Extrusion coated laminate of a polyester film having an aminofunctional silane primer

Abstract: Oriented polyester film primer coated with a hydrolyzed aminosilane IS disclosed. The aminosilanes in the unhydrolyzed state have the formula: (R¹) a Si(R²) b (R³) c wherein R¹ is a functional group with at least one primary amino group; R² is a hydrolyzeable group such as a lower alkoxy group, an acetoxy group, or a halide; and R³ is a nonreactive, nonhydrolyzeable group such as a lower alkyl or a phenyl group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero; and with a+b+c=4. The preferred silane is N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane having the formula: H₂N(CH₂)₂NH(CH₂)₃Si(OCH₃)₃. The hydrolyzed aminosilane is applied to the film as an aqueous solution at any suitable stage during manufacture of the film, i.e., before or during the stretching operation, or it may also be applied to the finished film. The resultant primed polyester film is found to exhibit excellent adhesion to other polymeric materials and can be readily laminated to other polymeric films.

Description: BACKGROUND OF THE INVENTION 1) Field Of The Invention The Present invention relates to oriented polyester film coated on at least one side with a water soluble primer coating composition comprising a hydrolyzed aminofunctional silane which renders the film more receptive to extrusion coating with other polymeric film materials such as polyolefins. This application is similar to another application by the same inventor, assigned to the same assignee, titled "Polyester Film Primed With An Aminofunctional Silane, And Film Laminates Thereof"; and having a European application No. 89110443.2 and a filing date of June 9, 1989. 2) Prior Art Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET), has been widely used as a packaging material or as a base for microfilm, reprographic films, proofing films, and the like. It has good optical clarity and toughness which renders it extremely suitable for such applications. A major use of PET film is in laminations with other polymers and with paper or foil. Often the PET film is extrusion coated with polyethylene or an ethylene copolymer in order to get heat sealing capability, adhesion to another material such as aluminum foil, additional bulk (thickness) or stiffness, or to achieve properties not possible with PET alone. Unfortunately, the surface of PET film is not very receptive to extrusion coating with other polymers. Numerous primer coatings are known in the prior art for application to the surface of polyester film to improve the adhesion of the film to various materials. Examples of such coatings include compositions based on vinylidene chloride polymers (U.S. Patent No. 2,698,240), thermosetting acrylic or methacrylic polymers (U.S. Patent No. 3,819,773) and like materials.Certain water dispersible copolyesters have also been disclosed as adhesives for laminating sheets of polyester film to one another or to sheets of nylon film, as taught in U.S. Patent Nos. 3,563,942 and 3,779,993.More commonly, solvent based polyurethane adhesives and polyethyleneimine primers are used with attendant problems with solvent emissions and safety. Corona discharge treatment is also used with and without a primer to achieve adhesion between the extrudates. In common practice, for extrusion coating, a converter will corona treat the PET film as received from the manufacturer, prime the corona treated film, dry the primer, and then extrusion coat another polymer onto the film. The need to prime the film introduced an additional process step that requires expensive additional equipment and affects yields. In addition, if a solvent based primer is required, additional safety and pollution equipment is required and an additional regulatory burden is imposed. For the above reasons it would be advantageous for the converter to be able to obtain an already treated or primed film that was directly extrusion coatable without additional priming or corona treatment. To keep from incurring the same costs and problems it is also advantageous that the directly extrusion coatable film be produced by treating or primer coating the film in-line during the manufacturing of the film. For an in-line process it would be ideal for the primer coating to be water based for safety and health reasons, reclaimable (i.e., not cause excessive yellowing or deterioration in physical properties when primer coated film scrap is