Paper and Surface Chemistry – Part 2- Coating and Printability Pedro

Paper and Surface Chemistry – Part 2- Coating and Printability

Pedro Fardim

Instituto de Química, Universidade Estadual de Campinas, SP, Brazil Present Address: Process Chemistry Group, Åbo Akademi University, Turku/Åbo, Finland

Abstract

Surface chemical interactions are important in many steps of paper manufacturing and converting and most of them are complex and not very well understood. Thus, many paper and coating formulations are designed according to an empirical trial-and-error approach. This work is the second part of a critical literature review concerning some interactions present in papermaking, coating and printing. In the first part interactions relating fiber surfaces and wet end chemicals were reviewed. In this second part, surface phenomena present in coating formulation and printing are discussed. Manufacture of paper is to create a surface. Different treatments, either mechanical such as calendering or chemical such as coating are applied to the paper surfaces in order to improve printability, optical properties, and strength. Coating is a mixture of minerals, colloidal particles and dissolved polymers which is applied onto the base paper surfaces requiring optimum flow properties, usually achieved with low solids and viscosity. However, high solids and viscosity are needed to obtain good printability and a uniform coating layer. A balance between coating application and printability must be found, and the understanding of surface chemical phenomena within coating formulation and rheology is required. A review concerning interactions between mineral particles with the dispersion media and also adhesion with latex particles is performed here. The influence of interactions between minerals, binder and co-binder on the coating water retention is discussed. Rheological properties and changes in coating color viscosity are interpreted using the DLVO theory. Formation and deposition of white pitch in paper machines are also discussed based in this theory. Interactions between printing inks and paper surfaces as well as penetration of the ink vehicle into coating layer are discussed in terms of Lucas- Washburn capillary model and thermodynamic work of adhesion. The concept of surface engineering is also suggested to be developed for coating formulation with the aim of improving adhesion with the paper base and eliminate problems such as varnish rejection, spots, cut dust and non uniform printability. The knowledge of surface chemical phenomena can be useful for the pulp and paper industry, not only for patents and product development, but also for improving in troubleshooting and customer support.

Key words: Papermaking, coating, printability, surface chemistry, surface engineering.

1. Produce a surface aiming at printability, optical properties and the best impression strength. These treatments can be mechanical, chemical or a Manufacture of paper is to create a combination of both and besides surface. Different superficial conferring greater economic value treatments are used in the process to the product, are primordial in a aiming at the improvement of the market of increasing competitiveness. The different softness and being affected by treatments used in the production of parameters related to the humidity paper and the concept of printability content, applied pressure and are presented in the next section. number of reels that the sheet passes. Coating is the application of a layer 1.1 Superficial treatments in the of ink prepared with pigments, paper manufacture adhesives, and other additives. This layer has the function to eliminate the lack of uniformity in the During the paper manufacture a set distribution of the printing ink that of well defined stages is followed. In is caused by the surface roughness the wet end section part of the and sites susceptible to the machine the stage of flocculation capillarity in regions intra- and occurs (Beghello, 1998, Swerin, inter-fibers (Allem, 1998; Ström et 1995), followed by drainage and al, 1995). The coating components sheet formation on the wire screen. form a complex mixture where the The next stages are the drying and pigment has the highest ratio and the superficial treatment in the dry each additive has a particular part of the machine. The main types function. The needs and the amount of superficial treatments are surface of an additive are designed sizing, calender in machine, according to the end use purposes of coating, and super-calender. the paper. Coating formulation is also directly related with the type Surface sizing and coating involve and characteristics of the applicator application of chemical components available. The mechanical forces on the paper surface while which the coating fluid is submitted calendering processes are only during application are determinant mechanical treatments. The surface for the formulation and also for a sizing process consists of the suitable process result (McGenity et application of a component layer al, 1992). that can be starch, polyvinyl alcohol, sodium alginate, carboxymethylcellulose, gelatins or glue. Sizing improves the fiber-fiber contacts by the fulfilling of empty pores and spaces and thus conferring better superficial strength and increasing the resistance to the water penetration (Verhoeff et al, Figure 1. AFM image of a coated paper 1963). Other polymers can be used surface after calendaring. Topographic mutually with the sizing agents (left) and phase (right) image (Fardim, aiming at the formation of a 1999). Calendering process caused a superficial film, as for example superficial flattening which increased the polystyrene films. Optical directional light reflection. brighteners are also used to increase The super-calendering process is the brightness and whiteness of often used off-machine for papers white papers. that already had been submitted to the coating process. It consists of Calendering in machine consists of running the paper between two reels running the dry paper onto a under high pressure. The process is frictional area between two steel similar to the one of calendering, reels under pressure. This process but it uses two types of reels, i.e., undergoes a plasticization and steel and a soft material, as for compression of the paper, increasing

