Method for the preparation of polyurethane elastomers based on polyethyleneglycol-bis(4-aminobenzoates)

A method for the preparation of polyurethane elastomers in which polyethyleneglycol-bis(4-aminobenzoate) is used as the curing agent to be admixed with the polyurethane formulations. This curing agent imparts considerably extended pot life to the ready-prepared polyurethane composition to be cured by heating into an elastomer having improved properties.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel method for the preparation of 
polyurethane elastomers by use of an aromatic diamine compound as the 
curing agent. In particular, the invention relates to a method for the 
preparation of polyurethane elastomers by use of a 
polyethyleneglycol-bis(4-aminobenzoate) as the curing agent. 
It is well known that polyurethane elastomers are usually prepared by the 
reaction of a polyisocyanate and a polyol admixed with a curing agent or 
by the reaction of a curing agent with a liquid polyurethane prepolymer 
having isocyanate groups at the chain ends prepared in advance by the 
reaction of a polyol and an excessive amount of a polyisocyanate. 
As a consequence of expanded application fields of polyurethane polymers in 
recent years, there is a growing demand for large quantities of 
polyurethane-based materials to be used for making large-sized shaped 
articles of polyurethane elastomers, for water-proofing in civil 
engineering and building construction, as pavement materials with 
elasticity, as sealing materials and the like. When a polyurethane 
composition is to be used in these applications, it is an essential 
requirement that the ready-prepared composition obtained by the admixture 
of the curing agent have a sufficiently long pot life in order to 
facilitate smooth casting or coating with the composition to give products 
or coating of uniform quality over a given work time. Some of the examples 
of the conventional curing agents are aromatic diamine compounds such as 
4,4'-methylene-bis(2-chloroaniline), 3,3'-dichlorobenzidine and the like, 
of which the first mentioned compound is widely employed since excellent 
polyurethane elastomers with a high hardness and elasticity as well as 
good abrasion resistance are obtained with this curing agent. 
The problem with 4,4'-methylene-bis(2-chloroaniline) is that the 
polyurethane composition when admixed with this curing agent has a rather 
short pot life leading to insufficient workability for fabrication of 
large articles or mass use in civil engineering or building construction. 
In order to overcome this defect of 4,4'-methylene-bis(2-chloroaniline), 
several kinds of improved curing agents are proposed in Japanese Patent 
Disclosure SHO No. 50-132096 including alkylenediol-bis(4-aminobenzoates) 
and the like. 
The problem with the alkylenediol-bis(4-aminobenzoates), on the other hand, 
is that the melting points of these compounds are so high that it is 
necessary to melt the curing agent beforehand in order to mix it with the 
prepolymer which should also be preheated at an elevated temperature in 
order to avoid precipitation of the curing agent during mixing. This 
necessity for heating of the curing agent and/or the prepolymer is 
disadvantageous not only due to the cost of heating and lower workability 
but also due to the danger of frequent degradation of the prepolymer by 
the prolonged heating at an elevated temperature resulting in inferior 
qualities of the final products. 
Thus there have hitherto been known no curing agents for polyurethane 
elastomers satisfying all of the above described requirements with respect 
to the melting point, pot life of the ready-prepared compositions and the 
properties of the final polyurethane products. 
SUMMARY OF THE INVENTION 
The object of the present invention is therefore to present a novel curing 
agent for polyurethane elastomers free from the problems in the prior art 
and also to provide a method for the preparation of polyurethane 
elastomers by the use of such curing agent. In particular, the present 
invention has been completed as a result of the extensive investigations 
undertaken by the inventors to develop an improved curing agent for 
polyurethane elastomers with a relatively low melting point and capable of 
giving sufficiently long pot life to the polyurethane compositions which 
can be cured into polyurethane elastomers with excellent properties. 