mixed with fresh polymer and reextruded, which is necessary for an in-line process because of low conversion efficiencies), and hard enough to allow the film to be wound into a roll (as would be necessary for a directly extrusion coatable film) without sticking or blocking. A directly extrusion coatable polyester film is disclosed in U.S. Patent No. 4,410,600 to P.T. McGrail. Disclosed is a biaxially oriented PET film coated in-line with a crosslinked styrene-maleic anhydride copolymer. The coated film, however, still requires corona discharge treatment prior to extrusion coating. A polyester film is disclosed in British Patent 818,176 in which a polyethylene layer is directly extrusion coated thereon. This patent discloses an isocyanate primer coating. The use of silane coupling agents to improve the bonding properties of polyethylene sheets and polyester sheets is known. For example, polyester sheets and polyethylene sheets primed with either vinyltrimethoxysilane or chloropropyltrimethoxysilane have been successfully laminated using a hot melt adhesive such as an ethylene/vinyl acetate terpolymer or an elastomeric polyester as disclosed by E. Plueddemann, "Bonding Through Coupling Agents", Plenum Press, New York, 1985. In addition, the use of N-2-aminoethyl-3-aminopropyltrimethoxysilane (sold under the trademark Z-6020 by Dow Chemical) as a primer coating to enhance the adhesion of an ionomer resin (salt of a copolymer of ethylene and methacrylic acid) to glass and to polycarbonate sheets is disclosed in U.S. Patent No. 4,663,228. U.S. Patent 3,477,901 discloses a primer coating for adhering polyolefin to various surfaces, the primer coating being a mixture of silane and either a monocarboxylic acid or a different silane. Related silanes such as 3-aminopropyltrialkoxysilanes are known to enhance the adhesion between polyurethane films and glass substrates, as disclosed in published European Patent Application NO. 171,917. None of these prior art disclosures addresses the problem of direct extrusion coatability of film, particularly of polyester film to polyethylene, ethylene ionomers and ionomers, or other polymer films, without additional primers or additional corona treatment. Accordingly, it is an object of this invention to provide an oriented, polyester film that can be directly extrusion coated with other polymers without the need for additional primers or additional corona treatment. Accordingly, it is also an object of this invention to provide a polyester film having a primer coating thereon in which the primer coated film scrap can be reclaimed without causing excessive yellowing or deterioration in properties of the reextruded polymer. Most prior art primer coatings employ volatile solvents, thus requiring associated equipment to handle the solvents, associated equipment to protect the workers, and associated equipment to dispose of any waste solvent. Accordingly, it is another object of the present invention to develop an aqueous primer coating that can be incorporated into most existing systems without the need for expensive associated equipment. SUMMARY OF THE INVENTION These and other objects of the invention may now be achieved by the provision of an oriented polyester film which has been primer coated with a hydrolyzed aminofunctional silane to render the film receptive to direct extrusion coating with other polymers. The aminofunctional silanes useful as a primer for the purposes of this invention are represented by the general formula in their unhydrolyzed state: (R 1 ) a Si(R 2 ) b (R 3 ) c wherein R 1 a functional group with at least one primary amino group, R 2 is a hydrolyzeable group such as a lower alkoxy group, an acetoxy group, or a halide, and R 3 is a nonreactive, nonhydrolyzeable group such as a lower alkyl or a phenyl group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero; and a+b+c=4. Generally, the aminofunctional silane is hydrolyzed in water and applied to one or more surfaces of the oriented polyester film by any conventional method such as spray coating or roll coating. Once the silane primer coating has dried, the primed polyester film is receptive to direct extrusion coating with one or more polymers. The extrusion coating can be conducted by any conventional process. The primer coating serves to bond the polyester film to the extrudate, thus forming a laminate. In the broadest sense, the present invention is directed to an oriented polyester film having an effective amount