2 example rubber. The purpose is to structure of the paper while those polish the paper surface and vehicles to be employed in significantly increase the sheet lithography require not formation of specular gloss. Image obtained in emulsion with water. Atomic Force Microscopy (AFM) of a coated paper surface after 2. Coating and surface chemistry calendering shows a flattening of superficial plateaus in the Coating used in paper surfaces is a topographical image (Fig.1). This material composed by a complex flattening reduces the diffuse mixture of components containing scattering of the light and increases colloidal particles and dissolved the specular reflection, thus polymers (Tab.1). A coating layer of increasing the gloss (Fardim, 1999). approximately 10 g/m2 is applied on 1.2 Printability: A Relevant quality the base paper in movement, parameter generally using a transfer reel. The excess is removed by a steel blade Printability is a parameter related and after coating transfer, the water with the level of paper quality when is removed by treatment with heat in it is printed. It is depend upon the the drying section. capacity of the paper sheet in "accept" the ink solid particles in a Compone Examples Function uniform way, without formation of nt films with irregular thickness and Pigments Kaolin, Support a also with good reproduction of Calcium structure of carbonate, fine porosity images that contain small tonality Titanium and form a variations. The printed dots must dioxide, surface clear and defined without Plastic which significant scattering and with pigments scatters the absence of ink migration during light. Adhesives Soluble in Bind the multi color printing. water (glues, pigment starches, particles Evaluation of printability is made gums, casein, between with specific tests, involving printing soy protein itself and the simulation in controlled conditions and others); coating Polymer layer onto and image analysis aiming at to emulsions the paper. quantify bleeding, dot gain, tonality, (latex, poly- Fulfill pores interface color-color, optical density, vinyl acetate, between the color scattering, printed area and acrylates) pigments. other parameters (Chen et al, 1995). Additives Polymers urea Make the Printing quality depends on paper for water formaldehyde coating less repellence sensitive to characteristics, printing process, and water. the printing ink composition. The Plasticizers Stearates, Improve the main printing processes in paper Paraffin flexibility of are: lithography, rotogravure, emulsions the coating flexography, typography, layer Thickeners Natural Control electrostatic, ink jet, and digital. polymers rheological The printing ink is composed Cellulose properties, basically by colored pigments derivatives viscosity, dispersed in a vehicle and both are and water specific for each printing process. retention. Dispersants Poliphosphate Optimize the For example, vehicles to be employed s, Ligno- pigment in rotogravure must have great sulphonates dispersion capacity of penetration in the Silicates

