Article pubs.acs.org/IECR
Drying of Porous Coating: Influence of Coating Composition Joel Songok,†,* Douglas Bousfield,‡ Cathy Ridgway,⊥ Patrick Gane,⊥,§ and Martti Toivakka† †
Laboratory of Paper Coating and Converting and Center for Functional Materials, Abo Akademi University, Porthaninkatu 3, FI-20500 Åbo/Turku, Finland ‡ Department of Chemical and Biological Engineering, University of Maine, 5737 Jenness Hall, Orono, Maine 04469, United States § School of Chemical Technology, Department of Forest Products Technology, Aalto University, P.O. Box 16300, FI-00076 Aalto, Finland ⊥ Omya Development AG, CH-4665 Oftringen, Switzerland ABSTRACT: The influence of the coating composition of a porous paper coating on the evaporation rate of water contained in the sample has been studied experimentally. For low-content latex samples, drying was found to be mainly controlled by capillarity, perhaps also in the form of thin film pore surface feature/wall wetting, which drew water from the connected pores to the drying surface or near the surface. This led to a lengthy constant drying rate period (CDRP) where nearly 70% of the saturated water was evaporated. High-content latex samples, characterized by low porosity and permeability showed shorter CDRP and lengthy falling drying rate period. The drying rate curve varied linearly with time in the CDRP and with the square root of time in the falling rate period, indicating a diffusive controlled mechanism. Low latex content samples took less time to dry, which can be inferred to mean they require less drying energy.
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INTRODUCTION The drying process of pigment-coated paper involves supplying heat to provide energy to vaporize the solvent. Evaporation during drying of paper is crucial in respect to the development of pore structure, optical properties, and printing quality. Depending on the drying concept, dryer adjustments and other operating parameters, the energy consumption in the coater dryer can vary from 4500 to10000 kJ/kg evaporated water.1 Hence, an immense amount of energy is required to effectively evaporate sufficient water to dry the coating. Therefore, an understanding of how vaporization of liquids from a paper coating, which is an irregular network of interpenetrating voids and solid, takes place from the microscale to macroscopic drying behavior could improve the energy efficiency. The drying of porous material has been studied in depth in the fields of food science, chemical engineering, and soil science. While the general mechanisms of liquid and vapor flow within a porous medium that is drying have been studied, for instance,2,3 the effect of the composition of a porous coating medium on drying is not fully understood. The mechanisms taking place during drying include phase change from liquid to vapor due to heating at the liquid−gas interfaces, heat transfer, capillary-driven flow of liquid, diffusive and convective vapor transport through the gas-filled pore space to the drying surface, and the receding of the meniscus under the combined effects of capillary and viscous forces.3,4 The complexity of pore structure, coating composition, and base-paper-coating interaction make the investigation of the drying of pigment coated paper challenging. In an effort to understand mechanisms that control the drying, researchers have tried to correlate binder migration with the drying behavior. It is found that mechanisms including soluble binder diffusion, convection, and capillary driven fluid flow determine how the binder distributes. Nowicki et al.5 used two-dimensional networks to simulate and analyze © 2012 American Chemical Society
liquid movement and potential binder migration in drying paper coatings. Pan et al.6 modeled a three-dimensional pore scale network using equations that describe the pore-level physics of meniscus-driven flow, vapor diffusion, and binder transport and deposition to examine the ranges of drying rate and binder diffusivity. The model showed that pores do not exclusively dry from the surface into the coating, but rather the pores empty in order of their accessibility and size, except when the rate of drying is high. In the drying of the paper coating, gravitational forces are negligible in comparison to the surface tension and are therefore ignored. This is because the Bond number, that is, the dimensionless ratio of gravitational force to surface tension force is small. For instance, for pore dimensions ranging from 100 to 0.1 μm the Bond number varies from 10−3 to 10−9. Capillarity describes the pressure difference across a curved meniscus separating two immiscible fluids