Surface Organic Modification of CaCO 3-TiO 2 Composite Pigment

To improve the properties and dispersibility of CaCO3-TiO2 composite pigments (CaCO3-TiO2) in organic matrices, the surface modification of CaCO3-TiO2 was performed with sodium stearate (SS) as an organic modifier by wet ultra-fine grinding in a stirred mill. The pigment properties of modified CaCO3-TiO2 and its dispersibility in organic media were tested and characterized. The binding mechanism between CaCO3-TiO2 and SS was explored by infrared spectrometry (IR) and X-ray photoelectron energy spectroscopy (XPS). The results showed that the mechanical grinding strength and SS dosage had a significant effect on the activation index and sedimentation rate of CaCO3-TiO2. After surface modification, the surface of CaCO3-TiO2 turned from a hydrophilic surface to a hydrophobic surface and the surface free energy was reduced. In addition, the hiding property and dispersibility of CaCO3-TiO2 in the organic medium were significantly improved. IR and XPS results indicated that the modifier SS was adsorbed on the surface of CaCO3-TiO2 by chemical combination.


Introduction
Calcium carbonate (CaCO 3 ) is an important non-metallic mineral and CaCO 3 powder prepared by grinding natural mineral (such as calcite) and chemical precipitation has become the most commonly used filler added in various industrial products, such as plastics, coatings, and paper [1,2].The preparation of composite pigments by uniformly coating TiO 2 on the surface of CaCO 3 particles is an important and efficient utilization of CaCO 3 mineral resources.Therefore, the related preparation processes have attracted wide attention [3,4].Meanwhile, the preparation of the composite pigments can improve the utilization rate of pigment TiO 2 and reduce the consumption of expensive TiO 2 [5,6].
The TiO 2 -coated CaCO 3 composite pigments can be prepared by several methods, such as mechanochemistry, carbonization reaction in a TiO 2 system, sol-gel, and hydrophobic aggregation methods, and the prepared CaCO 3 -TiO 2 all exhibit the equivalent hiding power, oil absorption, and whiteness to the pigment TiO 2 [7,8].However, the CaCO 3 -TiO 2 prepared from the above methods except the hydrophobic agglomeration method have hydrophilic surface properties.Therefore, prepared CaCO 3 -TiO 2 as a filler shows a poor dispersibility in organic products and its pigment function is largely affected [9,10].Although the surface of CaCO 3 -TiO 2 particles prepared by the hydrophobic aggregation method is hydrophobic, hydrophobic groups on the particle surface are often formed based on the combination of CaCO 3 and TiO 2 and the prepared composite pigment is not very compatible with the target application system.Moreover, industrially available CaCO 3 -TiO 2 produced by the mechanochemical method exhibits surface hydrophilicity.Therefore, it is necessary to conduct an organic modification of CaCO 3 -TiO 2 .In this way, CaCO 3 -TiO 2 can obtain a hydrophobic surface, thus improving the compatibility of the composite pigments with organic systems, the functions of CaCO 3 -TiO 2 , and the performance of the final products.Conventional surface modification technologies, such as the heating mixing modification and physical coating modification, have been applied in the treatment of fillers and pigments [11][12][13].However, the CaCO 3 -TiO 2 modified by the above two methods have some defects, such as an uneven dispersion between the modifier and materials and the absence of a reaction driving force caused by the weak stirring strength and low material mixing degree.Moreover, the hydrophobic modification effect is poor and the obtained products are unstable.Therefore, it is necessary to develop an efficient modification method.
Mechano-activated surface modification is a modification method utilizing the mechanochemical effect during the ultrafine grinding process.The method is thought to be more valuable and effective because the mechanochemical effect resulted from ultra-fine grinding can increase the activity of the mineral surface and enhance the reactivity in their interface [14,15].The efficient method had been proved by the modification of some powder, such as CaCO 3 and wollastonite [16,17].In this study, the surface organic modification of CaCO 3 -TiO 2 with SS as the modifier by the wet mechanochemical method was investigated, and the pigment properties of modified CaCO 3 -TiO 2 and its dispersibility in organic media were tested and characterized.Moreover, based on infrared spectrometry (IR) and X-ray photoelectron energy spectroscopy (XPS), the binding mechanism between CaCO 3 -TiO 2 and SS was also explored.

