Because of their rapid cure time, hot-melt adhesives are at present predominantly used in the sealing of corrugated fiberboard containers made from pre-cut flats. The rapid cure time of hot-melt adhesives provides economic advantages insofar as it enables shorter assembly lines and faster line speeds. But hot-melt adhesives also have a number of disadvantages, which have become increasingly onerous in recent years.
The material cost of hot-melt adhesives, which typically have an ethylene-vinyl acetate polymer base, has risen sharply over the past two decades, to the point where they may be several times more expensive than water-based adhesives. There is also a high energy cost associated with hot-melt adhesives, which must be heated to 250-350° F. before being applied to a corrugated fiberboard surface. The elevated application temperatures of hot-melt adhesives give rise to health and safety problems relating to burns and odors. High application temperatures often render hot-melt adhesives unsuitable for packaging of heat-sensitive products.
Equipment and maintenance costs are also significantly higher for hot-melt as compared with water-based adhesives. Temperature and cooling rate of the hot-melt adhesive must be controlled within a rather narrow range or the resulting bond is weakened. Hot-melt adhesives are difficult to spread and do not readily penetrate between the fibers of the substrate. On the other hand, due to their relatively low viscosity, water-based adhesive, such as polyvinyl acetate emulsions, can be sprayed or extruded in a thin layer that forms a strong adhesive bond. The relatively low viscosity of water-based adhesives also facilitates better penetration of a fibrous substrate for stronger bonding than is achievable with hot-melt adhesives.
The superior bonding of water-based adhesives also remains stronger over a greater range of temperatures than for hot-melt adhesives, which soften at temperatures over 165° F. and may become brittle below 40° F.
Finally, water-based adhesives offer an important environmental advantage as compared with hot-melts, since corrugated containers assembled with a water-based adhesive can be re-pulped and recycled without removing or separating the adhesive layers.
Despite all of the foregoing advantages of water-based adhesives, one factor has kept them non-competitive with hot-melt adhesives in the sealing of corrugated containers—their relatively slow curing time. Under the type of compression typically used on carton sealing lines, a hot-melt adhesive will set in about 1 to 3 seconds, as compared with 10 to 20 seconds for a water-based adhesive.
In an effort to overcome this deficiency, the prior art has largely focused on methods of heating the water-based adhesive prior to the compression/sealing stage. Examples of this approach are disclosed in Oakley, U.S. Pat. No. 4,490,129, Gong et al., Pub. No. US2004/0164135, and Nowicki et al., Pub. No. US2004/0166238. In a slight variation of the same theme, Lorenz et al., U.S. Pat. No. 7,238,149, teaches a process whereby a special aqueous-based adhesive, comprising a semi-crystalline vinyl acetate-ethylene polymer emulsion, is heated during the sealing stage.
These methods of shortening the curing time of water-based adhesives by applying heat to the adhesive have in practice proven ineffective, however. The heating of the water-based adhesive changes its physical properties and results in substantially less bonding strength.
An alternate approach to direct heating of the water-based adhesive has been to heat the water-based adhesive indirectly by pre-heating the cooperating surface that is to be bonded with the surface to which the adhesive is applied (the latter being hereinafter referred to as the “application surface”). The heat thus stored in the cooperating surface is then transferred to the water-based adhesive when the two surfaces are compressed together, thereby causing the adhesive to cure more quickly. Such a method is taught by Doman, U.S. Pat. No. 4,778,554. Here again, however, the water-based adhesive is being heated, albeit indirectly.
Besides weakening the resulting adhesive bond, the methods that attempt to reduce cure time by heating the water-based adhesive (whether directly or indirectly) effectively negate the water-based advantage of eliminating adhesive heating costs.
In Sewell et al., U.S. Pat. No. 4,375,383, the inventors have endeavored to shorten the curing time while retaining the advantages of the relatively low viscosity of water-based adhesive. They recognize that the strength of the adhesive bond is increased by achieving the two objectives of (a) applying the adhesive in a thin discrete pattern, and (b) inducing the adhesive to penetrate between the fibers of the substrate. They also recognize that both of these objectives can be achieved only if the inherent viscosity of the water-based adhesive is retained and not altered by heating.
The process taught by Sewell et al. comprises (i) spraying the water-based adhesive on the application surface, and then (ii) joining the application surface with the cooperating surface under high pressure (20-160 lbs/sq. in.) sufficient to force the adhesive into the interstices between the fibers of the two surfaces. Using this process, the inventors claim to reduce the adhesive curing time to 2 seconds or less.
There is, however, a major drawback to the Sewell process as applied to the sealing of corrugated cartons. The typical carton assembly line operates as follows: (1) the bottom and side walls of the box are first formed from a pre-cut flat pattern; then (2) the product is placed in the carton while the four top flaps of the carton remain open; then (3) the two innermost top flaps (referred to as the “minor flaps”) are closed; while (4) adhesive is applied to the lower surfaces of the two outermost top flaps (referred to as the “major flaps”); then (5) the major flaps are closed and compressed against the minor flaps long enough for the adhesive to set.
As can be seen in FIGS. 1-3 of the Sewell patent, the high pressure sealing process taught therein depends upon the presence of a rigid support below the bottom piece of corrugated cardboard. But in sealing the top flaps of a corrugated carton, there is no support under the minor flaps other than the product itself and associated packing materials, which usually are not rigid and which may be damaged by high pressure. Therefore, while the Sewell process may be suitable for sealing the bottom of a corrugated carton, it is not suitable for sealing the top flaps.
Consequently, there is a need, as yet unaddressed by the existing art, for a process and associated apparatus that will reduce the curing time of a water-based adhesive while not increasing the adhesive's viscosity. Such a process should be readily applicable to a conventional corrugated carton assembly line. The present invention fulfills this need by providing a process, with supporting apparatus, by which, prior to adhesive application, the application surface is briefly exposed to a heat source so as to open and expand the interstices between the fibers and thus render the surface more penetrable to a water-based adhesive. This process, which will be described in greater detail hereinbelow, is capable of reducing the curing time of water-based adhesives to 2 seconds or less while producing an adhesive bond that equals or exceeds the strength of a bond formed by a water-based adhesive cured for 10-20 seconds or more.