such as water and air within a pore. The strength of capillary forces at a given point in a pore is a function of the pore cross sectional area and the local contact angle. Pores with small diameters develop greater capillary forces than large diameter ones as defined in the Young−Laplace equation PC = 2γ cos θ/r, for a capillary of radius r, liquid surface tension γ, and contact angle θ.7 During drying of porous material, the meniscus recedes and pins at the pore region with smaller diameters. Menisci pinned in smaller pores therefore, tend to draw liquid from the larger pores, provided a continuous liquid route exists between them. Conventionally, the drying process of a porous medium is divided into constant drying rate period, first falling rate period, Received: Revised: Accepted: Published: 13680
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Table 1. Pore Intruded Volume, Porosity, Air Volume Fraction after Saturation and during Drying total specific intruded volume (cm3g−1)
porosity (%) before soaking (measured)
% air volume after soaking (calculated)
% water evaporated after 30 h
% water evaporated after 42 h
+ 2 ppha
0.150 0.140
29.19 27.44
−0.75 −0.70
76.15 68.02
75.30
+ 4 pph
0.151
28.26
−0.14
63.20
70.63
+ 6 pph
0.149
27.36
−0.78
44.98
+ 8 pph
0.139
26.27
−0.29
44.16
50.19
+ 10 pph
0.131
24.15
0.99
36.94
43.04
+ 12 pph
0.106
21.21
sample cbGCC cbGCC SA cbGCC SA cbGCC SA cbGCC SA cbGCC SA cbGCC SA a
pph refers to parts by weight of latex to 100 parts by weight of pigment.
and second falling rate period.8 During the constant drying rate period, the rate of evaporation is constant and controlled by environmental conditions such as air velocity and the relative humidity.8,9 The drying rate remains constant as long as the liquid film covers the pore networ, and this mainly depends on the connectivity between the bulk liquid front and evaporating surface.10 Capillary forces are responsible for pulling up the liquid to the drying surface to maintain a constant drying rate.8,11 As the evaporation proceeds, the pores empty out and the liquid film becomes thinner, hence the rate of drying decreases where else the menisci retreats deeper into the porous material giving rise to the first falling rate period. Liquid leaving the porous media is replaced by vapor-laden gas. In an isothermal drying, vapor diffuses out to the drying surface due to a concentration difference. As the diffusion path becomes more tortuous and grows in length, the rate of drying slows down. A point is reached when there is insufficient water to maintain a continuous film across the pores; the remaining water is relegated to small isolated pools in the corners and interstices of the pores. When this state is reached, the second falling rate period is initiated, during which the rate of drying is independent of the impinging hot air velocity, and is no longer a linear function of moisture content.3,5,8 The work of Gerstner12 tackles for the first time the factors of coating structure and the constituent coating components that control the thermal conductivity of porous multicomponent systems. The solid−solid connectivity provided by the bridging between the components, namely between pigment and binder has been identified as the main controlling factor in heat transmission. This paper is to identify whether this is a limiting factor for the efficiency of drying liquids from such porous structures. In our earlier work,13 we proposed three models to evaluate the role various parameters play on the drying rates of porous materials. The models were used as a first approximation to determine the heat transfer level requirements to the liquid, and sets out to determine the structural limiting factors during the drying process, applying for the first time the newly known values of coating thermal diffusivity, and hence conductivity.14,15 Although the models were limited to 2D, it was found that the thermal conductivity of the coating structure in today’s applications of heat delivery from hot air drying is not the limiting factor, but the drying efficiency limitation is due to the poor thermal transfer process at the structure surface. The value of the work was to underline the need for improvement in the thermal transfer process, in the case of hot air drying, before considering further measures
in designing increasing thermal conductivity of the porous medium. Continuing in this line of investigationwith the aim of understanding coating composition in respect to drying wetted paper coating, such as in printingwe carried out experiments to determine how coating composition affects the drying rate of porous pigmented samples.
