Synthesis and properties of imidazole-blockeddiisocyanatesA Sultan Nasar, S Subramani and Ganga Radhakrishnan*Polymer Division, Central Leather Research Institute, Adyar, Chennai 600 020, India

Abstract: Imidazole-, 2-methylimidazole- and benzimidazole-blocked hexamethylene diisocyanate

and isophorone diisocyanate have been prepared and characterized by elemental analyses, IR and

NMR spectroscopy. The structure±property relationship of these adducts has been established by

reacting with hydroxyl-terminated polybutadiene (HTPB). The cure rate of the adduct increases from

the imidazole to 2-methylimidazole and to the benzimidazole-blocked adduct. Also, the cure rate of the

adducts based on hexamethylene diisocyanate is higher than those based on isophorone diisocyanate.

Simultaneous TGA/DTA results also con®rm this trend. The gas chromatogram of the imidazole-

blocked isocyanate con®rms that the thermolysis products are blocking agent and isocyanate. The

solubilities of the adducts have been measured in polyether and hydrocarbon polyols: the 2-

methylimidazole and benzimidazole-blocked hexamethylene diisocyanate adducts show higher

solubility than the rest.

# 1999 Society of Chemical Industry

Keywords: imidazole; isocyanate; blocked isocyanates; hydroxyl-terminated polybutadiene; curing behaviour;structure±property relationship

INTRODUCTIONAny compound which can be described as a derivative

of an isocyanate could formally be considered as a

`blocked isocyanate' which apparently regenerates the

reactive isocyanate functionality by thermal split-

ting.1,2 The principle of this de®nition is used in

heat-curable systems, such as powder coatings and

heat-setting adhesives. A typical heat-curable system

consists of a blocked isocyanate prepolymer and

hydroxy or amine co-reactant. Upon heating, the weak

bond of the blocked isocyanate breaks and regenerates

isocyanate groups. The regenerated isocyanate group

can react with the substrate (co-reactant) in the

desired manner, forming thermally more stable bonds.

The overall reaction can be seen as:

Ar

H

jN C

kO

BDÿÿ! ÿÿ Ar NCO� BH

Ar NCO�HO R ÿÿ! Ar

H

jN C

kO

OR

where BH is a blocking agent.

The curing temperature of blocked isocyanate is a

crucial limiting factor in industrial applications. Gen-

erally 160°C with 30min duration or lower is

preferable. This temperature is speci®c for a particular

blocking agent. The minimum curing temperatures of

blocked polyisocyanates are as shown in Table 1.3,4

A few reviews3±5 have been published in which a

large number of patents describe the applications of

blocked polyisocyanates. Blocked polyisocyanates are

preferred for many technical and economical reasons.

They are essentially insensitive to moisture. The

storage stability of blocked isocyanate based systems

is generally high.3

Aromatic reactants are attractive to be used as

blocking agents for an isocyanate, because the

urethane linkages formed from aromatic reactants

are unstable at elevated temperatures. Phenols are

extensively studied for a variety of isocyanates.6±11 A

number of patents disclose that heterocyclic com-

pounds such as triazoles, imidazolines and imidazoles

have been used as blocking agents for isocyanates.12±15

Frisch and Damusis12 patented moulded elastomers

based on benzotriazole-blocked prepolymers cross-

linked with bisaromatic amine. Wegner and co-

workers16 disclosed the use of 1,2,4-triazole as

blocking agent for an isophorone diisocyanate pre-

polymer in a powder coating which is said to cure in

30min at 140°C. 2-Phenylimidazoline-blocked iso-

phorone diisocyanate is reported to unblock at a

temperature 20±30°C lower than the caprolactam-

Polymer International Polym Int 48:614±620 (1999)

* Correspondence to: Ganga Radhakrishnan, Polymer Division, Central Leather Research Institute, Adyar, Chennai 600 020, India(Received 10 December 1998; accepted 2 March 1999)

# 1999 Society of Chemical Industry. Polym Int 0959±8103/99/$17.50 614

blocked diisocyanate.13 2-Ethylimidazole-blocked iso-

cyanate has been patented for water soluble applica-

tions.17 A recent report18 discloses the use of imidazole

blocked 2,5-bis[(n-alkyloxy)methyl]-1,4-benzene dii-

socyanates to prepare rigid rod-like polyimides in

which the reaction of imidazole-blocked isocyanate

with pyromellitic dianhydride leads to polyimides.

