ENHANCED ADHESION OF POLYESTER COATINGS

Abstract: The invention concerns a method of enhancing adhesion of a polyester coating to a substrate using a water-dispersable modified polyester primer having pendant hydrophilic groups attached thereto. The primer allows for thinner polyester coatings, thereby decreasing the plastic content of various coated articles.

Description: TITLE OF THE INVENTION ENHANCED ADHESION OF POLYESTER COATINGS FIELD OF THE INVENTION The present invention relates to a process for enhancing the adhesion of polyester coatings to a substrate, a composite resulting therefrom, and an article containing the composite. The composite comprises a substrate, primer, and polyester coating. BACKGROUND OF THE INVENTION Polyester coatings are known to have unique properties for applications such as packaging. These properties include high heat resistance, grease resistance, low odor and low taste impingement. Thus, polyester homopolymers, polyester copolymers, and polyester blends are commonly extrusion coated onto various substrates to form composite structures useful in various packaging applications. An example of such an application is a polyester-coated paperboard structure for use as a formed tray to contain food that can be cooked in convection ovens and microwave ovens. One major drawback in a structure comprised of a polyester coating and a paperboard is poor adhesion of the coating to the substrate, especially to smooth-finished substrates such as clay-coated paperboards and metal foils. To obtain adequate adhesion of a polyester coating to these and other substrates, the polyester coat weight, or coating thickness, must be increased and/or the extrusion coating lamiπator speed must be reduced. Either approach is economically undesirable. Also, certain foreign countries have passed laws to limit the amount of plastic in a plastic/paperboard structure. Germany, for example, limits the plastic in an extrusion coated structure to no more than 5 weight percent. The minimum coat weight possible to achieve good bond with a polyester coating to paperboard is thought to be 40-grams per square meter, when coated onto a typical carton-grade board (300-400 g m 2 ), exceeding the 5 percent legal limit. Good bond between polyester and paperboard is usually considered to be 200-g/inch (80-g/cm) or greater, or an amount suficient to result in fiber tear when attempting to separate the coating. These legal limits on plastic content have caused a severe problem with manufacturers having otherwise excellent coated products. Thus, improved adhesion of polyester coatings to a substrate while allowing for thinner coating of the polyester is greatly desired. SUMMARY OF THE INVENTION The invention relates to a process for the use of water-dispersible amorphous polyester primers to enhance adhesion of polyester extrusion coating to a variety of substrates. The primers are coated on the substrate, followed by extrusion coating of a polyester onto the primed surface. The primers are polyesters modified by hydrophilic groups which allow the polyester to be water-dispersible. These primers permit thinner coatings of polyesters than could otherwise be achieved without the use of these primers. The invention also relates to the composites produced by the aforementioned process, and to articles containing the composite. Thus, an object of the present invention is to set forth a water- dispersible amorphous polyester primer, said primer being a polyester modified by having hydrophilic moieties attached thereto. Another object of the invention is to describe a primer useful for adhering extrusion coatings comprising polyesters to various substrates, wherein said polyesters may include polyester homopolymers, polyester copolymers, and polyester blends. A further object of the invention is to describe composites and articles comprising such composites, said composites comprising thin polyester coatings on a primed substrate, and wherein said coating has excellent adhesion to the substrate. Yet another object of the invention is to set forth an article comprising a substrate extrusion coated with polyester, wherein said article comprises no more than 5% by weight of plastic, said plastic comprising polyester. Still a further object of the invention is to describe a composite including a primed paperboard substrate extrusion coated with a composition comprising polyester, comprising less than 5% by weight plastic and wherein the adhesion of the coating to the substrate is sufficient to cause fiber tear when attempting to separate the coating. These and other objects, features, and advantages of the present invention will become apparent as reference is made to the following detailed description, preferred embodiments, and specific examples. DETAILED DESCRIPTION OF THE INVENTION The process according to the present invention comprises the dispersion in water of polyesters modified by having hydrophilic moieties attached thereto. The preferred polyesters are those modified by having - SO 3 M attached thereto. M may be hydrogen, NH 4 + , or a metal ion, such as Na + , and the polyester may be modified by mixtures of such ions. Such a polyester may be made, for instance, by a condensation process described in U.S. Patent No. 5,543,488, which is hereby incorporated