TRIBOELECTRIC MITIGATOR COATING

Abstract

A multilayer film is disclosed, which includes: a top layer; a bottom layer; and a triboelectric mitigator coating on an external surface of at least one of the top layer and the bottom layer, wherein the bottom layer is a different material having different triboelectric properties than the top layer and the triboelectric mitigator coating is effective to reduce static discharge from the multilayer film when it is unwound from a roll. A method of reducing static discharge during unrolling of a roll of film and a method for providing the multilayer film are also disclosed.

Claims

1. A multilayer film comprising: a top layer; a bottom layer; and a triboelectric mitigator coating on an external surface of at least one of the top layer and the bottom layer, wherein: the bottom layer is a different material having different triboelectric properties than the top layer; the triboelectric mitigator coating comprises a polyol ester selected from the group consisting of polyethylene glycol ester, glycerol ester and sorbitan ester; and the triboelectric mitigator coating is effective to reduce static discharge from the multilayer film when it is unwound from a roll.

2. The multilayer film of claim 1, wherein the top layer and the bottom layer independently comprise a member selected from the group consisting of polymers and metal oxides.

3. The multilayer film of claim 1, wherein the top layer and the bottom layer independently comprise at least one polymer selected from the group consisting of polypropylene, polyethylene, polyurethane, polyester, polycarbonate, polyamide, polyimide, polyvinyl alcohol, silicone, polyacrylate and fluoropolymer.

4. The multilayer film of claim 1, wherein the top layer and the bottom layer independently comprise at least one metal oxide selected from the group consisting of silicone oxide and aluminum oxide.

5. The multilayer film of claim 1, further comprising at least one inner layer comprising a polyolefin.

6. (canceled)

7. The multilayer film of claim 1, further comprising an ink adhered to the multilayer film, wherein the triboelectric mitigator coating does not interfere with adhesion of the ink to the multilayer film or release of the ink from the multilayer film.

8. The multilayer film of claim 7, which is rolled about itself in a form of a roll.

9. A method of reducing static discharge during unrolling of a roll of film, said method comprising rolling the multilayer film of claim 1 to provide the roll of film to be subsequently unrolled for further use.

10. The method of claim 9, wherein the roll of film generates less static discharge when unrolled than a reference roll of film identically unrolled and identical to the roll of film except that the reference roll of film comprises none of the triboelectric mitigator coating.

11. A method for providing the multilayer film of claim 1, comprising the steps of: providing a film comprising the top layer and the bottom layer; providing a triboelectric mitigator solution; applying the triboelectric mitigator solution to the external surface of at least one of the top layer and the bottom layer; and drying the triboelectric mitigator solution on the external surface to provide the triboelectric mitigator coating and the multilayer film.

12. The method of claim 11, wherein the top layer and the bottom layer independently comprise a member selected from the group consisting of polymers and metal oxides.

13. The method of claim 11, wherein the top layer and the bottom layer independently comprise at least one polymer selected from the group consisting of polypropylene, polyethylene, polyurethane, polyester, polycarbonate, polyamide, polyimide, polyvinyl alcohol, silicone, polyacrylate and fluoropolymer.

14. The method of claim 11, wherein the top layer and the bottom layer independently comprise at least one metal oxide selected from the group consisting of silicone oxide and aluminum oxide.

15. The method of claim 11, further comprising providing at least one inner layer comprising a polyolefin.

16. (canceled)

17. The method of claim 11, wherein 1-15 wt. % of the polyol ester is provided in a solvent comprising water.

18. The method of claim 11, wherein the multilayer film further comprises an ink adhered to the multilayer film, and the triboelectric mitigator coating does not interfere with adhesion of the ink to the multilayer film or release of the ink from the multilayer film.

19. The method of claim 11, wherein the triboelectric mitigator solution is applied to the external surface by Mayer rod, gravure, reverse gravure, dip, spray, or transfer coating, and the external surface is dried at room temperature or using heated air.