Raw Materials
CaCO 3 -TiO 2 was prepared by the mechanochemical method [18].In as prepared CaCO 3 -TiO 2 , TiO 2 (50%, w/w) was uniformly coated on the surface of CaCO 3 and its main physical performance indicators are listed in Table 1.The modifier sodium stearate (RCOONa, R stands for C 17 H 35 ) is a chemically pure reagent (Beijing Chemical Plant, Beijing, China) with a white solid powder appearance.It was mixed with warm water to form a 5% (w/v) solution before use.Kerosene, n-hexane, and ethanol are all chemically pure reagents.Deionized water was used in the experiment.

Modification Method
Firstly, 50 g CaCO 3 -TiO 2 was stirred with 200 g deionized water to form a slurry, which was ground together with an added 200 g grinding balls at 1000 rpm for 20 min.Three grinding balls with different diameters of 1.5 mm, 1.2 mm, and 0.8 mm were added according to the mass ratio of 1:1:1 in a GSDM-003 ultra-fine grinding mill (volume = 1 L).Secondly, SS solution was added into the CaCO 3 -TiO 2 slurry according to the different mass ratios of SS to CaCO 3 -TiO 2 powder (0%, 0.5%, 1%, 1.5%, 2%, and 3%) and the slurry was heated to different temperatures (20 • C, 50 • C, 80 • C, and 100 • C).Then, the slurry was ground to obtain the modified CaCO 3 -TiO 2 slurry.Thirdly, the modified CaCO 3 -TiO 2 slurry was separated from the grinding balls, dried, and then dispersed to obtain the modified CaCO 3 -TiO 2 .The organic modification degree of CaCO 3 -TiO 2 was evaluated based on the sedimentation rate in kerosene (organic liquid medium) and the activation index of CaCO 3 -TiO 2 .
The sedimentation rate of the powder in the liquid medium was measured with a self-assembled measurement device (Figure 1).During the test, the weight of the powder on the sedimentation disk at any time from the beginning to the end of the settlement was recorded and then the mass percentage of the sedimentation powder to the total powder in the sedimentation zone was calculated as the sedimentation rate.The low sedimentation rate of particles in kerosene indicates the poor agglomeration effect, good dispersion effect, and the good hydrophobic surface caused by surface modification of the particles with SS [19].