by reference in its entirety. The process described in 5,543,488 involves reacting an aromatic dicarboxylic acid with an aliphatic or cycloaliphatic glycol to form a polyester species. To this polyester species is added about five to eight side groups, the side groups being the -S0 3 M moiety. As used herein, the term "modified polyester" are polyesters modified with hydrophilic moieties, such as the -SO 3 M moiety. Such moieties allow for pendant cations such as H + , NH 4 + , and metal ions to be present in or on the polyester surface. The preferred modified polyesters are amorphous. Additional examples of useful modified polyesters are described in U.S. Patent Nos. 3,734,874, 3,779, 993, and 543,488, all of which are incorporated by reference. Of course it is to be understood that the modified polyesters may be modified by groups other than the -SO 3 M moiety, such as by -PO 3 M, - SiOM, and the like. In the present invention, the modified polyesters may be modified polyester homopolymers, modified polyester copolymers, and mixtures or blends thereof, and there may be a mixture of hydrophilic moieties. In addition, various additives and fillers may be incorporated, such materials per se known in the art as useful for enhancing properties, such as antifungal and antibacterial agents, and agents to enhance shelf- life. Also, dispersible crosslinking agents, such as urea formaldehyde, may be added to enhance the strength of the resulting finished product. In a preferred embodiment, the modified polyesters have a sodiosulfo substituent, -SO 3 Na + . The more preferred sodiosulfonated polyesters have Glass Transition Temperatures (Tg) of from about 28°F(-2°C) to about 100°F (+38°C). The ionic substituent permits the polyester to be water dispersible which in turn permits its use as an extrusion coating primer for various substrates. The modified polyester primers are applied to the substrate using, for instance, a liquid priming station equipped with a gravure applicator roll. Numerous methods of applying primers to substrates are known perse. Likewise, the extrusion coating or coatings are applied to the primed substrate in a manner perse known in the art. One of ordinary skill in the art, in possession of the present disclosure, can readily apply the primer according to the present invention followed by an extrusion coating, without undue experimentation. It is preferred that a in-line process is used including priming, followed by a dryer, followed by extrusion coating. The extrusion coatings may comprise any extrudable plastic but preferably comprise polyester homopolymers, polyester copolymers, and blends of polyesters. The substrate may be any substrate to which one desires to add an extrusion coating, particularly a polyester coating. Particular substrates include, but are not limited to, natural Kraft papers, photographic papers, clay-coated and natural board stock, polymeric films, and metal foils, such as aluminum foil. Examples The following examples are meant to illustrate the present invention. Numerous modifications and variations are possible, and it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. In laboratory trials, numerous substrates were extrusion coated with modified polyesters according to the present invention. Three polyesters having the sodiosulfonate moieties as modifying groups were prepared as set forth in the aforementioned U.S. Patent No. 5,543,488. The modified polyesters were characterized by having glass transition temperatures (Tg) of 28°F(-2°C), 84°F(+28°C), and 100°F(+38°C). Three specimens of each substrate were first separately primed with one of three water-dispersible polyester primers. From these trials, the inventors unexpectedly discovered that each substrate, ranging from paper to metal foil, prefers one polyester primer over another polyester primer in terms of obtaining maximum polyester coating adhesion and minimize polyester coat weight. For example, polyester adhesion to 40-lb Kraft paper is maximized using the polyester primer having a Tg of 100°F(+38°C); whereas, the use of the other two primers provides no better than about one-third the adhesion of the primer preferred by the Kraft paper. Evaluation of the utility of three aqueous polyester primers was performed as follows: Each primer was first dispersed in 185°F(85°C) water (75 weight percent water, 25 weight percent polyester). Each aqueous dispersion was applied to each of the following substrates using a liquid priming station equipped with a gravure applicator roll, then dried in a forced air oven at 302°F(150°C): (i) 40-lb natural Kraft paper; (ii) photographic paper obtained from Eastman Kodak; (iii) 160-lb Bleached paperboard; (iv) 185-lb Bleached paperboard; (v) 154-lb clay-coated paperboard; and (vi) Aluminum foil (i.e., substrates A through F, respectively, in Table 1 below). Primer weights on each substrate after drying were 0-g/m2, and 0.49- g/m 2 . Each primed substrate was then extrusion coated with a commercial extrusion coating polymer, EASTACOAT ® Polyester 9921 , available from Eastman Chemical Company, Kingsport, TN, having an intrinsic viscosity of 0.80. The polyester was applied to each primed substrate by extrusion from a slot die at a melt temperature of 590°F (310°C) with sufficient output to achieve a polyester