Laminating adhesive compositions for packaging applications

Abstract: An aqueous laminating adhesive composition is disclosed. The composition is adapted to bond difficult-to-adhere substrates. The composition comprises a vinyl ester/acrylic copolymer having a glass transition temperature from about 10 °C. to about -45 °C. The composition is free from organic cosolvent.

Description: The present invention relates to laminating adhesive compositions for packaging applications. In particular, the present invention relates to aqueous vinyl ester/acrylic-based laminating adhesive compositions free from organic cosolvent which are adapted to bonding difficult-to-adhere substrates such as for packaging applications. Major portions of the laminating adhesives industry such as, for example, the packaging adhesives require a laminating adhesive to permanently bond difficult-to-adhere substrates including, for example, plastic films such as, for example, polyethylene, polypropylene, and polyesters; and wax coatings. Present adhesives such as, for example, vinyl acetate/ethylene copolymers require the addition of volatile organic solvents, preferably chlorinated solvents, to obtain adequate adhesion or, alternatively, require the inclusion in the adhesive composition of expensive "soft" polymers, i.e., polymers with a glass transition temperature (Tg) below about-30°C. Attempts to eliminate volatile organic solvents from aqueous vinyl acetate/ethylene copolymer laminating adhesives have failed because a vinyl acetate/ethylene composition of sufficiently low Tg that volatile organic solvents are not needed requires a high level of ethylene, i.e., above about 30-40 weight % ethylene; such polymer compositions tend to crystallize which has the effect of making the polymer brittle and thereby exhibit poor adhesion performance. Vinyl acetate homopolymers have also been evaluated as laminating adhesives but it has not been possible to incorporate the levels of volatile organic solvent required to achieve adequate adhesion performance. Further, legislation and concerns regarding the use of volatile organic solvents make this approach undesirable. US-A-4,694,056 discloses an aqueous pressure sensitive adhesive containing a copolymer of an alkyl acrylate or vinyl ester, or both, an acid functional comonomer, and a polyfunctional copolymerizable monomer. The '056 patent does not disclose a laminating adhesive composition, which is used in a wet state, without cosolvent which exhibits good adhesive performance. US-A-5,100,944 discloses a water-borne packaging and converting adhesive which contains 10-98 parts dispersion of a vinyl acetate homo-/co-/ter-polymer, 2-30 parts by weight of plasticizer, and 1-20 parts by weight of ethylene glycol diacetate (EGDA) as the sole organic solvent. Also disclosed is the use of the EGDA in place of conventional halogenated solvents such as methyl chloroform to provide adhesive products with reduced environmental and health related problems. The '944 patent does not disclose a laminating adhesive composition without cosolvent which exhibits good adhesive performance. The present invention seeks to overcome the problems associated with the prior art. According to a first aspect of the present invention there is provided a method of laminating two substrates comprising (a) forming an aqueous, organic solvent-free laminating adhesive composition comprising a vinyl ester/acrylic copolymer having a glass transition temperature from 10°C to -45°C; (b) applying said composition to a first substrate; (c) contacting said applied composition with a second substrate; and, (d) after (c), drying said composition, wherein said first substrate and/or second substrate is a difficult-to-adhere substrate selected from the group consisting of plastic films and wax coatings. Preferably said glass transition temperature is from 10°C to -45°, more preferably 15 °C. to -35 °C.. Preferably said copolymer comprises a copolymerized acrylic monomer selected from the group consisting of n-butyl acrylate and 2-ethylhexyl acrylate, vinyl acetate and n-butyl acrylate, and 2-ethylhexyl acrylate and vinyl acetate. Preferably said copolymer further comprises a stabilizer selected from the group consisting of hydroxyethyl cellulose and polyvinyl alcohol. Preferably said copolymer is prepared with from 0.05% to about 0.75% of a chain transfer agent. Preferably said chain transfer agent is selected from the group consisting of n-dodecyl mercaptan and methyl mercaptopropionate. Preferably the laminating adhesive composition further comprises at least 5% clay by weight, based on the weight of said copolymer. Preferably the laminating adhesive composition further comprises an aqueous tackifier. The present invention therefore provides an aqueous vinyl ester/acrylic-based laminating adhesive compositions adapted to bond difficult-to-adhere substrates, which compositions are free from organic cosolvent. A method for laminating difficult-to-adhere substrates is also provided. The advantages of the present invention are that the aqueous laminating adhesive compositions incorporating vinyl ester/acrylic copolymers with a selected Tg from about 10 °C. to about -45 °C., preferably from about-15 °C. to about -35 °C. exhibit good adhesion performance with difficult-to-adhere substates without requiring any cosolvent. This invention is therefore directed to aqueous laminating adhesive compositions adapted to bond difficult-to-adhere substrates, the compositions incorporating vinyl ester/acrylic copolymers with a Tg from about 10 °C. to about -45 °C., preferably from about -15 °C. to about -35 °C., which compositions exhibit good adhesion performance without requiring organic solvent. Glass transition temperatures (Tgs) herein are those calculated as a weighted average of the homopolymer Tg values, that is, for calculating the Tg of a copolymer of monomers M1 and M2, Tg(calc.)