Surface Characteristics of Coated Paper Improved by Plastic Pigments

Ind. Eng. Chem. Res. 2005, 44, 9875-9883

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MATERIALS AND INTERFACES Surface Characteristics of Coated Paper Improved by Plastic Pigments and Synthetic Thickeners Samya El-Sherbiny*,† and Huining Xiao‡ Printing and Packaging Laboratory, Department of Chemistry, Faculty of Science, Helwan University, Cairo, Egypt, and Department of Chemical Engineering, University of New Brunswick, Fredericton, Canada E3B 5A3

Synthetic thickeners, either polyacrylate or associative types, have been extensively used in paper coating. However, systematic research on the effect of these thickeners on coated paper surface structure in the presence of plastic pigment is still inadequate. In the current work, we compared and contrasted several commercial plastic pigments (PP), focusing on two hollow sphere and two solid bead plastic pigments having various particle sizes. Two types of polymeric thickeners were also selected, and their influence on the roughness and surface structure of coated paper were investigated. The results confirmed that the incorporation of plastic pigment in the clay coating mixtures resulted in a substantial decrease in the roughness of coated paper. The effect was particularly pronounced with hollow sphere plastic pigments. The addition of polymeric thickeners further reduced the roughness of the coated paper, for both clay/PP and ground calcium carbonate/PP systems. In coating mixtures based on clay, it was found that hydrophobically modified alkali swellable emulsion (HASE) thickener has a stronger effect on reducing the roughness of clay-based coated paper than polyacrylate thickener. The trend is opposite in the case of a ground calcium carbonate (GCC) coating system. Introduction A pigment coating mixture may be considered to be a complex two-phase system consisting of solid pigment particles suspended in a fluid medium comprised of binders and soluble additives. During the coating process, the properties of a coating mixture may change with time, due to excessive dewatering or uncontrolled evaporation, which may be reflected in the behavior during the application of the mixture and in the properties of the coated layer.1-3 These changes are greatly influenced by the interactions between the pigment particles in the coating mixtures which, in turn, are mainly determined by the surface chemistry of the particles, their surface charge, the adsorption of different additives, and ion exchange equilibrium.4-6 The viscosity of the aqueous phase is known to be of considerable importance for the dewatering and immobilization of the coating layers and can be adjusted by the addition of either natural or synthetic watersoluble polymers with high molecular weights. Their effectiveness is judged by the amount of the polymer required to make the required change in viscosity. These thickeners provide a viscosity enhancing effect on the coating mainly by thickening the aqueous phase of the suspension, but some polymers also provide this effect by interacting with the other components in the mixture.7 * To whom correspondence should be addressed. E-mail: [email protected]. † Helwan University. ‡ University of New Brunswick.

Synthetic thickeners, either polyacrylate or associative thickeners, have found significant uses in the field of paper coating. Polyacrylates, in general, are alkalisoluble or swellable polymers consisting of acrylic acids or their salts, which provide the water affinity. Commercial products are usually copolymers containing acrylate (e.g., ethyl acrylate) and acrylic acid (or methacrylic acid).8 The associative thickeners consist of essentially hydrophilic, water-soluble polymers with strongly hydrophobic terminal groups or side chains (see Figure 1). The substitution of hydrophobic moieties on a water-soluble polymer gives rise to unique properties in aqueous media. That is, the dissolved polymer molecules become associated with one another in solution through hydrophobe-hydrophobe bonding. This type of intermolecular association can result in an increased hydrodynamic size for the polymer, thereby leading to an increased solution viscosity9,10 In our previous work, we have studied the adsorption of various synthetic thickeners on coating pigments (i.e., clay and ground calcium carbonate) and their influence on the surface and mechanical properties of coated paper.11,12 The results showed that the adsorption of selected thickeners on coating pigments, clay in particular, behaved differently, which in turn influenced the rheological properties of coating mixtures. High adsorption of thickener on pigments tended to increase the viscosity of coating slurries that led to high fiber coverage and a smooth coated paper surface. In our new study, we focus on the behavior of two synthetic thickeners showing extreme adsorption and viscosity properties with clay and ground calcium

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Figure 1. Molecular structure of model HASE associative polymers.

