Field of the Invention

The aqueous lattices of the invention can be used as laminating adhesives in joining film layers. The latex adhesives of the invention combine a blend of ethylenically unsaturated monomers, resulting in superior properties that the latex imparts to the laminated product.

Background of the Invention A great deal of attention has been directed in recent years to the preparation of aqueous lattices comprising polymers made from ethylenically unsaturated monomers that can be used in a variety of applications. The aqueous lattices of the invention exhibit improved properties in the emulsion form and in the laminate form when compared to solvent based polymer adhesives due to the lack of toxicity and flammability in the aqueous diluent. Further, aqueous materials have certain processing advantages over hot melt materials as well related to viscosity and machinability properties.

Aqueous lattices are prepared by the polymerization of ethylenically unsaturated monomers typically in a water dispersion using thermal or redox catalysts. A variety of monomer types, polymerization conditions, and catalysts have been investigated in the preparation of aqueous lattices. In the manufacture of aqueous latex adhesives for film lamination purposes, however, the search for a water-resistant self-emulsifiable, high solids, colloidally stable adhesive continues.

We have found that there is no adequate aqueous latex laminating adhesive available to date that has a blend of properties that adequately serves the needs of laminate manufacturers in the food and non-food flexible packaging

marketplace. To meet these needs, the preferred latex adhesives of the invention should have a solids content of greater than 50 wt-% and an aqueous viscosity of less than about 5,000 preferably less than 1,000 cP. The adhesive should also be moisture-resistant, mechanically stable, resistant to surfactant migration, exhibit wet and dry properties, able to be readily processed, and able to be manufactured at low cost.

Accordingly, a substantial need exists for an aqueous laminating latex adhesive composition that is self- emulsifiable, contains an appropriate anionic hydrophobic balance so as to obtain a good colloidal stability and high adhesion, wets typical film surfaces, is generally non- migrating, and has adequate adhesion properties so as to maintain the laminate structure.

Summary of the Invention We have found a novel laminating latex that is manufactured by polymerization of anionic and hydrophobic monomers, produces a polymer having a range of molecular weights and T. that is colloidally stable, and wets common film surfaces. The polymer latex of the invention is made by polymerizing a monomer mixture consisting of: "soft" monomers that upon polymerization result in a low T _g ; hard monomers such as vinyl acetate, methyl methacrylate, methyl acrylate, styrene, etc.; and a blend of anionic monomers that provide the desired lamination and stable dispersion properties. We have found that the resulting polymers are polydisperse and have an effective low molecular weight fraction such that the tendency of the polymer latex to wet common film surfaces is increased. We have found that the polymerization system has an appropriate anionic hydrophobic balance that aids colloidal stability but prevents surfactant migration. The resulting polymer, due to the blend hard monomers, vinyl acetate, methyl

methacrylate, methyl acrylate, styrene, and the "soft" monomers, has a T _g that ranges from about -65 °C to 15 °C with adequate flexibility and adhesive properties at room temperature. The solids content is greater than 50 wt-%, the viscosity is less than 1,000 and can be less than 500 cP, and the peel strength and T-shear peel strength show adequate utility for laminating applications.

The monomers useful in the preparation of the laminating latex of the invention comprise: (a) about 40 to 95 wt-% of at least one C ₂ _ ₁₀ aliphatic ester of acrylic or methacrylic acid; (b) about 0.1 to 5 wt-% of an alpha, beta olefinically unsaturated monomer; (c) about 0.1 to 5 wt-% of a monomer of the formula:

wherein R is hydrogen or a C ₅ alkyl, n is 1 to 12 and X is a proton, an alkali metal or an alkaline earth metal ion. The polymer formulation set forth above can also contain other monomers that can aid to optimize laminating properties. Further, the latex can contain additional ingredients that can act to further enhance or stabilize the latex. The invention also relates to a laminate comprising two

film layers bonded by the laminating latex of the inventio .

Detailed Description of the Invention As set forth above, the latex of the invention contains a C ₂ _ ₁₀ aliphatic alcohol ester of acrylic or methacrylic acid, methyl acrylate or methyl methacrylate, an olefinically unsaturated monomer containing a pendant carboxylic acid group, an ester of an alkanoic acid, and the sulfonic acid or sulfonate-containing monomer set forth above.

