The bonding of composite sheets, comprising a layer of cellulosic fibres and a layer adjacent thereto consisting in whole or in part of thermoplastic synthetic fibres, is accomplished by superimposing such sheets on the side of their respective cellulosic fibre layers and by inserting between such layers a film of a thermoplastic polymer having the following characteristics:… (a) a melting temperature lower than the melting temperature of the polymer forming the thermoplastic synthetic fibres contained in the layer adjacent to the cellulosic layer, intended for the adhesion, of the composite sheet;… (b) a viscosity in the molten state - measured in a capillary viscosimeter, at a velocity gradient equal to or lower than 10 -1 seconds, and at a temperature comprised between a temperature higher by 10 DEG C than the melting temperature of such polymer and a temperature higher by 20 DEG C than the melting temperature of the abovesaid thermoplastic fibres - lower than 1 . 108 poises, and preferably lower than 1 . 105 poises;… (c) a surface tension not exceeding by more than 5 dynes cm the surface tension of the polymer forming such thermoplastic fibres. measured at the same temperature;… (d) a solubility parameter, at a temperature of 25 DEG C, in a ratio of from 0.8 to 1.15, extremes included, to the solubility parameter of the polymer forming such thermoplastic fibres; and… by heating the whole at a temperature ranging from a temperature exceeding by 10 DEG C the melting temperature at the polymer forming such film to a temperature exceeding by 20 DEG C the melting temperature of the polymer forming the synthetic fibres existing in the layer which is adjacent to the cellulosic fibre layer, intended for the adhesion, of the composite sheet.
展开▼
