Use of Reactive Hot Melt Adhesive for Packaging Applications

There is a growing trend in the general "case and carton sealing" as well as the "carton and tray forming" market towards high speed production so as to provide lower unit costs and higher profit levels. However, the use of such higher machine speeds often excludes the use of normal water based adhesives, such as those based on dextrins, polyvinyl acetate and latex. The packaging industry has therefore moved towards higher technology products, such as hot melt adhesives which are fast setting, easy to apply, and clean to use. These adhesives provide a fair degree of high and low temperature resistance, but are attacked by aromatic oils, solvents and petrochemical vapors.

Consequently, some food containing cartons, particularly those containing aromatic oils and mint based foods, cannot be successfully sealed with hot melts. For example, certain teas contain oil of Bergamont. After the carton is filled with tea, it is sealed with hot melt and then overwrapped with polypropylene or similar film to prevent ingress of moisture, and loss of flavour. If cartons are examined within a few hours a good bond is obtained; however, after a week the hot melt will be seen to be weakening, and eventually total bond failure will result. This failure is due to the hot melt being attacked by the oil in the tea,

A similar situation also exists in non-food applications. These are typified by soap and detergent powder essences, fire lighter fuels, and light engineering oils. Here again, the hot melt is attacked, and bond failure results.

Bonding of silane, fluorocarbon and other treated stocks is also hampered by the use of hot melt adhesives which do not "wet" the coated surface sufficiently to insure adequate adhesion. In these cases, it is necessary to "block-out", i.e. leave uncoated, those portions of the paperboard stock which will subsequently be positioned in the sealed areas.

The development of "cork wads" has also been retarded by the lack of suitable adhesives. "Wads" is the term given to the liquid seal in the cap of a bottle. They are normally constructed from an inert plastic material bonded to either naturally occurring cork or a cork derivative. A laminate is produced, which is subsequently glued into the cap. It is this latter gluing operation which is often affected by the materials or chemicals contained within the bottle or jar. These chemicals can vary from solvents, through light engine oils to liquid medications. At present, most wads are retained in the caps by water based products, which take many days to set, since there is nowhere for the water to escape.

We have now found that the use of a specific class of low viscosity reactive urethane hot melt adhesive compositions for such packaging applications provides an adhesive coating which will be resistant to the effects of oils, solvents or petrochemical vapors and which will be effective on hard to bond surfaces, particularly silane or fluorocarbon treated surfaces.

The present invention is therefore directed to the use of a specific class of hot melt polyurethane adhesives for the sealing of cases and cartons comprising the steps of (1) erecting a case or carton blank; (2) filling the case or carton; (3) applying adhesive to one or more of the carton flaps; (4) compressing the flap(s); and (5) allowing the adhesive to set or cure.

The invention is also directed to the use of these adhesives for manufacturing case and carton blanks comprising the steps of (1) printing, cutting, folding and creasing boards to form a blank; (2) passing the resultant blank through a gluing machine; (3) applying adhesive to the resultant case or carton in the appropriate locations; (4) compressing and (5) allowing the adhesive to set or cure.

In addition, the invention contemplates a process for the production of paper and/or board trays comprising the steps of (1) printing, cutting, folding and creasing the board or paper to form a blank (2) passing the blank through a gluing machine; (3) applying adhesive to the appropriate portions of the blank; (4) forming the blank into a tray by compressing the flaps thereof; and (5) allowing the adhesive to set or cure.

The process of the invention may also be adapted to the gluing of cork wads into bottle caps to form an oil and solvent resistant liquid seal.

The specific hot melt adhesives for use herein are prepared by the addition of urethane prepolymers to low molecular weight polymers formed from ethylenically unsaturated monomers which do not contain active hydrogen, the adhesives being solid at room temperature and coatable at a viscosity of 3000 to 50,000 cps. at 120.degree. C. without the need for additional tackifiers or plasticizers. Adhesives for use herein are disclosed in copending U.S. Pat. Application Ser. No. 200,092, filed May 27, 1988, the disclosure of which is incorporated herein by reference.

In addition, we have found that the heat resistance of the hot melt adhesives used herein may be even further improved by utilizing an ethylenically unsaturated monomer which contains moisture reactive functional (crosslinking) groups. The resultant hot melt adhesive, upon curing, produces an adhesive consisting of a crosslinked polyurethane and a crosslinked ethylenically unsaturated polymer, i.e., a fully interpenetrating network (IPN) adhesive.