3 Preservativ Beta-naphthol Prevent Europe. Such pigments have es formaldehyde deterioration differences concerning between different morphological properties, chemical applications. composition, and surface charge. Anti-foams Long chain Control The latter plays an important role alcohols foam for colloidal stability and dispersion problems constancy. Pigment dispersion is the and base of coating preparation, being eliminate air bubbles the medium where the other Dyes Direct dyes, Use in formulation components are added Acid dyes, colored (Alince and Lepoutre, 1980). Colored coating Concentration of electrolytes and pH pigments formulations of the medium affect the dispersion, but specific characteristics of the Table 1 - Typical components of the coating color formulation for printing papers and pigment particles are also important paper-boards (Scott et al, 1995). (Sanders, 1992). The surface charge of calcium carbonate is During the coating color transfer, it negative and not dependent of pH, requires excellent properties of however this pigment can suffer fluidity, reached with low solids and decomposition in acidic media. The viscosity. However, to get a dry layer kaolin surface charge has different of uniform coating and with good polarities between the face and the printability characteristics, high lateral of the particle, being negative solids and viscosity are necessary in the face and positive in the (Van Gilder and Purfeerst, 1986), laterals. The charge of the laterals is which on the other hand, can cause dependent of pH, being positive in problems in the coating transfer acidic media and negative in such as deposits on the blade and alkaline media (Rand and Melton, coating marks (Reinbold and Ulrich, 1977). The pH at the iso-electric 1979). The needs are antagonic point is approximately 7.5 and in and require a balance condition values above of this pH, aiming at runnability and predominance of negative charge printability (Dahlvik, 2000). In occurs at the laterals (Diz and terms of surface chemistry, the Rand, 1989). In the acidic media properties required during the the particle deposition is favoured coating transfer are those of a due the electrostatic attraction system in colloidal stability, while between the positive laterals and the those excellent conditions of the negative faces (Dollimore and coating layer and printability are Horrige, 1973). The electrostatic observed when it has flocculation or attraction can be enough to exceed particle aggregation and formation the potential energy barrier, in of macro-flocks (Grön, 1998). The accordance with theory DLVO, knowledge of surface chemical discussed in the part I of this interactions is important to achieve revision. Presence of electrolytes in a balance between these trends. the medium also can favour the deposition due the compression of the double electric layer and 2.1 Pigments and its interaction reduction of the barrier of potential in the dispersion medium energy (Evans and Wennerström, 1999). In industrial applications, Kaolin and calcium carbonate are the pigment dispersions generally the pigments generally used in are kept in pH superior to 8.0 and a coating formulation in Brazil and polyelectrolyte is used as dispersion

4 agent (Dahlvik, 2000). The but it can be also used for polielectrolyte is adsorbed at the rotogravure and flexography. particle surface and functioning as a protective agent. The mechanism Binders are spherical particles with for stabilization of the dispersion diameter less than 1 µm and involves the increase in repulsion of different chemical compositions of the double electric layer, reduction the sphere nucleus. The of van der Waals attractive forces compositions can be of styrene- and effect of steric stabilization butadiene copolymer, styrene- (Shaw, 1980). Use of dispersion butylacrylate copolymer, cross- agents reduces the effect of the linked styrene-butadiene, and concentration of electrolytes and pH others. The components ratio in the of the medium in the dispersion, copolymer is generally 70 % of however, in extreme conditions the styrene and 30 % of butadiene or polyelectrolyte conformation is butylacrilate. The surface chemical modified, affecting the adsorption composition of the binder spheres is and favouring particle deposition also differentiated by the adsorption (Stenius et al, 1990). of acrylic acid or an anionic surfactant (Fig.2), which are used 2.2 Interactions between as stabilizing aids of the dispersion pigments, binder and co-binder in water (Joanicot et al, 1993).

The system of coating components is In ambient temperature, latex is an subject to interactions particle- amorphous and viscoelastic material particle, particle-solvent, and with a glass transition temperature particle-polymers. Different forces (Tg) between -7 and 40 °C, are involved in this system such as depending on component polymers. van der Waals attractive forces, During the drying of the coating, electrostatic repulsions, steric latex is melt producing a film that stabilization and also mechanical embraces pigment particles on the network (Whalen-Shaw, 1989; surface and thus forming the paper Fadat and Rigdahl, 1986). It is coating. Adhesion between latex important to stand out the and inorganic pigments are characteristics of adhesion and influenced by the coating drying cohesion between the mineral temperature, latex glass transition particles which form the coating temperature, and surface properties base structure. of latex and pigment. Studies using AFM to measure superficial Coating formulation uses adhesives adhesion (Granier et al, 1994) had in its composition (Tab.1) in a indicated that latex particles present proportion that can vary from 10-15 higher work of adhesion with parts per 100 pigment parts. The calcium carbonate than with mica function of adhesives is to promote and silicate plates. Latex presents the binding of pigment particles and acidic surface while calcium between these and the base paper carbonate alkaline surface, this surface (Kline, 1993), therefore is condition favours the adhesion also called binder. Styrene- between these particles. Silica has Butadiene (SB) latexes are an acid surface therefore the commonly used as coating binders observed adhesion is inferior to the in Brazil and Europe, especially for one with calcium carbonate. In the products destined to lithography case of kaolin, differences are printing due to the superficial observed between the crystal faces strength and printability required, and laterals. Faces have character