coat weight of 24-g/m2. at a haul off speed of 425-fpm (130-m/min). The molten polyester was drawn through a 5.25-inch (14-cm) air gap before striking each substrate and being quenched on a matte-finished chill roll at 64°F(18°C). In addition, the difficult-to-bond-to 154-lb clay-coated paperboard was primed with three primer coat weights using the single water-dispersible primer having a Tg of 100°F (38°C): 0.33-g/m2, 0.49-g/m2, and 0.66-g/m2. The three primed samples of clay-coated paperboard were then extrusion coated with the same commercial extrusion coating polymer, EASTACOAT ® Polyester 9921 , having an intrinsic viscosity of 0.80. The polyester was applied to each of the three primed substrates by extrusion from a slot die at a melt temperature of 590°F (310°C) with sufficient output to achieve three different polyester coat weights of 20-g/m2, 30-g/m2 and 40-g/m2 at respective haul off speeds of 510-fpm (157-m/min), 340-fpm (104-m/min), and 255-fpm (78-m/min). The molten polyester was drawn through a 5.25-inch (14-cm) air gap before striking each substrate and being quenched on a matte-finished chill roll at 64°F (18°C). In the examples to follow: Glass transition temperature (Tg) is measured in accordance with ASTM D5. Intrinsic viscosity is measured in accordance with ASTM D4603. Adhesion of the polyester extrusion coatings to each substrate is measured on an Instron Tensile Tester, Model No. 1122, equipped with a 10-lb (4540-g) load cell and equipped with Testworks computer software, version 3.0. Adhesion is determined on extrusion coated strips 1-inch (2.54-cm) wide. The coating is peeled 180° from the substrate at a rate of 0.1-inch/min (0.254-cm/min). The results are shown in Table 1 below. In the table, the numbers are a measure of the adhesion strength of the coating to the substrate, and take the units g/2.54-cm (or g/inch). TABLE 1 ADHESION OF 24-g/m 2 POLYESTER COATING TO PRIMED SUBSTRATES (At 0.49-g/m 2 Primer Weight) Example 1 Referring to Table 1 , Column A: Maximum polyester adhesion to primed 40-lb Kraft paper is achieved using a water-dispersible primer having a Tg of 100°F (38°C). Example 2 Referring to Table 1 , Column B: Maximum polyester adhesion to primed photographic paper is achieved using a water-dispersible primer having a Tg of 28°F (-2°C). Example 3 Referring to Table 1 , Column C: Maximum polyester adhesion to primed 160-lb bleached paperboard is achieved using a water-dispersible primer having a Tg of 100°F (38°C). Example 4 Referring to Table 1 , Column D: Maximum polyester adhesion to primed Kraft paper is achieved using a water-dispersible primer having a Tg of 28°F (-2°C). Example 5 Referring to Table 1 , Column E: Maximum polyester adhesion to primed 154-lb paper is achieved using a water-dispersible primer having a Tg of 28°F (-2°C). Example 6 Referring to Table 1 , Column F: Maximum polyester adhesion to aluminum foil paper is achieved using a water-dispersible primer having a Tg of 100°F (38°C). Examples 1-6 above show that the choice of a water-dispersible polyester primer in accepting a polyester extrusion coating to a given substrate is dependent upon the composition of that given substrate. These examples also show, in each case, that a polyester primer is essential to achieving satisfactory adhesion. This further leads to possibility of in-line priming on a paper making machine; that is, priming as the finished paperboard is wound. Examples 1 through 6 also show, unexpectedly, that the effectiveness of a water-dispersible primer on a given substrate cannot be predicted by its glass transition temperature. One would have predicted that the bond strengths afforded by the primer having a Tg of 84°F (29°C) would fall between those bond strengths afforded by those primers having Tg's of 28°F (-2 °C) and 100°F (38 °C). To the contrary, it proved to provide the weakest adhesion of the three primers. Nevertheless, its low bond strengths are considered acceptable for some polyester applications, and its use provides clearly superior results to the case where no primer is used. Furthermore, the use of the primers according to the present invention allows for thinner polyester coatings and thus decreases the percent plastic necessary for coated products. This is shown in Table 2, below. As in Table 1, the numbers take the units g/inch, and are a measure of the force necessary to peel the coating 180° from the substrate. TABLE 2 EFFECT OF PRIMER WEIGHT ON ADHESION TO 154-lb CLAY-COATED PAPERBOARD (Using a Primer with a Tg=38°C) Example 7 Referring to Table 2: Table 2 shows that by using a water-dispersible polyester primer, (a) polyester coat weights can be significantly reduced to as thin as 20-g/m2, and, (b) unexpectedly, if an extrusion coater desires to apply heavy coatings, e.g., 40-g/m2, of a polyester to a clay-coated paperboard, the primer coat weight should not exceed about 0.5-g/m2. This example shows that a critical range of primer coat weight exists for heavy polyester coatings on clay-coated paperboard —about 0.3-g/m2 to about 0.5-g/m2. Thus, the present inventors have discovered that a primer comprising water dispersible modified polyesters having hydrophilic pendant groups can provide enhanced adhesion of a polyester coating to a substrate, and allows for thinner coatings of the polyester.

