257

ADHESIVE CONTAMINANTS (STICKIES) AND METHODS FOR REMOVAL

JOHN H. KLUNGNESS* AND MAHENDHA R. DOSHI** *USDA Forest Service, Forest Products Laboratory, Madison, WI 53705 **D~shi & Associates Inc.. Marathon Engineers/Architects/Planners, Inc. Appleton, WI

ABSTRACT

A variety of adhesive contaminants (“stickies”) are encountered in wastepapers. They are broadly classified as hot melts, pressure-sensitive adhesives, and lattices. Their properties and control methods are discussed. Specifically, control methods include furnish selection, improved pulping end deflaking. well-designed screening end cleaning systems, and dispersion end additives to detackify or stabilize stickies, or both. A new technology is also discussed regarding it’s possible application for controlling stickie contaminants. Test methods for measuring stickies are also reviewed.

INTRODUCTION

Large amounts of wastepaper are generated every day in the United States and interest in its reuse has increased steadily due to environmental concerns and improved economics. In 1990, for example, 28.9 million tons of wastepaper were collected for recycling. By 1995, that amount is expected to increase to almost 40 million tons with a collection rate of almost 40% [1]. To facilitate the use of secondary fibers, sticky contaminants, or "stickies,” must be controlled [2].

CLASSIFICATION AND PROPERTIES OF STICKIES

Wastepaper bales usually contain extraneous materials such as sand, glass, staples, nails, inks, coatings, plastic, styrofoam, wax, EVA (ethylene vinyl acetate), SBR (styrene butadiene rubber), etc. A particularly troublesome contaminant in wastepaper is “stickies” which, in their original state, were used as paper adhesives. Inks end coatings can also be a source of stickies. Four primary components of inks include pigment, vehicle, binder, and modifier. Coating formulations contain several ingredients which can be classified as pigments, binders, and additives, such as plasticizers, thickeners, dispersants, dyes, preservatives, and defoamers. Note that both inks and coatings mixtures contain binders which contribute to the stickies problem. Common binders used in inks are hydrocarbon resins and rosin esters while those used in coating formulations include starch, soya protein, acrylics, end polyvinyl acetate [3,4]. An overview of Stickies was presented by Morelend and Scott [5,6].

Mat. Res. Soc. Symp. Proc. Vol. 266. © 1992 Materials Research Society 258

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 as it is similar to and an important ingredient of most hot melts. Contaminants such es plastics and styrofoam are also excluded from this discussion since they are not stickies.

Hot Melts

Hot melts are used in many applications such as book bindings and case sealing end as a moisture barrier. 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 end copolymers, tackifiers, and wax. The tackifier improves the nettability of the hot melt while wax is used as a bulking agent end to adjust the melting point of the hot melt. Tackifiera used in adhesives are summarized in Table 1 [7]. At room temperature, hot-melt adhesives are solid. They soften at 150° to 250° F, 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 0.9 end 1.0 g/cc.

Pressure-Sensitive Adhesives

Pressure-sensitive adhesives (PSA’S) are primarily used on labels, tapes, and self-sealing envelopes. An example is “Post-it Notesw which have become popular due to their convenience and ease of application. An important component of PSA is a rubber elastomer such as the widely used styrene butadiene rubber or styrene-isoprene-styrene block copolymer. A tackifier is added to improve wettability of the adhesive to the substrate end inorganic oxides are added as fillers. Like hot melts, most PSA’S are insoluble in water, mild acids, end alkalies. However, they dissolve readily in many organic solvents. Their density can range from O.9 to 1.1 g/cc.

Lattices

Lattices are widely used in foil lamination, heat-seal, and coating applications. They are also used on labels for varnished surfaces. Like PSA’S, lattices contain a rubbery component (a natural or synthetic rubber latex), and a tackifier. They are in the form of a colloidal suspension with appropriate additives used to prevent agglomeration. One peculiar property of lattices is that they become sticky at higher temperature (above approximately 200° F). Otherwise, their Properties - 259

similar to those of PSA’S. They are insoluble in aqueous media but dissolve readily in organic solvents.

PROBLEMS DUE TO STICKIES

When stickies are present in unacceptable amounts, they can cause problems with both paper machine operation end product quality. They deposit on wires, felts, press rolls, and drying cylinders. They prevent good fiber-to-fiber bonding end increase the risk of web breaks on the paper machine, particularly with newsprint end 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. The appearance of a product can be greatly influenced by the presence of stickies. Hot melts end wax present in the middle layer of a multi-ply boxboard can migrate to the top end bottom surface 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 end converting operations.

