HEAT RESISTANT THERMOPLASTIC COPOLYESTER HOT-MELT ADHESIVES This invention relates to thermoplastic copolyesters that are useful adhesives and form bonds exhibiting excellent resistance to debonding over a wide range of tempertures. More particularly, this invention relates to thermoplastic copolyesters derived from terephthalic acid, diethylene glycol and ethylene glycol, that are useful hot-melt adhesives which form bonds that exhibit excellent resistance to high temperatures .

Hot-melt adhesives are well known and their use commercially has grown rapidly in recent years. However, despite their many advantages, hot-melt adhesives, particularly those based on thermoplastic copolyesters, have not generally exhibited the resistance to high temperatures and bonding ^' properties needed to bond parts in "ovenable containers". Such containers can also be called oven heat resistant containers and are used to package ready-to-cook foods and are required to withstand heat of the intensity and for the duration of time normally encountered in cooking ovens, e.g. 204.4°C. (400°F.) for one hour. Ovenable paperboard trays are sometimes used as replacements for aluminum trays for packaging frozen foods. The ovenable board is often made by extrusion coating poly(ethylene terephthalate) onto paperboard, which is formed into trays or is made into folding cartons. The trays are generally covered with lids made of poly(ethylene terephthalate) coated board or film. Poly(ethylene terephthalate) coated board is sometimes sealed with a hot-melt adhesive to the ovenable tray, but inadequate seals are frequently obtained on trays in

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which food has been cooked, probably due to crystallization of the poly(ethylene terephthalate) coating during cooking. Quick setting latex adhesives are also used, but these adhesives often provide bonds which fail at low temperatures,, and the adhesive applicators must be cleaned very frequently.

Certain four and five component, amorphous polyesters can be coated from solution onto poly(ethylene terephthalate) lidding film, and these coated films heat sealed to poly(ethylene terephthalate) coated ovenable board. However, the lidded packages frequently debond when stored at low temperatures or when heated in an oven at 204.4 C. (400°F.). Accordingly, there is a need for an improved adhesive which can be coated by extrusion or solution techniques, which will bond to both crystallized and noncrystallized poly(ethylene terephthalate) coatings on paperboard trays and adhere to both poly(ethylene terephthalate) film and poly(ethylene terephthalate) coated paperboard lids. Thermoplastic copolyesters derived from terephthalic acid, diethylene glycol and ethylene glycol and their use as hot-melt adhesives are described in the prior art, as illustrated by U.S. Patent 4,156,774, issued May 29, 1979. The thermoplastic copolyester adhesives described in this patent comprise 80-100 mol % terephthalic acid, 0-20 mol X of aliphatic, cycloaliphatic, or other aromatic dicarboxylic acids, 30-60 mol X ethylene glycol, 30-69 mol X diethylene glycol, and 1-10 mol X polybutylene glycol having a minimum relative viscosity of 1.4 (minimum inherent viscosity (I.V.), as described hereinafter, of 0.34). Up to 20 mol X

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of the ethylene glycol or diethylene glycol can be replaced with alkylenediols having 3 to 10 carbon atoms and/or bisphenol dihydroxyalkyl ethers. Unfortunately, such four component copolyesters fail to form bonds exhibiting the necessary high heat resistance required for ovenable containers.

This invention provides thermoplastic . copolyesters that are useful hot-melt adhesives which form bonds that are resistant to high temperatures, i.e. 204.4°C. (400°F.), for one hour. In addition, unlike the thermoplastic copolyesters of U.S. Patent 4,156,774, issued May 29, 1979, the thermoplastic copolyesters of this invention are derived from only . three essential monomeric materials, i.e. terephthalic acid, diethylene glycol and ethylene glycol, which permits simple and less complicated copolyester synthesis and adhesive formulation procedures. Moreover, as illustrated by the following Examples 4 and 6, the mol percentages of each of the monomeric materials specified herein, are necessary to obtain the outstanding combination of heat resistance and sealing properties described in detail herein.

Accordingly, this invention provides a thermoplastic copolyester useful as a hot-melt adhesive, characterized in that the copolyester (1) consists essentially of an acid component of at least 99 mol percent terephthalic acid and a glycol component of (a) from 30 to 50 mol percent diethylene glycol and (b) at least 50 mol percent ethylene glycol and (2) has an inherent viscosity of from 0.4 to 1.0.

