Prepolymer-based polyurethane formulations for the production of holographic films

1. A polyurethane composition comprising

A) a polyisocyanate component comprising an NCO-terminated polyurethane prepolymer, wherein the NCO groups of said NCO-terminated polyurethane prepolymer are primarily aliphatically bonded and wherein said NCO-terminated polyurethane prepolymer is based on hydroxy-functional compounds having an OH functionality of from 1.6 to 2.05;

B) an isocyanate-reactive polyetherpolyol;

C) a urethane acrylate and/or urethane methacrylate comprising at least one aromatic structural unit and a refractive index of greater than 1.50 at 405 nm, wherein said urethane acrylate and/or urethane methacrylate is free of NCO groups and OH groups;

D) a free radical stabilizer;

E) a photoinitiator based on a combination of a borate salt and a dye having an absorption band which at least partly covers the spectral range of from 400 to 800 nm;

F) optionally a catalyst; and

G) optionally an auxiliary and/or an additive.

2. The polyurethane composition of claim 1 , wherein said NCO-terminated polyurethane prepolymer comprises one or more urethane, allophanate, biuret, and/or amide groups.

3. The polyurethane composition of claim 1 , wherein said NCO-terminated polyurethane prepolymer is a urethane or allophanate of an aliphatic isocyanate-functional compound and an oligomeric or polymeric polyol, wherein said urethane or allophanate has a number average molar mass in the range of from 650 to 8200 g/mol and an NCO functionality of from 2.0 to 3.2 or of from 3.9 to 4.2.

4. The polyurethane composition of claim 1 , wherein said NCO-terminated polyurethane prepolymer is an allophanate of HDI and a difunctional polyetherpolyol, wherein said allophanate has number average molar mass in the range of from 1900 to 4100 g/mol and an NCO functionality of from 2.0 to 3.2 or of from 3.9 to 4.2.

5. The polyurethane composition of claim 1 , wherein said NCO-terminated polyurethane prepolymer has a residual content of free monomeric isocyanate of less than 0.5% by weight.

6. The polyurethane composition of claim 1 , wherein B) comprises a poly(propylene oxide), a poly(ethylene oxide), or a combination thereof in the form of random or block copolymers.

7. The polyurethane composition of claim 1 , wherein B) comprises a difunctional polyetherpolyol based on propylene oxide and ethylene oxide, wherein the proportion of ethylene oxide is less than 10% by weight, based on the total weight of said polyetherpolyol, and wherein said difunctional polyetherpolyol has a number average molar mass in the range of from 2000 to 4200 g/mol.

8. The polyurethane composition of claim 1 , wherein said urethane acrylate and/or urethane methacrylate has a refractive index n D 20 of greater than 1.55.

9. The polyurethane composition of claim 1 , wherein said urethane acrylate and/or urethane methacrylate is based on (1) an aromatic isocyanate and (2) 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 4-hydroxybutyl acrylate, a polyethylene oxide mono(meth)acrylate, a polypropylene oxide mono(meth)acrylate, a polyalkylene oxide mono(meth)acrylate, or a poly(ε-caprolactone) mono(meth)acrylate.

10. The polyurethane composition of claim 1 , wherein E) is a mixture of E1) and E2, wherein E1) is selected from the group consisting of tetrabutylammonium tetrahexylborate, tetrabutylammonium triphenylhexylborate, tetrabutylammonium tris(3-fluorophenyl)hexylborate, and tetrabutylammonium tris(3-chloro-4-methylphenyl)hexylborate and E2) is selected from the group consisting of Astrazon Orange G, methylene blue, new methylene blue, azure A, pyrillium I, safranin O, cyanine, gallocyanine, brilliant green, crystal violet, ethyl violet, and thionine.

11. A process for producing a medium for recording visual holograms comprising applying the polyurethane composition of claim 1 to a substrate or in a mould and curing said polyurethane composition.

12. A medium for recording visual holograms produced by the process of claim 11 .

13. An optical element or image comprising the medium of claim 11 .