Interfacial properties of nano TiO 2 and cellulose paper coating

This paper examined different kinds of organic functional groups that were introduced onto the surface of nano TiO2 by surface modification with different types of zircoaluminate coupling agents. The modified nanoTiO2 products with different interfacial properties were obtained, and the impact of the interfacial properties of nanoTiO2 on the rheological behavior of paper coating and the properties of coated paper was systematically investigated. The steady shear rheological results showed that the paper coatings containing nano TiO2 exhibited a pseudoplastic fluid behavior, characterized as obvious shear thinning. Compared to the hydrophilic unmodified nano TiO2, modified nano TiO2 could contribute more to the viscosity of paper coatings. The study on the dynamic viscoelasticity revealed that, through the enhancing action among each component in paper coatings, the modified nano TiO2 with quaternary amine groups or carboxyl led to a higher dynamic elastic storage modulus and viscous loss modulus of paper coatings. In addition, SEM and AFM analyses indicated that adding modified nano TiO2 products in paper coating could improve the coating structure, thus ameliorating the optical properties and printability of coated paper. The results obtained could provide a good reference for the application of nano pigments in paper coating.


INTRODUCTION
In recent years, much attention has been paid to nano additives (e.g.2][3] Paper coating is a compact stereo network structure consisting of inorganic pigment particles, latex, and high molecular weight rheological agents.It has been reported that the addition of nano TiO 2 in paper coating not only could affect the rheological behavior of paper coating, 4 but it could also improve the surface strength and aging properties of coated paper. 5However, like other superfine materials, nano TiO 2 has strong surface polarity and high surface energy, resulting in the easy aggregation of nano TiO 2 particles. 6Moreover, nano TiO 2 is not compatible with polymer colloid in paper coating due to its highly hydrophilic surface. 5Therefore, surface modification is needed to achieve effective application of nano TiO 2 in paper coating. ][9] There are two different types of organic ligands selectively bounded to the backbone of zircoaluminates.One ligand imparts the hydroxylic and hydrolytic stability to the inorganic portion of zircoaluminates, and it can be α, β-or α, γ-glycol, or α-hydroxycarboxylic acid, with no more than six carbon atoms; the other (bridging ligand) contributes organo-functionality or reactivity to zircoaluminates, and it can include carboxyl, hydroxyl, mercapto, amino, or oleophilic groups, among others, for different purposes. 7,9 rcoaluminates are less water sensitive and cost-effective in the irreversible modification of all inorganic fillers/pigments than silanes and titanates.Therefore, zircoaluminates can lead to a remarkable decrease in the viscosity of coating resin or solvent and reduces particle agglomeration. 10,11  previous work, different types of zircoaluminate coupling agents were synthesized and characterized with different functional groups like carboxyl, fatty, and quaternary ammonium groups. 9The viscosity could be reduced by 50% after surface modification using 0.4% carboxyl zircoaluminates in a nano TiO 2 aqueous slurry with 30% solid content. 12The coated paper had better optical properties and printability compared to the control samples when the modified nano TiO 2 was added to the coating. 4,5 owever, it has not been investigated whether the interfacial properties of nano TiO 2 has an impact on the performance of the coating and the coated paper.Hence, in ongoing work, nano TiO 2 was modified with different zircoaluminates to obtain the modified nano TiO 2 with different surface functional groups i.e. different interfacial properties.Then, the influence of interfacial properties of nano TiO 2 products on the rheological behavior of paper coating and the characteristics of coated paper was fully investigated.The results obtained will be a practical reference for the application of nano pigment in paper coating.
The preparation of zircoaluminate coupling agents with different functional groups was conducted according to the method found within previous work. 9The surface modified nano TiO 2 products were prepared with different zircoaluminates following the previous procedure. 12The dosage of zircoaluminate was 0.4% based on the dry weight of nano TiO 2 .The names of zircoaluminates and nano TiO 2 products are listed in Table 1.Unmodified nano TiO 2 was named UM.Nano TiO 2 products modified by zircoaluminate of CA-C, CA-H, CA-M, CA-F, and CA-Q were named as CM, HM, MM, FM, and QM, respectively.
The base paper was a white paperboard (provided by Hongta Renheng Paper Co. Ltd., Zhuhai, China) with base weight of 225.8 g/m 2 , sheet density of 0.75 g/cm 3 , whiteness of 75.72%, opacity of 98.63%, PPS roughness of 7.44 μm, and gloss of 11.7%.
The coatings were prepared with the same method used from previous work. 4,13