carbonate (GCC) coating systems in the presence of plastic pigment. Plastic pigments have been accepted as a partial replacement of inorganic pigments. Heiser13 first described the use of polymeric latexes based on styrene as gloss additives for coated paper in 1972. These synthetic polymers were used to replace some of the inorganic pigments. Hence, the term “plastic pigments” was coined to describe this class of pigments. Many researchers such as Alince,14 Lepoutre,15 Hagymassey,16 Brown,17 and others have reported on the use of these thermoplastic pigments, their contribution to optical properties, and their response to varied finishing conditions. There are two general classes of plastic pigments used in the preparation of coatings for paper and paperboard: solid bead and hollow sphere. Both are available in a variety of particle sizes and compositions and, in the case of hollow spheres, in a range of void volumes. The use of plastic pigment as a partial substitute for conventional inorganic pigment is gaining commercial acceptance, and such a practice literally necessitates the use of the scanning electron microscope (SEM) for characterization of papers incorporated with these submicrometer particles, which also is one of our aims in this study. Experimental Section 1. Materials. This study examined a simple coating suspension comprising only clay/plastic pigment or GCC/plastic pigment, binder, and thickener using commercial grades. The four types of plastic pigment used, two hollow sphere and two solid bead types, are detailed in Table 1. The binder used was a copolymer based on n-butylacrylate, styrene, and acrylonitrile, Acronal S 360 D, supplied by BASF Germany. Two types of polymeric thickeners were selected from eight different ones. The selection was based on our previous work presented elsewhere.11,12 T1 is a copolymer of acrylic and acrylamide water in an oil emulsion, and T2 is a hydrophobically modified alkali swellable emulsion (HASE). Sodium polyacrylate (Polysalz S), supplied by BASF Germany, was used as a dispersant for the clay pigment. The amount of dispersant used was 0.25 parts of sodium polyacrylate per 100 parts (pph) of clay. Ground calcium carbonate, hydrocarb 90-ME 78%, predispersed slurry was supplied by OMYA Croxton +

Garry. The clay pigment SPS was obtained from Imerys U.K. More than 80% of the clay particles are smaller than 2.0 µm in diameter with a mean specific area of 10.5 m2/g. 2. Preparation of Coatings. The base pigment system consisted of an 80/20 ratio of clay or GCC to plastic pigment. The binder was used at 10 pph. The clay was dispersed in distilled water in the presence of 0.25 pph sodium polyacrylate at a solids content of 65% and a pH of 8.5. The solids content was adjusted to 50% for clay-based coating mixtures and 55% for GCC-based coating mixtures. The pH was adjusted to 8 using a 1 M NaOH solution. Various amounts of the selected thickeners (T1 and T2) were added to both clay/plastic pigment and GCC/plastic pigment systems. Additions of the thickeners were made by parts per hundred parts of pigments (pph), and then, the suspensions were stirred for another 5 min. 3. Preparation of the Samples. A laboratory blade coater was used for preparing the coated paper samples by coating pure chemical pulp paper. The average coat weight for all samples was approximately 8.0 m2/g. The coated paper was supercalendered at a temperature of 50 °C and a pressure of 87.5 kN/m in two nips using the pilot supercalender facility at the Department of Paper Science, UMIST, U.K. The samples for surface studies were supercalenered, but the samples for cross section were uncalendered. The coated samples were dried and stored at 23 °C and 50% relative humidity before SEM examination and roughness measurements were performed. 4. Surface Structural Analysis of Coatings Using a Scanning Electron Microscope. A TOPCON SM300 scanning electron microscope (SEM) was used to reveal the surface morphology of the coated samples. Coated samples of uniform and homogeneous surfaces were chosen and cut into specimens each 15 mm in diameter. All samples were coated with gold using the SEM coating unit E 5000. The SEM images were taken under the same conditionssthe acceleration voltage of the electron beam was 15 kV, and the time taken for a photograph was 60 s. 5. Characterization of Surface Roughness. A Parker Print-Surf (PPS) roughness tester (model ME 90) was used to obtain the S-10 values in accordance with Tappi T480. The measurements were carried out by clamping the paper sample against a standard soft black rubber backing under a pressure of 980 kN/m. The mean gaps between the sheets of paper and the flat rubber were measured, and the results of the test were expressed in micrometers. Results and Discussion 1. Effect of Plastic Pigment on the Surface Properties of Pigment-Coated Paper. The use of plastic pigment to improve the coated paper gloss and smoothness has been established.13-17 In the current work, we compared and contrasted several commercial plastic pigments which have been used in the paper coating industry, focusing on two hollow sphere and two solid bead plastic pigments having various particle sizes. The results shown in Table 2 indicate that the addition of plastic pigments, both hollow sphere and solid bead types, to clay and GCC coating systems causes a pronounced decrease in the coated paper roughness. The standard deviation of the roughness was about 2% based on mean values (e.g., 1.68 ( 0.03 for clay without plastic