Specifically, the present invention relates to a method for the preparation 
of a polyurethane elastomer in which a 
polyethyleneglycol-bis(4-aminobenzoate) represented by the general formula 
##STR1## 
where n is a positive integer from 2 to 4, is used as the curing agent 
which is reacted with a mixture of a polyisocyanate and a polyol or with a 
polyisocyanate prepolymer having isocyanate terminal groups at the chain 
ends to give a polyurethane elastomer. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The curing agent used in the method of the present invention is a 
polyethyleneglycol-bis(4-aminobenzoate) represented by the above-given 
general formula and this compound is obtained readily, for example, by the 
reaction of 2 moles of 4-nitrobenzoyl chloride and 1 mole of a 
polyethyleneglycol represented by the general formula HO--CH.sub.2 
CH.sub.2 O).sub.n H, where n has the same meaning as defined above, to 
give a polyethyleneglycol-bis(4-nitrobenzoate) which is subjected to 
reduction with hydrogen or with iron and acetic acid. 
The examples of the compounds in conformity with the above general formula 
in which n is 2, 3 or 4 are diethyleneglycol-bis(4-aminobenzoate) having a 
melting point of 119.degree.-121.5.degree. C., 
triethyleneglycol-bis(4-aminobenzoate) having a melting point of 
99.degree.-102.5.degree. C. and tetraethyleneglycol-bis(4-aminobenzoate) 
having a melting point of 64.degree.-67.5.degree. C., respectively. 
As is evident from the above, the melting point of the curing agent of the 
present invention decreases as the number n increases so that a 
possibility arises of selecting the curing agent with a melting point 
tailored to the particular need whereby the preheating temperature of the 
prepolymer can be decreased to avoid thermal degradation of the prepolymer 
at an excessively high temperature. 
Any of the conventional polyisocyanates and polyols used in the art may be 
employed in the method of the present invention. For example, 
polyisocyanate compounds are exemplified by hexamethylenediisocyanate 
(HMDI), cyclohexanediisocyanate, 2,4-tolylenediisocyanate (2,4-TDI), 
2,6-tolylenediisocyanate (2,6-TDI), mixtures of 2,4-TDI and 2,6-TDI, dimer 
and trimer of 2,4-TDI, xylylenediisocyanate (XDI), 
metaxylylenediisocyanate (MXDI), m-phenylenediisocyanate, 
4,4'-biphenyldiisocyanate, diphenylether-4,4'-diisocyanate, 
3,3'-ditoluene-4,4'-diisocyanate (TODI), dianisidinediisocyanate (DADI), 
4,4'-diphenylmethanediisocyanate (MDI), 
3,3'-dimethyl-4,4'-diphenylmethanediisocyanate, 1,5-napthalenediisocyanate 
(NDI), triphenylmethanetriisocyanate (TTI) and the like. 
Similarly, polyol compounds used in the method of the present invention may 
be any of the conventional ones exemplified by aliphatic polyesterglycols 
such as polyethylene adipate, polybutylene adipate, polypropylene adipate 
and the like with extended chain length obtained by the condensation 
reaction between an aliphatic glycol and a dicarboxylic acid; 
polyalkyleneether glycols such as polypropyleneether glycol, 
tetramethyleneether glycol and the like obtained by the ring-opening 
polymerization of cyclic ethers such as ethylene oxide, propylene oxide, 
tetrahydrofuran and the like; polyesterglycols obtained by the 
ring-opening polymerization of .epsilon.-caprolactone; diol compounds 
obtained by converting the terminal groups in polybutadienes into hydroxy 
groups; copolymers of two or more kinds of alkylene oxides; copolymers of 
two or more kinds of glycols and a dicarboxylic acid; polyester polyols 
obtained by the co-condensation of a dicarboxylic acid and a polyol such 
as aromatic glycols, long-chain diols, glycerin, trimethylolpropane and 
the like; and polyether polyols obtained by the ring-opening 
polymerization of a cyclic ether such as ethylene oxide, propylene oxide 
and tetrahydrofuran with a polyol such as glycerin or trimethylolpropane 
as the initiator. 
Further, polyurethane prepolymers having isocyanate terminal groups at the 
chain ends are prepared by the reaction of the above-mentioned polyol, 
usually a polyether glycol or polyester glycol, and an excess amount of 
the above-mentioned polyisocyanate compound and exemplified by those 
prepolymers prepared with polytetramethylene glycol and an excess amount 
of a tolylenediisocyanate; with polyethylene adipate and an excess amount 
of a tolylenediisocyanate; and with polycaprolactonediol and an excess 
amount of a tolylenediisocyanate. 