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MATERIALS AND METHODS A series of cylindrical tablets made from a coarse ground precoating grade of natural ground calcium carbonate (cbGCC), having a broad size distribution, used for paper and board coating (Hydrocarb 60, Omya Development AG, Switzerland), with an increasing amount of styrene acrylate (SA) latex (Acronal S360D, BASF AG), were analyzed. The tablets were formed by applying a constant pressure (15 bar for 60 min) to the suspension so that water was released by filtration through a fine 0.025 μm filter membrane, resulting in a compacted tablet of the pigment. The tablets then were dried in an oven at 60 °C for 24 h.16 The drying is made after immobilization and therefore, no crust can form due to movement of latex in the drying process. The tablets were ground not only to make the surfaces flat but also to remove a crust if any formed. This method resulted in tablets of 2 cm radius and 2 cm thickness. Previous studies have shown that the tablets reflect a pore structure that is quite homogeneous throughout.17−21 The tablets were weighed, and their sides were sealed with waterproof sealant silicone (SMP-polymer, Casco Marin & Teknik) and thereafter air-dried for at least 48 h. The tablets were saturated with distilled water for 24 h by inverting the tablets and contacting the top of each with a distilled water reservoir. In the liquid reservoir, the tablets were inclined to the base to allow a passageway for water underneath the tablet, thus facilitating the liquid absorption into the tablet. The tablets were taken out after soaking; excess water was wiped away using an inert cloth. The tablets were reweighed and the tablet bottom (area exposed to air during soaking) was sealed with the same SMP-polymer, after which the tablets were reweighed and analyzed. The drying analyses were carried out on a balance with a precision of 0.1 mg at standard temperature and humidity conditions of 23 °C and 50% RH, respectively. The balance was connected to a computer which automatically recorded the weight change of the tablets for at least 30 h. Table 1 shows measured pore characteristics before tablet saturation and calculated volume of the air remaining after the saturation before tablet drying. 13681
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in the base of the cell to guide the permeated liquid drops to the outlet. The continuous flow can be expressed in terms of the Darcy permeability constant, k, as
Permeability was measured using a technique described in Ridgway et al.20 A dried cuboidal piece of the tablet structure is placed into a mold and resin is poured around it as shown in Figure 1. (The PTFE-molds used to form the cylindrical
d V (t ) −kAδP = dt ηl
(1)
where dV(t)/dt is defined as the flux or volume flow rate per unit cross-sectional area, A, δP is the applied pressure difference across the sample, η is the viscosity of the liquid, and l is the length of the sample.
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RESULTS AND DISCUSSION A portion of each tablet (unsaturated) was characterized by mercury porosimetry for both porosity and pore size distribution using a Micromeritics Autopore IV mercury porosimeter. The maximum applied pressure of mercury was 414 MPa, equivalent to a Laplace throat diameter of 0.004 μm (∼nanometers). Figures 3 and 4 show the mercury intrusion
Figure 1. Preparation of pigment tablet for permeability measurement.20.
embedments were from Prüfmaschinen AG Giessenstrasse 15 CH-8953 Dietikon, Switzerland, and have an inner diameter of 30 mm. The resin used to embed the tablet samples was EpoThin Epoxy Resin: a product name from Bueher, Prufmaschinen AG Prufag, 8952 Schlieren, Switserland.) A quickly rising viscosity of the chosen curing resin results in a penetration of approximately 1 mm locally at the outer boundaries of the sample. This penetration depth is clearly visible because of the opacity change at the edge of the sample and can, therefore, be calibrated. The area free from resin was evaluated to establish the permeable cross-sectional area. The sample discs were placed in a dish containing the probe liquid in order to saturate the void network of the sample before the discs were placed in the apparatus. Hexadecane with density, ρ = 773 kg m−3 and viscosity, η = 0.0034 kg m−1 s−1 was used in the experiments to avoid any interaction with synthetic or natural binders. The sample disk is then placed in a specially constructed pressure cell. The cell design used for the pressurized permeability experiments is shown in Figure 2.
Figure 3. Mercury intrusion curves for the tablet samples.
