Process for bonding lignocellulose-containing raw materials with a prepolymer based on a urethane-modified diphenylmethane diisocyanate mixture

In the process for bonding lignocellulose-containing raw materials by the process comprising (1) coating the lignocellulose-containing raw material with a bonding agent and (2) reacting and curing the mixture at elevated temperature and pressure, the improvement comprising employing as bonding agent an isocyanate group-containing prepolymer which has a free isocyanate group content of about 15 to about 33.6 weight percent based on the weight of prepolymer and having a viscosity of 120 to 1000 mPas at 25.degree. C. which prepolymer is prepared by reacting at about 20.degree. C. to about 120.degree. C. about 0.05 to about 0.5 hydroxyl equivalent of

• (a) a polyol having 2 to 8 hydroxyl groups and a molecular weight of about 62 to about 2000
• per isocyanate equivalent of
• (b) a polyisocyanate mixture which contains 0 to about 50 weight percent polyphenyl polymethylene polyisocyanates and about 50 to 100 weight percent of diphenylmethane diisocyanate isomer mixture based on total weight of polyisocyanate mixture, said diphenylmethane diisocyanate isomer mixture containing about 10 to about 75 weight percent 2,4'-isomer and about 25 to about 90 weight percent 4,4'-isomer based on the weight of diisocyanate.