5 more acid than the laterals and the water molecules by hydrogen pH at the isoelectric point of the bonding involving the hydroxyl and kaolin surfaces has a value of 7.5 in carboxyl groups of the polymer comparison of 2.0 for silicate and chain (Dahlvik, 2000). 9.6 for calcium carbonate. A fraction of the restrained water is transferred to the paper surface during application, contributing for the adhesion between the base paper and coating layer (Young and Fu, 1991). However, if the amount of transferred water is very high, it can cause significant expansion of the fiber network or migration of latex particles and pigments (Malik and Kline, 1992, Engström and Morin, 1994). In cases where the water Figure 2. Binder spheres coated with acrylic acid (a) and anionic surfactant (b) transference is very low, problems in (Joanicot et al, 1993). the applied coating layer are observed (Barber, 1973). The effect Pigment particles are coated by a of the thickener on the formation of latex film when the temperature in a three-dimensional network in the drying conditions is higher than the dry coating layer is another latex Tg. Interactions between the important factor to be considered film and the particle are dependent (Grön and Kuni, 1995). Employ of on the pigment superficial groups anionic starch, for example, does and the amount of acidic groups on not contribute for formation of this the surface of the latex micro-sphere. kind of network, however, if cationic A poor adhesion is observed in starch is used instead, a high conditions of drying with interaction between this and the temperature lower than the latex Tg, pigment particles is observed and causing a partial surface coverage conditions for a three-dimensional of the pigment (Granier et al, 1994). aggregation are achieved (Grön and Beyond the pigment-binder Eklund, 1998). In case of CMC, a interactions there is a third partial adsorption onto pigment component which influences particles was considered (Wang et viscosity, water retention, the three- al, 1996), which contributes for dimensional arrangement and the network formation. Presence of consolidation of the coating layer, elastic flocks is also observed due to as well as the gloss and printed gloss interactions between adsorbed and (Grön et al, 1998) that it is the co- not adsorbed CMC (Whalen-Shaw binder or thickener (Tab.1). This and Gautam, 1990). These flocks component is generally a polymer are broken during the high shear soluble in water, for example conditions in coating application carboximetilcellulose (CMC), (Sandås and Salminen, 1991). cationic and anionic hydroxyetil Presence of high concentration of cellulose (EHEC), and starches Ca2+ ions harms the water retention (Eriksson and Rigdahl, 1994). The in coating colors which use CMC amount of thickener used in coating because this metal reduces the formulation varies from 0.1 up to number of carboxyl groups available 1.5 parts for 100 dry pigment parts. for the capture of water molecules Its effect on coating color viscosity is (Dahlvik et al, 1997). believed to be due to the capture of

6 2.3 Rheological properties of Fluid type Description Examples coating colors Pseudo Viscosity Inks, plastic decreases emulsions with and Coating colors are traditionally increasing dispersions studied using rheology techniques. in shear rate Rheology is the science of the flow Dilatant Viscosity De- and deformation of a substance that increases flocculated allows evaluate the performance of a with solids, starch increase in in water, coating color formulation during shear rate clay in application. A long range of shear water, sand rate must be tested for a complete in water evaluation and this constitutes an Plastic Fluid Tomato experimental difficulty, once that behaves as a ketchup solid under the current measurement devices are static suitable only for the extremes of low conditions, and high shear rates. The use of but if a force high-pressure capillary viscometers is applied a that employ extreme high shear flow is -1 induced (1000000 s ) was suggested (Gane which can et al, 1992) because coating is be either submitted to high shear rates in Newtonian, short intervals of time during the plastic, industrial application. Coatings are pseudo plastic or Non-Newtonian fluids (Krieger and dilatant Dougherty, 1957) or its viscosity Thixotropic Viscosity Greases, varies with the shear rate (Tab.2). decreases as printing inks Coating components are particles function of and coating not symmetrical that present size, time under colors constant form and cohesiveness as factors shear rate that affect the force necessary to put Rheopectic Viscosity Rarely found them into motion in a flow. These increases as not symmetrical particles are function of suspended colloidal and other such time under constant as kaolin, calcium carbonate, and shear rate titanium oxide. Non-Newtonian Table 2. Common categories of Non- fluids can be classified in different Newtonian fluids (Schramm, 1994). categories according to its response of shear stress as function of the Paper coating is a thixotropic fluid shear rate variation (Fig.3). and it must behave in a way that coating color flows only at the moment and at short instants after its application in conditions of high shear rates. The hysteresis behaviour of a coating color can be evaluated using a curve of shear stress versus shear rate (Fig.4). The flocculated structure of the fluid is destroyed as function of time during the shearing in a way that the viscosity is lower than the observed during the increase in shear rate. Flocks can reorganize again after the structure breakage and the time