In accordance with the general procedure for preparing adhesives suitable for use herein, the low molecular weight polymer may be added to a polyol component prior to reaction with the isocyanate components or it may be added to the already formed prepolymer. Suitable adhesives may also be formed through the simultaneous polymerization of the urethane prepolymer and the ethylenically unsaturated monomers. The polyurethane prepolymer may also be polymerized in the ethylenically unsaturated monomers, which are then subsequently polymerized to form the adhesive. Alternatively if a polyol is used to form the prepolymer, the ethylenically unsaturated monomers may be polymerized in the polyol using free radical polymerization procedures. In this case, the isocyanate components are subsequently polymerized with the mixture using conventional condensation polymerization procedures. This latter polymerization procedure has the advantage of excellent control of the molecular weight of the resulting vinyl polymer (as determined by intrinsic viscosity) and also produces a polymer which is free of detrimental impurities. In addition, the reduction in the handling and inventory of materials and elimination of intermediate packaging and storage bring about significant cost savings.

In the specific embodiment wherein an ethylenically unsaturated monomer containing moisture reactive functional groups is employed, the reactive nature of the crosslinking monomer requires that the monomer be added after the prepolymer has been formed. Thus, in this embodiment, the (co)polymer may be added in its polymerized form to the already formed prepolymer. Alternatively, and more preferably, the monomer or monomers are added to the already formed polyurethane prepolymer and polymerized therein using free radical polymerization procedures.

Any ethylenically unsaturated monomer capable of free radical polymerization and which does not contain active hydrogen may be used herein. Most commonly employed are the C.sub.1 to C.sub.12 esters of acrylic and methacrylic acids including, but not limited to methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-propyl or iso-propyl acrylate as well as the corresponding methacrylates. Mixtures of compatible (meth)acrylate monomers may also be used. Such mixtures, including mixtures of butyl and methyl methacrylate are well known in the art. Additional ethylenically unsaturated monomers such as vinyl esters (e.g., vinyl acetate and vinyl propionate), vinyl ethers, fumarates, maleates, styrene, acrylonitrile, ethylene, vinyl ethers, etc. may be used as may comonomers thereof.

When the ethylenically unsaturated monomer contains moisture reactive functional groups, any such monomer containing moisture reactive (curable) functional groups may be employed. In particular, monomers containing silane or reactive isocyanate functionality are particularly useful herein.

Typical of useful silane or isocyanate containing ethylenically unsaturated copolymers are those which contain only mers derived from an ethylenically unsaturated monomer containing silane or isocyanate functional groups or those which contain up to 99% by weight of mers derived from a non-silane or non-isocyanate containing ethylenically unsaturated monomer capable of free radical polymerization. Thus, the final ethylenically unsaturated silane or isocyanate containing copolymer may contain 1-100% of the polymerized ethylenically unsaturated silane or isocyanate monomer, preferably up to 50% by weight and more preferably (from an economic standpoint) 5-30% by weight. Indeed, levels of ethylenically unsaturated silane or isocyanate containing monomers in the range of 15% or less have been shown to provide potentially commercially significant results.

The ethylenically unsaturated silanes are preferably those which will participate directly in the reaction by free-radical polymerization and which do not contain active hydrogen. Representative commercially available silanes of this type include vinyltrichlorosilane, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltris(2-methoxyethoxy) silane, 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyltris(2-methoxyethoxy)silane, vinyltriacetoxysilane, 4-(3-trimethoxysilylpropyl)benzylstyrene sulfonate, 3-acryloxypropyltrimethoxysilane, allyltriethoxysilane, allyltrimethoxysilane, vinylmethyldiethoxysilane, vinyldimethylethoxysilane, vinylmethyldiacetoxysilane, 3-methyacryloxypropylmethyldiethoxysilane, 3-acryloxypropyldimethylmethoxysilane, etc.

The ethylenically unsaturated isocyanates are also preferably those which will participate directly in this reaction by free-radical polymerization and which do not contain active hydrogen. Representative commercially available isocyanates of this type include isocyanatoethyl methacrylate and m-isopropenyl - , - dimethylbenzyl isocyanate.

