PLASTIC RELEASE FILM WITH TEXTURED OR NON-PLANAR SURFACE

Abstract: A release film comprising an oriented polymeric substrate layer having a surface layer of an ethylene-propylene block copolymer on one side thereof and a release coating on the other side thereof. Where the release material comprises a silicone-acrylate release material it may include a quaternary ammonium salt therein.

Description: PLASTIC RELEASE FILM WITH TEXTURED OR NON-PLANAR SURFACE The present invention relates to a release film, a major utility of which is to protect and carry label stock u: 1 the label is put into use on an appropriate surface. For many years labels have been prepared with an adhesive applied to one side thereof. In instances where the adhesive is a pressure sensitive adhesive, some means had to be provided to protect the adhesive surface in advance of applying the label to the ultimate surface to be labeled. A technique has evolved over the years by which the pressure sensitive adhesive is protected by bringing the adhesive surface into contact with a release layer which has little tendency to aggressively adhere to the pressure sensitive adhesive surface. Thus, a second film having a release surface is brought into contact with the pressure sensitive surface carried by the label protecting the pressure sensitive adhesive until the label is applied to its ultimate surface. It is to be understood that the pressure sensitive adhesive can be applied to the release surface and the label stock thereafter applied to the adhesive surface. On removal of the label the pressure sensitive adhesive will preferentially adhere to the label stock. Release films can be manufactured separately from the label face stock. The release film is manufactured by applying a commercially available release material to one side of the release film and the combination is then convolutely wound into a roll stock. Ultimately this roll stock must be unrolled and slit to a width dimension suited to a particular label dimension. Next a pressure sensitive layer is applied over the release layer and this is mated with the underside of a sheet of label stock. Alternatively the label stock can have the pressure sensitive adhesive applied to it's undersurface which in turn is mated with the release label. Following this, label indicia is printed onto the surface of label and, thereafter, the label is die cut so that it can be separated from the release sheet-label stock combination. During the unwinding of release film roll stock for re-dimensioning purposes, serious problems can arise. After the release material is applied as a coating layer on the substrate and subjected to electron beam curing and during the convoluted winding of the stock, a portion of or a fraction of the release material is transferred to the back side of the release film. Later when this release roll is unwound to receive label stock thereon, and subsequently rewound, some release material or a fraction thereof transfers to the surface of the label stock prior to the label stock receiving printed indicia. This transferred release material causes imperfections to occur during the label printing process. The transferred release material interferes with printing. This leads to the need to discard the combination of material at a point when the product is almost completely ready for use or marketing. Furthermore, as a result of rapid unwinding, a high electrostatic potential develops at the point of unroll. This electrostatic potential can achieve dangerous levels which can lead to fire, explosion, or physical injury. Another problem is that the film will not always track in an even, unvarying manner as it moves employed in the manufacture of the product. This invention seeks to overcome the aforementioned difficulties. In one aspect, the present invention provides a release film comprising an oriented thermoplastic polymeric substrate layer having a surface layer comprising an ethylene-propylene block copolymer on one side thereof and a release coating on the other side thereof. While it is not completely understood, it is believed that the ethylene-propylene block copolymer surface has a distinctive texture which causes, during unwind of a roll of said film, a reduced electrostatic potential, in comparison to a smooth surface and said surface texture permits even, unvarying transport over rollers during manufacture and use. In another aspect, the invention provides a release film comprising an oriented thermoplastic polymeric layer having one side thereof a release coating comprising a silicone-acrylate release material, the coating including therein a quaternary ammonium salt. The substrates contemplated herein can be any polymeric film substrate which can effectively carry a release coating on one side thereof and an ethylene-propylene block copolymer surface on the other side thereof. Preferred substrates include, polyolefin films which are oriented and preferably biaxially oriented. The substrate can be a homopolymer, copolymer or a blend thereof. A preferred homopolymer is polypropylene. Particularly preferred are isotatic polypropylenes containing at least 80% by weight of isotatic polypropylene. This substrate layer can be homopolypropylene having a melting point range from 321° to 336°F. Commercially available materials of this description include Exxon 4612 and FINA 3371. It is to be understood that the substrate can be a single material or a combination of