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Ind. Eng. Chem. Res. 2004, 43, 712-719
Observation of Shrinkage during Evaporative Drying of Water-Based Paper Coatings Giuliano M. Laudone,† G. Peter Matthews,*,† and Patrick A. C. Gane‡ Environmental and Fluid Modeling Group, University of Plymouth, Plymouth PL4 8AA, U.K., and Omya AG, CH-4665 Oftringen, Switzerland
Coated paper for high-quality printing comprises a fine particulate mineral coating, applied as an aqueous suspension and fixed to the fibrous paper substrate with a binder. The shrinkage occurring while the coating layer dries onto the substrate has been measured by observing the deflection of strips of a synthetic substrate coated with ground calcium carbonate with different binders. The force acting on the surface of the strips to cause a given deflection has been calculated using the elementary beam theory. The porosities of the dry structures were measured by compression-corrected mercury porosimetry. We show that the shrinkage occurring during the drying of the coating layer is mainly due to capillary forces acting as the water recedes in the porous structure, while the binder can act to retain the stress resulting from such forces. Starch produced much higher stresses than latex. Introduction The application of a coating layer to a base sheet of paper or board improves its optical and printing properties such as uniformity in appearance, gloss, opacity, ink absorption, and spread. The voids within the porous coating have sufficient capillarity to absorb the solvent or diluent rapidly from ink and hence allow the ink to start to set in the brief intervals between successive ink applications, which are characteristic of a modern printing press. A coating layer typically consists of (i) water, so that the coating can be applied as a particulate suspension; (ii) pigments, such as ground (GCC) or precipitated (PCC) calcium carbonates, clays, polymeric pigments (such as polystyrene), titanium dioxide, silica, or talc; and (iii) binders, which are needed to provide good cohesion of the porous structure formed by pigments and adhesion of this structure to the substrate. A mixture of pigment and water is referred to as “slurry”, and a mixture of water, pigment, binder, and any other additives is known as a “coating color”. A commercial coating color has a solid content of between 50 and 70% (w/w). This solid content comprises 80-90% (w/w) of pigment and 10-20% (w/w) of binder. Other compounds, such as dispersants, are used in lower percentages to act as stabilizing agents and to make the components compatible in water suspensions. The particle diameters of the mineral pigments usually range from 0.01 to 10 µm. In this work, a layer between 5 and 10 µm thickness was applied to the substrate with a “draw-down rod”, which had a wire wound around it to apply a known volume of slurry or coating color. The layer then underwent progressive drying. Binders are usually divided into two types: natural (such as starch or protein) and synthetic [such as styrene-butadiene (SB), acrylic latex, or poly(vinyl * To whom correspondence should be addressed. Tel.: +44 1752 233020. Fax: +44 1752 233021. E-mail: pmatthews@ plymouth.ac.uk. † University of Plymouth. ‡ Omya AG.
acetate)]. Natural binders often result in coatings that have poor optical qualities, partly because of the way in which the coating shrinks during drying. Synthetic binders suffer less from this problem, and so, although they are more expensive, they are becoming increasingly popular. If the way in which a coating consolidates during drying can be understood, then the process can be controlled and the optical and printing properties optimized. Groves et al.1 have reviewed techniques for studying the process experimentally, and other workers have modeled it.2-4 Watanabe and Lepoutre5 divided the drying process into three phases, separated by two critical concentrations. The slurry or coating color dries until it reaches the first critical concentration (FCC), at which a threedimensional fluid-filled network is formed and particle motion is greatly restricted. In the second phase, the water-air interfaces recede into the surface pores, forming capillary elements and creating a differential capillary pressure that causes a shrinkage of the network. This continues until the second critical concentration (SCC) is reached, at which the structure is rigid and air enters as the liquid retreats. So, it is the structure ranging from FCC to SCC that is important with regard to shrinkage forces; before the FCC, particles are suspended in excess solution with no capillary forces present, and after the SCC, the positions of the particles are locked. Ignorance of this locking effect has often led to incorrect conclusions in the literature, especially concerning so-called migration of particulate material, for example, latex as reviewed by Groves et al.1 In real paper coating applications, the absorbency of the base sheet is a dominant factor in the absorption of liquid. Superimposed on this is the evaporation of liquid from the exposed surface as the coating is dried. The fibers of natural substrates have a partially plastic behavior and can rearrange during drying. The fluid entering the base sheet in real applications comprises water and dissolved chemicals such as pigment stabilizer, a solution known as “serum”.6 To reduce these experimental uncertainties and complexities, the samples
10.1021/ie0340138 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/10/2004
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in this study were coated onto an impermeable synthetic substrate, which had elastic but not plastic behavior. The weight loss on drying was therefore through evaporation of water from the surface only. We report a novel indirect method of measuring the forces acting during drying. We observed the extent of curl with respect to the longest dimension of a sample strip, measured with respect to weight loss.7 This was converted to a plot of drying stress with respect to percentage weight loss. The porosities of the dry structures were measured by mercury porosimetry, corrected for sample compression. Mechanisms for the drying within the different coating formulations are inferred from the stresses and porosities. Theory Each strip of synthetic substrate was assumed to be an elastic bending beam. Standard bending theory was applied, which relates the forces acting on a beam with the actual deflection of the beam itself, as detailed in the appendix. The forces acting on the surface, causing the deformation, could then be calculated if the Young’s modulus of the substrate is known. We made four main assumptions: (i) the weight of the substrate was negligible when compared with the forces acting on its surface, (ii) the mechanical properties of the substrate do not change during the drying of the coating layer, (iii) the substrate acts perfectly elastically in the direction of curl, and (iv) the resistance to bending of the drying coating color layer, which is 5-10 µm thick, is negligibly small when compared to that of the 100-µmthick substrate. If both the theory and assumptions are correct, then the equations derived in the appendix show that the bending moments should act to curl the samples into shapes following the arcs of circles in the xz plane. The bending did indeed cause such shapes exactly to within the precision of the measurement method. Clearly, there were structural transitions within the coating itself because it acted as a stress deliverer, and it is these that form the main subject of the investigation. Materials Substrate. The synthetic substrate was a synthetic laminate, Synteape8 (Arjo Wiggins), made by filling polypropylene with calcium carbonate and stretching it. Its slightly rough surface allowed the coatings to adhere. Its elastic behavior was shown to be anisotropic because of the stretching in one direction during the last stage of its manufacture. It was found necessary to cut the strips parallel to this stretch direction to establish a reproducible elastic behavior. The strips were formed 4.5 cm long (x axis) and 0.5 cm wide (y axis). The thickness of the substrate was 100 µm and its elastic modulus 1.73 GPa. Pigments. The pigments were based on GCC. This choice of pigment avoided complications such as the anisotropy of clay platelets and the acicularity of aragonitic PCC.9 The GCC was a dispersed limestone from Orgon, France, ground to two particle size ranges, named Hydrocarb 60 and Hydrocarb 90 (Omya AG, Oftringen, Switzerland). The samples contained respectively 60% and 90% (w/w) of particles with diameter
