Rotational molding process for insulating pipes

A process for insulating a pipe comprising:

• i) applying at least one insulating layer to said pipe by rotational molding, wherein the insulating layer is a rigid polyurethane foam-forming liquid reaction mixture comprising:
• a) a mixture of diphenyl methane diisocyanate and polyphenyl polymethylene polyisocyanates,
• b) a polyol component bearing on average at least isocyanate-reactive hydrogen atoms and containing
• 1) a polyether containing at least two hydroxyl groups and having a molecular weight of from 300 to 700,
• 2) an aliphatic, cycloaliphatic or aromatic polyamine having a molecular weight of from 32 to 1000 as crosslinking agent, and
• 3) a blowing agent,
• ii) allowing said reaction mixture to foam to reach its insulating thickness,
• iii) applying at least one solid polyurethane-forming liquid reaction mixture to said insulating layer by rotational molding, said solid polyurethane-forming reaction mixture containing no blowing agent, and comprising:
• a) an NCO-terminated prepolymer having an NCO content of from 5 to 20% by weight and being prepared by reacting:
• 1) 4,4'-diphenyl methane diisocyanate with
• 2) polyethers containing 2 to 4 OH groups and having molecular weights of from 1000 to 6000,
• b) a polyol component containing:
• 1) a polyether containing 2 to 4 isocyanate-reactive hydrogen atoms and having a molecular weight of from 1000 to 6000, and
• 2) 5 to 35% by weight of an aromatic diamine having a molecular weight of from 122 to 400.