= w(M1) x Tg(M1) + w(M2) x Tg(M2), wherein Tg(calc.) is the glass transition temperature calculated for the copolymer w(M1) is the weight fraction of monomer M1 in the copolymer w(M2) is the weight fraction of monomer M2 in the copolymer Tg(M1) is the glass transition temperature of the homopolymer of M1 Tg(M2) is the glass transition temperature of the homopolymer of M2 The glass transition temperature of homopolymers may be found, for example, in "Polymer Handbook", edited by J. Brandrup and E.H. Immergut, Interscience Publishers. The aqueous vinyl ester/acrylic copolymers of the composition of this invention contain at least one copolymerized vinyl ester. Vinyl esters are ethylenically unsaturated ester monomers which include monomers such as, for example, vinyl acetate, vinyl propionate, and vinyl versatate. Vinyl acetate is preferred. The aqueous vinyl ester/acrylic copolymers of the composition of this invention contain at least one copolymerized acrylic monomer. Acrylic monomers herein include esters of (meth)acrylic acid, esters of crotonic acid, amides of (meth)acrylic acid, nitriles of (meth)acrylic acid, and the like. Acrylic ester monomers such as, for example, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and lauryl methacrylate may be used, subject to the Tg restrictions on the copolymer as stated herein. Acrylic ester monomers are preferred as a result of their low Tgs and the ease of polymerization with vinyl ester monomers. Butyl acrylate and 2-ethylhexyl acrylate are more preferred. The aqueous copolymers of the composition of this invention may contain a copolymerized polar monomer such as, for example, a monomer containing a hydroxyl group such as, for example, hydroxyethyl acrylate and hydroxypropyl methacrylate; and a monomer containing a carboxylic acid group such as, for example, acrylic acid, methacrylic acid, itaconic acid, crotonic acid and fumaric acid; from 0% to about 10% by weight, based on the weight of the copolymer. Acrylic acid from about 0.2 to about 2% by weight, based on the weight of the copolymer is preferred. When low levels of precrosslinking or gel content are desired, low levels of multi-ethylenically unsaturated monomers such as, for example, allyl (meth)acrylate, diallyl phthalate, 1,4-butylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and the like, may be used from about 0.01% to about 5 %, by weight based on the weight of the copolymer. The aqueous vinyl ester/acrylic copolymers of this invention may be prepared by various addition polymerization techniques. Emulsion polymerization is preferred. The vinyl ester/acrylic emulsion copolymers may be be prepared by techniques for polymerizing ethylenically-unsaturated monomers which are well known in the art. Colloidal stabilization, anionic or nonionic surfactant stabilization, or mixtures thereof, may be used. Stabilization by a colloidal stabilizer such as, for example, hydroxyethyl cellulose, N-vinyl pyrollidone, polyvinyl alcohol, partially acetylated polyvinyl alcohol, carboxymethyl cellulose, and gum arabic is preferred. More preferred is stabilization by polyvinyl alcohol from about 0.05% to about 10% by weight, based on the weight of the emulsion copolymer. The polymerization reaction may be initiated by various methods known in the art such as, for example, by using the thermal decomposition of an initiator and by using an oxidation-reduction reaction to generate free radicals to effect the polymerization. Chain transfer agents including mercaptans, polymercaptans, and halogen compounds may be used in the polymerization mixture in order to moderate the molecular weight of the vinyl ester/acrylic emulsion copolymer. Generally, from 0% to about 1% by weight, based on the weight of the polymeric binder, of C 2 - C 20 alkyl mercaptans, 3-mercaptopropionic acid, or esters of 3-mercaptopropionic acid, may be used. Preferred is from 0.05% to about 0.75% by weight, based on the weight of the polymeric binder, of dodecyl mercaptan or methyl 3-mercaptopropionate. The particles of the vinyl ester/acrylic emulsion copolymer are from about 100 nanometers to about 4000 nanometers in diameter, as measured using a Brookhaven BI-90 Particle Sizer, which employs a light scattering technique. Further, broad particle size distributions and polymodal particle size distributions such as those disclosed in US-A-4,384,056 and US-A-4,539,361 may be employed. The solids content of the vinyl ester/acrylic emulsion copolymer may be from about 30% to about 70% by weight. Preferred is a solids content from about 45 to about 60% by weight. The viscosity of the vinyl ester/acrylic emulsion copolymer may be from about 200 mNsm -2 (cps.) to about 12,000 mNsm -2 (cps.), as measured by a Brookfield viscometer (Model LVT using spindle #3 at 12 rpm.). Preferred is a viscosity from about 1000 mNsm -2 (cps.) to about 5000 mNsm -2 (cps.). The aqueous laminating adhesive composition is free from organic solvent. "Organic solvent-free" as used herein means that the adhesive composition does not contain "volatile organic compounds" defined herein as having a boiling point at atmospheric pressure of