Surfactant-Based Separations - American Chemical Society

Chapter 23 D e i n k i n g o f Papers P r i n t e d with W a t e r - B a s e d Inks: A n Overview John K. Borchardt

Downloaded by EMORY UNIV on June 10, 2014 | http://pubs.acs.org Publication Date: November 16, 1999 | doi: 10.1021/bk-2000-0740.ch023

Shell Chemical Company, P.O. Box 1380, Houston, T X 77251-1380

Deinking paper printed with water-based inks results in several operational problems for deinking mills: • low pulp brightness due to the very small size of dispersed ink particles. • redeposition of ink onto cellulose fibers being greater than for offset ink particles that are larger. Redeposition can occur in all the mill unit operations but is probably greatest in the pulper when the concentration of dispersed ink particles and cellulose fibers is greatest. • dispersed flexographic ink particles are poorly sized for good flotation deinking efficiency. The optimum size range for best flotation efficiency may vary somewhat from one type of flotation cell to another. However, the optimum ink particle range for flotation deinking is considered to be about 10 microns up to about 100 microns. The absence of an oil vehicle makes agglomeration of the carbon black particles by fatty acid flotation agents more difficult than offset and letterpress ink agglomeration. • many dispersed flexographic ink particles are too small to be optimally sized for wash deinking. Washing is effective for particles in the 3-25 micron size range with the highest efficiency obtained for 5-15 micron particles. • dispersed flexographic ink particles are more difficult to remove from process water by dissolved air flotation and other conventional water clarification methods than are larger ink particles. As a result, more suspended small ink particles are present in recycled process water and are well sized for redeposition onto cellulose fibers.

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© 2000 American Chemical Society

In Surfactant-Based Separations; Scamehorn, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

385 There are three critical steps required for efficient deinking:


1. detachment of inkfromcellulose fibers and ink dispersal in the process water 2. separation of dispersed ink particlesfromcellulose fibers 3. removal of dispersed inkfromprocess water for water reuse The objective of this paper is to review results obtained when deinking paper printed with water-based inks, understand the factors limiting effectiveness of each of the above three processes, review methods proposed to overcome these limitations, and suggest directions for future research. Many articles have been published on flexographic newsprint deinking. In addition, reports in the detergency literature describe surface chemistry results that are relevant to the problem of flexographic newsprint deinking. A critical review and summary of this literature will aid in defining the causes of the difficulty in deinking flexographic newsprint. This improved understanding will aid in the development of improved chemicals, process equipment, and process designs for deinkingflexographicnewsprint. The major driving force in the use of water-based inks is the reduction in the emission of volatile organic carbon compounds associated with many conventional solvent-based inks. Printing is crisp and pleasing to the eye. The print does not rub off on readers' fingers. Bothflexographicinks used for printing newspapers and water-based gravure inks are commercially available and their deinking behavior appears similar (1). Deinking mill personnel are less pleased with this furnish. The difficulty in deinkingflexographicinksfromnewsprint compared to removal of letterpress and offset inks is a major concern in many mills deinking newsprint (2-11). The presence of as little as 10%flexographicnewsprint ir\ m old newsprint/old magazine furnish can substantially reduce theflotationdeinking step brightness gain (Figure 1) (12). The difficulties in deinking paper printed with water-based inks are related to the ink surface chemistry that is very hydrophilic. As a result of this hydrophilicity, water-based inks disperse into very small particles on pulping. This concern overflexographicink is coming even more serious as mills close their water loops. Without efficient water clarification that removes suspended ink, the ink load on the system increases reducing pulp brightness (13-15). This is a particularly serious problem with flexographic newsprint due to the very small ink particles formed on pulping. These suspended ink particles are particularly difficult to remove during clarification of mill process water. Being able to economically process furnishes containing flexographic inks would provide mill with a competitive advantage because being able to process furnishes containingflexographicnewsprint can significantly reduce furnish costs. Old newspapers collectedfromthe U.S. Pacific Coast and New



386

% FLEXO

Figure 1. Effect of Flexographic Ink Content of Furnish on Deinked Sheet Brightness


387 England areas in particular can contain substantial levels of flexographic newsprint. Much newsprintfromthese areas is exported to Canada for deinking. Reasons for the greater difficulty in separating dispersed flexographic inks (compared to letterpress and offset inks)fromfiber differ in part for wash and flotation deinking. Understanding these reasons is critical to developing effective deinking processes and chemicals for flexographic newsprint furnishes. Separation of the dispersed ink particlesfromprocess water for water recycling is also important to efficient deinking mill operation.


