Process for preparing moisture-resistant packing cardboard

A process for the preparation of mono- or multi-wave corrugated cardboard which consists of the following operations:

• (1) preparation of flat paper sheets containing from 20% to 80% by weight of cellulose fibers and from 80% to 20% by weight of synthetic thermoplastic polymer fibrils having a specific surface area greater than 1 sq.mt/gram;
• (2) heating said paper sheets, under pressure, and at a temperature equal to or higher than the melt temperature of the thermoplastic synthetic polymer forming said fibrils, or higher than the melt temperature of the polymer having the lowest melt temperature among those of the polymers forming said fibrils;
• (3) preparation of corrugated paper sheets starting from flat paper sheets obtained according to operations (1) or (2), by heating said flat paper sheets under pressure, on an undulated surface, at a temperature equal to or higher than the melt temperature of the synthetic thermoplastic polymer contained in said sheets, or higher than the melt temperature of the thermoplastic polymer having the lowest melt temperature among the synthetic thermoplastic polymers contained in said sheets;
• and
• (4) coupling the flat sheets obtained according to operation (2) to the corrugated sheets of paper, alternately the one to the other, with simultaneous mutual welding at the tips of the undulations, by binders or by melting of the synthetic thermoplastic polymer present in the sheets at the tips of the undulations.