of primer coating to render the film receptive to direct extrusion coating with one or more polymers; the primer represented in the unhydrolyzed state by the general formula: (R 1 ) a Si(R 2 ) b (R 3 ) c wherein R 1 is a functional group with at least one primary amino group, R 2 is a hydrolyzeable group, and R 3 is a nonreactive, nonhydrolyzeable group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero; and a+b+c=4. In the broadest sense the present invention is also represented by a laminate having an oriented polyester film, a primer coating, and one or more directly extruded polymer(s); the primer coating represented in the unhydrolyzed state by the general formula: (R 1 ) a Si(R 2 ) b (R 3 ) c wherein R 1 is a functional group with at least one primary amino group, R 2 is a hydrolyzeable group, and R 3 is a nonreactive, nonhydrolyzeable group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero; and a+b+c=4. DETAILED DESCRIPTION OF THE INVENTION Silanes are water soluble or water dispersible after hydrolysis, with aminofunctional silanes being especially water soluble. The present inventor has found that aminosilanes provide good adhesion of extrusion coated polymers to polyester film without further priming or corona treatment. The scrap from polyester film primed with an aminosilane coating can be reclaimed. The aminofunctional silanes useful as a primer for the purposes of this invention are represented in their unhydrolyzed state by the general formula: (R 1 ) a Si(R 2 ) b (R 3 ) c wherein R 1 is a functional group with at least one primary amino group, R 2 is a hydrolyzeable group selected from lower alkoxy group having 1-8 carbon atoms, an acetoxy group, or a halide, and R 3 is a nonreactive, nonhydrolyzeable group selected from a lower alkyl group having 1-8 carbon atoms or a phenyl group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero; and a+b+c=4. Examples of aminosilanes that fit this formula are N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 4-aminobutyltriethoxysilane, 4-aminobutyldimethylmethoxysilane, and p-aminophenyltrimethoxysilane. The preferred silane is N-2-(aminoethyl)-3-aminopropyltrimethoxysilane having the formula: H 2 N(CH 2 ) 2 NH(CH 2 ) 3 Si(OCH 3 ) 3 The hydrolyzed aminosilane is applied to the film as an aqueous solution at any'suitable stage during manufacture of the film, i.e., before or during the stretching operation, or it may also be applied to the finished film. The resultant primed polyester film is found to exhibit adhesion to extrusion coated polymeric films. The primer formulation is prepared by mixing the aminosilane with water at a level within the range of about 0.2 to about 6 percent by weight. Optionally a weak acid, such as acetic acid, may be added to facilitate hydrolysis. At least one of the hydrolyzeable groups of the silane is hydrolyzed into a silanol group (SiOH). It is believed that the hydrolysis product of the aminosilane has a partially hydrolyzed cyclized stucture, with the amino group probably forming ionic bonds to the silicon portion of the molecule. Thus, the term hydrolyzed as used herein also may refer to such partially hydrolyzed structures. The preferred oriented polyester film for the purposes of this invention is polyethylene terephthalate, although the invention is equally applicable to film based on a crystallizable polyester resulting from the polycondensation of a glycol such as ethylene glycol or butanediol, and mixtures thereof with terephthalic acid, or mixtures of terephthalic acid and other dicarboxylic acids such as isophthalic acid, diphenic acid and sebacic acid, or their polyester forming equivalents, which polyesters are made by methods well known in the art. The film may be produced by techniques also well known in the art using well known apparatus. For example, the polyester is melted and extruded as an amorphous sheet onto a polished revolving casting drum to form a cast sheet of the polymer. Thereafter, the film is stretched in one direction, either in the direction of extrusion (longitudinal) or perpendicular to the direction of extrusion (transverse) in the case of uniaxially oriented film, and in two directions in the case of biaxially oriented film, that is, the film is stretched in both the longitudinal and transverse directions. The first stretching, to impart strength and toughness to the film, can range from about 3.0 to about 5.0 times. The hydrolyzed aminosilane primer coating of this