A method of preparing a polymeric methylene diphenyl diisocyanate-polydialkylsiloxane resin by mixing at a temperature between 25° C. and 100° C.: (i) a polymeric methylene diphenyl diisocyanate with (ii) a polydialkylsiloxane selected from hydroxyalkyl dialkyl terminated polydialkylsiloxane having a viscosity of from 5 to 500 000 cSt at 25° C. or (hydroxyalkoxy)alkyl dialkyl terminated polydialkylsiloxane having a viscosity of from 5 to 500 000 cSt at 25° C. in an amount such that from 1 to 99% by weight of the total weight of (i) and (ii) is component (ii), optionally in the presence of a solvent; and subsequently extracting the solvent, if present.