THE EFFECT OF HIGH TEMPERATURE AIR DRYING ON EVAPORATION, RUNNABILITY AND COATED PAPER PROPERTIES. Pertti Heikkilä and Pasi Rajala

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1 Drying 2004 Proceedings of the 14th International Drying Symposium (IDS 2004) São Paulo, Brazil, 22- August 2004, vol. B, pp THE EFFECT OF HIGH TEMPERATURE AIR DRYING ON EVAPORATION, RUNNABILITY AND COATED PAPER PROPERTIES Pertti Heikkilä and Pasi Rajala Metso Paper, Inc., Air Systems, FIN-202, Turku, Finland Keywords: coated paper, impingement drying, backtrap mottle ABSTRACT Influence of effective impingement drying on the quality of blade-coated paper was studied with a 1.31 meter long one-sided impingement dryer that was installed on a pilot coating machine. Impingement temperatures to 550 C and velocities to m/s were used. Backtrap mottle was decreased when the drying power of the impingement dryer was increased. The gloss and smoothness of the coated samples were somewhat better when effective air drying was started immediately after the coating station. The high-low-high drying strategy (low drying rate at the consolidation stage) was beneficial for LWC papers, whereas the high-high-high drying strategy gave the best results for coated WF papers. The same dryer was used also for drying rate and web runnability experiments. The dryer was installed on a test rig consisting of an unwind, a frame for the dryer and a rewind, and means for controlling of web speed, tension etc. In the drying experiments rolls of moist paper were dried with the dryer. Web runnability was tested in the same test rig with rolls of initially dry paper. The objective was to find stable web transport and optimal nozzle-to-web distance at varying process conditions. INTRODUCTION This paper describes experiences of a new single-sided air impingement dryer for drying of coated paper and board. The dryer is aimed for positions where conventional double-sided air dryers are not applicable, for example after the coating station, or other positions where only one side of the web is available for drying. The new dryer offers also an economical alternative to infrared dryers that have been frequently used in places where it has not been possible to install a double-sided air flotation dryer. Air dryers are reliable and have low maintenance costs. The operating costs of an air dryer are formed mainly from the consumed energy. Air dryers usually have lower specific energy consumption than infrared dryers that makes air dryers more economical to operate. Paper and board are coated mainly in order to have an optimal surface for printing purposes. Drying 1295

2 of blade coated paper affects on the quality of the printed surface. Mottle represents uneven ink absorption in offset printing. This phenomenon can be described as low-contrast, low-frequency ( mm) unevenness on the printed surface. Mottle may be resulted, among other things, due to improper drying strategy. Engström et al. (1982) and Engström (1994) noticed that these drying effects are different with starch-containing and latex-containing colors. With starch colors the evaporation rate at the gel point (consolidation of color) is important, but with latex colors, efficient initial drying decreases mottle and is an even more important factor than the evaporation rate at the gel point (Watanabe and Lepoutre 1982). Hagen (1985, 1990) has studied different drying mechanisms and found that drying strategy and especially the drying rate near the gel point plays a vital role. EXPERIMENTAL METHODS The target of the research was to determine drying rate, web runnability and paper quality issues of the new single-sided air dryer. A pilot dryer was constructed for this purpose. The dryer is equipped with three impingement-floatation nozzles at 0 mm nozzle-to-nozzle spacing. Web path over the dryer is slightly curved, with a very large radius r. The same pilot dryer was used for drying rate measurements, for testing of web runnability, and later on it was installed on a pilot coating machine for coating trials. The coating trials were made on a pilot coater at the Coating Technology Center (CTC), Raisio, Finland, Figure 1. The pilot coater's drying section consists of a two rows IR dryer and three doublesided air dryers. The single-sided pilot dryer was installed 2 m after the coating station of the pilot coater. Figure 1. Layout of the pilot coater and impingement dryer. Drying rate RESUL AND DISCUSSION In the drying rate and runnability tests the dryer was installed on a test rig consisting of an unwind, a frame for the dryer, a rewind, and means for controlling of web speed, tension etc. In the drying experiments rolls of moist paper (38 gbd/m² LWC base paper and 93 gbd/m² Testliner) were run at relatively slow speed and dried with the dryer. Drying rates were determined from moisture and basis weight samples taken at the unwind and rewind rolls, and from web speed. The rolls were dried in the pilot dryer using impingement temperatures from 220 to 550 C and velocities from to 75 m/s. Drying rates from 30 to 112 kgh2o/m²/h were obtained, Figure 2. The measured drying rates were in well agreement with those calculated from impingement heat transfer coefficients of the nozzle and from the nozzle-to-paper temperature differences. 1296

