The presently disclosed and claimed inventive concepts relate to a composition for use in paper or paperboard applications. More specifically, the presently disclosed and claimed inventive concepts relate to a composition made by the hydrogenation or partial hydrogenation of an alkyl ketene dimer, alkenyl ketene dimer, or ketene multimer (collectively labeled H2-AKD). The presently disclosed and claimed inventive concepts also relate to a composition that when mixed with pulp or applied as a coating to paper or paperboard, increases the water and water vapor resistance of the paper and paperboard while maintaining good recyclability and repulpability.
Paper products are frequently sized or coated in order to form a barrier against gases such as moisture vapor and liquids such as water, oils, and greases. Wax coated paper and paperboard are widely used to protect fresh fruit, vegetables, fish, and poultry during storage and shipping. In addition to acting as a barrier, the coating also strengthens and stiffens the paper or paperboard under wet or humid storage conditions.
The wax coating formulations applied to paper and paperboard are well known in the pulp and paper industry. Coating waxes typically have hydrocarbon chains containing from about 20 to about 40 carbon atoms and melting points of from about 40 degrees Celsius (° C.) to about 85° C. Paraffin and microcrystalline waxes are two such waxes commonly used in coated paper and paperboard applications. Typical examples of such materials include natural and/or synthetic waxes as set forth, for instance, in U.S. Pat. No. 3,058,871 (Davis et al.); U.S. Pat. No. 2,859,110 (Sanford) and U.S. Pat. No. 3,021,252 (Hill et al.).
There are four commonly used methods of applying wax coatings to paper and paperboard. One method for coating at low wax addition levels (less than 5% by weight) uses a pre-made aqueous wax emulsion. The wax emulsion can be added to the wet-end of the paper machine, on a size press, or on an off-machine coater. In the other three methods, curtain coating, wax impregnation, and cascade coating, the coating is typically applied as a molten wax at addition levels of at least 3% by weight of the coated board. A curtain coater applies a thin layer of wax onto one side of the paper or paperboard.
Typical addition levels range from about 5% to about 15% wax based on the total weight of the coated paper or paper board. Wax impregnated paper or paperboard is made by passing the paperboard through a nip flooded with molten wax. Due to its low surface tension and the pressure applied in the nip, the wax penetrates evenly throughout the paper or paperboard. Wax addition levels for impregnated paper or paperboard range from about 12% to about 20% of the total weight of the coated paper or paperboard. Cascade wax coatings are applied to cut, glued, finished sections of corrugated paperboard (e.g. combined liner/corrugated medium/liner). A section of corrugated paperboard is passed under a stream of molten wax, completely coating the flutes and outside surfaces of the paperboard. Wax addition levels for cascade coatings can range from about 20% to about 50% of the total weight of the coated paper or paperboard.
It has generally been found that the waxes present on, and impregnated in, coated paper and paperboard are difficult to separate effectively without contaminating the pulp and the paper-making machinery. During repulping, the molten or semi-molten wax clings to the pulp fibers and repulping equipment. If it is not separated from the pulp fibers, the residual wax forms defects in the recycled paper or paperboard and deposits on the paper machine. Therefore, wax coated boxes cannot be recycled; they must be separated from recyclable boxes and either burned or used as landfill thereby increasing handling costs and creating environmental concerns.
A number of methods of removing wax from recycled paper and paperboard have been proposed. U.S. Pat. No. 3,058,871 (Davis, et. al.) and U.S. Pat. No. 2,703,754 (Myers) disclose the separation of hot melt coatings from the pulp by solvent extraction of the coating. Additionally, Myers teaches the separation of pulp and coating material as made by a combination of emulsification and solvent extraction. In U.S. Pat. No. 3,055,791 (Elias), solid absorbants are used in an attempt to recover pulp, whereas in U.S. Pat. No. 3,021,252 (Hill, et. al.) and U.S. Pat. No. 2,859,110 (Sanford), the coating is mechanically separated from the fiber. U.S. Pat. No. 3,822,178 (von Koeppen, et. al.); U.S. Pat. No. 2,614,922 (Hope); U.S. Pat. No. 2,859,110 (Sanford); and U.S. Pat. No. 2,959,513 (Savage), disclose procedures for recycling wax coated paper involving suspending the coating particles in a hot aqueous system. All of the above referenced patents are hereby incorporated herein by reference.
