WO2010104626A1

WO2010104626A1 - Coated polymer films

Info

Publication number: WO2010104626A1
Application number: PCT/US2010/022757
Authority: WO
Inventors: Pang-Chia Lu; Robert J. Eller
Original assignee: Exxonmobil Oil Corporation
Priority date: 2009-03-13
Filing date: 2010-02-01
Publication date: 2010-09-16

Abstract

Provided herein are coated polymer films. The polymer substrates are treated on one side with at least one treatment selected from flame treatment and plasma treatment and then coated with a solvent-based coating. The coated polymer films exhibit reduced roll blocking. The coated surface also provides for excellent ink adhesion.

Description

COATED POLYMER FILMS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Application Serial No. 61/160,159, filed March 13, 2009, the contents of which are incorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] This disclosure relates to coated polymer films. More particularly, this invention relates to polymer films that have been treated and coated on one side and are suitable for printing applications.

BACKGROUND OF THE INVENTION

[0003] Plastic films can be used as substrates for various printing applications, such as toner based electrostatic printing applications. Liquid toners are often used in printing processes, as the liquid medium allows for the use of very fine dye particles without the concern that the particles may become airborne during the printing process. Additionally, the use of liquid toners allows for high resolution copies to be made without needing the high temperatures that are required to fuse dry toners.

[0004] In order to make the film more receptive to inks and to provide better ink adhesion, the plastic film is often coated. Coatings are generally chosen based on how well they adhere to the film substrate and their affinity for the toner ink particle polymers. Often solvent-based coatings are used and applied to the plastic film in a multi-step process. The process involves unwinding the film, applying the coating uniformly at the desired thickness, waiting for the coating to dry, and rewinding the film into a uniform roll. [0005] In the solvent coating process, the coating polymer is dissolved in a solvent and the solvent is applied to the film. The solvent is evaporated once the coating is applied leaving behind a coated film. However, as the coating begins to dry it acts as a barrier to the remaining solvent making it difficult to drive off the last traces of residual solvent. Solvent- based coatings are often sensitive to ambient conditions and often absorb atmospheric moisture which may make the film tacky even after drying, thus leading to blocking problems.

[0006] U.S. Patent No. 5,827,627 discloses liquid toner printable thermoplastic films. The films are coated with an ethylene-acrylic acid copolymer based coating capable of electrostatic imaging with liquid toner. [0007] U.S. Patent No. 6,790,514 discloses substrates suitable for printing toner images thereon. The substrates comprise a sheet of plastic film; an underlayer coating which comprises a polymer chosen from the group consisting of amine terminated polyamide, a silane coupling agent, and amino propyl triethoxy silane; an overlayer coating comprising a polymer selected from the group consisting of ethylene acrylic acid copolymer, polyvinyl pyridine, and styrene butadiene copolymer, the overlayer coating is directly on the underlayer coating and has a surface to which a toner image can be fixed and fused. [0008] There is a need for a plastic film that can be coated and used in printing applications that has good ink adhesion and exhibits little to no roll blocking. The coated film should also exhibit good shelf life. SUMMARY OF THE INVENTION

[0009] In one aspect, this disclosure relates to a film comprising (a) a polymer substrate having a first side and a second side, wherein the first side has been surface treated with at least one treatment selected from flame treatment and plasma treatment and the second side has a surface energy less than or equal to 33 dynes/cm; and (b) a solvent-based topcoat that is on the first side of the polymer substrate.

[0010] In one embodiment, and in combination with any of the above disclosed aspect, the film further comprises a second coating that is intermediate the topcoat and the first side of the polymer substrate. [0011] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the film further comprises a print-image on the topcoat. [0012] In another aspect, this disclosure relates to a method of reducing back-side blocking on a film roll comprising (a) providing a polymer substrate, having a front side and a back side; (b) treating the surface of the front side of the polymer substrate with at least one treatment selected from flame treatment and plasma treatment; and (c) applying a topcoat to the front side of the polymer substrate, wherein the topcoat is a solvent-based coating. In one embodiment, the method further comprises the step of applying an undercoating to the front- side of the polymer substrate prior to applying the topcoat. The method may also further comprise the step of printing an image on the topcoat. [0013] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the first side of the polymer substrate has a surface energy greater than or equal to 35 dynes/cm. [0014] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the solvent-based topcoat is selected from a low molecular weight polyamide, an ethylene acrylic acid copolymer, and blends thereof.

[0015] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the second coating or undercoating comprises an amine -terminated polyamide, a silane coupling agent, and a silane.

[0016] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the polymer substrate comprises polypropylene, polyethylene, ethylene- propylene copolymers, propylene-butene copolymers, ethylene-propylene-butylene terpolymers, or blends thereof. [0017] In one embodiment, and in combination with any of the above disclosed aspects or embodiments, the polymer substrate is biaxially oriented.

