Pressure sensitive materials and methods of making same

In a pressure sensitive material, a printing paper sheet and a backing sheet of fibrous material having coextensive facing surfaces, a layer of adhesive material sandwiched between said coextensive facing surfaces of said paper sheet and said backing sheet, and a coating of an adhesive release agent on the surface of said backing sheet engaged with said adhesive layer, said release agent providing a predetermined releasability factor, said backing sheet having uniformly distributed undulations therein of a predetermined size and in a swirled waffle pattern such that a predetermined percentage of the total area of said adhesive layer is in engagement with said coating with such engagement being uniformly distributed throughout said surface of said backing sheet, said undulations being smooth and rounded to facilitate letter press printing and to minimize fracturing of fibers of said backing sheet and release of loose fibers into said adhesive material, said backing sheet also having a predetermined degree of stretchability and being contracted to a predetermined degree in forming said smooth and rounded undulations, said predetermined releasability factor and said predetermined percentage being high enough and said predetermined size of said undulations, said predetermined degree of stretchability and said predetermined degree of contraction being low enough to substantially obviate relative movement of said printing paper sheet and said backing sheet during letter press printing and to permit accurate letter press printing of said printing paper sheet without mottling, said predetermined releasability factor and said predetermined percentage being also low enough and the size of said undulations, said predetermined degree of stretchability and said predetermined degree of contraction being large enough to permit ready removal of said backing sheet by engagement of the finger of a user therewith, said predetermined releasability factor being on the order of 30-40 grams per lineal inch on the Keil scale, said predetermined percentage being from 40 to 60%, said predetermined degree of stretchability being less than 10% and said degree of contraction being from 2 to 10%.