CA2025368C - Method for heat sealing with an aqueous polyacrylate dispersion - Google Patents

Abstract

A method for heat sealing, especially for heat sealing coated aluminum foils to polystyrene or polyvinyl chloride, employing hot melt adhesive which is an aqueous dispersion of a polyacrylate polymer comprising at least 70 percent by weight of lower alkyl methacrylates, said dispersion further containing polymerized acrylamide or methacrylamide or aminoalkyl acrylate or methacrylate, present as a separate polymer and/or copolymerized into said polyacrylate polymer.

Description

-~ 202~3~8 Attorney Docket No.: 327410-2810 The present invention relates to a method for heat sealing employing an aqueous polyacrylate dispersion at the heat sealing adhesive.
Heat sealing is a technique that is widely used for bonding substrates of which at least one has a coating of a meltable polymer. The substrates are joined at a temperature and pressure at which the substrates themselves are not deformed or damaged but at which the polymeric coating melts and bonds to the other substrate. On cooling, the substrates are bonded to each other along the surfaces which were heated. Frequently only a portion of the area coated with the meltable polymer on one of the substrates is heat sealed to the other substrate in a dot or line pattern. For example, aluminum foil coated with a hot melt adhesive is heat sealed as cover to the rim of a container thermoformed from plastic sheeting A great many fusible polymers are used as hot-melt adhesives. They are selected on the basis of a number of criteria, including the requisite separation of their melting temperature from that of the substrates, the adhesion to the substrates, the required strength of the sealed seam, the conditions under which the hot melt adhesive has to be deposited on one of the substrates, and, in the case of packaging for food or pharmaceutical products, their safety from the legal and pharmacological points of view, respectively, and their resistance to the contents of heat sealed packages.

202~3~g The prior art The hot melt adhesives which have gained commercial acceptance on the basis of these criteria include polyacrylates.
These are predo~;n~ntly polymers of alkyl esters of acrylic and/or methacrylic acid having from 1 to 8 carbon atoms in the alkyl group with glass transition temperatures ranging from about 20C to 100C and weight average molecular weights of from 20,000 to 300,000. A proven polyacrylate of this type is a copolymer of methyl or butyl methacrylate. One of the advantages which polyacrylates have over other hot melt adhesives is their good adhesion to plastic substrates, and especially those based on polystyrene. These include, in addition to polystyrene itself, its copolymers with acrylonitrile, butadiene or acrylates, and impact resistant graft copolymers with rubberlike base polymers.
Up to now, the polyacrylates have mostly been coated onto the substrate to be heat sealed from solutions in organic solvents. However, this coating method requires measures for protection against fire and explosions as well as for the recovery of the solvent vapors. To permit these measures to be dispensed with, it is endeavored to use the polyacrylates in the form of aqueous dispersions which give off only water vapor as the coating dries, but no vapors posing a fire or environmental hazard.
Another known class of hot melt adhesives are polymers of vinylidene chloride. They may be applied in accordance with German patent 29 06 118 as an aqueous dispersion. To improve their adhesion to plastic film or sheeting or metal foil not provided with an undercoat, they can be mixed with special --2 ~

2~2~68 Anchoring or keying agents. Suitable for this purpose are water soluble polymers of functionally substituted acrylamides and/or methacrylamides or their copolymers with acrylamide and/or methacrylamide which are dissolved in the aqueous phase of the dispersion.
According to German patents 27 27 914 (corresponding to U.S. patent 4,291,090) and 28 55 147 (corresponding to U.S.
patent 4,299,748), hot melt adhesives based on polyacrylates can also be applied as aqueous dispersions. Frequently they are not composed only of esters of acrylic and/or methacrylic acid but may contain one or more comonomers providing the desired melting temperature or hardness or adhesive properties. Acrylamide and methacrylamide, and aminoalkyl esters of acrylic and/or methacrylic acid, are among the many comonomers mentioned, although no specific influence on the properties of the polymer are attributed to them.
The present invention seeks to improve the heat seal bond strength of hot melt adhesives based on polyacrylate dispersions, and especially the peel strength of the seal area after prolonged contact with water.
The invention provides a method for heat sealing a first surface of an aluminum foil having a primer coating of polyvinyl chloride thereon to a second surface of an unprimed plastic, which method comprises (1) applying an adhesive to the first surface as an aqueous dispersion of a polyacrylate polymer comprising at least 70 percent by weight of at least one alkyl ester of methacrylic acid having from 1 to 4 carbon atoms in the alkyl group, the balance of the adhesive comprising at least 0.1 A ~
A