Figure 2. Permeability measurement cell: (1) lid with pressure inlet, (2) sealing O-rings, (3) liquid cell; outer diameter = 40 mm, (4) porous sample embedded in resin disk of diameter = 30 mm, (5) fixing ring compresses the O-ring which seals the resin disk, (6) security shroud and drop collector, (7) drop captor (Teflon tubelet), (8) dish on microbalance.20 Figure 4. Pore size distributions of the tablet samples.
The use of the resin to embed the samples allows for rigid clamping and sealing of the sample into the pressure cell chamber. Gas overpressure is supplied from a nitrogen bottle. The pressure cell is fixed over a microbalance and a computer samples the balance data using specially developed software developed within Omya AG (Software can be obtained on request from Dr. C. J. Ridgway, Omya Development AG, CH 4665 Oftringen, Switzerland). A drop captor device was needed
curves and the pore size distributions for all the tablet samples. The data have been corrected using Pore-Comp for mercury and penetrometer effects and also for sample compression.22 After immersing the tablets in water for 24 h, it was taken that the tablets were fully saturated, after which calculations with density and measured weights were made as reported in Table 1. The negative values of the air volume fraction (in Table 1) indicate that either a small amount of excess water still 13682
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Figure 5. Drying time curves: a plot of time verses cumulative water loss.
disrupts the packing, it also fills up the crevices in the packing hence blocking the connectivity of the interstices and to the pores at the surface, as reflected in the structural changes shown in Figure 4. Additionally, the roundedness of the pore increases the flow resistance of a film on the pore wall and consequently the evaporation time. Chauvet and co-workers25,26 have analyzed in detail the effect of corner roundedness on evaporation controlled by liquid films. Prat27 has reported that the addition of binder fills in the corners of the pore spaces rounding or polygonalizing the pore. Similar observations were also made by Jin and Breuer28 who studied evaporation driven by mass transfer in rectangular microchannels. According to AlTuraif and Lepoute,29 as water evaporates during the coating structure formation, the latex spheres start to coalescence and form a smooth and continuous polymer film, which is able to fill up some of the detail geometry of connecting pores. Ransohoff and Radke30 showed that the roundedness of the corners diminishes the available area for flow, thereby causing greater flow resistance in the film. This is in agreement with experimental results presented in Figure 5, where samples with lower binder contents exhibit longer constant drying rate periods than is the case for the samples with high binder content. The tablets with higher binder contents offered higher flow resistance during drying therefore less amount of water was delivered to the surface to keep it wet leading to shorter constant drying rate periods. When the delivery of the water to the surface becomes inadequate or the vapor pressure at the surface equilibrates the vapor pressure within the surroundings, the evaporation starts to take place in the internal pore structure. The rate of drying in this period is influenced by a combination of effects, including capillary, viscous, and film flow, as well as gravity, thermal gradient, convection and diffusion of vapor. However, in the current investigation, it was recognized that not all these forces played a role. For a macroscopic tube, under the joint action of gravity and viscous effects, when the meniscus recedes into the pore structure, the evaporating film detaches from the pore mouth and starts to recede into the tube. As a result, vapor diffusion from the meniscus to the pore mouth is suppressed due to the increased transport resistance and hence the drying rate falls gradually. During the evaporation, the liquid/gas interface is cooled because of the energy used for the phase change resulting in a temperature difference. This temperature
remained on the surface of a tablet even after light wiping or the tablets swelled during soaking. The swelling, possibly of polyacrylate used to disperse the pigment, could result in additional water uptake. From Table 1, nearly 76% of absorbed water had been lost from the cbGCC tablet (without latex) after 30 h of drying whereas for cbGCC + 10pph SA tablet (with 10 pph of latex) only 37% was lost. Also, after 42 h, it was observed that tablets with lower latex content had lost more water than those with higher content of latex. Therefore, tablets with high contents of latex dried slower than the ones with low or zero latex content. Figure 5 below shows drying time curves for tablet samples. Two distinctive regions are seen that signify the two conventional drying rate periods: the constant drying rate period and falling drying rate period. The evaporation rate during the constant rate period is nearly the same for all the samples confirming that the experimental conditions were the same. Although vaporization at constant drying rate period is independent of