As used herein, the term terephthalic acid

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includes functional equivalents thereof, such as dimethyl terephthalate, as is known in the prior art. In practicing this invention it is preferred that the acid component be 100 mol percent terephthalic acid and the glycol component be 30 to 50 mol X diethylene glycol and 50 to 70 mol percent ethylene glycol. Most preferred is for the glycol component to be 32 to 40 mol X diethylene glycol and 60 to 68 mol X ethylene glycol. When the diethylene glycol content of the copolyester is less than 30 mol X, the copolyester is not readily soluble in solvents typically used to form hot-melt coating solutions. Also, the melting point of the copolyester is too high for it to be reactivated by heat sealing at 204.4°C. (400°F.).

Also, when the diethylene glycol content is greater than 50 mol X, the glass transition temperature (Tg) is low, which makes the copolyester difficult to handle in bulk and coated form. Also, bonds made with such materials frequently fail when heated at 204.4°C. (400°F.) for one hour which conditions simulate typical cooking conditions.

The inherent viscosity (I.V.) of the thermoplastic copolyesters is in the range of 0.4 to. 1.0, preferably, from 0.5 to 0.9, measured as described hereinafter. Copolyesters having such inherent viscosities exhibit satisfactory cohesive strength to enable bonding under minimum heat sealing conditions, e.g. 204.4°C. (400°F.) for 0.5 seconds while also permitting rapid dissolution in solvents for solution coating. Furthermore, copolyesters having such I.V.'s maintain bond integrity during baking up to one hour at 204.4°C. (400°F.)..

Copolyesters of this invention are soluble in chlorinated solvents and readily form coating solutions with such solvents. Suitable solvents include chlorinated solvents such as methylene chloride, chloroform, 1,1,2-trichloroethane, and

1,1,2,2-tetrachloroethane. The coating solutions can be made using 5 to 15 parts, by weight, of the - copolyester. Upon evaporation of the solvent, a solid adhesive layer remains on a substrate which may be adhered to another surface by melting the adhesive. Layers of adhesive as thin as 0.003 mm (0.1 mil) can be coated from the solution of the copolyester in solvent. Adhesive layers can also be extruded onto film, paper, or paperboard lids at thicknesses of 0.03 to 0.06 mm (1 to 2 mils). Lids adhered with these adhesives are resistant to delamination when lidded trays are quick-frozen at -40°C, are stored at -10 to -15°C, or are baked at up to 204.4°C (400°F) for one hour. The copolyesters of this invention are thermoplastic polymers, i.e. they can be rendered soft and pliable by the action of heat. They are amorphous materials, although crystallizable to a slight extent. Such copolyesters are readily prepared using typical polycondensation reaction conditions well known in the art, as illustrated by British Patent No. 1,047,072, complete specification published March 3, 1964. They can be prepared by either batch or continuous processes. Typical polyesterification catalysts that are used in such techniques include titanium alkoxides, ditutyl tin dilaurate, and combinations of zinc, manganese or magnesium acetates or benzoates with antimony oxide or antimony triacetate. The copolyesters can be

isolated from the reaction medium using any suitable procedure and put into the form of pellets, granules, chips, etc., for subsequent application to a substrate.

5 The copolyesters of thi-s invention are particularly adapted for use as adhesives on articles such as food containers that are subjected to high temperatures encountered in ovens. However, they can also be used on various types of paper, paperboard,

- _Q plastic, leather, metal, wood, ceramics, etc. They are especially suited for use with poly(ethylene terephthalate) or substrates coated with poly¬ ethylene terephthalate) .

Various addenda commonly used in adhesive

15 compositions to enhance bond strength, improve coatability, prevent sticking of the coated material in roll form and provide surface texture and reflection properties can be used with the thermoplastic copolyesters described herein. Such

20 addenda include colorants, plasticizers, antioxidants, fillers and waxes. Adhesive compositions comprising the thermoplastic copolyesters described herein and such addenda are conveniently formulated using any mixing method known 5 to the art.