Characterization
The average particle size and zeta potential of nano TiO 2 samples with a solid concentration of 0.1% were analyzed by a MalvernZEN3600 tester (Malvern Instruments Ltd., UK).All the sample suspensions were ultrasonically treated at 45 kHz and 180 W for 10 minutes prior to the analysis.For each sample, the tests were carried out in triplicate and the average data was reported.
The contact angles of nano TiO 2 samples were measured on a static contact angle analyzer (JC2000A, Shanghai Zhongchen Company, China) by dropping water droplets on nano TiO 2 tablets prepared in a pellet press under a pressure of 10 MPa for about 5 minutes.
The steady shearing and dynamic viscoelasticity tests of paper coatings were conducted on an AR550 rheometer (TA Company, US) at 25ºC.The test mode of the viscosity test was a continuous ramp flow.A 40 mm stainless steel parallel plate was used for the test at low shear rate (0-2,000 s -1 ), and a 20 mm stainless steel cone plate was used at high shear rate (2,000-10,000 s -1 ), the cone degree was 0.5º.With regard to dynamic strain sweep test, 40 mm stainless steel parallel plate was applied with fixed oscillation frequency and varied strain of 0.01-100%.The scanning frequency was in the range of 0.01-10 Hz.Elastic (storage) modulus (Gꞌ), visous (loss) modulus (Gꞌꞌ), and phase angle (δ) were characterized for dynamic viscoelasticity of coatings.
The coating of white paperboard was conducted on a K303 Multicoater (UK).After drying and calendering, the properties of coated paper were measured according to TAPPI test standards.The surface structure of coated paper was observed by scanning electron microscopy (SEM, S-3700N, Hitachi, Japan) and atomic force microscopy in tapping mode (AFM, Veeco, USA).b) The corresponding pH was 7.

Interfacial properties of nano TiO 2
Zircoaluminate is one of the low-molecular-weight inorganic polymers containing zirconium aluminate. 7ifferent organic functional groups can be introduced in the molecules of zircoaluminates due to the flexibility of its molecular design.In previous studies, many zircoaluminate products were synthesized with different functional groups. 9ano TiO 2 products with different interfacial properties were obtained by modifying raw nano TiO 2 with different zircoaluminates. 12As it can be seen from Table 1, the surface of unmodified nano TiO 2 (UM) was wholly hydrophilic, and its corresponding absolute value of zeta potential was relatively low (5), causing aggregation resulting in a larger average particle size (158 nm).In the event that the bridging ligand of zircoaluminate had hydrophilic groups or its molecule was small, the modified nano TiO 2 maintained good hydrophilicity.For instance, the water contact angle of CM was only 14°.The high portion of inorganic part (over 70 wt.%) of zircoaluminate might lead to the hydrophilicity of modified nano TiO 2 . 9,12 rthermore, some extent of lipophilicity due to the presence of hydrophobic groups in the bridging ligand of zircoaluminate was observed on the QM modified nano TiO 2 .The contact angle of QM was 57°.The dispersibility in water would not be affected because its contact angle was less than 90°. 10 In addition, after surface modification, the average particle size of nano TiO 2 was distinctly smaller compared to UM (Table 1).The size distribution of UM and CM, depicted in Figure 1 ranged from 50 nm to 400 nm, and 25 nm to 220 nm respectively.The volume fraction of a particle size below 100 nm had a discernible increase after surface modification and the particle size distribution became narrower.Hence, the dispersibility of nano TiO 2 was significantly improved in water after modification.The improved dispersibility was due to 1) the increased zeta potential (absolute value) and 2) steric hindrance generated by the adsorption of zircoaluminates on the surface of nano TiO 2 . 11,12 he increased dispersibility of nano TiO 2 could facilitate its dispersion in paper coatings.The coating viscosity as the function of shear rate in the range of 0-2,000 s -1 is shown in Figure 2. As it can be seen, the viscosity of paper coatings reduced as the shear rate increased, displaying shear thinning behavior.This phenomenon was similar with the paper coatings containing other kinds of nano additives like nano CaCO 3 , 11 nanocellulose, [14][15][16] and graphite. 17Shear thinning property is convenient for coating operation at high speed running.Moreover, at the same shear rate, the apparent viscosity of paper coating containing QM was the largest, followed by the one containing CM, FM, and HM, while the viscosity of the one containing UM was the lowest.Therefore, the strength of network structure of the coating containing QM was larger in comparison with other paper coatings, while the network for the one containing UM was easier to be destroyed.This phenomenon was probably related to the different interfacial properties of nano TiO 2 products, which would be further discussed in the following section of viscoelasticity of paper coatings.

