Process for surface treatment of foamed sheets

A process for the continuous modification of the surface portion only of a foamed plastic web to impart high mechanical resistance with retention of air permeability and elasticity, which comprises preheating the web to a temperature below its melting point, then subjecting the preheated web to shock heating by means of a device disposed at a distance of 5 to 10 mm from the web and at a temperature of from 150.degree. C. to 300.degree. C., and then passing the preheated and shock heated web through the nip of a calender comprising a pressure roller and a backing roller, the pressure roller being heated to a predetermined temperature above the melting point of the web and up to 500.degree. C., the web being fed to the calender nip in such a way that it is looped around the pressure roller through an angle of at least 30.degree. and is then withdrawn from the calender and wound up.