7 needed to this restructure affects the irreversible flocculation of the return of viscosity to the previous material. In this condition the values. If the flock restructure is production of a not homogeneous achieved fast then viscosity keeps a fluid occurs, causing changes in the constant platform, but if the linkages rheological characteristics and are restored slowly then the viscosity significant viscosity reduction. is reduced. The reestablishment of the linkages depends on the 2.4 Coating and white pitch interface between the dispersed phase and the liquid phase and also "White pitch" is a deposit formed by on the shape of the particles in the coating components, generally dispersed phase. originated from coated broke in the paper machine. The deposit many times contains latex, antifoams, pigment particles, and resins. This deposit reduces the machine efficiency due to the impregnation in the felt and consequent time off for cleaning. It can also occur in the coating application devices and Figure 3. Illustration of the behaviour of cause coating marks or defects. different Non-Newtonian fluid categories as During printing of a coated paper, observed in plot a shear stress (•) as "white pitch" can cause low function of shear rate (•) (Schramm, 1994). resistance to wet and dry picking in off-set and thus many other printability problems.

Formation of "white pitch" involves flocculation of latex particles which can contain or not other process components. Cationic polyelectrolytes can be used to induce a flocculation of latex particles with pulp fibers and thus Figure 4. Thixotropic hysteresis behaviour hindering the deposit formation. of coating color observed in a plot of shear stress as function of shear rate (Fardim, The surface phenomena involved 1999). both in deposit formation and well as on its remedy using According to the curve of potential polyelectrolytes are similar as it was energy of DLVO theory, the barrier already discussed in the part I of this of free energy cannot be exceeded so review. that the system is remained in the dispersed form. However, a weak 3. Surface chemistry and flocculation that keeps the dispersion printability occurs at the region of the secondary minimum. The formed flock is Interactions between printing inks subject to the disruption due the and paper involve a great number of action of shear forces, modifying the surface phenomena. Printability structure of the fluid and affecting parameters are affected by the viscosity. Brownian movement of physical structure of the sheet particles can also generate collisions surface, as for example roughness that provoke surpass on the free and also by physicochemical energy barrier and cause the parameters such as work of

8 adhesion and surface free energy. cure dominates. This may have been Many other parameters such as due to the long time required for ink grammage, thickness, opacity, absorption and the consequent whiteness, brightness, formation and problems of solvent evaporation strength also affect printability. discharged to the atmosphere (Scott et al, 1995). Printing ink is composed by polymers, colored pigments and, vehicles which can be water, oil or organic solvent. The lithography printing process uses inks that contain oils as vehicle, while flexography can use water and oil. Organic solvents of low boiling points are used in rotogravure printing while digital photocopy processes do not use liquid vehicles. The ink jet printing generally uses a mixture of water and ethanol as a vehicle.