2025~68 - percent by weight of at least one polymerized member selected from the group consisting of acrylamide, methacrylamide, aminoalkyl acrylates, and aminoalkyl methacrylates present as a built-in monomer unit of the polyacrylate polymer or as a water soluble polymer in addition to the dispersed polyacrylate or both, and drying the aqueous dispersion to form an adherent film on the first surface, and then (2) bonding the first surface to the second surface under heat and pressure.
Advantages of the invention The advantages offered by the invention are illustrated by the test results which follow. A detalled descri~ti n ~ the - 3a -~i 20253~8 preparation and testing of the specimens will be found in the examples at the end of this specification.
Aluminum foil provided with a PVC undercoat was coated with an aqueous dispersion of a copolymer of methyl and butyl methacrylate and heat sealed to polystyrene and rigid PVC sheets.
The peel strength of the seal area after immersion in water for 14 days and 28 days, respectively, was found to be inadequate.
But when the dispersion was mixed with 2 percent by weight (based on the solids content of the dispersion) of a water soluble copolymer of acrylamide, methacrylamide and hydroxyethyl methacrylate, in keeping with the invention, or a copolymer was used as polyacrylate whose composition included, in addition to n-butyl and methyl methacrylate in a weight ratio of about 4.5:1, 2 percent by weight of acrylamide, methacrylamide or dimethylaminoethyl methacrylate, the peel strength after immersion in water was considerably improved:

2 0 2 ~

TABLE

Polyacrylate of butyl and Peel strength (in N/15 mm) on:
methyl methacrylate (4.5:1) Polystyrene: Rigid PVC
after Nature of modification 0 14 28 0 14 28 (Percent are weight %) days of immersion in water No comonomer, no additive 9.1 1.1 0.9 6.5 1.3 1.6 No comonomer; 2% polymer 10.9 11.7 11.4 5.5 7.3 7.7 2% acrylamide as comonomer10.2 8.6 7.8 6.8 7.2 7.8 2% methacrylamide as comonomer 8.9 9.1 9.5 8.2 6.2 6.8 2% DMAEMA as comonomer 10.9 8.5 9.4 8.4 6.8 5.9 Comparative tests:

2% hydroxyethyl methacrylate as comonomer 8.6 4.4 3.8 9.9 3.1 3.2 Copolymer of 60% MMA, 38% BA, 2% methacrylamide 6.0 3.7 3.9 12.9 6.5 5.7 Key to abbreviations:
DMAEMA = Dimethylaminoethyl methacrylate MMA = Methyl methacrylate BA = Butyl acrylate As the comparative tests show, acrylamide and methacrylamide cannot be replaced by hydroxyalkyl esters, which also provide polar properties. The improvement in heat seal bond strength after immersion in water is not as great as required when the methacrylic esters amount to less than 70 percent by weight.
The effect of the polymerized acrylamide and/or methacrylamide and/or aminoalkyl acrylate or methacrylate cannot be explained from a knowledge of the prior art. What is surprising is, above all, that this effect seems to be produced regardless of whether said nitrogen containing monomer is present as a built-in monomer unit of the polyacrylate or as a water 20253~8 soluble polymer in addition to the dispersed polyacrylate.
In the preparation of the polyacrylate by emulsion polymerization of a monomer mixture of alkyl methacrylate and acrylamide or methacrylamide or aminoalkyl acrylate or methacrylate, it is likely that a portion of these nitrogen containing monomers will form in the aqueous phase a water soluble or possibly dispersed polymer and only the remainder will become, as a comonomer, a constituent of the dispersed polyacrylate. No attempt has been made to determine whether and to what extent the polymerized nitrogen cont~ining monomer falls into these two polymer categories. What is certain, however, is that the addition of the aforesaid water soluble polymer to a polyacrylate dispersion that was not prepared in the presence of nitrogen containing monomers results in about the same improvement as the addition of such monomers during emulsion polymerization. The invention thus includes the addition of polymerized amides or amino esters in any effective polymer form.
Practicing the invention The polyacrylate dispersion utilized in accordance with the invention may have been prepared in a manner known per se by emulsion polymerization in aqueous phase in the presence of anionic, cationic and/or nonionic emulsifiers and of a free radical forming initiator. Suitable methods of preparation include the polymerization of an initial charge of an aqueous monomer emulsion and the so-called gradual addition method, in which a homogeneous monomer phase or an aqueous emulsion of the monomers is added gradually, in step with the conversion, under polymerization conditions, to an initially introduced water phase. The dispersed polyacrylate preferably has a particle size - 202~3~8 ranging from 80 to 500 nanommeters (nm). The emulsion polymerization is preferably carried out in the presence of a chain transfer agent so that a polymer with an appropriate molecular weight assuring the requisite flowability in the molten state is obtained.
The alkyl esters of methacrylic acid contained in the polyacrylate dispersions are preferably the methyl, ethyl, n-propyl, isopropyl, n-butyl or i-butyl esters or mixtures thereof. Their quantity may considerably exceed the lower limit of 70 percent by weight and may be in excess of 80 percent by weight. As a rule, the composition of the polyacrylate does not include any monomers other than alkyl methacrylates and the nitrogen cont~; n; ng monomers which are essential to the invention. However, there is no reason why further monomers cannot be included to obtain special properties, for example, improved adhesion to metallic substrates, provided that they do not adversely affect the required properties. Usable comonomers are, for example, alkyl acrylates, higher alkyl methacrylates, acrylic and/or methacrylic acid or other unsaturated polymerizable mono- or dicarboxylic acids, hydroxyalkyl esters of acrylic and/or methacrylic acid, acrylo- or methacrylonitrile, vinyl pyrrolidone, vinyl imidazole, vinyl halides, and vinyl esters.
The monomers are selected so that the resulting polyacrylate has a glass transition temperature of from 20C to 100C, and preferably from 40C to 80C. Whenever possible, the minimum film forming temperature of the dispersion should not exceed 60C; preferably it ranges from 30C to 50C.
Among the nitrogen containing monomers which in accordance ^ 7--202~36~

