Chemistry Reference
In-Depth Information
For a mixture of polyols A and B,
ð
OH
Þ
mix
5 ð
OH
Þ
A
w
A
1 ð
OH
Þ
B
w
B
(3-17-iv)
where
w
is the weight fraction.
f
A
f
B
ð
M
n
Þ
mix
5
56
;
100
(3-17-v)
ð
OH
Þ
A
f
B
w
A
1 ð
OH
Þ
B
f
A
ð
1
2
w
A
Þ
the percent by weight of isocyanate (NCO) groups
present. The amine equivalent,
AE
, is the weight of the sample which reacts
with 1 gram equivalent weight of dibutyl amine (reaction 1-13).
Percent isocyanate
5
AE
5
M
n
=ð
no
:
of reactive groups
Þ
(3-17-vi)
%NCO
5
ð
4200
formula wt
:
of NCO
Þð
100 g compound
Þ
AE
5
(3-17-vii)
g NCO
4200
%NCO
5
for isocyanates:
E
5
AE
(3-17-viii)
For toluene diisocyanate (TDI, often 80 parts 2,4 isomer and 20 parts
2,6 isomer): %NCO
5
(42)2(100)/174
5
48, since 42
5
the formula weight
of the isocyanate NCO group.
Isocyanate index (index number)
5
100(actual amount of isocyanate
used)/(equivalent amount of isocyanate required). An excess of isocyanate
groups is used in some applications like flexible foam. The analytical
values required for isocyanate formulas are the isocyanate value, hydroxyl
number, residual acid value (acid number), and water content. The last
two parameters reflect the following reactions:
H
~NCO
+
−
COOH
→
~N
C
O
(3-17-ix)
O
(a) TDI is used to make a foam expanded with the carbon dioxide pro-
duced by reaction with 3 parts of water per 100 parts of a polyester
having hydroxyl and acid functionalities (a polyester polyol) with an
OH number
5
2.1 mg KOH/g. Calculate the amount of TDI required to provide iso-
cyanate indexes of 100 and 105.
5
62 mg KOH/g and an acid number (acid value)
HH
2
−
NCO
+
H
2
O
→ −
N
−
C
−
N
− +
CO
2
(3-17-x)
O
(b) Design an isocyanate-ended prepolymer (low-molecular-weight poly-
mer intended for subsequent reaction, as in Eq. 1-13, for example),
