Article pubs.acs.org/IECR
Role of Cellulose Nanofibrils in Structure Formation of Pigment Coating Layers Kyudeok Oh,†,‡ Jee-Hong Lee,† Wanhee Im,† Araz Rajabi Abhari,† and Hak Lae Lee*,†,‡ †
Department of Forest Sciences, College of Agriculture and Life Sciences, Seoul National University, Seoul 151-921, Korea Research Institute of Agriculture and Life Sciences, Seoul 151-921, Korea
‡
S Supporting Information *
ABSTRACT: The dried structure of paints and coatings are important to understand. In paper coatings, pigments and binder types are known to influence this structure. The influence of a new additive such as cellulose nanofibrils (CNF) on the interaction between the coating components is not thoroughly examined. In this study, the effect of CNF on the rheology of coating color and structure formation of the coating layer was investigated and compared to that of carboxymethyl cellulose (CMC). The addition of these two rheological modifiers made the dried coating layer porous, but the working mechanisms associated with these two additives were different. CMC, which flocculated coating components, limited the rearrangement of the components, resulting in a loosely packed coating structure in wet state. CNF, which did not significantly influence the interactions between the coating components, increased effective volume fraction by absorbing water. The water absorbing characteristics of CNF expanded the dried coating structure.
1. INTRODUCTION Suspensions composed of particles and polymeric binders are widely used in various industries to manufacture diverse products such as papers, paints, and batteries. In the case of the paper coating industry, a suspension composed of a pigment and binder, commonly called the coating color, is applied to the surface of dried base paper to form a coating layer. The structure of the coating layer has been recognized as one of the most important factors affecting the quality of coated paper such as printability. The types or composition of pigments and the binders used are main factors in determining the structure of the coating layer because these are the main ingredients of the coating suspension. When the pigment volume concentration is higher than the critical pigment volume concentration, void spaces are present for light scattering. Also, a reduction of binder faction increases the void spaces because the binder fills up the interparticular voids.1−3 The size, size distribution, and shape of the pigments influence the structure of the coating layer because these properties affect pigment packing in the coating layer.3−7 The film-forming behavior of the latex binder varies by the type and glass transition temperature of the latex, which in turn changes the structure of the coating layer.7−9 Even though pigments and binders are the major components of the coating formulation, some additives have a critical effect on coating runnability and coated paper properties. Thickener is a minor but essential component of the coating color because it controls the rheological properties and water retention of the coating color, which are two critical properties that control coating runnability. Several studies have © XXXX American Chemical Society
reported the effect of carboxymethyl cellulose (CMC), poly(vinyl alcohol), and hydroxyethyl cellulose on the rheological properties of the coating color.10−13 These conventional rheology modifiers are water-soluble polymers that increase the viscosity of the aqueous phase. It is also known that some thickeners change the coating structure because they can change the colloidal interactions of the coating components. Recently, there have been attempts to use cellulose nanofibrils (CNF) as a thickener in pigment coatings because of their gel-like and shear-thinning behaviors.14,15 Several studies reported the rheological properties of coating colors containing CNF.14−19 The viscosity and viscoelastic behaviors of the coating color prepared with CNF as an additive were analyzed under various evaluation conditions to predict spreading, leveling, and dewatering of the coating color.14,17,18 Coating color containing CNF has shown low elasticity and slow viscosity recovery after coating application compared to the coating color containing CMC, and this has been attributed to the much lower flocculation between the particles in CNF coating. Thus, the CNF coating color spreads easily, which increases the gloss of the coated paper.17 CNF is different from CMC because CNF is not a watersoluble additive. Most previous studies on CNF have examined its effect on the rheological properties of the coating color. It Received: Revised: Accepted: Published: A
July 5, 2017 August 1, 2017 August 7, 2017 August 7, 2017 DOI: 10.1021/acs.iecr.7b02750 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research has been shown that the information obtained with the coating suspension cannot always be used to predict the properties of the dry coatings.20 This suggests that a systematic study to correlate the suspension property with the dry coating property is necessary. Thus, there is a clear need to study the effect of CNF on the structure of coating layers and to correlate it with the suspension property. The principal aim of this study was to understand the effect of CNF on rheology and drying kinetics such as particle diffusion and structure formation. The microstructure of a coating color containing CNF was evaluated using a rotational rheometer to determine the effect of CNF on the suspension property. Drying kinetics of the coating layer were investigated using multispeckle diffusing-wave spectroscopy (MS-DWS), which can evaluate the Brownian motion of particles in suspension, thereby deriving structural changes of the coating color both in suspension and in drying states. The pore structure of the dried coating layer was also analyzed. Finally, a mechanism of structure formation by CNF is suggested.
