Study on blocking and deblocking kinetics of diisocyanate with e-caprolactam using ftir spectroscopy

Article abstract of DOI:10.14233/ajchem.2013.oh67

These blocking and deblocking reactions of toluene diisocyanate (TDI), diphenylmethane-4,4'-diisocyanate (MDI) and dicyclohexylmethylmethane-4,4'- diisocyanate (HMDI) blocked with e-caprolactam were directly traced by FTIR spectroscopy at different temperatures respectively. Taking advantage of the relationship between the concentration of reactants and absorbance of NCO group according to Lambert-Beer law, the change with time for concentration was calculated under different temperatures. The results showed that the reaction rate constants for toluene diisocyanate, MDI blocked with e-caprolactam were 4.05 × 10^-3 and 6.53 × 10^-2 at 60°C respectively, while the reaction rate constant for HMDI with catalyst was 3.56 × 10^-3 at 140°C, the class of reaction kinetics for diisocyanate (TDI and MDI) blocked with e-caprolactam were first order and second order respectively and the reaction of blocked HMDI was second order reaction selecting stannous octoate as catalyst. The Arrhenius equation was employed to acquire the activation energies of blocked diisocyanate (TDI, MDI and HMDI) respectively. The process of deblocking for blocked diisocyanate was also studied by TGA and FTIR.

Full text of DOI:10.14233/ajchem.2013.oh67

Asian Journal of Chemistry; Vol. 25, No. 10 (2013), 5703-5706
http://dx.doi.org/10.14233/ajchem.2013.OH67
Study on Blocking and Deblocking Kinetics of Diisocyanate with
ε-Caprolactam Using FTIR Spectroscopy†
1,2
1,2
2
2
YAN-FEI TANG , JIN LIU1,2,*, ZHEN LI , BI-YUN CHEN and SHENG-FU MEI
1Anhui Key Laboratory of Advanced Building Materials, Anhui University of Architecture, Hefei 230022, P.R. China
2School of Materials Science and Chemical Engineering, Anhui University of Architecture, Hefei 230022, P.R. China
*Corresponding author: E-mail: liujin@aiai.edu.cn
AJC-13310
These blocking and deblocking reactions of toluene diisocyanate (TDI), diphenylmethane-4,4'-diisocyanate (MDI) and
dicyclohexylmethylmethane-4,4'-diisocyanate (HMDI) blocked with ε-caprolactam were directly traced by FTIR spectroscopy at different
temperatures respectively. Taking advantage of the relationship between the concentration of reactants and absorbance of NCO group
according to Lambert-Beer law, the change with time for concentration was calculated under different temperatures. The results showed
that the reaction rate constants for toluene diisocyanate, MDI blocked with ε-caprolactam were 4.05 × 10-3 and 6.53 × 10-2 at 60 ºC
respectively, while the reaction rate constant for HMDI with catalyst was 3.56 × 10-3 at 140 ºC, the class of reaction kinetics for diisocyanate
(TDI and MDI) blocked with ε-caprolactam were first order and second order respectively and the reaction of blocked HMDI was second
order reaction selecting stannous octoate as catalyst. The Arrhenius equation was employed to acquire the activation energies of blocked
diisocyanate (TDI, MDI and HMDI) respectively. The process of deblocking for blocked diisocyanate was also studied by TGA and
FTIR.
Key Words: Diisocyanate, ε-Caprolactam, Blocking and Deblocking, Reaction kinetics.
amindines, caprolactam and related compounds have been
INTRODUCTION
reported as blocking agents. Among these blocking agents,
ε-caprolactam was one of the most studied blocking agents
because of the stability of blocking products and ε-caprolactam-
blocked products were good crosslinking agent to improve
performance of engineering materials. Many researchers6-10
have been engaged in the research of this aspect, but the blocking
and deblocking kinetics of diisocyanate with ε-caprolactam
were seldom reported.
Polyurethanes are synthetic multi-block co-polymer
materials, which can be prepared through chemical reaction
or polymeric processing into a series of products, like elasto-
mers, adhesives, foams, sealants and coatings widely used in
every engineering field1,2. Polyurethanes has become one of
hotpots in polymer materials at present and magical polyure-
thanes with special function appears almost daily. Due to the
high reactivity of isocyanates, polyurethanes was usually
prepared by two package reaction. Two packages polyurethanes
have placed an important role in application, but their usage
would be limited under certain conditions,which need to be
mixed just prior to the application. Blocked isocyanates have
several advantages, like marked reduction of moisture sensi-
tivity and elimination of toxicity associated with the free
isocyanates. It is widely used in kinds of powder coating,
elastomer and adhesive3. With the characteristic of fine particle,
well-distributed, simple configuration and harmless to the
environment, the industrial importance of blocked isocyanates
can be clearly seen from number of patents4,5. Several comp-
ounds such as phenols, oximes, amides, imides, imidazoles,
In this paper, the reaction kinetics of toluene diisocynate,
diphenylmethane-4,4′-diisocyanate (MDI) and dicyclohexyl-
methylmethane-4,4′-diisocyanate (HMDI) blocked with ε-
caprolactam were studied by FTIR. In addition, deblocking
reaction of a series of ε-caprolactam-blocked products and
thermodynamics of the reactions were also researched.
EXPERIMENTAL
The ε-caprolactam was from Sinopharm Chemical
Reagent Co. Ltd., AR) and the toluene diisocynate,
diphenylmethane-4,4'-diisocyanate (MDI) and dicyclohexyl-
methylmethane-4,4'-diisocyanate (HMDI) were supplied by
ANLIArtificial Leather Co. Ltd. The solvents of toluene (AR)
†Presented to the 6th China-Korea International Conference on Multi-functional Materials and Application, 22-24 November 2012, Daejeon, Korea