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Figure 2. Scanning electron micrograph of clay-coated paper without thickeners and supercalendered at 87.5 kN/m and 50 °C. Table 1. Plastic Pigment Types and Properties sample name

morphology

commercial name

particle size (µm)

supplier

PP1 PP2 PP3 PP4

hollow sphere hollow sphere solid bead solid bead

Ropaque BC-643 Ropaque Bright DPP 3740 DPP 3710

0.6 1.3 and 0.2, bimodal 0.45 0.17

Rohm and Hass Rohm and Hass Dow Chemical Dow Chemical

Table 2. Roughness (µm) of Coated Paper Having Various Types of Plastic Pigment for Both Clay and GCC Mixtures with plastic pigment (20%) clay roughness (µm) percentage decrease in roughness (%) GCC roughness (µm) percentage decrease in roughness (%)

without plastic pigment

PP4

PP3

PP2

PP1

1.68 ( 0.03 0 1.75 ( 0.035 0

1.51 ( 0.03 -10.1 1.49 ( 0.02 -15

1.49 ( 0.03 -11.3 1.55 ( 0.03 -11.4

1.39 ( 0.02 -17.3 1.52 ( 0.03 -13.1

1.46 ( 0.02 -13.1 1.49 ( 0.03 -15

pigment in Table 2). When a coating mixture is applied to the paper, the dewatering or water penetration into the base paper induces the immobilization of the coating layer. The coating layer is further immobilized during the subsequent drying process, leading to shrinkage of the structure.3,18 When a stiff and spherical particle, however, is incorporated in the coating mixture, the shrinkage of the coating layer during consolidation is counteracted. Apparently, plastic pigments act as spacers between the clay plates or the GCC particles and decrease the overall packing density (i.e., produce coated paper with loose packing), leading to high fiber coverage and a smooth surface of the coated paper.19,20 In contrast to conventional mineral pigments, the plastic pigments tend to be located on the surface due to their low density. As a result, the homogeneous arrangement of pigments on the surface significantly reduced the microroughness of the coated paper A. In Clay-Based Coating. Figure 2 presents the SEM images of the surface of the papers coated with clay mixed with four different types of plastic pigment (two hollow sphere and two solid bead). The figure illustrates that increasing the particle sizes of the plastic pigment leads to a decrease in the coated paper roughness for both hollow sphere and solid bead types. The greatest effect is observed when using the hollow

spherical ones. The thermoplastic property of this plastic pigment allows an even better response to the calendaring, producing a very smooth surface. It is shown in Figure 2a and b that a number of particles on the surface of the hollow sphere pigment are softened and flattened out under calendering, particularly those of relatively large size (i.e., the particles of 0.6 µm in PP1 and the particles of 1.3 µm in PP2). A reasonable explanation for their sensible response to supercalendering is that their hollow centers render them more easily deformed under calendering.21 This deformability leads to an improvement in the microsmoothness of the sheet. PP2 (Figure 2b) showed the lowest degree of coated paper roughness, and the decrease reached 17.3%. As this pigment consists of a mixture of two particle sizes (1.3 and 0.2 µm bimodal), some of the larger pigment flattens on the surface while the other small particles (0.2 µm) are distributed on the surface, filling the microvoids of the surface and creating a coated paper with high and homogeneous fiber coverage. In addition, larger particles can protrude above the uncalendered coating surface, improving the contact with the supercalender. A solid bead pigment of 0.45 µm (PP3) produces a smooth coated paper near to PP1 (hollow sphere), as

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Figure 3. Cross-sectional scanning electron micrograph of paper coated with clay/PP (without thickener): magnification ) 10 000×.

