GB753603A - Improvements in and relating to the manufacture of films - Google Patents

Abstract

<PICT:0753603/IV(a)/1> <PICT:0753603/IV(a)/2> <PICT:0753603/IV(a)/3> Thermoplastic film is stretched transversely whilst heated to a temperature that promotes drawing, the heat being applied locally in a line disposed in a general diagonal direction across the portion of the film being stretched. This localizes the stretch and results in a higher degree of orientation and greater strength for a given stretch ratio. To further localize the stretch, the line heater may be immediately followed by a line cooler. Two adjacent and parallel line heaters may be employed. Suitable line heaters include a hot metal wire bent to the required shape, in coiled form, if desired, and supported at intervals along its length; a metal tube containing hot liquid; a hot-air "knife" comprising a jet of hot air issuing through a slit orifice; or a suitably shaped infra-red heater. The film may travel in contact with or close to the heater. Suitable line coolers include a metal tube through which a cooling liquid flows, e.g. water; or a cold air "knife" comprising a jet of cold air issuing from a slit orifice. The process may be applied to travelling flat sheet or flattened tubular film, and is particularly suitable for materials which show a marked increase at elevated temperatures in the ease with which they can be drawn, e.g. polyethylene terephthalate. The film, which may have already been or may subsequently be drawn longitudinally, is preferably pre-heated before being subject to the transverse stretching. The line of heating may be straight, but this results in an uneven rate of stretching across the width of the film. In order to ensure an even rate of stretching throughout the width, the line along which heat is applied should have a particular shape, determined as follows:-In Fig. 4, AB represents the width of film before stretching, and A1B1 after stretching, the dotted line A1B representing the line heater whose shape is required. AB is divided into a number of equal parts by X, X1, X2, &c. and for even stretching such points should finish up as a similar number of points Z, Z1, Z2, &c. equally spaced along A1B1. As the film travels, the point X remains parallel to the edge A1A until it reaches the line of heat at Y, since no stretching of the film occurs before it is heated. The film is then locally heated, locally stretched and locally cooled, whereafter the point Y moves along the curve YZ parallel to the edge BB1, since no more stretching occurs after the heat is removed. Thus the correct shape of the heating line is the line joining the intersections of the series of curves XY, X1Y1, &c. parallel to A1A and the series of curves ZY, Z1Y1, &c. parallel to the edge B1B. Fig. 5 shows how the correct shape of the line of heating is obtained when it is desired to have two diverging line heaters X3A1 and X3B1 so that stretching commences at the centre of the travelling film, the symbols having the same significance as in Fig. 4. A suitable apparatus is shown in Fig. 6, wherein the film 1 passes over idler roll 2, its edges pass round mutually inclined pulleys 4, 41 against which its edges are gripped by a pair of endless belts 3, 31 whereby the film is stretched transversely, and over idler roll 5. In the stretching zone, i.e. between the points where the film first contacts pulleys 4, 41 and where it leaves them, the film is locally heated by a line heater 6 whose shape has been determined as described above, immediately behind which is a line cooler 13, parallel to the line heater. Instead of the edges of the film being gripped by the pulley and cord system shown, they may be gripped as in a tenter.