Patent GB-1137520-A (GB)
Title: Treatment of plastic films
IPC: B29C48/00
Filed: 1966-06-16
Abstract: 1,137,520. Films. MINNESOTA MINING & MFG. CO. 16 June, 1966 [17 June, 1965], No. 27019/66. Heading B5B. [Also in Division B6] A process for improving the anchorage characteristics of selected areas of plastic film, comprises blanketing a first surface portion of said film with a difficultly energizable gas, contacting a second surface portion of said film with a readily energizable gas, and subjecting said film to corona discharge while continuously blanketing said first surface portion with said difficultly energizable gas to cause a confined treatment of said second surface portions to improve the anchorage characteristics thereof. By difficultly energizable gases are meant gases which have a high dissociation energy and thus do not tend to ionize to form free radicals under the conditions attaining in conventional corona discharge treatment. Thus such gases are not reactive with polymeric film when contacted therewith in an electric field. Most such gases are also inert and non- oxidizing, and may also be described as " inert to the effects of corona discharge " or " non- treating " gases. Specified gases include nitrogen, helium, argon, krypton, neon, xenon, non- ionizing fluorocarbons, and water vapour, e.g. in the form of steam. Readily energizbale gases produce useful corona activation when contacted with film surfaces and are gases which are readily dissociable to form ions or free radicals, for example oxygen, carbon monoxide, nitric oxide and halogens. Such gases are generally strong oxidizing agents. Specified materials include polyolefin films, e.g. polyethylene, polypropylene, and ethylene-propylene copolymers, and in addition, polyurethanes, ethylene-vinyl acetate copolymers, polyamides, polyesters, polyethers, polyvinyl ethers, polyvinyl chlorides, and polyvinylidene chloride. In a preferred method the film may be extruded in the form of a hollow tubular column into an oxygen-containing atmosphere, e.g. air, the inner surface of said column being said first surface portion and the outer surface of said column being said second surface portion, a gaseous nitrogen atmosphere being maintained within said column, the column flattened, and corona discharge treatment applied to the outer surface of the flattened column to improve the anchorage characteristics of the outer surface without substantially altering the anchorage characteristics of the inner surface. In Fig. 1, a tubular column of extruded film 1 is extruded from die 2 which is provided with a circular orifice 3. The tubular column is inflated with a stream of difficultly energizable gas, e.g. nitrogen indicated by arrow 4, introduced through the central portion of the die. A series of collapsing rolls 5 are provided on opposite sides of the extruded film, column 1, to flatten the same prior to corona treatment. The flattened tube is passed over the earthed back-up roll 6 and corona discharge treated on one side by means of voltage applied to treater bar 7. The film may then pass over an idler 8 and then over a second earthed back-up roll 9 for corona discharge treatment on the opposite side by treater bar 10. The film is then further processed, e.g. by slitting and printing and wound up. In Fig. 2, a flat sheet 21 of extruded film from extrusion die 22 is provided on the lower side with a blanket of nitrogen or other difficultly energizable gas by means of spaced discharge tubes 23 along the path of travel of the film. Gas is supplied thereto through manifold 25. The discharge tubes are so located that the film is never permitted to come into contact with air or other readily energizable gas, i.e., the film surface portions to be protected from treatment are extruded directly into an atmosphere of difficultly energizable gas to prevent sorption or occlusion of readily energizable gases on such surface portions. The sheet of film passes over an earthed back-up roll 26 for corona discharge treatment by means of treater bar 27 after passing idler 28. The upper surface being treated is in contact with a readily energizable gas such as oxygen, air being suitable. A blanket of nitrogen is maintained between the sheet of film and the earthed roll as the film is fed under the treater bar. Plastic bags may also be treated by the method of the invention, the bags being first flushed out with the difficultly energizable gas, filled therewith and sealed, and then corona discharge treated. Examples are given.