Printable high barrier multilayer film

Abstract: A film combination comprising (a) an oriented polymeric substrate which either (i) comprises an intimate blend of polypropylene with maleic acid anhydride modified polypropylene or (ii) is otherwise susceptible of transmitting oxygen and water vapor and has a primer coating on at least one surface: (b) a layer of cross-linked polyvinyl alcohol on (i) the intimate blend or (ii) the primer coating: (c) a layer of a polymer comprising vinyl alcohol residues which may be as defined in (b) or an intimate blend of a homo- or copolymer of vinyl alcohol, with an ethylene acrylic acid copolymer; and, optionally (d) a layer as defined in (a) (i).

Description: This invention relates to a multilayer packaging film having good barrier resistance to the transmission of oxygen and water vapor, good printability; and the ability to receive a firmly bonded metal layer thereon. certain polymeric films e.g., polypropylene, employed for the packaging of foods, inherently permit the transmission of oxygen and water vapor from outside of the film to inside of a package made up of the film. Oxygen and water vapor permits rapid deterioration of foods packaged in containers made from such a film. Metal deposits on such films are desirable because of the improvement in appearance and in providing yet another layer which militates against the invasion of oxygen and water vapor. Thus, it is a principal object of the present invention to present a film which has excellent printability, can have a metal layer firmly bonded thereto and which has significantly decreased oxygen and water vapor transmission rates. The present invention relates to a process for forming a firmly bonded film laminate comprising: (a) preparing an intimate blend of polypropylene and a maleic acid anhydride modified polypropylene and forming a cast layer thereof; (b) forming said cast layer into a machine direction oriented film; (c) applying a coating of poly(vinyl alcohol) containing cross-linking means to the machine direction oriented film; (d) mating a second so-coated machine direction oriented film so that poly(vinyl alcohol) surfaces are in contact; (e) transverse direction orienting the mated films to form a film laminate of high delamination resistance. The invention is also concerned with a film combination, obtainable by the process above, comprising: (a) a pair of films each comprised of an intimate blend of polypropylene and a maleic acid anhydride modified polypropylene; (b) coatings of poly(vinyl alcohol) containing cross-linking means having been applied to a surface of each of said pair while said pair had been in a machine direction oriented condition; and (c) each coated film having been positioned with the poly(vinyl alcohol) layers in intimate contacting relationship and said coated contacting films had then been transverse direction oriented. The polypropylenes contemplated herein include homopolymers and copolymers of propylene. Particularly preferred are isotactic polypropylenes containing at least 80% by weight of isotactic polypropylene. The preferred base layer can be homopolypropylene having a melting point from 152 to 171°C (305 to 340°F), preferably from 161 to 169°C (321 to 336°F); an inherent viscosity from 1.4 to 4.0; an MFI (melt flow index) from 0.5 to 12; an MW (molecular weight) from 100,000 to 600,000; and a density from 0.89 to 0.91. Commercially available materials of this description include Exxon 4252 and ARCO W472 (registered trademarks). According to a preferred embodiment of this invention the polypropylene blends comprise from 90 to 99 wt%, preferably from 92.5 to 98.5 wt%, of a polypropylene resin as described herein, compounded with from 1 to 10 wt%, preferably from 1.5 to 7.5 wt%, based on the weight of the polypropylene resin, of a maleated polypropylene. The maleated polypropylene additive may be derived from maleic acid or its anhydride, copolymerized with polypropylene. Pertinent properties of maleated polypropylene marketed by Eastman Chemical are as follows: Ring and Ball Softening 157°C; MW 4500; Density 0.934; Acid Number 45; Brookfield Visc. 190°C.cp/370 Color Gardner Scale 11; Penetration Hardness, 100g/5 s/25°C tenths of mm.0-1 In addition, the term maleic anhydride modified polypropylene homopolymer is the product resulting from the reaction between maleic anhydride and the thermal degradation product of polypropylene. Examples of this material can be found disclosed in US 3,480,580. The polyvinyl alcohol employed herein can be any commercially available material. For example, ELVANOL 71-30, an E. I. DuPont product. The polyvinyl alcohol coating solution is prepared by dissolving the polymer in hot water, cooling and mixing both with a suitable cross-linking agent and an acid catalyst. The cross-linking agent can be a melamine- or urea-formaldehyde resin. Commercially available cross-linkers also would include PAREZ 613, a methylated melamine formaldehyde; CYMEL 373, a methylated melamine formaldehyde; CYMEL 401, a trimethylol melamine urea formaldehyde; glyoxal, borax, etc. An acid catalyst, e.g., ammonium sulfate, ammonium chloride, sulfuric acid, nitric acid and ammonium nitrate will effectively catalyze these systems. Cross-linking is carried out to make the polyvinyl alcohol less moisture sensitive but it is essential that this does not occur before the coating weight is applied and evenly distributed. This is effected by making up the aqueous solution so that the initial concentration is too low for this to occur but, as drying occurs, the concentration increases and the rate of cross-linking is heightened. Suitable concentrations have been found to be from 3 to 10, preferably from 4 to 8 wt% of the solution being polymer plus cross-linking agent plus catalyst. If the solids content is higher, the solution becomes too viscous; if lower, good water resistance is not obtained. From 10% to 30%, typically 15% cross-linking agent is used with from 0.5% to 4.5%, typically 2% of the acid catalyst. A preferred formulation comprises the following solids content: 85.5 wt% polyvinyl alcohol; 12.8 wt% methylated melamine formaldehyde; and 1.7 wt% ammonium chloride (NH 4 Cl). The aqueous polyvinyl alcohol solution was prepared by dissolving sufficient ELVANOL 71-30 in hot. water to form an 8 wt% solution which was then cooled. To this solution was added a 20 wt% aqueous melamine formaldehyde solution and a 5 wt% aqueous ammonium chloride solution to provide the preferred formulation recited above. By cross-linking the underlying layer of polyvinyl alcohol, the moisture resistance of the polyvinyl alcohol is dramatically improved. The following example illustrates the invention. EXAMPLE An intimate blend of polypropylene (95 wt%) and maleated polypropylene (5 wt%) was prepared and formed into a cast layer of approximately 0.09cm (35 mls) thick. This cast layer was then machine direction oriented to a thickness of about 0.013cm (5 mls) thick. A coating of the above-identified poly(vinyl alcohol) was applied to one surface each of a pair of the machine direction oriented polypropylene films. The poly(vinyl alcohol) coating contained 2.5 phr (parts per hundred resin) of ammonium chloride and 20 phr of Parez 613 malemine formaldehyde. The so-coated substrate films were mated together so that the poly(vinyl alcohol) layers were in contact. This was accomplished by running the film through rollers at room temperature. The film travelled at approximately 15 m/min (50 fpm) and was subjected to a pressure of 620 kPa (90 psi). Thereafter the film combination was heated to 160°C and transverse direction oriented. The finished film had been machine direction oriented 7 times and transverse direction oriented 5 times. The resulting laminated film strongly resisted delamination and was highly resistant to the transmission of oxygen and water vapour during a 24 h period. It is to be understood that instead of bringing together 2 separately formed coated machine direction oriented films a single poly(vinyl alcohol) coated film can be folded on itself so that the poly(vinyl alcohol) faces are in contact and, after edge trimming, the film can be pressure laminated to itself, heated and transverse direction oriented before final windup for shipment to a converter.