Property Test of Modified CaCO3-TiO2
The organic modification degree of CaCO3-TiO2 was evaluated based on the sedimentation rate in kerosene (organic liquid medium) and the activation index of CaCO3-TiO2.
The sedimentation rate of the powder in the liquid medium was measured with a self-assembled measurement device (Figure 1).During the test, the weight of the powder on the sedimentation disk at any time from the beginning to the end of the settlement was recorded and then the mass percentage of the sedimentation powder to the total powder in the sedimentation zone was calculated as the sedimentation rate.The low sedimentation rate of particles in kerosene indicates the poor agglomeration effect, good dispersion effect, and the good hydrophobic surface caused by surface modification of the particles with SS [19].The activation index refers to the mass proportion of the powder floating on water after powder is stirred in water [20].Compared with particles with a hydrophilic surface, fine particles with a hydrophobic surface showed the increased adhesion of the three-phase wetted periphery at the water-vapor interface and could float at the interface because the adhesive force could be decomposed into the upward direction component.The larger activation index indicates the stronger adhesion and the better hydrophobic modification effect of particles.The test method of the activation index is described as follows.The modified powder was dispersed in water and stirred for 20 min.Then, after standing for several minutes, the weight of the powder floating on the water-gas interface was measured.Finally, the activation index was calculated as the mass ratio of the floating powder to the added powder.
To evaluate the surface wettability of THE modified CaCO3-TiO2 and provide the basis for the calculation of their surface free energy, the contact angle between CaCO3-TiO2 and the liquid medium was measured by a contact angle meter (JC2000D, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd., Shanghai, China).The powder samples for the contact angle tests were processed by a tablet machine for measurement and the average of three measurements was adopted as the final result.Besides, the images of the CaCO3-TiO2 particles dispersed in different organic solvents were obtained by an image analyser (BT-1600, Bettersize Instruments Ltd., Dandong, China).The maximum resolution of the instrument was 0.1 μm and the images are the direct outputs of the instrument without processing.
The binding property between the modifier and CaCO3-TiO2 was explored by the analysis of Fourier transform infrared spectrum (FT-IR) and X-ray photoelectron spectroscope (XPS).The FT-IR spectra were recorded on an infrared spectrometer (Spectrum 100, PerkinElmer Instruments Co., Ltd., Waltham, MA, USA) in a scanning range of 400-4000 cm −1 .All samples were mixed with potassium bromide (KBr) according to a proportion of 1:100 to obtain the measurement slice.XPS measurements were carried out on an Escalab 250xi instrument (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatic Al Kα X-ray radiation and the XPS lines were calibrated with the Cls line at 284.6 eV.The morphology of unmodified CaCO3-TiO2 and modified CaCO3-TiO2 composite particles was observed by scanning electron microscope (SEM, S-3500N, HITACHI, Tokyo, Japan).The particles' The activation index refers to the mass proportion of the powder floating on water after powder is stirred in water [20].Compared with particles with a hydrophilic surface, fine particles with a hydrophobic surface showed the increased adhesion of the three-phase wetted periphery at the water-vapor interface and could float at the interface because the adhesive force could be decomposed into the upward direction component.The larger activation index indicates the stronger adhesion and the better hydrophobic modification effect of particles.The test method of the activation index is described as follows.The modified powder was dispersed in water and stirred for 20 min.Then, after standing for several minutes, the weight of the powder floating on the water-gas interface was measured.Finally, the activation index was calculated as the mass ratio of the floating powder to the added powder.
To evaluate the surface wettability of THE modified CaCO 3 -TiO 2 and provide the basis for the calculation of their surface free energy, the contact angle between CaCO 3 -TiO 2 and the liquid medium was measured by a contact angle meter (JC2000D, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd., Shanghai, China).The powder samples for the contact angle tests were processed by a tablet machine for measurement and the average of three measurements was adopted as the final result.Besides, the images of the CaCO 3 -TiO 2 particles dispersed in different organic solvents were obtained by an image analyser (BT-1600, Bettersize Instruments Ltd., Dandong, China).The maximum resolution of the instrument was 0.1 µm and the images are the direct outputs of the instrument without processing.
The binding property between the modifier and CaCO 3 -TiO 2 was explored by the analysis of Fourier transform infrared spectrum (FT-IR) and X-ray photoelectron spectroscope (XPS).The FT-IR spectra were recorded on an infrared spectrometer (Spectrum 100, PerkinElmer Instruments Co., Ltd., Waltham, MA, USA) in a scanning range of 400-4000 cm −1 .All samples were mixed with potassium bromide (KBr) according to a proportion of 1:100 to obtain the measurement slice.XPS measurements were carried out on an Escalab 250xi instrument (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatic Al Kα X-ray radiation and the XPS lines were calibrated with the Cls line at 284.6 eV.The morphology of unmodified CaCO 3 -TiO 2 and modified CaCO 3 -TiO 2 composite particles was observed by scanning electron microscope (SEM, S-3500N, HITACHI, Tokyo, Japan).The particles' size and specific surface area of CaCO 3 -TiO 2 composite particles were tested by a centrifugal sedimentation particle size analyzer (BT-1500, Bettersize Instruments Ltd., Dandong, China).
The pigment properties of unmodified CaCO 3 -TiO 2 and modified CaCO 3 -TiO 2 were evaluated based on the tested oil absorption, hiding power, and whiteness.Oil absorption refers to the minimum amount of varnish (linseed oil) required for completely wetting 100 g of pigment and can be tested according to China National Standard GB/T5211.15-2014[21].Hiding power refers to the minimum amount of pigment required for completely covering per unit of black and white checkerboard.The hiding power of a pigment can be tested according to the National Industry Standards HG/T3851-2006 (the test method of pigment hiding power) [22].The whiteness was tested with a whiteness meter (SBDY-1, Shanghai Yuet Feng Instrument Co., Ltd., Shanghai, China).The sedimentation rate in kerosene and the activation index of CaCO 3 -TiO 2 modified at different temperatures were investigated (Figure 2).The other experimental conditions were set as follows: SS dosage of 2.0 wt % (the mass ratio of SS to CaCO 3 -TiO 2 ), grinding speed of 800 rpm, and grinding time of 25 min.With the rise of the modification temperature, the activation index of the modified CaCO 3 -TiO 2 first increased gradually, reaching the maximum value (about 90%) at 80 • C, and then decreased slightly (Figure 2).With the rise of the modification temperature, the sedimentation rate of the modified CaCO 3 -TiO 2 in kerosene decreased firstly, then reached the minimum at 80 • C and then increased.Therefore, the optimum modification temperature was 80 • C. It might be interpreted as follows.The low modification temperature is not conducive to the dissolution of the modifier in water and the hydrolysis of RCOO − to RCOOOH, thus reducing the content of effective components of the modifier and leading to the poor modification effect.However, the solvents evaporate too fast at higher temperatures, thus leading to an increase in the solid content and viscosity of the slurry and affecting the full contact between the modifier and composite particles.

