Pressure-sensitive adhesive based on polyurethane, characterized in that the polyurethane is composed of the following starting materials which are reacted with one another in the stated proportions: a) at least one aliphatic or alicyclic diisocyanate having in each case an asymmetrical molecular structure, b) a combination of at least one polypropylene glycol diol and at least one polypropylene glycol triol, the ratio of the number of hydroxyl groups in the diol component to the number of hydroxyl groups in the triol component being between 0.7 and 9.0, preferably between 1.5 and 2.5, additionally the ratio of the number of isocyanate groups to the total number of hydroxyl groups being between 0.9 and 1.1, preferably between 0.95 and 1.05, and the diols and triols alternatively being selected and combined in each case as follows: diols having a molecular weight of less than or equal to 1000 are combined with triols whose molecular weight is greater than 1000, preferably greater than or equal to 3000, diols having a molecular weight of greater than 1000 are combined with triols whose molecular weight is less than 1000.