less than about 240°C., as may materially contribute to pollution, toxicity and/or volatile organic content of the adhesive compositions. "Organic solvent-free" composition is not to be taken as excluding compositions which contain very low levels of deliberately or adventitiously added volatile organic compounds such as, for example, solvents incorporated in certain commercial initiator or surfactant compositions and volatile organic compound impurities; in any event the level of volatile organic compounds is less than about 1% by weight, based on the dry weight of the vinyl ester/acrylic copolymer. The aqueous laminating adhesive composition may contain, in addition to the vinyl ester/acrylic emulsion copolymer, conventional treatment components such as, for example, emulsifiers, pigments, fillers, anti-migration aids, curing agents, thickeners, humectants, wetting agents, biocides, plasticizers, organosilanes, anti-foaming agents, colorants, waxes, and anti-oxidants. The aqueous laminating adhesive composition may be applied to a substrate by conventional techniques such as, for example, roll coating, wire wound rod coating, knife coating, gravure printing, curtain coating, or the like. The aqueous laminating adhesive composition, after it is applied to a substrate, may optionally be heated to effect drying and, optionally, curing. The duration and temperature of heating will affect the rate of drying, processability and handleability, and property development of the treated substrate. Heat treatment from about 25°C. to about 250°C. for a period of time between about 3 seconds to about 15 minutes may be carried out. The aqueous laminating adhesive composition may be used for applications such as, for example, packaging adhesives to permanently bond difficult-to-adhere substrates including, for example, plastic films such as, for example, polyethylene, polypropylene, polyvinyl chloride, and polyesters; and wax coatings; to themselves, to one another, or to other substrates such as, for example, paper and paperboard. The present invention will now be described only by way of examples. In the following Examples, the following abreviations are used: pli = x 5.6 kg/cm (equivalent to x 1 pound per linear inch - based on the calculation that 1 lb/in is equivalent to 0.18 kg/cm); CTA = chain transfer agent; °C. = degrees Centigrade; FT = fiber tear; OPP = oriented polypropylene; RH = relative humidity; PET = polyethylene terephthalate; DI = deionized; (meth)acrylate = acrylate or methacrylate; gli = x 2.6 g/cm (equivalent to x 1 gram per linear inch - based on the calculation that 1 g/in is equivalent to 0.39 g/cm); MMP = methyl 3-mercaptopropionate; and nDDM = n-dodecyl mercaptan. EXAMPLE 1. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 1. A stirred reactor containing 3,950 g. of deionized (D.I.) water was heated to 65 °C under nitrogen. Then 2 g of a 1 wt.% solution of iron (II) sulfate in water was added, followed by 50g of a monomer mixture consisting of: 2200 g of DI water, 2000 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 280 g of a 70 wt% solution of a nonylphenol 16 mole ethoxylate in water, 6000 g of vinyl acetate, 2080 g of n-butyl acrylate, 120 g of acrylic acid, and 13.8 g of diallyl phthalate. Then a mixture of 7.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 50 g of DI water was added. The remaining portion of the monomer mixture was fed according to the following schedule: 45 g/min for 15 min, then 65 g/min for the next 15 min, and then finish the feed at 87 g/min. Total feed time was approximately 3 hours. At the same time as the monomer mixture feed, the following two solutions were added: 22.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 150 g of DI water and 10.5 g of sodium sulphoxylate formaldehyde dissolved in 170 g of DI water, according to the following schedule: 1.1 g/min for 15 min, 0.75 g/min for the remainder of the monomer mixture feed, and 1.5 g/min after the completion of the monomer mixture feed. When 3,940 g of the monomer mixture remained, the following was added to it: 280 g of DI water, 770 g of butyl acrylate, 30 g of acrylic acid, and 1.2 g diallyl phthalate. At the completion of the monomer mixture feed an additional 200 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 10 wt% solution of sodium carbonate in water. The final product had a solids content of 53.7%, Brookfield viscosity (#3 spindle at 12 rpm) of 1,100 mNsm -2 (cps.), and Tg(calc.)= 6 °C. EXAMPLE 2. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 2. A stirred reactor containing 665 g. of deionized (D.I.) water was heated to 65 °C under nitrogen. Then 2 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 1.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 10 g of DI water. A feed of a monomer mixture consisting of: 520 g of DI water, 400 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 71 g of a 70 wt% solution of a nonylphenol 16 mole ethoxylate in water, 1000 g of vinyl acetate, 970 g of n-butyl acrylate, and 30 g of acrylic acid, was started. The monomer mixture was fed according to the following schedule: 5 g/min for 15 min, then 10 g/min for the next 15 min, 15 g/min for 15 min, and then the feed was finished at 20 g/min. Total feed time was approximately 3 hours. At the same time as the monomer mixture feed, the following two solutions were cofed: 4.