Chemistry Reference
In-Depth Information
with aniline which forms the isocyanate and then reacts with ethanol to form
the carbamate or urethane linkage. A competing reaction is the trimerization
of isocyanates. The reaction between an isocyanate and an alcohol is a rapid
reaction, but it is typically catalyzed, often with a tertiary amine. The catalyst
is often chosen to give the desired amount of trimerization.
O
O
C
H
NH
2
N
N
O
O
−
2HCl
EtOH
+
Cl
Cl
Aniline
Phosgene
Isocyanate
Carbamate
O
O
C
N
N
N
O
N
O
Isocyanate
Trimer
Diisocyanates are commonly used with either diols or a mixture of
diols and polyols. When diols are exclusively used and the trimerization
reaction is minimized, the polyurethane is substantially linear, but it becomes
increasingly three-dimensional with increasing levels of polyol or with
trimerization of the diisocyanate. The trimerization reaction and the use of
polyols enable thermoset polyurethanes. Common diisocyanates include
toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), naph-
thalene diisocyanate (NDI), and hexamethylene diisocyanate (HDI). TDI
is prepared by the nitration of toluene to give a mixture of mainly 2,4- and
2,6-dinitrotoluene, followed by reduction and reaction with phosgene. The
isomers are typically not separated and are used as a mixture.
CH
3
CH
3
CH
3
CH
3
CH
3
NH
2
H
2
N
+
NH
2
NO
2
O
2
N
NO
2
HNO
3
H
2
SO
4
+
NH
2
NO
2
CH
3
CH
3
CH
3
CH
3
NH
2
H
2
N
+
NH
2
NCO
OCN
NCO
COCl
2
+
NH
2
NCO
TDI
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