Thermoplastic film production

Abstract

In a process for extruding thermoplastic film means are provided for maintaining substantially free from deposit a wire electrode which is used to deposit an electrostatic charge on to the upper surface of the film. In a preferred embodiment the wire electrode is continuously unwound from a first reel on to a second reel. The wire electrode is kept substantially free of deposit by means of either a scraper plate or a current of air which is passed between the film and the wire electrode to prevent deposits from forming.

We claim: 
     
       1. A process for quenching a freshly extruded thermoplastic composition which comprises extruding a molten thermoplastic film forming polymeric composition on to a moving casting surface, depositing on the upper surface of the film an electrostatic charge from a wire electrode situated near to said film surface and maintained at a large electrostatic potential difference from the casting surface so that the film is caused to adhere to said casting surface, and maintaining said wire electrode substantially free from deposit arising from condensed vapors of volatile matter present in said film forming composition or the decomposition products of said volatile matter by continuously unwinding said wire electrode from a first reel positioned at one side of the die orifice from which the thermoplastic film is extruded, winding said wire on to a second reel positioned at the other side of the die orifice at a speed which prevents the dwell time of the wire electrode above the extruded film from being sufficient for substantially any deposit to accumulate on the said wire. 
     
     
       2. A process according to claim 1 in which said wire electrode is maintained substantially free from deposit by the means of a scraper plate having an orifice of sufficient diameter just to allow passage of the clean wire. 
     
     
       3. A process according to claim 2 in which said electrode wire is reciprocated through said scraper plate. 
     
     
       4. A process according to claim 2 in which said electrode wire is in the form of an endless band which is continuously passed through said scraper plate. 
     
     
       5. A process according to claim 1 in which said wire is maintained substantially free from deposit by means of a bath containing a solvent for the deposit which is periodically passed along the wire. 
     
     
       6. A process according to claim 5 in which said electrode wire is reciprocated through said bath. 
     
     
       7. A process according to claim 5 in which said electrode wire is in the form of an endless band which is continuously passed through said bath. 
     
     
       8. In a process for quenching a freshly extruded thermoplastic composition which comprises extruding a molten thermoplastic film forming polymeric composition on to a moving casting surface, depositing on the upper surface of the film an electrostatic charge from a wire electrode situated near to said film surface and maintained at a large electrostatic potential difference from the casting surface so that the film is caused to adhere to said casting surface the improvement consisting of maintaining said wire electrode substantially free from deposit arising from condensed vapors of volatile matter present in said film forming composition or the decomposition products of said volatile matter by passing a current of air between the upper surface of said thermoplastic film and the wire electrode so that said volatile matter is substantially prevented from reaching the electrode, said current of air being maintained at approximately the same temperature as the die orifice through which the thermoplastic film is extruded. .Iadd. 
     
     
       9.  In a process for quenching a freshly extruded thermoplastic composition which comprises extruding a molten thermoplastic film forming polymeric composition onto a moving casting surface and depositing on the upper surface of the film an electrostatic charge from an electrode situated near to said film surface and maintained at a large electrostatic potential difference from the casting surface so that the film is caused to adhere to said casting surface, the improvement which comprises preventing condensible vapors issuing from the molten film from condensing on said electrode by maintaining a current of heated air between the electrode and the film, said air being heated by means additional to any heat from the film. .Iaddend. .Iadd. 
     
     
       10.  Apparatus for preparing film from an extrudible thermoplastic composition which comprises extrusion means including a die orifice for extruding a film of said thermoplastic composition which issues condensible vapors; a moving casting surface; electrode means near said surface but spaced therefrom; means for maintaining said electrode means at a large electrostatic potential difference from said casting surface so that extruded film is caused to adhere to said surface, and means for maintaining the electrode means substantially free from deposit of condensed vapors issuing from the extruded film..Iaddend.