3 Table 1. Experimental conditions. Paper grade LWC Base Testliner Basis weight (gbd/m²) Initia dryness at unwind (%) Final dryness at rewind (%) Web speed (m/min) Web tension (N/m) Web width (mm) 0 0 Impingement temperature ( C) Impingement velocity (m/s) 75 Nozzle-to-web distance (mm) Specific evaporation (kg/m 2 /h) Impingement temperature ( C) m/s m/s 75 m/s Testliner Specific evaporation (kg/m 2 /h) Impingement temperature ( C) m/s m/s LWC Base Figure 2. Measured drying rates as a function of impingement temperature and velocity. Testliner (left) and LWC base paper (right). Nozzle-to-web distance 4 20 mm. Web stability It is absolutely important to achieve a stable and flutter free web transport over the dryer and to have an acceptable nozzle-to-web distance that enables efficient heat transfer. This has to be achieved in a sufficient wide range of running conditions. Web tension may vary depending on the paper grade, and there may be sudden disturbances in web tension. The cross direction tension profile of the paper web can be uneven, e.g., the paper edges are sometimes slack. Web runnability was tested in the above described test rig by running rolls of dry paper through the test facility. Different web speeds, web tensions, impingement / suction air ratios, impingement velocties were investigated. The objective was to find stable web transport and optimal nozzle-to-web distance at varying process conditions. Experimental conditions used in the runnabilty tests were: - Paper grade LWC Base - Web tension N/m - Web speed m/min - Web width 0 mm - Impingement velocity m/s The nozzle array of the pilot dryer forms a gently sloping arch. Web distance from the nozzle array is a balance between the affecting forces: web tension, centrifugal force, nozzle pad pressure and suction pressure in the return air slots between the nozzles. Following equations illustrate this balance. 1297

4 Web tension creates a force that presses the web towards the dryer nozzles. This force corresponds a pressure that is a a function of web tension and curvature T p T = (1) r Centrifugal force affects opposite to the tension force. In many cases the centrifugal force is quite negligible in practice. It is a function of basis weight, speed and curvature of the web 2 U p C = BW (2) r Pad pressure means average pressure directed to the web by the nozzle, compared to pressure in the exhaust air slots between the nozzles. Pad pressure depends on nozzle characteristics, nozzle-to-web distance, impingement velocity and impingement air density p = g h ρ w (3) where ( h) P ( ) 2 a a g is a function of the nozzle-to-web distance and it is possible to determine empirically (Figure 3). The above mentioned forces affect to the web in such a manner that the web seeks automatically a position where the forces are in balance p P + ps = pt + pc (4) Figure 3a shows distribution of the pad pressure measured across one nozzle at different nozzle-toweb distances. Figure 3b shows the integrated pad pressure, determined from the distribution curves in Figure 3a T a = C w a = 51 m/s Pressure (Pa) Pad pressure (Pa) L (mm) Nozzle-to-plate distance (mm) Figure 3. a) Pressure distribution over one nozzle box at 5 42 mm nozzle-to-web distances (left). b) The integrated pad pressure as a function of nozzle-to-web distance (right). Figure 4 displays schematically the balance between p P + ps and p T + pc. If we assume that web tension T is increased or decreased with a tension increment T while other parameters are kept unchanged, the nozzle-to-web distance will change slightly along the p P + ps curve. The system is self-stabilizing: As web tension increases, the web tends to get closer to the nozzles, and the web gets more straight ( r increases). Due to the decreased nozzle-to-web distance, the pad pressure is increased, and due to increased radius, the tension force is decreased. As web tension decreases, the same happens in the opposite way. This means, the web seeks a new position at a slightly increased distance. Suction pressure ( p S ) in the dryer enclosure is an efficient control parameter to stabilize the web at desired distance from the nozzle array. 1298

5 T + T T p T + p C p (Pa) T - T p P + p S Nozzle-to-plate distance (mm) Figure 4. The nozzle-to-web distance is a balance between the nozzle pad pressure, web tension and centrifugal forces. Pilot coater trials A series of coating trials were run with different paper grades (triple coated WF, double coated WF, single coated LWC and double coated board), Table 2. Coating color formulations of the top coatings are shown in Table 3. Blowing air temperatures and velocities used in the impingement unit were and C, and, and m/s respectively for all the paper grades, except the triple coated WF was dried with 550 C, and and m/s only. As a reference some trial points were run without the impingement unit, having a 2-row electrical infrared dryer in the same position. The first double sided air dryer was used in constant conditions (200 C 20 m/s). The second IR dryer located before the double sided air dryer was kept in constant conditions (50 % for WF and LWC, and 0 % for board). The final moisture content was measured with on-line IR Scanner and with laboratory samples taken from each trial point. The final moisture content of WF paper was 4.5 %, for LWC 5.5 % and for the board 7%. Dried LWC and WF papers were supercalendered with the pilot supercalander at the Coating Technology Center Raisio, Finland. The pilot supercalender was equipped with a 10-roll fiber stack. The double coated board trial points were soft-calandered using the on-line soft-calander of the pilot machine. Samples were printed on a 4-color sheet-fed offset printing press at the Central Laboratory (KCL) Espoo, Finland. The mottle on the printed surface was determined by image analysis (KCL method). Table 2. Coating trial conditions. Paper grade Base paper Precoating (g/m 2 ) Top coating (g/m 2 ) Top coat color Speed (m/min) Triple coated WF WF1 80 g/m Double coated WF WF1 80 g/m , LWC LWC1 g/m 2 none , LWC LWC2 45 g/m 2 none Double coated board Board g/m 2 10 () 12 () 5,

