RECENT DEVELOPMENTS IN THE USE OF FOAMED AQUEOUS I NKS IN ROTOGRAVURE PRINTING EDWARD C. NORMAN FOAMINK COPHPANY, INC.

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1 RECENT DEVELOPMENTS IN THE USE OF FOAMED AQUEOUS I NKS IN ROTOGRAVURE PRINTING EDWARD C. NORMAN FOAMINK COPHPANY, INC. L I o N v I I- 1- E, P E N N s Y LV A N I A The Asroclalion Dedlcaied to Air Pollullon Conirol and Hazardour Waste Management For Presentation at the 80th Annual Meeting of APCA New York, New York June 21-20,1987

2 The rotogravure printing process consists of rotating an engraved cylinder in a bath of ink, removing ink from the unengraved portions of the cylinder with a doctor blade, and then contacting a moving web with the cylinder in such a manner as to transfer the ink contained in the engravings to the web. The web then passes through a dryer. The arrangement of a typical rotogravure press unit is shown in Figure 1. The cylinders used are engraved either mechanically or by chemical etching. In both cases, tiny discrete pits called "cells" are created in the cylinder. Each cell prints a dot. Typically, there are 20-30,000 cells per square inch of engraved surf ace. The advantages of the rotogravure process include its capability for good quality four color process work and its ability to be operated at high web speeds. Typical web speeds utilized in publication gravure are around 2000 fpm. End products of rotogravure printing fall into three general classes, listed below: 1. Publication - Free standing advertisement supplements and Sunday magazines for newspapers, catalogs, etc. 2. Packaging - Beer six packs, cartons, foil and plastic packaging materials 3. Product - Decorative substrates for vinyl and melamine laminates, gift wrap, postage stamps Rotogravure printing is an important source of VOC emissions, as the inks conventionally used are thinned with mixtures of aliphatic and aromatic hydrocarbons, or in some cases, pure toluene. While most plants are equipped with solvent recovery systems, the efficiency of these units is at best, about 90%. Older units may recover as little as 65% of the solvent. For economic and transportation reasons, most rotogravure printing operations are located in VOC non-attainment areas. As a practical matter, this makes it impossible to expand these operations. Also, it is not possible to build new plants in non-attainment areas, which,are, for the reasons cited above, the most desirable locations. Aqueous inks have been evaluated extensively by the rotogravure industry and have found some use in packaging and product gravure, where slow press speeds and heavy printing substrates have made it possible to overcome the slow drying and 2

3 web deformation characteristics of aqueous inks. However, there are other disadvantages which have proved difficult to overcome. These are: 1. VOC content of aqueous inks - Most aqueous ink systems contain 10-20% of an alcohol, usually n-propanol, to aid in drying and improve print quality. 2. Clean up problems - Most aqueous systems are based on emulsion polymers, which are either not resoluble or are resoluble only with difficulty. 3. Printability - The rheological characteristics of aqueous inks are such that print quality is not as good as that obtained with solvent systems. "Worminess" and "snow-f laking" are the problems most commonly encountered. For many years, foamed aqueous inks have been used n the printing of textiles by a process similar to rotogravurei. The objective here is to reduce the energy required for drying by avoiding saturation of the web and by, in effect, diluting the ink with air rather than water. These inks are formulated to produce a foam with a very fine bubble structure and uniform microscopic bubbles, which actually fill the engravings in the printing cylinder. This works well at the low web speeds at which textile printing is done. However, at the high web speeds encountered in printing on paper, this type of foam encounters many problems. A pressurized fountain is necessary to fill the engravings with bubbles. The foam must be continuously reprocessed to maintain the small bubble size, as the larger bubbles formed by the shearing action of the rotating cylinder tend to cannibalize the the smaller bubbles. The rheology necessary for a liquid to produce a uniform mass of small bubbles is not ideal for high speed printing, as ink transfer and flow-out problems result. Textile printing foams, like liquid aqueous ink systems, contain emulsion polymers. As noted above, these cause clean-up problems. Consequently, this type of ink has never had any commercial success in the rotogravure printing industry. This paper describes the development of foamable aqueous inks suitable for rotogravure printing of paper webs at high speed. EXPERIMENTAL METHODS AND RESULTS The objective of the work was to develop an ink with the following characteristics: 3