Influences of Modification Conditions on the Activation Index and Sedimentation Rate of
size and specific surface area of CaCO3-TiO2 composite particles were tested by a centrifugal sedimentation particle size analyzer (BT-1500, Bettersize Instruments Ltd., Dandong, China).
The pigment properties of unmodified CaCO3-TiO2 and modified CaCO3-TiO2 were evaluated based on the tested oil absorption, hiding power, and whiteness.Oil absorption refers to the minimum amount of varnish (linseed oil) required for completely wetting 100 g of pigment and can be tested according to China National Standard GB/T5211.15-2014[21].Hiding power refers to the minimum amount of pigment required for completely covering per unit of black and white checkerboard.The hiding power of a pigment can be tested according to the National Industry Standards HG/T3851-2006 (the test method of pigment hiding power) [22].The whiteness was tested with a whiteness meter (SBDY-1, Shanghai Yuet Feng Instrument Co., Ltd., Shanghai, China).

Influences of Modification Temperature
The sedimentation rate in kerosene and the activation index of CaCO3-TiO2 modified at different temperatures were investigated (Figure 2).The other experimental conditions were set as follows: SS dosage of 2.0 wt % (the mass ratio of SS to CaCO3-TiO2), grinding speed of 800 rpm, and grinding time of 25 min.With the rise of the modification temperature, the activation index of the modified CaCO3-TiO2 first increased gradually, reaching the maximum value (about 90%) at 80 °C, and then decreased slightly (Figure 2).With the rise of the modification temperature, the sedimentation rate of the modified CaCO3-TiO2 in kerosene decreased firstly, then reached the minimum at 80 °C and then increased.Therefore, the optimum modification temperature was 80 °C.It might be interpreted as follows.The low modification temperature is not conducive to the dissolution of the modifier in water and the hydrolysis of RCOO − to RCOOOH, thus reducing the content of effective components of the modifier and leading to the poor modification effect.However, the solvents evaporate too fast at higher temperatures, thus leading to an increase in the solid content and viscosity of the slurry and affecting the full contact between the modifier and composite particles.