Claims: A process for forming a film laminate comprising: (a) preparing an intimate blend of polypropylene and a maleic acid modified polypropylene and forming a cast layer thereof: (b) forming said cast layer into a machine direction oriented film; (c) applying a coating of poly(vinyl alcohol) containing cross-linking agent to the machine direction oriented film; (d) mating a second so-coated machine direction oriented film so that poly(vinyl alcohol) surfaces are in contact; (e) transverse direction orienting the mated films to form a delamination resistant film laminate. A process according to claim 1 wherein the mating step (d) is carried out under a mating force of 207 to 896 kPa (30 to 130 psi). A process according to claim 1 or claim 2 in which the machine direction orientation is from 2 to 7 times and the transverse direction orientation is from 3 to 8 times. A film laminate obtainable by a process comprising: (a) forming a pair of films, each film being formed by: providing an intimate blend of polypropylene and a maleic acid modified polypropylene, forming a cast layer thereof, monoaxially orienting the layer in the machine direction, applying to a surface of the monoaxially oriented layer a coating of poly(vinyl alcohol) containing a cross-linking agent, and (b) positioning the pair of films with the poly(vinyl alcohol) layers in intimate contacting relationship, and orienting the contacting films in the transverse direction. A film laminate according to claim 4 obtainable by a process in which the positioning is accompanied by a mating force of 207 to 896 kPa (30 to 130 psi). A film laminate according to claim 4 or claim 5 obtainable by a process in which the orientation is from 2 to 7 times MD and from 3 to 8 times TD.

Inventors: KNOERZER ANTHONY ROBERT, REID LELAND WALLACE, KNOERZER, ANTHONY ROBERT, REID, LELAND WALLACE
Applicants: EXXONMOBIL OIL CORP, EXXONMOBIL OIL CORPORATION
Application year: 1992
Main classification: B32B27
IPC codes: B32B27, C08L51, C08L23, B29C55, C08J7