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Ind. Eng. Chem. Res. 2003, 42, 5656-5661
GENERAL RESEARCH Washing of Pollution-Preventing Lithographic Inks from a Roller Drum Chandrakant S. Maji† and Ashok N. Bhaskarwar* Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, 110 016 India
The share of lithography is sizable in the printing industry. Conventional lithographic inks usually contain a volatile organic compound (VOC) as a solvent. Ink rollers and plates are cleaned in place with a solvent, usually a VOC. Emissions of VOCs become a major cause for air pollution. A newly developed water-washable ink system eliminates the use of petroleum-based solvents and avoids emissions of VOCs during printing and cleaning of presses. The presses are cleaned by washing with water at a slightly elevated pH. Washing kinetics of this ecofriendly ink on a prototype roller drum are discussed. An empirical correlation between the mass-transfer coefficient and the process variables is sought based on experimental data on washing of the prototype roller drum. This correlation may be useful in deciding the optimum resin concentration in ink, the pH of the wash solution, and the speed of roller drum during cleaning of the presses. The mass-transfer coefficients obtained on the roller drum are very large as compared to those on a revolving watch glass in the low-shear experiments reported earlier (Bhaskarwar, A. N.; Cussler, E. L. Chem. Eng. Sci. 1997, 52 (19), 3227-3231; Nair, S.; Bhaskarwar, A. N. Chem. Eng. Sci. 2000, 55 (10), 1921-1923). Introduction The share of lithography is sizable in the printing industry. This accounts for as much as 47-50% of all conventionally printed materials.1 The lithographic process consists of four major steps: prepress, make-ready, press, and postpress.1 Prepress operations involve several steps in which artwork or design for the printed image is converted into an image carrier, namely, the printing plate. During this step, raw materials such as photoprocessing chemicals and solutions are used. The make-ready step includes the steps taken to prepare the press to print. This involves attaching printing plates to the press, adding ink and fountain solutions to each print unit, testing the press to make sure the image is aligned properly, and printing according to specifications. The press step is the actual printing operations in which inks, cleaning solvents, and substrates are used. Postpress involves any finishing work performed to the printed product. This includes cutting, trimming, binding by gluing or stitching, and final packaging. Conventional lithographic inks usually contain a volatile organic compound (VOC) as a solvent. Cleaning of the press occurs most often during the make-ready step as adjustments are made to the press and plates, during the actual press run, between press runs, and at the end of the day. The frequency of press washes * To whom correspondence should be addressed. Tel.: (09111)-26591028. Fax: (091-11)-26581120. E-mail: ashoknb@ chemical.iitd.ernet.in. † Research scholar, presently working as Manager (Environmental Engineering) for MECON LIMITED, India.
depends on many factors including paper dust and dried ink accumulation, the quality of the paper, and the habits of the particular press operator. Ink rollers and plates are cleaned in place with a solvent, usually a VOC. Residual ink is dissolved in the solvent and scraped off from the roller with a knife blade. The inks, cleaning solvents, and fountain solutions all contain VOCs (xylenes, ketones, alcohols, or aliphatics). The amount of VOCs in inks depends on the lithographic process and the ink type. Cleaners (roller, blanket, and press washes) are petroleum-based with products containing naphtha, mineral spirits, methanol, and toluene. Many cleaners contain almost 100% VOCs. These compounds readily evaporate at room temperature and have become a major cause for air pollution. They often also produce ozone in the lower atmosphere. VOC emissions account for more than 90% of the releases from printing presses.1 The average VOC emission at printing press is 2 tons/ year. In lithography, a majority of the VOCs from the ink stays with the product. With nonheat set inks, approximately 95% VOCs from the ink stays with the product, while in heat set inks, approximately 60% remains with the product.1 It has been estimated that in the USA 130 000 tons of VOCs are emitted into the atmosphere annually via printing inks. The European commission reports that printing presses are responsible for some 187 000 tons of VOCs emission in the EU.2 A newly developed water-washable ink system eliminates the use of petroleum-based solvents and thus avoids emissions of VOCs during printing as well as cleaning of presses.3-6 This new ecofriendly ink does not contain aliphatic solvents, and hence its use produces
10.1021/ie020908t CCC: $25.00 © 2003 American Chemical Society Published on Web 09/26/2003
Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003 5657 Table 1. Properties of Alkyd Resins Synthesized sample no.