Interaction of the ink with the paper can be divided in three stages (Fig.5). In the initial stage (1) a fresh film of ink is placed on the sheet surface; in the intermediate stage there is an agglomeration of pigments to form a partially dry film and in the final stage there is a consolidation of a dry film with pigments agglomerated. Figure 5. Different stages of printing ink drying at the paper surface (Scott et al, 1995). Vehicle penetration into the paper Mechanisms of drying of printing (A), cure by oxidation of polymers which inks can be summarized in three aggregates the pigments (B), and vehicle main phenomena. Absorption of evaporation (C). ink is the phenomenon that occurs when the vehicle penetrates the Paper coating composition also structure of the paper (A) and affects the printing ink drying (Zang evaporation is the phenomenon that and Aspler, 1998). The adhesive occurs when the vehicle evaporates used in coating color can reduce the to the atmosphere, with or without penetration of water if its heating (C). Reactions of oxidation hydrophobic character or and polymerization also occur and concentration is increased. The the oxygen of air, a catalyzer, porous structure of coating also ultraviolet light or an electron beam affects the penetration (Donigian et can promote the cure of a polymer al, 1997). During printing, a part that assemble the pigments (B). In of the ink is immobilized on the the drying process the three surface by the paper roughness and phenomena can occur with in coated papers the roughness is predominance of one of them, limited, therefore the phenomena of however, the increase in the printing ink drying have important speed has required the use of inks performance. The penetration rate where the phenomenon of polymeric dh/dt of the vehicle in a cylindrical

9 pore of ray R is suggested by the adhesion. Such components can be equation of Lucas-Washburn: estimated using values of surface tension and measurements of dh/dt = Rγcosθ/4ηh [ 2.1 contact angle, with some ] approximations (Berg, 1993) for pure liquids. Nevertheless for multi Where h is the depth of penetration, t component systems such as printing is the time, γ is the liquid surface ink this estimative are complex. tension, θ is the contact angle Interactions between the printing between the liquid and the wall of ink and the paper surface are still the pore and η is the fluid viscosity not much investigated. Research (Aspler, 1993). The term γcosθ/η concerning the adhesion between the ink components and the paper presents good correlation with the surface has been started just for a ink jet printing quality using water few years (Wickman, 1998) using based ink. surface chemistry concepts. The Water penetration in uncoated development of new products and printing quality parameters are in papers depends if the product was many cases still performed using submitted to sizing or not. An inter- trial-and-error approach, frequently fiber absorption occurs initially being followed by the absorption of based in subjective methods. the fiber wall when the paper is not sized and the intra-fiber absorption 4. Surface engineering, coating is followed by the inter-fiber and printability absorption when the paper is sized. The printing ink penetration occurs Surface modification of the base in a homogeneous mixture between paper with purpose of increase the pigment and vehicle; however, when adhesion with the coating layer as the vehicle is oil based and contains well as the development of a dissolved polymer it remains on formulations of coating color and the paper surface (Aspler, 1993). printing inks can be improved with the application of the surface Adhesion between components of engineering concept. Better printing ink and paper surface adhesions between ink and paper depends on the work of adhesion can be achieved if thermodynamic which is related with dispersion and parameters are combined permanent dipole forces between the considering the different printing molecules at the interface. The and coating types. Problems usually currently used expression to describe observed such as spots, varnish the work of adhesion (Berg, 1993; rejection, formation of cut dust and Erbil, 1997) considers acid-base non uniform printability could be interactions and formation of eradicated in an objective and hydrogen bonding in a component lasting way. Catalyzers for printing AB ink drying could be incorporated in Wa and related interactions of physical character to the forces of the coating formulation, reducing dispersion in a component W LW. the amount of vehicle used in a printing and thus improving AB LW printability. There are many Wa = Wa + Wa [ 2.2] possibilities, including the formulation of "digital" coatings LW through the incorporation of The component Wa is termed Lifshitz-van der Waals and Wa is conductive polymers, already termed thermodynamic work of available, and then make possible

