Polymer International
Polym Int 56:1521–1529 (2007)
Synthesis, characterization and thermal
dissociation of 2-butoxyethanol-blocked
diisocyanates and their use in the synthesis of
isocyanate-terminated prepolymers
Iftikhar Ahmad,1∗ Javid H Zaidi,2 Rizwan Hussain1 and Arshad Munir1
1 National
Engineering and Scientific Commission, PO Box 2216, Islamabad, Pakistan
of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
2 Department
Abstract: A series of blocked diisocyanates has been synthesized from toluene diisocyante (TDI), isophorone
diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 4,4 -diphenylmethane diisocyanate (MDI) and 2butoxyethanol. The synthesis of blocked diisocyanate adducts was confirmed by Fourier transform infrared,
1
H NMR, electron impact mass spectrometry and nitrogen analysis. Differential scanning calorimetry (DSC),
thermal gravimetric analysis (TGA) and carbon dioxide evolution were used to determine the minimum deblocking temperatures. De-blocking temperatures determined by these three techniques were found to be in the
order DSC > TGA > CO2 evolution. The effect of different metal catalysts on thermal de-blocking reaction of
the blocked diisocyanates was studied, using the carbon dioxide evolution method. It was found that iron(III)
oxide has the maximum catalytic activity on de-blocking. The solubility of the blocked diisocyanate adducts was
determined in different solvents. The study revealed that at 30 ◦ C blocked IPDI and HDI adducts show better
solubility than adducts based on TDI and MDI. Isocyanate-terminated prepolymers of blocked diisocyanates
and hydroxyl-terminated polybutadiene (HTPB) were prepared. The storage stability and gelation times of the
prepolymers were studied. Results showed that all the diisocyanate-HTPB compositions are stable at 50 ◦ C for
more than three months. However, aliphatic diisocyanate-HTPB compositions require greater gelation time than
aromatic diisocyanate-HTPB compositions at their respective de-blocking temperatures.
 2007 Society of Chemical Industry
Keywords: blocked disocyanates; hydroxyl terminated; prepolymer; 2-butoxyethanol
INTRODUCTION
Polyurethanes are one of the most versatile class
of polymers ever invented.1 – 3 Their unusual versatility stems not only from their excellent properties but also because of their easily tailor-made
nature.4,5 Polyurethane preparation can be achieved
from a rich choice of starting materials or building blocks that can be combined by diisocyanate
polyaddition processes.6,7 Polyurethanes have a wide
range of applications such as coatings, adhesives,
foams, textiles, membranes, elastomers and rubber
adhesion promoters.8 – 10 A rapid reaction between a
polyfunctional isocyanate and a hydroxyl-terminated
oligomer leads to a urethane linkage. The high reactivity and high toxicity of isocyanates do not allow
their storage or use in one-component systems. A
solution to overcome these problems, particularly
used in the coating and paint industries, is the use
of blocked isocyanates.11 Blocked isocyanates have
a bright future in the field of powder coatings12
and heat setting adhesives.13 – 16 They are particularly suitable building blocks for light-stable twocomponent urethane coatings,17 and single-package
blocked adduct urethane coatings.18 Both aliphatic
and aromatic isocyanates can be blocked by a variety
of blocking agents.19 – 22 The most widely commercially used agents are phenols, oximes, alcohols,
ε-caprolactam and dibutyl malonate.12 The temperature range characterizes these blocking agents
where the de-blocking reaction is expected.22 Blocking and de-blocking of isocyanates are equilibrium reactions according to the following equation:
O
Tb
R
N
C
O + BH
C
B
Td
BH = Blocking agent
Tb = Blockimg temperature
Td = De-blocking temperature
At elevated temperatures the reaction tends to proceed
from right to left.23 The curing temperature of blocked
isocyanates is of vital importance in industrial applications. Applications12,19,20,24 of blocked isocyanates
have been widely reported in the literature. The
mechanism of reaction of blocked isocyanates with
∗
Correspondence to: Iftikhar Ahmad, National Engineering and Scientific Commission, PO Box 2216, Islamabad, Pakistan
E-mail: Iftikhar.Ahmad@sheffield.ac.uk
(Received 18 September 2006; revised version received 29 December 2006; accepted 20 February 2007)
Published online 13 June 2007; DOI: 10.1002/pi.2295
 2007 Society of Chemical Industry. Polym Int 0959–8103/2007/$30.00
R
H
N
I Ahmad et al.