It has also been suggested that the addition of dispersants during repulping can improve the removal of wax by mechanical methods. However, these approaches are not economically feasible as the process requires treatment of the entire recycled pulp furnish with expensive chemical additives regardless of the amount of wax present.
While some of these methods have been commercially successful, a more general solution that can be adopted by the paper industry with minimal capital investment is needed. The use of fatty acids, and other organic carboxylic acids, in wax coatings for paper and paperboard is disclosed in U.S. Pat. No. 3,629,171 (Kremer). More recently, E. L. Back, “Corrugated Paperboard Project Researches Self-Dispersing Wax,” Tappi Journal, volume 74, no. 4, pages 37-38, July 1992; J. Michelman, “Method of Dispersing Wax from a Hot Melt Wax-coated Paper,” U.S. Pat. No. 6,273,993, Aug. 14, 2001; and Fuller et al., “Recyclable Wax-coated Container,” U.S. Pat. No. 5,539,035, Jul. 23, 1996; proposed incorporating a fatty acid or other dispersant into the wax coating formulation to simplify repulping and recycling.
U.S. patent application Ser. No. 07/907,173 (Michelman) describes the addition of a “latent dispersant” to wax coatings. Fuller et al., “Recyclable wax-coated container,” U.S. Pat. No. 5,539,035, Jul. 23, 1996, describe repulpable wax coatings made from mixtures of paraffin wax, fatty acid, and a compatible hydrophobic polymer additive such as polyethylene propylene rubber. Hassan et al., “Repulpable wax,” U.S. Pat. No. 6,811,824, Nov. 2, 2004, disclose repulpable water resistant coatings for paper and paperboard based on hydrogenated triglyceride fatty acid esters.
Narancic et al., “Method of Repulping Repulpable and Recyclable Moisture Resistant Coated Articles,” U.S. Pat. No. 6,416,620, Jul. 9, 2002, describe the addition of inorganic mineral fillers to improve the repulpability of wax coatings. Narancic, et al. teach that the addition of a fatty acid or surfactant to a wax coating can cause corrosion problems on handling equipment and reduce the coating's resistance to water and water vapor.
Ideally, a repulping additive for wax coatings should be physically compatible with wax over a wide range of addition levels. To simplify handling, it should have a melting point close to that of commercial wax coatings. Additionally, the additive should not negatively affect the water and water vapor resistance of the wax coating. The resulting repulpable wax coating should maintain its water and water vapor resistance for long periods of time under the wet (neutral pH), humid, and high temperature conditions encountered during the storage and shipping of produce. For example, standard “tropical” moisture vapor transmission rate (MVTR) testing of wax coated paperboard is carried out at about 38° C. and 90% relative humidity. Also, the ideal wax repulping additive should be based on renewable raw materials.
For recycling purposes, the wax coated paper or paperboard should be repulpable under pH and temperature conditions readily available to the paper maker without large capital investment. To minimize deposition on repulping equipment, the wax coated paper or paperboard should be repulpable at temperatures below the melting point of the wax. Finally, any additive and/or wax should not cause corrosion problems on the coating, repulping, or paper making equipment.
The hydrogenated alkyl ketene dimer, alkenyl ketene dimer, or ketene multimer (collectively identified as H2-AKD) of the current composition offer several advantages over the related art. The fatty acids, anionic surfactants, and cationic surfactants described by Back, Michelman, and Fuller are hydrophilic salts under the neutral pH conditions encountered by wax coated paper and paperboard during storage and shipping. Similarly, the nonionic surfactants described in the prior art are hydrophilic and water miscible. Adding these hydrophilic materials inevitably reduces the water resistance of the wax coating. Because of the warm, humid conditions encountered during the shipping and storage of produce, it is unlikely that the coatings described by Hassan, which are comprised primarily of triglycerides, can resist microbial growth for long periods of time under these conditions.
By contrast, H2-AKD is hydrophobic under end-use conditions and can be added directly to wax at the levels needed to improve repulpability without compromising the water and water vapor resistance of the coating. Additionally, since H2-AKD is a non-polar hydrophobic wax, it is not corrosive to coating or paper making equipment.
As described below, H2-AKD addition levels as low as 5% by weight of total coating can improve the repulpability of a wax coating. Also advantageous is that H2-AKD is based on renewable, fatty acid raw materials.
Additional objects, advantages, and features of what is claimed will be set forth in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the technology. The objects and advantages of the presently disclosed and claimed inventive concepts will be realized and attained by means of the compositions and methods particularly pointed out in the appended claims, including the functional equivalents thereof.