Figure 1. TEM images of cellulose nanofibrils (the solids content of CNF for TEM image was 0.001%).
Table 1. Formulation of the Coating Colorsa
pigment latex CMC CNF
2. EXPERIMENTS 2.1. Materials. Ground calcium carbonate (GCC, Setacarb 77K, Omya Korea) in slurry form was used as a coating pigment, which was dispersed in production using polyacrylatebased anionic colloidal stabilizer. The particle size of the pigment was evaluated using a light diffraction spectroscope (Mastersizer 2000, Malvern Instruments, UK). The median size of the GCC was 0.78 μm (50 vol %), and 98 vol % of the particles were below 2 μm. Styrene−butadiene (S/B) latex provided by LG Chem. was used as a coating binder. The size, glass transition temperature, and gel content of the S/B latex were 123 nm, −6 °C, and 82%, respectively. CMC and CNF were used as additives for the coating color. The average molecular weight, the degree of substitution, and charge density of CMC were 45 000 g/mol, 0.78, and −11.3 mequiv/g (pH 7), respectively. CNF was prepared by grinding bleached eucalyptus kraft pulp with a grinder (Super Masscolloider, Masuko Co.). Grinding is the most common method for the preparation of CNF. Before grinding, the pulp was pretreated to 450 mL CSF (Canadian Standard Freeness) using a laboratory Valley beater. The average size of the CNF agglomerates determined at 0.1 wt % was about 3.8 μm, and the zeta potential of CNF was −38.9 mV at pH 9. Charge density of CNF was −0.122 mequiv/g at pH 7. The water retention value of CNF was measured in accordance with ISO/ DIS 23714. CNF gel that was formed by filtration was centrifuged at 900g for 30 min. The weight of the centrifuged CNF gel was measured before and after drying at 105 °C. The weight change was converted to the water retention value of CNF. As seen in Figure 1, CNF was highly agglomerated because of its high length-to-diameter ratio. 2.2. Formulation of the Coating Colors. Formulations of the coating colors used in this study are shown in Table 1. Total solids content of the coating colors was 64 wt %. Coating colors were prepared by mixing S/B latex with pigment slurries and then CMC or CNF was added and thoroughly mixed. When both CNF and CMC were used as additives, CNF was added after the addition of CMC. Mixing was performed for 30 min to obtain complete mixing of all ingredients. The gel structure of CNF disappeared after 5 min of mixing. Finally, the coating color was filtered using a 100-mesh wire, and then, the pH was adjusted to 9.3 with 0.1 N NaOH solution.
reference
CMC 0.1
CMC 0.3
CMC 0.5
CNF 0.2CMC 0.1
CNF 0.4CMC 0.1
100 12 0 0
100 12 0.1 0
100 12 0.3 0
100 12 0.5 0
100 12 0.1 0.2
100 12 0.1 0.4
a
Component amounts are given in parts per hundred (pph; by weight) based on 100 parts of pigment.