a polyol is believed to proceed as the dissociation
of a blocked isocyanate, to give the blocking agent
and free isocyanate group which then reacts with
the alcoholic hydroxyl group (elimination–addition)
(Scheme 1). Alternatively, the reaction could proceed
by addition of the alcohol to the blocked isocyanate
to yield a tetrahedral intermediate, followed by elimination of the blocking agent (addition–elimination)
(Scheme 2).11,25
Many factors affect the rate, extent and mechanism
of the reaction. These include the structure of the
blocking agent, the external nucleophile, the structure
of the isocyanate employed, the presence or absence
of catalyst, the solvent polarity and the reaction
temperature. Among these factors, catalysts play an
important role in the de-blocking mechanism of
O
R
k1
B
C
N
R
k-1
O + BH
C
N
blocked isocyanates. Organometallic compounds and
tertiary amines lower the de-blocking temperature and
the time of de-blocking reaction as compared to noncatalyzed systems.26 – 28 Alcohols have been widely
used as blocking agents for isocyanates.12,29 The
use of fluorine-containing alcohol-blocked isocyanates
in dust-resistant polyurethane powder coatings30 has
been reported.
Keeping in view the widespread applications of
alcohol-blocked isocyanates, 2-butoxyethanol was
tried as a blocking agent for diisocyanates. 2Butoxyethanol-blocked toluene diisocyanate (TDI),
isophoron diisocyanate (IPDI), 4,4 -phenylmethane
diisocyanate (MDI) and hexamethylene diisocyanate
(HDI) adducts were prepared, characterized and
their thermal dissociation studied. It was found that
their de-blocking temperature was low, and that the
blocking agent had better homogeneity in the presence
of co-reactants.
H
O
R
N
C
O
+
R1 OH
k2
C
N
R
O R1
H
Scheme 1. Elimination–addition mechanism.
OH
O
R
N
C
B
+
R1 OH
H
k3
OH
R
N
H
C
R
k-3
N
C
H
OR1
B
O
B
OR1
K4
R
N
C
H
Scheme 2. Addition–elimination mechanism.
EXPERIMENTAL
Materials
The materials used in this study are presented in
Table 1. These were all laboratory-grade chemicals
and were used as received except for hydroxylterminated polybutadiene (HTPB), which was prepared in our laboratory and was dried in vacuum at
100 ◦ C for 10 h before use. All the solvents used were
purified and dried according to standard procedures.31
OR1 + BH
Measurements
Fourier transform infrared (FTIR) spectra of adducts
were recorded using an EQUINOX 55 IR analyzer.
The samples were prepared as pellets with potassium
bromide. Proton NMR spectra were recorded using
a Bruker Advance AV 500 spectrometer. In all the
NMR experiments, the solvent used was DMSO-d6
and TMS as internal standard. Mass spectra were
Table 1. Materials used
Designation
TDI
IPDI
MDI
HDI
2-BuEt
DBTL
TPB
TBF
Fe2 O3
DMF
DMSO
Glycerol
PTMG-1000
Caster oil
Ba(OH)2
HTPB
1522
Chemical description
Toluene diisocyante (mixture of 80/20 wt% 2,4- and
2,6-toluene diisocyanate)
Isophrone diisocyante
4,4 -Phenylmethane diisocyanate
Hexamethylene diisocyanate
2-Butoxyethanol
Dibutyltin dilaurate
Triphenyl bismuth
Tert-butyl ferocine
Iron(III) oxide
Dimethylformamide
Dimethylsulfoxide
1,2,3-Trihydroxypropane
Poly(oxytetramethylene glycol)
Hydroxyl value ≥ 150 mg mmol−1
Barium hydroxide
Hydroxyl-terminated polybutadiene; Mn = 3000 g mol−1 ,
hydroxyl value ≥ 720 mg mmol−1
Supplier
Aldrich, USA
Aldrich, USA
Aldrich, USA
Aldrich, USA
Merck, Germany
Aldrich, USA
Aldrich, USA
TCI, Japan
Panreac, Spain
Fluka
Merck, Germany
Panreac, Spain
Aldrich, USA
Panreac, Spain
Merck, Germany
Prepared in our laboratory
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
2-Butoxyethanol-blocked diisocyanates
recorded using a MAT312 mass spectrometer. DSC
thermograms were recorded using a Perkin Elmer
DSC-7 thermal analyzer. TGA was carried out using
a Perkin Elmer TGA-7, from 0 to 900 ◦ C at a rate of
20 ◦ C min−1 . Elemental analysis was carried out using
a Perkin Elmer series-11, 2400 elemental analyzer.
