Casting of polymeric film

#The present invention relates to the production of films from organic thermoplastic polymeric materials and in particular to a process and apparatus for electrostatically pinning a film to a casting surface.

#Polymeric film may be made by extrusion from a slot die on to a casting surface, such as a rotating drum or moving belt, which may be cooled so as to quench the molten film into a solidified form suitable for subsequent film making operations. In the case of certain crystallisable polymeric materials, such as polyethylene terephthalate, it is important to rapidly quench the extruded film to a temperature below its second order glass transition temperature to minimise the onset of excessive crystallinity in the film which would cause embrittlement and interfere with subsequent film making operations. A known method of quenching molten film involves a technique commonly known as electrostatic-pinning in which electrostatic charges are deposited upon the molten film as it reaches the casting surface so that the film is electrostatically attracted to the casting surface, which is usually earthed but may be at a potential of either polarity.

#U.S. Pat. No. Re. 27771 discloses such a process of electrostatic-pinning and in particular is concerned with a process in which a direct current supply is used to develop the electrostatic charges. The specification states that a positive or negative current, but not both, may be used and that it is possible to use a pulsating supply superimposed upon a direct current supply if the polarity of the resultant current does not undergo any change, i.e. remains either positive or negative.

#In order that the invention may be more readily understood it is described further with reference to the accompanying drawings, wherein:

#FIG. 1 is a perspective view of apparatus which may be used according to this invention; and

#FIGS. 2 to 5 illustrate various "trapezoidal" or substantially "square" wave voltage supplies which may be used according to this invention.

#FIG. 1 of the drawings illustrates the casting of a molten film 1 on to a rotating polished steel casting drum surface 2 which is cooled by the passage of a quenching fluid through the inlet 3 and outlet 4 which are coincident with the axis about which the drum rotates. The quenched film 6 is removed from the drum surface 2 over a roll (obscured in the drawing by the drum) and is passed to a conventional molecular orienting and heat setting unit (not illustrated).

#A pinning wire electrode 8 is supported in tension close to the film surface in the proximity of the touch down region of the molten film on the casting drum surface 2. The wire electrode 8 is made from a nickel-chromium stainless steel (8%/18%) and has a diameter of approximately 0.007 inch.

#The molten film web 1 is narrower than the casting drum surface 2 so that bare margins 9 and 10 of the metal casting drum surface are exposed beneath the pinning wire beyond the edges of the web 1. Accordingly the end sections of the pinning wire 8 which are located above these exposed margins 9 and 10 are insulated with polytetrafluoroethylene tubular insulators 12 and 13 which just overlap the edges of the film web 1.