Verfahren zur Herstellung flexibler Polyurethanschaumstoffe

Flexible cellular polyurethanes are prepared by reacting together a polyfunctional polyol having a hydroxyl number of at least 70 and an equivalent weight of less than 800, an organic polyisocyanate, and water, in proportions such that the initial ratio of isocyanate groups to the active hydrogen groups in the water and the polyol (the NCO/[H] index), is less than 0.9 : 1, and such that give a product with a ratio of urea groups/urethane groups of less than 3 : 1. Appropriate polyols have -OH numbers of 70-350, preferably 100-200, an equivalent weight of 180-800, preferably 280-560, and are preferably trifunctional (i.e. triols); suitable polyols are polymers of alkylene oxides (e.g. ethylene oxide, propylene oxide, butylene oxide, and mixtures thereof), and addition products of alkylene oxides with polyfunctional alcohols (e.g. glycerine, trimethylol propane, hexane triol, pentaerythritol). Also mixtures of polyols, wherein some components have -OH numbers outside the preferred range, but together have an average -OH number within the preferred range, may be employed. Polyisocyanates used are preferably liquid di-isocyanates. The preferred amount of polyisocyanate used is partly dependant upon the proportion of water employed, this in turn being varied according to the -OH number and molecular weight of the polyol, and also to some extent according to the properties required for the foam. Generally, the higher the hydroxyl number of the polyol, the lower is the preferred ratio of isocynate groups to total active hydrogen groups present, in order to preserve flexibility of the product. This NCO/[H] index ranges from about 0.9 : 1 for an -OH number of 70 to less than 0.5 : 1 for an -OH number of above 200. The proportion of water preferred decreases as the -OH number of the polyol rises, and water content is desirably kept at the minimum compatible with final hydrolytic stability of the foam. The density of the foam may be controlled by incorporation of a blowing agent at up to 20% of the weight of the polyol used; this is suitably a fluorinated saturated aliphatic hydrocarbon, volatile at polymerization temperatures and inert to the other reactants and the foamed product. Other ingredients may be introduced into the reaction mixture, such as polymerization catalysts (e.g. tertiary amines, and organo tin compounds), fillers, extenders, fire retardant agents, and emulsifiers such as sulphonates, lecithin, polyethylene phenol ether, polyalcohol carboxylate es