with the invention are present in polymerized form, acrylamide and/or methacrylamide are preferred. Their N-substituted derivatives are usually less effective and may therefore have to be used in a somewhat larger amount. Examples of usable aminoalkyl acrylates or methacrylates and substituted acrylamides or methacrylamides are: 2-dimethylaminoethyl acrylate and methacrylate, 3-(N,N-dimethylamino)propyl acrylate and methacrylate, 4-(N,N-dimethylamino)butyl acrylate and methacrylate, triethanolamine monoacrylate and monomethacrylate, 2-(dimethylaminoethyloxy)ethyl acrylate and methacrylate, 2-imidazolylethyl acrylate and methacrylate, 2-piperazinylethyl acrylate and methacrylate, (1,2,2,6,6-pentamethylpiperidyl-4) acrylate and methacrylate, morpholinoethyl acrylate and methacrylate, N,N-dimethylaminoneopentyl acrylate and methacrylate, 2-(N,N-dibutylaminoethyl acrylate and methacrylate, 2-piperazinylethyl acrylamide and methacrylamide, 3-(N,N-dimethylamino)propyl acrylamide and methacrylamide, N,N-dimethylaminoneopentyl acrylamide and methacrylamide, 3-morpholinopropyl acrylamide and methacrylamide, 2-4-diethylamino-1-methylbutyl acrylamide and methacrylamide, and methylacrylamidoglycolate methyl ether.
The amount of polymerized acrylamide and/or methacrylamide or aminoalkyl acrylate and/or methacrylate generally ranges from 0.1 to 10 percent by weight, and preferably ranges from 0.5 to 5 percent by weight, based on the dry weight of the polyacrylate.
If the nitrogen cont~in;ng monomers are to be built in as monomer units of the polyacrylate, they are simply added, in an amount as mentioned above, to the monomer compositions used to prepare the polyacrylate. However, if they are to be introduced 20253~8 into the aqueous phase of the dispersion in the form of a water soluble polymer, this polymer has to be prepared separately.
This is most effectively done by dissolving the nitrogen contAin;ng monomers in water and polymerizing them by means of a free radical forming initiator. Suitable polymerization methods resulting in aqueous polymer solutions having a concentration from, for example, 10 to 50 percent by weight, are well known in the art.
The water soluble polymer may be composed solely of said nitrogen containing monomers. However, water soluble copolymers of at least one monomer selected from the group consisting of acrylamide, methacrylamide, aminoalkyl acrylates, and aminoalkyl methacrylates, and of at least one monomer selected from the group consisting of hydroxyalkyl esters of acrylic acid and of methacrylic acid are suitable and are sometimes preferred. The percentages of the nitrogen contAin;ng monomers is usually more than 50, and preferably 60 to 90, percent by weight of the water soluble copolymer. In addition to, or instead of, these hydroxyalkyl esters, still further comonomer units may form part of the copolymer provided it r~mA;nC water soluble. Easily water soluble monomers such as acrylic acid, methacrylic acid, or N-vinyl pyrrolidone may be used in higher percentages within the region mentioned above than may difficultly water soluble or insoluble monomers such as the alkyl esters of acrylic or of methacrylic acid, acrylonitrile, methacrylonitrile, or styrene.
The molecular weight of the water soluble polymer is not critical; suitable molecular weights range from about 10,000 to 1 million and preferably from 20,000 to 200,000. Usually, the polymer is added to the polyacrylate dispersion in the aqueous _ q _ - 202~368 solution in which it has been prepared. However, if desired, the polymer may be added in solid form and dissolved in the aqueous phase of the dispersion.
To coat the substrate, the dispersion is used with a solids content from 30 to 60, and preferably from 40 to 50, percent by weight, depending on the coating method. As a rule, the coating mixture will contain no additives other than the auxiliary agents needed for emulsion polymerization, although minor amounts of thickening agents or film forming aids may be used in special cases. The coating is applied by spray coating, spread coating, cast coating, dip coating, knife coating, roll coating or in another suitable manner in such thickness that on drying a hot melt adhesive layer of from about 5 to 20 microns is formed. For drying, the coated substrate is exposed in a drying oven or continuously traversed tunnel dryer to a temperature between 100C and 200C, optionally at reduced pressure. The higher the drying temperature, the shorter the required drying time will be. The drying time may range from 5 seconds to 5 minutes, for example. To produce relatively thick hot melt adhesive coatings, the coating operation can be repeated several times without running the risk that the layer deposited earlier will lift.
The polyacrylate dispersion is preferably applied to nonporous, unbroken substrate surfaces, particularly to plastic film and sheeting or metal foil. When adequate adhesion or uniform wetting cannot be achieved with very smooth films or foils, it is advisable to first apply a suitable primer or to use films or foils which are commercially available with a prime coat. For example, aluminum foil is given a PVC-based prime coat 202~368 and can then readily be coated and heat sealed in keeping with the invention.
The coated substrate can be heat sealed in accordance with the invention to other substrates of suitable surface condition.
To this end, too, a prime coat which in itself is not heat sealable may prove advantageous, although heat sealing normally is done to substrates lacking such a prime coat.
Heat sealing is preferably done to plastic films or sheeting or to articles made from them. Good heat sealability is exhibited by rigid PVC, for example, and especially by polystyrene-based plastics. Heat sealing requires that the glass transition temperature of the polyacrylate be exceeded in the coating. The shorter the contact time and the poorer the thermal conductivity through the substrate layer, the higher the temperature of the sealing bars must be above the required sealing temperature. Thin metal foils have very good thermal conductivity and allow the sealing bars to be held at a temperature just slightly higher than the melting temperature of the polyacrylate, although in practice considerably higher sealing bar temperatures ranging from 120C to 200C, for example, are usually chosen to permit fast sealing. The glass transition temperature of the plastic substrate may impose limits on the sealing bar temperature. To achieve a high strength seal, a sealing bar pressure of at least 1, and preferably from 3 to 6, kp/cm2 should be used.
A better understanding of the present invention and of its many advantages will be had be referring to the following specific examples, given by way of illustration.