As used herein, the term "inherent viscosity" (I.V.) refers to viscosity determinations made at 25°C using 0.25 gram of polymer per 100 ml. of a solvent comprising 60 weight percent phenol and 0 40 weight percent tetrachloroethane.

Unless otherwise indicated, all parts, percentages and ratios described herein are on a weight basis .

The "melting point" (T ) of the 5 copolyesters described in this application are

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readily obtained with a Differential Scanning Calorimeter.

The "heat of fusion" ΔH^ of a polymer is the amount of heat absorbed when crystallizable polymers are melted. ΔH _£ values are readily obtained using conventional Differential Scanning Calorimeters (DSC) such as the Perkin-El er "DSC-1B" Differential Scanning Calorimeter. For example, one convenient method for determining ΔH _£ of the copolyester of this invention is described in the

Journal of Applied Polymer Science, 2 , 1209 (1976). Measurement of ΔH _£ is also described in duPont Thermal Analysis Bulletin No. 900-8 (1965). Qualitatively, it is possible to compare the degree of crystallinity of polymers by comparing their ΔH^ values.

The strength of the adhesive bonds described herein is determined according to a "Peel Test" which is based on a modification of the ASTM T-Peel Test se forth on pages 63 and 64 of the 1964 edition of the BOOK OF ASTM STANDARDS, published by the American Society for Testing Materials, and more specifically identified as Test Number D-1876-61-T.

To evaluate their ability to withstand heat of the intensity and for the duration normally encountered in cooking ovens, bonds formed by the thermoplastic copolyesters described in certain of the following examples were subjected to a simulated cooking test. In carrying out this test, a bonded ? mple (laminate) was simply subjected to a temperature of 204.4°C (400°F.) for one hour. A sample observed as showing no substantial debonding under these conditions passes this "cooking test".

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The strength of the bonds formed by the thermoplastic copolyester described in the following Example 6, Run B were evaluated using a somewhat more objective test than that described in the preceding paragraph. This test involved more stringent conditions than are normally encountered in cooking ovens since it measured the peel failure temperature of a bonded sample which had a 100 g. weight attached to the sample in a peel configuration. In carrying out this test, the bonded sample (laminate) was suspended in a circulating air oven at ambient temperature with a 100 g. weight attached to one of the laminated members of the sample in a peel configuration (T-Peel Test ASTM D-1876-T) . The temperature of the oven was increasd in 10°C increments each ten minutes until the bond failed by peeling. This temperature is the peel failure temperature for the sample analyzed.

The following examples are submitted for a better understanding of the invention. EXAMPLE 1

500 ml flask; the following were placed in a

135.8 g (0.70 mol) of dimethyl terephthalate, 68.0 g (1.1 mol) of ethylene glycol, 40.0 g (0.38 mol) of diethylene glycol, and 4.7 ml of acetyl titanium isopropoxide catalyst solution, (163 g of ethylene glycol, 0.43 g acetic acid, and 2.0 g titanium tetraisopropoxide) to give a concentration of 67 ppm of titanium based on predicted polymer weight. The flask was flushed with nitrogen, fitted with a motorized stirrer, and connected to a vacuum pump. The flask was heated in a bath of low melting metal alloy to 200°C. for one hour, while the reactants were stirred, under a nitrogen atmosphere. Methanol

was evolved and collected in a trap as transesterification proceeded. Next, the temperature of the bath was raised to 215°C. for one hour while transesterification continued. Finally, the temperature of the bath was raised to 277°C. When this temperature was reached, the flask was evacuated to less than 0.07 kPa (0.5 torr) with the vacuum pump to remove excess glycol components. The reaction was run under these conditions for 75 minutes, with constant stirring. As the molecular weight of the polymer increased, the stirring speed was reduced to maintain contact of the polymer with the flask walls. After the 75 minute polyesterification period, the flask was removed from the heating bath and repressurized with nitrogen. The resulting copolyester was allowed to cool and was removed from the flask by breaking away the flask. The copolyester had an I.V. of 0.66 and a diethylene glycol content of 37 mol X (by hydrolysis/gas chromatography analysis) . The copolyester was tough, slightly yellowish, and clear and it had a DSC melting point of 183°C. and a glass transition temperature (Tg) of 57°C. measured on a Differential Scanning Calorimeter (DSC) lib instrument at a heating rate of 20°C/minute.