The alcohol ester of acrylic or methacrylic acid can comprise ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl methacrylate, n- butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, isooctyl, amylacrylate, isoamylacrylate, etc. The ester of an alkanoic acrylate acid can comprise vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, etc. The carboxylic acid-containing monomer useful in the manufacture of the latex polymers of the invention include monomers such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, a vinyl benzoic acid, and dicarboxylic acid half esters. The monomer blend useful for manufacturing the latex of the invention can also contain a variety of other ethylenically unsaturated monomers including styrene, ethylmethylstyrene, acrylonitrile, methacrylonitrile, vinyl toluene, vinyl chloride, vinylidene chloride, vinyl acetate, vinyl butyrate, and other well known vinyl monomers. The blend of monomers used in the latex preparation can be adjusted using well known polymerization techniques to obtain the desired T _g .

The polymer latex of the invention also contains a sulfonic acid or sulfonate-containing monomer having the formula:

wherein R is a hydrogen or a C _x _ ₅ alkyl, n is 1-12 and X is a proton, an amine salt, an alkali metal or an alkaline earth metal ion. Such monomer, in combination with the carboxylic acid pendant monomer, provides anionic functionality which copolymerizes with the hard and soft monomer components to form the preferred adhesive latex systems of the invention. Such a blend of monomers produces the molecular weight distribution, hydrophobic anionic balance, and adhesive properties necessary to achieve a level of success required for commercial adhesive latex material. Examples of suitable monomers include a sulfonic acid-substituted ethyl acrylate, sodium sulfonate- substituted n-butyl methacrylate, sodium sulfonated- substituted ethyl methacrylate, 1-propane sulfonic acid, 2- hydroxy-3-(2-propenyloxy)-sodium vinyl sulfonate, etc. This functional monomer contains a vinyl group to allow for incorporation into the base polymer, and a sulfonic acid or sulfonate end group to provide emulsion stability without

significantly affecting water sensitivity of the dried adhesive laminate. A particularly preferred monomer of this type is a 2-sulfoethyl methacrylate known as Sipomer 2-SEM, which is available from W. R. Grace. Certain compositions of the invention can include surfactants, chain transfer agents, and polymerization initiators. A low level of a free surfactant enhances the emulsion stability under shear forces of pumping or coating operations without adversely affecting the water sensitivity of the adhesive laminate. Any of the known surfactants can be used, as for example. Aerosol MA-80, which is a dihexyl ester of sodium sulfosuccinate and is available from American Cyana ide. Other surfactants include monophenol ethoxylates, sodium dodecyl benzene sulfonates, sodium dioctyl sulfosuccinate, octyl phenol ethoxylates, octyl phenol ethoxylate sulfonates, the ammonium salt and mixtures thereof, and the like. The amount of surfactant in the composition is within the range of about 0.1 to 1 wt-%, preferably about 0.3 wt-%, based upon the total weight of the composition. Surfactants can be added prior to polymerization or after polymerization is complete.

A chain transfer agent is used in certain preferred compositions to modify the molecular weight of the polymer. A particularly preferred such agent is lauryl mercaptan, e.g., n-dodecyl or t-dodecyl mercaptan. Other useable chain transfer agents include butyl mercaptan, amyl mercaptan, octyl mercaptan, etc. The chain transfer agent is generally included in the compositions at a relatively low level, i.e., within a range of about 0.01 to 2 wt-% based on the total weight of the emulsion.

A polymerization initiator that is thermally activated is used to advantage in certain preferred emulsions of the invention as a source of free radical species. The initiator is typically present in an amount of about 0.01

to 4 wt-%, based on the total weight of the emulsion. Sodium persulfate is a particularly useful initiator. Others include potassium persulfate, ammonium persulfate, hydrogen peroxide, and t-butyl hydroperoxide and Vazo type irritations such as WACO 50.