5704 Tang et al.
Asian J. Chem.
from Xilong Chemical Co. Ltd. and the acetone from Shanghai
B & C Chemical Co. Ltd. were dried by 4 A molecular sieve
for 4 h before using.
toluene diisocynate and the structure of ε-caprolactam similar
to benzene ring lead that the steric hindrance was stronger in
the blocking reaction. The reaction rate depended on second
process of the blocking reaction. It was related to toluene
diisocynate. The constant of reaction rate κ1 was obtained from
the slope listed in Table-3.
Measurements: The process of blocking and deblocking
were observed by WQF-300 FTIR. The characteristic peak of
NCO from diisocyanate at 2274 cm-1 was sensitive to reaction,
which was used as tracking. The absorbance of C-H in reaction
didn't vary with time, which was used as inner standard peak.
The absorbance of NCO group was recorded during the reaction
with the control of the data collection software. With the
constant temperature heating device, the temperature controller
confined the temperature to change within 5-10 ºC during the
best process of blocking and deblocking. The fluctuation
should be within 0.5 ºC. The reaction kinetics was calculated
by Lambert-Beer law using the initial concentration corres-
ponded with initial absorbency of NCO group and the change
of absorbency responded to the change of sample concentration
indirectly.
0.5
2274 cm-1
0 s
2929 cm-1
0.4
60 s
0.3
120 s
0.2
180 s
240 s
0.1
300 s
TGA adopted STA409PC thermal analyzer to measure
weight loss and thermal performance. The heating rate was
kept at 10 K/min and high pure N2 as shelter gas. The range of
temperature was controlled from 50 ºC to 400 ºC.
0.0
3000
2800
2600
2400
2200
Wavenumbers (cm-1)
Fig. 2. FTIR curves of NCO absorption band at 60 ºC
RESULTS AND DISCUSSION
Reaction Kinetics of blocked toluene diisocynate with
ε-caprolactam: The reaction for toluene diisocynate blocked
with ε-caprolactam was shown in Fig. 1. The initial concen-
tration of NCO in this reaction system was C0 = 4.72 × 10-3
mol g-1. Fig. 2 illustrated that the change with time for the
absorbance of NCO peak at 2274 cm-1. The concentration of
NCO was figured out at every moment and defined the result
of log(C0/C) to be as the corresponding figure for time. The
plots of log(C0/C) against time for blocking reaction of toluene
diisocynate with ε-caprolactam at different temperatures were
shown in Fig. 3.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
60 ºC
70 ºC
80 ºC
slow
CH3
CH3
CH3
O
C
0
100
200
300
400
500
O
C
O
H
N
NH
NCO
NCO
N C
T (s)
Fig. 3. Plots of log(C0/C) against time at different temperatures
+
¡÷
∆
¡÷
∆
NCO
NH
NH
C
N
O
C
N
O
O
fast
O
C
C
Deblocking reaction for blocked toluene diisocynate
with ε-caprolactam: The product of the blocked toluene
diisocynate was analyzed by TGA, which estimated that the
deblocking reaction emerged at 160 ºC. This process was
confirmed by FTIR.
Fig. 1. Scheme of the reaction of TDI deblocked with ε-caprolactam
Toluene diisocynate was deblocked by ε-caprolactam at
60, 70, 80 ºC respectively. The concentration of ε-caprolactam
was more than that of toluene diisocynate at molar 2.2:1. The
absorbency of NCO group continuous became low with time
in Fig. 2, which indicated that the blocking reaction was
proceeding. Combining with the results of the linear relation-
ship of log(C0/C) against time under different temperatures
shown in Fig. 3, it made clear that the class of the reaction was
first order. At the beginning of the reaction, ε-caprolactam
reacted with one NCO group from toluene diisocynate, this
process was fast. As the reaction continuous, ε-caprolactam
reacted with the other NCO group to achieve deblocked
products, this process was slower. The plenty of groups of
The blocked product was dissolved by toluene and then
coated on monocrystalline silicon piece. Temperature was
controlled at 160 ºC and 165 ºC with heating device. The
change of NCO group was observed by FTIR.
The absorbance of NCO group appeared at 160 ºC during
deblocking reaction shown in Fig. 5, this result was consistent
with TGA. The absorbance of NCO group almost did not vary
with time after 165 ºC. The kinetics research for the deblocking
reaction of blocked toluene diisocynate with ε-caprolactam
had more sophisticated process and the temperature should
be controlled much more accurate in range of 5 ºC.