Such a reaction of imidazole-blocked isocyanate

differs from the reaction of polyurethane formation,

and this may open a new area of application. In a

recent report,19 we have described the synthesis and

properties of imidazole-blocked toluene diisocyanates.

In the present investigation we describe the prepara-

tion and properties of some imidazole-blocked hexa-

methylene diisocyanate and isophorone diisocyanate

adducts that may be used as crosslinkers in many

thermally curable systems.

EXPERIMENTALMaterialsHexamethylene diisocyanate (HMDI) (Fluka, Swit-

zerland), isophorone diisocyanate (IPDI) (Fluka),

imidazole (Qualigens, India), 2-methylimidazole

(Fluka), and benzimidazole (Merck, Germany) were

used without further puri®cation. Propylene-oxide-

based polyols and hydroxyl-terminated polybutadiene

(HTPB) containing 0.1% moisture were obtained

from Manali Petrochemicals Ltd (Chennai, India) and

the Vikram Sarabhai Space Center (Trivandrum,

India), respectively. Solvents were puri®ed by stan-

dard procedures.20

Preparation of blocked diisocyanatesIn a typical synthesis, 50ml of 1.6M solution of the

blocking agent was taken in a three-necked ¯ask ®tted

with a condenser and a magnetic stirrer. Dry nitrogen

was passed through the other neck for 1min. Then

50ml of 0.8M diisocyanate solution was added drop

by drop over a period of 2h at re¯ux temperature, and

the reaction was continued for an additional hour. The

product was effectively precipitated in petroleum ether

(60±80°C) and dried in air. Details of the reaction

conditions and results are given in Table 2.

Characterization methods for the blockeddiisocyanatesIR spectra of the adducts were recorded by the KBr

pellet method in a Nicolet impact-400 FTIR spectro-

photometer (USA). 1H NMR spectra were recorded in

a Bruker 300MHz spectrometer (Germany). Elemen-

tal analyses were carried out with a Heraeus CHN

RAPID analyser. The melting points of the adducts

were determined in a Toshniwal melting point

apparatus (Mumbai, India).

Thermal analysisTGA and DTA were carried out simultaneously in a

Seiko TGA/DTA 200 thermal analyser using alumina

as a reference. The sample mass was 3±5mg. The work

was performed from 30 to 600°C at a heating rate of

10°Cminÿ1 in a nitrogen atmosphere with a gas ¯ow

rate of 100mlminÿ1.

GC analysisThe gas chromatograph traces for N,N '-dimethyl-

propyleneurea, HMDI, N,N '-dimethylpropyleneurea

solution of imidazole and imidazole±HMDI adduct

were recorded on a Hewlett Packard-5890 instrument

(USA) using an Apiezon column. The ¯ow rate of

Table 1. Minimum curing temperaturesof blocked polyisocyanates

Combination

Temperature

(°C)a

Benzotriazole-blocked isocyanates�hydroxy functional resins 170

Phenol-blocked polyisocyanates�hydroxy functional resins 160

Caprolactam-blocked polyisocyanates�hydroxy functional resins 160

Caprolactam-blocked polyisocyanates�cycloaliphatic diamines 140

Butanoneoxime-blocked isocyanates�hydroxy functional resins 140

Malonate- and acetoacetate-blocked isocyanates�hydroxy functional resins 120

a 30min cure.

Table 2. Preparation of imidazole-blocked isocyanates

Elemental analysis (%)

Calculated Found

Melting

Blocking agent Isocyanate Solvent Temperature Yield (%) C H N C H N point (°C)

Imidazole HMDI CHCl3 Re¯ux 90 55.2 6.6 27.6 54.7 6.2 26.7 104±108

2-Methylimidazole HMDI CHCl3�DMF (9:1) Re¯ux 85 57.8 7.2 25.2 57.8 8.0 25.0 ±

Benzimidazole HMDI CHCl3�DMF (9:1) Re¯ux 90 65.3 5.9 20.7 64.4 5.6 20.1 138±140

Imidazole IPDI CHCl3 Re¯ux 80 60.3 7.3 23.4 59.4 7.4 22.9 ±

2-Methylimidazole IPDI CHCl3�DMF (9:1) Re¯ux 80 62.1 7.8 21.7 61.5 8.6 20.9 ±

Benzimidazole IPDI CHCl3�DMF (9:1) Re¯ux 80 68.0 6.5 18.3 67.3 7.3 18.5 ±

Polym Int 48:614±620 (1999) 615

Imidazole-blocked diisocyanates

nitrogen was 20mlhÿ1. The heating programme was

set at 200°C for 0.1min and 275°C for 5min; the

heating rate was 20°C minÿ1.

