Polyurethane adhesives having high peel strength at 70.degree. C

An adhesive composition which comprises a polyurethane comprising (a) units derived from a diisocyanate, (b) units derived from a poly (.epsilon.-caprolactone) diol having a hydroxyl value of from 25 to less than 30, and (c) units derived from an alkane diol chain lengthening agent containing at least 2 carbon atoms, the mole ratio of alkane diol:poly (.epsilon.-caprolactone) diol being from greater than 0.7:1.0 to less than 1.2:1.0. The adhesive may be used in solid form or dissolved in an organic solvent. The adhesive has improved hot bond strength and is particularly useful for bonding shoe soles to shoe uppers.

The present invention relates to polyurethanes and more particularly to 
adhesive compositions comprising polyurethanes derived from 
poly(.epsilon.-caprolactones) and a process for their preparation. 
British Pat. No. 1,268,138 describes a solvent-containing adhesive which 
comprises a hydroxyl-containing polyurethane prepared from a di-isocyanate 
and a polyester having a molecular weight of more than 2000 by 
polycondensation of a hydroxy alkane monocarboxylic acid containing at 
least 5 carbon atoms or by polymerisation of its lactone and a finite 
amount, up to 0.7 moles of an alkane diol containing at least 4 carbon 
atoms being used per mol of polyester, as a chain lengthening agent. The 
polyester may be derived from .epsilon.-caprolactone. The only polyester 
derived from .epsilon.-caprolactone which is exemplified has a hydroxyl 
value of 52.1, that is a molecular weight of about 2150. 
British Pat. No. 1338790 describes a solution for use as an adhesive, the 
solution comprising a polyester urethane which may be produced by reacting 
together a diisocyanate with a bifunctional polyester having a hydroxyl 
value of 30 to 100, preferably about 50. The bifunctional polyesters 
include those derived from .epsilon.-caprolactone and of these the 
polyesters exemplified have hydroxyl values ranging from 45.6 to 54.5, 
that is, molecular weights ranging from 2460 to 2055. 
U.S. Pat. No. 3,660,357 describes polyurethane adhesives prepared from 
poly(.epsilon.-caprolactones) having molecular weights of from 1800 to 
2200, that is, hydroxyl values of from 62 to 51. 
According to the present invention there is provided an adhesive 
composition which comprises a polyurethane comprising (a) units derived 
from a diisocyanate, (b) units derived from a poly(.epsilon.-caprolactone) 
diol having a hydroxyl value of from 25 to less than 30, and (c) units 
derived from an alkane diol chain lengthening agent containing at least 2 
carbon atoms, the mole ratio of alkane diol:poly(.epsilon.-caprolactone) 
diol being from greater than 0.7:1.0 to less than 1.2:1.0. 
A hydroxyl value of from 25 to less than 30 for the 
poly(.epsilon.-caprolactone) diol represents a molecular weight range of 
from 4480 to 3750. 
We have found that in general polyurethane adhesives according to the 
invention tend to have advantageous properties, for example improved high 
temperature bond strengths, when compared with adhesives comprising 
polyurethanes prepared from poly(.epsilon.-caprolactone) diols having 
hydroxyl values above 30. 
We have found that for the polyurethane adhesive according to the invention 
to be sufficiently stable in storage it is necessary for the ratio of the 
moles of the alkane diol to moles of the poly(.epsilon.-caprolactone) to 
be less than 1.2:1.0. Furthermore if this ratio is equal to or less than 
0.7 then the adhesive properties of the polyurethane are inferior. 
Preferably the ratio is in the range of from greater than 0.7:1.0 to 
1.0:1.0 and most preferably from 0.8:1.0 to 0.9:1.0. 
According to a further embodiment of the present invention there is 
provided a process for preparing an adhesive composition which comprises a 
polyurethane by reacting together a diisocyanate, a 
poly(.epsilon.-caprolactone) diol having a hydroxyl value of from 25 to 
less than 30, and from greater than 0.7 to less than 1.2 moles per mol of 
the poly(.epsilon.-caprolactone) diol of an alkane diol chain lengthening 
agent containing at least 2 carbon atoms. 