10 the long-distance printing by using adhesion and related phenomena. Nord. a micro-antenna on the paper Pulp Pap. Res. J., 8(1), 75. surface. Chen, C.Y., Pruett R.J. e Wygant, R.W. (1995): A review of techniques for 5. Conclusion characterizing paper coating surfaces, structures and printability. Proc. Tappi Concepts of surface chemistry are Coating Adv. Fund. Symp.,1. practically involved in all stages of Dahlvik, P. (2000): Thermal thickening of manufacture and printing of paper. paper coatings. Doctoral Thesis, Åbo Coating color formulation and its Akademi University, Turku/Åbo. performance also present important surface interactions. Coating Dahlvik, P., Ström, G. e Eklund, D. (1997): Variations in calcium ion concentration rheological properties which are and pH influencing coating colour important during application can be rheology, dewatering and immobilization. interpreted in some extent by the Nord. Pulp Pap. Res. J., 12(1), 61. DLVO theory. Coating structure and its effects on printability can also be Diz, H.M.M. e Rand, B. (1989): The variable nature of the isoelectric point of the interpreted considering edge surface of kaolinite. Br. Ceram. Trans. thermodynamic properties between J., 88, 162. mineral and adhesive particles. Paper printability can be Dollimore, D. e Horridge, T.A. (1973): investigated and be improved Dependence of flocculation behavior of china clay – polyacrylamide suspensions on according to a surface chemistry the suspensions pH. J. Coll. Interf. Sci., approach, though many researches 42(3), 581. are still based on subjective methods of evaluation. The concept of Donigian, D.W., Ishley, J.N. e Wise, K. surface engineering can also be (1997): Coating pore structure and offset applied in coating and printing ink printed gloss. Tappi J., 80(5), 163. formulations and thus makes Elftonson, J. e Ström, G. (1995): Penetration possible the development of paper of aqueous solution into models for coating with higher economic benefits. layers. Proc. Tappi Coating Adv. Fund. Symp., 17.

6. References Engström, G. e Morin, V. (1994): Quantitative description od the increase in Alince, B. e Lepoutre, P. (1980): Porosity surface roughness of the base paper during and optical properties of clay coatings. J. coating. Nord. Pulp Pap. Res. J., 9(2), 106. Coll. Interf. Sci., 76(2), 439. Erbil, H.Y. (1997): Calculation of spreading Allem, R. (1998): Characterization of paper pressure of water on cellulosic films from coatings by scanning electron microscopy contact angle data. Turkey J. Chem., 21, and image analysis. J. Pulp Pap. Sci., 332. 24(10), 329. Eriksson, U. e Rigdahl, M. (1994): Aspler, J. S. (1993): Interactions of ink and Dewatering of coating colors containing water with the paper surface in printing. CMC or starch. J. Pulp Pap Sci., 20(11), Nord. Pulp Pap. Res. J.,8(1), 68. 333.

Barber, E.J. (1973): Effect of hydrocolloids Evans, D. F. e Wennerström, H. (1999). The on coating color operability and coated colloidal domain, where physics, chemistry, paper properties. Tappi J., 56(1), 52. biology and technology meet. Willey, New York, p.426. Beghello, L. (1998): Some factors that influence fiber flocculation. Nord. Pulp Fadat, G. e Rigdahl, M. (1986): Interactions Pap. Res. J., 13(4), 274. in coating colors Part 2. Effects of ionic strength. Nord. Pulp Pap. Res. J., 1(4), 37. Berg J.C. (1993): The importance of acid base interactions in wetting, coating,

11 Fardim, P. (1999): “Surface chemical components on rheology, water retention composition of eucalyptus kraft pulp: and runnability. Proc. Tappi Coating Conf., Caharacterisation and influences on 133. strength and physicochemical properties of fibres”. PhD Thesis, Instituto de Química, Perry, S.S. e Somorjai, G. A. (1994): Universidade Estadual de Campinas, Characterization of organic surfaces. Anal. UNICAMP, Campinas. Chem., 63(7), 403.