shown in Figure 2c. Although the response is limited, compared with the hollow sphere ones, the solid bead particles have sufficiently high concentration on the surface to fill in the microroughness on the surface. The SEM image in Figure 2d reveals distinct nonuniformity in the surface structure of the coating mixture containing PP4, compared to the other types of plastic pigment. This mixture produces a rough surface with an irregular orientation of clay platelets and could not completely cover the underling fibers. The decrease in roughness only reached about 10.1%. Parts a-d of Figure 3 show the cross-sectional SEM micrographs of uncalendered clay-coated papers having PP1, PP2, PP3, and PP4, respectively. It is shown that the particle integrity and void network extended uniformly throughout the entire coating. This void network is truly three-dimensional, interconnecting not just in the Z direction but in the machine and cross machine directions as well, thus providing a loose coating layer packing. All the particles below the surface of the coating are protected by the surrounding mineral pigment particles. This effect is enhanced as the particles sizes increase, as shown in Figure 3b (PP2). Parts c and d of Figure 3 show a dense and compact coated paper and reveal the higher roughness of the solid bead pigment than that of the hollow sphere type. B. In GCC-Based Coating. The SEM images of the surface of the papers coated with GCC/PP are shown in Figure 4. The results presented in Table 2 along with the SEM images indicate the significantly different influence of plastic pigment on GCC pigment, compared with the clay/PP system. Although PP2 shows the lowest roughness value in the case of clay pigment (Figure 2b), PP1 and PP4 show the highest decrease in coated paper roughness, and the decrease reached about 15%, compared to 13.1 and 11.4% for PP2 and PP3, respectively. As shown in Figure 4b, the large particles flatten on the surface but do not reach to the valley of the GCC base, leaving some roughness on the surface. Clearly, the GCC coating system does not give a bulky coating

layer as in the case of clay (i.e., provided dense packing is present), as shown in the cross section micrographs (Figure 5b). As a result, the larger particles can protrude far above the uncalendered coating surface (higher hill) during the supercalendering. The particles on the surface (the hills) flatten but the valleys remain relatively unaffected. The cross-sectional images (Figure 5a) explicitly show that PP1 creates a bulky coated paper that has high fiber coverage. The image also reveals a cushion effect under supercalendering, which may explain the reason this type of pigment leads to an improved smoothness. Despite the fact that clay coating has a greater degree of shrinkage compared to GCC, it is shown that GCCbased coated paper has a higher surface roughness than the clay-based coated paper. This difference in roughness can be attributed to the clay platelets having a tendency to flatten and align on the paper surface, providing a smoother surface.22 2. Synergetic Effect of Polymeric Thickener and Plastic Pigment on the Surface Properties of Pigment-Coated Paper. Figure 6 illustrates the effect of the polymeric thickener T1 on coated paper roughness using a clay/plastic pigment mixture. The percentage decreases in coated paper roughness for both of the thickeners T1 and T2 are further detailed in Tables 3 and 4. The standard deviation for all samples was within 2% relative to the mean values. It is shown that an increase in the addition level of both thickeners 1 and 2 led to a decrease in the coated paper roughness for all types of plastic pigment. The highest effect was observed for the clay/PP2 system. The presence of thickener in coating mixtures apparently increases their viscosities, depending on the type of thickener. The pronounced thickening effect of thickeners is the result not only because of their interaction with pigment particles but also due to their larger hydrodynamic size causing more coating flocculation. The flocculation effect is particularly profound with polymers that strongly adsorb onto pigments. A higher degree of flocculation


Figure 4. Scanning electron micrograph of paper coated with GCC/PP (without thickener) and supercalendered at 87.5 kN/m and 50 °C.

Figure 5. Cross-sectional scanning electron micrograph of paper coated with GCC/PP (without thickener): magnification ) 5000×.

often results in a stronger coating structure and a higher coating viscosity at low shear rates, leading to higher fiber coverage and creating a smooth and uniform coated paper surface.8 Additionally, a flocculated state of suspension effectively limits the mobility of particles during consolidation. Shear rates are relatively low during drying and are not expected to break-up the flocculated structures. Flocculation induces the formation of flocs consisting of large entities made up of several particles. The large entities have very limited mobility in the coating layer. Comparing between the

two thickeners, thickener 2 has a higher effect in reducing the coated paper roughness. In our previous work,11,12 it has been found that thickener 2 (HASE) has a higher affinity toward clay pigment and more substantially increases the viscosity of the clay coating mixture than thickener 1 (polyacrylate). The higher thickening efficiency of thickener 2 is achieved by the self-associating properties of the attached hydrophobic groups, yielding a network structure that enhances the viscosity of the formulation and decreases the dewatering velocity of the coating mixture. Dewatering velocity