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 42 g of DI water and 2.1 g of sodium sulphoxylate formaldehyde dissolved in 46 g of DI water, according to the following schedule: 0.2 g/min for 15 min, 0.25 g/min for 15 min, 0.3 g/min for 15 min, 0.25 g/min for 15 min, and then 0.2 g/min for the remainder of the solutions. At the completion of the monomer mixture feed an additional 45 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt.% solution of sodium carbonate in water. Sample 2 had a solids content of 53.7%, and Tg(calc.)= -8.6 °C. EXAMPLE 3. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 3. Sample 3 was prepared according to the process of Example 2 using the following monomer mixture: 520 g of DI water, 400 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 71 g of a 70 wt% solution of a nonylphenol 16 mole ethoxylate in water, 700 g of vinyl acetate, 1270 g of n-butyl acrylate, and 30 g of acrylic acid. Sample 3 had a solids content of 55.3%, Brookfield viscosity (#3 spindle at 12 rpm) of 3,100 mNsm -2 (cps.), and Tg(calc.)= -21.5 °C. EXAMPLE 4. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 4. Sample 4 was prepared according to the process of Example 2 using the following monomer mixture: 520 g of DI water, 400 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 71 g of a 70 wt% solution of a nonylphenol 16 mole ethoxylate in water, 400 g of vinyl acetate, 1570 g of n-butyl acrylate, and 30 g of acrylic acid. Sample 4 had a solids content of 53.5%, Brookfield viscosity (#3 spindle at 12 rpm) of 5,900 mNsm -2 (cps.), and Tg(calc.)= -34.4 °C. EXAMPLE 5. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 5. Sample 5 was prepared according to the process of Example 2 using the following monomer mixture: 520 g of DI water, 400 g of a partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 71 g of a 70 wt% solution of a nonylphenol 16 mole ethoxylate in water, 500 g of vinyl acetate, 1480 g of n-butyl acrylate, and 20 g of acrylic acid. The cofed hydrogen peroxide solution was 3.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 42 g of DI water. Sample 5 had a solids content of 54.6%, a Brookfield viscosity (#3 spindle at 12 rpm) of 4,300 mNsm -2 (cps.), and Tg(calc)= -30.9 °C. EXAMPLE 6. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 6. A stirred reactor containing 500 g. of deionized (D.I.) water was heated to 65 °C under nitrogen. Then 2 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 0.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 10 g of DI water. A feed of a monomer mixture consisting of: 590 g of DI water, 300 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 30 g of a nonylphenol 10 mole ethoxylate, 500 g of vinyl acetate, 1480 g of n-butyl acrylate, and 20 g of acrylic acid, was started. The monomer mixture was fed according to the following schedule: 5 g/min for 15 min, then 10 g/min for the next 15 min, 15 g/min for 15 min, and then the feed was finished at 20 g/min. Total feed time was approximately 165 min. At the same time as the monomer mixture feed, the following two solutions were cofed: 4.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 43 g of DI water and 2.1 g of sodium sulphoxylate formaldehyde dissolved in 46 g of DI water, according to the following schedule: 0.2 g/min for 15 min, 0.25 g/min for 15 min, 0.3 g/min for 15 min, 0.25 g/min for 15 min, then 0.2 g/min until 15 min after the completion of the monomer mixture feed, and then 0.5 g/min for the remainder of the solutions. At the completion of the monomer mixture feed an additional 45 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt% solution of sodium carbonate in water. Sample 6 had a solids content of 55.3%, Brookfield viscosity (#3 spindle at 12 rpm) of 2,600 mNsm -2 (cps.), and Tg(calc.)= -30.9 °C. EXAMPLE 7. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 7. A stirred reactor containing 500 g. of deionized (D.I.) water was heated to 65 °C under nitrogen. Then 2 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 0.3 g of 30 wt% hydrogen peroxide (in water) dissolved in 10 g of DI water. A feed of a monomer mixture consisting of: 590 g of DI water, 300 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 30 g of a nonylphenol 10 mole ethoxylate, 500 g of vinyl acetate, 1480 g of n-butyl acrylate, and 20 g of crotonic acid, was started. The monomer mixture was fed according to the following schedule: 5 g/min for 15 min, then 10 g/min for the next 15 min, 15 g/min for 15 min, and then the feed was finnished at 20 g/min. Total feed time was approximately 3 hours. At the same time as the monomer mixture feed, the following two solutions were cofed: 2.4 g of 30 wt% hydrogen peroxide (in water) dissolved in 44 g of DI water and 1.26 g of sodium sulphoxylate formaldehyde dissolved in 46.5 g of DI water, according to the following schedule: 0.2 g/min for 15 min, 0.25 g/min for 15 min, 0.3 g/min for 15 min, 0.25 g/min for 15 min, then 0.2 g/min for the remainder of the solutions. At the completion of the monomer mixture feed an additional 45 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt.% solution of sodium carbonate in water. Sample 7 had a solids content of 55.3%, Brookfield viscosity (#3 spindle at 12 rpm) of 500 mNsm -2 (cps.)s, and Tg(calc.)= -30.9°C. EXAMPLE 8. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 8. A stirred reactor containing 3,020 g. of deionized (D.I.) water and 150 g of a nonylphenol 10 mole ethoxylate of was heated to 66 °C under nitrogen. Then 5.1 g of a 0.6 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 2.