Preferably the diisocyanate is added to a mixture of the molten 
poly(.epsilon.-caprolactone) diol and the alkane diol. The reaction is 
carried out preferably at a temperature of from 100.degree. to 140.degree. 
C, most preferably 110.degree. C to 130.degree. C and particularly 
120.degree. C for a period of from 1/2 hour to 24 hours and most 
preferably 1 hour to 10 hours. The resulting polyurethane product is 
cooled and the resulting solid may be post-cured, if desired, at room 
temperature for, for example 24 hours. The solid polyurethane product may, 
if desired, be granulated. If required the polyurethane product may be 
dissolved in a suitable organic solvent. 
The ratio of moles of diisocyanate to the total of the number of moles of 
poly(.epsilon.-caprolactone) diol and moles of alkane diol, that is, the 
NCO:OH ratio, should be in the range of 0.97:1.0 to 1.04:1.0 and most 
preferably 1.0:1.0, so that there is no substantial excess of isocyanate 
nor of terminal hydroxyl groups remaining. 
NCO:OH ratios of 1.0:1.0 or greater are generally necessary when the 
polyurethane is to be used as a single-pack adhesive, that is, one which 
can be used as such, without the need for further additives before use, so 
that there is enough isocyanate present to ensure that, in use, sufficient 
cross-linking occurs for the attainment of adequate bond strengths. 
We have found that if the NCO:OH ratio is less than 0.97 then the adhesive 
properties of the polyurethane tend to be inferior, for example, the 
ultimate bond strengths attainable are diminished. Generally speaking for 
ratios of from 0.97:1.0 to less than 1.00:1.00 it is normally necessary to 
add a trifunctional isocyanate to the polyurethane before use in order to 
ensure sufficient cross-linking. Thus, the polyurethane forms part of a 
two component adhesive system. Suitable trifunctional isocyanates are 
reaction products from alkane triols, particularly trimethylolpropane, and 
an excess of a diisocyanate, particularly tolylene-di-isocyanates. 
Suitably 15% to 25% by weight of the trifunctional isocyanate is added, 
based on the weight of the polyurethane. 
If the NCO:OH ratio is greater than 1.04:1.00 then in general the 
polyurethane tends to have poor solubility in the organic solvents 
commonly used for polyurethane adhesives. 
We have found that when there is any residual or excess isocyanate present 
in the polyurethane adhesives of this invention, then on storage the 
polyurethane tends to cross-link to give an intractable insoluble 
composition which is unsuitable as an adhesive. Even if, in its 
preparation, the NCO:OH ratio is less than 1.0:1.0, there may still be a 
trace of residual isocyanate because of incomplete reaction. If the 
initial NCO:OH ratio used is greater than 1.0:1.0, then, of course, there 
is necessarily excess isocyanate. With a ratio of 1.04:1.00 then the 
storage life of the adhesive may be only about one month. Storability is 
an important consideration because there may be a time lapse of several 
months between manufacture of the polyurethane adhesive and its use. 
We have found that it is possible to improve the storage stability of the 
polyurethane adhesives of this invention by the addition of an isocyanate 
blocking agent thereto. By the term "isocyanate blocking agent" is meant a 
compound which reacts with the residual or excess isocyanate and so 
prevents or substantially reduces the ability of this isocyanate to cause 
cross-linking under storage conditions, but which, when the adhesive is 
heated in use, dissociates from the isocyanate so that the isocyanate is 
then free to perform its normal function. 
According to another embodiment of the present invention, therefore, there 
is provided an adhesive which comprises a polyurethane as hereinbefore 
defined and an isocyanate blocking agent as hereinbefore defined. 