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Figure 6. Roughness (µm) of paper coated with a clay/PP coating mixture having various levels of thickener 1. *PP1 and PP2 are hollow sphere pigments. **PP3 and PP4 are solid bead pigments. Table 3. Percentage Changes of Clay/PP-Coated Paper Roughness Using Thickener 1 coating mixture

thickener concentrations (pph of dry pigment) 0.0 0.3 0.5 1.0 1.5 2.0

clay/PP1 clay/PP2 clay/PP3 clay/PP4

0.0 0.0 0.0 0.0

-5.5 -12.2 -6.7 -6.0

-17.8 -15.1 -12.8 -12.6

-19.9 -24.6 -13.4 -12.6

-24.7 -28.9 -19.5 -11.9

-25.3 -31.7 -18.8 -15.2

Table 4. Percentage Changes of Clay/PP-Coated Paper Roughness Using Thickener 2 coating mixture

thickener concentrations (pph of dry pigment) 0.0 0.3 0.5 1.0 1.5 2.0

clay/PP1 clay/PP2 clay/PP3 clay/PP4

0.0 0.0 0.0 0.0

-17.1 -13.7 -12.1 -17.2

-18.5 -20.3 -15.4 -17.9

-20.5 -26.8 -16.1 -17.9

-28.1 -30.9 -22.1 -21.2

-28.8 -34.0 -24.2 -27.2

is considered to be one of the most important factors controlling structure formation. The presence of thick-

ener slows down the dewatering rate, leaving more time for the pigment particles to pack efficiently. Consequently, the pigment platelets have a chance to align themselves parallel to the paper surface, creating a smooth coating.22 It should be noted that, for clay-based coatings, the effect of the thickener depends strongly on the type of plastic pigment used. For instance, for clay/PP2, increasing the thickener level up to 2 pph leads to a continuous reduction in the PPS roughness. However, for other clay/PP combinations, the effect of the thickener level on the PPS roughness is rather small after 1 pph. Figures 7-10 are the SEM micrographs of paper coated with clay/plastic pigment mixtures containing 0.2 pph of thickeners 1 and 2, respectively. The images show that the addition of thickener led to a significant decrease in coated paper roughness compared to that of coated paper without thickener (Figure 2). In the presence of the thickeners, the clay platelets tend to orient themselves horizontal and parallel to the paper surface. The plastic pigment on the surface is arranged uniformly; the hollow spherical ones flatten more significantly in the absence of thickener. Comparing the two types of thickeners, it was found that thickener 2 gave a very smooth surface due to its strong interaction with the clay pigment and also facilitated the flattening of plastic pigments. This effect is particularly evident in the case of PP2. Figure 11 explicates the effect of thickener T1 on the paper roughness for GCC-based systems. The percentage decreases in coated paper roughness for both thickeners T1 and T2 are also detailed in Tables 5 and 6. The standard deviation in roughness for GCC-coated samples is similar to that for clay-coated paper. Clearly, there is a significant difference between these two

Figure 7. Scanning electron micrograph of paper coated with a sample clay/PP1 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ≈ 10 000×.




Figure 10. Scanning electron micrograph of paper coated with a sample clay/PP4 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ≈ 10 000×. Table 6. Percentage Changes of GCC/PP-Coated Paper Roughness Using Thickener 2 coating mixture GCC/PP1 GCC/PP2 GCC/PP3 GCC/PP4

Figure 11. Roughness (µm) of paper coated with a GCC/PP coating mixture having various levels of thickener 1. Table 5. Percentage Changes of GCC/PP-Coated Paper Roughness Using Thickener 1 coating mixture GCC/PP1 GCC/PP2 GCC/PP3 GCC/PP4

thickener concentrations (pph of dry pigment) 0.0 0.3 0.5 1.0 1.5 2.0 0.0 0.0 0.0 0.0

