to 2000 the preferred polyol component is selected from among the polyether polyols having a molecular weight of approximately 450 obtained by condensation of 1 2 propylene oxide with 2 2 bis hydroxymethyl 1 butanol and having a content of free hydroxyl groups of approximately 10 5 to 12 by weight a slightly branched polyester polyol based on trimethylol propane 1 6 hexanediol adipic acid and o and i phthalic acid having a content of oh radicals of 3 to 5 by weight a trifunctional lactone polyester polyol based on trimethylol propane or glycerol and u03b5 caprolactone having a content of oh groups of approximately 8 to 12 by weight other than the catalyst for the trimerization reaction in situ the reaction mixture advantageously contains a second catalyst which specifically promotes the formation of polyurethane by the condensation of the triisocyanate formed in situ and the polyol component the catalyst is an organometallic compound for example particularly an organic tin compound such as dibutylstannous dilaurate in order to assure trimerization of the diisocyanate prior to the reaction with the polyol the quantity of the catalyst which is employed to specifically promote trimerization is greater than the quantity of the catalyst employed for the polyurethane formation reaction thus the quantity of the trimerization catalyst employed can be advantageously more than 10 times greater than the quantity of the catalyst the quantities of diisocyanate on the one hand and polyol on the other hand in the reaction mixture are preferably calculated to have a final ratio of nco oh between 0 7 and 1 2 another advantage of the present process is that the trimerization reaction utilizes the temperature conditions employed in the production of a polyurethane layer by reactive flow of the reaction mixture of components with a functionality greater than.