The isocyanate blocking agent may be selected from lactams, malonates, 
bisphenols, pyrocatechol and diphenylamine. Preferably a 5 to 7-membered 
lactam is used, most preferably .epsilon.-caprolactam. The blocking agent 
may be used in an amount of from 0.1 to 10% by weight, preferably .05 to 
5.0% by weight, based on the weight of the polyurethane. 
The isocyanate blocking agent may be incorporated into the polyurethanes by 
any convenient means. For example, it may be added to the molten 
polyurethane shortly after the formation of the latter, or, alternatively 
it may be mixed with granules of the polyurethane and the mixture worked, 
for example by milling, at elevated temperature, for example at 80.degree. 
- 120.degree. C, such that the granules soften sufficiently for the 
isocyanate blocking agent to be blended in. 
In general, we have found that, in use, there is no discernible difference 
between the ultimate properties of the adhesives of this invention which 
do contain the isocyanate blocking agent and those which have otherwise 
identical formulations but do not contain the isocyanate blocking agent. 
In general, the only difference is that the polyurethane adhesives which 
do contain the isocyanate blocking agent remain usable for a longer 
period, that is, have a longer storage life. 
The poly(.epsilon.-caprolactone) diols suitable for use in this invention 
are the polyester reaction products of .epsilon.-caprolactone and glycols 
containing from 2 to 10 carbon atoms, for example, ethylene glycol, 
trimethylene glycol, tetramethylene glycol and hexamethylene glycol or 
mixtures thereof. Most preferably tetramethylene glycol (butane 1,4 diol) 
is used. Most preferably the reaction between the glycol and 
.epsilon.-caprolactone is carried out at a temperature not exceeding 
160.degree. C, in the presence of a conventional catalyst. We have found 
that the poly(.epsilon.-caprolactone) diol product prepared under these 
conditions leads to the most advantageous polyurethane adhesives. 
The diisocyanates which may be used in the process of this invention 
include for example 4,4.sup.1 -di-iso-cyanatodiphenyl methane (MDI), 
tolylene-di-isocyanates particularly dimerised tolylene di-isocyanates, 
di-isocyanate-dicyclohexylmethane, hexamethylene-di-isocyanate and 
naphthalene1,5-di-isocyanate. MDI is particularly preferred. 
Any alkane diols or mixtures thereof, which contain at least 2 carbon atoms 
are suitable for use as chain lengthening agents and include for example, 
ethane-1,2-diol, propane-1,3-diol, butane-1,4-diol, pentane-1,5-diol and 
hexane-1,6-diol. Butane 1,4-diol is particularly preferred. 
The polyurethane adhesives according to this invention may be used in the 
conventional manner. For example, they may be applied to surfaces to be 
adhered together from solutions in suitable solvents such as acetone, 
methyl ethyl ketone, tetrahydrofuran, dioxane or dimethylformamide. 
Solution in methyl ethyl ketone is preferred. The viscosity of the 
solution can be adjusted to the particular requirements of the bonding 
process or to the materials which are bonded by varying the concentration 
of the polyurethane. The adhesive coated surfaces are preferably heated, 
for example to about 80.degree. to 90.degree. C, to evaporate the solvent 
and to activate the adhesive. The surfaces may then be pressed together 
immediately. Prior to the application of the adhesive solution the 
surfaces may have been preheated to 80.degree. to 90.degree. C and 
roughened to prepared in some other way, for example, by wiping with a 
solvent. Alternatively the polyurethane adhesives may be applied in solid 
form, for example as a film or tape, and heated, for example to 80.degree. 
to 90.degree. C, to soften and activate the polyurethane and induce 
adhesion to the surface. 
Generally speaking it is preferred to apply the adhesives from solution. 
The polyurethane adhesives of this invention generally have excellent 
adhesive properties immediately on contact and readily crystallise to form 
the desired hard but flexible structural adhesive, reaching their initial 
hardness within 24 minutes of being heat softened. 