invention in the form of an aqueous solution may be applied in-line at one of three stages during the film manufacture, namely; the pre-draw stage at the point between the casting of the amorphous sheet and the first stretch such as disclosed, for example, in British Patent No. 1,411,564; the inter-draw stage subsequent to the uniaxial drawing but prior to biaxial drawing such as disclosed, for example, in the U.S. Patent No. 4,214,035; or the post draw stage subsequent to biaxial stretching, but prior to winding the film. Normally, the heat applied to the film during the stretching or final conditioning stages is sufficient to evaporate the water and other volatiles, and dry the primer coating, although a separate drying step would be required if the coating was applied subsequent to such heating steps. In one preferred embodiment, the primer coating is applied after the film is uniaxially stretched, that is after the film is stretched in one direction, but before the film is stretched in the orthogonal direction. In another preferred embodiment, the polyester film is first stretched in the longitudinal direction prior to coating. In this preferred embodiment, after longitudinal stretching, the film is coated by any of the well known techniques employed in the art. For example, coating may be effected by roll coating, spray coating, slot coating or immersion coating. In a preferred embodiment, the polyester film is coated by means of gravure roll coating. Also, the uniaxially drawn film may be subjected to a corona discharge by a corona discharge apparatus prior to coating as is known in the art. The corona discharge treatment decreases the hydrophobic character of the polyester film surface, which permits the water based primer coating to more easily wet the surface and thus improve the adhesion of the primer coating to the surface. The hydrolyzed aminosilane of the present invention is applied to the film as an aqueous solution at a concentration of from about 0.2 to about 6 percent by weight of the hydrolyzed aminosilane. A weak acid such as acetic acid, phosphoric acid, or the like is then added at about 0.2 percent by weight to facilitate hydrolysis. The preferred concentration of the hydrolyzed aminosilane is within the range of about 0.25 to 2.5 percent by weight. The preferred concentration is such to yield a final dry primer coating weight of about 0,5 mg/m 2 being the target weight. The coating weight may be considerably greater if the coating is applied off-line (that is, applied to the finished film in a separate coating operation) with dry coating weights of 49 mg/m 2 or greater giving good results. The coating of this invention may be applied to one or both sides of the film, or it may be applied to one side and a different coating such as a thermosetting acrylic or methacrylic coating as taught in U.S. Patent No. 4,214,035 applied to the opposite side. The coating may also be applied over a different primer coating to which it will adhere and which is already present on the surface of the film, such as a thermosetting acrylic coating as described in U.S. Patent No. 3,819,773. The coating composition may also contain other ingredients so long as such ingredients do not detract from the adhesion promoting action of the hydrolyzed aminosilane. Such would include minor amounts of colloidal silica, dyes, pH regulating agents, wetting agents, and the like. The primer coating is present on the film surface as a continuous coating, which term is also intended to include where the primer may form a plurality of islands or segregated regions of coating. Scrap film made during production that is coated with the primer coatings of this invention can be ground and mixed with fresh polyester, remelted, and reextruded to produce oriented films. Such film produced, containing significant quantities of primed scrap reclaim, exhibits very little degradation of physical properties due to the presence of the coating impurity and color generation is also low. Thus the primed film of this invention offers commercial advantage to the film manufacturer over many other primed films, such as films primed with vinylidene chloride containing polymers as disclosed in U.S. Patent Nos. 2,627,088 and 2,698,240, which tend to degrade and discolor when reclaimed as set forth above. Laminates may be formed by the well known process of extrusion coating wherein a molten sheet of polymer is continuously deposited on the primer coated surface