Gel time studiesHTPB (2�10ÿ3M) was taken separately in three

beakers, each of 30mm diameter. To this, 2�10ÿ3M

of blocked diisocyanates were added and mixed

thoroughly. Then the beakers were placed in an oil

bath maintained at 160°C. The beakers were inverted

at regular time intervals to observe the ¯ow behaviour

of the solutions. The time at which the solution ceased

to ¯ow was taken as the gel time. A duplicate

experiment was conducted for each adduct to ensure

the accuracy of the data collected.

Solubility testThe solubilities of the adducts in various polyols were

determined according to the reported procedure.9

RESULTS AND DISCUSSIONThe reaction mixtures of experiments conducted with

2-methylimidazole and benzimidazole were found to

be heterogeneous even at the re¯ux temperature of

chloroform. N,N '-Dimethylformamide was added to

homogenize the reaction mixture. The blocking agents

used in this study are highly reactive with isocyanate

and are basic in nature, so no catalyst was added to

avoid any undesirable side reactions, such as dimer-

ization or trimerization of isocyanate.

Typical FTIR spectra of the imidazole blocked

HMDI and IPDI adducts are given in Fig 1. All the

spectra are identical and do not show absorption in the

2250±2270cmÿ1 range. This indicates that the NCO

groups of the original HMDI and IPDI molecules are

completely blocked with imidazoles. Strong absorp-

tions at 1690±1725cmÿ1 (C=O stretching), 3200±

3400cmÿ1 (NÐH stretching), 1530±1560cmÿ1

(NÐH bending) and 1210±1240cmÿ1 (the stretching

vibration of the C=O group of urea combined with

the NÐH group)21 con®rm the formation of imida-

zole-blocked HMDI and IPDI adducts.

The 1H NMR spectra of the imidazole-blocked

isocyanate adducts commonly show peaks at four

different chemical shift values. The solvent and the

chemical shift values for the individual compounds are

given in Table 3 and are consistent with the assigned

protons and structure of the compounds.

The elemental analyses data for the blocked

isocyanates are included in Table 2. The results agree

well with the calculated values, indicating that the

compounds are pure.

To study the structure±property relationship of the

synthesized imidazole-blocked HMDI and IPDI ad-

ducts, they were reacted with HTPB, which is a novel

binder used in solid rocket propellants. The reason for

choosing HTPB is that it makes a short pot-life

possible for a system involving an aromatic isocyanate

because of the high reactivity of the primary hydroxyl

groups of HTPB towards the isocyanate groups.

Aliphatic isocyanates give a higher pot-life with HTPB

than aromatic isocyanates because the electrophilicity

of the aliphatic ÐNCO groups is less and hence they

react slowly with hydroxyl groups. IPDI has been

adopted as curing agent for HTPB in a number of

propellent binder systems.22 The use of blocked

isocyanates instead of isocyanates as such, along with

HTPB, is another way to increase the pot-life. Hence

blocked aliphatic isocyanates will give cumulative

effect. At elevated temperatures the functionality of

the blocked isocyanates will be regenerated and a cure

reaction will take place as follows:

During the course of the reaction, the viscosity

increased gradually and at once the free ¯ow was

arrested. The times required for gelation of the

blocked isocyanate±HTPB mixture to occur are given

Figure 1. FTIR spectrum of (a) imidazole-HMDI adduct and (b) imidazole-IPDI adduct.

616 Polym Int 48:614±620 (1999)

A Sultan Nasar, S Subramani, G Radhakrishnan

in Table 4. The gel time of both the isocyanate based

adducts decreases from imidazole to 2-methylimida-

zole and to benzimidazole. Considering isocyanates,

adducts based on HMDI show a lower gel time than

those based on IPDI, but the adducts based on both

HMDI and IPDI show a higher gel time than those

based on toluene diisocyanate which have been

recently reported.19 According to previous reports by

the authors9,19 and others,23 the bond formed between

the carbonyl carbon of isocyanate and oxygen or

nitrogen of blocking agent is labile and sensitive to the

electronic and steric effects that are operational in the

structure of the compounds. In the present study, the

low gel time of benzimidazole-blocked isocyanates is

due to the presence of aromatic substituent on the

blocking agent which drains the electron density from

the nitrogen of imidazole moiety and leaves a partial

positive charge on it. The carbonyl carbon already

carrying a partial positive charge makes the bond more

labile. The low gel time of 2-methylimidazole-blocked

isocyanate is the result of the steric effect of the methyl

substituent. The accelerating effect of the substituent

in the 2-position to the functional group of different

types of blocking agents has been reported by the

authors9,19 and others.23,24

The high gel time of the IPDI-based adducts is due

Table 3. 1HNMR chemical shifts of imidazole-blocked HMDI and IPDI adducts

Blocked isocyanate Chemical shifts (ppm)