coextruded substrate materials. For example, polypropylenes of different melt indexes can be employed to yield excellent handling characteristics. The substrate can be provided in any thickness which facilitates either human or machine handling. A preferred thickness range can be from 0.5 mils to 3 mils. The substrate material can be composed of one or a plurality of layers and may be transparent or opaque. The ethylene-propylene block copolymer can have an ethylene content of less than 20% by weight; for example, from 2 to 20 and preferably from 3 to 15%, and a melt flow index from 1 to 100 g/10 minutes, preferably from 3 to 50 g/10 minutes. A preferred ethylene-propylene block copolymer is commercially available as Himont 8523, having a melt flow index of about 6.0 g/10 minutes. The release film substrate can be formed for example by coextruding the polypropylene film of choice with the above identified ethylene-propylene block copolymer followed by biaxially orienting the combination 4.3 times in the machine direction and 8 times in the transverse direction. The block copolymer coated substrate is now in condition to receive a commercial release coating on the side opposite to the ethylene-propylene block copolymer. The selected release coating can be applied by any convenient technique, for example, by differential offset gravure coating. Thereafter, if the proprietary release coating needs to be cured it can be done so by any prior art technique. A convenient class of release coatings is described in U.S. Patent No. 4,678,846 and 4,306,050. In essence, these release coatings are silicone polymers having acrylic functional end groups. The material is applied to the opposite side of the block copolymer coated polypropylene film by differential offset gravure coating and the coating is cured by exposure to energized electrons at the rate of 2.5 Mrads in a RPC electron beam processor. It is to be understood than curing can be accomplished within the range from 1 to 5, preferably from 2 to 3 Mrads. The release material of the present invention includes (meth)acrylate ester modified organopolysiloxane mixtures, comprising essentially equilibrated organopolysiloxanes with, on the average, more than 25 to fewer than 200 silicone atoms and which additionally contain 2 to 30 weight percent of organopolysiloxanes with, on the average, 2 to 25 silicone atoms and 2 to 30 weight percent of organopolysiloxanes with, on the average, 200 to 2,000 silicone atoms. As used herein, the organopolysiloxane fractions having more than 25 to less than 200 silicone atoms are sometimes referred to as the first fraction, those having 2 to 25 silicone atoms are referred to as the second fraction, and those having 200 to 2,000 silicone atoms are referred to as the third fraction. Especially preferred are those (meth) acrylate ester modified organopolysiloxane mixtures which contain, besides the essentially equilibrated organopolysiloxane with, on the average, more than 25 to fewer than 200 silicone atoms, 5 to 20 weight percent of organopolysiloxanes with, on the average, 5 to 25 silicone atoms and 5 to 20 weight percent of organopolysiloxanes with, on the average, 200 to 2,000 silicone atoms. The organopolysiloxanes contained in the organpolysiloxane mixtures preferably correspond to the formula : R a -Si-Q b °4- (a+b) in which R is an alkyl radical with 1 to 4 carbon atoms, preferably, the methyl radical. R may, however, also be a vinyl or phenyl radical. Moreover, the R radical may also represent different moieties in the molecule, so that some R radicals represent methyl radicals and the other R radicals may, for example, represent a vinyl or a phenyl radical. Preferably, at least 90% of the R radicals are alkyl radicals, especially methyl radicals. Q is an organic radical which contains the acrylate or methacrylate ester groups. Preferably, the acrylate or methacrylate ester radical is linked to the silicone atom through an SiOC bridge. The acrylate or methacrylate radical may moreover be derived from pentaerythritol triacrylate, pentaerythritol trimethacrylate, dipentaerythritol pentaacrylate, dipentaerythritol pentamethacrylate, ditrimethylolpropane triacrylate, ditrimethylolpropane trimethacrylate, ditrimethylolethane triacrylate, ditrimethylolethane trimethacrylate, trimethylolpropane di(meth)acrylate, trimethylolethane di(meth)acrylate, glycerin di(meth)acrylate, hydroxyethyl acrylate, hydroxpropyl acrylate and neopentyl glycol monoacrylate. The acrylate or methacrylate ester radical may, however, also be linked through an Sic bridge with the organopolysiloxane. a has a value of 1.0 to 2.2; b has a value of 0.001 to 1.6. The fractions of different average molecule weight contained in the inventive acrylate or methacrylate ester modified organopolysiloxane mixtures may differ in respect to the R and Q radicals and the a and b subscripts. Organosilicone species in the above referenced silicone acrylate release coatings are believed to migrate from the coating to adjacent contacting surfaces. The cause of this migration may be nonfunctionalized silicone remaining in the release coating. These migratable species are not bound or fixed in the cured coating and thus will migrate to adjacent contacting surfaces. It has been found that by including a quaternary ammonium salt in the silic e acrylate material prior to applying it to the release substrate that by some reaction the migratable material is either prevented from