the solid geometry and dependent on environmental conditions (such as humidity, air velocity, and air temperature) as stated by Marshall et al.23 and also by Richardson et al.,24 pore sizes and pore shapes play a role in holding and pulling up the liquid from the porous material to the surface of the tablet to keep it wet. Samples with low binder content exhibited longer constant rate periods than samples with high latex content. This period varied from approximately 4 to over 20 h. The latex-free sample showed the longest constant rate period. The inverse relationship between the latex content with the time it takes drying to reach the critical point can be attributed to the pore shape and pore size. Different pore sizes generate different capillary forces within the pore structure. Small pores exhibit high capillary forces and vice versa. The liquid contained in the porous tablets results in a vapor pressure that is determined by the pore size. The tablets were dried in an environment of lower vapor pressure; hence, the tablets continuously lost liquid through evaporation until the vapor pressure reached equilibrium with the atmosphere. In this process, the large pores at or near the surface run out of liquid first; hence the gas−vapor interface recedes while the vaporization from the small pores at the surface draws water from the large pores in the structure. The draining of water from large pores by the smaller ones depends on accessibility to the former in the pore network. Although latex addition initially 13683
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It took nearly 16 h for the sample with 4 pph of latex to lose 50% of the initial volume fraction of water and almost 54 h for the sample with 10 pph latex content, whereas it took only 17 h for the latex-free sample to lose the same quantity. The shorter time for the sample with 4 pph and possibly 2 pph can be because of an increase in the pore size and the pore volume due to the addition of latex, which consequently affects the permeability as observed in Figure 8. The addition of latex up to nearly 4 pph causes a more open structure due to the disruption of the pigment particle packing resulting in a porous structure, which affects more the permeability and less the surface area.21,31 The retention of water in the structure is attributed to small pore sizes that result in high capillary forces and low permeability. Work done by Gane and Ridgway31 on moisture uptake into calcium carbonate-based pigment coating shows that the rate of water adsorption correlates with the permeability within the shorter time region. Similar observation was seen in this work, even though it involves two different mechanisms: drying and wetting. It is suggested that the connectivity of the pore space determines the drying time under vapor transport. Therefore, we may infer that the final drying times, and hence the rate of drying, are determined by the path the vapor takes to reach the surroundings. A more tortuous path results in a slower drying rate. This tortuosity can be brought about by a broad range of particle sizes and also by the addition of an ingredient such as latex in the pigment coating, which in turn fills in the pore spaces.
difference across the liquid layer was considered insufficient to give rise to Marangoni and convectional effects. The limiting mechanism for evaporation was therefore the diffusion of vapor inside the pore to the surface. The path the vaporised liquid (vapor) takes out to and from the surface depends on the permeability of the tablets. Figure 6 shows that the drying time
Figure 6. Drying time curves indicating a nearly constant rate for samples cbGCC + 4pph SA and cbGCC + 10pph SA. The time axis is plotted as a square root of time.
curves for the different samples during the falling rate period to be decreasing linearly with the square root of time. The small difference in gradients was attributed to narrow differences in the pore size ranges and permeability of the samples. Figure 7 shows the declining volume fraction of water and corresponding rising air volume fraction as the drying proceeds.
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CONCLUSION The drying of porous material with varied physical and chemical properties (pore size, porosity, surface area, and permeability) has been experimentally investigated. This is critical for a wide range of technological applications not only limited to drying of pigment coatings but also to applications involving phase change in confined environments. The complex phenomena taking place in such applications can be understood by investigating a far simpler case such as weight loss measurements as a function of time, using a precision balance. Sample tablets made from ground calcium carbonate with increasing amounts of latex content have been investigated. More rapid drying related to longer constant drying rate periods were observed for samples with low latex content. Furthermore, the water menisci in the samples with high latex content detached from the pore mouths earlier (sooner) than
Figure 7. Water and air volume fractions (vf) as drying proceeds of samples cbGCC + 4pph SA and cbGCC + 10pph SA.
