Process for making security paper from film-fibril sheets

Claim 1

In an improved process for preparing security paper, the process being of the type wherein webs containing bondable thermoplastic fibers are assembled with a discontinuous intercalary layer of identifying material and are then bonded, the improvement comprising the following steps in sequence: (1) assembling the intercalary layer of identifying material with a first and a second unbonded, lightly consolidated, nonwoven, polyethylene film-fibril sheet, each sheet having a basis weight in the range of 25 to 75 grams per square meter and a density in the range of 0.15 to 0.3 grams per cubic centimeter; (2) passing the sheet assembly through an unheated nip which applies a compression in the range of 17 to 85 kilograms per centimeter width of sheet assembly to form a lightly laminated sheet assembly; (3) self-bonding the lightly laminated assembly by passing the assembly, while under compressive restraint, through a heating zone and raising the temperature of one face of the assembly sufficiently to cause fusion of surface film fibrils so as to obtain an abrasion resistance of at least five cycles and cooling the assembly, while under restraint, to a temperature below that at which the assembly distorts or shrinks substantially and then repeating the procedure of this step to treat the other face of the sheet assembly and to obtain a self-bonded sheet assembly having an opacity of at least 70% and a delamination resistance of at least 60 grams per centimeter.