 || 287.8
|-
!scope="row" |[[Tetraethylene glycol]]
| 194.2 || 1.123 || −9.4 || 325.6
|-
!scope="row" |[[Propylene glycol]]
| 76.1 || 1.032 || [[Supercooling|Supercools]] || 187.4
|-
!scope="row" |[[Dipropylene glycol]]
| 134.2 || 1.022 || Supercools || 232.2
|-
!scope="row" |[[Tripropylene glycol]]
| 192.3 || 1.110 || Supercools || 265.1
|-
!scope="row" |[[1,3-Propanediol]]
| 76.1 || 1.060 || −28 || 210
|-
!scope="row" |[[1,3-Butanediol]]
| 92.1 || 1.005 || — || 207.5
|-
!scope="row" |[[1,4-Butanediol]]
| 92.1 || 1.017 || 20.1 || 235
|-
!scope="row" |[[Neopentyl glycol]]
| 104.2 || — || 130 || 206
|-
!scope="row" |[[1,6-Hexanediol]]
| 118.2 || 1.017 || 43 || 250
|-
!scope="row" |[[1,4-Cyclohexanedimethanol]]
| — || — || — || —
|-
!scope="row" |[[HQEE]]
| — || — || — || —
|-
!scope="row" |[[Ethanolamine]]
|61.1 || 1.018 || 10.3 || 170
|-
!scope="row" |[[Diethanolamine]]
| 105.1 || 1.097 || 28 || 271
|-
!scope="row" |[[Methyldiethanolamine]]
| 119.1 || 1.043 || −21 || 242
|-
!scope="row" |[[Phenyldiethanolamine]]
| 181.2 || — || 58 || 228
|-
|rowspan="4"| Hydroxyl compounds – trifunctional molecules
!scope="row" |[[Glycerol]]
| 92.1 || 1.261 || 18.0 || 290
|-
!scope="row" |[[Trimethylolpropane]]
| — || — || — || —
|-
!scope="row" |[[1,2,6-Hexanetriol]]
| — || — || — || —
|-
!scope="row" |[[Triethanolamine]]
|149.2 || 1.124 || 21 || —
|-
|rowspan="2"| Hydroxyl compounds – tetrafunctional molecules
!scope="row" |[[Pentaerythritol]]
| 136.2 || — || 260.5 || —
|-
!scope="row" |''N'',''N'',''N''′,''N''′-Tetrakis<br />(2-hydroxypropyl)<br />ethylenediamine
| — || — || — || —
|-
|rowspan="2"| Amine compounds – difunctional molecules
!scope="row" |[[Diethyltoluenediamine]]
| 178.3 || 1.022 || — || 308
|-
!scope="row" |[[Dimethylthiotoluenediamine]]
| 214.0 || 1.208 || — || —
|}