In the preparation of the aqueous latex of the invention, the aqueous medium is typically maintained at an acid pH of less than about 6, preferably less than about 3. The pH can be adjusted with commonly available bases. A preferred base is ammonium hydroxide. The polymerization is typically conducted under conventional polymerization conditions using adequate agitation, typical reactor and reaction temperature control, and maintaining the temperature between about 45 °C to 100 °C. Generally, the aqueous polymerization system contains greater than about 40 wt-%, preferably greater than about 50 wt-%, solids based on the total emulsion. As stated above, conventional free radical initiators of the thermal and redox type can be used. Also, chain transfer agents can be employed conventionally, to control molecular weight and distribution.

The major thrust of the latex system of the invention is the blend of hard monomers, soft monomers, carboxylic functional monomers, and sulfonic acid substituted monomers which copolymerize to form the useful adhesive of the invention. To enhance the properties of the latex of this invention, minimal amounts of additional emulsifiers can be used in the preparation of the emulsion. Resin can be recovered if desired, preferably by spray drying, coagulation, dewatering or drying to yield a hard particulate resin. Preferably, however, the material is sold in aqueous concentrate form.

Film Substrate Materials Films that can be used for forming the laminates of the invention are commonly organic film-forming compositions that can be formed from a variety of common polymeric films including printed papers, metallized papers, metal foils, or various chemical or energy surface treated films.

Thermoplastic resins are also useful in the laminate films of the invention. Useful addition polymers include poly alpha-olefins, polyethylene, polypropylene, poly 4- methyl-pentene-1, ethylene/vinyl copolymers ethylene vinyl acetate copolymers, ethylene acrylic acid copolymers, ethylene methacrylate copolymers, ethyl-methylacrylate copolymers, etc.; thermoplastic propylene polymers such as polypropylene, ethylene-propylene copolymers, etc.; vinyl chloride polymers and copolymers; vinylidene chloride polymers and copolymers; polyvinyl alcohols, acrylic polymers made from acrylic acid, methacrylic acid, methylacrylate, methacrylate, acrylamide and others. Fluorocarbon resins such as polytetrafluoroethylene, polyvinylidiene fluoride, and fluorinated ethylene- propylene resins. Styrene resins such as a polystyrene, alpha-methylstyrene, high impact polystyrene acrylonitrile- butadiene-styrene polymers and others. Preferred material for use in the laminate of the invention are polyester film materials such as poly- ethylene-teraphthlate, polybutylene teraphthlate and polyimide materials. These film materials are sold by duPont, Allied-Apical, Tiejin, Kanega-fuchi, as Mylar , Kapton ^® , Apical , Upilex , etc., films.

In somewhat greater detail, the polymerization process of the invention includes a first step of introducing the monomers singly, or in the mixture, into the aqueous solution contained within a conventional polymerization reactor. Typically, the water contains a small amount of a

free surfactant. The monomer feed and polymerization reaction is generally carried out at a temperature of from about 25 °C to about 100 °C, preferably at a temperature within the range of about 60 °C to 90 °C. More preferably, the reactor charge is heated to about 70 °C to 80 °C. A small amount of the monomer mix is then added (about 2%). The monomer mix typically contains all monomers and the chain transfer agent, but not the sulfonate-containing component if it is not soluble in the particular mix of monomers. The time of reaction is not critical but depends on many variables such as particular monomer blends, concentration of initiator, heat removal capacity, reactor size, batch size, etc. After a small amount of the monomer mix has been added, the initiator is then added in an amount within the range of about 0.1 to 4 wt-%, preferably at about 2 wt-% of the total weight of the emulsion. The reactor charge is then further heated to about 78 °C to 82 °C. The remainder of the monomer mix, the catalyst, and the sulfonate-containing component are then added separately and slowly to the reactor charge. After a hold period of about 30 minutes to one hour, the reactor contents are cooled to ambient and the emulsion is filtered.

The latex prepared by the process of the invention and can contain up to 70 wt-% of the polymer based on the total weight of the latex, and preferably from about 45 to 65 wt- %. The latex adhesive produced provides great flexibility in producing quick tack, peel strength, shear strength, viscosity, emulsion stability, etc. Generally, the lattices prepared using the process of the invention have relatively high surface tension due to the absence of free surfactant in the latex. Depending on the polymer film and other variables in the lamination process, the surface tension may be modified by adding small amounts of surfactants.

