obtained it has also surprisingly been found that a further improvement in adhesion can be obtained by pretreating the surface of the highly elastic material with polar or apolar solvents for 10 seconds to several minutes polar or apolar solvents which may be used for this purpose include non etching liquids such as water alcohols preferably ethanol methanol and isopropanol ketones such as acetone and methylethylketone chlorinated hydrocarbons such as ccl 2 u2550 ccl 2 ccl 4 ch 3 ccl 3 and ccl 2 h ccl 2 h hydrocarbons such as n hexane n pentane ligroin petroleum ether and cleaning spirit dimethylformamide dimethylsulfoxide and mixtures thereof neutral anionic and cationic surfactants and quaternary amines may be added to the solvents in quantities of from 0 1 to 5 0 wt to enhance their cleaning effect tetrabutylammonium bromide dimethyl benzyl ammonium chloride sodium benzenesulfonate and isononylphenol heptaglycol ether are all suitable for this purpose the process of the present invention has the following important advantages over the conventional pretreatment with oxidizing media 1 the mechanical properties of the highly elastic material are not adversely affected by the plasma treatment 2 the polyurethane foam layers applied by the process according to the invention have a bond strength according to din 53 273 of u2267 20 n cm 3 there are no wastewater problems i e no risk of environmental pollution and 4 the plasma treatment involves relatively little expense methods for plasma treatment are known plasma treatment may be carried out with conventional plasma reactors such as for example hf high frequency plasma in the frequency range from 27 12 to 13 56 mhz with microwave plasma reactors in the frequency range of for example 2 45 ghz see for example m.