Moisture-resistant polyurethane-based pressure-sensitive adhesives

A moisture-resistant, polyurethane-based, pressure-sensitive adhesive comprising a substantially uniform mixture of:

• (1) the reaction-product of 1.7 to 1.1 equivalents of an aromatic polyisocyanate and 1 equivalent of polyols of molecular-weight between 1500 and 5000 and hydroxyl number between 30 and 100;
• (2) the reaction-product of 1.7 to 1.1 equivalents of an aromatic polyisocyanate and 1 equivalent of a liquid hydroxylated elastomer of hydroxyl number between 20 and 60 in the amount of between 6 and 25 percent of the combined weight of the products of (1) and (2);
• (3) compatible tackifying resins in the amount of 40 to 150 percent of the combined weight of the products of (1) and (2) and selected from the class consisting of (a) the methyl, triethylene glycol and diethylene glycol esters of rosin, tall oil, hydrogenated rosin and stabilized rosin and (b) the glycerol and pentaerythritol esters of rosin, tall oil, hydrogenated and stabilized rosin;
• the proportions of the resins of (a) and (b) being between 10 and 40 percent of the combined weight of (a) and (b) resins;
• (4) active polyurethane catalyst in the amount of between 0.3 and 3 per cent of the combined weight of (1) and (2).