Two-component polyurethane adhesive for lamination

1. Two-component polyurethane-based adhesive composition comprising an —NCO component and an —OH component, such that:

the —NCO component is a composition A comprising polyurethane prepolymers A1 comprising at least two —NCO end groups, A1 being obtained by polyaddition reaction between:

a composition A1.C1 of diphenylmethanediisocyanate (MDI) comprising at least 35% by weight of the 4,4′ elastomer, said percentage being expressed on the basis of the total weight of said composition, and

a composition A1.C2 of polyols comprising a polyether diol PE A 1 with a molar mass Mn of between 800 and 1200 g/mol and a polyether triol PE A 2 with a molar mass Mn of between 350 and 550 g/mol, in an amount corresponding to a PE A 2 /(PE A 1 +PE A 2 ) weight/weight ratio of between 15 and 30%,

the amounts of A1.C1 and A1.C2 used being such that the —NCO/—OH molar equivalent ratio is between 2.5 and 6.5;

the —OH component is a composition B comprising polyurethane prepolymers B1 comprising at least two —OH end groups, B1 being obtained by polyaddition reaction between:

a composition B1.C1 of diphenylmethanediisocyanate (MDI) comprising at least 35% by weight of the 4,4′ elastomer, said percentage being expressed on the basis of the total weight of said composition, and

a composition B1.C2 of polyols with a functionality of 2 comprising, on the basis of the total weight of said composition:

from 38 to 42% by weight of a composition B1.C2.D comprising an aliphatic polyester diol PES B 1 with a molar mass Mn of between 800 and 1200 g/mol and a partially aromatic polyester diol PES B 2 with a molar mass Mn of between 1000 and 1400 g/mol, in an amount corresponding to a PES B 1 /PES B 2 weight/weight ratio of between 0.35 and 0.45; and

from 58 to 62% by weight of an aliphatic polyether diol PE B with a molar mass Mn of between 350 and 550 g/mol;

the amounts of B1C1 and B1C2 used being such that the —NCO/—OH molar equivalent ratio is between 0.05 and 0.50;

it also being specified that the amounts of the —NCO and —OH components of said two-component adhesive composition are such that the —NCO/—OH molar equivalent ratio is within a range extending from 1.5 to 1.7.

2. Two-component adhesive composition according to claim 1 , characterized in that the 2,2′ isomer content of the MDI composition A1.C1 used in the preparation of A1 is less than 1% by weight, on the basis of the total weight thereof.

3. Two-component adhesive composition according to claim 1 , characterized in that the MDI composition A1.C1 comprises at least 49% by weight of the 4,4′ isomer.

4. Two-component adhesive composition according to claim 1 , characterized in that the polyether diol PE A 1 and the polyether triol PE A 2 used in the polyol composition A1.C2 are propylene glycols with the respective functionalities 2 and 3.

5. Two-component adhesive composition according to claim 1 , characterized in that the —NCO end group weight content of the polyurethane prepolymers A1 varies in a range extending from 9 to 18%.

6. Two-component adhesive composition according to claim 1 , characterized in that the composition A, as —NCO component, consists essentially of the polyurethane prepolymers A1.

7. Two-component adhesive composition according to claim 1 , characterized in that the composition A, as —NCO component, also comprises an aliphatic polyisocyanate A2, the —NCO end group functionality of which is within a range extending from 2 to 3.5 and the —NCO content of which, expressed by weight on the basis of the weight of A2, is between 17 and 27%.

8. Two-component adhesive composition according to claim 7 , characterized in that the aliphatic polyisocyanate A2 bears an allophanate.

9. Two-component adhesive composition according to claim 1 , characterized in that the total —NCO group content, expressed as % by weight on the basis of the total weight of the composition A, is within a range extending from 13 to 18%.

10. Two-component adhesive composition according to claim 1 , characterized in that the —OH group content, expressed as % by weight on the basis of the total weight of the composition B, is within a range extending from 3 to 5%.

11. Two-component adhesive composition according to claim 1 , characterized in that the viscosity, measured at 23° C., of each of the two —NCO and —OH components is less than or equal to 25 Pa·s.

12. Two-component adhesive composition according to claim 1 , characterized in that the viscosity thereof, measured at 50° C., at the time of mixing its 2 components, is within a range extending from 500 to 2 000 Pa·s.

13. Multilayer film comprising 2 thin layers of material linked to one another by a continuous layer, characterized in that said layer is constituted by the two-component adhesive composition as defined in claim 1 , in the crosslinked state, in an amount of less than 10 g/m 2 .

14. Process for continuously preparing the multilayer film as defined in claim 13 , comprising the sequential steps of:

(i) combining the —NCO and —OH components in the form of an adhesive mixture made flowable by heating at a suitable temperature,

(ii) coating, with said adhesive mixture, a first thin layer of material in the form of a substantially continuous layer,

(iii) laminating a second thin layer, on the first thin layer coated in accordance with step (i), then

(iv) crosslinking the adhesive mixture.

15. Flexible packaging comprising the multiplayer film according to claim 1 .