4 1. Foamable with simple equipment 2. Foam produced must be sufficiently stable that continuous refoaming is not necessary 3. Print quality equal to or better than solvent inks 4. VOC content less than 1% 5. Capable of being run at standard press speeds without dryer modification 6. Easily resoluble for clean-up 7. Cost equal to or less than solvent inks The formulation of inks, like most formulation research, is largely an Edisonian proposition. While there are certain general parameters that must be observed, within those parameters there is wide latitude in the selection of raw materials. The requirements given above narrow the choice somewhat. Laboratory testing is performed using unsophisticated methods and equipment. Color strength and shade comparisons are made visually after making side-by-side "draw-downs" on a sheet of standard paper using a wide bladed putty knife. Viscosity is measured with Zahn cups or with a Brookfield viscosimeter. Foamability and foam stability ars evaluated using methods developed for fire fighting foams. Printability can be partially evaluated on a laboratory scale rotogravure press. A full evaluation of printability can be made only by running tests on an actual production press. Early in the project, a key discovery was made. This was that a properly formulated foamed ink, rather than filling a rotogravure cell with bubbleg,5would form a film, or lamella, across the mouth of the cell '. See Figure 2. This lamella will then transfer to the paper. Because the lamella is under tension due to the surface tension of the ink, it is very easily transferred as compared to emptying a cell filled with either liquid or small bubbles. Another implication of this discovery was that it is not necessary to have a uniform, small bubble foam, because it is not. necessary for the bubbles to fill the cells. As the engraved printing cylinder rotates in a bath of foamed ink, it becomes coated with the foam which is then doctored off with a steel blade. The action of the foam coated cylinder passing under this doctor blade creates the lamellae. See Figure 3. Our pr in t ing inks which have been successfully used in commercial have the f o 1 low i ng,c h a r a c t e r i s t i cs : Foam Expansion Ratin - Between 5 to 1 and 7 to 1 Foam Stability - At least 6 minutes before the first drop of drainage appears. Rheology (liquid) - Pseudoplastic VOC content - Less than 1% 4

5 5. Resolubility - Easily resoluble in dilute solutions of ammonium hydroxide. Many factors which are not fully understood from a theoretical standpoint come into play in determining the printability of a rotogravure ink. The engraved cylinder deposits the ink as discrete dots. In areas where a solid coverage is desired, the ink must flow out to fill in the space between the dots. Failure to do so results in white voids or "snow flaking". It must not, however, flow out so much that it smears, a condition known as "worminess". Ink rheology, surface tension, wetting power for the substrate and wetting power for the cylinder are all important factors in determining printability. Ingredients which affect these characteristics are binder resins, surfactants and rheology control additives. Complex interactions between these materials were observed. The optimum formulation contains three resins, three surfactants and a rheology modifier. Drying speed has been the most difficult property for aqueous inks of any type to achieve. The high latent heat of vaporization of water and the affinity of water for cellulosic substrates are important factors. Foamed inks of the type which we have developed add much less water to the substrate than is the case with liquid inks. This is because a lamella has much less volume than a cell filled with liquid. Equivalent color strength is achieved by using a somewhat higher pigment strength in the foamable ink and by formulating the foamable ink to give less penetration of the substrate than is the case with liquid aqueous inks. Production runs using foamed aqueous inks have shown that no reduction in press speed or increase in dryer capacity is necessary as compared to solvent type inks. The lower water add-on and less penetration also solved another difficult problem encountered with aqueous inks, that of web deformation. Water swells cellulosic materials, causing web growth, which in turn, causes register problems, curling and waffling. We encountered all of these problems at various times in our work. The key to their solution is the avoidance of ingredients which cause the ink to penetrate the web. This is a bit of a challenge when the system contains anionic resins, which are wetting agents in their own right, and surfactants for foam production and stabilization. A further complication is that there is no good laboratory test to measure web penetration or its undesirable effects. We did a lot of experimentation on a running commercial press. The only volatile components of these inks are watcr (ca. 75%), ammonia (ca. 1%) and lower alcohols (ca. 0.5%). Ammonia emissions are not presently regulated. The lower alcohols, 5