As discussed above, any ethylenically unsaturated monomer capable of free radical polymerization and which does not contain active hydrogen may be employed as a comonomer in forming the final silane or isocyanate containing copolymer. Most commonly employed are the C.sub.1 to C.sub.12 esters of acrylic and methacrylic acids including, but not limited to methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-propyl or iso-propyl acrylate as well as the corresponding methacrylates. Mixtures of compatible (meth)acrylate monomers may also be used. Such mixtures, including mixtures of butyl and methyl methacrylate are well known in the art. Additional ethylenically unsaturated monomers such as vinyl esters (e.g., vinyl acetate and vinyl propionate), vinyl ethers, fumarates, maleates, styrene, acrylonitrile, ethylene, vinyl ethers, etc. may be used as may copolymers thereof.

The urethane prepolymers are those conventionally used in the production of polyurethane hot melt adhesive compositions. Most commonly, the prepolymer is prepared by the condensation polymerization of a polyisocyanate with a polyol, most preferably the polymerization of a diisocyanate with a diol. The polyols used include polyhydroxy ethers (substituted or unsubstituted polyalkylene ether glycols or polyhydroxy polyalkylene ethers), polyhydroxy polyesters, the ethylene or propylene oxide adducts of polyols and the mono-substituted esters of glycerol.

In addition, the urethane prepolymers may be prepared by the reaction of a polyisocyanate with a polyamino or a polymercapto-containing compound such as diamino polypropylene glycol or diamino polyethylene glycol or polythioethers such as the condensation products of thiodiglycol either alone or in combination with other glycols such as ethylene glycol, 1,2-propylene glycol or with other polyhydroxy compounds disclosed above. Also, small amounts of low molecular weight dihydroxy, diamino, or amino hydroxy compounds may be used such as saturated and unsaturated glycols e.g. ethylene glycol or condensates thereof such as diethylene glycol, triethylene glycol, and the like; ethylene diamine, hexamethylene diamine and the like; ethanolamine, propanolamine, N-methyldiethanolamine and the like.

Any suitable organic polyisocyanate may be used such as, for example, ethylene diisocyanate, ethylidene diisocyanate, propylene diisocyanate, butylene diisocyanate, hexamethylene diisocyanate, toluene diisocyanate, cyclopentylene-1,3'-diisocyanate, cyclohexylene-1,4-diisocyanate, cyclohexylene-1,2-diisocyanate, 4,4'-diphenylmethane p-phenylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, xylylene diisocyanate, 1,4-naphthylene diisocyanate, 1,5-naphthylene diisocyanate, diphenyl-4,4'-diisocyanate, azobenzene-4,4'-diisocyanate, diphenylsulphone-4,4'-diisocyanate, dichlorohexamethylene diisocyanate, furfurylidene diisocyanate, 1-chlorobenzene-2,4-diisocyanate, 4,4',4"-triisocyanato-triphenylmethane, 1,3,5-triisocyanato-benzene, 2,4,6-triisocyanato-toluene, 4,4'-dimethyldiphenylmethane-2,2',5,5-tetraisocyanate, and the like.

The polyisocyanate and polyol, polyamino or polymercapto components are combined in proportions so as to yield a urethane prepolymer characterized by an isocyanate content of from about 0.25 to about 15%, preferably to about 10%, and most preferably from 1.5 to 5%. In addition, the ratio of isocyanate equivalents to hydroxyl, amino or mercapto equivalents (known as the isocyanate index) should be greater than 1 but no more than about 2. By maintaining the low isocyanate index, we are able to reduce the level of free isocyanate content in the final hot melt adhesive composition to less than about 4%, preferably less than 1%. It will be recognized that the presence of higher levels of free isocyanate has a detrimental effect on a hot melt formulation since it causes toxic fumes to be released when the adhesive is heated to application temperature. The higher levels of free isocyanate may also cause reduction in viscosity and poorer initial bond strength of the adhesive. The precise amount of the polyisocyanate used in the polymerization will depend on the equivalent weight and amount of the non-isocyanate components, and the particular polyisocyanate employed. In general, the amount of the polyisocyanate needed to achieve the isocyanate content will vary from about 5 to about 55% of the final prepolymer.