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 15 g of DI water. A feed of a monomer mixture consisting of: 2602 g of DI water, 10 g of sodium acetate, 1660 g of a 21% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 2500 g of vinyl acetate, 7400 g of n-butyl acrylate, and 100 g of acrylic acid, was started. The monomer mixture was fed at 86 g/min for 165 min. At the same time as the monomer mixture feed, the following two solutions were cofed: 20.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 311 g of DI water and 11.4 g of sodium sulphoxylate formaldehyde dissolved in 320 g of DI water, at 1.7 g/min for 190 min. At the completion of the monomer mixture feed an additional 45 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 10 wt% solution of sodium carbonate in water. Sample 8 had a solids content of 54.8%, Brookfield viscosity (#3 spindle at 12 rpm) of 1,560 mNsm -2 (cps.), and Tg(calc.)= -30.9 °C. EXAMPLE 9. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 9. A stirred reactor containing 575 g. of deionized (D.I.) water and 30 g of a nonylphenol 10 mole ethoxylate was heated to 65 °C under nitrogen. Then 2 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 0.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 5 g of DI water. A feed of a monomer mixture consisting of: 560 g of DI water, 300 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 900 g of vinyl acetate, 1080 g of 2-ethylhexyl acrylate, and 20 g of acrylic acid, was started. The monomer mixture was fed at 17.5 g/min for 165 min. At the same time as the monomer mixture feed, the following two solutions were cofed: 4.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 62 g of DI water and 2.1 g of sodium sulphoxylate formaldehyde dissolved in 65 g of DI water, at 0.32 g/min for 180 min. At the completion of the monomer mixture feed an additional 30 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt% solution of sodium carbonate in water. Sample 9 had a solids content of 53.6%, Brookfield viscosity (#3 spindle at 12 rpm) of 500 mNsm -2 (cps.), and Tg(calc.)= -30.4 °C. EXAMPLE 10. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 10. Sample 10 was prepared according to the method of Example 9 using the following monomer mixture: 560 g of DI water, 300 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 700 g of vinyl acetate, 1280 g of 2-ethylhexyl acrylate, and 20 g of acrylic acid. Sample 10 had a solids content of 54.5%, Brookfield viscosity (#4 spindle at 12 rpm) of 16,000 mNsm -2 (cps.), and Tg(calc.)= -42 °C. EXAMPLE 11. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 11. Sample 11 was prepared according to the method of Example 9 using the following monomer mixture: 560 g of DI water, 300 g of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 800 g of vinyl acetate, 1180 g of 2-ethylhexyl acrylate, and 20 g of acrylic acid. Sample 11 had a solids content of 54.7%, Brookfield viscosity (#3 spindle at 12 rpm) of 650 mNsm -2 (cps.), and Tg(calc.)= -36.3 °C. EXAMPLE 12. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 12. A stirred reactor containing 211 g. of deionized (D.I.) water and 12 g of a nonylphenol 10 mole ethoxylate was heated to 65 °C under nitrogen. Then 0.8 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 0.2 g of 30 wt% hydrogen peroxide (in water) dissolved in 2 g of DI water. A feed of a monomer mixture consisting of: 211 g of DI water, 122 g of a 19.6% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 200 g of vinyl acetate, 592 g of n-butyl acrylate, 8 g of acrylic acid, and 0.8 g of n-dodecyl mercaptan, was started. The monomer mixture was fed at 6.9 g/min for 165 min. At the same time as the monomer mixture feed, the following two solutions were cofed: 1.6 g of 30 wt% hydrogen peroxide (in water) dissolved in 39.8 g of DI water and 0.8 g of sodium sulphoxylate formaldehyde dissolved in 41 g of DI water, at 0.2 g/min for 180 min. At the completion of the monomer mixture feed an additional 12 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt% solution of sodium carbonate in water. Sample 12 had a solids content of 54.5% (#3 spindle at 12 rpm) of 1,850 mNsm -2 (cps.), and Tg(calc.)= -30.9 °C. EXAMPLE 13 Preparation of vinyl ester/acrylic copolymer Preparation of Sample 13. Sample 13 was prepared according to the method of Example 12 using the following monomer mixture: 211 g of DI water, 122 g of a 19.6% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 200 g of vinyl acetate, 592 g of n-butyl acrylate, 8 g of acrylic acid, and 4 g of n-dodecyl mercaptan. Sample 13 had a solids content of 54.4%, Brookfield viscosity (#3 spindle at 12 rpm) of 2,000 mNsm -2 (cps.), and Tg(calc.)= -30.9 °C. EXAMPLE 14. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 14. Sample 14 was prepared according to the method of Example 12 using the following monomer mixture: 211 g of DI water, 122 g of a 19.6% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 200 g of vinyl acetate, 592 g of n-butyl acrylate, 8 g of acrylic acid, 0.8 g of methyl 3-mercaptopropionate. Sample 14 had a solids content of 54.6%, Brookfield viscosity (#3 spindle at 12 rpm) of 1,500 mNsm -2 (cps.), and Tg(Calc.)= -30.9 °C. EXAMPLE 15. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 15. Sample 15 was prepared according to the method of Example 12 using the following monomer mixture: 211 g of DI water, 122 g of a 19.6% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 200 g of vinyl acetate, 592 g of n-butyl acrylate, 8 g of acrylic acid, and 4 g of methyl 3-mercaptopropionate. Sample 15 had a solids content of 54.5%, Brookfield viscosity (#3 spindle at 12 rpm) of 1,700 mNsm -2 (cps.), and Tg(calc.)