[0018] These and other features, aspects, and advantages of the present disclosure will become better understood with regard to the following description and appended claims. DETAILED DESCRIPTION OF THE INVENTION [0019] Various specific embodiments, versions and examples of the invention will now be described, including preferred embodiments and definitions that are adopted herein for purposes of understanding the claimed invention. While the following detailed description gives specific preferred embodiments, those skilled in the art will appreciate that these embodiments are exemplary only, and that the invention can be practiced in other ways. For purposes of determining infringement, the scope of the invention will refer to any one or more of the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. Any reference to the "invention" may refer to one or more, but not necessarily all, of the inventions defined by the claims. [0020] As used herein, "intermediate" is defined as the position of one layer of a film structure wherein said layer lies between two other identified layers. In some embodiments, the intermediate layer may be in direct contact with either or both of the two identified layers. In other embodiments, additional layers may also be present between the intermediate layer and either or both of the two identified layers. For example, in some embodiments an undercoating layer may be intermediate the top coating layer and the polymer substrate. [0021] Solvent-based coatings suitable for electrostatic printing, such as materials with acidic or basic functional groups, tend to absorb atmospheric moisture upon aging, making the coating sticky. This sticky coating tends to block to the backside of the polymer substrate if the backside has a relatively high surface energy. To overcome the aging problem associated with solvent-based coated films used in liquid toner printing processes, the backside of the polymer substrate to be coated (i.e., the side of the polymer substrate that is not coated) should have a very low surface energy to minimize the affinity of the front-side functional coating, such as a polyamide coating, to the backside surface during aging or when the film is in a moist environment. When the backside of the polymer substrate has a low surface energy and when the front-side coating is properly dried, the coating will have low affinity to the backside of the film substrate thus reducing roll blocking. [0022] In order to get good coating adhesion to the polymer substrate, the polymer substrate's surface is usually treated with corona treatment in order to create a high surface energy. Without corona treatment, coatings have somewhat lower adhesion to the substrate and often peel. However, when the front-side of the polymer substrate is subjected to corona treatment this indirectly also causes backside substrate treatment. This backside treatment thus increases the backside's surface energy, often resulting in aged film blocking. [0023] Provided herein is a method to minimize the backside blocking when the polymer substrate is coated with a solvent-based coating. The method to reduce back-side blocking on a film roll comprises the steps of providing a polymer substrate, having a front side and a back side; treating the surface of the front side of the polymer substrate with at least one treatment selected from flame treatment and plasma treatment; and applying a topcoat to the front side of the polymer substrate, wherein the topcoat is a solvent-based coating. [0024] When only flame treatment or plasma treatment is used to treat the polymer substrate, only the surface energy of the side of the film being treated is increased. The surface energy on the other side of the polymer substrate will not be affected. The flame and/or plasma treatment still allows for good coating adhesion of the solvent-based coating to the polymer substrate, and during aging there is no film blocking since the coating does not block to the untreated surface of the polymer substrate that has a low surface energy. The coated surface also provides for excellent ink adhesion.

[0025] Also provided herein are film substrates suitable for printing a toner image. The film comprises a polymer substrate having a first side and a second side, wherein the first side has been surface treated, and a solvent-based coating applied to the first side of the polymer substrate. The coating is suitable for printing thereon. In some embodiments, a second coating is intermediate the polymer substrate and the solvent-based coating. [0026] In some embodiments one of the outermost surfaces of the film substrate may be metallized. Application of a metal coating layer may be accomplished by vacuum deposition, or any other metallization technique, such as electroplating or sputtering. The metal may be aluminum, or any other metal capable of being vacuum deposited, electroplated, or sputtered, such as, for example, gold, zinc, copper, silver, chromium, or mixtures thereof. Polymer Substrate [0027] The polymer substrate to be coated may be a single layer film or a multilayer film. In some embodiments, the polymer substrate is a multilayer film that comprises a core layer, one or more optional tie layers, and one or more skin layers. For example, in some embodiments, the polymer substrate may comprise a core layer, one or more skin layers on either side of the core layer, and/or one or more tie layers disposed between the core layer and the one or more skin layers.

[0028] The polymer substrate may include any film-forming polyolefm. For example, the film substrate may comprise one or more polymers selected from polypropylene, isotactic polypropylene ("iPP"), high crystallinity polypropylene ("HCPP"), ethylene-propylene copolymers, ethylene propylene random copolymer, ethylene-propylene block copolymers, propylene-butene copolymers, ethylene-propylene -butylene terpolymers, polyethylene, high density polyethylene ("HDPE"), medium density polyethylene ("MDPE"), low density polyethylene ("LDPE"), linear low density polyethylene ("LLDPE"), syndiotactic polypropylene (sPP), polyesters, polyamides, and combinations thereof. The polymers may be produced by Ziegler-Natta catalyst, metallocene catalyst, or any other suitable means. [0029] In one embodiment, the polymer substrate comprises a LDPE having a density in the range of about 0.915 to 0.93 g/cm and a melt index (I_2-16) ("MI") in the range of about 0.1 to 15 g/10 min, or in the range of 0.3 to 10 g/10 min, or about 7 g/10 min. A polymer's MI (I_2-16) is the melt flow rate at 19O⁰C under a load of 2.16 kg, and is determined according to ASTM D- 1238, condition E. MI is reported in the units of g/10 min, or the numerically equivalent units of dg/min. The polymer's density may be measured per the ASTM D-792 test method.

[0030] In another embodiment, the polymer substrate comprises a LLDPE having a density in the range of about 0.90 g/cm³ to about 0.94 g/cm³, or more preferably in the range of about 0.910 g/cm³ to about 0.926 g/cm³. The LLDPE may have a melt index in the range of about 1 to about 10 g/10 min, or in the range of 0.5 to 10 g/10 min. The LLDPE may be a copolymer of ethylene and a minor amount of a higher olefin comonomer containing 4 to 10 carbon atoms, such as for example, butene-1, hexene-1, or octene-1. [0031] In yet another embodiment, the polymer substrate comprises a MDPE having a density in the range of about 0.926 to about 0.940 g/cm³, or in the range of about 0.93 to about 0.94 g/cm³.

[0032] In a further embodiment, the polymer substrate comprises a HDPE. HDPE is a substantially linear polyolefm having a density of about 0.940 g/cm³ or more, or preferably 0.952 g/cm³ or more. The HDPE may have a density in the range of about 0.952 g/cm³ to about 0.962 g/cm³. The HDPE may have a MI in the range of about 0.2 to about 10.0 g/10min, or preferably in the range of about 0.5 to about 2.0 g/10min, and a melting point in the range of about 13O⁰C to about 148⁰C. The polymer's melting point may be measured by the Differential Scanning Calorimetry ("DSC") procedure described herein. [0033] In one embodiment, the polymer substrate comprises a syndiotactic polypropylene ("sPP") having an isotacticity of less than 25%, or less than 15%, or less than 6%. The mean length of the syndiotactic sequences may be greater than 20, or greater than 25. The isotacticity of the polymer may be determined by ¹³C nuclear magnetic resonance ("NMR"). [0034] In another embodiment, the film- forming polyolefm may be an iPP which has an isotacticity in the range of about 93% to about 99%, a crystallinity in the range of about 70% to about 80%, and a melting point in the range of about 145⁰C to about 167⁰C. The polymer's percent crystallinity may be measured by the DSC procedure described herein. The thermal energy for the highest order of propylene is estimated at 189 J/g (i.e., 100% crystallinity is equal to 189 J/g).

