Ink and Speck Dispersion and Removal
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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. INK 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 printing 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 papers, conventional methods fail to disperse inks 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 “stickies,” affects paper 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 surfactant, 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 resin composed on styrene end butyl acrylate polymer [2]. Other adhesives 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 pulp 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 (soap), 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.