-6.0 -10.5 -5.2 -10.7

-12.1 -13.8 -6.5 -15.4

-12.8 -15.8 -7.1 -16.1

-14.1 -18.4 -14.2 -18.8

-14.8 -19.1 -18.7 -18.1

thickeners; the trend is opposite to that obtained with clay-coated paper. Thickener 1 decreases the coated paper roughness for all types of plastic pigment, but thickener 2 has no effect on the coating mixture containing PP1 and PP3. With PP2 and PP4, the roughness started to decrease only at the addition levels of 1.5 and 2 pph. This may be due to the low adsorption of thickener 2 onto GCC and its lesser impact on rheological properties, which has been found in our previous work.11 The SEM images shown in Figures 12-15 present the comparison between the effect of the two selected

thickener concentrations (pph of dry pigment) 0.0 0.3 0.5 1.0 1.5 2.0 0.0 0.0 0.0 0.0

0.0 -3.3 0.6 0.0

0.7 -3.3 -0.6 -1.3

0.0 -5.9 -1.9 -0.7

1.3 -8.6 -3.2 -9.4

0.7 -9.2 -3.9 -13.4

thickeners in GCC/PP coating systems. The images reveal that a smooth and uniform surface is observed for the systems consisting of thickener 1 and plastic pigment, regardless of the type of plastic pigment. The most significant effect was obtained with PP2 and PP4; the percentage decrease in coated paper roughness reached about 19 and 18%, respectively. Overall, the data presented in Tables 3-6 indicate that the percentage decreases in paper roughness for the clay-based coating system are higher than those for the GCC-based system when either thickener 1 or thickener 2 is used. It has been reported23 that clay pigment has a higher interaction and adsorption capability, compared with GCC. This leads to a strong network structure within the coating mixture, increasing the fiber coverage. Conclusions The results of this study led to the following conclusions: (i) The addition of plastic pigments, both hollow sphere and solid bead, to clay and GCC coating systems has a pronounced synergistic effect in decreasing the coated paper roughness. However, the synergy depends on the type of mineral pigments. (ii) In clay coating systems, increasing the particle sizes of the plastic pigment led to a decrease in the coated paper roughness for both hollow sphere and solid bead. The strongest

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Figure 12. Scanning electron micrograph of paper coated with a sample GCC/PP1 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ) 10 000×.

Figure 13. Scanning electron micrograph of paper coated with a sample GCC/PP 2 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ) 10 000×.

Figure 14. Scanning electron micrograph of paper coated with a sample GCC/PP 3 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ) 10 000×.

Figure 15. Scanning electron micrograph of paper coated with a sample GCC/PP4 pigment supercalendered at 87.5 kN/m and 50 °C with 0.2 pph of thickener 1 (a) or 2 (b): magnification ) 10 000×.

effect was obtained when using the hollow spherical ones. Among the selected plastic pigments, PP2 (hollow sphere pigment) created a coated paper of the lowest degree of roughness due to its unique bimodal structure. (iii) In GCC-based coatings, a different influence of

plastic pigment was obtained compared to the case of the clay/PP system. Although PP2 generated the lowest roughness value in the case of the clay pigment, PP1 (hollow sphere pigment) and PP 4 (solid bead pigment) produced the highest decrease in the coated paper


roughness in the GCC-based system. (iv) The surface structure and roughness of the coated paper were significantly influenced by the presence of synthetic polymer thickeners, for both clay/PP and GCC/PP systems. The strongest synergistic effect was observed for the clay/PP2 system. (v) Thickener 2 (HASE) has a stronger effect on reducing the coated paper roughness due to its higher affinity toward clay pigment, compared with thickener 1 (polyacrylate). However, in GCC coating systems the trend is opposite. Acknowledgment The authors wish to thank the World Laboratory Organization in Switzerland for part of the financial support for this work; the Paper Science Department at UMIST in Manchester, U.K., for use of the facilities; and Rohm & Hass and Dow Chemicals for providing the thickeners. Literature Cited (1) Rutanen, A. Optimisation of Rheology and Water Retention Using Carboxymethyl Cellulose (CMC). Proceeding of PITA Coating Conference, Edinburgh, 2001; 99. (2) Chonde, Y.; Roper, J.; Salminen, P. A Review Of Wet Coating Structure: Pigment/Latex/Cobinder Interaction And Its Impact On Rheology And Runnability. Advanced Coating Fundamentals Symposium Proceedings; Tappi Press: Atlanta, GA, 1995; p 57. (3) Dahlvik, P.; Lohmander, S.; Lason, L.; Rigdahl, M. Relations between the Rheological Properties of Coating Colour and their Performance in Pilot-Scale Coating. Nord. Pulp Pap. Res. J. 2000, 15, 106. (4) Jarnstrom, L.; Rigdahl, M. Modified Starches in Coating Colours. Nord. Pulp Pap. Res. J. 1995, 10, 183. (5) Wang, X. Q.; Gron, J.; Eklund, D. Adsorption Of Modified Starches On Clay And Its Effect On Wet Coating Structure. Nord. Pulp Pap. Res. J. 1996, 11, 137. (6) Husband, J. C. The Adsorption Of Coating Starch Derivatives Onto Kaolin. Advanced Coating Fundamentals Symposium; Tappi Press: Atlanta, GA, 1999; p 1. (7) Hermansson, E.; Dahlvik, P. 1998 coating/papermakers conference; Tappi Press: Atlanta, GA, 1998; p 109. (8) Young, T.; Burdick, C. L. In Paper Coating Additives; Kane, R. J., Ed.; Tappi Press: Atlanta, GA, 1995.

