Polyurethane binding agents having a low content of highly volatile monomers

A polyurethane binder with a low content of readily volatile isocyanate-functional monomers comprising components A and B, wherein:

• (a) component A comprises a polyurethane propolymer containing at least two isocyanate groups or a mixture of two or more polyurethane prepolymers containing at least two isocyanate groups, wherein, component A is produced by an at least two stage process comprising:
• (i)a first stage, wherein a first polyurethane prepolymer is prepared by reaction of a first at least difunctional isocyanate having a first isocyanate group and a second relatively slower reacting isocyanate group, compared to a reactivity of the first isocyanate group to isocyanate reactive groups, with an at least first polyol component at a :OHNCO ratio less than 2 to provide a prepolymer having free OH groups and a predominate percentage of the second relatively slower reacting isocyanate groups;
• (ii) reacting in a second stage, a second at least difunctional isocyanate and a second polyol component with the polyurethane prepolymer from the first stage, the isocyanate groups of the second at least difunctional isocyanate having a higher reactivity to isocyanate-reactive groups than the predominant percentage of the isocyanate groups in the prepolymer from the first stage, wherein, the second polyol component can be the same as the first polyol component, the polyurethane prepolymer containing at least two isocyanate groups or the mixture of two or more polyurehane prepolymers containig at least two isocyanate groups contian at least two attached different types of isocyanate groups wherein at least one type of isocyanate groups has a lower reactivity to isocyanate-reactive groups than a second type of isocyanate groups;
• and
• b) component B comprises an at least difunctional isocyanate having a molecular weight lower than a molecular weight of the polyurethane prepolymers present in component A and which isocynate groups have a higher reactivity to isocyanate-reactive compounds, than the isocyanate group of relatively low reactivity present in component A wherein, component B is present in the polyurethane binder as a whole in an amount of at least 5% by weight.