Five grams of the copolyester was dissolved in 95 g. of chloroform at 50°C, with stirring. The copolyester remained in solution as the mixture cooled to room temperature. The solution was then coated onto 0.05 mm (2.0 ail) thick pol (ethylene terephthalate) film with a wire wound coating rod. The coating, after drying for two hours at room temperature followed by two days at 50°C , measured

0.005 mm (0.2 mil) thick. Samples of the coated film were heat sealed at 204. °C. (400°F.) for 0.5 second to samples of paperboard previously coated to a thickness of 0.03 mm (1-1/4 mils) with poly(ethylene terephthalate). ^' The poly(ethylene terephthalate) coated paperboard samples were either amorphous (as prepared) or crystallized (15 minutes at 150°C to simulate cooking prior to lidding) . The bonds were cooled and aged two days at 50°C The bonds were evaluated as described previously herein, by peeling the film from the poly(ethylene terephthalate) coated board using a 90° peel angle and measuring the maximum peel force required using a spring loaded force gauge. Bonds of the film to the crystallized pol (ethylene terephthalate) samples had a strength of 0.14 kg/cm (0.8 pounds/inch) with failure at the interface. Similar bonds made to the noncrystallized polyethylene terephthalate coated samples had a strength of 0.63 kg/cm (3.5 pounds/inch) with failure being cohesive in the poly(ethylene terephthalate) film. The bonded samples prepared in this example pass the "cooking test". EXAMPLE 2

A copolyester was prepared by the method of Example 1 using 135.8 g (0.7 mol) of dimethyl terephthalate, 65.8 g (1.05 mol) of ethylene glycol, 38.9 (0.37 mol) of diethylene glycol, and 4.7 ml σf the catalyst solution of Example 1. The polymer which resulted had an I.V. of.0.42 and a diethylene glycol content of 34 mol X (by hydrolysis/gas chromatography analysis). The copolyester was tough, had a melting point (DSC) of 193°C (ΔH _f of 3.9 cal/g.) and a Tg of 60°C. The polymer was pale yellow and opaque. A solution of 5 weight percent,

of the copolyester in ethylene chloride, was prepared and coated onto 0.01 mm (0.5 mil) polyethylene terephthalate film with a coating rod. The coated film was bonded at 204. °C (400°F) for 0.5 second to poly(ethylene terephthalate) coated board samples that were either, crystallized or noncrystallized. Bonds to the noncrystallized .. poly(ethylene terephthalate) coating failed cohesively by breaking the film at 0.45 kg/cm (2.9 pounds/inch); bonds to the crystallized poly(ethylene terephthalate) coating failed by peeling at a peel strength of 0.2 kg/cm (1.1 pounds/inch). The bonded samples prepared in this example pass the "cooking test". EXAMPLE 3

A hot-melt adhesive containing terephthalic acid, ethylene glycol, and diethylene glycol was made by the process of Example 1 and contained 33 mol X diethylene glycol. The adhesive copolyester had an I.V. of 0.5 and was coated onto 0.05 mm (2 mil) polyester film at a thickness of approximately 0.01 mm (0.5 mil). The adhesive was heat sealed to samples of crystallized and noncrystallized poly(ethylene terephthate) coated paperboard at several tempera¬ ture/time conditions. When the bonds were tested as described in Example 1, the following results were obtained:

Temperature, T-Peel, Strength, kg/cm (pounds/inch) °C (°F) Crystallized Noncrystallized

Time (Second) Coating Coatins

19ϋ.ό(375)/ϋ.5 ϋ ^" l8(1.0) 0.52(2.9)

204.4(400)/0.5 0.29(1.6) 0.54(3.0)

204.4(400)/0.2 _ 0.21(1,2) 0.46(2.6)*

204.4(400)/0.1 0.34(1.9) 0.41(2.3)

218.3(425)/0.5 0.30(1.7) 0.52(2.9)

218.3(425)/0.2 0.41(2.3) 0.34(1.9)* 218.3(425)/0.1 0.36(2.0) 0.64(3.6)*

*Denotes substrate cohesive failure.