DMSO-d6 1.2±1.6 (12H, 1); 7.6 (2H, 2); 7.0 (4H, 3); 8.2±8.5 (2H, 4)

DMSO-d6 1.2±1.6 (12H, 1); 2.2±2.3 (6H, 2); 6.8 (4H, 3); 8.2±8.5 (2H, 4)

DMSO-d6 1.2±1.6 (12H, 1); 7.6±7.7 (2H, 2); 7.2±7.4 (8H, 3); 8.4±8.5 (2H, 4)

DMSO-d6 0.7±1.3 (18H, 1); 7.6 (2H, 2); 7.0 (4H, 3); 8.4±8.5 (2H, 4)

DMSO-d6 1.0±1.5 (18H, 1); 2.6 (6H, 2); 7.0 (4H, 3); 8.8±9.1 (2H, 4)

DMSO-d6 0.6±1.3 (18H, 1); 7.6±7.8 (2H, 2); 7.1±7.4 (8H, 3); 8.8±8.9 (2H, 4)

Table 4. Gel time and decomposition temperature of imidazole-blockedisocyanates

Decomposition

temperature (°C)

Blocked isocyanate Gel time (h) Initial Final

Imidazole±HMDI >24 184 227

2-Methyl imidazole±HMDI 24 143 366

Benzimidazole±HMDI 5.5±6 201 255

Imidazole±IPDI >24 187 237

2-Methyl imidazole±IPDI >24 159 405

Benzimidazole±IPDI 24 216 280

Polym Int 48:614±620 (1999) 617

Imidazole-blocked diisocyanates

to the secondary ÐNCO group and geometry of the

IPDI molecules. There are two aspects which should

be considered for the curing reaction of a blocked

isocyanate with a hydroxy compound: the reactivity of

the blocked isocyanate group on deblocking reaction

and the reactivity of the regenerated isocyanate group

towards the hydroxyl group. As a fact which resulted

from the electronic effect, blocked isocyanates based

on aromatic isocyanates deblock at lower temperature

than those based on aliphatic isocyanates, and thus the

reactivity of the regenerated aromatic ÐNCO group is

higher than that of the aliphatic ÐNCO group towards

the hydroxyl group. In the present investigation, when

compared to HMDI, the IPDI molecule comprised cis

and trans isomers in the ratio of 3:1 (Fig 2). Wright

and co-workers25 studied the reactivity of different

ÐNCO groups of IPDI molecules and concluded that

the ÐNCO group present at the equatorial position of

the cis isomer showed higher reactivity than the

ÐNCO group present at the axial position of the

trans isomer. Coutinho and cavalheiro26 have con-

®rmed this in addition to the differential reactivity of

primary and secondary ÐNCO groups of the IPDI

molecule. Also, these authors have studied the

reactivity of IPDI and HMDI towards the hydroxyl

group and found that IPDI is less reactive than HMDI

because of the presence of one secondary ÐNCO

group per IPDI molecule. Thus the observed low (high

gel time) reactivity of IPDI based adducts is not a

surprise because the secondary ÐNCO group and its

geometry play a role opposite to HMDI.

The TGA and DTA traces obtained simultaneously

for HMDI- and IPDI-based adducts are given in Figs

3 and 4, respectively. The adducts based on imidazole

and benzimidazole follow single-stage dissociation

whereas the adducts based on 2-methylimidazole

follow multiple-stage dissociation. The initial and ®nal

dissociation temperatures determined by the extra-

polation of the TG curves are given in Table 4. Like

gel±time studies, thermal analyses also re¯ect the

electronic and steric effects that are operational in the

adducts, except for those based on benzimidazole. The

high initial dissociation temperatures found with

benzimidazole-blocked HMDI and IPDI may be due

to low volatility of the blocking agent after deblocking,

because its melting temperature is higher than the rest

of the imidazoles. Compared to DTG curves, DTA

traces of imidazole-, 2-methylimidazole- and benzimi-

dazole-blocked HMDI adducts show an additional

peak at 115°C, 133°C and 141°C, respectively

because of the melting of the compounds. Among

these three endotherms, the one that corresponds to

the benzimidazole-blocked HMDI adduct is very

sharp and pronounced. In accordance with the melting

endotherm of the rest of the HMDI-based adducts, the

Figure 2. Configurational structures ofIPDI.

