Moisture- and heat-crosslinkable polyurethane-based adhesive composition comprising a low content of isocyanate monomer

1. A crosslinkable adhesive composition comprising:

a) from 40 to 60 wt % of at least one polyurethane obtainable by a polyaddition reaction:

of at least one aromatic or aliphatic diisocyanate that is:

a1) isophorone diisocyanate,

a2) 2,4-toluene diisocyanate,

a3) diphenylmethane-2,4′-diisocyanate,

a4) an allophanate derivative of hexamethylene diisocyanate of formula (I):

in which:

p is an integer in the range from 1 to 2;

q is an integer in the range from 0 to 9;

R represents a hydrocarbon chain, saturated or unsaturated, cyclic or acyclic, linear or branched, comprising from 1 to 20 carbon atoms;

R 3 represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms;

or a mixture of said aromatic or aliphatic diisocyanates,

with at least one polyether diol, with or without the presence of at least one reaction catalyst, at a reaction temperature T1 below 95° C., in anhydrous conditions, and in amounts of diisocyanate(s) and of polyether diol(s) leading to an NCO/OH molar ratio, designated r 1 , in the range from 1.6 to 1.9; said polyurethane having formula (II)

in which:

R 1 represents an aliphatic or aromatic divalent group:

a1′) a divalent group derived from isophorone diisocyanate:

a2′) a divalent group derived from 2,4-toluene diisocyanate:

a3′) a divalent group derived from diphenylmethane-2,4′-diisocyanate:

or

a4′) a divalent group derived from an allophanate of hexamethylene diisocyanate (HDI) of formula (III):

in which:

p is an integer in the range from 1 to 2;

q is an integer in the range from 0 to 9;

R represents a hydrocarbon chain, saturated or unsaturated, cyclic or acyclic, linear or branched, comprising from 1 to 20 carbon atoms;

R 3 represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms;

R 2 , which may be identical to or different from R 3 , represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms;

n is a non-zero integer such that the number-average molecular weight of the polyether block of formula [OR 2 ] n — is in the range from 2000 to 12000 g/mol;

m is an integer such that the number-average molecular weight of the polyurethane is in the range from 3000 to 21000 g/mol;

b) from 39 to 59 wt % of at least one compatible tackifying resin, with a number-average molecular weight in the range from 200 Da to 5000 Da, and selected from the resins obtained by one of the following methods:

b1) polymerization of terpene hydrocarbons in the presence of Friedel-Crafts catalysts, followed by reaction with phenols,

b2) polymerization of alpha-methylstyrene, followed by reaction with phenols; and

c) from 0.01 to 1 wt % of at least one crosslinking catalyst, said percentages being expressed by weight relative to the total weight of the adhesive composition.

2. The adhesive composition according to claim 1 , wherein the allophanate derivative of hexamethylene diisocyanate of formula (I) is in the form of a composition in which it is comprised at a level of at least 99.5 wt %, said composition comprising less than 0.5 wt % of hexamethylene diisocyanate relative to the total weight of said composition.

3. The adhesive composition according to claim 1 , wherein the at least one polyether diol has a number-average molecular weight in the range from 2000 to 12000 g/mol.

4. The adhesive composition according to claim 1 , wherein the at least one polyether diol is a polyoxypropylene diol with a polydispersity index in the range from 1 to 1.4.

5. The adhesive composition according to claim 1 , comprising:

a) from 40 to 60 wt % of the polyurethane of formula (II), in which:

R 1 represents the divalent group derived from 2,4-toluene diisocyanate,

R 2 represents the divalent propylene group,

n is a non-zero integer such that the number-average molecular weight of the polyether block of formula [OR 2 ] n — is in the range from 3500 to 8500 g/mol,

m is an integer such that the number-average molecular weight of the polyurethane is in the range from 4600 to 16800 g/mol;

b) from 39 to 59 wt % of the compatible tackifying resin, with a number-average molecular weight in the range from 200 Da to 5000 Da, that is a resin obtained by method b2); and

c) from 0.01 to 1 wt % of the least one crosslinking catalyst,

relative to the total weight of the adhesive composition.

6. The adhesive composition according to claim 1 , comprising less than 0.5 wt % of unreacted aliphatic diisocyanate monomer, and less than 0.1 wt % of unreacted aromatic diisocyanate monomer, relative to the total weight of the adhesive composition.

7. The adhesive composition according to claim 1 , having a total content of NCO group in the range from 0.15 to 1.7 wt % relative to the total weight of the adhesive composition.

8. A method of preparing a composition as defined in claim 1 , comprising mixing the at least one polyurethane the at least one tackifying resin and the at least one crosslinking catalyst.

9. A self-adhesive support obtainable by a method comprising:

(i′) preheating the adhesive composition as defined in claim 1 to a temperature T4, in order to make it liquid, and then

(ii′) coating said adhesive composition on a support layer, and

(iii′) crosslinking said adhesive composition by heating the coated support to a temperature T5 in the range from 70° C. to 150° C., in the presence of humidity.

10. A label and/or self-adhesive tape comprising the self-adhesive support as defined in claim 9 .