Rheological behavior of paper coatings containing nano TiO 2 3.2.1 Steady shear rheological behavior
Figure 3 shows the flow curves of paper coating viscosity at high shear rate (2,000-10,000 s -1 ).It is evident from Figure 3 that all the paper coatings retained shear thinning properties at high shear rate.However, the rate of change in thinning gradually smoothened.Likewise, there was a clear breakpoint for each viscosity curve of paper coating with the increase of shear rate, and this phenomenon demonstrated that the strong shear at the breakpoint caused substantial damage on the network structure of paper coating.The viscosity difference of paper coatings at high shear rate was congruent with the ones at low shear rate (Figure 2), but the difference was more evident.For example, the paper coating containing QM presented the highest viscosity and the corresponding breakpoint of viscosity curve was not as distinct.This indicated that QM with quaternary ammonium groups had strong interactions with other components in paper coating.However, paper coating containing UM showed the lowest viscosity at the same shear rate, indicating the weakest shearing resistance.After surface modification, nano TiO 2 particles could be well dispersed in paper coatings, and the smaller size could lead to the increased specific surface area, thus improving the interactions between nano TiO 2 particles and other components in paper coating. 10,16 n the other hand, the positive groups on the surface of QM could also enhance the interactions of the coating components.
The Herschel-Bulkley model (expressed in Eq. ( 1)) and Cross model (expressed in Eq. ( 2), data before breakpoint was used) can be used to obtain a better fit of the flow curves of paper coatings at low and high shear rate, respectively.
In Eq. ( 1) τ is the shear stress, γ is the shear rate, τ 0 is the yield stress, k is the consistency coefficient, and n is the fluid characteristic index (FCI).In Eq. ( 2) η is the viscosity, λ is time constant, and η 0 and η ∞ are zero shear viscosity and limit viscosity, respectively.As shown in Table 2, the paper coating containing QM had the highest yield stress (18.08 Pa), reflecting its strong network structure.Furthermore, all the FCI of paper coatings were less than 1, indicating that all paper coatings containing nano TiO 2 were pseudoplastic fluid.The FCI of paper coatings at high shear rate were all larger than 1, as can be seen from Table 3.This indicated that the paper coatings had a shearing thickening trend.However, the FCI values were all close enough to 1. Thus, the rheological behavior of the paper coatings at high shear rate displayed the characteristics of near Newtonian fluids.