Melting points of blocked diisocyanate adducts were
recorded using a Gallenkamp model P-57/B-2 melting
point apparatus.
Synthesis of blocked diisocyanate adducts
2-Butoxyethanol-blocked
diisocyanate
adducts
(Scheme 3) were synthesized using a reported
procedure29 with a slight modification. 2-Butoxyethanol (0.1 mol L−1 ) in n-hexane (100 mL) and dibutyltin
dilaurate (three drops) were stirred at 0 ◦ C under
a constant flow of nitrogen in a three-neck roundbottom flask. To this stirred solution, the diisocyanates
(0.05 mol L−1 ; solution in 25 mL of n-hexane) were
added dropwise using a dropping funnel over a period
of 45 min. The reaction mixture was stirred at 0 ◦ C
for an additional 1 h. Then, the reaction content was
stirred at room temperature for 3–24 h depending
on the diisocyanate used. The progress of the reaction was monitored by taking out the samples at
regular time intervals and determining the percentage of isocyanate content left using dibutylamine
titration. After completion of the reaction, all the
blocked diisocyanate adducts were extracted with ethyl
acetate, except the 2-butoxyethanol–HDI adduct.
Ethyl acetate was removed using a rotary evaporator
and the resulting products were dried under vacuum. The 2-butoxyethanol–HDI adduct was filtered,
washed with ethyl acetate and dried under vacuum.
The purity of the blocked diisocyanate adducts was
established by nitrogen content analysis of the adducts.
Physical data of the blocked diisocyanate adducts are
presented in Table 2.
Synthesis of isocyanate-terminated prepolymers
For the synthesis of isocyanate-terminated prepolymers (Scheme 4), 2-butoxyethanol-blocked diisocyanate adducts (0.03 mol L−1 ) were dispersed in
HTPB (0.02 mol L−1 ) in a two-neck round-bottom
flask under a constant flow of nitrogen. To this dispersion, dibutyltin dilaurate (three drops) was added
and the mixture was stirred for 3 h at 50 ◦ C. After 3 h,
the reaction mixture was poured into Teflon moulds
O
O
R2O
C
R1
NH
NH
OR2 + OH
(
C
)
n OH
I (a - d)
I
HTPB
Blocked diisocyanate adducts
Heat
DBTL
O
OCN
R1 ( NH
C
O
O
C
NH )n R1
NCO
O
III (a - d )
R2
R1
CH3
OCN R1
NCO + R2 OH
I (a - d)
DBTL / Inert
Atmosphere
II
0 °C
O
O
OR2
R2O C NH R1 NH C
III (a - d)
R1
R2
CH3
CH3
CH3 (CH2)3 O (CH2)2
(a)
CH2 CH2 O CH2 CH2 CH2 CH3
a =
b =
CH3
CH3
CH3
(b)
CH3
CH3
c =
CH2
(c)
d =
(d)
CH2
(CH2)6
(CH2)6
Scheme 4. Synthesis of isocyanate-terminated prepolymers using
blocked diisocyanates.
Scheme 3. Synthesis of 2-butoxyethanol-blocked diisocyanates.
Table 2. Physical data of 2-butoxyethanol-blocked diisocyanate adducts
Blocked adduct
Solvent
2-Butoxyethanol–TDI
n-Hexane
2-Butoxyethanol–IPDI
n-Hexane
2-Butoxyethanol–MDI
n-Hexane
2-Butoxyethanol-HDI
n-Hexane
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
Reaction
parameters
Yield
(%)
Melting
point (◦ C)
C, calc.
(found) (%)
H, calc.
(found) (%)
N, calc.