Influences of Modifier Dosage
Figure 3 shows the influences of the SS dosage on the activation index and sedimentation rate of CaCO 3 -TiO 2 .The modification temperature was 80 • C and the other experiment conditions were the same with those in Section 3.1.1.When the SS dosage increased from 0 to 2.0 wt %, the activation index of the modified products increased greatly, but the sedimentation rate decreased greatly (Figure 3).When the modifier dosage was more than 2.0 wt %, the activation index and sedimentation rate tended to be stable.The above results indicated that the minimum optimal dosage of SS was 2 wt %.It might be interpreted as follows.The modifier adsorbed on the surface of CaCO 3 -TiO 2 had reached the saturation state under the dosage of 2 wt % and the excess modifier could not be adsorbed.
Minerals 2019, 9 FOR PEER REVIEW 5 Figure 3 shows the influences of the SS dosage on the activation index and sedimentation rate of CaCO3-TiO2.The modification temperature was 80 °C and the other experiment conditions were the same with those in Section 3.1.1.When the SS dosage increased from 0 to 2.0 wt %, the activation index of the modified products increased greatly, but the sedimentation rate decreased greatly (Figure 3).When the modifier dosage was more than 2.0 wt %, the activation index and sedimentation rate tended to be stable.The above results indicated that the minimum optimal dosage of SS was 2 wt %.It might be interpreted as follows.The modifier adsorbed on the surface of CaCO3-TiO2 had reached the saturation state under the dosage of 2 wt % and the excess modifier could not be adsorbed.

Influences of the Mechanical Strength in Grinding
In the process of mechanical activation modification, the mechanical grinding force plays an important role in promoting the contact and bonding between the modifier and particle surface and the mechanical grinding strength is mainly determined by the grinding speed and grinding time.Therefore, the influences of the grinding speed and grinding time on the activation index and sedimentation rate of CaCO3-TiO2 were investigated (Figure 4).Under 2.0 wt % SS at 80 °C, the other experimental conditions were the same as those in Section 3.1.2.The grinding speed had little effect on the activation index and sedimentation rate of CaCO3-TiO2 (Figure 4a).In terms of the changes of the activation index and sedimentation rate, the modification effect increased with the increase of the grinding speed, but the increasing amplitude was not large.The optimum grinding speed was 1000 rpm.As the grinding time increased, the activation index of the modified CaCO3-TiO2 increased firstly and then decreased, whereas the sedimentation rate decreased firstly and then increased (Figure 4b).After 15-min of modification grinding, both the activation index and sedimentation rate indexes reached their optimal values.Obviously, the optimum modified grinding time was 15 min.It might be interpreted as follows.If the grinding time was too short, the mechanical strength was too low to activate the surface of CaCO3-TiO2, thus resulting in a weak reaction between CaCO3-TiO2 and SS (RCOO − or RCOOH).If the mechanical strength was too high, the modified products would be stripped and even the composite structure of CaCO3-TiO2 would be partly destroyed.

Influences of the Mechanical Strength in Grinding
In the process of mechanical activation modification, the mechanical grinding force plays an important role in promoting the contact and bonding between the modifier and particle surface and the mechanical grinding strength is mainly determined by the grinding speed and grinding time.Therefore, the influences of the grinding speed and grinding time on the activation index and sedimentation rate of CaCO 3 -TiO 2 were investigated (Figure 4).Under 2.0 wt % SS at 80 • C, the other experimental conditions were the same as those in Section 3.1.2.The grinding speed had little effect on the activation index and sedimentation rate of CaCO 3 -TiO 2 (Figure 4a).In terms of the changes of the activation index and sedimentation rate, the modification effect increased with the increase of the grinding speed, but the increasing amplitude was not large.The optimum grinding speed was 1000 rpm.As the grinding time increased, the activation index of the modified CaCO 3 -TiO 2 increased firstly and then decreased, whereas the sedimentation rate decreased firstly and then increased (Figure 4b).After 15-min of modification grinding, both the activation index and sedimentation rate indexes reached their optimal values.Obviously, the optimum modified grinding time was 15 min.It might be interpreted as follows.If the grinding time was too short, the mechanical strength was too low to activate the surface of CaCO 3 -TiO 2 , thus resulting in a weak reaction between CaCO 3 -TiO 2 and SS (RCOO − or RCOOH).If the mechanical strength was too high, the modified products would be stripped and even the composite structure of CaCO 3 -TiO 2 would be partly destroyed.