[0035] Polypropylene copolymers, if used in the film substrate, may include one or more comonomers. Preferably the comonomer is selected from one or more of ethylene or butene. The propylene is generally present in such co- or terpolymers at greater than 90 wt%. [0036] The procedure for DSC determinations is as follows. About 0.5 grams of polymer is weighed and pressed to a thickness of about 15 to 20 mils (about 381-508 microns) at about 140-150⁰C, using a "DSC mold" and MYLAR™ film as a backing sheet. The pressed polymer sample is allowed to cool to ambient temperatures by hanging in air (the MYLAR™ film backing sheet is not removed). The pressed polymer sample is then annealed at room temperature (about 23-25⁰C) for about 8 hours. At the end of this period, a 15-20 mg disc is removed from the pressed polymer sample using a punch die and is placed in a 10 microliter aluminum sample pan. The disc sample is then placed in a DSC (Perkin Elmer Pyris 1 Thermal Analysis System) and is cooled to about -100⁰C. The sample is heated at about 10°C/min to attain a final temperature of about 200⁰C. The thermal output, recorded as the area under the melting peak of the disc sample, is a measure of the heat of fusion and can be expressed in Joules per gram (J/g) of polymer and is automatically calculated by the Perkin Elmer system. Under theses conditions, the melting profile may show one or more maxima and the maxima at the highest temperature is taken as the melting point within the range of melting of the disc sample relative to a baseline measurement for the increasing heat capacity of the polymer as a function of temperature. [0037] The polymer substrate may include one or more additives. Examples of useful additives include, but are not limited to, opacifying agents, pigments, colorants, cavitating agents, slip agents, antioxidants, anti-fog agents, anti-static agents, anti-block agents, moisture barrier additives, gas barrier additives, hydrocarbon resins, hydrocarbon waxes, fillers such as calcium carbonate, diatomaceous earth, and carbon black, and combinations thereof. Such additives may be used in effective amounts, which vary depending upon the property required. If the polymer substrate is a multilayer film, the additive(s) may be included in any one or more of the layers. [0038] The film substrate may be monoaxially or biaxially oriented. Orientation in the direction of extrusion is known as machine direction ("MD") orientation. Orientation perpendicular to the direction of extrusion is known as transverse direction ("TD") orientation. Orientation may be accomplished by stretching or pulling a film first in the MD followed by the TD. Orientation may be sequential or simultaneous, depending upon the desired film features. Preferred orientation ratios are commonly from between about three to about seven times in the MD and between about four to about ten times in the TD. [0039] Blown films may be oriented by controlling parameters such as take up and blow up ratio. Cast films may be oriented in the MD direction by take up speed, and in the TD through use of tenter equipment. Blown films or cast films may also be oriented by tenter- frame orientation subsequent to the film extrusion process, in one or both directions. Typical commercial orientation processes are BOPP tenter process and LISIM technology. Surface Treatment

[0040] The outer exposed surfaces of the film substrate may be surface-treated to increase the surface energy of the film. The surface treatment may aid in rendering the film more receptive to metallization, coatings, printing inks, and/or lamination. Surface treatment can be carried out according to any method known in the art, such as, corona discharge, flame treatment, plasma treatment, chemical treatment, or treatment by means of a polarized flame. [0041] In preferred embodiments, only one side of the polymer substrate is surface treated. Therefore, preferred methods of surface treating the film include those that only treat the side of the film that the surface treatment is directed towards and do not indirectly treat the other side of the film. Preferred surface treating methods include flame treatment and plasma treatment. In preferred embodiments, corona treatment is not used to treat the surface of the film.

[0042] In some embodiments, the film may first be treated, for example by flame treatment, and then be treated again in the metallization chamber, for example by plasma treatment, immediately prior to being metallized. [0043] In preferred embodiments, the polymer substrate has a first side and a second side, wherein only the first side of the polymer substrate is surface-treated. The first side is preferably the side of the polymer substrate that is to be coated, as described below. The first side of the polymer substrate may have a surface energy greater than or equal to 35 dynes/cm, or greater than or equal to 37 dynes/cm, or greater than or equal to 38 dynes/cm, or greater than or equal to 40 dynes/cm. In some embodiments, the first side may have a surface energy in the range of 35 to 50 dynes/cm, or in the range of 37 to 45 dynes/cm, or in the range of 38 to 42 dynes/cm. The second side of the polymer substrate may have a surface energy that is less than 35 dynes/cm, or less than or equal to 34 dynes/cm, or less than or equal to 33 dynes/cm, or less than or equal to 32 dynes/cm, or less than or equal to 30 dynes/cm, or less than or equal to 28 dynes/cm. In some embodiments, the second side of the polymer substrate has a surface energy in the range of 20 to 34 dynes/cm, or in the range of 25-33 dynes/cm, or in the range of 27 to 32 dynes/cm. The surface energy of the film may be measured by ASTM D2578. Coating [0044] The polymer substrate is coated on one or both sides with a coating composition which may be applied by any means known in the art as a continuous film or as a pattern. In preferred embodiments, only one side of the polymer substrate is coated. Preferred coatings include solvent-based coatings. Usually coatings used for improved ink adhesion contain acidic or basic functional groups, such as ethylene acrylic acid copolymers, polyamides, polyurethane, and epoxy based coatings. In coated areas, the application rate of the coating can be between 0.05 and 5 grams/msi. Economics generally favor thinner coating layers, however, one might choose to use thicker layers of coatings to impart stiffness, moisture or gas barrier, seal strength, or optical effects (e.g., color, opacity, or a matte finish) to the plastic film. [0045] Before applying the coating composition to the polymer substrate, the outer surface of the film to be coated may be treated to increase its surface energy. This treatment may help to ensure that the coating layer will be strongly adhered to the outer surface of the film, and thus reduce the possibility of the coating peeling or being stripped from the film. Preferred methods of treatment include those that only treat one side of the polymer substrate, i.e., those that only treat the side of the polymer substrate to be coated and do not treat the other side of the polymer substrate. In preferred embodiments a method of surface treatment is used such that only the surface energy of the side of the film to be coated is increased, and the surface energy of the opposite side of the film is not affected. Preferred treatment methods include flame treatment and plasma treatment. After surface treatment, the coating composition may then be applied thereto. In embodiments where the coating is applied to the surface of the film substrate that has been metallized, the metal layer may be surface treated prior to applying coating, although such treatment is typically not necessary due to the relatively high surface energy of freshly metallized surface. [0046] In some embodiments there is a first coating and a second coating, wherein the second coating is intermediate the polymer substrate and the first coating. [0047] The coatings are preferably applied by a solution or emulsion coating technique, but may also be applied by co-extrusion, and/or lamination. In preferred embodiments, the coating is a solvent-based coating. The coating composition may be applied to the film as a solution. For example, an aqueous or organic, e.g. ethanol, ketone, ester, etc., solvent solution may be used. If a double coating apparatus is available, the first coating may be applied immediately over a dried second coating.