(found) (%)
1 h at 0 ◦ C
8 h at 30 ◦ C
1 hur at 0 ◦ C
24 h at 30 ◦ C
1 h at 0 ◦ C
8 h at 30 ◦ C
1 h at 0 ◦ C
3 h at 30 ◦ C
93
Thick liquid
61.46 (60.45)
8.29 (9.04)
6.82 (5.99)
89
Thick liquid
62.88 (62.0)
5.24 (5.06)
6.11 (6.66)
91
Gummy mass
66.66 (67.6)
7.81 (7.91)
5.76 (5.86)
88
78
59.40 (58.5)
9.9 (10.16)
6.93 (6.23)
1523
I Ahmad et al.
and cured at 100 ◦ C as thin films. The gelation time
for each isocyanate-terminated prepolymer was determined. The prepolymers prepared were dried at 80 ◦ C
under vacuum before further analysis.
Storage stability
In order to determine the storage stability of
the 2-butoxyethanol-blocked diisocyanate adducts,
they (0.03 mol L−1 in 75 mL of ethyl acetate) were
dispersed in HTPB (0.02 mol L−1 ). Then 50 mL of
each dispersion was placed in air-tight glass bottles
and placed in an air-circulated oven at 50 ◦ C. The
viscosity of each dispersion was measured after regular
intervals of time.
RESULTS AND DISCUSSION
Characterization of blocked diisocyanate
adducts
FTIR spectroscopy
FTIR spectroscopy has been successfully employed
to characterize blocked diisocyanate adducts as
well as to measure their minimum de-blocking
temperature.25 FTIR spectra of all 2-butoxyethanolblocked diisocyanate adducts are identical and show
no absorption in the 2250–2270 cm−1 range, which
indicates that the NCO groups of the diisocyanate
molecules are completely blocked by the blocking
agent. Formation of a urethane linkage is easily
identified by four main characteristic bands:32 – 34 the
strong bands at 3311–3332 cm−1 correspond to the
stretching vibrations of NH; the very strong bands
at 1685–1715 cm−1 correspond to the stretching
vibrations of carbonyl (C=O) groups; the bands at
1533–1539 cm−1 correspond to carbamate; and the
strong band at 1223–1267 cm−1 correspond to the
starching vibrations of C=O combined with NH in
all the spectra. The absorption frequencies of the
individual adducts are shown in Table 3. Formation
of a urethane linkage and the disappearance of bands at
2250–2270 cm−1 are strong evidence for the blocking
of isocyanate groups of the diisocyanates. A typical
spectrum of a blocked diisocyanate adduct is shown in
Fig. 1.
NMR spectroscopy
1
H NMR spectra of blocked diisocyanate adducts
showed that for 2-butoxyethanol-blocked TDI, IPDI
and MDI, all the methyl protons of the 2butoxyethanol, TDI and IPDI moieties appear as
multiplets at δ = 0.79–0.98 ppm, whereas methyl
protons of the 2-butoxyethanol–HDI adduct appear
at δ = 0.84–0.87 ppm as a triplet. Aromatic protons
Ha, Hb, Hc of TDI appear at δ = 7.04–7.49 ppm
each as a doublet, whereas aromatic protons of
the MDI moiety appear at δ = 6.92–7.36 ppm as a
multiplet. NH protons of the 2-butoxyethanol–TDI
and 2-butoxyethanol–MDI adducts appear at δ =
8.87–9.61 ppm as singlets whereas NH protons of the
2-butoxyethanol–IPDI and 2-butoxyethanol–HDI
adducts appear at δ = 7.01–7.14 ppm as triplets and
at δ = 7.07–7.18 ppm as a doublet. Table 4 shows
the complete 1 H NMR spectral assignments of the
2-butoxyethanol-blocked diisocyanate adducts. The
Table 3. Characteristic FTIR frequencies of 2-butoxyethanol-blocked diisocyanate adducts
Adduct
2-Butoxyethanol–TDI
2-Butoxyethanol–IPDI
2-Butoxyethanol–MDI
2-Butoxyethanol–HDI
Urethane (S) str
(<N–H) (cm−1 )
Carbonyl (VS) str
(<C=O) (cm−1 )
Carbamate (S)
(–NH–COO–) (cm−1 )
Amide (S) str
(–NH–CO–) (cm−1 )
3311
3330
3317
3327
1715
1705
1699
1685
1534
1535
1533
1539
1228
1244
1223
1267
S = strong; VS = very strong; str = stretch.
Figure 1. FTIR spectrum of 2-butoxyethanol-blocked IPDI adduct.