Pigment Properties of Modified CaCO3-TiO2
The changes in the main pigment properties of CaCO3-TiO2 before and after modification are presented in Table 2.After wet mechanochemical modification, the hiding power of CaCO3-TiO2 decreased from 19.6 g/m 2 to 16.9 g/m 2 under 1.5 wt % SS and 16.7 g/m 2 under 2 wt % SS, indicating that the hiding property of CaCO3-TiO2 was improved significantly by wet mechanochemical modification.The oil absorption of CaCO3-TiO2 decreased significantly and the whiteness remained unchanged.Obviously, the surface organic modification by the mechanochemical method significantly improved the pigment performance of CaCO3-TiO2.According to the analysis of the pigment properties, the improvement in the hiding property of CaCO3-TiO2 by modification was mainly caused by the improvement in particle dispersion.

Dispersion Properties
Figure 5 shows the images of unmodified CaCO3-TiO2 and modified CaCO3-TiO2 dispersed in ethanol (organic polar solvent) and kerosene (organic non-polar solvent).Unmodified CaCO3-TiO2 existed in the form of large-scale aggregates in ethanol and kerosene and the distribution of CaCO3-TiO2 was uneven (Figure 5a,b).The sizes of the aggregates in ethanol and kerosene were, respectively, larger than 100 μm and 200 μm, indicating the poor dispersion effect.As for the modified CaCO3-TiO2, although there were still some aggregates in ethanol and kerosene, the size of the aggregates significantly decreased below 10 μm (Figure 5c,d).The distribution of particles was more uniform, indicating that the dispersion of modified CaCO3-TiO2 in organic media was significantly improved.Undoubtedly, modification is an important factor for the improvement in the performance of CaCO3-TiO2.The result is consistent with Table 2.The changes in the main pigment properties of CaCO 3 -TiO 2 before and after modification are presented in Table 2.After wet mechanochemical modification, the hiding power of CaCO 3 -TiO 2 decreased from 19.6 g/m 2 to 16.9 g/m 2 under 1.5 wt % SS and 16.7 g/m 2 under 2 wt % SS, indicating that the hiding property of CaCO 3 -TiO 2 was improved significantly by wet mechanochemical modification.The oil absorption of CaCO 3 -TiO 2 decreased significantly and the whiteness remained unchanged.Obviously, the surface organic modification by the mechanochemical method significantly improved the pigment performance of CaCO 3 -TiO 2 .According to the analysis of the pigment properties, the improvement in the hiding property of CaCO 3 -TiO 2 by modification was mainly caused by the improvement in particle dispersion.

Dispersion Properties
Figure 5 shows the images of unmodified CaCO 3 -TiO 2 and modified CaCO 3 -TiO 2 dispersed in ethanol (organic polar solvent) and kerosene (organic non-polar solvent).Unmodified CaCO 3 -TiO 2 existed in the form of large-scale aggregates in ethanol and kerosene and the distribution of CaCO 3 -TiO 2 was uneven (Figure 5a,b).The sizes of the aggregates in ethanol and kerosene were, respectively, larger than 100 µm and 200 µm, indicating the poor dispersion effect.As for the modified CaCO 3 -TiO 2 , although there were still some aggregates in ethanol and kerosene, the size of the aggregates significantly decreased below 10 µm (Figure 5c,d).The distribution of particles was more uniform, indicating that the dispersion of modified CaCO 3 -TiO 2 in organic media was significantly improved.Undoubtedly, modification is an important factor for the improvement in the performance of CaCO 3 -TiO 2 .The result is consistent with Table 2.