[0048] The coating may be applied by any one of the available methods of coating, for example, using a rotogravure coating system. Alternatively a flexographic printing system can be used to apply the coating with the settings of the printer controlling the thickness of the coating. However any other suitable coating process (double blade direct applicator, etc.) can be used. Other solution concentrations and other variations on the above described method can also be used. [0049] After application of the solution coating the coating may be dried, preferably by using the dryer integral in the coating apparatus. Other drying methods can also be used.

[0050] The coating composition may be applied in such an amount so that there will be deposited upon drying a smooth, evenly distributed layer. The coating may be dried by hot air convection, electron beam, radiant heat (e.g., ultraviolet or microwave), or by any other conventional means. Generally, the coating composition is on the order of 0.1 μm to 25 μm in thickness, or in the range of about 0.381 μm to about 16.8 μm, or in the range of 0.31g to 5.43g of coating per square meter of film. First Coating [0051] In preferred embodiments, one side of the polymer substrate is coated with a first coating. This first coating is the "topcoat" or the "overcoating." Preferably the first coating is a solvent-based coating. In preferred embodiments, the first coating contains a material having acidic or basic functional groups. The first coating may comprise ethylene acrylic acid copolymers, polyamides, polyurethane, and epoxy based coatings. [0052] In one embodiment, the first coating comprises an amine terminated polyamide. Examples of commercially available amine terminated polyamide include Macromelt 6239 (Henkel), Versamid 973 (Henkel), and Uni-Rez 2200 (Union Carbide). In one embodiment, the polyamide has a ball and ring softening point in the range of 50 to 200⁰C, or in the range of 70 to 18O⁰C, or in the range of 80 to 15O⁰C; according to ASTM D36. In another embodiment, the polyamide has a viscosity at 225⁰C in the range of about 50 to 100 cPs.

[0053] In another embodiment, the first coating layer may be a urethane coating. Suitable urethane topcoat materials include copolymers of a glycol, a propanoic acid, and a polyisocyanate, for example a copolymer of a polybutylene glycol, a dimethylolpropionic acid, and isophorone diisocyanate, e.g. a copolymer of α-hydro-ω-hydroxypoly(oxy-l,4- butanediyl), 3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid, and 5-isocyanato-l- (isocyanatomethyl)-l,3,3-trimethylcyclohexane.

[0054] In a further embodiment, the first coating is an epoxy coating. For example, the epoxy coating may be the reaction product of an epoxy resin and an acidified aminoethylated vinyl polymer, which is used as a hardener or curing agent. The epoxy resin may be glycidyl ethers of polyhydroxy compounds. Typical polyhydroxy compounds which may be used include bisphenol A, ring substituted bisphenol A, resorcinol, hydroquinone, phenol- formaldehyde, novolac resins, aliphatic diols, such as ethylene glycol, propylene glycol, 1 ,A- butanediol, 1,6-hexane-diol, glycerol, lower alkyl hydantoins, and mixtures thereof. For example, the epoxy resin may be made by the glycidation reaction between epichlorohydrin and bisphenol A. Epoxy resins of this type are commonly classified by their epoxy equivalent weight ("EEW"), which is defined as the weight of resin in grams which contains one gram equivalent of epoxy groups. Resins with an EEW in the range of 170 to 280, or in the range of 180 to 210 may be used. [0055] In yet another embodiment, the first coating comprises a carboxylic acid copolymer component. The carboxylic acid copolymer component of the coating, typically, comprises a copolymer of acrylic acid or methacrylic acid or ester of those acids. The acrylates contemplated contain lower alkyl groups such as those ranging from about 1 to about 16 carbon atoms, specific examples include methyl, ethyl, butyl, lauryl and stearyl. The acrylic copolymer includes a functional comonomer, typically having an average molecular weight of at least about 10,000. The acrylic copolymer usually includes about 5 wt% of this comonomer. [0056] A particularly useful thermoplastic copolymer is ethylene-acrylic acid copolymer. The ethylene copolymer may be a copolymer of about 65 to 95 wt%, typically about 75 to about 85 wt% of ethylene and, for example, about 5 to 35 wt%, typically about 15 to about 25 wt% of acrylic acid or methacrylic acid. The copolymer may have a number average molecular weight of about 2,000 to 50,000, preferably about 4,000 to 10,000. [0057] The copolymer is often supplied as a solution or fine dispersion of an ammonium salt of the copolymer in an ammoniacal water solution. When the copolymer is dried, ammonia is given off and the ionized and water sensitive carboxylate groups are converted to largely unionized and less water sensitive free carboxyl groups.

