Process for the preparation of polyurethane resins using low molecular weight polyhydroxyl compounds prepared by the condensation of formaldehyde

In a process for the preparation of cellular or non-cellular polyurethane resins by the reaction of

• (a) polyisocyanates with
• (b) compounds with a molecular weight of between 32 and 400 having at least two active hydrogen atoms, and optionally
• (c) compounds with a molecular weight of between 400 and 10,000 having at least two active hydrogen atoms, and optionally
• (d) blowing agents, catalysts and other known additives,
• the improvement which comprises using as component (b) products prepared by the condensation of formaldehyde in an aqueous reaction medium in the presence of metal compounds as catalysts and of compounds capable of enediol formation as cocatalysts, by introducing synthesis gases containing formaldehyde continuously or discontinuously at temperatures of between 10.degree. and 150.degree. C. into an absorption liquid comprising
• (a) 5 to 99% by weight of water,
• (b) 0.1 to 90% by weight of compounds capable of enediol formation as cocatalysts,
• (c) 0 to 20% by weight of soluble or insoluble metal compounds as catalysts optionally bound to high molecular weight carriers, and
• (d) 0 to 60% by weight of one or more monohydric or higher hydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds
• and having a pH of 3 to 10, and the formaldehyde being condensed at the same time or, if the absorption solution contains no catalyst, the formaldehyde being condensed by the addition of catalyst, stopping the condensation of formaldehyde with itself by cooling and/or by inactivation of the catalyst with acids in known manner when the reaction mixture has a residual formaldehyde content of from 0 to 10% by weight;
• removing the catalyst in known manner;
• and reducing the aldehyde and keto groups present in the reaction product to hydroxyl groups.