The latex produced by the process of the invention may also contain other additives well known to those skilled in the art including pH control agents, foam control agents, salts and organic solvents, dyes, perfumes, etc. The latex adhesive compositions of the invention may be used to produce laminates from a wide variety of sheet-like flexible webs.

The preferred laminates are made from polyethylene, polypropylene, polyesters such as polyethylene teraphthlate, cellophane, polyimide polyvinyl chloride, polyvinylidiene dichloride, nylon, paper, woven and nonwoven fabrics made from fibers comprising cotton, polyester polyolefin, polyamide, polyimide, and the like can be used. Additionally, metallic sheet-like materials such as aluminum foil, metallized film, paper and paperboard, foam materials, etc., can be used.

Laminates can be formed using the adhesive of the invention using conventional techniques employed to apply aqueous based adhesive emulsions to a continuous web. Thus, the adhesive may be applied to film web by any standard mechanical coating process such as road coating, 3 roll coating gravure processing, etc. Most commonly, the adhesive is coated on the primary web and is heated to remove residual water. The adhesive is then laminated with an additional film web or substrate.

The product of the present invention exhibits excellent mechanical stability, good wetting to a variety of film substrates, and minimal foaming. As with many water-based products, the laminating adhesive latex composition of the present invention may exhibit varying leveling qualities during application. An addition of about 2 wt-% or less of n-propyl alcohol or isopropyl alcohol to the composition can improve the leveling qualities. In addition, higher machine speeds as surfactants such as acetylnic glycol or sodium dioctyl sulfosuccinate can produce better leveling.

In some cases a smoothing bar may be used to enhance leveling.

The following examples and test data illustrate the preparation of the emulsion adhesives of the invention and their use in the preparation of film laminates.

Example 1 The laminating adhesive surfactant-free latex composition made according to Example 1 was coated at 1.5 to 2.0 lbs/3000 ft ² dry on 2.0 mil thick polyester and laminated to one of the following substrates: LDPE, aluminum foil, polyprolylene, surlyn, PVDC cellophane. This was also done for a solvent-based system and for a surfactant-stabilized system. These laminations were aged for three days under ambient conditions prior to exposure to humidity or further testing. The data listed below under the "Dry Peel" results were for samples that were aged for 21 days at ambient temperature and humidity. The data listed below under the "Wet Peel" results were for samples that were aged for 21 days at room temperature and 100% relative humidity.

The surfactant-based system contains 30.84% 2-ethyl hexyl acrylate, 16.84% vinyl acetate, 0.7% 2-hydroxy ethyl acrylate, 0.36 lauryl mercaptan, 47.26% deionized water, 1.04% sodium lauryl sulfate, 1.08% nonylphenoxypoly(ethyleneoxy)ethanol (HLB = 13.0), 1.14 hydroxyehtylcellulose, 0.28 ammonium persulfate initiator, 0.39 sodium bicarbonate buffer, 0.07% preservative. The material has a theoretical Tg of -41 °C and contains 52% solids.

Example 2 A preferred laminating adhesive surfactant-free latex composition was prepared in the following manner. At room temperature, deionized water (32.65 parts by weight).

Aerosol M-80 (0.07 parts by weight), and aqueous ammonia (26° Braume, 0.14 parts by weight) were charged to a reactor equipped with a nitrogen purge. With mixing, the reactor charge was heated to 75.5°C. A monomer mix was prepared from 29.47 parts by weight 2-ethyl-hexyl-acrylate, 9.97 parts by weight methyl methacrylate, 7.25 parts by weight vinyl acetate, 1.00 parts by weight glacial acrylic acid, and 0.14 parts by weight lauryl mercaptan. About 2% of the monomer mix wa then added to the reactor. Sodium persulfate (0.27 parts by weight) in deionized water (1.39 parts by weight) was then added to the reactor and the contents were immediately heated to 82.5°C. with steam and held at that temperature (plus or minus about 1.5°C.)_ for about 15 minutes. The remainder of the monomer mix, sodium persulfate (0.14 parts by weight) in deionized water (4.18 parts by weight), and 2-SEM (0.42 parts by weight) in deionized water (4.18 parts by weight) with Aerosol MA-80 (0.33 parts by weight) were then added separately and slowly to the reactor charge. The remaining monomer mix was added over a period of 3 hours, the catalyst (sodium persulfate) was added over a period of 3.5 hours, and the 2-SEM mixture was added over a period of 3 hours. The temperature was maintained with the range of 81.5°C. to 83.5°C. during the period of addition and after all additions were made for an additional hour. The reactor contents were cooled to room temperature (about 25°C). Defoamer (0.50 parts by weight, available from Colloids) and antimicrobial (0.05 parts by weight, available from ICI), were then added and the emulsion mixed for 10 minutes. The emulsion was then filtered through a 75 micron filter.