composition for alkyd synthesis
acid no. ) 56.1VN/Wa
specific gravity
viscosity, cP
1. 2. 3. 4. 5. 6. 7. 8.
oleic acid + acetic anhydride + glycerol + castor oil castor oil + glycerol + phthalic anhydride castor oil + glycerol + phthalic anhydride castor oil + glycerol + phthalic anhydride castor oil + glycerol + phthalic anhydride oleic acid + glycerol + castor oil oleic acid + acetic anhydride + glycerol + castor oil castor oil + glycerol + phthalic anhydride
48 47 68 46 64 23 58 44
0.95 1.16 1.11
274 5702
1.18 0.98 1.09 1.14
2397 332 159 2856
a
average molecular weight
1307 832 921
V ) volume of the NaOH solution. N ) normality of the NaOH solution ) 0.1 N. W ) weight of the resin (g).
Figure 1. Experimental setup for the study of washability and washing kinetics of alkyd resin based inks.
virtually no emissions during printing. Also, this ink contains vegetable oil based alkyd resins, which are biodegradable, and hence it generates no wastewater threat of a secondary pollution during washing of presses. It acts as a good emulsifying agent when it is brought in contact with an aqueous basic wash solution. The new printing ink contains no volatile solvent, and the presses are cleaned by washing with water at a slightly elevated pH. In this paper, the washing kinetics of the recently developed ecofriendly ink on a prototype roller drum are discussed. An empirical correlation between the masstransfer coefficient and the process variables is sought, based on the data obtained on washing of the prototype roller drum. Experimental Section Synthesis. Materials. Three different types of alkyd resins were synthesized in the laboratory by a fatty acid-oil method for formulating the new low/no-VOCs inks.7 The synthesis of resins included castor oil of LR
grade (G.S. Chemical Testing Lab. and Allied Industries, Bombay/New Delhi, India), glycerol (glycerine) about 98% purified (E. Merck, India Limited), acetic anhydride (E. Merck, India Limited), phthalic anhydride of LR grade (Ranbaxy Laboratories Limited, India), and oleic acid, extra pure (LOBA CHEMIE Private Limited, India). The constituents and physicochemical properties of alkyd resins synthesized are reported in Table 1. Washability Measurements. (a) Materials. For carrying out the washability study, the resins chosen are sample nos. 6-8 of Table 1. Aqueous solutions of sodium hydroxide, prepared using doubly distilled water, have been used as wash solutions. (b) Experimental Setup. The experimental setup for studying the washability and washing kinetics of alkyd resin based inks on the prototype roller drum is shown in Figure 1. The apparatus consists of a roller drum of 11.5 cm diameter and 10.2 cm length made of mild steel, an electrically driven 1/12-hp motor, with an ac supply through a dimmerstat. Constant flow of the aqueous
5658 Ind. Eng. Chem. Res., Vol. 42, No. 22, 2003
Figure 2. Mass-transfer coefficient describing washing of ink from the roller drum combining data of resin sample nos. 6-8 of Table 1 for variable speed.
