Adhesive Contaminants in Secondary Fibre Utilisation John H. Klungness, Chemical Engineer USDA Forest Service Forest Products Laboratory One Gifford Pinchot Drive Madison, WI 53705-2398 U.S.A. October 1992 Prepared for publication in Proceedings of the Pira Conference Paperboard-the Technology and the Future, Birmingham, West Midlands, UK. 11-12 November 1992 Keywords: Adhesion, adhesives, classification, control systems, hot melts, impurities, lattices, properties, reclaimed fibres, wastepapers. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. ADHESIVE CONTAMINANTS IN SECONDARY FIBRE UTILISATION1 John H. Klungness Chemical Engineer USDA Forest Service Forest Products Laboratory2 One Gifford Pinchot Drive Madison, WI 53705-2398 U.S.A. ABSTRACT A variety of adhesive contaminants (stickies) are encountered in wastepapers. To use wastepaper in paperboard production, stickies must be controlled. The properties and control methods of adhesive contaminants are discussed here. Specifically, control methods include furnish selection, improved pulping and deflaking, welldesigned screening and cleaning systems, and dispersion or additives to detackify or stabilize stickies, or both. The possible application of a new technology for controlling stickies is also discussed. Also, test methods for measuring contaminants in pulps are reviewed. INTRODUCTION To facilitate the use of wastepaper in paperboard production, stickies (sticky contaminants from synthetic adhesives) must be controlled [1]. Large amounts of wastepaper are used in U.S. paperboard manufacture (Table 1), and the amount is projected to increase. The current level of 36 x 106 t of paperboard production per year is projected to reach 40 x 106 t per year in 1995. This is about a 12% increase in production. But this includes an even greater percentage increase in the use of wastepaper from the current 13 x 106 t to 17 x 106 t over the same period, for a 30% increase [2]. Most grades of wastepaper available are used in the production of the various grades of paperboard. 1Portions of this paper are taken directly, by permission, from Progress in Paper Recycling, Vol. No. 1, pp. 54-63, November 1991, and from Materials Interactions Relevant to Recycling Wood- Based Materials, published by Materials Research Society. 2The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. 2 CLASSIFICATION AND PROPERTIES OF STICKIES Wastepaper bales usually contain extraneous materials such as sand, glass, staples, nails, inks, coatings, plastic, Styrofoam, wax, ethylene vinyl acetate (EVA), and styrene butadiene rubber (SBR). Particularly troublesome contaminants in wastepaper are stickies which, in their original state, were used as paper adhesives. Inks and coatings can also be a source of stickies. Four primary components of inks are pigment, vehicle, binder, and modifier. Coating formulations contain several ingredients that can be classified as pigments, binders, and additives, such as plasticizers, thickeners, dispersants, dyes, preservatives, and defoamers. Both inks and coatings contain binders that contribute to the stickies problem. Common binders used in inks are hydrocarbon resins and rosin esters; those used in coatings include starch, soya protein, acrylics, and polyvinyl acetate [3,4]. An overview of stickies was presented by Moreland and Scott [5,6]. 3 Stickies can generally be classified into three categories: hot melts, pressure-sensitive adhesives, and lattices. Although wax can be a contaminant, it is not included separately because it is similar to and a main ingredient of most hot melts. Contaminants such as plastics and Styrofoam are also excluded from this discussion because they are not stickies. Hot Melts Hot melts are used in book bindings, case sealings, and moisture barriers. Many boxboards are coated with hot melts to prevent the transport of moisture. They are applied at high temperature and form bonds upon cooling. Three primary components of hot melts are vinyl acetate polymers and copolymers, tackifiers, and waxes. The tackifier improves the wettability of the hot melt, and wax is used as a bulking agent and to adjust the melting point of the hot melt. Tackifiers used in adhesives are summarized in Table 2 [7]. At room temperature, hot melts are solid. They soften at 70°C to 120°C, depending on the amount of waxes and other ingredients. Most hot melts are insoluble in water, acidic, or alkaline solutions but dissolve readily in many organic solvents such as dichloromethane and toluene. The density of hot melts ranges between 900 and 1,000 kg/m3. Pressure-Sensitive Adhesives Pressure-sensitive adhesives (PSA) are primarily used on labels, tapes, and self-sealing envelopes. An important component of PSA is rubber elastomer, such as the widely used SBR or styrene-isoprene-styrene block copolymer. A tackifier improves wettability of the adhesive to the substrate, and inorganic oxides are