[0058] A suitable ethylene-acrylic acid copolymer coating is available commercially under the tradename Michem®, particularly Michem®-4983, an aqueous dispersion having 25% solids content and obtained from a reaction between 15 mole % acrylic and 85 mol % ethylene, by Michelman Corporation. Ethylene-acrylic acid is, typically, produced by high pressure copolymerization of ethylene and acrylic acid. When ethylene is copolymerized with acrylic acid, the molecular structure is significantly altered by the random inclusion of bulky carboxylic acid groups along the backbone and side chains of the copolymer. The carboxyl groups are free to form bonds and interact with any poly substrate. Another commercially available ethylene-acrylic acid copolymers is Primacor™ 4983 sold by Dow Chemical Co. [0059] The total amount of the copolymer present in the entire coating composition can range from about 15% to about 100%, or from about 30% to about 95% by weight based on the entire weight of the coating composition. [0060] The coating can also include a mixture of copolymer and a polymer of a carboxylic acid containing vinylic unsaturation and an acrylic polymer. A specific concentration of polymer to copolymer is in the range of about 5 to 50% polymer and from about 95 to about 50% copolymer based on the weight of the copolymer. [0061] In one embodiment, the coating can contain a neutralizing metal ion, for example, an alkali metal. In practicing this aspect of the invention there is added to the solution or dispersion of the copolymer an amount of ions of at least one metal from Group Ia, Ha or lib of the Periodic Table, preferably, sodium, potassium, lithium, calcium or zinc ions, and most preferably sodium ions, e.g., in the form of their hydroxides. The quantity of such metallic ions may be in the range sufficient to neutralize, for example, about 2 to 80%, preferably about 10 to 50% of the total carboxylate groups in the copolymer. As an example, sodium ions are added as sodium hydroxide. The amount of sodium hydroxide added corresponds to the foregoing percentages of carboxylate groups which are to be neutralized, for example, about 0.33 to 8.8 phr, preferably about 1.1 to 5.5 phr, where "phr" stands for parts by weight per hundred parts of the total resin, which is the same as ethylene copolymer when no other resin is present. For the purpose of determining the phr of various additives present in the coating, all the carboxylate groups of the ethylene copolymer are assumed to be in their free carboxyl (--COOH) form. [0062] In addition to the partially neutralized ethylene copolymer, the first coating may also contain an antiblock/slip agent. Typically, this is a relatively large particle size wax. Wax is known to be a low melting organic compound of relatively high molecular weight that is generally a solid at room temperature. The wax provides further lubricity properties. Contemplated waxes are natural wax such as animal wax including beeswax, lanolin and shellac wax, vegetable wax such as carnauba wax, candelilla, bayberry and sugar cane wax, mineral waxes such as fossil or earth wax including oxocerite, ceresin and montan wax. Synthetic waxes are also contemplated such as ethylenic polymers and polyol ether-esters, chlorinated naphthalenes and hydrocarbon waxes such as those derived from the Fischer- Tropsch synthesis. Both natural and synthetic microcrystalline wax are also contemplated. A particularly preferred wax is carnauba wax. The wax may be present in the coating in an amount of in the range of about 1 to about 20%, or in the range of about 2 to about 10% based on the entire weight of the coating.

[0063] In addition to functioning as an anti-blocking material, the wax when incorporated into the first coating may also function to improve the "slip" properties of the films coated therewith, i.e., the ability of a film to satisfactorily slide across surfaces at about room temperatures.

[0064] Usually, the first coating also contains a relatively inert particulate filler additive. A filler which has found specific utility in the first coating is fumed silica. The fumed silica is composed of particles which are agglomerations of smaller particles and which have an average particle size of in the range of about 2 to 9 microns, or in the range of about 3 to 5 microns. Generally any finely divided inorganic solid materials such as silica is contemplated as a useful filler for purposes of the present coating. These include talc, calcium carbonate, diatomaceous earth, calcium silicate, bentonite and clay. The total amount of filler typically ranges from about 0.1% to about 80%, or from about 0.3% to about 7.0% based on the entire weight of the coating. When a clear film is contemplated, the particulate concentration in the coating will be relatively low so as to not hinder clarity, for example it may range from about 0.1% to about 10%, or from about 0.3% to about 7.0%. The particulates are generally small in size, typically ranging from about 1 μm to about 10 μm, specifically from about 3 μm to about 7 μm. Further examples of fillers include kaolin, silica, aluminum silicates, clay and talc. Pulp is also contemplated.

[0065] Preferred among the foregoing fillers are those that function as antiblock/slip agents. Silica is a specific example of a filler which is found to function as in this manner. [0066] Opacity enhancing particulates may also be employed. These are relatively inert substances. Calcium carbonate is extensively used in thermoplastics, it is relatively inexpensive and easy to use. It can be used in its natural form but "precipitated calcium carbonate" which is prepared by chemical processes can be employed. Sometimes, particles of calcium carbonate are coated with a resin to reduce plasticizer absorption and this form can also be employed.

[0067] The filler can also include pigment-imparting particulates. Pigments contemplated are organic or inorganic substances. Typical pigments include carbon black and titanium dioxide. Calcium carbonate can also act as a pigment. Other pigments not to be excluded by this invention are metallic pigments such as particles of aluminum, copper, gold, bronze or zinc. These pigments are usually flake shaped particles which reflect light when incorporated into the coating vehicle.

[0068] The fillers, including inert particulate slip/antiblock agents, opacifying agents and/or pigments can be used in combination, depending upon the desired degree of translucency or opacity. Typically when the opacifying particulates and/or pigments are used the concentration is less than about 70% of the total particulate concentration of the coating, specifically about 20% to about 50% of the total particulate concentration of the coating. [0069] Further specific examples of particulates which may be employed in addition to those noted above include acetylene black, alpha cellulose, aluminum silicates, barium sulfate, calcium silicate, calcium sulphate, cellulose, clays, diatomite, glass flake, keratin, lignin, lithophone, mica, microballoons, molybdenum disulfide, nepheline syenite, paper, pulp, quartz, shell flour, talc, vermiculite and wood.

