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Recyclable Barrier Coatings Offer Alternative
March 1, 2009 By: Charles P. KlassA number of factors have combined to create renewed interest in recyclable barrier coatings as an alternative to waxed boxes and for other applications of paperboard packaging.
OCC is in tight supply and increasing in price due to export demand for new mills in China and Vietnam. Walmart’s “Packaging Scorecard” is providing incentives to develop recyclable packaging materials. Tipping fees for non-recyclable waste going to landfill or incinerators is increasing. As much as 10 percent of a supermarket company’s profits can be spent separating non-recyclable waxed boxes and other non-recyclable packaging materials.
At present, regulatory requirements are not driving the U.S. market. In Europe, however, taxes are levied on quantity and size of packaging and composite packaging, due to problems with recycling. This has created demands for smaller, lightweight packaging that is “mono – material” as opposed to extrusion coated and waxed products.
There is increasing concern about the carbon footprint of materials used in making paperboard packaging. Petrochemical wax is made from old carbon.
Any new generation of recyclable barrier coated products must also address environmental concerns about recovery and recycling. They must be repulpable, biodegradable, not affect reprocessing in the paper mill, result in lower energy consumption, recover treatment chemicals and possibly be chlorine-free. If food contact approval is essential for broad-scale market acceptance, they must be minimal in migration, low in toxicity and organoleptically inert.
Barrier Coating Development And New Materials
Through most of the 1990s, acrylic latex was the primary product used for barrier coating because it provides rapid formation of a strong, pinhole-free film, and is non-tacky even at elevated temperature and humidity. Acrylic latex films are flexible and low odor, but are relatively expensive.
While the superior film-forming properties of SBR latex have long been known, it tends to yellow when exposed to UV light. It can also have an objectionable odor and can be somewhat tacky. Modified SBR latex could provide some significant advantages in barrier coatings.
In the mid to late 1990s, styrene acrylate latex was developed to combine some of the advantages of styrene and acrylic monomers. It forms a tight film and combines some of the advantages of SBR and acrylic.
In the past five years, new engineered latex products have included latices with controlled particle size, varied Tg and a varied degree of carboxylation. This is being done with all of the common latex chemistries, including styrene-butadiene, polyvinyl acetate, acrylic and styrene acrylate.
Polyvinyl alcohol is an excellent film former. It is available in a range of viscosities and varying degrees of hydrolysis, and also provides oil and grease resistance. Polyvinyl alcohol films are hard and can be brittle, and a major drawback is that they must be cooked on site.
Aqueous dispersions and emulsions of resins can be useful in barrier coating. The two primary resins are PVDC (polyvinyladene chloride) and PET (polyethylene terephthalate). PVDC is applied to the sheet as a suspension or emulsion of discrete particles. When heat is applied to the coating to drive off the water, the PVDC particles coalesce to form a film. PET resin dispersions and emulsions are similar in action to PVDC. They are applied as discrete particles and subsequently coalesce and self-crosslink as the coating is dried.
Polyethylene extrusion coating is used on SBS board for application in frozen food boxes and other food packaging applications.
New Barrier Coating Materials
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As noted above, all of these materials are petrochemical based and have a significant carbon footprint. In addition, recycling of board treated with synthetic emulsion polymer latices can cause deposit problems in the paper mill. The latest developments in barrier coating materials include biolatex and very high aspect ratio pigments.
EcoSphere® Biolatex™ from EcoSynthetix LLC is made by a proprietary extrusion process that breaks starch into nanoparticle latex as shown in Figure 1. It does not require cooking and is readily dispersed in water to form crosslinked water-swollen nanoparticles that are sterically stabilized. When the resulting latex is formulated with the proper crosslinker, coated linerboard with excellent water resistance can be produced. Biolatex has a minimal carbon footprint as shown in Figure 2.
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Barrisurf™ engineered kaolin clay pigment from IMERYS has a very high aspect ratio <100:1. It is very effective in plugging pores in a coating structure and increases tortuosity to resist penetration by water, grease and oil as illustrated in Figure 3. It also reduces transmission of water vapor and other gases.
