Preferably, polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol and glycerol, alkanolamines such as diethanolamine and triethanolamine, polyoxyalkylene type polyols having a hydroxyl value of at least 200, and polyamines such as t-butyltolylenediamine, diethyltolylenediamine and chlorodiaminobenzene, are employed.