1524
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
2-Butoxyethanol-blocked diisocyanates
Table 4. 1 H NMR data of 2-butoxyethanol-blocked diisocyanate adducts
Structure
Assignment and chemical shifts (δ, ppm)
1.28–1.35 (m, 4H, 1), 1.49–1.51 (m, 4H, 2), 3.39 (t, 4H, 3), 3.59 (t,
4H, 4), 4.16 (t, 4H, 5), 7.49 (d, 1H, a), 7.17 (d, 1H, b), 7.04 (d, 1H,
c), 8.87 (s, 1H, e), 9.61 (s, 1H, f), 0.87 (m, 9H, methyl)
O
CH3
e
c
R
C
NH
H
Ha
H
b
R
C
NH
f
O
4
3
2
1
5
R = O CH2 CH2 O CH2 CH2 CH2 CH3
O
R
C
5
b
HN
a
CH3
NH
1
1
CH3
1.27–1.48 (m, 15H, 1), 3.09 (t, 4H, 2), 3.37–3.56 (t, 4H, 3), 4.03 (t,
4H, 4), 2.73–2.9 (d, 2H, 5), 7.01–7.05 (t, 1H, b), 7.07–7.18 (d,
1H, a), 0.79–0.98 (m, 15H, methyl)
O
C
R
1
CH3
1
4
3
2
R = O CH2 CH2 O CH2 CH2 CH2 CH3
O
R
C
3
CH2
HN
2
O
NH
C
R
1.28–1.51 (m, 12H, 1), 3.40–3.57 (m, 8H, 2), 2.5 (S, 2H, 3),
6.92–7.36 (m, 8H, Ar), 9.5 (s, 2H, NH), 0.84–0.88 (m, 6H, methyl)
1
R = O CH2 CH2 O CH2 CH2 CH2 CH3
1
O
R C
HN
5
5
CH2 CH2 CH2 CH2 CH2 CH2
O
NH
C
R
1.2–1.47 (m, 16H, 1), 2.90–2.94 (q, 4H, 5), 3.34–3.37 (t, 4H, 2),
3.47–3.49 (t, 4H, 3), 4.0–4.02 (t, 4H, 4), 7.12–7.14 (t, 2H, NH),
0.84–0.87 (t, 6H, methyl)
1
4
3
2
R = O CH2 CH2 O CH2 CH2 CH2 CH3
Figure 2. 1 H NMR spectrum of 2-butoxyethanol-blocked TDI adduct.
assignments fully confirm the chemical structure of
the blocked diisocyanate adducts. A typical spectrum
of a blocked disocyanate adduct is shown in Fig. 2.
Mass spectrometry
The common fragments found in the electron impact
mass spectra of 2-butoxyethanol-blocked adducts are
listed in Table 5. In all cases the intensity of the
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
molecular ion (M+ ) peak is between 2.0 and 2.23%,
which is very low. This low intensity of the molecular
ion peak can be attributed to the low stability of the
blocked diisocyante adducts due to the scission of
the –NH–CO–O– linkage.35 Dissociation of blocked
adducts generates two molecules of blocking agent
and one molecule of diisocyanate. The peak for
regenerated TDI and MDI appeared with intensity
of 33.39 and 15.51%, respectively, but in the case of
IPDI there was no peak at m/z 222. The absence of
a peak for IPDI is due to the complex fragmentation
pattern usually observed for cycloalkanes.36 Similarly,
in blocked HDI, the peak intensity at m/z 168 is
only 5%, which is again due to the same reason.
The base peak in all cases appeared at m/z 57, which
corresponds to the scission of – CH2 –O–CH2 – bond
of 2-butoxyethanol, the blocking agent regenerated
(Fig. 3).
De-blocking temperature
Many analytical techniques have been applied to
study de-blocking temperatures. It should be noted
that reported de-blocking temperatures frequently
depend on the method of analysis, heating rate and
other variables. Different analytical techniques can
1525
I Ahmad et al.