of a moving web of film. Laminates of polyester with polyethylene, ethylene-vinyl acetate copolymers, polyvinyl alcohol, polyvinyl acetate, and other polymers may be readily made by the extrusion coating process. A film of PET which is extrusion coated with polyethylene has particularly useful properties such as good heat sealing capability or adhesion to other materials such as aluminum foil. Although the inventor is not certain, it is theorized that the amino group in the primer reacts with the heated, oxidized polyethylene when it is extruded onto the PET film, thus firmly bonding the primer to the polyethylene. The thickness of polyester film suitable for use in this invention may generally range from about 6 to about 250 µm or more. The following examples are illustrative of the invention: FILM PREPARATION N-2-(aminoethyl)-3-aminopropyltrimethoxysilane (AE-APTMS) (sold by Dow Corning as Z-6020 and by Union Carbide as A-1120) or N-3-aminopropyltrimethoxysilane (APTMS) was dispersed in ordinary tap water to form a concentration of from about 0.25 percent to 1.5 percent by weight AE-APTMS (or other aminosilane, if indicated). Acetic acid was added at a concentration of 0.2 percent by weight to facilitate hydrolysis. Polyethylene terephthalate polymer was melted and extruded through a slot die onto a casting drum maintained at a temperature of about 20°C. The melt solidified to form a cast sheet. The cast sheet was longitudinally stretched at a draw ratio of approximately 3.5 to 1 while maintained at a temperature of about 80°C. The longitudinally drawn film was corona treated by a corona discharge apparatus and thereafter coated by reverse gravure with the solution of hydrolyzed aminosilane prepared above. The corona treated, longitudinally drawn, coated film was dried at a temperature of about 100°c. Thereafter, the film was stretched in the transverse direction at a draw ratio of 3.9 to 1 to produce a biaxially drawn film. The thickness of the drawn film was from about 12 µm to about75 µm. The biaxially drawn film was then heat set at a maximum temperature of 230°C. The dry weight of the coating was about 2,5 mg/m 2 of film. EXAMPLES 1 - 8 Polyethylene terephthalate film was made and coated as described in FILM PREPARATION and the film was 12 µm thick. The film was coated with an aqueous coating of an amino silane as shown in Table 1. A composite roll was made containing 30 m of film for each coating formulation and the roll was run through an extrusion coater and coated with about 25 µm of low density polyethylene (LDPE) (USI resin with melt index of 14). The melt temperature was 325 °C and the die height above the film was approximately 200 mm. No additional corona treatment or priming was used. Film Coating Formulation (% By Weight) Coating Weight (mg/m 2 ) Example 1 Uncoated control - Example 2 0.25% AE-APTMS 1,3 Example 3 0.5% AE APTMS 2,7 Example 4 1.0% AE-APTMS 5,4 Example 5 1.5% AE-APTMS 8,0 Example 6 0.5% APTMS 2,0 Example 7 1.0% APTMS 4,0 Example 8 1.5% APTMS 6,0 The adhesion of the polyethylene to the uncoated control of Example 1 was 16 mN/mm using ASTM tests D882 and E4. The adhesion of the polyethylene to the films of Examples 2-8 was so good that the two layers could not be separated in order to do a peel test. Separation at the PET/LDPE interface could not be achieved with hot water, toluene, or tetrahydrofuran. Adhesion was excellent with both APTMS (3-aminopropyltrimethoxysilane) and AE-APTMS (N-2-aminoethyl)-3-aminopropyltrimethoxysilane). EXAMPLES 9 - 28 Polyethylene terephthalate film was made and coated as described in FILM PREPARATION and the film was 12 µm thick. The film was coated with an aqueous coating of an amino silane as shown in Table 2. A composite roll was made containing 30 m of film for each coating formulation and the roll was run through an extrusion coater and coated with about 19 µm of SURLYN® 1652, an ionomer resin. The melt temperature was 307 °C. Half of each sample was corona treated at 2.5 kVA prior to extrusion coating and the other half was not corona treated. Another composite roll of the same film was coated with 19 µm of Norchem 1014, a low density polyethylene resin, extruded at 330 °C. As with the SURLYN®, half of each sample was corona treated and the other half was not. The peel strength was measured using an Instron tester and ASTM tests D882 and E-4. (During the extrusion coating operation strips of untreated polyester were inserted between the ends of the