CONTROL OF STICKIES

There are five approaches used to combat stickies. They include furnish selection, improved pulping and deflaking operations, screening and cleaning, dispersion, and additives. Some of these have been discussed by McKinney [10] and Doshi [11].

Furnish Selection

One of the easiest ways to avoid stickies problems is to prevent their entry into the mill. It is very important that criteria [12,13,14] be established for acceptable end unacceptable wastepaper. This information should, in turn, be communicated to wastepaper dealers to maintain the quality of incoming paper. Depending on the nature of the furnish, final product end specific problems or customer needs, it may be desirable to measure the concentration of stickies, plastics or clay, the brightness, freeness, groundwood content or fiber length distribution. One or more of these indicators of wastepaper quality may be used, depending on a mill’s particular circumstances.

Improved Pulping and Deflaking

Once the wastepaper is accepted, it goes to the pulper where, through proper operating conditions and accessory equipment, it is fiberized without significant disintegration of contaminants. Important parameters include stock consistency, temperature, low vs. high pulping intensity, end configuration of the pulper. 260

Most modern 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 end string, a junker for large contaminants, and a secondary pulper. A stream is bled off at the secondary pulper and is subjected to mild fiberizing. 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 while most newer pulpers come equipped with secondary pulpers [15]. Another pulper which is gaining in popularity, particularly for newsprint deinking, is the drum pulper [16.17,18]. Due to 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 end are rejected by the associated rotary screen. Recently, steam explosion pulping has been proposed for defibering wastepapers [19.20]. In this pulper, loose wastepapers are subjected to steam at about 400 psi end 400° F. When the pressure is released, the materiel explodes and defibers. Contaminants, stickies, end inks are also dispersed in this pulper. Long term performance of the pulp on the paper machine remains to be evaluated.

Screening and Cleaning

Coarse screens with holes and fine screens with slots are used to remove contaminants, based primarily on their size [21-30]. Holes are generally 0.062 in. (1.55-mm or larger), although some screens are used with a O.055 in. (1.38 mm) hole size. Fine screens have slots with widths ranging from 0.008 in. to 0.024 in. (0.20 to 0.60 mm). It is important to remember that, as slot size decreases, contaminant removal increases but so also does fiber loss [31, 32]. Moat pressure screens operate with mess reject ratios of 15-30%. TO minimize fiber loss, second and third screening stages must be utilized. How these stages are arranged is very important to the contaminant removal efficiency of the system. In a conventional cascade system (Fig. 1), re-circulation of contaminants between stages is quite common but this can be detrimental to overall system efficiency. The use of a forward-flow arrangement is recommended when ever possible to avoid this problem [34,34,35,36]. One way of accomplishing a forward flow is shown in Fig. 2. Note that both the primary end secondary stages have coarse end fine screens with openings of identical size. Secondary slotted screen accepts are moved forward. Depending on the nature of the furnish and concentration of contaminants, a scalping screen (a coarse screen with a larger hole size then those found in the primary or secondary stages) is used in the third stage, followed by a coarse screen and fine slotted screen. By moving third-stage, slotted-screen accepts forward, the re-circulation of contaminants has been avoided. An added benefit is the reduced capacity requirements in all three stages by not recycling accepts from the secondary or tertiary stages back to the primary stage. 261

After the screening operation, cleaners are used to remove contaminants based on their density differences. Cleaners are classified as either high, medium, or low density, with their use dependent on the density and size of the contaminants they are removing. As shown in Table 2 [37], a high-density cleaner is used to remove nuts, bolts, paper clips, end staples. These high-density or forward cleaners are usually located immediately after the pulpers. For medium-density contaminants, smaller-diameter hydrocyclones are used. As the hydrocyclones diameter decreases. its efficiency in removing small-size contaminants increases. For practical and economic reasons, the 3-in. (75-mm) diameter cyclone is the smallest cleaner used in the paper industry. Reverse hydrocyclones or throughflow cleaners are used to remove low-density contaminants. A disadvantage of reverse hydrocyclones is that 55% of the flow is constrained to the reject stream. Therefore, secondary and tertiary stages are needed to recover the usable fiber [38]. This problem does not occur in throughflow cleaners. The accepts end rejects come out at the same end in throughflow cleaners. The reject stream is only 10% by volume and 2% by mass[38, 39, 40]. However, the contaminant removal efficiency of reverse hydrocyclones is usually higher than that of the throughflow cleaners. Another problem with throughflow cleaners is that they are somewhat prone to plugging due to the narrow gap at the exit. Rotating body cleaners, introduced recently in North America, are effective for the removal of low-density contaminents [41]. The reject ratio of these cleaners is so low that there is no need for a second or third stage. There are other types of cleaners available. One is the core bleed cleaner where both high- and low-density contaminants are removed in a single unit operation. High-density contaminants are removed through underflow while low-density contaminants are concentrated in the center and removed through the core tube. Accepts are removed through the annular space. These cleaners are discussed further by Moreland [42].