2.3. Sedimentation of the Coating Colors. A 10 g portion of the coating color was transferred into a Falcon tube and centrifuged at 3 000g for 90 min for complete sedimentation. The level of sediment was recorded after centrifugal sedimentation. 2.4. Dewatering and Rheological Properties of the Coating Colors. An Åbo Akademi Gravimetric water retention device (ÅA-GWR) was used to measure the dewatering amount of the coating colors under pressure. A 10 cm3 of the coating color was poured into a cylindrical vessel placed on a membrane (mixed cellulose ester membrane filter, pore size 0.2 μm, Advantec) and blotter papers. Dewatering of the coating color was performed under a pressure of 2 bar for 60 s. The weight difference of the blotter paper was measured before and after dewatering, and the water retention value was calculated. The viscosity and microstructure of the coating color were measured using a stress-controlled rotational rheometer (CVO, Bohlin Instruments) with a cone−plate geometry (R = 40 mm, angle = 4°). Prior to measurement, the coating color was presheared at a shear rate of 10 s−1 for 5 min, followed by a rest time of 10 min. The viscosity was evaluated as a function of shear rate from 0.1−100 s−1. The microstructure of the coating color was investigated with oscillatory tests by measuring storage (G′) and loss modulus (G″) as a function of the frequency (0.01−10 s−1). Before the frequency sweep test, the linear viscoelastic range of the coating color was confirmed from an amplitude sweep test at a constant angular frequency (1 s−1) as a function of shear stress (0.03−10 Pa). 2.5. Viscosity of the CMC Solution. The zero-shear viscosity of the CMC solution was evaluated using a stresscontrolled rotational rheometer (Kinexus lab+, Malvern Instruments) with a cone−plate geometry (R = 50 mm, angle = 2°). 2.6. Drying Kinetics of the Coating Layer. MS-DWS (Horus, Formulaction, France) was used to investigate the drying process of the coating layer. A laser light illuminating the wet coating sample is scattered by the particles in the coating. When the scattered light is observed using a camera without a B
DOI: 10.1021/acs.iecr.7b02750 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research lens, an image called a speckle image, which is composed of dark and bright spots by the interferences of backscattered light caused by Brownian motion of the particles, is obtained. In other words, the motion of the particles causes the intensity change of the speckle image. The schematics of the apparatus and intensity change of the speckle image are shown in Figure 2.
x = dimx − 1 y = dimy − 1
d2 =
∑
∑
x=0
y=0
(I2(x , y) − I1(x , y))2 (1)
The wavelength of the laser was 655 nm, and the camera, for which maximum frame rate was 30 images/s, could take images consisting of 320 × 240 pixels. The wet coating layer was applied using a blade doctor. The thickness of the wet film was 50 μm, and its area was 10 cm2. The wet coating layer was dried in a constant temperature and humidity room (23.0 ± 0.5 °C, RH 50 ± 3%). Weight loss during drying was measured using an analytical balance (Radwag, Poland) whose resolution was 0.1 mg. 2.7. Pore Structure of the Coating Layer. The pore structure of the coating layer was analyzed by mercury intrusion porosimetry (AutoPore IV 9500, Micromeritics Instruments Corporation). The coating layer was formed on a PET film of 100-μm thickness using an application bar with a gap size of 100 μm. The wet coating layer was dried at room temperature. The thickness of the dried coating was about 50 ± 5 μm. Porosity and pore size distribution of the dried coating were evaluated by measuring the intruded mercury into the pores of the coating layer. The range of the pore size investigated was from 10 to 1 000 nm. The pores were assumed to be of cylindrical shape, and their size was depicted as the diameter of the cylinder. The Washburn equation (eq 2) was used to obtain the pore diameter from the external pressure data. P=
−4γ cos θ d
(2)
2.8. Characterization of the Coating Layer. The surface and cross-sectional images were acquired using a field-emission scanning electron microscope (FE-SEM, AURIGA, Carl Zeiss). A 5 nm Pt coating was applied on all samples for FE-SEM to reduce charging. The surface of the coating layer was observed after Pt coating. The sample for the cross-sectional image was pretreated with OsO4 (Heraeus, Germany) for staining of the latex binder. The sample was exposed to OsO4 for 48 h. After staining, the sample was embedded in an epoxy resin (EpoFix, Struers) and hardener (EpoFix Hardener, Struers) for 5 min at 60 °C. Excessive resin was wiped up, and the sample was dried for 48 h at room temperature. Through a series of processes, it was possible to distinguish clearly the boundary of the coating layer surface and pores in the cross-sectional observation. The
Figure 2. (a) Schematic of the MS-DWS apparatus and (b) intensity change in speckle images.