Table 5. Mass spectroscopic data of 2-butoxyethanol-blocked diisocyanate adducts
2-Butoxyethanol–TDI
2-Butoxyethanol–IPDI
2-Butoxyethanol–MDI
2-Butoxyethanol–HDI
Fragment
m/z
I (%)
m/z
I (%)
m/z
I (%)
m/z
I (%)
Mol. ion
M+ − C6 H14 O2
M+ − C6 H13 O2
M+ − 2C6 H14 O2
M3 − NCO
M4 − NCO
M5 + H
M6 − CH3
M7 + H
410
292
293
174
132
90
91
75
76
2
13.11
2.26
33.39
5.13
–
3.59
9.3
2.04
458
340
223
222
180
138
139
124
109
2.27
–
–
–
–
2.55
42.87
11.15
5.16
486
368
369
250
208
166
167
152
153
1.5
10.88
2.23
15.51
2.88
–
–
–
–
404
286
287
168
126
84
85
70
71
2.23
–
5
5
6
4
25
2.06
2.85
Peak
+
M
M1
M2
M3
M4
M5
M6
M7
M8
12
100
H
60
BI-05 = 2-Butoxyethanol - HDI
8
H
6
NH C R
O
R = O CH2 CH2 O CH2 CH2 CH2 CH3
%
BI-04 = 2-Butoxyethanol - MDI
10
O
NH C R
Heat flow
H
80
CH3
40
4
BI-05
2
0
-2
-4
20
BI-04
-6
-8
0
50
100
150
200
250
300
350
400
450
M/z
Figure 3. Mass spectrum of 2-butoxyethanol-blocked TDI adduct.
give different de-blocking temperatures for the same
sample. The most common methods for determining
de-blocking temperatures follow some change in
physical properties. Examples are gel time, FTIR,
FTIR in combination with dynamic mechanical
analysis, TGA, DSC, solid-state NMR and carbon
dioxide evolution.25 In the present investigation, we
studied the de-blocking temperature of the blocked
diisocyanate adducts using DSC, TGA and carbon
dioxide evolution.
Urethane scission takes place on applying heat;37
therefore, there should be an endothermic transition in the DSC curve of blocked diisocyanate
adducts (Fig. 4). The DSC thermogram of 2butoxyethanol–HDI shows two endothermic transitions at 76 and 264 ◦ C. The first sharp transition at 76 ◦ C corresponds to the melting point of
the adduct; the second sharp transition at 264 ◦ C
corresponds to de-blocking of the blocked adduct.
All other DSC thermograms of 2-butoxyethanolblocked diisocyanate adducts show only one endothermic transition, which indicates that melting point
and initial de-blocking temperatures are the same
(Table 6). The de-blocking temperature of the
blocked diisocyanate adducts determined by DSC
are in the following order: 2-butoxyethanol–HDI>2butoxyethanol–IPDI>2-butoxyethanol–TDI>2-but1526
-10
0
50
100
150
200
250
300
350
Temperature / οC
Figure 4. DSC thermograms of 2-butoxyethanol-blocked
diisocyanate adducts.
oxyethanol–MDI. This reflects the fact that aromatic diisocyanates de-block at lower temperature
than aliphatic diisocyanates. Since the carbonyl carbon of the urethane group has a partial positive charge,
the bond between the carbon and oxygen of the blocking agent will be more labile due to the reduction
of the negative charge density of the carbonyl carbon through resonance in the aromatic ring. It was
reported38 that the de-blocking temperature decreased
from 200 to 180 ◦ C on introducing an aryl component
in the blocking agent whereas introduction of an aryl
isocyanate reduces the dissociation temperature from
180 to 120 ◦ C. TGA is very useful for determining
the de-blocking temperature of blocked isocyanates.
However, TGA cannot be used for compounds that do
not exhibit volatility over the de-blocking temperature
range. The TGA curves presented in Fig. 5 show that
initial de-blocking of all the 2-butoxyethanol-blocked
diisocyanate adducts occurred in the temperature
range 180–200 ◦ C, very close to the boiling point
of 2-butoxyethanol, the blocking agent.