Gane, P.A.C., Waters, P. e McGenity, P. Rand, B. e Melton, I. E. (1977): Particle (1992): Factors influencing the runnability interactions in aqueous kaolinite pf coating colors at high speed. Proc. Tappi suspensions 1. Effect of pH and electrolyte Coating Conf., 117. upon the mode of particle interaction in homoionic sodium kaolinite suspensions. J. Granier,V.V., Sartre, A., Joanicot M. M. Coll. Interf. Sci., 60(2), 308. (1994): Adhesion of latex particles on inorganic surfaces. Tappi J., 77(5), 419. Reinbold, I. e Ulrich, H. (1979): Possibilities and limitations of high solids coating Grön, J. e Eklund, D. (1998): Temperature colors. Proc. Tappi Coating Conf., 31. effect on rheological and dewatering properties for starch-based coating colors. Sanders, N.D. (1992): The effect of surface Nord. Pulp Pap. Res. J., 13(1), 10. modification of pigments on colloidal stability and structural performance. J. Pulp Grön, J. e Kuni, S. (1995): The impact of Pap. Sci., 18(5), 169. different coating pigments and molecular weight of carboxymethyl cellulose on Sandås, S. e Salminen, P. (1991): Pigment- coating structure. Proc. Tappi Coating cobinder interactions and their impact on Conf., 447. coating rheology, dewatering and performance. Tappi J., 74(12), 179. Grön, J. Dahlvik, P. e Ström, G. (1998): Influence of starch modification on the Schramm, G. (1994): A practical approach chemical composition and structure of to rheology and rheometry. Haake GmbH coated layers. Nord. Pulp Pap. Res. J., 13(2), Publication, Karlsruhe. 119. Scott, W.E., Abbott, J.C. e Trosset, S. (1995): Grön, J. (1998): Coating suspension Properties of paper: an introduction. Tappi structure and rheology. Doctoral Thesis, Press, Atlanta, p.161. Åbo Akademi University, Turku/Åbo. Shaw, D. J. (1980): Introduction to colloid Joanicot, M., Wong, K. e Cabane, B. (1993): and surface chemistry. Butterworth, Structure of latex films. Proc. Tappi Coating Londres, p. 208. Conf., 383. Stenius, P. Järnström, L e Rigdahl, M. Kline, J.E. (1993): Fundamental aspects of (1990): Aggregation in concentrated kaolin latex mobility in paper coatings. Proc. suspensions stabilized by polyacrylate. Coll. Tappi Adv. Coating Fund. Symp., 93. Surf., 51, 219.

Krieger,I.M. e Dougherty, T.J. (1957): A Ström, G.,Härdin, A-M e Salminen, P. mechanism for non-Newtonian flow in (1995): A novel approach for improving suspensions of rigid spheres. Trans. Soc. fibre coverage during blade coating. Nord. Rheol., 3,137. Pulp Pap. Res. J., 10(4), 227.

Lepoutre, P. and Lord, D. (1990): Swerin, A. (1995): Flocculation and fiber Desestabilized clay suspensions: flow curves network strength in papermaking and dry film properties. J. Coll. Interf. Sci., suspensions flocculated by retention aids 134(1), 66. systems. PhD Thesis, Royal Institute of Technology, Stockholm. Malik, J.S. e Kline, J.E. (1992): A study of the effect of water soluble polymers on water Van Gilder, R.L. e Purfeerst, R.D. (1986): holding and binder migration tendencies of The effect of coating color solids on coatings. Proc. Tappi Coating Conf., 105. properties and surface uniformity. Tappi J., 69(5), 119. McGenity, P.M., Gane, P.A.C., Husband, J.C. e Engley, M.S. (1992): Effect of Verhoeff, J., Hart, J.A. e Gallay, W. (1963): interactions between coating color Sizing and the mechanism of penetration of

12 water into paper. Pulp Pap. Mag. Can., 64, T509.

Wang, X. Q., Grön, J. e Eklund, D. (1996): Temperature dependence for adsorption of carboxymethyl cellulose on clay. Proc. Tappi Coating Conf., 79.

Whalen-Shaw, M. e Gautam, N. (1990): A model for colloidal and rheological characteristics of kaolin latex NaCMC formulations. Proc. Tappi Coating Conf., 371.

Whalen-Shaw, M. (1989): Coating structure – Part I: A mechanistic view to the development of wet coating structure. Proc. Tappi Coating Conf., 9.

Wickman, M. (1998): Surface chemical behavior of alkyd resins in offset printing inks. PhD Thesis. Royal Instute of Technology, Stockholm.

Young, T.S. e Fu, E. (1991): Associative behavior of cellulosic thickners and its implications on coating structure and rheology. Tappi J., 74(4), 197.

Zang, Y-H. e Aspler, J.S. (1998): The effect of surface binder content on print desnity and ink reciptivity of coated paper. J. Pulp Pap. Sci. 24(5), 141.

13