Patent Number: 047913037
Section: summary

The present invention is directed to methods and apparatus for laminating polymeric sheet material in the absence of application of a separate adhesive and/or application of heat, and in particular is directed to methods and apparatus for effectuating the lamination of at least two polymeric sheets utilizing application of gaseous cold plasma, preferably comprising cold argon plasma, to effectuate such adhesiveless bonding through the generation of free radicals, and also in the substantial avoidance of any melting or heat degradation of such polymeric properties thereof in strength, stability, etc. It has been desirable in the packaging arts, and particularly in the packaging of comestibles to utilize clear polymeric materials for visual observation of the particular food product being displayed, in order to enhance the marketability thereof and to provide the consumer with a full opportunity to observe the condition of the food product. Unfortunately, certain polymeric materials have had properties rendering such materials desirable for some necessary aspects of packaging, but have had properties which were less than optimal in other areas. Specifically, for example, some polymeric sheet materials have permitted excellent transparency, but have been lacking in barrier protection against the infusion of oxygen and/or moisture from the atmosphere, etc. Thus, it has been desirable to utilize two or more polymeric sheet materials bonded together in the form of a laminate in order to achieve the hybridized or synthesized properties of several different polymeric sheet materials. As a further example, some polymeric sheet materials have been excellent in their performance as barrier materials, but have lacked printability on the surface thereof, which has been detrimental to their use as packaging material. In formulating such laminated materials, two basic prior art techniques have been principally utilized. In one technique, adhesive has been used as a lamination material between two separate sheets of different polymeric materials. A number of disadvantages and/or deficiencies have been associated with the use of adhesives for this purpose. Such adhesives have resulted in a relatively high scrap rate, comprising between in some instances 5% and 25%. Also, such prior art adhesive lamination has rendered necessary in most instances the presence of solvents or unreacted monomers, which has introduced another area of uncertainty, pursuant inter alia to Food and Drug Administration rules and regulations regarding toxicity, into packaging which has been proposed to be particularly utilizable for the packaging of comestibles. In addition, the machinery which has been developed for use in adhesively bonding together polymeric sheets to form such laminates has been relatively costly, and has involved high depreciation costs and excessive maintenance attention. Another form of laminate formation has been the hot co-extrusion of two different webs of polymeric materials, and/or application of a layer of heat melt adhesive, such that two or more webs are joined by such heat lamination. Such heat utilization techniques have resulted in relatively high scrap levels for relatively short runs of laminate material. The capital outlays have been relatively large for such equipment, and the depreciation costs involved have also been relatively high. In addition, the different types of polymeric material which have been utilizable in such hot co-extrusion and related techniques have been relatively limited. Moreover, in some polymeric materials the utilization of heat has tended to degrade the desirable physical properties of such polymeric materials. In the method and apparatus techniques of the present invention involving cold plasma lamination of two or more polymeric sheets, a number of advantages have been realizable over prior art techniques. One such advantage is that many different types of polymeric materials can be readily combined to form laminate materials having a wide variety of hybrid characteristics, as compared with the prior art. In addition, the energy utilization required in such systems of such cold plasma lamination techniques has been relatively low as compared with prior art systems, such as adhesive lamination and/or hot melt or hot co-extrusion techniques. Also, the cold plasma lamination methods and apparatus of the present invention are readily adaptable to more economical short production runs, and which have further involved relatively low scrap rates. Moreover, the use of solvents or monomers which would complicate bonding techniques and thereby reduce predictability and uniformity in the finished product, and which would also result in additional regulatory difficulties because of the utilization of such solvents or monomers, has been beneficially eliminated by means of the present invention. Accordingly, and in view of the above deficiencies and difficulties with prior art techniques, methods, handling and application apparatus, it is an object of the cold plasma lamination methods and apparatus of the present invention to reduce materially such difficulties and deficiencies associated with such prior art techniques. Additionally, it is an object of the cold plasma methods of the present invention to utilize cold plasma in exposing polymeric films thereto to effect adhesion between two such polymeric films. It is an additional object of the cold plasma methods of the present invention to utilize very high voltage sources across the anode and cathode of the cold plasma reactor apparatus hereof, and in particular to utilize voltages in the approximate range of 180,000 to 500,000 volts. It is also a further object of the cold plasma methods of the present invention to utilize pulses