The conditions for the preparation of the polyurethane elastomer according 
to the present invention are about the same as in the conventional 
procedures for the preparation of similar elastomers except for the 
possibility of decreasing the preheating temperature of the prepolymer and 
the melting temperature of the curing agent. The method of the present 
invention may be performed either by the so-called prepolymer process or 
by the one-shot process using a polyisocyanate and a polyol instead of a 
prepolymer. In the prepolymer process, the curing agent melted at an 
elevated temperature of 64.degree.-122.degree. C. according to the value 
of n in the general formula is admixed with the prepolymer preheated at a 
temperature at which the curing agent will not solidify during mixing to 
advantageously effect the curing reaction. 
The amount of the curing agent in this case is in the range from 0.8 to 1.2 
equivalents or, preferably, from 0.9 to 1.0 equivalent based on the 
isocyanate groups in the prepolymer. The curing schedule of the thus 
formulated polyurethane composition depends largely on the types of the 
prepolymer but it is usually sufficient to heat the mixture at a 
temperature of 70.degree.-80.degree. C. for about 3-5 hours to complete 
the curing reaction. 
In the method of the present invention, a somewhat longer pot life than in 
the prior art is readily obtained regardless of the kind of the curing 
agents so that fabrication of large articles or working in large areas in 
civil engineering and building construction can be facilitated with the 
polyurethane compositions prepared according to the invention. Further, 
the curing agents used in the present invention are not toxic to humans. 
In addition, those curing agents which are solid at room temperature 
exhibit remarkable super-cooling so that, once melted, they can be cooled 
down below their melting points, say to temperatures in the range of 
40.degree. to 100.degree. C., as the case may be, or even to about 
20.degree. C. without solidifying. Therefore an additional advantage is 
obtained by blending a super-cooled curing agent in liquid state obtained 
by gradually cooling the molten curing agent, with a prepolymer kept at a 
relatively low temperature where no problem arises of thermal degradation 
of the prepolymer.

Following are the examples to illustrate the present invention in further 
detail. 
In the Examples, the procedures for the measurement of the properties of 
the cured polyurethane elastomers and the pot life of the ready-prepared 
compositions were as follows. 
The polyurethane prepolymer specified in each of the Examples was admixed 
with the curing agent melted by heating at its melting point or above with 
stirring and the mixture was cast into a metal mold of 2 mm thickness 
preheated at 70.degree. C. and kept for 5 hours at the same temperature or 
for 3 hours at 80.degree. C. to effect curing. The test specimen obtained 
by taking out the above cured product from the metal mold with subsequent 
post-curing for 15 hours in an air oven at 100.degree. C. was subjected to 
the measurement of the mechanical properties in accordance with the 
procedure specified in JIS K 6031. The pot life was determined by the time 
to the disappearance of fluidity of the above prepared mixture of the 
prepolymer and the curing agent kept in an air oven at 90.degree. C. 
PREATION 1 
[Preparation of diethylene glycol-bis(4-aminobenzoate)] 
Into a reaction vessel of 1-liter capacity were taken 40.0 g (0.377 mole) 
of diethylene glycol, 76.3 g (0.754 mole) of triethylamine and 200 ml of 
toluene and a solution of 139.9 g (0.754 mole) of 4-nitrobenzoyl chloride 
in 300 ml of toluene was dropped into the above mixture kept at a 
temperature of 10.degree. C. or below under agitation over a period of 
about 15 minutes followed by heating at 60.degree. C. for 3 hours to 
effect the reaction. The reaction mixture thus obtained was filtered while 
hot to remove the hydrochloride of triethylamine and subjected to 
distillation to remove toluene leaving 150.0 g of diethylene 
glycol-bis(4-nitrobenzoate) having a melting point of 
94.0.degree.-96.0.degree. C. The yield was 98.4% of the theoretical based 
on the diethylene glycol. 