[0070] Other optional additives which can be used, include cross-linking agents such as melamine formaldehyde resins which may be present in an amount, for example, of less than about 25 wt%, anti-static agents such as poly(oxyethylene) sorbitan monooleate which may be present in an amount, for example, of less than about 10 wt% and antifoam agents such as silicone oil or fluorocarbon which may be present in an amount of less than about 0.1 wt%, based on the entire weight of the coating. [0071] The coating may be made by combining all the ingredients sequentially or at the same time and mixing or blending them at room temperature and atmospheric pressure conditions in a conventional mixing apparatus. Typically, the coating is in an aqueous media having a solids content of about 1 to about 60%, specifically about 5 to about 50% based on the entire weight of the final coating composition. [0072] The first coating may be applied to the polymer substrate as an aqueous emulsion in-line after the machine direction orientation but before the transverse direction orientation. This procedure is described in U.S. Patent No. 5,451,460. The uniaxially drawn film may be subjected to surface treatment prior to coating. [0073] Alternatively, the coating may be applied off-line, by any conventional method. For example, the film can be coated by roller coating, spray coating, slot coating or immersion coating. Gravure roll coating or reverse direct gravure coating are acceptable methods. The excess coating solution can be removed by squeeze rolls or doctor knives. [0074] Regardless of these methods, the amount should be such that upon drying a smooth, evenly distributed layer is obtained. A typical coating weight ranges from about 0.1 to about 10 g/m².

[0075] In one embodiment of the invention, the coating can be applied by coextrusion. [0076] For some applications it is useful to reduce the degree of acidity of the ethylene acrylic acid copolymer, preferably by saponification to a level of preferably between 8 and 18 wt% of acrylic acid comonomer, more preferably between 10 and 16 wt%. Unmodified material typically has an acrylic acid comonomer weight percent of 20%. In preferred embodiments of the invention, the overlayers are free of silica, talc or other such fillers and free of wax or pigments. Second Coating

[0077] In preferred embodiments, the polymer substrate is optionally coated with a second coating prior to being coated with the first coating described above. The second coating is intermediate the polymer substrate and the first coating. This second coating is also known as the "undercoating."

[0078] In one embodiment, a polyamide coating can be applied as an undercoating with an ethylene acrylic acid as the top coating.

[0079] In some embodiments, the undercoating can be a primer under the top-coating used to enhance the adhesion of the top-coating to the substrate. Suitable undercoatings include polyethylene imine, epoxy coating, or urethane coatings.

[0080] A primer may be useful in applications where even greater adherence is desired between the topcoat layer and the polymer substrate, i.e. greater than that which would result from surface-treatment alone of the metal layer. Before applying the primer the film may first be treated to provide increased active adhesion sites on the film's surface (thereby promoting primer adhesion). Then a continuous coating of a primer material may be applied to the treated film surface. The primer provides an overall adhesively active surface for thorough and secure bonding with the subsequently applied coating composition. The primer may be applied to the film by conventional solution methods, for example, by roller application. [0081] In one embodiment, the primer coating layer may be poly(ethyleneimine), which is applied as either an aqueous or organic solvent, e.g. ethanol, solution, or as a solution in a mixture of water and organic solvent, containing from about 0.1 to about 0.6 wt% of the imine. [0082] In another embodiment, the primer may be a urethane coating. Suitable urethane primers include copolymers of a glycol, a propanoic acid, and a polyisocyanate, for example a copolymer of a polybutylene glycol, a dimethylolpropionic acid, and isophorone diisocyanate, e.g. a copolymer of α-hydro-ω-hydroxypoly(oxy-l,4-butanediyl), 3-hydroxy-2- (hydroxymethyl)-2-methylpropanoic acid, and 5-isocyanato-l-(isocyanatomethyl)-l,3,3- trimethy Icy clohexane . [0083] In a further embodiment, the primer may be an epoxy coating. For example, the epoxy coating may be the reaction product of an epoxy resin and an acidified aminoethylated vinyl polymer, which is used as a hardener or curing agent. The epoxy resin may be glycidyl ethers of polyhydroxy compounds. Typical polyhydroxy compounds which may be used include bisphenol A, ring substituted bisphenol A, resorcinol, hydroquinone, phenol- formaldehyde, novolac resins, aliphatic diols, such as ethylene glycol, propylene glycol, 1 ,A- butanediol, 1,6-hexane-diol, glycerol, lower alkyl hydantoins, and mixtures thereof. For example, the epoxy resin may be made by the glycidation reaction between epichlorohydrin and bisphenol A. Epoxy resins of this type are commonly classified by their epoxy equivalent weight ("EEW"), which is defined as the weight of resin in grams which contains one gram equivalent of epoxy groups. Resins with an EEW in the range of 170 to 280, or in the range of 180 to 210 may be used. Industrial Application [0084] The polymer substrate may be prepared by any suitable means. Preferred methods comprise co-extruding, then casting and orienting the film. In one embodiment, the polymer substrate may be formed by co-extruding the one or more layers through a flat sheet extruder die at a temperature in the range of 200⁰C to 26O⁰C, casting the film onto a cooling drum and quenching the film. The sheet is then stretched 3 to 7 times its original size, in the machine direction (MD), followed by stretching 4 to 10 times its original size in the transverse direction (TD). The drawing temperature for the biaxial orientation may be in the range of about 100⁰C to about 200⁰C.

[0085] In a preferred embodiment, the coated polymer substrate is used for printing applications. Preferably toner images and more preferably liquid toner images are used in the printing application. Preferred toners are based on one or more of ethylene vinyl acetate (EVA) copolymers, copolymers of ethylene and α-,β-ethylenically unsaturated acid selected from the group consisting of acrylic and methacrylic acids and ionomers such as are produced under the trade name of Surlyn®, by DuPont. [0086] Provided herein are films suitable for printing a toner image thereon. The film comprises a polymer substrate having a first side and a second side, wherein the first side has been surface treated and the second side has a surface energy less than or equal to 33 dynes/cm, and a solvent-based top-coating that is on the first side of the polymer substrate, wherein a toner image may be fused and fixed to the outer surface of the top-coating. In some embodiments, an optional undercoating is intermediate the polymer substrate and the top-coating.

[0087] Also provided herein is a method of producing a coated substrate to which a toner image can be adhered. The method comprises the steps of providing a polymer substrate having a first side and a second side; treating the surface of the first side of the polymer substrate, coating a treated first side of the polymer substrate with a top-coating, wherein the top-coating has an outer surface to which a toner image can be adhered and fixed. In some embodiments, the method further comprises the step of coating the first side of the treated polymer substrate with an undercoating prior to applying the top-coating, and then coating the undercoating with the top-coating.