= -30.9 °C. EXAMPLE 16. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 16. A stirred reactor containing 3,950 g. of deionized (DI) water was heated to 65 °C. Then 2 g of a 0.1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 0.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 5 g of DI water. A feed of a monomer mixture consisting of: 400 g of DI water, 500 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol 205) in water, 30 g of TRITON X-100, 1200 g of vinyl acetate, 780 g of n-butyl acrylate, and 20 g of acrylic acid, was started. The monomer mixture was fed according to the following schedule: 5 g/min for 15 min, then 10 g/min for the next 15 min, 15 g/min for 15 min, and then finished at 20 g/min. Total feed time was approximately 3 hours. At the same time as the monomer mixture feed, the following two solutions were cofed: 4.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 43 g of DI water and 2.1 g of sodium sulphoxylate formaldehyde dissolved in 46 g of DI water, according to the following schedule: 0.2 g/min for 15 min, 0.25 g/min for 15 min, 0.3 g/min for 15 min, 0.25 g/min for 15 min, and then 0.2 g/min for the remainder of the solutions. At the completion of the monomer mixture feed an additional 45 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt% solution of sodium carbonate in water. Sample 16 had a solids content of 55.7% and a Tg(calc.)= -1°C. EXAMPLE 17. Preparation of vinyl ester/acrylic copolymer Preparation of Sample 17. A stirred reactor containing 2,950 g. of deionized (DI) water was heated to 65 °C under nitrogen. Then 1 g of a 1 wt.% solution of iron (II) sulfate in water was added, followed by a solution of 2.5 g of 30 wt% hydrogen peroxide (in water) dissolved in 15 g of DI water. A feed of a monomer mixture consisting of: 2,800 g of DI water, 1,500 g of a 20% solution of partially hydrolyzed polyvinyl alcohol (Airvol-205) in water, 150 g of TRITON X-100, 2,500 g of vinyl acetate, 7,400 g of n-butyl acrylate, and 100 g of acrylic acid, was started. The monomer mixture was fed according to the following schedule: 25 g/min for 15 min, then 50 g/min for the next 15 min, 75 g/min for 15 min, and then finished at 100 g/min. Total feed time was approximately 3 hours. At the same time as the monomer mixture feed, the following two solutions were cofed: 20.0 g of 30 wt% hydrogen peroxide (in water) dissolved in 143 g of DI water and 10.5 g of sodium sulphoxylate formaldehyde dissolved in 155 g of DI water, according to the following schedule: 0.7 g/min for 15 min, 0.88 g/min for 15 min, 1.05 g/min for 30 min, 0.88 g/min for 15 min, 0.7 g/min until 15 min after the completion of the monomer mixture feed, and then 1 g/min for the remainder of the solutions. At the completion of the monomer mixture feed an additional 225 g of DI water was added to the reaction. At the completion of the hydrogen peroxide and sodium sulphoxylate formaldehyde feeds the reaction was cooled and adjusted to pH 4.5 with a 15 wt% solution of sodium carbonate in water. Sample 17 had a solids content of 55.2%, a Brookfield viscosity (#3 spindle at 12 rpm) of 7,000 mNsm -2 (cps.)., and a Tg(calc.)= -30.9 °C. EXAMPLE 18. Preparation of Adhesive Composition The adhesive compositions used were the vinyl ester/acrylic copolymers of this invention or comparative polymers in the form of the neat copolymer in all cases. EXAMPLE 19. Evaluation of adhesive performance Adhesives, Samples 1-5 and Comparative sample A, were evaluated for adhesion to low energy surfaces - polypropylene and wax-coated surfaces. Adhesion to wax-coated surface. Michelman Coating 40E wax coating was applied to cardboard with a rubber squeegee (2 passes). The coated board was cured at 105 °C for 1 minute. The board was cut into 5.1 cm by 10.2 cm (2 inch by 4 inch) coupons. A thin coat of adhesive was applied to the coated board with a paint brush. The coated coupon was married to a coupon of virgin board and a 1kg weight was applied for 4 minutes. Three coupons were peeled at 15 second intervals and percent fiber tear was determined visually. After 24 hours the fourth coupon was peeled and percent fiber tear was determined visually. Adhesion to wax-coated surface. Michelman Coating X300 wax coating was applied to cardboard with a rubber squeegee (2 passes). The coated board was cured at 105 °C for 1 minute. The board was cut into 5.1 cm by 10.2 cm (2 inch by 4 inch) coupons. A thin coat of adhesive was applied to the coated board with a paint brush. The coated coupon was married to a coupon of 45.4 kg (100 lb.) untreated Kraft liner and a 1kg weight was applied for 4 minutes. The sample was dried overnight and samples were peeled using an Instron tester at a rate of 25.4 cm/minute (10 inches/minute) at an angle of 180 degrees. Polypropylene Adhesion. Adhesives were applied to polypropylene film (Hercules B 503; 50 gauge) with a Gardiner knife to yield 0.002 dry cm (0.8 dry mils) of adhesive. Greaseproof paper (Federal Base Stock from International Paper) was married to the adhesive-bearing surface and roll-laminated with a 1.1 kg (2.5 lb.) rubber roller. The laminate was dried overnight and bond strength was measured on 2.54 cm (1 inch) wide strips on the Instron tester at a rate of 25.4 cm/minute (10 inches/minute) in a T-peel configuration. Adhesion to wax-coated substrate (Michelman Coating 40E) (% Fiber Tear) Sample 4 minute dwell 24 hr dwell 1 35 95 2 35 30 3 40 100 5 80 80 Comparative A 30 30 Adhesion to wax coated substrate (Michelman Coating X300) (180 degree Peel-p.l.i.) Sample neat with 1.5%GR5M with 3% GR5M 2 0.5 1.6 2.0 4 1.1 2.1 2.4 Comparative A 0.3 0.9 1.2 Adhesion to Polypropylene (Hercules B 503) Sample Adhesion to OPP(pli) 2 0.4 5 0.85 Comparative A 0.1 Samples 1-5 of this invention exhibit superior adhesion to substrates to which it is difficult to adhere such as, for example, wax-coated surfaces and polypropylene. Comparative A is a commercial copolymer (Tg=0 °C.) which is substantially composed of vinyl acetate/ethylene. EXAMPLE 20. Evaluation of adhesion to metal and water resistance.of PVOH-stabilized vinyl ester/acrylic copolymers One inch wide strips of 80 square cotton were immersed in the adhesive for 10 seconds. As the cotton was removed excess adhesive was doctored from the cotton using glass stirring rods. The strips were wet laminated to the substrate under study, covered with untreated PE and rolled with a 0.9 kg (2 lb.) rubber roller (2 passes). Samples were conditioned as follows: Dry Conditioning-samples were dried in a (24°C (75 F)/50% RH) room for 7 days. Wet Conditioning-samples were dried in a (24°C (75 F)/50% RH) room for 6 days, then immersed in DI water for 24 hrs. Peel strength was measured immediately after removal from the water. All samples were tested on an Instron Tensile Tester at a rate of 25.4 cm/minute (10 inches/minute) at an angle of 180 degrees. Adhesion to metal and metallized film substrates Bond(g.l.i.) after dry conditioning Sample Comparative A. Sample 16 Sample 17 Aluminum Foil 1030 substrate destruct Metallized PET - 1136 6688 Metallized OPP - 1024 480 Bond(g.l.i.) after wet conditioning Comparative A Sample 16 Sample 17 Auminum Foil 0 0 64 Metallized PET not tested 0 32 Metallized OPP not tested 128 512 Adhesion to metal surfaces and metallized films for Sample 17 of this invention is superior to that of Comparative sample A. EXAMPLE 21. Evaluation of compatibility of PVOH-stabilized vinyl ester/acrylic copolymers with low molecular weight alcohols and solvent esters. Ten grams of the solvent is added, all in one shot, to 100 grams of Sample 8, while stirring. The emulsion is then inspected for the presence of ropes of gel. Lack of compatibility is defined as the formation of gelled 'ropes' that appear in the emulsion after the solvent addition. Compatibility of copolymers with low molecular weight alcohols and solvent esters. Polymer Sample 8 Comparative A Methanol OK ropes Ethanol OK ropes PM Acetate OK ropes Sample 8 of this invention exhibits superior compatibility with low molecular weight alcohols and solvent esters, the incorporation of which results in increased setting speed. These alcohols and solvent esters may be used in sitations where substantially no volatile organic content is not a requirement. Comparative A is a commercial copolymer (Tg= 0 °C.) which is substantially composed of vinyl acetate/ethylene. EXAMPLE 22. Evaluation of effect of tackifier on adhesion performance of PVOH-stabilized vinyl ester/acrylic copolymers Copolymer emulsions, to which 25% tackifier (Aquatack 6025), by weight based on the dry weight of copolymer, had been addded, were coated onto polypropylene film with a #8 wire wound rod, then closed to paper and rolled with a 1.13 kg (2.5 lb.) rubber roller. Initial peels were measured after 30 minutes using an Instron Tensile Tester at a rate of 25.4 cm/minute (10 inches/minute). Typically, tackifiers are not used with vinyl acetate/ethylene compositions due to incompatibility of the two. To achieve the beneficial effects of a tackifier there must be a degree of compatibility between the base resin and tackifier. Due to the presence of acrylic, PVOH stabilized vinyl/acrylics are compatible with aqueous tackifiers. As a result, adhesion can be enhance by adding aqueous tackifier emulsions. Adhesion of untreated polypropylene to paper: Sample 8 Comp. A neat w/Tack. neat w/Tack. initial peel(p.l.i) 0(zips) paper tear 0(zips) 0(zips) 24hr peel(p.l.i) 0(zips) 0.8(slt tear) 0(zips) 0(zips) Sample 8 of this invention exhibits enhanced adhesion through incorporation of tackifier. It is believed that there is an essential degree of compatibility between PVOH-stabilized vinyl ester/acrylic copolymers and aqueous dispersed tackifier resins, which contributes to enhanced adhesion properties. Comparative A is a commercial copolymer (Tg= 0 °C.) which is substantially composed of vinyl acetate/ethylene. EXAMPLE 23. Effect of in-process chain transfer agent in vinyl ester/acrylic copolymer preparation on improved adhesion to plastic films and wax coatings. Plastic Adhesion. Neat emulsion polymer was applied to oriented polypropylene (OPP) plastic film with a #18 wire wound rod. Greaseproof paper(Federal Base Stock from International Paper) was married to the coated film and the laminate was rolled with a 1.13 kg (2.5 lb.) rubber roller. The laminate was dried overnight at ambient temperature and bond strength was measured on 1 inch wide strips using an Instron tester at a rate of 25.4 cm/minute (10 inches/minute) in a T-peel configuration. Wax coating Adhesion . A wax coating (Michelman Coating 40E) was applied to cardboard with a rubber squeegee (2 passes). The coated board was dried at 105 °C. for 1 minute. The board was cut into 5.1 cm by 5.1 cm (2 inch X 2 inch) coupons. A thin coat of adhesive was applied to the coated board with a aluminum bar notched with 0.16 cm (1/16") deep notches (to simulate a extrusion coated adhesive when applied). The coated coupon was married to a coupon of virgin