In the broadest sense, the ethylenically unsaturated monomer may be polymerized using conventional free radical polymerization procedures to a relatively low molecular weight. For purposes of clarification herein, by "low molecular weight" we mean weight average molecular weights in the range of approximately 10,000 to 30,000. The low molecular weight is obtained by careful monitoring and controlling the reaction conditions and, generally, by carrying out the reaction in the presence of a chain transfer agent such as dodecyl mercaptan. There is a recognized correlation between intrinsic viscosity and molecular weight and we have found that, in general, monomers polymerized to an intrinsic viscosity of 0.1 to 0.4 (I.V. as measured in a 9:1 mixture of tetrahydrofuran and alcohol) are particularly preferred for use herein. In this embodiment, the low molecular weight polymer is then blended either with the polyol and dissolved therein prior to reaction with the isocyanate component or the low molecular weight polymer is dissolved in the already formed urethane prepolymer. In the case of polyamino or polymercapto containing prepolymers, in-situ vinylic polymerization must be performed only in the pre-formed prepolymer. In either case, low molecular weight polymer is combined with the isocyanate terminated urethane prepolymer in a proportion such that the reactive curing hot melt adhesive contains about 5 to 90% of the urethane prepolymer and 95% to 10% of the low molecular weight polymer. Care should be taken in storing and handling the low molecular weight polymer to avoid contamination with ambient moisture or other factors which might affect the stability of the prepolymer system.

In accordance with an alternate method for preparing the urethane prepolymers, where the ethylenically unsaturated monomers do not contain reactive functional groups, and wherein a polyol is employed, the monomers are combined in an amount of about 2 to 90% by weight with 10 to 98% by weight of the polyol and polymerized therein using conventional free radical polymerization procedures in the presence of a chain transfer agent such as dodecyl mercaptan to achieve the low molecular weight polymer dissolved in the polyol. Subsequent to the polymerization of the ethylenically unsaturated monomer(s), the polyisocyanate and any additional ingredients required for the urethane prepolymer forming reaction are added and that reaction is carried out using conventional condensation polymerization procedures. In this manner, the resultant isocyanate terminated urethane prepolymer forms the reactive curing hot melt adhesive described above which contains about 5 to 90% of the urethane prepolymer and 95 to 10% of the low molecular weight polymer

It is also possible in the case of monomers which do not contain isocyanate reactive functional groups to polymerize the low molecular weight polymer in the presence of the already formed isocyanate terminated urethane prepolymer. This method has the drawback of subjecting the prepolymer to unnecessary heating during the acrylic polymerization, heating that might result in branching, viscosity increase, depletion of needed isocyanate groups and possible gellation. Although these disadvantages are subject to control, more stringent control of conditions are required as compared to polymerization in the non-isocyanate functional urethane components. When the reaction is run in the polyol or other non-isocyanate containing component, there is also the advantage of lower reaction viscosities and reduced exposure to isocyanate vapors because of the lesser amount of heating required.

As discussed previously, the resultant hot melt adhesives which are solid at room temperature have appropriate application viscosities within the range of 3000 to 50,000 cps at 120.degree. C. (which corresponds to about 1500 to 25,000 at 135.degree. C. and 10,000 to about 100,000 at 108.degree. C.) and thus require no additional tackifiers and/or plasticizers in order to achieve these application viscosities. It is noted, however, that small amounts of tackifiers or plasticizers may be added so long as their presence does not serve as a detriment to the desired adhesive properties.

The resulting hot melt adhesive compositions, as described hereinabove, are typically applied at temperatures of from about 230.degree. to 250.degree. F. and a corresponding melt viscosity of from about 5000 to 40,000 centipoises. Coatings having a wet film thickness of from about 5 to 30 mils are ordinarily utilized in preparing.

This invention can be further illustrated by the following examples of preferred embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

The following example illustrates the preparation of an adhesive composition for use herein.

A one liter reaction vessel was set up equipped with a condenser, gas inlet tube, slow addition tube, thermometer, stirrer, and provisions for heating/cooling. The ingredients of the reaction consisted of the following:

The reaction vessel was purged with dry nitrogen and a slow stream of dry nitrogen was bubbled subsurface throughout the reaction. Ingredients designated 1, 2, 3, 4, 6, 8 and 9 were added to the vessel and the temperature raised to 80.degree. C. After 1/2 hr at 80.degree. C. ingredients 5 and 7 were added uniformly over a period of 1 hr. The reaction was held at 80.degree. C. for an additional three hours, at which time 10 was added. The reaction was held an additional 2 hrs. @80.degree. C. and 11 was added; then the temperature was raised to 100.degree. C. and held for three hours. At this point a vacuum of 120mm to 130mm was applied to the vessel for 20 minutes to 30 minutes and the reaction poured hot from the flask.

