Process for preparing rigid, mainly open-celled foams containing urethane and/or isocyanurate groups

A process for the production of rigid, very substantially open-celled foams containing urethane and/or isocyanurate groups by reacting polyisocyanates with compounds containing at least two isocyanate-reactive groups whose equivalent weight is from about 9 to 800 per reactive group, in the presence of catalysts, surfactants and optionally physical blowing agents, flameproofing agents and other known auxiliaries and finishes, the reaction being carried out in the presence of liquid, relatively high-molecular-weight, unsaturated hydrocarbons which are free from isocyanate-reactive groups, in amounts of from 0.01 to 3.0% by weight, based on the reaction batch. Particularly preferred hydrocarbons are polybutadiene and polyoctenylene having a mean molecular weight of from 800 to 10,000 g/ mol. The rigid foams have high dimensional stability due to the predominance of open cells.