Flexographic Ink Chemistry Morphology of offset and flexographic ink printing appears very similar in environmental scanning microscope studies (16). Representative micrographs of paper printed with these two types of inks are shown in Figures 2 and 3. So it is most unlikely that ink morphology differences between offset and flexographic inks can account for the greater difficulty in deinking flexographic furnishes. In contrast, the surface chemistry of flexographic inks differs greatly from that of other newsprint inks. Letterpress and offset inks are oil-based. Flexographic inks are water-based. Simplified black newsprint ink compositions are summarized in Table 1. Table 1. Newsprint Ink Compositions* (17) Ink Type Flexographic

Offset Letterpress

Carrier (Vehicle)

_Additives_

Carbon Black (12-16%)

Acrylic Resin Water (9-12%) (65-77%)

Defoamers Waxes Dispersants

Carbon Black (16-20%)

Alkyd Resin (0-15%)

Pigment

Binder

Mineral Oil (55-80%)

Waxes

Carbon Black Various Resins Mineral Oil (10-15%) (0-15%) (70-90%)

Waxes

a. All percentages are by weight Recent reports describe the composition of water-based inks in some detail (18,19). The carbon black pigment in flexographic inks are grades having a specific surface area of about 100 m^/g and an average particle size of 25 nm (20). (1 nanometer = 1000 micrometers (microns).) Oxidation is often used to increase the polarity offlexographicink carbon black. This increases its hydrophilic character.



388

Figure 2. Environmental Scanning Electron Microscope Image of Flexographic Ink Character on Newsprint

Figure 3. Environmental Scanning Electron Microscope Image of Offset Character on Newsprint In Surfactant-Based Separations; Scamehorn, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

389


The water vehicle offlexographicinks is readily dispersible in aqueous pulping media. The basic pH used in newsprint pulping can convert the acrylate groups in the acrylic binder resins to anionic carboxylic acid groups. Charge repulsion and polarity effects make the binder resin readily dispersible and/or soluble in alkaline pulping media. As a result of these factors, detachment of flexographic inksfromcellulose fibers is not particularly difficult. So difficulty in detachingflexographicinkfromcellulose fibers is not responsible for the difficulty in deinkingflexographicnewsprint. Water-based inks are most strongly dispersed at a pH of approximately 8 (21), a value commonly encountered in deinking and water clarification. This dispersion plus steric hindrance to the approach of collectors makes ink agglomeration difficult (4). This steric hindrance is due to binder resin adsorbed onto carbon black particles. Ink Particle Size After Pulping Factors contributing to the formation of very small ink particles when pulping flexographic newsprint include: • electrostatic repulsion of ionized acrylic acid binder resin carboxylate groups during alkaline pulping promotes formation of small dispersed ink particles (4). • efficient contact between the carbon black surface and the alkaline medium promotes ionization of carbon black carboxylate groups. This ionization creates electrostatic repulsion between carbon black particles thus promoting carbon black dispersion (22). Pulpingflexographicnewsprint results in the formation of many more small ink particles than obtained when pulping offset newsprint (Figure 4). The number offlexographicnewsprint ink particles after pulping was increasing rapidly as the lower detection limit of the image analysis system (about 1 micron in the longest ink particle dimension) was approached (23, 24). One report indicates that, after pulping, 90% of theflexographicink particles are smaller than 5 microns in diameter (25). Using a different type of particle size analyzer than used in deinked handsheet analysis indicated the average particle size of dispersed flexographic inks is about 200 nanometers (nm). This is about three times greater than the size of individual carbon black particles (21). (1 micron = 1,000 nanometers.) Scanning probe microscopy (SPM) indicated ink particles as small as individual carbon black particles are formed upon pulping newspapers printed with flexographic ink (16). Thus oil-based ink droplets dwarf the particles of a flexographic ink dispersion. The specific area of a 10 micron oil-based ink droplet can be as much as 300-times greater that that of the smallest flexographic ink particle, a single carbon black particle (20). Figure 5 shows an SPM image of a small portion (0.3 microns in width) of a character on newsprint printed withflexographicink (16). The individual carbon black particles contained in the ink are clearly seen as being less than 100 nm in size. Samples of a 75:25 offset:flexographic newsprint after In Surfactant-Based Separations; Scamehorn, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