To determine the minimum de-blocking temperatures of the blocked diisocyante adducts using the carbon dioxide evolution method, a reported procedure39
was followed with a slight modification. An amount of
0.2–0.3 g of adduct was placed in 10 mL of DMSO
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
2-Butoxyethanol-blocked diisocyanates
Table 6. De-blocking temperatures of 2-butoxyethanol-blocked
diisocyanate adducts
Table 7. Effect of different metal catalysts on de-blocking
temperature of 2-butoxyethanol–IPDI adduct
De-blocking temperature (◦ C)
Adduct
Carbon dioxide
evolution method
DSC method
110
125
98
130
232
242
216
264
2-Butoxyethanol–TDI
2-Butoxyethanol–IPDI
2-Butoxyethanol–MDI
2-butoxyethanol–HDI
110
100
BI-02 = 2-Butoxyethanol –TDI
BI-03 = 2-Butoxyethanol –IPDI
BI-04 = 2-Butoxyethanol –MDI
BI-05 = 2-Butoxyethanol -HDI
90
Weight % (%)
80
70
60
50
40
BI-04
30
BI-05
20
10
BI-03
0
25
100
200
300
400
BI-02
500
600
700
800
900
Temperature (°C)
Figure 5. TGA curves of 2-butoxyethanol-blocked diisocyanate
adducts.
along with 2.5 g of molecular sieves (4 Å) saturated
with moisture (ca 24% by weight) at 25 ◦ C in a 25 mL
two-neck round-bottom flask, with a magnetic stirrer.
One neck of the flask was connected to a supply of
dry, carbon dioxide-free nitrogen and the other neck
was connected to a purging tube, which was immersed
in a saturated solution of barium hydroxide. The system was continuously purged with a slow stream of
carbon dioxide-free, dry nitrogen gas. The flask was
heated in a silicone oil bath, at a rate of 3 ◦ C min−1 .
As de-blocking takes place, regenerated NCO reacts
with the available moisture from the molecular sieves,
librating carbon dioxide. This carbon dioxide then
reacts with the saturated solution of barium hydroxide,
causing turbidity due to the formation of insoluble barium bicarbonate. The minimum temperature at which
detectable turbidity appears was taken as the minimum de-blocking temperature. The results are shown
in Table 6. It was observed that de-blocking temperatures determined by the three different techniques
were in the order DSC > TGA > CO2 evolution.
Effect of catalysts on de-blocking temperature
Catalysts are usually included in blocked isocyanate
formulations but commonly without consideration
of what reaction or reactions in which they might
be involved. Despite the development of several
other metal-based catalysts, inorganic and organic
tin compounds are still the most common catalysts
for polyurethane reactions, although most organotin
catalysts are very sensitive towards hydrolytic stability.
In the presence of moisture, they hydrolyze, reducing
Polym Int 56:1521–1529 (2007)
DOI: 10.1002/pi
Catalysts
No catalyst
Dibutyltin dilaurate (DBTL)
Triphenyl bismuth (TPB)
Ferric oxide (Fe2 O3 )
Tert-butyl ferocine (TBF)
De-blocking temperature (◦ C)
125
97
110
90
100
the catalytic activity.40 Another disadvantage is that
most tin catalysts can promote the hydrolysis of
the ester groups of polyester polyols. In addition
some organotin compounds have shown high aquatic
toxicity.41 Organometallic compounds having iron
as metal were found to be potential substitutes for
usual catalysts in the two-step preparation of polyester
urethanes.42 Besides being more environmentally
friendly than organotin compounds, the use of these
compounds allows, in some cases, the preparation of
urethanes with better mechanical properties.
Inorganic metal oxides have been used in heterogeneous catalysis as supports, replacing polymer
supports. Iron(III) oxide has been successfully used to
catalyze methyl methacrylate polymerization;43 however, the use of iron(III) oxide for polyurethane
formation reactions has not yet been reported. It is
commonly accepted that urethane formation results
from the activation of one or both reactants (isocyanate and hydrogen-containing compound) by a
species that contains the metal center. Of two possible reaction paths, one is a Lewis acid mechanism
in which the isocyanate is firstly activated by coordination to the metal via oxygen or nitrogen atom44,45
followed by the nucleophilic attack of the hydroxyl
group from the alcohol;46 the other is the prior activation of the alcohol by the metal catalyst, followed by
the complexation to the isocyanate.47
To study the effect of catalysts on the de-blocking
temperature of the 2-butoxyethanol–IPDI adduct,
four different metal catalysts were employed in DMSO
as solvent, while maintaining all other conditions
the same. The catalysts used and the de-blocking
temperatures of the 2-butoxyethanol–IPDI adduct are
shown in Table 7. The amount of catalyst (0.01 g)
used was maintained constant in each experiment.
