Prepolymer-based polyurethane formulations for producing holographic media
View Patent ↗The present invention relates to a polyurethane composition comprising A) a polyisocyanate component comprising an exclusively diol-based NCO-terminated polyurethane prepolymer, wherein the NCO groups are bonded in a primary manner, B) an isocyanate-reactive polymer, C) a compound having groups which react by polymerizing with an ethylenically unsaturated compound under the action of actinic radiation (radiation-curing groups) wherein the compound is free of NCO groups, D) a free-radical stabilizer, E) a photoinitiator, F) optionally a catalyst, G) optionally assistants and additives.
1. A polyurethane composition consisting of
A) 10 to 55% by weight of a polyisocyanate component comprising an exclusively diol-based NCO-terminated polyurethane prepolymer, wherein the NCO groups are bonded in a primary manner, and wherein the prepolymer comprises a residual contents of free monomeric isocyanate of less than 0.5% by weight,
B) 15 to 79.989% by weight of an isocyanate-reactive polymer,
C) 10 to 70% by weight of a compound having groups which react by polymerizing with an ethylenically unsaturated compound under the action of actinic radiation (radiation-curing groups) wherein the compound is free of NCO groups,
D) from 0.001 to 2% by weight of a free-radical stabilizer,
E) from 0.01 to 7.5% by weight of a photoinitiator,
F) optionally up to 3% by weight of a catalyst,
G) optionally up to 50% by weight of assistants and additives.
2. The polyurethane composition as claimed in claim 1 , wherein the prepolymer in A) comprises NCO-functional prepolymers having groups selected from the group consisting of urethane, allophanate, biuret, amide and mixtures thereof.
3. The polyurethane composition as claimed in claim 1 , wherein the prepolymer in A) comprises urethanes, allophanates, and biurets which are formed from an aliphatic isocyanate-functional compound and oligomeric polyols, polymeric polyols, or polyamines having a number-average molar mass from 500 to 8500 g/mol.
4. The polyurethane composition as claimed in claim 1 , wherein the prepolymer in A) comprises allophanates formed from HDI or TMDI and difunctional polyether polyols having a number-average molar mass from 1000 to 8200 g/mol.
5. The polyurethane composition as claimed in claim 1 , wherein the isocyanate reactive polymer in B) comprises homopolymers or random or block copolymers based on ethylene oxide, propylene oxide, or mixtures thereof, optionally in combination with tetrahydrofuran, butylene oxide or ε-caprolactone as monomer units, wherein the homopolymers or random or block copolymers have an OH functionality from 1.5 to 6 and a number-average molar mass from 200 to 18000 g/mol.
6. The polyurethane composition as claimed in claim 1 , wherein the compound in C) has a refractive index n D 20 of greater than 1.55.
7. The polyurethane composition as claimed in claim 1 , wherein the compound in C) comprises a urethane acrylate or a urethane methacrylate which is based on an aromatic isocyanate and a compound selected from the group consisting of 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 4-hydroxybutyl acrylate, polyethylene oxide mono(meth)acrylate, polypropylene oxide mono(meth)acrylate, polyalkylene oxide mono(meth)acrylate and poly(ε-caprolactone) mono(meth)acrylates.
8. A process for producing media for recording visual holograms, comprising applying the polyurethane composition as claimed in claim 1 to a substrate or to a mold and curing the polyurethane composition and the substrate or mold.
9. A medium for recording visual holograms obtained by the process as claimed in claim 8 .
10. An optical element or image comprising the medium as claimed in claim 9 .
11. A process comprising the step of recording a hologram onto the medium as claimed in claim 9 in an exposure process to produce a hologram for optical applications in the entire visible and near UV ranges.