A number of materials such as for example, paper, cardboard, leather, wood, 
glass, metal, thermoset and thermoplastic materials, and rubber materials 
for example natural, synthetic and urethane rubbers, including poromeric 
rubbers, can be bonded very firmly with the polyurethane adhesives 
according to the invention. The adhesives are particularly useful for 
bonding rubber materials, PVC and particularly plasticised PVC, and 
leather to one another, especially for bonding shoe soles made of these 
materials to shoe uppers made of leather or of synthetic materials which 
simulate leather. 
Certain embodiments of the present invention will now be illustrated more 
fully by way of example. Example 5 is a comparative example and is not 
according to the invention.

EXAMPLE 1 
Preparation of Polyurethane Adhesive 
The quantities of reactants used and their mole ratios are given in Table 
1. 
The poly(.epsilon.-caprolactone) diol which was used had a hydroxyl value 
of 27.25 (molecular weight 4010) and was prepared from 
.epsilon.-caprolactone and tetramethylene glycol at a temperature of 
160.degree. C. This material is available from Interox Chemicals Limited 
under the Trade Mark CAPA 240X. 
The MDI used was DESMODUR 44 MS (Bayer). 
TABLE 1 
______________________________________ 
Butane 
Ratio of Ratio of 
1,4 diol 
Moles MDI 
Moles B. 
Ad- MDI Poly (.epsilon.-capro- 
(B.D) to Moles D. to 
he- % lactone) diol 
% (Polyol Moles 
sive w/w (Polyol) % w/w 
w/w +B.D.) Polyol 
______________________________________ 
1 10.20 88.05 1.74 0.99 0.88 
2 10.29 87.97 1.74 1.00 0.86 
3 10.62 87.65 1.73 1.03 0.80 
______________________________________ 
The required quantity of poly(.epsilon.-caprolactone) diol was placed in a 
vessel, melted and held at 70.degree. to 80.degree. C. The required 
quantity of butane diol was mixed in with stirring. The required quantity 
of MDI in the form of flake was added rapidly with vigorous stirring to 
give a homogeneous mixture. The mixture was poured into PTFE lined trays 
preheated to 120.degree. C and the trays were placed in an oven at 
120.degree. C for 1 hour. After this 1 hour reaction time the trays, now 
containing the polyurethane product, were removed from the oven, allowed 
to cool to room temperature and held at room temperature for 24 hours. The 
polyurethane product was then removed from the trays and reduced to 
granules. 
The polyurethane reactives 1, 2 and 3 were stable in storage for between 
one and three months. 
EXAMPLE 2 
Preparation of Adhesive Solutions 
(a) One Component Adhesive 
15 parts by weight of the polyurethane adhesives 1, 2 and 3 prepared as 
described above in Example 1 were dissolved in 85 parts by weight of 
methyl ethyl ketone. These solutions are hereinafter referred to as 1(a), 
2(a) and 3(a). 
(b) Two Component Adhesives 
15 parts by weight of the polyurethane adhesives 1, 2 and 3 prepared as 
described above in Example 1 were dissolved in 85 parts by weight of 
methyl ethyl ketone, to which solution was added 3 parts by weight of 
DESMODUR L (Bayer). DESMODUR L is a triisocyanate compound. 
These solutions are hereinafter referred to as 1(b), 2(b) and 3(b). 
PROPERTIES OF THE ADHESIVE SOLUTIONS 
Bonding tests were carried out using 1 cm wide strips of a 1.25 mm thick 
PVC coated shoe upper material, comprising a heavy gauge cotton twill 
fabric base coated with a plasticised, pigmented PVC. Two strips of this 
material were first prepared by wiping the PVC surface with acetone and a 
1 cm length of the surface of each strip was then coated with a thin layer 
of adhesive solution. The adhesive coated surfaces were heated at 
90.degree. .+-. 5.degree. C under an I.R. lamp for several minutes to 
evaporate the solvent and soften the polyurethane. The adhesive coated 
portions of each strip were then pressed together and placed under a 6 kg 
load for 3 minutes to make the bond. 