EXAMPLE 4

This is a comparative example which illustrates that a copolyester having less than the required 30 to 50 mol percent diethylene glycol does not form a satisfactory hot melt adhesive. A copolyester was prepared by the method of Example 1 from dimethyl terephthalate 135.8 g (0.7 mol), ethylene glycol 76.0 g (1.24 mol), diethylene glycol 29.0 g (0.27 mol), using 4.7 ml of acetyl titanium isopropoxide catalyst solution. The copolyester contained 28 mol X diethylene glycol and had an I.V. of 0.57. This copolyester did not dissolve in either methylene chloride or chloroform, which are typical solvents used in forming hot melt adhesive coating solutions; so no solution coatings were made. The copolyester melted too high to bond at 204.4°C. (400°F.).

Poly(ethylene terephthalate) modified with 28 mol X diethylene glycol was extrusion coated onto paperboard at 234°C. This coated board failed to heat seal to polyethylene terephthalate coated paperboard at 204.4°C. (400°F.) for 0.5 second. EXAMPLE 5

A copolyester was prepared by the method of Example 1 from a mixture of 135.8 g of dimethyl terephthalate, 66.3 g of ethylene glycol, 39.8 g of diethylene glycol, using 100 ppm of titanium catalyst (7.6 g of the catalyst solution of Example 1). The resulting copolyester contained 38 mol X diethylene glycol and had an I.V. of 0.96. This copolyester was extruded onto kraft paper at 225°C. The adhesive layer was 0.03 mm (1-1/2 mils) thick. The coated paper was heat sealed to samples of crystallized and

noncrystallized polyethylene terephthalate coated paperboard at 218.3°C. (425°F.) for 0.2 seconds. Bonds were evaluated as described herein using a 90° peel angle configuration and found to have a peel strength of 0.23 kg/cm (1.3 pounds/inch) on the crystallized substrate and 0.05 kg/cm (2.8 pounds/inch) with delamination of the coated paperboard, on the noncrystallized substrate. The bonds pass the cooling test. EXAMPLE 6

Run A This is a comparative example which illustrates that a copolyester having more than the required 30 to 50 mol percent diethylene glycol is not a suitable hot-melt adhesive as described herein.

A copolyester was prepared as described in Example 1 from a mixture containing 135.7 g (0.70 mol) of dimethyl terephthalate, 21.7 g (0.35 mol) of ethylene glycol, 111.3 g (1.05 mol) of diethylene glycol, and an n-butanol catalytic solution of titanium tetraisopropoxide (2,4 weight percent titanium) to give 100 ppm titanium based on the weight of polymer prepared. The copolyester contained 75 mol X diethylene glycol (by hydrolysis/gas chromatography analysis), had an I.V. of 0.62 and a glass transition temperature (Tg) of 30°C. This copolyester was very difficult to process into pellet form. Also, the pellets became tacky and exhibited cold flow when they were slightly warmed. Paperboard co-ted with this adhesive was tacky and coated sheets blocked severely when stacked. Bonds made to poly(ethylene terephthalate) coated paperboard at 204°C. (400°F.) for 5 seconds do not withstand the "cooking test".

Run B A copolyester was prepared by the method of Example 1 and analyzed to contain 52 mol X of diethylene glycol, 48 mol X of ethylene glycol, and 100 mol X of terephthalate acid. This copolyester had an I.V. of 0.77, was amorphous and had no melting point, and had a glass transition temperature (Tg) of 48°C.

The copolyester was readily soluble in methylene chloride at 10X solids at 60°C. It was coated from this methylene chloride solution onto 0.01 mm (0.5 mil) thick poly(ethylene terephthalate) film. The coated film was heat sealed at 204.4°C. (400°F.) for 0.5 sec. to poly(ethylene terephthalate) coated paperboard. Duplicate samples of these bonded laminates peeled at 100°C. (Peel Failure Temperature). In contrast, samples using comparable copolyesters containing 44 and 37 mol percent, respectively, each exhibited a Peel Failure Temperature of 150°C.

This run clearly demonstrates that copolyester adhesives having more than 50 mol X of diethylene glycol have sharply reduced peel failure temperatures in comparison to those having the 30-50 mol percent diethylene glycol content required in this invention.

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