Figure 8. Liquid permeation. 13684
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(17) Gane, P. A. C.; Koivunen, K. Relating liquid location as a function of contact time within a porous coating structure with optical reflectance. Transp. Porous Media 2010, 84, 587−603. (18) Schoelkopf, J.; Ridgway, C. J.; Gane, P. A. C.; Matthews, G. P.; Spielmann, D. C. Measurement and network modeling of liquid permeation into compacted mineral blocks. J. Colloid Interface Sci. 2000, 227, 119−131. (19) Schoelkopf, J.; Gane, P. A. C.; Ridgway, C. J. Observed nonlinearity of Darcy-permeability in compacted fine pigment structures. Colloids Surf. A: Physicochem. Eng. Aspects 2004, 236, 111−120. (20) Ridgway, C. J.; Schoelkopf, J.; Gane, P. A. C. A new method for measuring the liquid permeability of coated and uncoated papers and boards. Nord. Pulp Pap. Res. J 2003, 18, 377−381. (21) Gerstner, P.; Veikkolainen, S.; Gane, P. A. C. In Effective Thermal Conductivity of Liquid Saturated Coatings and Their Liquid Vaporisation Behaviour; Tappi Press: Atlanta, GA, 2010. (22) Gane, P. A. C.; Kettle, J. P.; Matthews, G. P.; Ridgway, C. J. Void space structure of compressible polymer spheres and consolidated calcium carbonate paper-coating formulations. Ind. Eng. Chem. Res. 1996, 35, 1753−1765. (23) Marshall, W. R., Jr. Drying. Ind. Eng. Chem. 1953, 45, 47−54. (24) Richardson, J. F.; Harker, J. H.; Backhurst, J. R. In Rate of Drying; Chemical EngineeringParticle Technology and Separation Processes; Butterworth Heinemann: Oxford, 2002; Vol. 2, pp 904−918. (25) Chauvet, F.; Duru, P.; Geoffroy, S.; Prat, M. Three periods of drying of a single square capillary tube. Phys. Rev. Lett. 2009, 103, 1−4. (26) Chauvet, F.; Duru, P.; Prat, M. Depinning of evaporating liquid films in square capillary tubes: Influence of corners’ roundedness. Phys. Fluids 2010, 22, 1−14. (27) Prat, M. On the influence of pore shape, contact angle, and film flows on drying of capillary porous media. Int. J. Heat Mass Transfer 2007, 59, 1455−1468. (28) Jin, S.; Breuer, S. K. In in Diffusion-Limited Evaporation in Long Microchannels; ASME: 2003; pp 673−677. (29) Al−Turaif, H.; Lepoutre, P. Evolution of surface structure and chemistry of pigmented coatings during drying. Prog. Org. Coat. 2000, 38, 43−52. (30) Ransohoff, T. C.; Radke, C. J. Laminar flow of a wetting liquid along the corners of a predominantly gas-occupied noncircluar pore. J. Colloid Interface Sci. 1988, 121, 392−401. (31) Gane, P. A. C.; Ridgway, C. J. In Moisture Pickup in Calcium Carbonate Coating Structures: Role of Surface and Pore Structure Geometry; Tappi Press: Atlanta, 2008.
was the case for the low content latex samples. Thereby, the high latex content samples entered the falling drying rate period early, which led to longer duration of drying. The drying time and drying rate were found to be related to the porous structure permeability, and the limiting mechanism for drying was found to be the diffusion of vapor. Finally, the drying curves obtained for the porous tablets exhibited the characteristic breakdown into the expected mechanistic drying periods.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +358 2 215 4232. E-mail: jsongok@abo.fi. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support of TEKES, the Finnish Funding Agency for Technology and Innovation (financing decision 40124/08), is gratefully acknowledged.
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REFERENCES
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