Peel strength tests were then carried out on the bonds as follows: 
(i) Immediate peel strength (green strength). Immediately after the bond 
had been made as described above, the force required to peel the bond 
apart was determined. 
(ii) 20.degree. C Peel strength. After the bond had been made it was 
allowed to cure for 24 hours at 20.degree. C and the force required to 
peel it apart was then determined. 
(iii) 70.degree. C peel strength. After the bond had been made it was 
allowed to cure at 20.degree. C for 24 hours, after which time it was 
heated at 70.degree. C for 30 minutes and then the force required to peel 
it apart was determined. 
(iv) Creep test. The bond area in this case was 1 inch .times. 1 inch. 
After the bond had been made it was allowed to cure at 20.degree. C for 24 
hours, after which time it was heated to 70.degree. C and maintained at 
this temperature whilst a peeling force of 1 kg was applied to it. The 
time to failure was determined. 
The results are given in Table 2. 
TABLE 2 
______________________________________ 
Adhesive Solution 
1(a) 1(b) 2(a) 2(b) 3(a) 3(b) 
______________________________________ 
Immediate peel 
strength (green 
1.3 3.2 3.6 6.8 5.4 5.6 
strength) in 
kg/cm. 
20.degree. C peel strength 
in kg/cm. 6.4 12.3 15.2 &gt;16 6.4 19.2 
70.degree. C peel strength 
in kg/cm. 0.4 0.4 2.5 15.2 2.4 7.3 
Creep test -- -- N.M. N.M. N.M. N.M. 
______________________________________ 
N. M. = no movement in 24 hours 
EXAMPLE 3 
The procedures of Example 1 were repeated. The granules so obtained were 
then mixed in a 2-roll mill with 3% by weight of .epsilon.-caprolactam, 
based on the weight of polyurethane, at a temperature of 100.degree. C. 
After thorough blending the product was removed as anide and granulated. 
The three polyurethane adhesive products so obtained were stable in storage 
for more than six months. 
EXAMPLE 4 
The procedures of Example 1 were repeated up to and including adding the 
required quantity of MDI, in the form of flake, rapidly with vigorous 
stirring to give a homogeneous mixture. 
The mixture was then poured into PTFE lined trays preheated to 120.degree. 
C and the trays were placed in an oven at 120.degree. C for 3/4 hour. 
After this period of reaction, the trays now containing the polyurethane 
were removed from the oven and 3% by weight .epsilon.-caprolactam; based 
on the weight of the polyurethane, preheated to 80.degree. C was 
thoroughly mixed in. The trays were placed in an oven at 100.degree. C for 
1 hour and then removed and allowed to cool to room temperature and held 
at room temperature for 24 hours. The polyurethane product was then 
removed from the trays and reduced to granules. 
The three polyurethane adhesive products so obtained were stable in storage 
for more than 6 months. 
EXAMPLE 5 
The procedures of Example 1 were repeated but using a poly 
.epsilon.-caprolactone diol having an hydroxyl value of 36.8 (molecular 
weight 3024). This material is available from Interox Chemicals Limited 
under the trade name CAPA 230. 
The quantity of reactants used and the mole ratios are given below; 
______________________________________ 
wt % moles 
______________________________________ 
MDI 12.68 0.051 
Poly .epsilon.-caprolactone diol 
85.25 0.028 
Butane 1,4 diol 2.03 0.022 
Ratio moles BD/polyol = 0.8 
Ratio moles MDI/BD + polyol 
= 1.02 
______________________________________ 
The granulated product obtained was dissolved in methylethyl ketone as 
described in Example 2(a) and tested as described in Example 2 with the 
following results: 
______________________________________ 
Immediate peel strength 
Kg/cm 4.2 
20.degree. C peel strength 
Kg/cm &gt;16.0 
70.degree. C peel strength 
Kg/cm 0.3 
______________________________________ 
It is readily apparent that the 70.degree. C peel strength is poor when 
compared with the adhesives according to the invention.