| abstract |
Polyurethanes are prepared by reacting together under slightly acid conditions at below 100 DEG C., preferably at from 50 DEG C. to 100 DEG C., a substantially linear polyhydroxy compound of M.W. greater than 800 and preferably less than 5000, and an excess of an organic diisocyanate having an acidity below 0.02% determined by adding a measured weight of diisocyanate to an excess of n-propanol, standing the mixture for 10 minutes to allow the n-propanol and diisocyanate to react and then titrating with a standard solution of KOH in alcohol; heating the product so obtained to 60 DEG to 150 DEG C. and mixing with a diol having a M.W. of at most 250 supplied to the mixing vessel at from 15 DEG C. to 150 DEG C. with agitation for 20 seconds to 3 minutes, thereafter discharging the mixture from the mixing vessel, heating it at from 70 DEG to 130 DEG C. until it solidifies and heating the solidified mixture at a temperature up to 120 DEG C. for 6 to 40 hours. The preferred polyhydroxy compounds are polyethers and polyesters especially polypropylene ethers, polytetrahydrofurans and polyesters having an acid value less than 3 and derived from glycols having two to eight carbon atoms and dicarboxylic acids having four to ten carbon atoms. Polyetherthioethers and polyacetals may also be used. The preferred diisocjanates are those having symmetrical structures such as hexamethylene diisocyanate and 4,41-diphenymlethane diisocyanate. Suitable diols are ethylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-bis-(b -hydroxyethoxy)benzene and 1,6-cyclohexanediol. If necessary a mineral or organic acid, suitably adipic acid, may be used in the first stage to bring the acidity to the required level. Conventional catalysts for isocyanate reactions, e.g. organic and inorganic bases and organometallic compounds, may be added with the diol. |