added as fillers. Like hot melts, most PSA are insoluble in water, mild acids, and alkalies. However, they dissolve readily in many organic solvents. The density of PSA ranges from 900 to 1,100 kg/m3 Lattices Lattices are commonly used in foil lamination, heat-seal, and coating applications. They are also used on labels for varnished surfaces. Like PSA, lattices contain a rubbery component (natural or synthetic rubber latex) and a tackifer. They are in a colloidal suspension, and appropriate additives are used to prevent agglomeration. One peculiar property of lattices is that they become sticky at high temperatures (higher than approximately 90°C). Otherwise, their properties are similar to those of PSA. They are insoluble in aqueous media but dissolve readily in organic solvents. 4 5 PROBLEMS CAUSED BY STICKIES When stickies are present in pulps in unacceptable amounts, they cause problems with both paper machine operation and product quality. They deposit on wires, felts, press rolls, and drying cylinders. They prevent good fibre-to-fibre bonding and increase the risk of web breaks on the paper machine, particularly with newsprint and tissue grades [8,9]. Wax or hot melts can form a thin film on linerboards, giving them a slippery surface. Consequently, when the linerboard is wound, a telescoping roll results. Stickies can greatly influence the quality of a product, Hot melts and wax in the middle layer of a multi-ply boxboard can migrate to the top and bottom surfaces when the board is dried. After the board is wound, adjacent layers adhere to each other. The roll is then shipped to a customer who discovers a hole or defect in the board. Stickies also cause problems in high-speed printing and converting operations. CONTROL OF STICKIES There are five approaches to controlling stickies. These are furnish selection, improved pulping and deflaking operations, screening and cleaning, dispersion, and additives. Some of these approaches were discussed by McKinney [10] and Doshi [11]. Furnish Selection One of the easiest ways to avoid stickies problems is to prevent them from entering the mill. Criteria must be established for acceptable and unacceptable wastepaper [12,13,14]. This information should, in turn, be communicated to wastepaper dealers to maintain the quality of incoming paper. Depending on the nature of the furnish, the final product, and specific problems or customer needs, measuring the concentration of stickies and plastics or clay and the brightness, freeness, groundwood content, or fibre length distribution may be desirable. One or more of these indicators of wastepaper quality may be used, depending on the particular circumstances of the mill. Improved Pulping and Deflaking Once the wastepaper is accepted at the mill, it goes to the pulper where, through proper operating conditions and accessory equipment., it is fibrized without significant disintegration of 6 contaminants. Important parameters of this process include stock consistency, temperature, low and high pulping intensities, and configuration of the pulper. Most modem pulpers are equipped with auxiliary equipment to remove contaminants before they are broken down into small pieces. The auxiliary equipment includes a ragger to remove wire and string, a junker for large contaminants, and a secondary pulper. A stream is bled off the secondary pulper and subjected to mild fibrizing. High-density contaminants accumulate in a chamber with a double-valve arrangement while stock is sent back to the pulper or is screened in a rotary screen. Many older pulpers are being retrofitted with these accessories [15]. Another pulper that is gaining popularity, particularly for newsprint deinking, is the drum pulper [16,17,18]. Because of the absence of a high-speed rotor, the drum pulper does not have any cutting action. As a result, many contaminants, like plastics and book bindings, remain virtually intact and are rejected by the associated rotary screen. Recently, steam explosion pulping has been proposed for defibring wastepapers [19,20]. In this pulper, loose wastepapers are subjected to steam at about 2,800 Wa and 200°C. When the pressure is released, the material explodes and defibres. Contaminants, stickies, and inks are also dispersed in this pulper. Long-term performance of the pulp on the paper machine remains to be evaluated. Screening and Cleaning In the screening and cleaning approach, come screens with holes and fine screens with slots are used to remove contaminants, based primarily on their size [21-30]. Holes are generally 1.55 mm wide or larger, although some screens have a 1.38-mm-wide hole. Fine screens have slots with widths ranging from 0.20 to 0.60 mm. As slot size decreases, contaminant removal increases but so does fibre loss [31,32]. Most pressure screens operate with mass reject ratios of 15% to 30%. To minimize fibre loss, second and third screening stages must be utilised.