Laminated films

In a process of manufacturing a continuous elongated web of rib-patterned cross-laminate from at least one cross-sandwich web, each such cross-sandwich web being formed from at least two films of orientable thermoplastic polymer material wherein the films are each uniaxially or unbalanced biaxially oriented to impart thereto a major direction of orientation and at least one of the films exhibits an angle between its longitudinal direction and its major direction of orientation, and said at least two films are sandwiched together in a criss-crossing arrangement of the respective major directions of orientation, in which said cross-sandwich web is stretched in the direction transverse to its length by passage of the same lengthwise between grooved rollers and said at least two films are laminated together, the improvement wherein the transverse stretching is effected by forming said cross-sandwich web while at a temperature below the melting point of the thermoplastic material into undulating cross-sectional shape;

• during or after said undulating forming process, stabilizing in their curved shape tip portions of the undulations on at least one side of the cross-sandwich web;
• and transversely stretching the stabilized cross-sandwich web in regions between the stabilized tip portions by the use of grooved rollers, this latter stretching being adapted to retain in the stabilized portions either said undulating shape or at least a memory of said undulating shape;
• and if said stabilized portions retain only a memory of said undulating shape, subsequently heat-treating the cross-sandwich web so that the material in local regions between the stabilized tip portions shrinks along a hypothetical plane lying substantially midway between the surfaces of the cross-sandwich web in a direction perpendicular to the direction of movement of the web, so that said stabilized tip portions which retain only said memory of the undulating shape are reshaped at least in part, thereby to create a rib patterned cross-laminate web with a pattern of elongated lengthwise ribs of generally shallow U-shaped transverse cross-section, said ribs having a maximum thickness significantly greater than the average thickness of the cross-laminate as a whole.

A method of biaxially stretching an elongated web of polymeric sheet material by the steps of:

• 1) subjecting the web of material to a first simultaneous transverse stretching and forming operation in which opposite surfaces of the material are compressed towards toward each other over at least laterally spaced longitudinal regions thereof by passing the material through compressionally working opposed grooved rollers from which the material emerges in a transversely undulating configuration,
• 2) downstream of the compressionally working grooved rollers stretching the web longitudinally by passing the same through opposed rollers rotating at a higher circumferential rate of speed than said compressionally working grooved rollers, and
• 3) subjecting the longitudinally stretched web to a second transverse stretching operation by passing the same through additional opposed grooved rollers, wherein the stretching conditions are selected to remove from the web material a portion of said undulations while retaining at least a trace of said undulations therein;
• and said second stretching operation is correlated to the undulations of said first operation by either:
• a) adjusting the separation between the undulations in the web to match the separation between the grooves of said additional grooved rollers, or
• b) utilizing for each of said first and second operations opposed grooved rollers having substantially the same separation between the grooves thereof and locating any intervening pair of rollers through which the web passes between said first and second operations sufficiently near a next upstream pair of rollers that the separation between the undulations of the web remains constant.

Apparatus for biaxially stretching an elongated film of polymeric material which comprises in sequence:

• 1) A first transverse stretching and film forming station comprising a first set of driven opposed grooved rollers each having on the periphery thereof a pattern of circular or helical grooves and intervening peaks with the peaks of one roller in intermeshing relation with the grooves of the opposite roller, the grooved rollers of said first set being urged together to compressionally work the film passing therebetween, to thereby transversely stretch the film and simultaneously form the same upon emergence from the first roller set into a transversely undulating configuration;
• 2) A stretching station to stretch the undulating film lengthwise and partially but not completely remove the undulations therefrom;
• 3) A second transverse stretching and forming station comprising a second set of driven grooved rollers each having on the periphery thereof a pattern of circular or helical grooves and intervening peaks with the peaks of one roller in intermeshing relation with the grooves of the opposite roller, to thereby transversely stretch the film and simultaneously form the same upon emergence from the second roller set into a transversely undulating configuration;
• and
• 4) registration means for maintaining the undulations in the film emerging from the second roller set in substantial transverse registration with the undulations in the film emerging from the first roller set.