Particle Characteristics
Figure 6 shows the scanning electron microscope (SEM) images of unmodified and modified CaCO3-TiO2.The unmodified and modified CaCO3-TiO2 samples exhibited a good coating morphology.Fine TiO2 particles were uniformly and compactly coated on the surface of CaCO3 particles, indicating that organic modification did not significantly change the structure

Particle Characteristics
Figure 6 shows the scanning electron microscope (SEM) images of unmodified and modified CaCO 3 -TiO 2 .The unmodified and modified CaCO 3 -TiO 2 samples exhibited a good coating morphology.Fine TiO 2 particles were uniformly and compactly coated on the surface of CaCO 3 particles, indicating that organic modification did not significantly change the structure and morphology of composite particles.Additionally, d 50 and d 90 of unmodified CaCO 3 -TiO 2 were, respectively, 0.78 µm and 1.72 µm, and the d 50 and d 90 of modified CaCO 3 -TiO 2 were, respectively, 0.72 µm and 1.65 µm.The specific surface areas of unmodified and modified CaCO 3 -TiO 2 were, respectively, 6.29 g/m 2 and 6.41 g/m 2 .Short-time grinding in the modification process did not cause a significant decrease in the particle size or a significant increase in the specific surface area of the composite particles, indicating that the composite particles were not significantly pulverized.

Particle Characteristics
Figure 6 shows the scanning electron microscope (SEM) images of unmodified and modified CaCO3-TiO2.The unmodified and modified CaCO3-TiO2 samples exhibited a good coating morphology.Fine TiO2 particles were uniformly and compactly coated on the surface of CaCO3 particles, indicating that organic modification did not significantly change the structure and morphology of composite particles.Additionally, d50 and d90 of unmodified CaCO3-TiO2 were, respectively, 0.78 μm and 1.72 μm, and the d50 and d90 of modified CaCO3-TiO2 were, respectively, 0.72 μm and 1.65 μm.The specific surface areas of unmodified and modified CaCO3-TiO2 were, respectively, 6.29 g/m 2 and 6.41 g/m 2 .Short-time grinding in the modification process did not cause a significant decrease in the particle size or a significant increase in the specific surface area of the composite particles, indicating that the composite particles were not significantly pulverized.

Wetting Contact Angle of CaCO3-TiO2
The wetting contact angles of unmodified CaCO3-TiO2 and modified CaCO3-TiO2 (SS dosage = 0.5% and 1.5%) with distilled water, glycerol, and n-hexane are presented in Table 3.The contact angles of modified CaCO3-TiO2 with water and glycerol were greatly increased compared with that of unmodified CaCO3-TiO2 and the angles were further increased proportionally with the increase in SS dosage.However, the contact angle of modified CaCO3-TiO2 with n-hexane decreased, indicating that the organic modification of CaCO3-TiO2 resulted in a much weaker degree of wetting and the weaker interaction between CaCO3-TiO2 and the organic solvents (water and glycerol), whereas the interaction between CaCO3-TiO2 and n-hexane was enhanced.It could be inferred that the modification converted the polar surface of CaCO3-TiO2 into an organic non-polar surface, which was similar to n-hexane.Obviously, the conversion was ascribed to the adsorption of SS on the surface of CaCO3-TiO2.The wetting contact angles of unmodified CaCO 3 -TiO 2 and modified CaCO 3 -TiO 2 (SS dosage = 0.5% and 1.5%) with distilled water, glycerol, and n-hexane are presented in Table 3.The contact angles of modified CaCO 3 -TiO 2 with water and glycerol were greatly increased compared with that of unmodified CaCO 3 -TiO 2 and the angles were further increased proportionally with the increase in SS dosage.However, the contact angle of modified CaCO 3 -TiO 2 with n-hexane decreased, indicating that the organic modification of CaCO 3 -TiO 2 resulted in a much weaker degree of wetting and the weaker interaction between CaCO 3 -TiO 2 and the organic solvents (water and glycerol), whereas the interaction between CaCO 3 -TiO 2 and n-hexane was enhanced.It could be inferred that the modification converted the polar surface of CaCO 3 -TiO 2 into an organic non-polar surface, which was similar to n-hexane.Obviously, the conversion was ascribed to the adsorption of SS on the surface of CaCO 3 -TiO 2 .Note: X% in the modified CaCO 3 -TiO 2 -X% represents the dosage of SS.