wash solution is obtained from an aspiratory bottle mounted at the top. (c) Procedure. A weighed quantity of alkyd resin (average ∼ 70 mg) was placed on the surface of the roller drum over a fixed area of 1.5 cm2 (1 cm × 1.5 cm strip). The ink was a mixture of castor oil and resin, without any dye/pigment. The drum was rotated, with the help of a v-belt, with an electrically operated 1/12-hp motor. Different speeds of the drum were obtained using a dimmerstat from Automatic Electric Limited, Mumbai, India. The speed was measured with the help of a phototachometer from Electronic Automation Private Limited, Bangalore, India [type DT 2001 A with a fivedigit, 0.3-in. liquid crystal display and with a range of 5-100 000 rpm (one reflecting mark) and a resolution of 0.1-9999 rpm]. An aqueous wash solution with a known initial pH was allowed to flow through a nozzle (internal diameter of 0.05 cm) continuously with a constant flow rate onto the rotating roller drum coated with the ink until the washing of ink was completed. The wash solution used during the cleaning of the roller drum was collected in a trough kept beneath the drum. The time required for complete washing was recorded with a stopwatch. The volume of the wash solution collected in a trough was measured with a measuring cylinder. The final pH of the solution after completion of washing was recorded with a model LI 120 pH meter from ELICO, India. The variables studied in the experiment are the ink composition (varied from 10% to 100% resin), roller drum speed (varied from 52 to 313 rpm), and pH of the wash solution (varied from 7.18 to 12.70). Results and Discussion The data obtained from the experiments for different speeds of the roller drum, pHs of the wash solution, and resin concentrations are organized in terms of an average mass-transfer coefficient with the help of fol-
lowing equation:
kavg )
Q(C0 - Cf) AC0
(1)
where kavg ) average mass-transfer coefficient (m‚s-1), Q ) flow rate of the wash solution (m3‚s-1), C0 ) initial hydroxide concentration in the wash solution (mol‚m-3), Cf ) final hydroxide concentration in used wash solution after completion of ink washing (mol‚m-3), and A ) area of ink spread on the roller drum (m2). The mass-transfer coefficient calculated through eq 1 is correlated to the speed of the roller drum, the initial hydroxide concentration in the wash solution, and the resin concentration in the ink. Bhaskarwar and Cussler5 suggested a correlation between the mass-transfer coefficient and process variables when resin is the limiting reagent. This limit of excess base is given as
(ka)avg ∝ (ω[OH-]0/µ[resin]0)
(2)
where ω ) stirring speed (s-1), [OH-]0 ) initial hydroxide concentration of the wash solution (mol‚m-3), [resin]0 ) concentration of resin in the ink (mol‚m-3), and µ ) viscosity (cP). Equation 2 is checked experimentally for washing of inks from a roller drum. The log-log graphs of (ka)avg versus ω, (ka)avg versus [OH-]0, and (ka)avg versus µ[resin]0 were plotted by combining the data of resin sample nos. 6-8 of Table 1 for each variable (Figures 2-4). Each of these plots was linear. The log of experimental (ka)avg values obtained through eq 1 for variables ω, [OH-]0, and [resin]0 were plotted against the log of the group (ω[OH-]0/µ[resin]0) to check the validity of eq 2. A linear correlation, inspired by eq 2, with a regression coefficient of 0.3971 resulted upon combining data
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Figure 3. Mass-transfer coefficient describing washing of ink from the roller drum combining data of resin sample nos. 6-8 of Table 1 for variable pH.
Figure 4. Mass-transfer coefficient describing washing of ink from the roller drum combining data of resin sample nos. 6-8 of Table 1 for variable resin concentration.
of roller drum experiments and new data on watch glass experiments (excess base, resin sample nos. 6-8 of Table 1) with error bounds as shown in Figure 5. An empirical correlation, based on powers deduced from Figures 2-4, is compared with all data on washing of inks from the roller drum as well as the watch glass in Figure 6. It shows a better regression coefficient of 0.4691, and error bounds of +50% and -60%. The rather wide scatter of the mass-transfer coefficients, which characterize the washing of inks, with the present three parameters, viz., ω, [OH-]0, and [resin]0, suggests that other parameters may also be
varying. Two probable parameters, which appear to be of particular concern at this stage, are the characteristic length or the resin-film thickness and the diffusion coefficient of hydroxide ions through the soap layer. The resin-film thickness, for instance, may be different in the experiments on the roller drum versus those on the watch glass and could be different between experiments as well despite the precautions taken. Also, the hydroxide diffusivity could vary considerably in the various samples depending on their physical properties. These observations may have an important bearing on the
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Figure 5. Empirical correlation of the mass-transfer coefficient suggested by eq 2 for washing of ink combining data of the roller drum and watch glass experiments (excess base, resin sample nos. 6-8 of Table 1), with error bounds.