This example illustrates the addition of a commercially available low molecular weight polymer to the urethane prepolymer to produce a hot melt adhesive. Elvacite 2013 is a 64% butylmethacrylate/36% methylmethacrylate copolymer of I.V. 0.2 available from DuPont. The Elvacite was vacuum dried in a desicator for 24 hours immediately prior to use herein.

All the ingredients listed above were added to the vessel and heated at 100.degree. C. until the Elvacite was dissolved. After 4 hours at 100.degree. C. the reaction was poured hot from the vessel. The samples had the following properties:

In order to demonstrate the applicability of the hot melt of Example II to packaging operations, it was subjected to the following test procedures to test the resistance of the adhesive to migratory oils.

1. The test pieces were prepared such that one surface in contact with the adhesive had no varnish/ink or other pretreatment. The other surface was always the outside of the carton and could have any combination of protective varnish, ink, etc. Strips of the test material were cut. A bead of adhesive was run along the length of the test piece and the top piece was placed on the bead and compressed. The bonded piece was left for a period of time (commonly 24 hours) before testing destructively. In all cases, fiber tear was obtained as one of the surfaces tore or the (varnish and/or the printed ink) was pulled away from the board underneath.

2. Heat resistance was tested by making test pieces in a lap shear type of bond, and suspending them in a circulated air oven. Weights (usually 200 g) were hung on the bottom and the temperature increased from 70.degree. C. to 110.degree. C. over several days. No failures were obtained.

3. Low temperature performance was tested by making bonds as in Step (1) and putting them in a freezer at -40.degree. C. After 3 days, the bonds were tested as in (1). When the bonds were left at ambient conditions for six hours, and then frozen, the performance was excellent. If, however, the bonds were frozen immediately, the adhesive did not cure, and the adhesion was poor rendering this particular formulation unsuitable for frozen foods. Use of other ethylenically unsaturated monomers may overcome this deficiency if frozen food applications are required.

4. Solvent/oil migration. The bonds were made as in (1) and suspended over a container of oil, etc., and sealed. In the case of solid materials (i.e., chocolate, tea leaves, etc.) the bonds were in physical contact with the material. They were then tested over a period of time.

For this test, the polyurethane hot melt of the example was compared against conventionally employed ethylene vinyl acetate (EVA) and polyamide hot melt adhesives which are widely regarded as having good oil resistance.

The test results were as follows.

The results indicate that polyurethane hot melts of the invention are entirely resistant to migratory oils often found in packaging applications and are much more reliable than EVA or polyamide hot melts, which may fail after a period of time.

The adhesives were also tested on production equipment to ascertain their behavior characteristics. The adhesive was dispensed into the glue pot at 120.degree. C., and was applied to the carton flaps by a rotating grooved wheel, additionally using a spinner wheel to prevent the adhesive from stringing.

The open time and compression time varied according to the speed of the machine. The machine was set to apply adhesive to the edge of the flap.

The trial was highly successful with very little stringing apparent and it was clear that the product would be effective under standard conditions at production levels of about 150 cartons per minute.

On cartons that were well folded, fiber tear was possible after 1 minute, which was unexpected in view of the fact that the adhesive was still in its open time period.

This example describes the synthesis of a fully IPN hot melt adhesive for use as a packaging adhesive using the basic procedure described in Example I.

The sample was prepared following the procedure of Example 1. The properties of the sample were summarized as follows:

The resulting hot melt adhesive, as well as hot melts prepared from monomers containing other moisture reactive functional groups, may be successfully used to form or seal food cartons, treated or printed stocks and cork wads as described hereinabove.

In addition to the packaging applications described above, the hot melt adhesives of the invention may be used for other applications where resistance to oil is needed such as the manufacture of fiber drums for oil products, or automotive oil containers (a composite of board and metal foil).

It will be apparent that various changes and modifications may be made in the embodiments of the invention described above, without departing from the scope of the invention, as defined in the appended claims, and it is intended therefore, that all matter contained in the foregoing description shall be interpreted as illustrative only and not as limitative of the invention.