Abstract:
The disclosed invention provides an adhesive composition that can be applied to a paperboard carton via water-based flexographic or gravure printing, that can subsequently be activated (i.e., melted) by RF radiation during a carton sealing operation, and that provides water-resistant bonding.

Description:
[0001]    This non-provisional application relies on the filing date of provisional U.S. application Ser. No. 60/787,570 filed on Mar. 30, 2006, which is incorporated herein by reference, having been filed within twelve (12) months thereof, and priority thereto is claimed under 35 USC § 1.19(e) 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a composition useful in a method for sealing cartons. More particularly it relates to application of an adhesive composition on one or more flaps of an unfolded carton by printing said composition using a water-based flexographic or gravure process. Most particularly, the invention relates to activating the printed adhesive with radio frequency radiation and folding the flaps to form the sealed carton. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hot melt adhesives are widely used to seal paperboard cartons such as those used to store and transport beer cans, soda bottles, and similar items. The hot melt adhesives have a very rapid set time which makes them highly suited to the high-speed assembly machinery that is used for filling and closing the cartons. 
         [0004]    However, hot melt application equipment is subject to frequent plugging. This plugging is the primary cause of stoppages and down time on carton filling machinery. 
         [0005]    It would therefore be desirable if an adhesive could be pre-applied to the flaps of cartons that are to be sealed and then be activated in some manner at the point where the sealing is to take place. The ideal way to apply such an adhesive would be during the process wherein the cartons are printed. This would be far more efficient that applying it in a separate manufacturing step. Since most paperboard cartons are printed by flexographic or gravure processes using water-based inks, a reactivatable adhesive that could be applied by this process would be most desirable. 
         [0006]    A number of possible ways of producing reactivatable adhesives have been considered in the past, but they have been rejected for various reasons. For example, water-remoistenable adhesives set too slowly because of the high amount of energy needed to drive off the water. Adhesives containing microencapsulated activators have been tried, but their activation speed is limited by the mass transport rate of the encapsulated component. 
         [0007]    One way of rapidly reactivating an adhesive would be to melt it by heating with radio frequency (hereinafter RF) radiation. U.S. Pat. No. 6,348,679 and U.S. Pat. No. 6,600,142 disclose methods of sealing various substrates by reactivating a pre-applied adhesive with RF radiation. The adhesive coatings taught in these patents comprise two components: A polar polymer, such as a sulfonated polyester, and a highly polar, non-aqueous carrier. While such adhesives are readily activated by RF radiation due to the aromatic ester bonds in the sulfonated polyesters, they are completely unsuitable for application by flexographic printing due to the presence of the polar carrier. The polar solvents described as carriers U.S. Pat. No. 6,348,679 and U.S. Pat. No. 6,600,142 have very high boiling points whereas flexographic inks require relatively low boiling point solvents in order to dry properly on the press. Typically, in water-based flexography or gravure printing the solvent consists of water and small amounts alcohols containing 1-4 carbon atoms. The polar “carriers” described in the prior art would not evaporate under typical flexographic or gravure printing conditions, and their presence in the printed film would render it tacky and cause blocking problems when the printed cartons are subsequently handled. 
         [0008]    An important consideration for an adhesive used on beverage cartons is that the final bond be waterproof. Such cartons are regularly placed in contact with ice, which may be partially melted at times, and it is important that the adhesive holding the carton together not become unstuck. This is not a problem with conventional hot melt adhesives, which are usually water-resistant. However, the compositions disclosed in U.S. Pat. No. 6,348,679 and U.S. Pat. No. 6,600,142, being mixtures of highly polar polymers and highly polar solvents, are very water sensitive and will not stand up to the “dunk test” usually applied to such cartons. As the name implies, this test consists of immersing a carton full of beverage can in water for a set period of time and observing whether any of the adhesive bonds fail. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the current invention is to provide an adhesive composition that can be applied to a paperboard carton via water-based flexographic or gravure printing, that can subsequently be activated (i.e., melted) by RF radiation during a carton sealing operation, and that provides water-resistant bonding. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    The object of the present invention is met by providing an aqueous composition comprising two resinous components: (a) an aromatic carboxylic ester-modified rosin derivative that is insoluble in water but soluble in aqueous base and (b) an acrylic or styrenic latex. 
         [0011]    Aromatic esters, such as isophthalates and terephthalates will absorb in the RF range. This range includes 13.56 MHz and its harmonics (e.g., 27.12 MHz, 40.68 MHz, etc.) which have been licensed by the FCC for industrial use. Commercial irradiating units have been produced for various applications with operating frequencies as high as 120 MHz. For example, RF at these frequencies can be used to melt polyethylene terephthalate plastics for processing. 
         [0012]    Partial esters of maleated or fumarated rosin with various polyols are commonly referred to in the art as maleic resins. Maleic resins are described in detail in  Coating and Ink Resins,  by W. Krumbhaar (Reinhold, New York, 1947), chapter III, which is incorporated herein by reference. Maleic resins with acid numbers of above about 140 are usually soluble in aqueous base but insoluble in water at a neutral pH. 