Figure 6. Empirical correlation of the mass-transfer coefficient for washing of ink combining data of the roller drum and watch glass experiments (excess base, resin sample nos. 6-8 of Table 1), with error bounds.
future work on these systems, which will certainly need estimates of diffusion coefficients and film thicknesses as well. Conclusions Washing of printing presses contributes to the emissions of VOCs because various surfaces of the presses are coated with the inks containing VOCs. The cleaning solutions also contain aliphatic solvents and mixtures of aliphatic and aromatic solvents. A new pollutionpreventing ink system avoids these emissions completely. Washing of the new ink from the roller drum
seems to obey the following proportionalities:
(ka)avg ∝ ω0.20 (ka)avg ∝ [OH-]00.20 (ka)avg ∝ (µ[resin]0)-0.83 where 0.20, 0.20, and -0.83 are the slopes obtained from Figures 2-4, respectively. These dependencies (weaker than those implied in proportionality (2)) can be combined in the form of a
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correlation for the mass-transfer coefficient characterizing the washing kinetics on the roller drum as
(ka)avg ∝ (ω2[OH-]02/(µ[resin]0)8.3)0.1
(3)
The proportionality (3) thus arrived at is similar to, but weaker than, that in eq 2 given first by Bhaskarwar and Cussler5 for emulsification kinetics of the ink under low-shear conditions where the power was found to be between 0.93 and 1, with the chief difference being that in the characteristic group of process variables the stirring speed is now raised to power 2.0 (instead of 1 of their correlation) signifying the turbulent conditions during washing on the roller drum. The lower magnitude of the exponent in the range between 0.10 and 0.11 for the roller drum suggests a lesser sensitivity to the combination of process variables. The mass-transfer coefficient obtained on the roller drum is, however, very large and ranges between 6 and 297 times that on a revolving watch glass in the low-shear experiments of Bhaskarwar and Cussler.5 These correlations (eqs 2 and 3) for the mass-transfer coefficient may be useful in deciding the optimum resin concentration in ink, the pH of the wash solution, and the speed of the roller drum during cleaning of the presses. Acknowledgment The authors are grateful to the reviewers for their astute and kind comments, which have helped in presenting a clearer perspective and providing a direction for future work on these important new ink systems. Nomenclature a ) surface area of the ink per volume of wash solution, m-1 k ) overall mass-transfer coefficient, m‚s-1
kavg ) average mass-transfer coefficient, m‚s-1 Q ) flow rate of the wash solution, m3‚s-1 C0 ) initial hydroxide concentration in the wash solution, mol‚m-3 Cf ) final hydroxide concentration in used wash solution after completion of ink washing, mol‚m-3 A ) area of ink spread on the roller drum, m2 [OH-]0 ) initial concentration of hydroxide in the wash solution, mol‚m-3 [resin]0 ) initial concentration of resin in ink, mol‚m-3 Greek Letters µ ) soap-layer viscosity, cP ω ) stirring speed, rpm Subscripts 0 ) initial ∞ ) after infinite time avg ) average f ) final
Literature Cited (1) Anderson, C. L.; Epstein, L. N. The Great Printers Project. Pollution Prevention Review, Summer, 1996, p 47. (2) Trenal Ltd. Vegetable oil-based printing inks for newspaper printing. . Contact address: rue R Magritte 165, 7860 Lessines, Belgium. (3) Pennaz, T. J. Ink composition and recovery. U.S. Patents 5,308,390 and 5,338,351, 1994. (4) Pennaz, T. J. Development of a VOC-free lithographic printing system. TAGA Proc. 1994, 324-338. (5) Bhaskarwar, A. N.; Cussler, E. L. Pollution-preventing lithographic inks. Chem. Eng. Sci. 1997, 52 (19), 3227-3231. (6) Nair, S.; Bhaskarwar, A. N. Washing Kinetics of pollutionpreventing lithographic inks. Chem. Eng. Sci. 2000, 55 (10), 19211923. (7) Kirk, R. E.; Othmer, D. F. Alkyd resin. Encyclopedia of Chemical Technology, 2nd ed.; John Wiley & Sons: New York, 1963; Vol. 1, pp 851-882.
Received for review November 12, 2002 Revised manuscript received August 4, 2003 Accepted August 4, 2003 IE020908T