Claims: CLAIMS 1. A method to enhance adhesion of polyester coating to a substrate comprising applying a water-dispersable modified polyester having hydrophilic pendant groups as a primer on said substrate to create a primed substrate surface and then extrusion coating a polyester onto said primed substrate surface. 2. The method according to Claim 1, wherein said water-dispersable modified polyester is a polyester modified with pendant groups having a cation selected from the group consisting of hydrogen, NH 4 + , a metal ion, and mixtures thereof. 3. The method according to Claim 1 , wherein said water-dispersable modified polyester is a polyester modified with the group SOaNa. 4. The method according to Claim 1 , wherein said water-dispersable modified polyester is a polyester having a sodiosulfo substituent, having a Tg of from about -2°C to about 38°C. 5. The method according to Claim 1 , wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay-coated board stock, natural board stock, polymeric film, and metal foil. 6. The method according to Claim 4, wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay-coated board stock, natural board stock, polymeric film, and metal foil. 7. A composite comprising a substrate having coated thereon a primer and then a polyester layer, said primer comprising a water-dispersable modified polyester having at least one pendant hydrophilic group attached thereto. 8. The composite according to Claim 7, wherein said primer is a water- dispersable modified polyester having pendant hydrophilic groups comprising a cation selected from the group consisting of hydrogen, NH 4 + , a metal ion, and mixtures thereof. 9. A composite according to Claim 7, wherein said primer is a water- dispersable modified polyester having -SOsNa groups attached thereto. 10. A composite according to Claim 7, wherein said water-dispersable modified polyester is a polyester having a sodiosulfo substituent, having a Tg of from about -2°C to about 38°C. 11. A composite according to Claim 7, wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay-coated board stock, natural board stock, polymeric film, and metal foil. 12. A composite according to Claim 10, wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay- coated board stock, natural board stock, polymeric film, and metal foil. 13. An article comprising a substrate coated with a primer and then coated with polyester, wherein said primer is a water-dispersable modified polyester having pendant hydrophilic groups attached thereto. 14. The article according to Claim 13, wherein said water-dispersable modified polyester has pendant hydrophilic groups including a cation selected from the group consisting of hydrogen, NH 4 + , a metal ion, and mixtures thereof. 15. The article according to Claim 13, wherein said water-dispersable modified polyester is a polyester modified with the SO 3 Na moiety. 16. The article according to Claim 13, wherein said water-dispersable modified polyester is a polyester having a sodiosulfo substituent, having a Tg of from about -2°C to about 38°C. 17. The article according to Claim 13, wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay-coated board stock, natural board stock, polymeric film, and metal foil. 18. The article according to Claim 16, wherein said substrate is selected from the group consisting of Kraft paper, photographic paper, clay-coated board stock, natural board stock, polymeric film, and metal foil.

Inventors: BEALL WHITNEY KAY, EDWARDS RAY, FARNHAM DIANE HINES, FOSTER BRUCE WILLIAM, BEALL, WHITNEY, KAY, EDWARDS, RAY, FARNHAM, DIANE, HINES, FOSTER, BRUCE, WILLIAM
Applicants: EASTMAN CHEM CO, EASTMAN CHEMICAL COMPANY
Application year: 1998
Main classification: C09D167
IPC codes: B05D7, B29C47, B32B27, C09D5, C09D167