of very high frequency discharge, and in the radio wave frequencies, to activate such cold plasma, and in particular in preferred embodiments frequency pulses on the order of 12.56 megaHertz (12,560,000 cycles per second). It is a yet further object of the cold plasma methods of the present invention to maintain such gaseous cold plasma in the reactive zone for treating such polymeric sheet material under pressures of approximately 1.1 Torr or greater in such reaction zone. It is also a yet further object of the cold plasma methods of the present invention to utilize an approximately 1% to 2% noble gas in air mixture as the plasma medium, and to utilize argon gas in such concentrations in some preferred embodiments due to its superior plasma formation characteristics and relatively lower cost. It is yet another object of the cold plasma methods of the present invention to automatically control the power to the electrode and the pressure in the reactor relative to one another, and in particular to do so by means of an adiabatic pressure valve to function to minimize pressure changes and automatically to vary power in response to voltage variation at the anode, which variation results from changes in pressure. In addition to the above, it is a material object of the cold plasma reactor apparatus of the present invention to provide a pressurized plasma chamber substantially and/or relatively sealed on one side and defining one side of such chamber by means of the moving polymeric film which is simultaneously being treated by the plasma, and especially by means of the use of a rolling anode comprising a conductive roller to force the polymeric film down against the chamber end walls to form such plasma chamber. Moreover, it is a material object of the cold plasma reactor apparatus of the present invention to utilize insulating ingress and egress rollers on either side of the plasma chamber forming walls and also on either side of such conductive roller to facilitate movement of the film over the chamber forming walls and into and away from such conductive roller. It is also a material object of the cold plasma reactor apparatus of the present invention to utilize very sharp edged, thin cathodes, such as for example razor blade-like cathodes, to serve to generate such gaseous cold plasma. It is a yet further additional material object of the cold plasma reactor apparatus of the present invention to utilize two spaced and elongated rows of such razor thin cathodes on either side of gas inlets for the plasma generating gaseous medium to insure an evenly pressurized zone of plasma between the two rows of such razor thin cathodes. It is a yet further additional supplemental material object of the cold plasma reactor apparatus of the present invention to utilize selectedly and preferably uniformly gaseous medium inlets in the cathode frame to feed the plasma generating gaseous medium, preferably comprising an argon in air mixture, to the plasma treatment chamber. Further additional material objects of the cold plasma methods and apparatus of the present invention will become known to those of ordinary skill in the art upon the review of the following summary of the invention, brief description of the drawing, detailed description of preferred embodiments, appended claims and accompanying drawings. SUMMARY OF THE INVENTION The present invention is directed in one aspect thereof to various methods for laminating at least two polymeric sheets into a laminate composite having improved properties. A broad embodiment of such inventive method comprises a first step of providing a first sheet of polymeric material, and thereafter exposing a surface thereof to gaseous cold plasma to activate the surface thereof. Such activation is effectuated by the formation of free radicals on the surface thereof. Next, the activated surface of such first sheet is disposed into intimate proximity with a second sheet of polymeric material, and then the two sheets are pressed together, thereby to form a laminated composite, and to do so in the absence of any substantial heating of either sheet, and without the application of a separate adhesive to either of such sheets. In addition to the above methods, the present invention is also directed to a reactor apparatus for activating the surface of a first sheet of polymeric film material to form free radicals thereon, essentially utilizing the methods as described above. Such reactor apparatus includes a cold plasma generating cathode and anode disposed in respective spaced proximity with respect to the surface of the first sheet of polymeric material to be treated. Means for flowing a stream of cold plasma generatable gaseous medium past the cold plasma generating cathode are provided. Means are further provided for effectuating relative movement between such cathode and the surface of the first sheet of polymeric material for exposing a substantial portion of the surface of such first sheet to such cold plasma. In some of its more specific embodiments, the above cold plasma reactor apparatus includes a preferably pressurized cold plasma chamber which is defined by and substantially relatively sealed on one side thereof by the moving polymeric film which is simultaneously being treated by the plasma. In such preferred embodiments, a rolling anode in the form of a conductive roller is utilized to urge the treated film downwardly against the non-conductive chamber forming walls. In such preferred embodiments non-conductive ingress and egress rollers are disposed on opposite sides of such conductive roller to urge the film to be treated over such treatment chamber for treatment thereof by such cold plasma contained with such chamber. Further aspects of the cold plasma methods and apparatus are described in greater detail, infra.