Il ~

-EXAMPLES
The making and testing of the heat seal specimens whose strength values are given in the table in the section "Advantages of the invention" will now be described. The polyacrylate of the first line of the Table corresponds to Example 1 (A). The next four specimens correspond to Examples 1 (B) and 2 to 4. The two comparative tests which follow relate to Comparative Examples 1 and 2.
The specimens were made by applying a 10 percent by weight solution of a commercial vinyl chloride/vinyl acetate/maleic acid copolymer ("Vinylite VMHC", Union Carbide) in ethyl acetate with a spiral knife coater to a flexible aluminum foil 40 microns thick in such thickness that after drying for 1 minute at 180C a prime coat of 2 microns was present. To form the hot melt adhesive layer, the corresponding dispersions were also applied with a knife coater and for film formation introduced promptly into a forced-air drying oven, preheated to 180C, before the water had evaporated from the dispersion layer. The knife had been selected so that the dried hot melt adhesive layer had a thickness of from 6 to 8 microns.
The coated aluminum foils were heat sealed over a 1 cm wide strip along one edge by means of a laboratory heat sealer (manufactured by Brugger) to polystyrene and PVC films, respectively. The temperature of the sealing bars was 180C, the sealing pressure, 6 kp/cm2, and the sealing time, 1 second.
To measure the heat seal bond strength in conformity with DIN 51,221 after storage under st~n~rd climatic conditions (23C, 50% relative humidity), 15 mm wide strips were cut at right angles to the seal area, folded, and clamped in a tensile 202~368 testing machine in such a way that one pair of grips was holding the free end of the aluminum foil and the other pair of grips the free end of the plastic film and the seal area projected roughly at right angles from the stretched test strip. The force necessary to pull the foil and film apart over a width of 15 mm is designated the heat seal bond strength.
To determine the water resistance of the seal area, the test strips were immersed at room temperature in water for 14 and 28 days, following which their heat seal bond strength was measured as described above while they were still wet.
Example 1 (A) In a one liter round bottomed flask equipped with stirrer and contact thermometer, 160 parts of fully desalted water and 0.05 part of sodium diisooctyl sulfosuccinate were heated to 80C with stirring and mixed with 2.4 parts of a 10 percent by weight ammonium persulfate solution. At that temperature, 240 parts of fully desalted water;

4 parts of sodium diisooctyl sulfosuccinate;
1 part of ammonium persulfate;
328 parts of butyl methacrylate;
72 parts of methyl methacrylate; and 0.4 part of dodecyl mercaptan were then added dropwise as an emulsion over a period of 4 hours.
After another 2 hours at 80C, the batch was cooled to room temperature. A stable dispersion free of coagulate was obtained.
Solids content: 50%.
Average particle diameter: 320 nm.
Viscosity: 40 mP s.

20~368 pH: 3.5.

