In: Rowell, Roger M.; Laufenberg, Theodore L.; Rowell, Judith K., eds. Materials interactions relevant to recycling of wood-baaed materials: Proceedings of Materials Research Society symposium;. 1992 April 27–29; San Francico, CA. Pittsburg, PA: Materials Research Society; 1992:249-256. Vol. 266. AND SPECK DISPERSION AND REMOVAL

MAHENDRA R. DOSHI* AND JOHN H. KLUNGNESS** ● Marathon Enginers/Architects/Planners, Inc., Doshi and Associates Inc., Appleton, WI **USDA Forest Service, Forest Products Laboratory, One Gifford Pinchot Dr., Madison, WI 53705

ABSTRACT

Advances in technology have posted serious problems for the deinking industry. When ink is fused, as in laser printing end the photocopy process, when water resistant binders are included in coatings or ink formulations, end when varnishes are applied to printed , conventional methods fail to disperse and specks. The objective of this article is to review different approaches available for removing these inks and specks. Copying processes, coatings, and varnishing processes are discussed prior to discussing deinking and speck removal processes. Deinking ideally involves intense mechanical, chemical, and thermal action needed to detach non-dispersible inks from fibers. The detached ink is then removed by screens, cleaners, washing, end the flotation process. If higher brightness and cleanliness are desired, high-consistency dispersion, together with bleaching or the use of agglomerating chemicals (for photocopy-laser toners), may have to be considered. For coating specks, increasing the PH, increased pulping time, and removal by cleaners are typically used to reduce the number end size of specks. Varnish specks can be more difficult to remove than coating specks, as they are resistant to chemical treatment and are not easily removed by centrifugal cleaners. Flotation removal holds promise for removing varnish specks if particle size is controlled.

INTRODUCTION

Recycling is challenging due to the ever-changing nature of contaminants associated with various grades of wastepaper. One class of troublesome contaminants, called “,” affects machine operation end product quality. The other class of concern end the one we will address in this review is inks end specks. In the early days of recycling, commonly used oil-based or solvent-based inks could be emulsified and dispersed by pulping wastepaper at high temperature using alkali, a , and a dispersant [1]. However, advances in printing technology have posed serious problems for the deinking industry. When ink is fused, as in laser printing and the photocopy process, conventional methods fail to disperse it. One purpose of this paper is to review different approaches available for removing these types of ink. The other purpose of this paper is to review methods for removing coating and varnish specks. Coating specks result from binding between the pigment 250

and the coating material in printed coated papers. Varnish specks are mainly from UV cured varnishes on magazine covers and lead to specks of large size.

OBJECTIVE

The objective of this review is to understand the characteristics of non-dispersible ink particles and summarize the available approaches to detach ink from fibers. We will begin with an introduction to the copying or printing process which fuses ink to the fiber. This will serve as a background for the deinking literature reviewed subsequently. The same approach will also be used to review the removal of specks.

DEINKING

We will briefly review some of the recent and relevant articles on laser printing, photocopying, and associated deinking methods. The review is not meant to be comprehensive but will, rather, provide necessary background information. Seldin [2] and Quick and Hodgson [3] present an excellent introduction to the photocopying and laser printing processes. According to Seldin, the image creation step end the final copy from photocopying end laser printing are identical and so are the corresponding deinking problems. We will first discuss the copying process itself before reviewing some of the deinking approaches.

The Copying Process

The copying process involves a photoconductive cylinder which is charged by a high-voltage corona discharge [2]. An important property of the cylinder surface is that it will maintain its charge in the dark, while exposure to light will dissipate the charge. Thus, when light from the unprinted areas of the original document falls on the cylinder, a charged image is formed which then attracts oppositely charged “dry ink” or “toner” particles. Plain paper enters the copier. a charge is applied on the back of the paper, again by corona discharge, transferring the toner image from the cylinder to the paper. The last step involves permanent attachment of the toner particles to the paper. This is achieved by applying heat and also pressure in high-speed machines. A laser beam serves as a source of light in a laser printer. A computer’s digital data are used to manipulate the laser beam and generate an image on the photo conductive cylinder [3]. The remaining steps of forming a visible image on the cylinder, transferring it onto paper, end fixing it by fusion are identical to those used in the photocopying process. There are other processes that utilize a dielectric system or “liquid ink” [2,3]. However, they are not widely used at this time end we will not consider them further. 251