6 which in this case are n-propanol and isopropanol, are present in very small amounts. Because foams behave differently from liquid inks, they create certain mechanical problems which must be addressed in order to achieve commercial operation. These problems and the solutions which we developed are described briefly below: Because foam is much lighter than the liquid from which it is made, and because it is a Bingham plastic, a layer of air forms between the rapidly rotating engraved cylinder and the ink. This causes skipping, as the cylinder is not properly inked. The solution to this problem is to install a flexible plastic blade in contact with the cylinder below the surface of the foam in the fountain. This blade prevents the formation of the air layer. Without the air layer present, the cylinder tends to drag the foam in the direction of its rotation, causing a build-up of foam in the back of the fountain under the doctor blade carrier, which oscillates back and forth during operation. This oscillation pumps the foam out from under the ends of the doctor blade carrier onto the floor. This problem was solved by the installation of flexible seals on the ends of the doctor blade carrier. When using liquid inks, level control in the fountain is achieved by using an inking pan in the fountain which is continuously supplied with ink from a sump. It overflows, and the overflow ink is drained back to the sump. This does not work with foam. Therefore, level controls had to be developed. We have been successful with both ultrasonic and capacitance type level detectors. The ink is supplied as a liquid at press side. The foam generating equipment on the press is controlled by the level detectors to maintain the proper level of foam in the fountain. No inner inking pan is used. If the press has to be shut down for any period of time for web breaks, form changes, mechanical breakdowns, etc., liquid ink will drain out of the foam as it collapses. We have made a provision to collect this drainage from the bottom of the fountain, and recycle it through the foam generator when the press starts up again. A typical rotogravure press producing newspaper advertising supplements consumes approximately 600 pounds of ink per hour, or 14,400 pounds per 24 hour period. If foamed aqueous inks were in use, total emissions per 24 hours would be 144 pounds of 6

7 ammonia and 72 pounds of VOCs. By comparison, if the press were using solvent type ink, which contains approximately 60% volatile components, total VOC emissions would be 1382 pounds per day assuming 85% solvent recovery, which is the New Source Performance Standard for publication rotogravure presses. Many existing recovery systems have considerably lower efficiencies. The cost of utilizing foamed aqueous inks is roughly the same as that of using solvent type inks. While the inks themselves are somewhat more expensive than solvent inks, extensive mileage tests which have been run for periods of over 7 days have shown that the wet add on of foamed inks is approximately 30% less than that of solvent inks. If the economic calculations assume that recovered solvent is available at no cost, the ink cost per thousand copies is the same for both inks. If the cost of operating and maintaining the solvent recovery unit is included, there is a 7% cost advantage for the foamed aqueous ink. Other possible cost savings include lower insurance rates, the elimination of the need for explosion proof electrical equipment, etc. CONCLUSIONS It is possible to formulate foamable aqueous inks which give technical and economic performance equal to or better than the solvent type inks currently in use in the rotogravure printing industry. The use of these inks can reduce VOC emissions from rotogravure printing operations by 95%. REFERENCES 1. Kumins et a1 US 2,971, Gaqliardi et a1 US 3,400, NFPA 11 Standard for Low Expansion Foam and Combined Agent Systems Appendix A Section A National Fire Protection Association., Ouincv. - MA Rosner, C.A. US 4;474, Norman, E.C. US 4,482,648 NOTE TO EDITORS Under the new federal copyright law, publication rights io this paper are retained by the author($). 7

8 IMPRESSION ROLL \ TYPICAL ROTOGRAVURE PRESS UNIT r'igure 1. Typical Rotogravure Press Construction a

9 Tiguie 2. uatnellas 9

10 Figure 3. Formation of Lanilas 10