Low free MDI prepolymers for rotational casting

A polyurethane prepolymer rotational casting composition comprising:

• a) an isocyanate-terminated polyurethane prepolymer prepared by reacting an organic diisocyanate monomer with a polyol, which prepolymer comprises a reaction product of polytetramethylene ether glycol and 4,4′-diisocyanato diphenylmethane, which prepolymer comprises less than 1.0% by weight of free 4,4′-diisocyanato diphenylmethane, based on the total weight of the prepolymer;
• and
• b) a curative comprising, based on the total weight of the curative,
• i) about 10 wt % to about 90 wt % of a polyol comprising polytetramethylene ether glycol;
• ii) about 10 wt % to about 90 wt % of an aromatic diamine comprising diethyl toluene diamine and/or dimethylthio-toluene diamine;
• iii) about 0.1 wt % to about 1.5 wt % of a thixotropic aliphatic amine selected from the group consisting of ethylene diamine, 1,6-hexanediamine, 1,12-dodecane diamine, 1,4-cyclohexane diamine, isophorone diamine, diethylene triamine, triethylene tetramine, amine-terminated polyoxypropylenes, xylene diamine, and piperazine;
• and
• iv) about 1.0 wt % to about 10 wt % of a thixotropic colloidal additive selected from the group consisting of fumed silica, clay, bentonite, and talc,
• wherein the total active hydrogen content of the curative agent is equal to about 80-115% of the total isocyanate content of the isocyanate-terminated polyurethane prepolymer.