Constituents and properties of toner particles are of interest from the point of view of deinking. An important ingredient of toner is a thermoplastic composed on styrene end butyl acrylate polymer [2]. Other are used for toner compounding as well. Other ingredients, listed in Table 1, are pigments and additives to control the charge. Toner particles are not water soluble but will dissolve in many organic solvents. During the copying process, when heat is applied, the paper temperature is about 300° F, melting the toner particles and fusing them together onto paper. In the case of high-speed machines, temperatures can reach up to 400°F at 50 to 100 psi during a contact time of about 20-30 milliseconds. Silicone oil may be applied for ease of releasing paper from the hot surface. Photocopies from high-speed machines may contain 5 to 8 microliters of silicone oil [2]. Some of the important physical properties of toner particles are summarized in Table 2. Individualized toner particles are 8 to 15 microns in diameter. The melting point and softening point (glass transition temperature) of toner particles vary considerably among copy machines. The surface tension of toner particles is relatively low (compared to water) and, therefore, in theory, one should be able to remove them by flotation. Above the softening temperature, the toner should “dissolve” or fuse with other material having similar volubility parameters. How the surface tension and the volubility parameters of the toner particles changes after heat fusion during the copying process is not clear.

Deinking

One of the first patents on the deinking of photocopy paper was issued to C.J. Green of Xerox Corporation [4]. Green proposed the addition of an immiscible organic solvent to the flotation cell containing the slurry. With mild agitation, toner particles accumulate in the thin organic solvent layer, which can then be skimmed off. Flotation temperature was maintained between 150° to 250° F. Typical solvents included benzene, toluene, cyclohexanone. tetrachloroethylene, and methyl isobutylketone. Green emphasized the importance of mild agitation - without vortexing. Intense agitation with vortexing can redistribute the toner particles and lower the efficiency of ink removal. Pulping can be done at temperatures between 70° and 212° F, with or without dispersing chemicals. Pfalzer [5] conducted flotation deinking experiments for photocopy and carbonless papers. He recommended pulping ground-wood-free photocopy papers with 2% alkali, 1.5,% surfactant (), and 0.2% dispersing agent at about 70° F. After pulping, chemicals were allowed to react for 60 min. followed by the removal of ink by flotation. In contrast to low temperature pulping suggested by Pfalzer [5], he end his coworkers, in a subsequent German patent [6], recommended briefly heating end defibering the stock above 185°F prior to flotation. In 1985, a patent was granted to Shell Oil Co. for a deinking agent for photocopy papers [7]. The deinking agent was a mixture of one or more 8-16 carbon alkanols and a mixture of alcohol ethoxylates to be added in the amount of 0.2,% to 2.0% by weight on fibers. Pulping was carried out under an alkaline PH condition at 105° to 212° F. According to the authors, ink could then be removed by either washing or flotation. Quick and Hodgson [3] studied the fundamentals of photocopy paper deinking. After discovering that conventional deinking methods did not work, they looked at a number of alternatives. Three processes appeared satisfactory and are sumarized below: (a) Washing/Flotation: In this process, ink was dispersed in a hot pulper loop at 10% consistency and 160° F with 2.5% alkali, 1% silicate, and 1% dispersant, followed by removal of the ink by washing end flotation. (b) Two-Stage Flotation: Photocopy paper was pulped at 10% consistency and 100° F, with 5% alkali and 0.25% nonionic soap. The ink was removed by flotation in two stages. A one-hour soaking period was allowed between the two stages. (c) Adsorption: In this patented process [8], photocopy paper was pulped at 5.5% consistency and 120° F with 2% alkali. Pulp was then soaked with 1.5% c-18 primary alcohol and 1% polystyrene foam prills at 1600 F. The alcohol acted as an agglomerating agent and the foam prills acted as a collector of ink particles. The prills were then removed by a slotted screen. All three methods appear to be effective in removing photocopy ink, as shown in Table 3. Two stage processes (a and b above) are quite efficient but the shrink loss is also higher. The patented adsorption process is not as efficient in removing ink but has a much lower shrink. A translation of Makita’s article [9] was not available and the method given in en English abstract was not clear. It states “Ubix Printing wastepaper could be treated completely with a mixed solution of 0.3% SnowWhite, 0.33% xylene, and 1.7% NaOH. Photocopy wastepaper could be treated with 1% NaOH at 40° C.” We were not sure if Ubix stands for UV ink and whether the method for photocopy wastepaper is just 1% alkali or whether it also includes xylene and Snow White. Researchers at PPG Industries Inc. developed a compound that can be used in en agglomerating process for photocopy ink [10,11,12]. Agglomerated ink can then be removed by slotted screens end forward hydrocyclones, eliminating the need for a conventional washing or flotation operation to remove ink. Recently, Betz PaperChem also developed a compound for agglomerating these inks [13,14]. LaPointe, et al. [15] studied the effect of surfactant double bonds on the deinking of photocopy papers. They considered three 18-carbon-chain :