20. The method of claim 11, wherein the multilayer film is provided as a roll.

21. The method of claim 11, wherein the triboelectric mitigator coating of the multilayer film comprises 0.75 to 20 wt. % of at least one polysorbate.

22. The multilayer film of claim 1, wherein the triboelectric mitigator coating comprises 0.75 to 20 wt. % of at least one polysorbate.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0036] The invention flows from the discovery that providing a coating of a triboelectric mitigator on at least one surface of a film with two dissimilar surfaces can eliminate the static discharge that occurs immediately upon film layer separation as in the act of unwinding a roll of film at high speed.

[0037] Multilayer Film

[0038] In a first aspect of the invention, a multilayer film is provided. The film comprises: a top layer; a bottom layer; and a triboelectric mitigator coating on an external surface of at least one of the top layer and the bottom layer, wherein the bottom layer is a different material having different triboelectric properties than the top layer and the triboelectric mitigator coating is effective to reduce static discharge from the multilayer film when it is unwound from a roll.

[0039] In certain embodiments, at least one inner layer is provided between the top layer and the bottom layer. It should be understood that the particular composition(s) of the inner layer(s) do(es) not constitute a limitation on the broadest aspects of this invention. In preferred embodiments, the at least one inner layer comprises a polyolefin. More preferably, the at least one inner layer comprises a polypropylene homopolymer, which is most preferably a high crystalline polypropylene. The polyolefin preferably comprises at least 50 wt % of the at least one inner layer.

[0040] In one non-limiting embodiment of this invention, the at least one inner layer is a core layer having a nominal thickness of 330 ga. (82.5 microns). In another non-limiting embodiment, the at least one inner layer is a core layer having a nominal thickness of 280 ga. (70 microns). The specific thickness of the at least one inner layer does not constitute a limitation on the broadest aspects of the invention. However, when a core layer is included, it is most preferably the thickest layer in the structure; far exceeding the thickness of the top layer and bottom layer in a three-layer structure. In another non-limiting embodiment of this invention, the at least one inner layer is a core layer including over 50% and more preferably approximately 70% of a high crystalline polypropylene, sold under the designation Braskem Inspire 6025 by Braskem America (Philadelphia, Pa., USA); approximately 7.5% of a propylene/ethylene copolymer including 2.5% by weight ethylene sold under the designation Braskem DS6D21 by Braskem America; CaCO.sub.3 (voiding agent) and TiO.sub.2 (whitening agent) in percentages conventionally employed to provide a white, voided structure, and a small percentage of one or more slip agents, such as Erucamide and Behenamide, as is well known in the art.

[0041] The number of layers of the multilayer film can be varied in accordance with the broadest aspects of this invention. However, the multilayer film should comprise at least two layers, namely the top layer and the bottom layer. At least one inner layer is optional. The number of inner layers can be 1, 2, 3, 4, 5 or more.

[0042] In preferred embodiments, the top layer and a bottom layer independently comprise a member selected from the group consisting of polymers and metal oxides. In certain embodiments, the top layer and the bottom layer independently comprise at least one polymer selected from the group consisting of polypropylene, polyethylene, polyurethane, polyester, polycarbonate, polyamide, polyimide, polyvinyl alcohol, silicone, polyacrylate and fluoropolymer. In certain embodiments, the top layer and the bottom layer independently comprise at least one metal oxide selected from the group consisting of silicone oxide and aluminum oxide.

[0043] As the invention relates to the mitigation of the triboelectric effect, the multilayer film has a structure that would suffer from (or suffer more from) the triboelectric effect but for the inclusion of the triboelectric mitigator in the film. Thus, the top layer and the bottom layer must be sufficiently different so as to result in the triboelectric effect during roll unwinding. In preferred embodiments, the top and bottom layers are constructed of substantially distinctly different materials not merely different levels of the same composition. The invention is particularly useful for films where one of the top layer and bottom layer comprises a polyamide, which is very high on the triboelectric scale with a high tendency to reject electrons to many other polymer surfaces including polypropylene, polyethylene, and polyurethane. However, the particular combinations of surface layers (i.e., top and bottom layers) is not limited since a difference in electron donor or acceptor behavior is always observed between two dissimilar materials.