migrating or converted to a material which has no adverse effect on label printing. What is known is that the inclusion of the quaternary ammonium salt solves this expensive problem. The silicone acrylate release coating normally is applied to the substrate in a quan ~y of from about 0.2-1.2 grams/1000 i .n2, preferably about 0.7 g/1000 i.n2. The quaternary ammonium salt can be added to the silicone acrylate release coating material in a proportion of from 0.2-8 wt %. The substrate can be from 1.8 to about 2.2 mils thick. Examples of quaternary ammonium salts include the following: o Dimethylaminoethyl methacrylate dimethyl sulfate ("Madquat Q-5" by Norsolac, Inc.) o (3-Lauramidopropyl)trimethyl ammonium methyl sulfate ("Cyastat LS" by American Cynamid) o Stearamidopropyl-dimethy1-2-hydroxyethyl ammonium nitrate ("Cyastat SN" by American Cynamid) o N,N-bis(2-hydroxyethyl)-N-(3-dodecycloxy-2- hydroxypropyl)methyl ammonium methyl sulfate ("Cyastat 609" by American Cynamid) o Stearamidopropyl-dimethyl-2-hydroxyethyl ammonium dihydrogen-phosphate ("Cyastat SP" by American Cynamid) o Trialkylalkyletherammonium salts (e.g. Emerstat 6660-A from Henkel Corp.) Examples of the release film in accordance with the present invention in comparison with a prior art release film is shown below. The release compositions were coated onto a 2.0 mil biaxially oriented polypropylene and cured by conventional electron beam radiation technique. The release coating was a silicone-acrylate identified as RC450 from Th. Goldschmidt of Essen, Germany, to which was added 2,4 and 8% by weight of trialkylalkyletherammonium salt. The release force (grams/inch) was measured using a TESA4154 tape and a TLMI release force tester. The film was convolutely wound and so held for a period of 1 week at 125°F. , so that the magnitude of release material transfer to the back side of the film could be measured. The film was then unrolled and the rating of ink wetting was measured using a #8800 Berol magic marker. Rating 1 = high silicone migration, causing a severe ink wetting problem; rating 10 = no silicone migration , i.e., no ink wetting problem. The results are shown in the table below. TABLE The above data shows the outstanding decrease in the amoun * ' of transferred or migrated release material to the back side of the release film. The data also shows that the quaternary ammonium salt modified silicone-acrylate release material decreased the release force to a moderate extent, thus maintaining its release effectiveness. The silicone-acrylate coated release liner is then convolutely wound into a roll which is approximately from 60 to 80 inches in width. For test purposes the roll stock of the present invention was unrolled at speeds from 300 to 1500 feet per minute, simulating the unroll speed for purposes of slitting the roll stock to form lower dimension rolls. During this unrolling no dangerous electrostatic discharge levels were noted. In addition, the film tracked in an even, unvarying manner.

Claims: CLAIMS 1. A release film comprising an oriented thermoplastic polymeric substrate layer having a surface layer comprising an ethylene-propylene block copolymer on one side thereof and a release coating on the other side thereof. 2. A release film according to claim 1 wherein the substrate comprises a polyolefin film and the release coating comprises a silicone-acrylate coating. 3. A release film comprising an oriented thermoplastic polymeric substrate layer having on one side thereof a release coating comprising a silicone-acrylate material, the coating including a quaternary ammonium salt therein. 4. A release film according to claim 3 wherein the quaternary ammonium salt is present in the silicone-acrylate material in an amount from 0.2 to 8% by weight. 5. A release film according to claim 3 or 4 wherein the oriented thermoplastic polymeric substrate has a surface layer comprising an ethylene-propylene block copolymer on the other side thereof. 6. A release film according to any preceding claim wherein the polyolefin substrate comprises a homopolymer, a copolymer or a blend thereof. 7. A release film according to claim 6 wherein the homopolymer is polypropylene. 8. A release film according to any preceding claim wherein the ethylene-propylene block copolymer has an ethylene content of less than 20% by weight and a melt flow index from 1 to lOOg/10 minutes. 9. A release film according to claim 8 wherein the block copolymer has an ethylene content from 3 to about 15% by weight. 10. The release film of claim 1 wherein said ethylene-propylene block copolymer surface has a distinct surface texture which causes, during unwind of a roll of said film, a development of reduced electrostatic potential in comparison to a smooth surface and a film which tracks in an unvarying manner. 11. Use in a release film to reduce development of electrostatic potential during unrolling of a surface layer comprising an ethylene propylene block copolymer. 12. Use in a release film to improve tracking of a surface layer comprising an ethylene-propylene block copolymer. 13. Use in a release coating comprising a silicone-acrylate material to prevent transfer of release material to a label surface of a quaternary ammonium salt therein.

Inventors: KIM HACHUL CHRISTOPHER, LEBEL ANDREW PHILIP, KIM, HACHUL, CHRISTOPHER, LEBEL, ANDREW, PHILIP
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1992
Main classification: B32B13
IPC codes: B32B27, C08J7, C09J7