l Stearic acid (zero double bond)

l Oleic acid (one double bond) * Linoleic acid (two double bonds)

Their results showed that oleic acid with one double bond is a better flotation deinking aid than the other two surfactants.

Summary

Several investigators have developed methods for deinking photocopy and laser-printed papers. Those proposed by Green [4] or the adsorption method of Quick [8] are not readily adaptable to a mill situation. Specific chemicals [7,9,10,11,12.14], together with the use of conventional equipment, can be 253

useful in mill applications. All investigators except Quick [8] and those at PPG and Betz [10,11,12,14] recommend the use of flotation and/or washing for the removal of ink. PPG and Betz propose an agglomerating process so that ink can be removed with screening and cleaning equipment only.

SUGGESTED DEINKING APPROACHES

For deinking non-dispersible inks, one should start with a high-consistency (14-18%) batch pulper with a helical rotor, followed by screening, cleaning, flotation, and washing (not necessarily in that order). Intense shearing action in the helical rotor pulper, together with the effect of thermal energy and chemicals, will detach most inks from fibers. Some industry sources say that low consistency pulping will also detach ink particles from fiber, but that separating the ink from the pulp slurry is the challenge. That is, both low and high consistency pulping have enough shear to cause particle detachment, but neither has adequate shear to effect the detached ink particle size distribution. The particle size of inks from laser-printed and photocopied papers is usually greater than 25 microns. These particles cannot be washed out effectively end must be removed from the stock by flotation. Some agglomerated ink particles can also be removed by forward hydrocyclones. For certain products, like tissue, towels, and napkins, the next steps are thickening end bleaching. For products requiring greater cleanliness, the above steps should be followed by thickening, dispersion, flotation, washing, and bleaching stages, An important step here is high consistency (25-30%) dispersion at 180°to 200°F to detach ink from the fibers [16-21]. Bleaching chemicals may be added either prior to dispersion or after that step has been completed. Another approach is to use specialized chemicals in the pulper to agglomerate fused ink particles which can subsequently be removed by slotted screens end forward cleaners. Long-term data on this approach is not available at this time. In summary, intense mechanical, chemical, and thermal action is necessary to detach and control particle size distribution of nondispersible inks. The detached ink is then removed by screens, cleaners, and the flotation process. If higher brightness and cleanliness are desired, high-consistency dispersion and flotation, together with bleaching may have to be considered. An alternative approach is the use of agglomerating chemicals so that the fused ink particles can be removed with screening and cleaning equipment.

SPECK REMOVAL

Galland, et al [22] present an excellent introduction to the origin of specks. According to Galland, specks in deinked pulp originate from printed coated papers, and from varnished wastepapers. Another source of specks not discussed by Galland et al is UV cured ink. 254

Printed coated papers

Specks from printed coated papers appear when large coating particles are covered with ink from the printing process. Two factors contribute to the creation of the specks: water resistance of the coating end water resistance of the ink and its adhesion to the coating. A number of factors can contribute to the water resistance of ink and coatings. These include: type of ink, coating formulations (including type of ), and age of paper after coating.

