Patent GB-1044068-A (GB)
Title: Production of coated biaxially oriented linear highly crystalline polyolefin films
IPC: C08J7/052
Filed: 1963-09-27
Abstract: Heat sealable polyolefine film is formed by coating at least one surface of a linear highly crystalline polyolefine film with an olefine polymer having a lower melting point than the highly crystalline polyolefine, stretching the coated film in both the machine and transverse directions while maintaining it at a temperature above that at which it becomes drawable and orients when stretched and below that at which it thins out without appreciable orientation when stretched, said temperature being also above the melting point of the olefine coating polymer; and cooling the stretched film to room temperature.  The coated film may be maintained at a temperature 5 to 35 DEG C. below the melting point of the high crystalline polyolefine film.  The linear highly crystalline polyolefine may be polypropylene and the coating polymer may be polyethylene having a density of less than 0.94 gram per cubic centimetre.  The resulting coated film may be maintained at a temperature of between about 130 and about 165 DEG C. during the stretching step.  The film may be of tubular form and may be made by feeding a molten highly crystalline polyolefine, e.g. polypropylene, by an extruder 4 illustrated in Fig. 1 (not shown) to a double orifice die 8 from which the polymer issues in the form of a seamless tubing 10.  Simultaneously an olefine polymer having a melting point below that of the first polymer is fed by the extruder 14 to the die 8 and is extruded through a second annular die orifice concentric with and surrounding the first annular die orifice to form a seamless tubing 12 contiguous to and surrounding the tubing 10, thus forming a seamless tubing 13 having a two ply wall.  The tubing 13 is drawn vertically upward from the die 8 by a pair of squeeze rolls 18 through a cooling zone 20 and the portion of the tubing between the die and these rolls is distended to its desired diameter by means of pressurized air.  The flattened tubing is then drawn vertically upwards between guide rolls 21 and through a heating device 22 by a second pair of squeeze rolls 24 rotating at a higher peripheral speed than the squeeze rolls 18.  The heater 22 heats the tubing 13 to a temperature above that at which the inner layer 10 of the tubing becomes drawable and orients when stretched, but below that at which the layer merely thins out when stretched without appreciable orientation, and preferably to a temperature from about 5 DEG C. to 35 DEG C. below the melting point of the inner layer. The temperature to which the tubing 13 is heated during its passage through the heater 22 is also above the melting point of the outer layer 12 of the tubing 13 and below the degradation temperature of this layer. Air or other gas at super-atomospheric pressure is maintained within the portion of the tubing 13 between the squeeze rolls 18 and 24 to inflate and thereby stretch the tubing transversely at the same time that it is stretched longitudinally, the tubing thereby being converted into a tubular film which is hauled off and flattened by the squeeze rolls 24. The biaxial stretching of the tubing 13 during its passage through the heater 22 causes the inner and outer layers 10 and 12 of the two ply tubing to bond firmly together, whereas they can otherwise readily be separated and peeled one from the other. The polyolefine coating should have a melting point of at least 5 DEG C. below, preferably 20 to 90 DEG C. below, that of the highly crystalline polyolefine.  For polypropylene film a polyethylene coating of density below 0.94 and melting point below 125 DEG C. is preferred.  The low density polyethylene coating can be applied to preformed highly crystalline polyolefine film by applying the polyethylene as an emulsion or dissolved in a volatile organic solvent.  Alternatively the polyethylene may be preformed into a film and laminated to the preformed highly crystalline polyolefine film. The highly crystalline polyolefine may have a crystalline content of at least 55% by weight, an isotactic content of 57 to 82% by weight, a melt index of 0.8 to 2.7, an intrinsic viscosity of at least 1.3 and preferably 2.85 to 3.18, and a density of at least 0.88, preferably 0.90 to 0.905.  Other highly crystalline polyolefines which may be used are polyethylene having a crystalline content of at least 70% by weight, and a density of at least 0.94, polybutene and copolymers of ethylene and/or polypropylene with unsaturated compounds such as vinyl chloride and vinyl acetate.  The low density polyethylene need not necessarily be of a type to form self-supporting films and may be a relatively low molecular weight polyethylene such as a polyethylene wax.