Example 3 A preferred laminating adhesive surfactant-free latex composition was prepared in the following manner. At room

temperature, deionized water (32.65 parts by weight) and aqueous ammonia (26° Baume, 0.14 parts by weight) were charged to a reactor equipped with a nitrogen purge. With mixing, the reactor charge was heated to 75.5°C. A monomer mix was prepared from 41.82 parts by eight n-butyl acrylate, 9.97 parts by weight methyl methacrylate, 3.20 parts by weight vinyl acetate, 1.00 parts by weight glacial acrylic acid, and 0.14 parts by weight tert-dodeyl mercaptan. About 2% of the monomer mix was then added to the reactor. Sodium persulfate (0.27 parts by weight) in deionized water (1.39 parts by weight) was then added to the reactor and the contents were immediately heated to 82.5°C. with steam and held at that temperature (plus or minus about 1.5°C.) for about 15 minutes. The remainder of the monomer mix, sodium persulfate (0.14 parts by weight) in deionized water (4.18 parts by weight), and 2-SEM (0.42 parts by weight) in deionized water (4.18 parts by weight) with Aerosol MA-80 (0.33 parts by weight) were then added separately and slowly to the reactor charge. The remaining monomer mix was added over a period of 3 hours, the catalyst (sodium persulfate) was added over a period of 3.5 hours, and the 2-SEM mixture was added over a period of 3 hours. The temperature was maintained within the range of 81.5°C. to 83.5°C. during the period of addition and after all additions were made for an additional hour. The reactor contents were cooled to room temperature (about 25°C). Defoamer (0.05 parts by weight, available from Colloids) and antimicrobial (0.05 parts by weight, available from ICI) were then added and the emulsion mixed for 10 minutes. The emulsion was then filtered through a 75 micron filter.

Example 4 A preferred laminating adhesive surfactant-free latex composition was prepared in the following manner. At room

temperature, deionized water (32.65 parts by weight), and Aerosol OT-75 (0.07 parts by weight), 0.14 parts by weight) were charged to a reactor equipped with a nitrogen purge. With mixing, the reactor charge was heated to 75.5°C. A monomer mix was prepared from 36.24 parts by weight n-butyl acrylate, 6.97 parts by weight 2-ethyl hexyl acrylate, 9.97 parts by weight methyl methacrylate, 1.81 parts by weight vinyl acetate, 1.00 parts by weight glacial acrylic acid, and 0.14 parts by weight lauryl mercaptan. About 2% of the monomer mix was then added to the reactor. Potassium persulfate (0.27 parts by weight) in deionized water (1.39 parts by weight) was then added to the reactor and the contents were immediately heated to 82.5°C. with steam and held at that temperature (plus or minus about 1.5°C.) for about 15 minutes. The remainder of the monomer mix, potassium persulfate (0.14 parts by weight) in deionized water (4.18 parts by weight), and 2-SEM (0.42 parts by weight) in deionized water (4.18 parts by weight) with Aerosol OT-75 (0.33 parts by weight) were then added separately and slowly to the reactor charge. The remaining monomer mix was added over a period of 3 hours, the catalyst (potassium persulfate) was added over a period of 3.5 hours, and the 2-SEM mixture was added over a period of 3 hours. The temperature was maintained within the range of 81.5°C. to 83.5°C. during the period of addition and after all additions were made for an additional hour. The reactor contents were cooled to room temperature (about 25°C). Defoamer (0.05 parts by weight, available from Colloids) and antimicrobial (0.05 parts by weight, available from ICI) were then added and the emulsion mixed for 10 minutes. The emulsion was then filtered through a 75 micron filter.

