Polyurethane prepolymers comprising NCO groups and a low content of monomeric polyisocyanate

1. A polyurethane prepolymer comprising an NCO content of 2% by weight to 10% by weight (as determined by Spiegelberger's method, EN ISO 11909), an OH value of 0 and a monomeric polyisocyanate content of at most 2% by weight produced by the process of

A) mixing

I) at least one reactive component (I) containing urethane groups with

II) at least one low-monomer polyisocyanate (II) containing urethane groups, the low-monomer polyisocyanate (II) containing urethane groups produced by reacting at least one monomeric polyisocyanate with at least one polyol in an NCO:OH ratio of 2:1 to 10:1 and subsequently removing unreacted monomeric polyisocyanate,

wherein the percentage by weight of the reactive component (I) containing urethane groups in the mixture of component (I) and polyisocyanate (II) is in the range from 20 to 90% by weight,

B) and, optionally reacting components (I) and (II);

wherein component (I) contains at least one of:

both NCO groups and isocyanate-reactive functional groups as component (Ia);

NCO groups as component (Ib) and is characterized by:

i) a molecular weight of 500 to 20,000 g/mol, as determined by gel permeation chromatography (GPC),

ii) a Brookfield viscosity of 500 to 25,000 mPas (ISO 2555) at a temperature of 20° C. to 100° C.,

iii) an NCO content in the range from 1 to 10% by weight, as determined by Spiegelberger's method (EN ISO 11909),

iv) an OH value of 0, and

v) a monomer concentration of greater than 2% by weight;

isocyanate-reactive functional groups as component (Ic) and is characterized by;

i) a molecular weight of 500 to 20,000 g/mol, as determined by gel permeation chromatography (GPC),

ii) a Brookfield viscosity of 500 to 25,000 mPas at a temperature of 20° C. to 100° C., as determined by ISO 2555

iii) an OH value as determined by ISO 4326 in the range from 5 to 50 and

iv) an NCO content of 0.

2. The polyurethane prepolymer of claim 1 wherein the monomeric polyisocyanate content is at most 1 % by weight.

3. The polyurethane prepolymer of claim 1 wherein the prepolymer has a Brookfield viscosity at 100° C. of 100 mPas to 25,000 mPas, as measured to ISO 2555.

4. The polyurethane prepolymer of claim 1 wherein component (I) containing urethane groups contains both NCO groups and isocyanate-reactive functional groups as component (Ia).

5. The polyurethane prepolymer of claim 4 wherein component (Ia) is obtained by reacting at least one compound containing isocyanate-reactive functional groups with at least one nonsymmetrical monomeric polyisocyanate selected from the group consisting of all isomers of toluene diisocyanate (TDI) either in the form of the pure isomers or mixtures of several isomers; 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethyldiisocyanate (isophorone diisocyanate, IPDI); 2,4-diphenylmethane diisocyanate.

6. The polyurethane prepolymer of claim 4 wherein component (Ia) is characterized by:

a) a molecular weight of 500 to 20,000 g/mol, as determined by gel permeation chromatography (GPC),

b) a Brookfield viscosity of 500 to 25,000 mPas (ISO 2555) at a temperature of 20° C. to 100° C.,

c) an NCO content in the range from 1 to 10% by weight, as determined by Spiegelberger's method (EN ISO 11909) and

d) an OH value in the range from 5 to 50, as measured by ISO 4326.

7. The polyurethane prepolymer of claim 1 wherein component (I) containing urethane groups contains NCO groups as component (Ib) and is characterized by:

a) a molecular weight of 500 to 20,000 g/mol, as determined by gel permeation chromatography (GPC),

b) a Brookfield viscosity of 500 to 25,000 mPas (ISO 2555) at a temperature of 20° C. to 100° C.,

c) an NCO content in the range from 1 to 10% by weight, as determined by Spiegelberger's method (EN ISO 11909),

d) an OH value of 0, and

e) a monomer concentration of greater than 2% by weight.

8. The polyurethane prepolymer of claim 1 wherein component (I) containing urethane groups contains isocyanate-reactive functional groups as component (Ic) and is characterized by:

a) a molecular weight of 500 to 20,000 g/mol, as determined by gel permeation chromatography (GPC),

b) a Brookfield viscosity of 500 to 25,000 mPas at a temperature of 20° C. to 100° C., as determined by ISO 2555

c) an OH value as determined by ISO 4326 in the range from 5 to 50 and

d) an NCO content of 0.

9. The polyurethane prepolymer of claim 1 wherein the low-monomer polyisocyanate (II) containing polyurethane groups has an unreacted monomeric polyisocyanate content of 0 to 0.5% by weight, based on (II), and an OH value of 0.

10. A process for the production of the polyurethane prepolymer claimed in claim 1 comprising the steps of:

preparing at least one reactive component (I) containing urethane groups in a first reaction step as

a) component (Ia) by reacting at least one nonsymmetrical monomeric polyisocyanate with at least one compound containing isocyanate-reactive functional groups in an NCO/OH ratio of 1.05 to 1.8:1 until the more reactive NCO groups of the monomeric polyisocyanate have reacted almost completely with part of the isocyanate-reactive functional groups available and/or

b) preparing component (Ib) by reacting at least one monomeric polyisocyanate with at least one compound containing isocyanate-reactive functional groups, preferably with at least one polyol, in an NCO/OH ratio of 1.05 to 3:1 until the isocyanate-reactive functional groups have almost completely reacted and/or

c) preparing component (Ic) by reacting at least one monomeric polyisocyanate with at least one compound containing isocyanate-reactive functional groups in an NCO/OH ratio of 0.1 to 0.8:1 until the isocyanate groups have almost completely reacted; and

preparing at least one low-monomer polyisocyanate (II) containing urethane groups in a second step by reacting at least one monomeric polyisocyanate with at least one polyol in an NCO/OH ratio of 2:1 to 10:1, subsequently removing unreacted monomeric polyisocyanate and then mixing the reactive component (I) containing urethane groups and polyisocyanate (II); in steps a) and c), the low-monomer polyisocyanate (II) containing urethane groups being added and allowed to react in excess, based on the free isocyanate-reactive functional groups of component (Ia) or (Ic) containing urethane groups.

11. A process for the production of the monomeric polyisocyanate (II) containing urethane groups of claim 10 wherein the excess monomeric diisocyanate is removed from the reaction mixture by distillation, extraction, chromatographic or crystallization processes either individually or optionally in combination with one another.

12. A method of bonding plastics, metals or paper comprising applying the polyurethane prepolymer produced in accordance with claim 1 in combination with hardeners and/or moisture and optionally in the presence of organic solvents, accelerators and additives, onto the surfaces of the plastic, metal or paper.

13. A method of bonding plastics, metals or paper comprising applying the polyurethane prepolymer produced in accordance with claim 10 in combination with hardeners and/or moisture and optionally in the presence of organic solvents, accelerators and additives, onto the surfaces of the plastic, metal or paper.

14. A method of bonding flexible packaging comprising applying the polyurethane prepolymer of claim 1 onto the surface of the flexible packaging.

15. A bonded article of manufacture produced in accordance with the process of claim 12 .

16. A bonded article of manufacture produced in accordance with the process of claim 13 .

17. A bonded article of manufacture produced in accordance with the process of claim 14 .