[0044] The surface layers preferably have low electrical conductivity.

[0045] The triboelectric mitigator coating of the multilayer film is effective to reduce or eliminate static discharge during unrolling of a roll of the film. The triboelectric mitigator coating preferably comprises a polyol ester selected from the group consisting of polyethylene glycol (PEG) ester, glycerol ester and sorbitan ester. Suitable PEG esters include but are not limited to PEG fatty acid esters, PEG glycerol fatty acid esters, such as PEG oleate, PEG cocoate, PEG stearate, PEG acrylate, PEG glycerol cocoate and PEG 40-hydrogenated castor oil. Suitable glycerol esters include but are not limited to glycerol stearate, glycerol oleate, glycol acrylate and polyglycerol esters of fatty acids. Suitable sorbitan esters include but are not limited to sorbitan laurate, sorbitan stearate and sorbitan palmitic. The preferred polyol esters are from the class of polysorbates such Polysorbate 20 and Polysorbate 80 also referred to commercially as Tween 20 and Tween 80. This material also enables maintenance of food grade purity for films where that requirement is necessary or preferred.

[0046] The triboelectric mitigator is a liquid at normal use conditions and remains so even after the coating is dried to remove the water or alcohol used for dilution and accommodating the method of coating. A liquid coating would have been expected to be undesirable for application uses that require adhesion to the film. This is one of the surprising observations regarding the triboelectric mitigator coating of the invention. Even though it exists as a liquid at room temperature, the coating appropriately applied does not impact ink adhesion or ink application.

[0047] Ink application and adhesion can be determined as outlined in the test procedure below. A UV curing screen ink namely, Norcote 80-1019 Opaque Black UV Screen (Norcote International, Crawfordsville, IN, USA) is applied to the film by first placing a screen mesh 355 34-micron thread @ 90° 10×10 new CD flat frame with a 3×3 inch square image (Sefar Group Heiden, Switzerland) on the film surface. The ink is then drawn across the screen with a Ryonet Ergo Force Squeegee 6 inches wide with a 70 durometer (Ryonet Corp Vancouver, WA, USA). The screen is then removed. The ink area should be uniform in color. The extent of uniformity is assessed by the tester as a percentage of ink coverage called the Percent Ink Application. 100% indicates that the ink has been successfully transferred to the surface uniformly. Lesser percentages indicated the degree to which the ink was poorly transferred. The inked film is run through a UV light tunnel (Fusion LC6B Heraeus Group, Hanau, Germany) with a D bulb at 50 ft/min where the ink is completely cured. Four types of adhesive tape applied to the ink surface, namely, 800, 810, 600, and 610 Scotch tape (3M Corporation Maplewood, MN, USA). The tape is pulled from the ink surface at a moderate rate and a 45° angle. It is desirable that on average over the four tapes, at least 90% of the ink remains adhered to the film surface, called the Percent Ink Adhesion. The percentage is assessed visually by the tester.

[0048] The triboelectric mitigator coating preferably does not interfere with ink adhesion to or ink release from the multilayer film.

[0049] The multilayer film can be clear or opaque.

[0050] Method of Reducing Static Discharge

[0051] A second aspect of the invention is a method of reducing static discharge during unrolling of a roll of film, said method comprising rolling the inventive film to provide the roll of film to be subsequently unrolled for further use. The roll of film preferably generates less static discharge when unrolled than a reference roll of film identically unrolled and identical to the roll of film except that the reference roll of film comprises none of the triboelectric mitigator coating.

[0052] Method of Providing a Multilayer Film

[0053] A third aspect of the invention is a method for providing the inventive film, comprising the steps of: providing a film comprising the top layer and the bottom layer; providing a triboelectric mitigator solution; applying the triboelectric mitigator solution to the external surface of at least one of the top layer and the bottom layer; and drying the triboelectric mitigator solution on the external surface to provide the triboelectric mitigator coating and the multilayer film.