layers to provide a nonadhered section where the two layers could be separated for the peel test.) Adhesion (mN/mm) Film Coating Formulation (% By Weight) Corona SURLYN® LDPE Example 9 0.25% AE-APTMS YES 458 215 Example 10 0.25% AE-APTMS NO 497 213 Example 11 0.50% AE-APTMS YES 380 250 Example 12 0.50% AE-APTMS NO 744 238 Example 13 1.00% AE-APTMS YES 424 215 Example 14 1.00% AE-APTMS NO 424 227 Example 15 1.50% AE-APTMS YES 438 247 Example 16 1.50% AE-APTMS NO 445 249 Example 17 0.25% APTMS YES 558 224 Example 18 0.25% APTMS NO 506 250 Example 19 0.50% APTMS YES 392 219 Example 20 0.50% APTMS NO 548 207 Example 21 1.00% APTMS YES 277 215 Example 22 1.00% APTMS NO 313 226 Example 23 1.50% APTMS YES 340 292 Example 24 1.50% APTMS NO 383 222 Example 25 Copolyester YES 180 77 Example 26 Copolyester NO 3,1 14 Example 27 Uncoated YES 184 180 Example 28 Uncoated NO 1,75 19 With the uncoated PET film and with the copolyester coated film, corona treatment prior to extrusion coating is necessary to get some adhesion of the SURLYN® 1652 and Norchem® 1014 to the polyester film. Even with corona treatment, adhesion to the uncoated PET and the copolyester coated film was not as good as to the aminisolane coated film. Also, corona treatment of aminosilane coated film was not necessary and made no difference in the adhesion. Adhesion was good for both AE-APTMS and APTMS at a concentration as low as 0.25 percent by weight.

Claims: A laminated film comprising: an oriented polyester film having a primer coating composition on at least one side thereof, characterized in that said primer coating composition consists of the dried residue of a hydrolyzed aminosilane compound having the formula in the unhydrolyzed state: (R 1 ) a Si(R 2 ) b (R 3 ) c wherein R 1 is a functional group with at least one primary amino group, R 2 is a hydrolyzable group selected from the group consisting of a lower alkoxy group having 1 - 8 carbon atoms, an acetoxy group, or a halide group, and R 3 is a nonreactive, nonhydrolyzable group selected from the group consisting of a lower alkyl group having 1 - 8 carbon atoms, or a phenyl group; with (a) being greater than or equal to 1; (b) being greater than or equal to 1; (c) being greater than or equal to zero, and with a+b+c=4, and that said primer coating composition is present at a weight of from 0.5 to 100 mg/m 2 of film surface, on a dry weight basis, to improve the adhesion of one or more extrusion coated polymers to said polyester film; and an extrusion coated layer of another polymer adhered to the primer coated surface of said primed film, said extrusion coated layer is selected from polyester, copolyester, polyolefin, polyamide, ethylene vinyl acetate, polyvinyl alcohol, or polyvinyl acetate. The film of claim 1 characterized in that the hydrolyzable group (R 2 ) is an alkoxy, selected from the group of a methoxy or an ethoxy. The film of claim 1 or 2 characterized in that the amino functional group (R 1 ) is selected from the group of a primary amino group, a diamine, or a triamine. The film of claim 1 characterized in that the said aminosilane is selected from the group consisting of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane and N-bis-(2-aminoethyl)-3-aminopropyl trimethoxysilane. The film according to any of claims 1 to 4 characterized in that said oriented polyester film is biaxially oriented polyethylene terephthalate. The film according to claim 5 prepared by melt extruding a substantially amorphous polyester film and thereafter orienting the film by stretching it sequentially in two directions and heat setting the film, characterized in that said primer coating composition being applied to said film as an aqueous solution prior to stretching the film in one direction, or subsequent to stretching in one direction but prior to stretching in a mutually perpendicular direction, or subsequent to stretching in two direction. The film of claim 6 characterized in that said film is subjected to a corona discharge treatment prior to the application of said primer coating composition. The film of claim 6 characterized in that said primer coating composition is applied subsequent to stretching in one direction but prior to stretching in a mutually perpendicular direction.

Inventors: SWOFFORD HOWARD WAYNE, SWOFFORD, HOWARD WAYNE
Applicants: HOECHST CELANESE CORP, HOECHST CELANESE CORPORATION
Application year: 1989
Main classification: C08J7
IPC codes: B29K67, B29C55, B29L9, B29C47, C08J7, B32B27