Figure 3. TG/DTA curve of (a) imidazole-HMDI adduct(b) 2-methylimidazole-HMDI adduct and (c) benzimidazole-HMDI adduct.

618 Polym Int 48:614±620 (1999)

A Sultan Nasar, S Subramani, G Radhakrishnan

melting point is dif®cult to detect using a melting point

apparatus. The endotherm corresponding to the

deblocking reaction is obviously broad and starts after

completion of the melting endotherm. This pattern

con®rms that all the HMDI-based adducts dissociate

above the melting temperature. Unlike this trend, the

adducts based on IPDI do not show melting en-

dotherms and start to dissociate without melting. This

was again con®rmed with melting point apparatus

where no melting temperatures were observed for the

compounds.

No report has been found in the literature dealing

with product analysis of imidazole-blocked isocyanates

under thermolysis conditions; therefore, in this study

the imidazole-blocked HMDI adduct was subjected to

gas chromatographic analysis applying the conditions

at which the adduct undergoes dissociation. A

representative chromatogram along with references is

given in Fig 5. The peak at retention time 1.98min

corresponds to blocking agent released, because it

coincides with the retention time of reference imida-

zole. The HMDI possibly appeared with lower

intensity than the blocking agent, because the con-

centration of regenerated isocyanate is only half that of

imidazole released and the corresponding peak is

merged with the solvent.

The solubility of the blocked isocyanates is a

Figure 5. Gas chromatogram of (a) imidazole (b) HMDI and (c) imidazole-HMDI adduct.

Figure 4. TG/DTA curve of (a) imidazole-IPDI adduct(b) 2-methylimidazole-IPDI adduct and (c) benzimidazole-IPDI adduct.

Table 5. Dissolution temperatures of imidazole-blocked isocyanates in polyols

Dissolution temperature (°C)

Blocked isocyanate PPGa-400 PPG-1000 PPG-2000 Empeyol F-3000b HTPB

Imidazole±HMDI Not completed at 160 Not completed at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

2-Methyl imidazole±

HMDI

160 Not completed at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

Benzimidazole±HMDI 150 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

Imidazole±IPDI Not completed at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

2-Methyl imidazole±

IPDI

Not completed at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

Benzimidazole±IPDI Part. soluble at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160 Part. soluble at 160

a PPG, poly(propylene glycol).b Empeyol F-3000, a glycerol based triol.

Polym Int 48:614±620 (1999) 619

Imidazole-blocked diisocyanates

limiting factor for uniform curing with the hydroxy co-

reactants. The solubility tests for blocked HMDI and

IPDI adducts were carried out separately in different

polyols and the results are summarized in Table 5.

Even though all the adducts were derived from

aliphatic isocyanates, the adducts prepared with

HMDI show better solubility than those prepared

with IPDI. This may be because of the rigidity of the

cyclic structure of the IPDI molecule. 2-Methylimi-

dazole-blocked HMDI dissolves more readily than

imidazole-blocked HMDI due to the presence of the

aliphatic substituent. Improvement in the solubility of

blocked isocyanate using a methyl-substituted block-

ing agent has been reported.9 Higher solubility of the

benzimidazole-blocked HMDI adduct over other

imidazole-blocked HMDI adducts may be the result

of dissociation rather than dissolution of the adduct in

the polyols. It is also found that the solubility of the

adducts decreases with increasing molecular weight of

the polyols.

CONCLUSIONSImidazole-, 2-methylimidazole- and benzimidazole-

blocked HMDI and IPDI adducts were prepared and

characterized. The structure±property relationship of

the adducts was established by determination of curing

time with HTPB and the dissociation temperatures

using the TG/DTA technique. It was found that the

thermal stability decreased from imidazole- to

2-methylimidazole- and to benzimidazole-blocked

isocyanate. Also, it was found that the adducts based

on HMDI showed lower thermal stability than those

based on IPDI. Among six adducts prepared, only

2-methylimidazole- and benzimidazole-blocked

HMDI adducts showed good solubility in the polyols.

ACKNOWLEDGEMENTOne of the authors (ASN) thanks the Council of

Scienti®c and Industrial Research, India for ®nancial

assistance.

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