Into a reaction vessel of 2-liter capacity were taken 268.0 g (4.80 moles) 
of powdered iron metal, 5.0 g (0.08 mole) of acetic acid, 268.0 g (14.87 
moles) of water and 400 ml of toluene and a solution of 150.0 g (0.37 
mole) of the above obtained diethylene glycol-bis(4-nitrobenzoate) in 350 
ml of toluene was dropped into the above mixture under reflux with 
agitation over a period of 1 hour followed by further refluxing for 
additional 2 hours. The thus obtained reaction mixture was admixed with 
6.7 g of sodium hydrogencarbonate to neutralize the acetic acid and 
filtered while hot to remove the sludge of iron powder and the toluene 
solution taken by separation on standing from the aqueous phase was 
subjected to distillation to remove toluene leaving a crystalline product. 
The crystalline product after recrystallization from an ethyl alcohol 
solution was light brown in color weighing 90.0 g and had a melting point 
of 119.0.degree.-121.5.degree. C. to be identified as the objective 
diethylene glycol-bis(4-aminobenzoate). The yield was 70.6% of the 
theoretical based on diethylene glycol-bis(4-nitrobenzoate). 
PREATION 2 
[Preparation of triethylene glycol-bis(4-aminobenzoate)] 
The procedure for the preparation of triethylene 
glycol-bis(4-nitrobenzoate) was about the same as in the preparation of 
diethylene glycol-bis(4-nitrobenzoate) described in Preparation 1 above 
excepting the use of 56.6 g (0.377 mole) of triethylene glycol in place of 
40.0 g (0.377 mole) of diethylene glycol. The product had a melting point 
of 98.0.degree.-99.5.degree. C. and the yield of 167.5 g correspond to 
99.1% of the theoretical based on triethylene glycol. 
The reduction of the above obtained triethylene glycol-bis(4-nitrobenzoate) 
was conducted in the same manner as in Preparation 1 excepting the use of 
165.9 g (0.37 mole) of triethylene glycol-bis(4-nitrobenzoate) in place of 
150.0 g (0.37 mole) of diethylene glycol-bis(4-nitrobenzoate). The product 
thus obtained was a white powder having a melting point of 
99.0.degree.-102.5.degree. C. and identified to be triethylene 
glycol-bis(4-aminobenzoate). The yield of 102.0 g corresponded to 71.0% of 
the theoretical based on triethylene glycol-bis(4-nitrobenzoate). 
PREATION 3 
[Preparation of tetraethylene glycol-bis(4-aminobenzoate)] 
The procedure for the preparation of tetraethylene 
glycol-bis(4-nitrobenzoate) was about the same as in the preparation of 
diethylene glycol-bis(4-nitrobenzoate) described in Preparation 1 above 
excepting the use of 73.2 g (0.377 mole) of tetraethylene glycol in place 
of 40.0 g (0.377 mole) of diethylene glycol. The product had a melting 
point of 49.0.degree.-52.0.degree. C. and the yield of 182.8 g 
corresponded to 98.5% of the theoretical based on tetraethylene glycol. 
The reduction of the thus obtained tetraethylene 
glycol-bis(4-nitrobenzoate) was conducted in the same manner as in 
Preparation 1 excepting the use of 182.2 g (0.37 mole ) of tetraethylene 
glycol-bis(4-nitrobenzoate) in place of 150.0 g (0.37 mole) of diethylene 
glycol-bis(4-nitrobenzoate). The product thus obtained was a light brown 
powder having a melting point of 64.0.degree.-67.5.degree. C. and was 
identified to be tetraethylene glycol-bis(4-aminobenzoate). The yield of 
113.3 g correspond to 70.8% of the theoretical based on tetraethylene 
glycol-bis(4-nitrobenzoate). 
EXAMPLE 1 (Experiments No. 1 to No. 5) 
Either one of the curing agents di-, tri- and tetra-ethylene 
glycol-bis(4-aminobenzoates) (Experiments Nos. 1, 2 and 3, respectively) 
prepared in the above Preparations 1, 2 and 3, respectively, in an amount 
indicated in Table 1 below was melted at 130.degree. C. and blended with 
100 g of a polyurethane prepolymer Adiprene L-100 (product of E. I. DuPont 
de Nemours Co.) having a content of isocyanate groups of 4.19% by weight 
preheated at 80.degree. C. with stirring and the mixture was cast and 
cured in the manner as described before with the curing schedule of 5 
hours at 70.degree. C. 