(B) To 196 g of the dispersion there was added with stirring 8 g of a 25% aqueous solution of an acrylamide/methacrylamide/hydroxyethyl methacrylate copolymer (60:10:30 percent by weight).
Example 2 By the procedure described in Example 1 (A), a dispersion was prepared from the following monomers:
320 parts of butyl methacrylate;
72 parts of methyl methacrylate; and 8 parts of acrylamide.
Solids content: About 50%.
Average particle diameter: 280 nm.
Viscosity: 38 mPa s.
pH: 2.6 Example 3 By the procedure described in Example 1 (A), a dispersion was prepared from the following monomers:
320 parts of butyl methacrylate;
72 parts of methyl methacrylate; and 8 parts of methacrylamide.
Solids content: About 50%.
Average particle diameter 340 nm.
Viscosity: 40 mPa s.
pH: 3.6.
Example 4 160 parts of fully desalted water and 0.05 part of sodium diisooctyl sulfosuccinate were heated to 80C and mixed with 5 2û25368 -parts of a 10 percent by weight 4,4-azobis-(4-cyanovaleric acid) sodium salt solution. At that temperature, 240 parts of fully desalted water;
4 parts of diisooctyl sulfosuccinate;
1 part of 4,4-azobis-(4-cyanovaleric acid) sodium salt;
320 parts of butyl methacrylate;
72 parts of methyl methacrylate;
8 parts of 2-dimethylaminoethyl methacrylate; and 0.4 part of dodecyl mercaptan were then added dropwise as an emulsion over a period of 4 hours.
Solids content: About 50%.
Average particle diameter: 410 nm.
Viscosity: 20 mPa s.
pH: 8.2.
Comparative Example 1 By the procedure described in Example 1 (A), a dispersion was prepared from the following monomers:
320 parts of butyl methacrylate;
72 parts of methyl methacrylate; and 8 parts of 2-hydroxyethyl methacrylate.
Solids content: About 50~.
Average particle diameter: 280 nm.
Viscosity: 22 mPa s.
pH: 2.2.
Comparative Example 2 By the procedure described in Example 1 (A), a dispersion was prepared from the following monomers:
240 parts of methyl methacrylate;

- 20~68 152 parts of butyl acrylate; and 8 parts of methacrylamide.
Solids content: About 50%.
Average particle diameter: 300 nm.
Viscosity: 20 mPa s.
pH: 3.2.

Claims (6)

1. A method for heat sealing a first surface of an aluminum foil having a primer coating of polyvinyl chloride thereon to a second surface of an unprimed plastic, which method comprises (1) applying an adhesive to said first surface as an aqueous dispersion of a polyacrylate polymer comprising at least 70 percent by weight of at least one alkyl ester of methacrylic acid having from 1 to 4 carbon atoms in the alkyl group, the balance of said adhesive comprising at least 0.1 percent by weight of at least one polymerized member selected from the group consisting of acrylamide, methacrylamide, aminoalkyl acrylates, and aminoalkyl methacrylates present as a built-in monomer unit of the polyacrylate polymer or as a water soluble polymer in addition to the dispersed polyacrylate or both, and drying said aqueous dispersion to form an adherent film on said first surface, and then (2) bonding said first surface to said second surface under heat and pressure.

2. A method as in claim 1 wherein said polymerized member is entirely or predominantly present as a comonomer in said polyacrylate polymer.

3. A method as in claim 1 wherein said polymerized member is entirely or predominantly present as a separate polymer, together with said polyacrylate polymer, in said hot melt adhesive.

4. A method as in claim 3 wherein said polymerized member is present completely or partially dissolved in said aqueous dispersion.

5. A method as in claim 4 wherein said polymerized member is a water soluble copolymer of at least one monomer selected from the group consisting of acrylamide, methacrylamide, aminoalkyl acrylates, and aminoalkyl methacrylates and at least one monomer selected from the group consisting of hydroxyalkyl esters of acrylic acid and hydroxylalkyl esters of methacrylic acid.

6. A method as in claim 1 wherein said unprimed plastic is polystyrene or hard polyvinyl chloride.