Example 5 A preferred laminating adhesive surfactant-free latex composition was prepared in the following manner. At room temperature, deionized water (32.65 parts by weight), Aerosol M-80 (0.07 parts by weight), and aqueous ammonia (26 degrees Baume, 0.14 parts by weight) were charged to a reactor equipped with a nitrogen purge. With mixing, the reactor charge was heated to 75 °C. A monomer mix was prepared from 43.21 parts by weight n-butyl acrylate, 9.97 parts by weight methyl methacrylate, 1.81 parts by weight vinyl acetate, 1.00 parts by weight glacial acrylic acid, and 0.14 parts by weight lauryl mercaptan. About 2% of the monomer mix was then added to the reactor. Sodium persulfate (0.27 parts by weight) in deionized water (1.39 parts by weight) was then added to the reactor and the contents were immediately heated to 82 °C with steam and held at that temperature (plus or minus about 1 °C) for about 15 minutes. The remainder of the monomer mix, sodium persulfate (0.14 parts by weight) in deionized water (4.18 parts by weight), and 2-SEM (0.42 parts by weight) in deionized water (4.18 parts by weight) with Aerosol MA-80 (0.33 parts by weight) were then added separately and slowly to the reactor charge. The remaining monomer mix was added over a period of 3 hours, the catalyst (sodium persulfate) was added over a period of 3.5 hours, and the 2-SEM mixture was added over a period of 3 hours. The temperature was maintained within the range of 81 °C to 83 °C during the period of addition and after all additions were made for an additional hour. The reactor contents were cooled to room temperature (about 25 °C) . Defoamer (0.05 parts by weight, available from Colloids) and antimicrobial (0.05 parts by weight, available from ICI) were then added and the emulsion mixed for 10 minutes. The emulsion was then filtered through a 75 micron filter.

Example 6 A preferred laminating adhesive surfactant-free latex composition was prepared in the following manner. At room temperature, deionized water (32.65 parts by weight), Aerosol M-80 (0.07 parts by weight), and aqueous ammonia (26° Baume, 0.14 parts by weight) were charged to a reactor equipped with a nitrogen purge. With mixing, the reactor charge was heated to 75.5°C. A monomer mix was prepared from 44.55 parts by weight n-butyl acrylate, 3.48 parts by weight methyl methacrylate, 6.96 parts by weight styrene, 1.00 parts by weight glacial acrylic acid, and 0.14 parts by weight tert-dodecyl mercaptan. About 2% of the monomer mix was then added to the reactor. Potassium persulfate (0.27 parts by weight) in deionized water (1.39 parts by weight) was then added to the reactor and the contents were immediately heated to 82.5°C. with steam and held at that temperature (plus or minus about 1.5°C.) for 15 minutes. The remainder of the monomer mix, potassium persulfate (0.14 parts by weight) in deionized water (4.18 parts by weight), and 2-SEM (0.42 parts by weight) in deionized water (4.18 parts by weight) with Aerosol MA-80 (0.33 parts by weight) were then added separately and slowly to the reactor charge. The remaining monomer mix was added over a period of 3 hours, the catalyst (potassium persulfate) was added over a period of 3.5 hours, and the 2-SEM mixture was added over a period of 3 hours. The temperature was maintained within the range of 81.5°C. to 83.5°C. during the period of addition and after all additions were made for an additional our. The reactor contents were cooled to room temperature (about 25°C). Defoamer (0.05 parts by weight, available from Colloids) and antimicrobial (0.05 parts by weight, available from ICI) were then added and the emulsion mixed for 10 minutes. The emulsion was then filtered through a 75 micron filter.

The above specification, examples, comparative examples and data provide detailed explanation and discussion of the current embodiment of the claimed invention. However, many embodiments to the invention can be made without departing from the spirit and scope of the invention. Accordingly, the invention resides on the claims hereinafter appended.