[0088] Preferably, the sheet polymer substrate comprises polypropylene, polyethylene, ethylene -propylene copolymers, propylene-butene copolymers, ethylene-propylene-butylene terpolymers, or blends thereof. In preferred embodiments, the polymer substrate is biaxially oriented. [0089] In a preferred embodiment, the top-coating comprises an ethylene acrylic acid copolymer. Preferably, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of less than 18%, or less than 16%. Alternatively or additionally, the ethylene acrylic acid copolymer has an acrylic acid comonomer percentage weight of more than 8%, or more than 12%. [0090] In one embodiment, the top-coat has a weight of between 0.1 and 10 grams per square meter. Alternatively or additionally, the topcoat has a weight of between 0.2 and 2 grams per square meter, or between about 0.25 and about 0.35 grams per square meter. [0091] In a preferred embodiment, the undercoating comprises an amine terminated polyamide. [0092] In one embodiment, the undercoating has a weight of between 0.1 and 1 grams per square meter. Alternatively or additionally, the undercoating has a weight of between about 0.3 and 0.5 grams per square meter.

[0093] In one embodiment the top-coating is substantially wax and pigment free. [0094] In another embodiment, the top-coating is substantially free of particulate matter. Preferably, the undercoating is substantially free of particulate matter. More preferably, both the top-coating and the undercoating are free of particulate matter.

[0095] In preferred embodiments, the film comprises only two coating layers, the top- coating and the undercoating. [0096] Also provided herein is a printing method comprising: providing a film substrate, as described above; and printing a toner image on the film substrate. Preferably, the toner image is a liquid toner image. Alternatively or additionally, printing comprises transferring the toner image to the substrate using heat and pressure. Alternatively, printing comprises electrostatically transferring the toner image to the substrate. [0097] In one embodiment, the printing method comprises: forming the image on an image forming surface; transferring the image from the image forming surface to an intermediate transfer member, and transferring the image from the intermediate transfer member to the substrate. [0098] Liquid toners have been developed in which the toner is dispersed in a solvent. The solvent is removed in the last printing step by the mechanism of the press. Because of the liquid medium, very fine dye particles can be employed without concern for the particles becoming airborne. Thus, copies of very high resolution can be made and high temperatures needed to fuse dry toners are not required. Examples of liquid toners for electrostatic imaging are described in U.S. Patent Nos. 5,225,306; 5,276,492; 5,346,769; and 5,407,771, all of which are incorporated herein by reference.

[0099] The term "liquid toner" covers a composition in which liquid toner particles are dispersed in a liquid base. Typically the liquid base is non-polar such as an aliphatic hydrocarbon fraction. Typical toners of this kind, and processes for using them in imaging, are described in U.S. Patent Nos. 5,225,306; 5,276,492; 5,346,769; and 5,407,771, all of which are incorporated herein by reference. The coextruded films of this invention are capable of receiving toner derived from these liquid toner compositions. [00100] Preferred toners are based on one or more of ethylene vinyl acetate (EVA) copolymers, copolymers of ethylene and α-,β-ethylenically unsaturated acid selected from the group consisting of acrylic and methacrylic acids and ionomers such as are produced under the trade name of Surlyn, by DuPont.

[00101] The toner can be applied to the coated film using any electrostatic printer adapted or designed to use liquid toner such as the Indigo brand "Omnius" or "E-Print Model 1000" printers. EXAMPLES

[00102] The coated polymer films will now be further described with reference to the following non-limiting examples.

[00103] In the Examples, the following test method was used to determine the amount of roll blocking. This test can be used to predict how the film in roll form will unwind and function in final packaging applications. A Carver Laboratory Press, Model C, was used. The film samples to be tested should be wrinkle and contamination free. The Press is set to a temperature of 140 ⁰F ± 2 ⁰F (6O⁰C ± 1.11⁰C). The films samples are cut as wide or slightly wider and longer than the Press's blocking plates (with the longer dimension parallel to the machine direction). The film sample should be long enough to permit the unblocked ends to be clamped in the testing device, approximately 6 inches in the machine direction and 2 inches in the transverse direction (15.2 cm x 5.1 cm).

[00104] The desired test surfaces of two layers of the film are placed together, i.e., in/out, coated/uncoated. A piece of aluminum foil is placed on the bench top. The aluminum foil is as wide or slightly wider and longer than the Press's blocking plates. The aluminum foil should be smooth and flat. The 2-layered specimen is then placed on the foil. To prevent the test specimen from blocking to the foil, certain film types may require the two-layered specimen to be sandwiched between two sheets of slip film. This results in a "sandwich" composed of foil/slip film/test specimen/test specimen/slip film/foil.

[00105] Another piece of aluminum foil is placed on top of the two-layered specimen. The foil/specimen/specimen/foil stack is placed between the Press's matched set of room temperature blocking plates. The chromed surfaces of the blocking plates are placed together with the sample stack visually centered between the plates in the long direction or one edge of the stack should be aligned with the edge of the plates. The plates are then placed in the center of the blocking press. The blocking press is set to 6000±200 lbs (2722±91 kg) for 60 minutes. After one hour the samples are removed from the press. The specimens are carefully removed from the plates, and the foil is slowly peeled from the specimen. The foil is peeled from the specimen using a 120° to 180° angle of pull. [00106] A Sintech testing unit is then used to pull the blocking specimen. The Sintech testing unit is set to a 1 inch (2.54 cm) jaw separation with a 5 Ib. (2.27 kg) load cell and a cross head speed of 5 inches/min (12.7 cm/min). The Sintech testing unit should be calibrated and the unit load cell should be warmed up 20 minutes prior to calibration. Insert one layer of the specimen in the upper jaw perpendicular to the jaw face and tighten the jaw. The other layer is placed squarely in the lower jaw and the jaw is tightened. The specimen is pulled approximately ¹A inch (1.27 cm) into the blocked area while the specimen tail is held perpendicular to the direction of pull. The Sintech testing unit performs the blocking test and reports the blocking value in grams per inch. [00107] Table 1 lists various components used in the examples. TABLE 1 - Various Com onents Used in the Exam les