Viscoelasticity of paper coatings
Paper coating has a linear and a nonlinear viscoelastic regions, and the viscoelasticity is highly dependent upon the structure of the coating system. 5First, the strain sweep of all paper coatings was conducted with a fixed oscillating angular frequency of 10 rad/s to determine the critical strain.It was found that the linear viscoelastic regions of paper coatings were all in the strain range of 0.3-2.0%(Figure 4).dynamic viscoelasticity of paper coatings were tested at a fixed strain.In this work, the fixed strain of 0.4% was selected so that the frequency sweep of paper coatings could be conducted in the linear viscoelastic region, where the applied strain could not damage the structure of coating system.Figure 5 shows the variation curves of dynamic viscoelasticity of paper coatings.The paper coatings containing modified nano TiO 2 had higher elastic (storage) modulus (Gꞌ) and visous (loss) modulus (Gꞌꞌ), compared to the paper coating containing UM, from Figures 5a and 5b.This suggested that the addition of modified nano TiO 2 could enhance the strength of the coating network structure.
The viscoelasticity of pigment particles at the low region was very sensitive to the formation and changes of coating network structure.The strength of the three dimensional network structure of paper coatings was mainly related to the nature of particles and the interactions among particles and polymer colloids.If the interactions were strong enough, a compact network structure could also be formed even when the coating has a low solids content which could be sensitively reflected by viscoelasticity at the low frequency region (Figure 6a).As for the paper coatings containing modified nano TiO 2 , the interactions among particles could be very strong and easily form a steady network structure due to the larger specific surface area and better dispersion of modified nano TiO 2 in the coating system.Therefore, paper coatings containing modified nano TiO 2 showed a unique behavior of dynamic viscoelasticity.Similar results were also reported for paper coatings containing nano CaCO 3 . 11,18 gure 5 also shows that the viscoelasticity of paper coatings containing different modified nano TiO 2 displayed a clear difference, which was consistent with the results of the steady shear behavior (Figures 2 and 3).As presented in Figure 6a, paper coating containing QM had the highest Gꞌ and showed Gꞌ plateau at the relatively higher frequency region, followed by the ones containing CM, FM, and HM.However, the paper coating containing MM had a relatively lower Gꞌ, but was higher in comparison to the one containing UM.This was expected because for a smaller average particle size and larger specific surface area, the introduced organic functional groups on modified nano TiO 2 surface could cause a large difference of interfacial properties, leading to the different interactions between modified nano TiO 2 and polymer colloids.QM with quaternary ammonium groups of long chain aliphatic hydrocarbons was positively charged, which could be better adsorbed onto the surface of pigments; its hydrophobicity could lead to a stronger interaction between QM particles and polymer colloids in coatings.Hence, paper coating containing QM presented the highest viscoelasticity.For CM with the smallest particle size, carboxyl groups of short chain aliphatic hydrocarbons existed on the surface of CM (Table 1).Carboxyl could react with hydroxyl on the surface of pigments and adhesive molecules, thus enhancing the strength of coating network structure.Yet, the double bonds on the surface of MM might have a weak interaction with the coating components, resulting in a lower viscoelasticity.Therefore, it could be concluded that the interfacial property of nano TiO 2 had a big impact on the viscoelasticity of paper coatings.

(a) (b) (c)
In addition, as shown in Figure 5c, the phase angles (δ) of all paper coatings were less than 35º.Hence, the paper coatings containing nano TiO 2 displayed strong elastic solid properties, but the viscous liquid properties were relatively weak.

Analysis of coated paper properties
The effect of different nano TiO 2 products on the properties of coated paper is presented in Table 4.It can be seen that, the optical properties and printability of coated paper containing modified nano TiO 2 products were clearly improved in comparison with the one containing UM.The amelioration of optical properties (e.g.brightness, gloss) and printability (such as surface strength and ink absorptivity) achieved by QM and CM was more significant, followed by FM and HM.These results were in line with the results of the viscoelasticity analysis (Figure 5).For instance, at the frequency of 6.2 Hz, the Gꞌ of the coating containing QM was 41.57Pa, while the Gꞌ for the one with UM was only 23.14 Pa.The increase of Gꞌ could lead to the decrease of sheet density, i.e. the increment of bulk, thus improving the ink absorptivity.The effective tuning of coating structure achieved by adding modified nano TiO 2 in paper coatings improved the optical properties of coated paper.