The pot life of the ready-prepared mixture and the mechanical properties of 
the cured polyurethane elastomer were as set out in Table 1. 
For comparison, similar experiments were undertaken with conventional 
4,4'-methylene-bis(2-chloroaniline) (Experiment No. 4) and 
1,3-propanediol-bis(4-aminobenzoate) (Experiment No. 5) and the results 
are set out in Table 1. 
TABLE 1 
______________________________________ 
Experiment No. 
1 2 3 4 5 
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Curing agent taken, g 
16.3 18.4 20.5 12.7 14.2 
Pot life, minutes 
20 22 30 15 30 
Hardness (JIS, 
90 89 90 88 89 
type A) 
Tensile strength, 
460 565 321 438 413 
kg/cm.sup.2 
Ultimate elongation, 
660 610 670 440 530 
% 
Tear strength, kg/cm 
105 93 102 95 97 
Elastic resilience, % 
52 55 57 54 51 
______________________________________ 
EXAMPLE 2 
(Experiments No. 6 to No. 10) 
Polyurethane prepolymer compositions were prepared by admixing at 
80.degree. C. either one of the melted curing agents di-, tri- and 
tetraethylene glycol-bis(4-aminobenzoates) prepared in Preparations 1, 2 
and 3 above (Experiments Nos. 6, 7 and 8, respectively), 
1,3-propanediol-bis(4-aminobenzoate) (Experiment No. 9) and 
4,4'-methylene-bis(2-chloroaniline) (Experiment No. 10) in amounts as 
indicated in Table 2 below with 100 g of a prepolymer Cyanaprene A-8 
(product of American Cyanamid Co.) prepared by the reaction of 
polyethylene adipate and a tolylenediisocyanate with a content of 
isocyanate groups of 3.1% by weight and the mixtures were cast and cured 
in the manner as described before with the curing schedule of 3 hours at 
80.degree. C. 
The pot life of the ready-prepared compositions and the mechanical 
properties of the cured polyurethane elastomers are set out in Table 2. 
TABLE 2 
______________________________________ 
Experiment No. 
6 7 8 9 10 
______________________________________ 
Curing agent taken, g 
11.4 12.9 14.3 10.4 8.9 
Pot life, minutes 
17 20 22 20 13 
Hardness (JIS, 
82 81 81 81 79 
type A) 
Tensile strength, 
593 564 531 521 418 
kg/cm.sup.2 
Ultimate elongation, 
850 830 810 810 650 
% 
Tear strength, kg/cm 
85 81 79 79 76 
Elastic resilience, % 
42 42 43 40 41 
______________________________________ 
EXAMPLE 3 
(Experiments No. 11 to No. 15) 
The same experimental procedures were repeated as in Example 2 by use of 
the same curing agents di-, tri- and tetraethylene 
glycol-bis(4-aminobenzoates) (Experiments Nos. 11, 12 and 13, 
respectively), and two kinds of the same comparative curing agents 
1,3-propanediol-bis(4-aminobenzoate) and 
4,4'-methylenebis(2-chloroaniline) (Experiments No. 14 and No. 15, 
respectively) except that the polyurethane prepolymer in this case was 
Pandex 305E (product of Dainippon Ink Kagaku Co.) prepared by the reaction 
of polycaprolactonediol and a tolylenediisocyanate with a content of the 
isocyanate groups of 5.2% by weight instead of Cyanaprene A-8. The results 
are set out in Table 3. 
TABLE 3 
______________________________________ 
Experiment No. 
11 12 13 14 15 
______________________________________ 
Curing agent taken, g 
19.2 21.6 24.1 17.5 14.9 
Pot life, minutes 
10 12 13 10 6 
Hardness (JIS, 
91 91 90 90 87 
type A) 
Tensile strength, 
511 493 459 470 424 
kg/cm.sup.2 
Ultimate elongation, 
700 690 650 640 460 
% 
Tear strength, kg/cm 
95 95 89 91 87 
Elastic resilience, % 
35 34 31 32 29 
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