(9) Hanciogullari, H. Pigment coating and surface sizing of paper. In Papermaking Science and Technology; Gullichsen, J., Paulapuro, H., Eds.; Fapet Oy: Helsinki, Finland, 2000; Vol. 11. (10) Fadat G. The Influence of Associative Rheology Modifiers on Paper Coating. Nord. Pulp Pap. Res. J. 1993, 8, 191. (11) El-Sherbiny S.; Xiao, H. Effect of Polymeric Thickeners On Pigment Coatings: Adsorption, Rheological Behaviour And Surface Structures. J. Mater. Sci. 2004, 39, 4487. (12) El-Sherbiny S.; Xiao, H. Characteristics of Clay and GCC Pigment Coatings Containing synthetic Polymeric Thickeners. Appita J. 2005, 58, 119. (13) Heiser, E. J.; Shad, A. Lightweight Polymeric PigmentProperties of Pigments. Tappi J. 1973, 56, 70. (14) Alince, B.; Lepoutre, P. Plastic Pigment in Paper CoatingsThe Effect of Particle Size on Porosity and Optical Properties. Tappi J. 1980, 63, 49. (15) Lepoutre, P.; Alince, B. Effect of Posttreatment on the Optical Properties of Plastic Pigment Coatings. Tappi J. 1981, 64, 67. (16) Hagymassey, J.; Haynes, J. U. Comparison of all Clay Mineral Pigment Systems to Clay/Plastic Pigment Combinations under Varying Finishing Conditions. Tappi J. 1977, 60, 126. (17) Brown, J. T. The Relationship between Geometry and Optical Performance of Finished Paper Coatings. TAPPI 1991 Coating Conference Proceedings; Tappi Press: Atlanta, GA, 1991; p 113. (18) Whalen-Shaw, M., Ed. Binder Migration in Paper and Paperboard Coatings; Tappi Press: Atlanta, GA, 1993. (19) Camilla, R.; Eriksson, U.; Rigdahl, M. Consolidation Behavior and Gloss of Paper Coatings Based on Plastic Pigments. Nord. Pulp Pap. Res. J. 1994, 9, 254. (20) Hoshino, F.; Fukaya, S.; Yanagihara, T. Study on Sheet Gloss Development by Addition of Polystyrene Latex Particles. Proc. Adv. Coating Fundam. Symp. Tappi Press: Atlanta, GA, 1993; p 81. (21) McDonald, C.; Devon, M. Hollow latex particles: synthesis and applications. Adv. Colloid Interface Sci. 2002, 99, 181. (22) Stanislawska, A.; Lepoutre P. Consolidation of Pigmented Coatings: Development of Porous Structure. Coating Conference; Tappi Press: Atlanta, GA, 1995; p 67. (23) Jarnstrom, L. The Polyacrylate Demand In Suspension Containing Ground Calcium Carbonate. Nord. Pulp Pap. Res. J. 1993, 8, 27.

Received for review January 17, 2005 Revised manuscript received July 4, 2005 Accepted October 1, 2005 IE050061B

Surface Characteristics of Coated Paper Improved by Plastic Pigments

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