Higher Aspect Ratio Pigment; Lower Aspect Ratio Pigment
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Linerboard And Corrugated Containers
To help board retain structural integrity in humid and wet conditions, wax is typically applied with curtain coating or cascading. However, wax creates many recycling and machine runnability problems and can saturate fibers and affect bonding. As little as 1 to 2 percent wax in a pulp furnish can decrease strength by 10 percent. On the paper machine, wax saturated fibers contribute to wet end deposits and felt filling. Wax that remains on sheet fibers entering the hot dryer section is drawn to the sheet surfaces by capillary action and results in “slippery” board that causes problems in printing, converting and warehouse handling. For these reasons papermakers refuse to recycle waxed boxes—forcing supermarkets and others to segregate them for landfill or burning.
Nevertheless, wax treatment is relatively inexpensive and functional. A recyclable barrier coating must be able to provide equivalent functionality at competitive cost. Many of the attempts have focused only on treatment of the liners by coating but were doomed by lack of treatment of the medium. Use of wet strength medium and water-resistant corrugating adhesive will improve performance. However, wet strength medium creates problems in recycling.
Capability to treat medium is limited in the U.S. because very few medium machines have size presses due to the good quality and relatively high strength of North American OCC. There are a few mills that make wet strength medium, but problems in broke handling severely limit this. Since the quality of secondary fiber in Europe is significantly lower, size presses are common on medium machines, and they typically use starch to improve strength.
When making medium for use in recyclable barrier-coated wax replacement boxes, special size press treatments are used. Lignosulfonate size press treatment adds not only strength but also water resistance to medium, and this higher-cost medium can be used with coated liner for applications that match the performance of cascade waxed boxes. Size press treatment with starch cross-linked with glyoxal provides adequate performance for recyclable barrier-coated boxes that replace curtain-coated waxed boxes. Both of these medium treatments are repulpable and recyclable.
Wet end addition of biolatex with the proper combination of other additives can be used to produce wax-alternative medium on paper machines not equipped with a size press.
Another approach is to use an inline saturator to treat the medium in the corrugator. This approach not only requires installation of new equipment but also increases the drying load on the corrugator. The additional water must be evaporated and pass through liners that have been treated to resist water transport. This approach could also increase potential problems with warp and wash-boarding.
SBS Folding Carton Board
A considerable amount of solid bleached sulfate (SBS) paperboard is PE-extrusion coated for application in frozen food boxes and other food packaging applications. The PE-extrusion coating also provides heat seal. There has been very little development in the U.S. to try to replace PE-extrusion coating on SBS. There is considerably more activity in this area in Europe in an attempt to avoid the taxes on composite packaging materials. The board is coated with acrylic latex and high aspect ratio pigment on dedicated off-machine coating lines. Biolatex in combination with high aspect ratio pigment shows promise in these applications.
Multi-Ply Recycled Boxboard
The most important application of barrier coatings on multi-ply recycled board is boxes for powdered detergent. This is a very demanding application that requires a combination of Kit Level 8-12 folded grease resistance to resist surfactants and MVTR to prevent caking. One mill in Brazil is making detergent boxes with backside coating of specialty latex and high aspect ratio pigment that meets the needed criteria and is recyclable. It is being used commercially in Brazil.
| Hyper-Platy WBBC Formulations: Beyond The Fluorocarbon Barrier The increased focus on recycling and sustainable packaging is driving the trend for water-based barrier coatings (WBBC) to replace fluorocarbon and extruded polymer barrier coatings. Hyper-platy kaolins, incorporated with appropriate polymers, can provide effective barrier performance against water, gases and grease. The use of these hyper-platy kaolins improve the barrier performance above and beyond typical latex coatings.
As the platy particles of BARRISURF™ align during the application and drying of the water-based barrier coating, they form a tortuous path, giving a signficantimprovement in barrier to moisture, oil and grease and oxygen. "With extremely platy particles, moisture or grease can't work their way through," says Tony Lyons, director of Research, Imerys Pigments for Paper Business Group. “Our hyper-platy clays deliver valuable physical properties to the coating, as well as overall cost savings and higher environmental performance.” Paper and board treated with a water-based barrier coating can be re-pulped and is therefore recyclable. The cost of disposal to a landfill can be transformed into income from the sale of recyclable paper and board, giving BARRISURF sustainability credentials. Environmental concerns, based upon the use of fluorocarbons, are no longer an issue. |
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