Reaction kinetics of blocked MDI with ε-caprolactam:
Reaction kinetics of blocked HMDI with ε-caprolactam:
Deblocking reaction for blocked MDI with ε-capro-
Vol. 25, No. 10 (2013)
Study on Blocking and Deblocking Kinetics of Diisocyanate with ε-Caprolactam 5705
110
100
90
2
50 ºC
10
55 ºC
60 ºC
TG
0
8
6
4
2
0
80
DSC
70
60
-2
-4
50
159.80°C
40
30
20
237.66°C
50
100
150
200
250
300
350
T/°C
20
40
60
80
100
120
140
160
180
200
Fig. 4. TG and DSC curves of the deblocking reaction of blocked TDI
T / s
Fig. 7. Plots of (1/C-1/C0) against time for blocked MDI
0.20
2929cm-1
2274cm-1
360 s
The absorbance of NCO group which changed with time
at different temperatures was shown in Table-1. The absorbance
of NCO group continuously became low with time, which
indicated that the blocking reaction kept proceeding. It illus-
trated that the blocking reaction for MDI was the second order
reaction demonstrated by linear relationship under different
temperatures shown in Fig.7. The steric hindrance of MDI
was weak and the reactivity of MDI was so high that MDI
mixed together with ε-caprolactam reacted quickly at 50 ºC,
the blocking reaction was in relationship with MDI and ε-
caprolactam. The constant of reaction rate κ2 from the straight
slope was listed in Table-3.
300 s
240 s
0.15
0.10
0.05
0.00
-0.05
180 s
120 s
60 s
2950 2900 2850 2800
255025002450240023502300225022002150
Wavenumber/cm-1
Fig. 5. FTIR curves of NCO absorption band with time in the deblocking
reaction at 160 ºC
lactam: The blocked MDI was relatively stable due to the
high reactivity of MDI. Thus the absorbance peak of NCO
group for deblocking reaction appeared slightly under 200 ºC.
The side reaction of the MDI under high temperature with
high reactivity was complex. It considered that the deblocking
reaction for blocked MDI was irreversible.
The reaction of MDI blocked with ε-caprolactam was shown
in Fig. 6. The initial concentration of NCO in this reaction
system was C0 = 4 × 10-3 mol·g-1. Table-1 illustrated that the
absorbance peak of NCO at 2274 cm-1 changed with time.
The concentration of NCO was figured out at every moment
and defined the result of (1/C-1/C0) to be as the corresponding
figure for time in Fig. 7.
The reaction of HMDI deblocked with ε-caprolactam was
shown in Fig. 8. The reactivity of HMDI was so low that the
stannous octoate was put into experiment as the catalyst,
because it was aliphatic isocyanates. The initial concentration
of NCO in this reaction system was C0 = 3.9 × 10-3 mol·g-1.
Fig. 9 illustrated that the absorbance peak of NCO at 2274
cm-1 changed with time. The concentration of NCO was fig-
ured out at every moment and defined the result of (1/C-1/C0)
to be as the corresponding figure for time (Fig. 9).
TABLE-1
ABSORBANCE OF BLOCKED MDI AT
DIFFERENT TEMPERATURES
Absorbance
T (ºC)
50
55
60
Wavenumbers∗
2934
2274
2934
2274
2934
2274
-1
(cm )
O
30
60
0.452
0.278
0.558 0.176 0.247 0.047 0.599 0.066
0.238
0.113
0.593
0.216
C
stannous octoate
NCO + 2
OCN
C
H2
HN
Time
90
120
¡÷
∆
(s)
0.098
0.073
0.009
0.002
0.015
0.006
O
O
150
O
C
O
C
C
H2
C
H
N
H
N C N
N
O
H2
C
C
OCN
NCO
+ 2
HN
¡÷
∆
Fig. 8. Scheme of the reaction of HMDI diblocked with CL
O
O
O
C
O
C
H2
C
The absorbance changed with time at different tempe-
ratures for blocked HMDI with ε-caprolactam was shown in
Table-2. The absorbance of NCO group continuous became
H
N C N
H
N C N
Fig. 6. Scheme of the reaction of MDI diblocked with CL