Example 1

[00108] In Example 1, various films were made using a variety of polymer substrates, coatings, and surface treatments. In this Example, Coating A was 2.7% V-973 in n-PrOH; Coating B is the same as Coating A but was applied to give a thicker coating; Coating C was V-973 + 0.1 pph Tospearl; and Coating D was V-973 + 0.2 pph Tospearl. The films were made with a line speed of 250 fpm (76.2 m/min) and an oven temperature of 200 ⁰F (93°C). The 0.03 g/msi (47g/m²) weight coatings were applied with a 460 Quad gravure roll, and the 0.06 g/msi (93g/m²) weight coatings were applied with a 200 Quad gravure roll. The films were tested to determine the amount of blocking, with the results shown in Table 2. In Table 2, the Front Side Surface Treatment refers to surface treatments that were applied directly to the front side of the film prior to applying the coating; the Backside Surface Treatment refers to surface treatments that were applied directly to the back side of the film prior to applying the coating.

[00109] As seen in Table 2, when the polymer substrate was treated on both sides with a corona surface treatment, the film exhibited blocking, leading to either skin delamination or destruction. The films that had only front side flame treatment and no treating on the back side exhibited very little to no blocking (e.g., 10 g/inch (3.9 g/cm) or lower), even when the film was first metallized prior to coating.

[00110] When the front side coating was applied over an acrylic coating (e.g., Films 3, 6,

10, and 15), the front side coating exhibited little blocking with the back side acrylic coating.

This is most likely due to the slip and anti-block additives in the acrylic coating. While these films exhibited little blocking, it is generally not preferred to have an acrylic coating on the back side of the film during liquid toner digital printing. Because the acrylic coating has a low melting point, there may be a tendency for the acrylic coating to block to the hot printing equipment surface.

[00111] In the films without a front side acrylic coating, adding the antiblock additive only helped to reduce blocking very slightly and was not enough to overcome the blocking problem.

Example 2

[00112] In Example 2, various films were made using different surface treatments. The polymer substrate of the films was a 1 mil (25.4 microns) clear oriented polypropylene film.

A front-side coating of 2.4% Macromelt-6239 in a 98/2 blend of n-PrOH/H₂O with 50 ppm of Tospearl was applied to the polymer substrate (after the surface treatment if a surface treatment was used) at 0.06 g/msi (93 g/m²) coating weight. The films were tested to determine the amount of blocking, with the results shown in Table 3.

TABLE 3 - Example 2 Film Structures and Blocking Data

** Sporadic coating appearance, but able to make blocking measurement.

[00113] It was difficult to get a uniform front-side coating applied to the films that were not surface treated prior to applying the front-side coating. Films exhibited blocking, even though only the front-side was corona treated and the back side was not treated. It is believed that the front side corona treatment indirectly caused the backside's surface energy to increase leading to film blocking.

Example 3

[00114] In Example 3, comparative water-based ethylene-acrylic acid coatings were applied to 1 mil (25.4 microns) clear oriented polypropylene film substrates, after the film substrate's front-side was surface treated. The films were tested to determine the amount of blocking, with the results shown in Table 4. The film showed no blocking to the backside even when the front side was corona treated, and water-based ethylene-acrylic acid coating was used.

TABLE 4 - Example 3 Film Structures and Blocking Data

[00115] All patents and patent applications, test procedures (such as ASTM methods, UL methods, and the like), and other documents cited herein are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted.

[00116] When numerical lower limits and numerical upper limits are listed herein, ranges from any lower limit to any upper limit are contemplated. While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.

[00117] The invention has been described above with reference to numerous embodiments and specific examples. Many variations will suggest themselves to those skilled in this art in light of the above detailed description. All such obvious variations are within the full intended scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A film comprising: a. a polymer substrate having a first side and a second side, wherein the first side has been surface treated with at least one treatment selected from flame treatment and plasma treatment and the second side has a surface energy less than or equal to 33 dynes/cm; and b. a solvent-based topcoat that is on the first side of the polymer substrate.

2. The film of claim 1, wherein the first side of the polymer substrate has a surface energy greater than or equal to 35 dynes/cm.

3. The film of claim 1, wherein the solvent-based topcoat comprises a low molecular weight polyamide .

4. The film of claim 1, wherein the solvent based topcoat comprises ethylene acrylic acid copolymer.

5. The film of claim 1, wherein the film further comprises a second coating that is intermediate the topcoat and the first side of the polymer substrate.

6. The film of claim 5, wherein the second coating comprises an amine -terminated polyamide, a silane coupling agent, and a silane.

7. The film of claim 1, wherein the polymer substrate comprises polypropylene, polyethylene, ethylene -propylene copolymers, propylene -butene copolymers, ethylene- propylene-butylene terpolymers, or blends thereof.

8. The film of claim 1 , wherein the polymer substrate is biaxially oriented.

9. The film of claim 1 , wherein the film further comprises a print-image on the topcoat.

10. A method of reducing back- side blocking on a film roll comprising: a. providing a polymer substrate, having a front side and a back side; b. treating the surface of the front side of the polymer substrate with at least one treatment selected from flame treatment and plasma treatment; and c. applying a topcoat to the front side of the polymer substrate, wherein the topcoat is a solvent-based coating.

11. The method of claim 10 further comprising applying an undercoating to the front-side of the polymer substrate prior to applying the topcoat.

12. The method of claim 11, wherein the undercoating comprises an amine-terminated polyamide, a silane coupling agent, and a silane.

13. The method of claim 10, further comprising printing an image on the topcoat.

14. The method of claim 10, wherein the back-side of the polymer substrate has a surface energy less than or equal to 33 dynes/cm.

15. The method of claim 10, wherein the front-side of the polymer substrate has a surface energy greater than or equal to 35 dynes/cm.

16. The method of claim 10, wherein the topcoat comprises ethylene acrylic acid.