Varnished papers

Varnishes are used for two reasons: to protect the printed surface and to add gloss to the product. Varnishes can be applied directly in the printing process, or by a varnishing machine. In offset printing varnish can be applied as a fifth color and there are two options: wet offset or dry offset. Wet offset varnish is applied only on the printed area with about 1.5 to 2-g/m2 varnish applied. For dry offset varnish the entire surface is covered with a resulting addition of about 3 to 5 g/m2 varnish. Offset varnishes consist of either a conventional varnish made of phenol formaldehyde resin or UV cured epoxy-acrylate based adhesive. The UV exposure polymerizes the adhesive. Another method for applying varnish during the printing process is by using the moistening pan. Varnishes applied by this technique tend to be q ore glossy end the deposit is about 7 g/m2. These varnishes give stiffness to the sheet and are water based products. Nitrocellulose varnishes are used for post cards and rotogravure printed packaging protection. Water-based varnishes are used in packaging especially food grade packaging. Two-component (polyurethane) varnishes are used for non-slip properties and are often used with flexo printed packaging or gloss on rotogravure printed papers. Varnishes cured with UV are similar in composition to the UV varnishes used in offset applications (described above), but the coat weight is usually higher. The use of this material is increasing. These varnishes are used on magazines, advertising print, and postcards. These varnishes contain a silicone based oil which makes printing application easier.

UV Ink Printing

A major ingredient of UV-ink is vinyl or acrylate monomer which serves es a vehicle for the pigment (Table 4). On exposure to UV radiation, free radical initiated polymerization results in medium to high molecular weight polymer film [23,24], in a fraction of a second. The ink film produced is resistant to attack by alkali, acids, and alcohols. The ink film is physically strong and resists abrasion. UV-cured ink printing is fast and provides a smooth printed surface. It does not cause odor or pollution problems. All these features make UV-cured ink printing an attractive choice for many applications. 255

Speck removal deinking

Galland has reported [22] experiments comparing coating speck to varnish speck removal. Specks from coatings have densities similar to coating particle density (about 1.6 g/cm3 ). Specks from varnish coating are much less dense. Varnish density is 1.1 before curing, after polymerization the density is 1.25. Laboratory measurements show a density of about 1.35. Experiments using cleaners have not shown good results in removing UV varnishes by cleaning. Varnish specks tend to be thin as opposed to coating specks which contain coating and ink. Varnishes are friable and can be broken up as opposed to the flexible coated specks according to Galland. Water resistant coating specks can be broken down by chemical treatments whereas varnish specks are resistant to chemical treatment. Usually coating specks are broken down merely by raising the PH in the pulper. Particle size is reduced for both varnish and coating specks by deflakers, but is not used primarily because of the reduction in size for stickies which makes stickie removal more difficult. Cleaners seem to be q ore effective for coating speck removal than for removing specks derived from varnishes. Flotation can remove varnish specks effectively if particle size range is controlled. This correlates with measured decreases in surface energy of varnish coated surfaces, especially those with additives used for increased spreading ease. A combination of forward centrifugal cleaners and flotation prefered by McCool and Silveri [25]. Information on deinking of UV-ink papers is limited. Vanderhoff [26,27] performed some preliminary experiments end proposed that UV-ink papers can be deinked by the use of alkali, peroxide, and flotation. The effect of agglomeration chemicals is not clear. According to Forester [28], UV-ink particles are strongly negatively charged over the pH range 6 to 10. He observed that for his specific floation experimental conditions, best results were obtained at PH 6 and temperatures between 105 end 115°F. Ultrasonic deinking [29,30,31] seems to be succesful on a laboratory scale, but has not been proven on a pilot scale.

Summary

Galland et al [22] have concluded that the two main causes for specks in deinked pulps are derived from water resistant paper coatings and UV cured varnishes. An increase in pulping pH together with a slight mechanical treatment is usually sufficient to break-up specks from water resistant coatings. Recently, W cured varnish specks have been appearing in deinked Pulps. The use of these varnishes are expending. UV cured varnish specks are more difficult to remove that coating specks. W varnish specks are resistant to chemical treatment, and centrifugal cleaning is not highly effective. However, flotation deinking is more effective for UV cured varnish specks then for water resistant coating specks. 256

LITERATURE CITED

Printed on Recycled Paper