How do you add value to a brown box or carton so that you satisfy your customer's performance requirements and also improve your margins and profitability?
Converters of paperboard have long faced the challenge of incorporating barriers to water, grease and other hazards through a variety of processes, while still being able to successfully complete the finishing of these boxes.
Wax, foil, fluorochemicals, polyethylene and other polymers have done the job in the past. However, over the past ten years, environment, cost and performance issues have driven the development of functional coatings as a viable alternative for many of these materials.
Michelman, Inc. of Cincinnati, OH has trademarked the phrase “expanding the possibilities of paper.” From their viewpoint, functional and decorative coatings are an exciting way to add value to the final package. Michelman's product line illustrates the range of functionalities available to converters: water resistance and/or reduced moisture vapor transmission rate (MVTR), grease and oil resistance, flame retardance, anti-skid properties, abrasion resistance, conductive coatings to protect electronic parts and colors.
Water is the enemy of corrugated. All boxes are affected by humidity that changes the moisture content of the board and thus negatively impacts compression and stacking strength. In addition, almost 20% of box shipments require some degree of water or moisture resistance in order to effectively protect the end-use product and box integrity in distribution. Key end-uses that require boxes with varying degrees of water resistance are poultry, fresh produce, meat, fish, horticultural products, frozen foods, beverages intended for refrigerated distribution and products intended for outside storage like building materials and military supplies. Non-tissue paper products for printing also require moisture resistance, although often ream wrap fills that need.
The corrugated industry has long supplied boxes with resistance to both water and moisture. The oldest methods use wax as part of the post-corrugator converting processes. Wax can be coated onto the box, the box can be dipped into molten wax, or the wax can be cascaded onto the board so that it seeps down into the flutes as well as over the facings. In addition, some liner and fluting mills still produce wet strength grades by chemically treating the furnish in the wet end of the machine. The chemicals used are mostly synthetic polymeric substances such as latexes and polyacrylamides, either alone or in combination with starches and gums. The combination of wax with wet strength containerboard, especially wet strength fluting, can create a truly waterproof box. Yet waxed cases and those with one or more sheets of wet strength cannot be repulped, creating all sorts of issues for board producers and box users. In the past five years, functional coatings have progressed to the point where wax and wet strength containerboard are losing share in the markets where moisture matters.
Water resistance is also a key attribute for beverage carrier board, but for most folding carton converters, grease resistance is the greater challenge, impacting end-users in a variety of industries, particularly margarine and butter, confectionery, pet food, lotion-treated tissue, baked goods, food service and liquid packaging. European environmental regulations dictate the need to shift to monomaterials and source reduction, driving the development of barrier coatings to enable compliance in the final package. StoraEnso, a European-based global leader in liquid packaging boards, summarizes the challenge as follows: “Barrier coatings applied onto the packaging board give protection against undesirable effects caused by such factors as light, oxygen, humidity, grease or heat. In addition, selecting the most suitable coating and baseboard combination saves material, improves printability and visual quality, simplifies converting and packaging and reduces waste.”
Fluorochemicals and polyethylene extrusion have dominated the specifications for grease resistant paperboard in the past. The former is applied in the wet end of the paper machine, but most mills do not really like to use fluorochemicals in their process. The market leader, 3M, discontinued their line of products using perfluoroctanyl chemistry at the end of 2000, making fluorochemically treated boxboard for cartons an endangered species, despite the scramble to fill 3M's exit by those who still supply fluorochemicals. Again, functional coatings have made inroads. Several boxboard mills like Simkin's Lowe mill in New Jersey, and Caraustar's mill in Rittmann, OH produce coated recycled boxboard with on-machine application of grease-resistant coatings. Converters with roll-fed rotogravure or flexographic printing equipment can apply functional coatings with gravure or roll coaters, although high levels of grease resistance still elude these methods.
There are three main ways to apply functional coatings to paperboard — on the paper machine, on the corrugator or carton converter's printing press or on an off-line coater. Spectra-Kote of Gettysburg, PA has generic patents covering the application of all types of acrylic based functional and decorative coatings on the paper machine, whether the application is done in the wet end, the size press, calendar stacks or coating station. Some containerboard producers are thus able to apply these coatings on-machine, although this trend is even stronger in the cartonboard arena where several key producers are applying grease-resistant coatings on machine, as noted earlier.
If applied on the corrugator, the type of coating dictates whether it will be applied on the wet or dry end. The weight range of coating deposit, coating viscosity and coating speed determines the type of coater. Speed is determined largely by drying requirements. Corrugator application is usually limited by drying capacity, making blade application much more doable since the heavier laydowns achieved by rod coaters cannot be dried while maintaining normal throughput.
Functional coatings applied to linerboard need precise and uniform laydown, using knife or bar coaters that meter the material to remove excess coating, only allowing a predetermined amount of material to pass through. However, on-corrugator application slows down the corrugator, usually by at least 10% and at times as much as 50%. Most plants do not want to lose that time, even in the current slow growth box market, or deal with the variation in machine settings that different types of coating application require.
The installation of advanced robotics and electronic controls also make on-corrugator application of coatings problematic, as does the need to keep drive shafts and knives free of coating contamination. In those circumstances, the boxmaker turns to off-line coating, sometimes at the supplying mill, or, more likely, a third party converter who specializes in coating rollstock. This latter option is increasingly popular as converters recognize both the economic, quality and service advantages of out-sourcing the application of functional and decorative coatings to those who focus on this business.
Liner quality is a critical factor, with 100% recycled sheets at a disadvantage because of fiber formation that fails to create the dense, smooth surface that enables coating film formation without pinholes or other voids. This does not necessarily mean that only virgin kraftliner can be used, but it does mean that the surface of recycled liner needs to be made receptive. Often, a first bump of a lower cost base coat converts the recycled sheet into a receptive substrate for the higher cost coating. The combined cost will still be less than applying a heavier deposit of the coating onto a virgin sheet. If the board is first coated on an off-line coater, the right material may be further densified by the heat and pressure on the corrugator, which further enhances the integrity of the film of protective coating. Whether the converter applies the coatings on-corrugator or on-press, or sources through a third party coater, further hurdles remain in finishing.
The functional coating must be able to accept printing inks, starches, glues, hot and cold melts. The challenge of printing a water-based flexographic ink on a water-resistant coating or an oil-based lithographic ink on a grease-resistant coating are hurdles that have been addressed by suppliers like Spectra-Kote and Michelman.
Decorative coatings are increasingly used in place of higher cost printing inks for solid colors, particularly in corrugated. In the market for waxed fresh produce boxes, the traditional use of white-top and coated white-top outer liners for aesthetics is now challenged by white coatings. These white coatings will provide the aesthetics of white-top, but retain a higher level of brightness after the box is waxed. Custom colors that help define a brand identity are also a growth area.
Will functional coatings ultimately prevail? Can off-line coating be more cost-effective once all aspects of efficiency, downtime, scrap and performance are included in the cost analysis? Will the European initiative towards monomaterials come to North America?
Regardless of the answers, the initiatives from Spectra-Kote and Michelman and their customers make it certain that functional and decorative coatings have a rosy future. Boxmakers now have the materials they need to satisfy customer requirements by adding value that is truly meaningful to everyone.