Metal oxide (iron(III) oxide) was found to be the
most suitable of all the catalysts tested for de-blocking
the 2-butoxyethanol–IPDI adduct. As the mechanism
of the reaction is still not known, it may follow the
paths described earlier. However, attempts will be
made to study the mechanism of de-blocking, initiated
by iron(III) oxide, to gain an insight into its better
catalytic activity.
Gelation time of isocyanate-terminated
prepolymers
Gelation times for diisocyanate–HTPB prepolymers
were determined. Gelation time depends upon the
reactivity of diisocyanates. As aromatic isocyanates
1527
I Ahmad et al.
Table 8. Solubility (% w/v g (100 mL)−1 ) of 2-butoxyethanol-blocked diisocyanate adducts
Adduct
2-Butoxyethanol–TDI
2-Butoxyethanol–IPDI
2-Butoxyethanol–MDI
2-Butoxyethanol–HDI
DMF
DMSO
1,2-Dimethoxyethane
Polyol
Glycerol
Caster oil
43
30
35
25
57
49
53
47
61
39
45
35
32
19
21.1
15.1
27
17
23.2
2
22
15
17
6.6
are more reactive than aliphatic isocyanates, gelation
time is a clear reflection of this. The reactivity of
isocyanate arises mainly from the resonance hybrids
of the –NCO groups. In aromatic isocyanates the
– NCO group is directly attached to the aromatic
ring, enhancing the electrophilic character of carbon atoms by conjugation with aromatic residue. The
– NCO group is particularly prone to undergo attack
by nucleophilic agents. Conjugation with aromatic
nucli makes aromatic isocyanates particularly reactive,
especially if electron-withdrawing ring substituents are
present, and even more so when they are present
in the ortho or para position to the – NCO group.
Conversely electron-releasing substituents depress the
reactivity.48 – 50 It was observed that the gelation times
of the isocyanate-terminated prepolymers was in the
order IPDI–HTPB prepolymer > HDI–HTPB prepolymer > TDI–HTPB prepolymer > MDI–HTPB
prepolymer.
Storage stability
The viscosity of each dispersion at 50 ◦ C was
determined after one week regularly, for three
months. Investigations showed that no measurable
change occurred in the viscosity of all dispersions.
This indicates that the 2-butoxyethanol-blocked
diisocyanate adducts are stable at 50 ◦ C for more than
three months.
de-blocking temperatures of the blocked diisocyanate
adducts were determined using DSC, TGA and
carbon dioxide evolution. De-blocking temperatures
determined by these three techniques were in the
following order: de-blocking temperature determined
by DSC > de-blocking temperature determined by
TGA > de-blocking temperature determined by
carbon dioxide evolution. The effect of catalysts on
de-blocking temperature was studied. Iron(III) oxide
was found to be the most effective catalyst for deblocking. The solubility of the blocked diisocyante
adducts was determined in different solvents. IPDIand HDI-based adduct showed better solubility than
TDI- and MDI-based adducts.
The minimum de-blocking temperature was confirmed by preparing isocyanate-terminated prepolymers of the blocked diisocyante adducts and HTPB.
The gelation time for these prepolymers was determined to be in the order IPDI–HTPB prepolymer >
HDI–HTPB prepolymer > TDI–HTPB prepolymer
> MDI–HTPB prepolymer.
The storage stability of the blocked diisocyante
adducts was determined. It was observed that the
blocked diisocyante adducts are stable for more than
three months at 60 ◦ C. It was concluded that 2butoxyethanol can be successfully used as a blocking
agent for diisocyanates.
REFERENCES
Solubility of blocked diisocyanate adducts
For uniform curing of blocked diisocyanate adducts
with hydroxyl co-reactants, the solubility of the
blocked diisocyanates is a limiting factor. Attempts
were made to determine the maximum percentage
(w/v) solubility of blocked diisocyanates in different
solvents/polyols at 30 ◦ C. No qualitative percent (w/v
g (100 mL)−1 ) solubility data have been reported for 2butoxyethanol-blocked diisocyante adducts so far. The
results are summarized in Table 8. It is observed that
blocked IPDI and HDI adducts show better solubility
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CONCLUSIONS
2-Butoxyethanol-blocked TDI, IPDI, MDI and HDI
adducts were prepared and fully characterized. Initial
1528
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