5706 Tang et al.
Asian J. Chem.
10
to block at 160 ºC or higher temperature with catalyst. For the
deblocking reaction, the temperature should be much higher.
There was no absorbance peak of NCO group under 230 ºC in
experiment. ε-Caprolactam would have ring-opening reaction
if the temperature was too high. Thus the reaction for blocked
HMDI with ε-caprolactam was irreversible.
180 ºC
160 ºC
140 ºC
8
6
4
2
0
Activation energy of blocked diisocyanate with ε-capro-
lactam: The experiment was observed by FTIR and analyzed
by TGA; the calculation for the blocking reaction was carried
on. The constant of reaction rates were listed in Table-3.
According to Arrhenius equation:
lnκ = lnA-Ea/RT
(1)
The reaction rate was converted into logarithm (lnκ) which
was employed to against temperature (1/T). Then the activation
energy Ea was acquired shown in Table-3.
0
100
200
300
400
500
T / s
Fig. 9. Plots of (1/C-1/C0) against time for blocked HMDI
Conclusion
Results showed that the class of blocking reaction for
blocked toluene diisocynate with ε-caprolactam was first order,
while for MDI and HMDI were both second order. The blocked
toluene diisocynate could deblock at 160 ºC, the blocking
reaction was reversible, while the reactions of MDI and HMDI
blocked with ε-caprolactam were irreversible. The activation
energy of blocked toluene diisocynate with ε-caprolactam was
143 kJ·mol-1 and that of blocked MDI and HMDI with ε-capro-
lactam were 0.0952 and 30.5 kJ·mol-1 respectively. The
reactivity of blocked isocyanates with ε-caprolactam were
influenced by both electronic effect and steric effect, which
directly reflected by the level of the activation energy. Com-
pared with MDI, the activation energy of blocked toluene
diisocynate with ε-caprolactam was higher and the steric
hindrance was stronger due to the plenty of groups of toluene
diisocynate. Compared with HMDI, the activation energy of
MDI was lower because of the electron-withdrawing tendency
for aromatics than aliphatics.
low, which indicated that the blocking reaction kept procee-
ding. The reactivity of HMDI was so low that the reaction con
dition had to be high temperature. The dimerization and
trimerization of isocyanates and isocyanates itself reacting with
the blocked isocyanates generated alliphanate or biuret under
high temperature, which were thermal decomposition through
different kinetics process. Approximately, it considered that
the differences for the reaction mainly depend on the differ-
ences for the reactivity of the NCO group in the reaction. The
second order reaction of initial catalytic blocking reaction of
HMDI with ε-caprolactam was testified. The rate constants κ3
achieved from the straight slope was listed in Table-3.
TABLE-2
ABSORBANCE OF BLOCKED HMDI AT
DIFFERENT TEMPERATURES
Absorbance
Temp. (ºC)
140
160
180
Wavenumbers
(cm-1)
2934
2274
2934
2274
2934
2274
ACKNOWLEDGEMENTS
60
0.263
0.198
0.292
0.183
0.399
0.245
0.169
0.128
0.114
This work was financially supported by the Natural
Science Foundation of China (No. 21171004) and the Natural
Science Foundation ofAnhui Province Education Department
(KJ2011A064, KJ2011A070).
120
Time
(s)
180 0.271 0.167 0.349 0.111 0.452
240
300
0.149
0.131
0.073
0.043
REFERENCES
TABLE-3
KINETICS DATA AND ACTIVATION ENERGY
OF BLOCKING REACTIONS
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161 (2002).
Activation
energy
(kJ mol-1)
3. D.A. Wicks and Z.W. Wicks Jr., Prog. Org. Coat., 43, 131 (2001).
4. F.N. Jones, V. Swarup, A.I. Yezrielev and R.A. Subrayan, US Patent
6103826 (2000).
Systems
Temperature and rate constant κ
T (ºC)
κ1/s-1
60
4.05×10-3 4.71×10-3 5.14×10-3
50 55 60
2.29×10-2 4.51×10-2 6.53×10-2
70
80
5. L.A. Flood, R.B. Gupta, T. Iyengar, D.A. Ley and V.K. Pai, US Patent
6063922 (2000).
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MDI/ CL
HMDI/CL
143
0.0952
30.5
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T (ºC)
κ2/g(mol s)-1
T (ºC)
κ3/g(mol s)-1
140
160
180
3.56×10-3 1.64×10-2 4.38×10-2
TDI = Toluene diisocynate; CL= ε-Caprolactam
Deblocking reaction for blocked HMDI with ε-capro-
lactam: The reactivity of HMDI was so low that it only reacted