         [0013]    We have found that it is possible to modify such soluble maleic resins for use in the present invention by replacing the polyol with a hydroxyester formed from a polyol and an aromatic carboxylic acid. This enables the incorporation of aromatic ester functionality into the rosin derivative, thereby making it an antenna for RF radiation. 
         [0014]    The preferred way of making the aromatic-modified maleic resin is first to react a polyol with an aromatic polycarboxylic acid to form an oligomeric ester. The polyol/polycarboxylic acid molar ratio should be chosen so that there is an excess of polyol, thereby yielding an oligomer that is terminated with hydroxyl groups. 
         [0015]    The aromatic polycarboxylic acids suitable for use in this step are generally those that contain from two to about four carboxyl groups. Examples include, but are not limited to phthalic acid, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acids, trimellitic acid, pyromellitic acid, benzophenonetetracarboxylic acid, and their anhydrides, and mixtures thereof. The preferred acids are isophthalic acid and terephthalic acid. 
         [0016]    Polyols suitable for use in this step are generally those containing from two to about six hydroxyl groups and from two to about twelve carbon atoms. Examples include, but are not limited to ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, cyclohexanedimethanol, neopentyl glycol, butanediol, hexanediol, glycerol, trimethylolethane, trimethylolpropane, ditrimethylolpropane, pentaerythritol, dipentaerythritol, and mixtures thereof. In place of glycols, the analogous epoxides, such as ethylene oxide or propylene oxide, or the analogous alkylene carbonates, such ethylene carbonate, propylene carbonate, or glycerol carbonate, may be used. 
         [0017]    Esterification catalysts are well known in the art, and these may be used to shorten the reaction time for this step. Typical examples of suitable esterification catalysts include, but are not limited to, sulfonic acids, tertiary amines, and organotin compounds. 
         [0018]    In addition to the direct esterification of aromatic carboxylic acids or anhydrides with polyols, epoxides, or alkylene carbonates, suitable hydroxyl-terminated aromatic esters can be made by transesterification of simple alkyl esters of the aromatic acids, such as the methyl or ethyl esters with the various polyols noted above. Dimethyl isophthalate and dimethyl terephthalate are the preferred simple alkyl esters. The transesterification can be facilitated by the addition of known transesterification catalysts such as tetraalkyl titanates. 
         [0019]    In a second step, rosin is reacted with maleic acid, maleic anhydride, or fumaric acid in a known manner as described in the Krumbhaar reference cited above. Generally, the rosin is melted and mixed with the desired amount of unsaturated acid or anhydride, and the mixture is heated at from about 180 to about 230° C. until Diels-Alder adduction of the unsaturated acid or anhydride to the rosin is complete. This usually takes for one to four hours. The rosin used may be gum rosin, tall oil rosin, or wood rosin. 
         [0020]    In a third step, the maleated or fumarated rosin from step two and the aromatic hydroxyester from step one are reacted together in an esterification reaction to form the aromatic-modified maleic resin. Typical conditions for this esterification reaction are described in the Krumbhaar reference cited above. In general, the reaction mixture is heated at a temperature of about 180 to about 240° C. until the desired acid number is obtained. Typically, this takes from about one to about ten hours. The preferred acid number for the product is from about 140 to about 300, more preferred is from about 160 to about 240. 
         [0021]    Other orders of addition and reaction are possible, but they are generally suitable for only certain combinations of reactants. Other combinations tend to give inhomogeneous products. The preferred method is the most generally applicable. 
         [0022]    The aromatic ester-modified, rosin-based resins that are described above are utilized in the present invention in the form of aqueous solutions, where they are dissolved in water at a pH of about 7.5 or higher, preferably a pH of from about 8.0 to about 10.0. The desired pH is obtained by the addition of a fugitive base, such as ammonia, an amine, or an alkanolamine. Typical amines that can be used include, but are not limited to, mono-, di- and trialkylamines containing from one to about 12 carbon atoms, morpholine, and N-alkylmorpholines, where the alkyl group contains from one to about four carbon atoms. Typical alkanolamine that are suitable for use in the invention include, but are not limited to, monoalkanolamines, dialkanolamines, trialkanolamines, alkyldialkanolamines, and dialkylalkanolamines containing from two to about 12 carbon atoms. The use of alkaline solutions of rosin-based resins in flexographic and gravure inks and varnishes well known in the art, and one skilled in the art could readily choose an appropriate amine as needed to adjust the drying time and viscosity of the ink or varnish to meet required press conditions. 
         [0023]    The rosin or rosin derivative provides tackiness to the activated adhesive, allowing quick bond formation. It also enhances the water resistance of the final bond because when the neutralizing base evaporates upon drying of the adhesive, the rosin or rosin derivative becomes water-insoluble. 