In each speckle image, using eq 1, d2 was calculated based on a standard image, which was the first image captured by the camera. The d2 values were plotted as a function of time from which correlation time was determined. The speckle rate was the reciprocal of the correlation time. More detailed information on the instrument and the underlying principle can be found in the literature.21,22
Figure 3. Amplitude sweep of coating colors for a range of stress (0.03−10 Pa) at a constant angular frequency (1 Hz): (a) CMC, (b) CNF, (closed symbols) storage modulus (G′), (open symbols) loss modulus (G″). C
DOI: 10.1021/acs.iecr.7b02750 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 4. Frequency sweep of coating colors for a range of stress (0.01−10 Hz): (a) CMC, (b) CNF, (closed symbols) storage modulus (G′), (open symbols) loss modulus (G″). Shear stress for frequency sweep (no additive 0.05 Pa, CMC 0.1:0.3 Pa, other conditions 0.5 Pa). (c) Complex modulus (G*) of coating color.
of CNF is far below that required to cause depletion flocculation to occur in the coating color. G*, which shows a similar tendency compared to G′ is shown in Figure 4(c). G* increased and the slope of the G* decreased with the addition of CMC and CNF. The slope of G* of CMC coating color was lower than that of CNF coating color at the same additive content, which means that structured CMC color showed more solid-like behavior than CNF color. The differences in flocculation of the coating components induced by CMC and CNF were also confirmed in the sedimentation experiment. Figure 5a shows the coating color
porosity of the coating in the cross-sectional image was obtained by image analysis. The analysis procedure is presented in the Supporting Information (Figure S1).
3. RESULTS AND DISCUSSION 3.1. Rheological Properties of the Coating Color. It has been recognized that the use of CMC improves the dispersion of CNF in coating suspensions.15,23 Highly charged anionic CMC appears to make the CNF disentangle in the suspension.23 An amplitude sweep and a frequency sweep test were performed to evaluate the interaction between the coating components. To find the linear viscoelastic region, storage (G′) and loss (G″) moduli were evaluated by changing the shear stress at a constant angular frequency (1 Hz), and they are depicted in Figure 3. A linear viscoelastic region was obviously present, and G′ was larger than G″ in all coating colors. As the content of CMC and CNF in the coating color increased, G′ and G″ also gradually increased, as did the critical stress at which the structure was destroyed, indicating the presence of a stronger interaction between the coating components. G′ and the critical stress of the CMC coating colors were higher than those of the CNF coating colors, which indicated that the CMC coating is more solid-like than the CNF coating color. The addition of CNF also increased G′ and the critical stress of the coating color. CNF used in this study absorbs about 6.4 g of water per gram of CNF whereas the pulp fiber absorbs about 1.6 g of water per gram of fiber. The high water absorption ratio of CNF decreases the freely moving liquid phase in the coating color by uptake of the aqueous phase, and this gives the same effect of increasing the coating solids. The frequency sweep test was performed to confirm the microstructure of the coating color in the linear viscoelastic region. G′ and G″ of the coating color are depicted as a function of frequency in Figure 4. G′ and G″ increased with an increase of frequency indicating the formation of a weakly flocculated structure in the coating color. In the case of 0.3 and 0.5 pph of CMC, G′ and G″ did not change much regardless of the frequency, indicating that a strongly flocculated structure was formed. As the CMC content increases, the coating components flocculate by the depletion mechanism of CMC.10−13 CNF does not flocculate the coating components by the depletion mechanism because it is much lower in charge density and smaller in number than CMC. For example, when the density, diameter, length, and aspect ratio of CNF were 1.5 g/cm3, 20 nm, 4 μm, and 200, respectively, the ratio of the number of CMC to CNF was about 2 500:1. Thus, the number
Figure 5. Sedimentation heights of the coating colors vs the addition rate of CMC and CNF: (a) after centrifugation and (b) after decantation of the supernatant.