6 Table 3. Coating color formulations. Color Fine clay (HG 90) Ground fine carbonate (HC 90) Delaminated clay (Astraplate) SB latex A, Tg SB latex B, Tg +15 SB latex C, Tg VinAcetAcrylic latex D, Tg CMC Oxidized Starch 6 Hardener Lubricant Optical brightener Solids Print mottle Figures 5 7 show the measured values of print mottle (grainess in 0.2 to 2.0 mm scale) of the LWC, WF and board trial points. For LWC with clay coatings (colors 7 and 8) there is a clear trend so that print mottle is decreased when drying rate of the impingement unit is increased. For LWC with color 3 print mottle is rather constant regardless of drying rate in the impingement dryer. When the impingement dryer is compared to the electrical IR dryer (reference point), the impingement dryer gave less mottle when high temperature and / or velocity were used ( color 7). For double coated WF the print mottle is in all trial points very low, the mottle values are in the interval 3 to 4 %, when values below 4 % are generally regarded as visually unifom printing, Figure 6. There is a very slight trend so that mottle is decreased when drying rate of the impingement unit is increased. The reference with IR gave about similar mottle level as the corresponding trial points with the impingement unit with high drying rate. For the triple coated WF print mottle was also low, from 4.0 to 4.3, and the mottle was slightly decreasing with increasing drying rate. With double coated board at a machine speed of 500 m/min the drying time in the impingement dryer was so long (144 ms) that optimal initial drying was achieved. The printed samples have excellent print quality, Figure 7. Mottle is constant or decreases slightly as drying rate of the impingement unit is increased. The coating was expected to consolidate completely in the impingement unit, giving uniform surface properties. Mottle (%) LWC 2 color 8 color 7 color 7 Ref Figure 5. Print mottle of LWC paper. The numbers above each column indicate impingement velocity (m/s) and temperature ( C) of the impingement unit. Mottle values below 4% are considered to be visually even print. 1

7 4.0 color 3 Mottle (%) 3.8 Ref Figure 6. Print mottle of double coated WF. The values above each column indicate impingement velocity (m/s) and temperature ( C) of the impingement unit. Mottle values below 4% are considered to be visually even print. Mottle (%) Board color 5 Board color Figure 7. Mottle of double coated board. 3.4 Paper smoothness and gloss Smoothness (PPS10) and gloss (Hunter) were measured for the calendered trial points. For LWC the gloss was slightly improved when drying rate of the impingement unit is increased. For double coated WF there was a more clear trend showing increasing gloss with increasing drying rate of the impingement unit. Gloss of double coated board was rather constant at all impingement unit drying rates. Figures 8-10 show the measured smoothness values. A general trend is that increasing initial drying improves smoothness. This can be observed for all paper grades, most clearly for LWC. Trial points with impingement unit at full power gave clearly better smoothness than the corresponding reference points with IR dryer. PPS LWC 2 color color 7 color 7 Ref PPS color 3 Ref 0.9 Figure 8. PPS10 smoothness of calendered LWC paper 0.8 Figure 9. PPS10 smoothness of calendered double coated WF paper 1301

8 Board color 5 Board color 6 PPS Figure 10. PPS10 smoothness of calandered double coated board. 0.7 CONCLUSIONS A single sided high temperature impingement unit was tested regarding drying rate, web runnability and quality of produced paper and printed surface. Impingement temperatures up to 550 C were tested. For a part of the studied paper grades print mottle was decreased with increasing drying intensity in the impingement unit, while for a part of the studied grades mottle level was rather constant regardless of intensity of initial drying. Quite similar mottle level was obtained when the initial drying was made with the impingement unit or with the infrared dryer, when drying intensity is similar. Increasing of drying rate in the impingement unit in most cases improved paper smoothness. Somewhat better smoothness was obtained with the impingement unit at high drying rate than with the infrared dryer. Paper gloss was either increased or it remained unchanged, when drying intensity of the impingement unit was increased. NOTATION BW web basis weight kg/m 2 p pressure Pa r radius m T tension N/m U web speed m/s ρ air density kg/m³ LITERATURE Engström, G. (1994), Formation and consolidation of a coating layer and the effect on offset-print mottle, Tappi Journal, Vol 77, no. 4, pp Engström G., Persson A., Fineman I., Åkesson R. (1982), How Ink Mottling in Offset-Printing is Affected by the Drying Conditions during Coating - a Comparison between Starch and CMC/SB Latex Binders, Proceedings of Tappi Coating Conference, pp Hagen K.G. (1985), A Fundamental Assessment of the Effect of Drying on Coating Quality, Proceedings of Tappi Coating Conference, pp Hagen K.G. (1990), Fundamentals of drying blade coatings, Tappi 1990 Blade coating seminar notes, pp Watanabe J. and Lepoutre, P. (1982), A mechanism for the consolidation of the structure of clay-latex coatings, Journal of Applied Polymer Science, Vol. 27, pp