Dispersion

The objective in dispersion is to break up contaminants end inks further so they will be invisible in the final product [43,44,45]. Important parameters to consider in dispersion are consistency, temperature, and pressure. Consistencies of 25-30% are used while temperatures range from 160-180° F at atmospheric pressure. In some instances, higher temperatures are used. An application of dispersion is in breaking up waxes or hot melts in old corrugated container (OCC) stock. Another application is in deinking of waste paper furnishes. With the advances in printing technology, some of the newer inks such se non-contact toner are believed to be bonded to the paper, end these inks appear as specks in the final product. However, dispersion of ink particles into smaller particles does decrease sheet brightness. When this occurs, dispersion should be followed by en ink removal step to improve brightness. 262

Additives

A final strategy to combat stickies involves the use of chemical additives. There are many different types of additives available on the market end they can be classified according to their physical state end chemical nature. Some of these additives include talc, solvents and dispersants, cationic polymers, synthetic fibers, zirconium compounds, and alum sequestering agents.

Talc

Talc consists of superimposed layers of magnesium sheets sandwiched between two silica sheets. These layers are held together by weak van der Waal’s forces, giving talc a soft and slippery feel. One of the important characteristics of talc is its hydrophobic surface end hydrophilic edge. The hydrophobic surface has an affinity for stickies while the hydrophilic edge allows easy dispersion of the talc in water. In order for talc to be effective, it should be es pure es possible because impurities will reduce the affinity of the surface for organic material. Talc should be added to a deink storage chest to allow time for the talc to contact end detackify the stickies. About 0.6 to 1.9% of talc is generally used, based on o.d. weight of the fibers. Talc is not effective on stickies which are riot tacky at the headbox temperature but become tacky at the dryer temperature. It is important that most of the talc be retained in the sheet so es to avoid excessive concentration in the white water.

Solvents end dispersants.

Dispersants are classified as either anionic or nonionic surfactants/polymers [5, 47-50]. Anionic dispersants keep small stickies suspended in a slurry by inducing a negative charge on them which repels other negatively charged stickies. Nonionic dispersants are generally long-chain molecules with one hydrophobic end one hydrophilic and. When a secondary fiber slurry is mixed with a nonionic dispersant, its hydrophobic end will attach to a stickie, leaving the hydrophilic end exposed to water with no affinity for stickies. Thus, agglomeration of stickies is prevented. Dispersants mixed with appropriate solvents can help in the defibering of wet-strength papers. Some dispersants are sensitive to pH,temperature, end the presence of other chemicals end should be selected based on their compatibility with the existing system. Environmental concerns like toxicity, odor, end flammability should also be addressed when selecting these chemicals. 263

Cationic polymers

Stickies and other colloidal particles in the pulp slurry can be adsorbed onto fibers by the use of cationic polymers [48]. These polymers can be added at the thick stock chest, fen pump or at the headbox. The addition of cat ionic polymers can thus avoid build-up of stickles in the white water system. Another application of cationic polymers involves spraying them on paper machine wires to minimize the deposition of stickies [51]. These polymers passify the wire. Some products are also available for spraying on felts.

Synthetic fibers

Synthetic fibers, such as polypropylene fibers, have en affinity for ink end organic contaminants and can be used to scavenge them from a system [7]. They are especially useful with tacky stickies. Like all additives, synthetic fibers should be added well ahead of the headbox. The recommended initial dosage is about 0.1%, based on o.d. fibers, increasing to 0.3% if necessary. Synthetic fibers are effective only at the temperature and PH where stickie tackiness is high. Therefore, like talc, synthetic fibers may not be very effective on stickies which are not tacky at headbox temperatures but become tacky at dryer temperatures. These fibers do not bond well with cellulose fibers and may cause problems during paper machine operation. Because of this, their use has been limited.

Zirconium compounds

Zirconium compounds detackify stickies from hot melts and pressure-sensitive adhesives, with most of them ending up in the final product [52]. Small amounts can also be found in white water due to their adsorption on fines. These compounds are available in liquid form and any excess can accumulate in the white water where they prevent the buildup of stickies due to white water closure. Zirconium compounds are moat effective at high dosages, with 1 kg/ton recommended. Like all additives, they should be added as far upstream from the headbox es possible and should be selected for the pH range of interest. They are not as effective in detackifying stickies when a combination of hot melts and pressure-sensitive adhesives are encountered.