390

Figure 4. Ink Particle Size Distributions for Offset and Flexographic Newsprint Ink After Pulping (based on data taken from reference 23).

Figure 5. Scanning Probe Microscopy of Flexographic Ink Printing on Newsprint


391


pilot mill pulping andflotation- wash deinking were also analyzed by SPM. Figure 6 depicts residual ink particles less than 50 nanometers in diameter. Some of the particles are approximately 20 nanometers in diameter. Schematic representations of the data may be more indicative. The ink particles appear as small hemispherical objects on the sample surface depicted in Figure 7. The size of these discrete particles indicates that they are probably individual carbon black particles that adsorbed on paper fibers after being dispersed during pulping. The deinking agent can have an effect on ink particle size obtained in pulping. In laboratory pulping of aflexographicink printed furnish, a lower hydrophilic:lipophilic balance (HLB) alcohol ethoxylate (HLB 11.8) provided a brightness of 44.5 while a more dispersing alcohol ethoxylate with a higher HLB (HLB 14.3) provided a lower brightness, 39 (26). When higher shear pulping conditions were used, the brightness provided by the lower HLB surfactant decreased to 39.5 while that provided by the higher HLB surfactant was essentially unchanged. Thus the brightness difference between surfactants was substantially less under conditions promoting greater ink particle dispersion. Unsurprisingly, pulping conditions, either chemical or mechanical, promoting greater ink particle higher dispersion resulted in lower pulp brightness. Extended pulping time can both reduce ink particle size (promoting increased particle redeposition (see below) and allow more time for redeposition processes to occur (26). Detergency studies have indicated that carbon black deposition onto cotton fabric increases linearly with time (27). Pulping at an acidic pH of 5 provided a brightness value 5 points higher than pulping under the same conditions at pH 9. Afterflotation,the brightness differential decreased to 1.6 points but increased to 4.6 points after subsequent washing. Redeposition of Ink Particles on Fiber Carbon black particle redeposition increases with decreasing particle size (27). The very small size offlexographicink particles (20-to-50 nm as confirmed by the scanning probe microscopy studies) make them quite susceptible to redeposition onto fibers. By interacting with the binder resins, water hardness ions reduce the electrostatic repulsion between the suspended particles and the cellulose fibers thus increasing ink particle redeposition (28). A study has been performed to directly study deposition of dispersed flexographic ink particles onto cellulosefibers(26). Theflexographicink was printed on Mylar® plastic film using a Dow Lab Coater with a rod. The thickness of the ink on the Mylarfilmwas about one micron, the approximate thickness offlexographicink on paper. After printing and aging, the ink was detached from the plastic and pulverized to form fine particles. Two flexographic inks were used but only the chemical composition of only one was available (Table 2).



392

Figure 6. Scanning Probe Microscopy Image of Residual Flexographic Ink Particles After Deinking



Figure 7. Line Drawing of Figure 6


394 Table 2. Composition Flexographic Ink 1 (26). Ingredient Percent by Weight carbon black styrene-acrylic acid resin alcohol defoamer sodium hydroxide water

18.0


3

5.65 1.0 0.01

Surfactant-Based Separations - American Chemical Society

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