board and a 1 kg weight was applied for 4 minutes. Ten coupons were peeled after 3 hours and percent fiber tear was determined. Effect of chain transfer agent on adhesion to plastic films Peel Strength (p.l.i.) OPP/greaseproof paper PET/greaseproof paper Sample 8 1.2 1.0 (no CTA) Sample 12 1.5 >2.0 (0.1 nDDM) Sample 14 2.2 2.2 (0.1 MMP) Sample 13 1.2 1.2 (0.5 nDDM) Sample 15 0.6 0.5 (0.5 MMP) Effect of chain transfer agent (CTA) on adhesion to wax coating (Michelman Coating 40E) (% Fiber Tear) Sample 8 40 (no CTA) Sample 12 70 (0.1 nDDM) Sample 14 80 (0.1 MMP) Sample 13 83 (0.5 nDDM) Sample 15 72 (0.5 MMP) When 0.1% of n-DDM or MMP are aded to the polymer during the polymerization process (Samples 12-15 of this invention) peel adhesion increases significantly. When MMP was added at a concentration of 0.5%, adhesion to plastic films is decreased. However the improved adhesion to wax coatings (eg., Michem Coat 40E) is maintained at both 0.1 and 0.5% chain transfer agent. EXAMPLE 24. Effect of addition of clay to vinyl ester/acrylic copolymer-containing composition on high temperature shear. A thin bead of adhesive was applied to carton stock (printed clay-coated board from Westvaco) using a wooden applicator stick. The glue lap was folded such that the adhesive came in contact with the carton. A 4.5 kg (10 lb.) weight was placed on the carton for 1 minute. Initial fiber tear of laminate was determined after 1 minute. One inch strips were cut for use in the high temperature shear test. The high temperature shear test was begun after 24 hr RT dwell using two test specimens of 2.54 cm by 0.64 cm (1 in. X 1/4 in.) X 1 kg. Testing was carried out after 60°C (140 F) for 24 hrs, alternatively followed by 93°C (200 F) for 16 hours. The fiber tear of the laminate was tested. Effect of clay addition on high temperature shear Shear(hours) Adhesion(% FT) Sample 60°C (140 F) 93°C (200 F) initial 24hr 2 >24 0.5,>16 fair fair+ 3 >24 0.5,0.5 fair good 100pts 3 + 50pts VA >24 0.5,>16 fair+ fair 4 >24 0.5,0.5 excel. good 100pts 4 + 50pts VA >24 >16,>16 fair fair+ 100pts 4 + 10pts clay 2 >24 >16 good excellent Samples 3 and 4 of this invention may be hardened by the addition of vinyl acetate homopolymer to achieve the shear performance of the harder copolymer (sample 2) of this invention; however, adhesion is compromised. Incorporation of clay into the vinyl ester/acrylic adhesive composition contributes high temperature shear without adhesion loss. EXAMPLE 25. Evaluation of adhesive performance of 2-ethylhexyl acrylate/vinyl acetate copolymers Samples 9-11 were evaluated for adhesion to a wax coating (Michelman Coating 40E), oriented polypropylene (OPP), and polyester (Mylar) according to the methods of Example 19. Adhesion to wax coating (Michelman Coating 40E) (% Fiber Tear) 3 hour dwell 24 hr dwell Sample 9 88 63 Sample 10 91 83 Sample 11 82 95 Comparative A 14 11 Samples 9-11 of this invention, which contain 2-ethylhexyl acrylate and vinyl acetate, exhibit excellent adhesion to a wax coating. Comparative A is a commercial copolymer (Tg= 0 °C.) which is substantially composed of vinyl acetate/ethylene. Adhesion to plastic films Adhesion OPP(pli) Mylar(pli) Sample 9 paper tear 0.95 Sample 10 0.81 1.1 Sample 11 paper tear 1.1 Samples 9-11 of this invention, which contain 2-ethylhexyl acrylate and vinyl acetate, exhibit good adhesion to plastic films.

Claims: A method of wet-laminating two substrates comprising (a) forming an aqueous, organic solvent-free laminating adhesive composition comprising a vinyl ester/acrylic copolymer having a glass transition temperature from 10 °C. to -45 °C.; (b) applying said composition to a first substrate; (c) contacting said applied composition with a second substrate; and, (d) after (c) drying said composition, wherein said first substrate and/or second substrate is a difficult-to-adhere substrate selected from the group consisting of plastic films and wax coatings. A method as claimed in claim 1 wherein said vinyl ester/acrylic copolymer has a glass transition temperature from 10 °C. to -35 °C.. A method as claimed in claim 2 wherein the glass transition temperature is from -15 °C. to -35 °C.. A method as claimed in any preceding claim wherein said copolymer comprises a copolymerized acrylic monomer selected from the group consisting of n-butyl acrylate and 2-ethylhexyl acrylate, vinyl acetate and n-butyl acrylate, and 2-ethylhexyl acrylate and vinyl acetate. A method as claimed in any preceding claim wherein said copolymer further comprises a stabilizer selected from the group consisting of hydroxyethyl cellulose and polyvinyl alcohol. A method as claimed in any preceding claim wherein said copolymer is prepared with from 0.05% to 0.75% of a chain transfer agent. A method as claimed in claim 6 wherein said chain transfer agent is selected from the group consisting of n-dodecyl mercaptan and methyl mercaptopropionate. A method as claimed in any preceding claim wherein said composition further comprises at least 5% clay by weight, based on the weight of said copolymer. A method as claimed in any preceding claim wherein said composition further comprises an aqueous tackifier.

Inventors: BRICKER MARK CHARLES, EISENHART ERIC KARL, GRAZIANO LOUIS CHRISTOPHER, LIN LONG-JIN, VAN RHEENEN PAUL RALP, BRICKER, MARK CHARLES, EISENHART, ERIC KARL, GRAZIANO, LOUIS CHRISTOPHER, LIN, LONG-JIN, VAN RHEENEN, PAUL RALP
Applicants: ROHM & HAAS, ROHM AND HAAS COMPANY
Application year: 1993
Main classification: C09J131
IPC codes: B32B7, C09J131, C09J133, C08J5