Alum sequestering agents

Alum is used to ensure good retention or uniform sizing. However, an excessive amount can lead to coagulation of stickles in the secondary fiber stock. Also, when the pH changes suddenly, excess alum can precipitate on solid surfaces. Sequestering agents can be used to scavenge excess aluminum ions which could later be available if alum concentration decreased [53]. 264

However, agglomeration of stickies occurs even in the absence of excess alum. Stickies also accumulate and agglomerate in stagnant areas of tanks, pipes, and partly open valves. In those cases, the addition of sequestering agents will be of little help.

NEW TECHNOLOGY

High-shear-field separation under laminar flow conditions has been investigated at the USDA Forest Products Laboratory. The results have been reported in a series of publications [54-58]. Experiments to date have used a disk geometry to demonstrate the separations possible with high-shear-field separation. A disk separator with a 152-mm diameter end disk rotation at around 5000- rpm exhibits good contaminant removal for many contaminants from pulp slurries at up to about 1.0 percent consistency. The researchers involved report that high-shear-field separation is strongly indicated es a process for fiber recovery from reject streams from papermills that recycle wastepaper. Subjecting the reject Pulp slurries to high-shear-field separation se a final process step before discarding the rejects could be beneficial to paper manufacturers in two ways. First, the most obvious benefit for paper manufacturers would be the combined reduction of disposal costs plus the value of the recovered fiber. Ideally the recovered fiber would be sufficiently clean to feed forward in the papermaking system. Failing that, the recovered fiber could be re-introduced at the beginning of the stock preparation process for a complete stock processing again. Another alternative would be for the recoverd fiber to be sold to a nearby mill producing a grade with higher tolerances for contaminants. The second benefit would be in the amount of wastepaper, the quality of wastepaper, or both that an efficient fiber recovery system would permit the paper manufacturer to use. For most paper manufacturers, the coat of fiber is the single highest manufacturing cost. An efficient fiber recovery system would permit significant savings to the paper manufacturer by permitting the manufacturer to purchase lower cost fiber. Throughput capacity for a single 152-mm diameter disk is at 0.25 metric tons per day on a dry pulp basis. Efforts to increase the throughput to commercial scale throughputs of 3 to 5 metric tons (dry basis) using the disk geometry have not been successful.

TEST METHODS

No single satisfactory contaminant-measuring method exists, nor has any one method been widely adopted [59]. Factors that contribute to the complexity of method requirements include the great variations between the many wastepaper grades, which contain heterogeneous and ever-changing contaminants, end the wide range of paper grades made from recycled wastepaper. which vary vastly in their tolerance for contaminants. Also, contaminant-measuring methods are used by different groups (for example, those 265 engaged in manufacturing, technical service, and equipment development), and each group has different masuring needs. Usually stickie test methods are developed for a specific application. A compilation of atickie test methods has recently been published by Forrester [60]. He has compiled 11 methods in this group which cover the range of methods usually needed. People in need of a teat method specifically designed for a given situation can refer to these methods for ideas on how to develop a suitable test method. Ling end co-workers [61], modified a method originally developed by Doshi [11]. The Ling method measures the weight of stickies attracted to microfoam collectors from a pulp slurry. The Ling method has been used for three major types of stickies: hot melts, contact adhesives, and laser inks. Hacker [62], described a method for contrast enhancement of stickies in handsheet samples prior to image analysis. The method used an aqueous ink to dye the pulp fiber to enhance the contrast between stickies end pulp. Hacker documents how the method was used to evaluate a stock preparation system improvement for recycling old corrugated containers.

CONCLUSIONS

Stickies are one of the major problems encountered in recycling wastepaper, many approached can be used for their control. These include both mechanical methods (screening, cleaning, end dispersion) end chemical methods (solvents, synthetic fibers, zirconium compounds. end other additives). By understanding the kinds of stickies that maybe encountered end their properties, their impact on the papermaking process end resulting product can be greatly minimized. In the future, cooperation and communication between representitives of paper mills, equipment suppliers, manufacturers of inks, coatings, adhesives, and chemical suppliers can greatly benefit the advance of recycling technology. The ultimate goal should be to formulate inks, coatings, end adhesives so that they are effective end economical and, at the same time, allow for easy removal of stickies during the recycling process.

LITERATURE CITED

In: Rowell, Roger M.; Laufenberg, Theodore L.; Rowell, Judith K., eds. Materials interactions relevant to recycling of wood-based materials: Proceedings of Materials Research Society symposium; 1992 Apri127-29; San Francico, CA. Pittsburg, PA: Materials Research Society; 1992: 257-267. Vol. 266.

Printed on Recycled Paper