Surface structure of coated paper
The SEM images of coated paper surface containing nano TiO 2 are given in Figure 6.As reported, the formation of more effective void size in optics facilitated the improvement of light scattering efficiency of coatings, thus ameliorating the optical properties of coated paper. 19Same as the particle size with the most effective light scattering, the diameter at about 0.2 μm of coating voids caused the best light scattering 5,20 (i.e.QM).As it can be seen from Figure 6a, the pigment particles on the coating surface arranged in a disorderly manner, the coating structure was loose, the distribution of coating voids was inhomogeneous, the void size was relatively large, and the diameter of the largest void was over 500 nm.However, for the paper coating surface with QM, the distribution of voids was more homogeneous, and the size of most of the voids was approximately 200 nm (Figure 6b).This type of void structure provided a benefit to the achievement of the largest light scattering, leading to higher brightness and opacity in coated paper.Figure 6b also shows that the coating surface was flat, kaolin particle arranged fittingly, and the small particles of size below 100 nm could be identified.The smooth surface contributed to a higher smoothness and gloss.
Figures 7a and 7b are the AFM images of coating surface containing UM and QM, respectively.The scanning range was 0.5μm × 0.5μm.As it can be seen, the coating surface containing UM was coarser, while the one containing QM was flatter.The corresponding arithmetical mean deviations of the profile scanned for the coating surface with UM and QM were 7.364 nm and 2.684 nm, respectively.Lower roughness of coating surface could lead to a higher gloss of coated paper. 21This was in agreement with the results listed in Table 4.In addition, for the coating surface with QM, the number of micropores with the diameter of about 200 nm had a distinct increase, leading to a better light scattering of coating.This further improved the optical properties of coated paper.Moreover, many modified nano TiO 2 particles of less than 100 nm were homogeneously distributed on the surface of coated paper (Figure 7b), indicating a well dispersion of modified nano TiO 2 in paper coatings.Enhanced interactions among coating components improved the surface strength of coated paper.

CONCLUSIONS
Different interfacial properties of nano TiO 2 could be achieved by surface modification using zircoaluminates with different organic functional groups.The results showed that the interfacial properties of modified nano TiO 2 had a substantial impact on the rheological behavior of paper coating and the quality of coated paper.It was found that the modified nano TiO 2 with surface quaternary ammonium groups or carboxyl groups could increase the shear resistance of coating by the reinforcement of interactions with coating components.In this manner, better bulk and light scattering could be obtained by effectively tuning coating void structure, thus improving the optical properties and printability of coated paper.

Fig. 2 .
Fig. 2. Flow curves of paper coating viscosity at low shear rate.

Fig. 3 .
Fig. 3. Flow curves of paper coating viscosity at high shear rate.

Fig. 4 .
Fig. 4. Dynamic strain sweep data for paper coatings containing UM.After determining the linear viscoelastic region, the
TiO 2 products modified by zircoaluminate of CA-C, CA-H, CA-M, CA-F, and CA-Q were named as CM, HM, MM, FM, and QM, respectively.

Fig. 6 .
Fig. 6.SEM images of coated paper surface (a: surface of coated paper containing UM; b: surface of coated paper containing QM).

Fig. 7 .
Fig. 7. AFM images of coated paper surface (a: surface of coated paper containing UM; b: surface of coated paper containing QM).

Table 1 .
The properties of different nano TiO 2 products.

TiO 2 samples a Surface modifier (zircoaluminates) Bridging ligand in zircoaluminate Average particle size (nm) Functional group on nano TiO 2 surface Contact angle (°) Zeta potential b (mV)
a) Unmodified nano TiO 2 was named UM.Nano TiO 2 products modified by zircoaluminate of CA-C, CA-H, CA-M, CA-F, and CA-Q were named as CM, HM, MM, FM, and QM, respectively.

Table 2 .
Rheological parameters of Herschel-Bulkley model fitted to flow curves of paper coatings at low shear rate.Unmodified nano TiO 2 was named UM; Nano TiO 2 products modified by zircoaluminate of CA-C, CA-H, CA-M, CA-F, and CA-Q were named as CM, HM, MM, FM, and QM, respectively.Solid content of paper coatings was 50%.b) FCI: fluid characteristic index.c) SD: standard deviation.

Table 3 .
Rheological parameters of Cross model fitted to flow curves of paper coatings at high shear rate.Unmodified nano TiO 2 was named UM; Nano TiO 2 products modified by zircoaluminate of CA-C, CA-H, CA-M, CA-F, and CA-Q were named as CM, HM, MM, FM, and QM, respectively.Solid content of paper coatings was 50%.

Table 4 .
Effect of different nanoTiO 2 products on the properties of coated paper.
a) Unmodified nano TiO 2 was named UM.Nano