Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1998-07-22
2001-05-29
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S210000, C156S336000
Reexamination Certificate
active
06238504
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to the manufacture of paperboard products, in particular the manufacture of corrugated paperboard or boxboard products.
DESCRIPTION OF RELATED ART
Corrugated paperboard or boxboard products are made by adhering one or more sheets of linerboard to one or both sides of a fluted sheet of corrugating medium, which is typically a fluted sheet of paper, in large machines called corrugators. The corrugators apply adhesive to the corrugating sheet, assemble it with the sheets of linerboard, and heat the resulting assembly to dry and cure the adhesive. Viewed in cross section, the resulting boxboard contains alternating tips and valleys, having adhesive applied to the tips on one or both sides where they come into contact with, and are bonded to, the linerboard. Boxboard having a single sheet of linerboard bonded to the corrugating medium is generally referred to as “single face” board. An additional sheet of linerboard can then be applied to the single face to form a sandwich of corrugating material between two sheets of linerboard, known as “double back” board. Alternatively, two sheets of single face board can be bonded together, with the flat surface of one linerboard sheet bonded to the exposed surface of the corrugating medium of the other. An additional sheet of linerboard can be bonded to the remaining exposed surface of the corrugating medium to form what is known as “double wall.”
The adhesives used to prepare corrugated boxboard are often starch-based, which are formulated and cooked by the corrugated board plant for use on their particular manufacturing equipment. In most corrugating plants, the adhesive is formulated, pumped to the corrugator, and re-circulated through various application stations. This approach can create a number of problems, however.
For example, the composition and properties of the adhesive may vary to some extent, depending on the quality of raw material used to formulate the adhesive and the precision with which the adhesive is formulated. Often, the starch-based adhesive has a rheology such that it does not remain only on the tips of the flutes of the corrugating material in the areas that come into contact with the linerboard. When this occurs, the adhesive can flow onto the inner surface of the adjacent valley. This results in excessive use and waste of adhesive, as well as the production of wetter board that requires longer drying time (decreasing production and increasing the chance of producing warped board production).
Addressing these problems by reducing the amount of starch-based adhesive applied to the corrugating medium is difficult, because this amount is difficult to control precisely. Most corrugators use a starch coating roll to apply the adhesive to the corrugating medium, and use a doctoring or metering roll near the applicator roll and separated therefrom by a variable gap to control the amount of adhesive that is applied. Decreasing the gap results in the application of a lower volume of adhesive, thereby decreasing adhesive consumption. However, this decreased volume of adhesive also reduces the performance of the boxboard as measured by pin number and edge crush tests. As a result, it is difficult to control the amount of adhesive applied with sufficient precision to properly balance adhesive consumption with board performance.
Another approach to decreasing starch consumption is to lower the solids content of the adhesive composition by diluting with water. Even if the viscosity of the starch composition is maintained, the use of additional water increases drying time, causing the corrugator to run more slowly and/or producing a wetter, poorer quality board.
Starch based adhesives of the type generally used in producing corrugated boxboard can generally be foamed only with difficulty. Unintended foaming, which occurs with some frequency with some other adhesives (see U.S. Pat. No. 3,775,145 at column 8, lines 50-65) is not commonly encountered, except for the occasional formation of large bubbles in the pans through which adhesive passes in the corrugated paper board-making process. This foaming is generally regarded as undesirable by corrugated board manufacturers, and the difficulty with which starch-based adhesives form a foam has been considered to be beneficial.
In certain circumstances, it is desirable to adhere labels to the exposed flute tips of corrugated board having a single sheet of linerboard bonded to the corrugating medium (called “single face” board). Often, white labels are affixed using either vinyl acetate or dextrin starch-based adhesives. The application of these adhesives can result in problems similar to those described above for the corrugating process itself.
Accordingly, there is a need in the art for a method of making corrugated board products that decreases adhesive consumption without compromising board quality or throughput. There is also a need in the art for a method of making corrugated board that provides stronger, higher quality board. Finally, there is a need for a process that achieves these objectives without the need for large, expensive additional machinery, and that can easily be connected to existing corrugator installations.
SUMMARY OF THE INVENTION
The needs described above are met by the process, apparatus, and corrugated board produced by the process of the present invention.
The present inventors have discovered that air can be dispersed into starch-based adhesives as a diluent and rheology modifier. In one aspect, the present invention relates to a method of preparing a corrugated sheet by incorporating an amount of air into a liquid adhesive that is sufficient to form a stable foam, applying this foamed adhesive to the tips of the fluted corrugating medium, placing the adhesive-coated tips in contact with a sheet of flat stock, and drying the adhesive for a sufficient time to adhere the flute tips to the flat stock.
The process utilizes air dispersion equipment to form the foam using either direct introduction of air, mechanical or static means to incorporate air into the starch compound and form a foam via fluid shear or turbulence, or a combination of these techniques. The amount of air dispersed can vary depending upon the particular adhesive formulation used, but is typically between approximately 10% and 30% (by volume). In one embodiment of the invention, the starch-based adhesive is saturated with dispersed air. The foam obtained by the process of the present invention can be easily and precisely applied to the tips of the corrugating medium, and the amount of adhesive applied can be more accurately controlled.
In one embodiment of the invention, the foam-forming characteristics of the process of the invention can be improved by the addition of a surfactant, such as sodium alkyl sulfonates.
Corrugated paper board manufacturing plant data shows acceptable performance on all flute and linerboard combinations using a foamed starch-based adhesive in accordance with this invention. The air injection process allows the corrugated board manufacturer to reduce starch consumption by applying the same physical volume of adhesive at reduced solids level. Since the air alters the adhesive rheology without altering liquid adhesive viscosity, the quantity of adhesive applied remains on the flute tips without migrating into the flute valleys. Because the adhesive is located properly and remains in the proper location, the process of the present invention uses less starch, yet gives a stronger bond, and thus a stronger sheet, than prior processes. The process of the present invention results in boxboard having increased pin numbers on both single face and double backed board, and the corrugated board products produced by the process of the present invention forms another aspect of the invention.
The process of the present invention achieves improved board quality with decreased adhesive consumption, increased running speeds, and avoids common corrugator problems. While it requires additional equipment, the saving
Brown W. Lamar
Simmons, Jr. Raymond C.
Ball Michael W.
Gray Bruce
Johnson Kristin L.
Kilpatrick & Stockton LLP
Musser Barbara J.
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