after centrifugation, and Figure 5b shows the sedimentation height after removal of the supernatant. The sedimentation height increased with the addition of CMC, indicating that CMC induced flocculation of the coating components. The coating color containing 0.5 pph of CMC gave transparent supernatant, which suggested that almost no particles were present in the supernatant. In contrast, the supernatants of the other coating colors were turbid, suggesting incomplete D
DOI: 10.1021/acs.iecr.7b02750 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
stage can be divided into three regimes as shown in Figure 7. In regime 1, which begins just after application of the coating
sedimentation of the pigment particles. This showed that flocculation of all pigment particles into a size large enough to cause sedimentation was obtained when 0.5 pph of CMC was used as the additive, whereas the addition of CMC of less than 0.5 pph resulted in turbid supernatant because of the incomplete sedimentation of small pigment particles. The sedimentation height of the coating colors containing CNF was similar to that of CMC 0.1, indicating that no additional flocculation of pigments resulted with CNF. In other words, it appeared that CNF in the coating color did not cause any interaction with the coating components. This is supported by the findings of Dimic-Misic et al.15 The viscosity of the coating color as a function of shear rate is shown in Figure 6. Shear-thinning behavior was observed for
Figure 7. Regimes depending on the change in speckle rate during drying.
color on the glass plate, the speckle rate of the film decreases gradually by the evaporation of water, and then a sudden change in the speckle rate begins that coincides with the beginning of regime 2.22 The high speckle rate in regime 1 indicates that fast motion of the particles in the coating layer still prevails.22 The movement of the particles is restricted by the neighboring particles with increasing solids volume fraction of the coating by drying in regime 2. In other words, the structure of the coating layer is almost fixed in regime 2.22 A critical time (tc) can be determined by the sudden change of the speckle rate between regime 1 and 2. The solids content of the coating layer at the starting point of regime 3 ranges from 96− 98%. The speckle rate and film weight change of the coating layer by drying time are shown in Figure 8. The initial speckle rate in regime 1 and tc were greatly reduced with an increase of CMC content (Figure 8a). Coating color without any additive and CMC 0.1 showed a similar speckle rate in regime 1 and tc. The solids contents at tc were 79, 77, 70, and 66 wt % when the CMC contents were 0, 0.1, 0.3, and 0.5 pph, respectively. This clearly showed that CMC caused early immobilization of coating components in the wet coating layer. When CNF was used along with CMC, the speckle rates in regime 1 and tc were decreased as shown in Figure 8b. The decrements of the speckle rate and tc for the CNF coating were lower than those for the CMC coatings because of the lower structure-forming property of the CNF coating. The solids contents at tc were 74 and 72 wt % when the CNF contents were 0.2 and 0.4 pph, respectively. The changes of speckle rate in regime 1 and tc can be explained by the Stokes−Einstein diffusion coefficient. The diffusion coefficient D is defined as eq 3:
Figure 6. Viscosity of coating color as a function of the shear rate (shear rate = 0.1−100 s−1).
all coating colors. The viscosity increased with increasing CMC and CNF content. CMC colors gave higher viscosity than CNF coatings because of the flocculation of the coating component. The viscosity of the coating color containing 0.5 pph of CMC remained constant at low shear rate, which indicated that this coating color had a highly structured, solid-like behavior below a shear rate