[0054] The triboelectric mitigator is preferably applied as a coating to at least one external surface of the film. The coating solution is created by dissolving the mitigator in a solvent, such as water, to a sufficient concentration which depends on the method of application. The coating solution optionally comprises a small amount of defoamer. The coating solution optionally further comprises an alcohol, such as methanol, ethanol, propanol or butanol, to aid in wetting.

[0055] Suitable coating methods include but are not limited to: Mayer rod, gravure, reverse gravure, dip, spray and transfer coating. The coating is dried to remove the water or alcohol at room temperature or using heated air. The method of drying is not critical to the invention.

[0056] The lower limit of addition is determined by the reduction of electric discharge at the point of film separation upon unwind. The upper limit is determined by achievement of acceptable ink application and adhesion. In certain embodiments, the triboelectric mitigator solution comprises 0.75 to 20 wt. %, or 1 to 15 wt. %, or 2 to 12 wt. % or 5 to 10 wt. % of the triboelectric mitigator.

[0057] Preferably, the triboelectric mitigator coating does not interfere with ink adhesion to or ink release from the multilayer film of the invention. For example, at least 50% or 75% or 100% of ink adheres to the multilayer film and at least 50% or 75% or 100% of ink releases from the multilayer film of the invention.

EXAMPLES

[0058] The invention will be illustrated in more detail with reference to the following Examples, but it should be understood that the present invention is not deemed to be limited thereto.

[0059] Commercial film (PPGBCE 65) from TaghleefIndustries was used in the examples. The film has a urethane coating on one surface and a polyamide polymer on the opposing surface. The urethane coating is designed for excellent printability.

[0060] The urethane surface of the film was coated with a solution of water and polyolester. The specific polyol ester was polysorbate 20 (Organic Health and Beauty Inc. Westchester, IL, USA). The coating solution was fed to a full scale commercial reverse gravure coating head. Because this coating method does not deliver a metered amount of coating to the surface, the quantity of the polysorbate 20 delivered to the surface is not known in absolute terms. However, by changing the concentration the relative amount can be inferred. The coated rolls were approximately 500 kg, contained 6400 m of film, and were 1.2 meters wide. The experimental control roll was not coated with a polysorbate 20 solution.

[0061] The rolls were coated and dried at 60° C. and allowed to sit for 3 weeks. The rolls were then rewound at 600 m/min. The extent of electric discharge at the point of film separation from the roll was observed. The results are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Triboelectric Mitigator Effect on Static Discharge Concentration of polysorbate 20 Static in the coating Discharge % UV ink % UV ink Example solution (wt. %) at Rewind application adhesion 1 0 Severe 100 100 2 0.1 Severe 100 100 3 0.5 Severe 100 100 4 1 Moderate 100 100 5 5 None 100 100

[0062] Interestingly the static decay was measured on the film that eliminated the static discharge at rewind and found to be rather low, only achieving 500V of decay in 2 to 8 seconds. This is a very modest dissipation capability, yet it was very effective at eliminating the triboelectric effect from taking place at the instant where the film surfaces separate.

[0063] Laboratory coatings using a Mayer rod coating technique on hand sheet samples of film showed similar UV ink Adhesion results. Because higher concentrations were difficult to run using the full scale commercial reverse gravure technique, the laboratory coating process was used to demonstrate the problem of excessive coating on ink application and adhesion. The coating was applied with a #4 Mayer rod winding. The coating was dried in a convection oven for 2 minutes at 80° C. Table 2 below shows the result of higher % polysorbate 20 in the water solution that could not be run on the commercial coater.

TABLE-US-00002 TABLE 2 Ink Application and Adhesion Wt. % polysorbate % ink % ink Example 20 in water application adhesion 6 1 100 100 7 5 100 100 8 10 100 100 9 20 75 50 10 30 60 50 11 50 25 50

[0064] While the invention has been described in detail and with reference to specific examples thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.