         [0024]    The acrylic or styrenic latex component of the present invention also contributes to the water-resistance of the final bond. It also increases the drying rate of the composition during flexographic printing. These latices are manufactured from styrene, substituted styrenes, acrylic esters, and methacrylic esters by well-known emulsion polymerization methods. Any of the latices of this type that are commonly used in water-based flexographic inks are suitable for use in the present invention. Typical products of this type are commercially available under trade names such as JONREZ (MeadWestvaco), JONCRYL (S.C. Johnson), and RHOPLEX (Rohm &amp; Haas). These acrylic polymers are available with a wide range of glass transition temperature (Tg), generally from −40 to +100° C. For the purposes of the present invention, it is useful to select a resin that is sufficiently hard (high Tg) so that the reactivatable coating will not block when unsealed cartons are stacked during processing or shipment. At the same time, the Tg should not be too high, or it will raise the melting point of the overall coating and thereby increase the reactivation time. The selection of a latex with an appropriate Tg will also depend on the softening points of the other components of the coating, as discussed above, and the relative ratios of the ingredients. One skilled in the art of formulating water-based flexographic and gravure printing inks, where blocking is also an issue, will already be familiar with the properties of these acrylic and styreneic latices, and should be able to select an appropriate latex for a given application. 
         [0025]    The two components of the present invention, the aromatic ester-modified, rosin-based resin and the acrylic or styrenic latex can be blended together to make the water-based, flexographically printable by first dissolving the rosin derivative in aqueous base and then mixing the resulting solution with the latex. Alternatively, sufficient water and base to dissolve the rosin derivative can first be added to the latex and the rosin derivative can then be dissolved in the mixture in the presence of the latex. 
         [0026]    To function well on a flexographic or gravure printing press, the final blended coating should have a viscosity of about 8 to about 40 seconds as measured with a No. 2 Shell cup. A viscosity of about 18 to about 25 seconds is most preferable. The viscosity can be adjusted by the addition of water or small amounts of thickeners, such as polyvinyl alcohol or associative thickeners, whose use will be familiar to one skilled in the art of formulating flexographic or gravure inks. 
         [0027]    For stability during the printing process, the pH of the printable adhesive composition should be maintained in a range of about 7.5 to about 10.0. The pH can be adjusted using any of the fugitive bases described above. 
         [0028]    With some rosin derivatives, particularly those derived from tall oil, some of the rosin may tend to crystallize out of the mixture on storage. This problem can be eliminated by adding small amounts (preferably &lt;10%) of alcohols containing from one to about four carbon atoms. 
       EXAMPLES 
       [0029]    The invention may be illustrated by the following examples, which are not to be construed as limiting the invention in any way. 
       Preparation of an Aromatic Ester-Modified Rosin-Based Resin 
       [0030]    Fourteen hundred twenty-five grams of ROSIN SS (a tall oil rosin supplied by the Specialty Chemical Division of MeadWestvaco) was charged to a 2-liter flask equipped with a mechanical stirrer and a heating mantle and melted by heating to 160° C. Then 327.8 grams of maleic anhydride and 3.56 grams of dibutyltin oxide were charged, and the batch was heated for one hour at 185° C. Then 242.8 grams of bis(2-hydroxyethyl) terephthalate (the reaction product from a 1:2 molar ration of terephthalic acid and ethylene glycol) was added, and heating was continued at four at 200° C. to produce an alkali-soluble, resinous rosin derivative with an acid number of 190. 
       Preparation of a Flexographically Printable, RF Activatable Adhesive 
       [0031]    An aqueous solution of the above resin was prepared by dissolving 821 grams of the resin in a mixture of 275 grams of concentrated aqueous ammonia and 1505 grams of deionized water. 
         [0032]    One hundred two grams of the above resin solution thus obtained was added with stirring to 100 grams of JONREZ E-2069, an acrylic latex supplied by the Specialty Chemical Division of MeadWestvaco, to produce the adhesive. 
       Printing the RF-Activatable Adhesive 
       [0033]    The composition prepared above was successfully printed on 18 point Carrier Kote board (a grade of paperboard supplied by MeadWestvaco that is used to manufacture beverage cartons) using a Comco Captain pilot scale flexographic printing press. No problems were seen in the transfer of the material into the cells of the anilox roll, transfer of the material from the anilox roll to the plate, or transfer of the material from the plate to the board substrate. 
       Testing RF Response 
       [0034]    Board coated with the composition prepared above was tested for RF activation by placing a strip of coated board against a strip of uncoated board, placing these two strips between two blocks of UHMW polyethylene, and securing the blocks with rubber bands (polyethylene and rubber are transparent to RF). The assembly was then subjected to RF radiation of 100 MHz frequency for five seconds in a THERMALL Model 950 machine manufactured by Radio Frequency Company of Millis, Mass. Upon disassembly of the blocks, it was found that there was a fiber-tearing bond between the two pieces of board. 
         [0035]    This demonstrates that the invention adhesive compositions useful in this process can both be printed by flexographic or gravure processes and subsequently be activated by radio frequency radiation.