Calculation of Surface Free Energy
When the solid particle is wetted by liquid, the relationship among the surface free energy of the solid, the surface free energy of the liquid, and the contact angle can be expressed as [23][24][25]: where θ represents the wetting contact angle for the solid-liquid interface; γ L and γ S respectively represent the surface free energy of the liquid and solid; γ L + and γ L − are, respectively, the electron acceptor part and the electron donor part in the polar component of γ L ; γ S + and γ S − are, respectively, the electron acceptor part and the electron donor part in the polar component of γ S ; γ L LW and γ S LW are, respectively, the non-polar components of γ L and γ S .The polarity component of γ S AB of γ S can be calculated from γ S + and γ S − : The γ S can be calculated from γ S AB and γ S LW : The surface free energy parameters (γ L , γ L LW , γ L + , and γ L − ) of water, glycerol, and n-hexane are presented in Table 4 [26].The values of γ S and its components can be obtained by substituting the parameters in Table 4 and the wetting contact angle in Table 3 into Equation (1) based on Equations ( 2) and ( 3).The calculation results are shown in Table 5.
Table 5 shows that the surface free energy of CaCO 3 -TiO 2 is greatly reduced after modification by SS and the decrease in amplitude is increased with the increase in SS dosage.When the dosage of SS was 1.5%, the γ S value decreased from 74.21 mJ/m 2 to 11.62 mJ/m 2 , and the decrease was as high as 62.59 mJ/m 2 .From the perspective of the composition of γ S , the decrease in the γ S of CaCO 3 -TiO 2 was mainly caused by the decrease of the polar component (γ S AB ), whereas the non-polar component, γ L LW , was slightly improved.Due to the modification, the properties of the polar components on the particle surface were masked, whereas the non-polar properties of the alkyl chain of modifier were displayed.Since the change in the surface free energy of the particles was an important factor affecting the interface free energy and dispersion behavior of particles in the medium, it was considered that the above result was consistent with the change in the pigment performance of CaCO 3 -TiO 2 (Table 2).3.4.Binding Properties between CaCO 3 -TiO 2 and Modifier

Infrared Spectral Analysis
To investigate the binding properties between the modifier and CaCO 3 -TiO 2 , the infrared spectra of unmodified CaCO 3 -TiO 2 , modified CaCO 3 -TiO 2 , and SS were analyzed (Figure 7).In the infrared spectra of CaCO 3 -TiO 2 , the absorption band at 3277 cm −1 is ascribed to the stretching vibration absorption band of hydroxyl groups, indicating that several hydroxyl groups or a small amount of coordination water molecules are adsorbed on the particle surface.The broadness of the peak indicates the existence of the association between hydroxyl groups.This is undoubtedly the consequence of the Minerals 2019, 9, 112 9 of 12 dehydroxylation between CaCO 3 and TiO 2 and the hydration of TiO 2 in CaCO 3 -TiO 2 .The absorption bands at 1433 cm −1 and 874 cm −1 can be assigned to the asymmetric stretching vibration and bending vibration of CO 3 2− [27].The broad band around 651 cm -1 is ascribed to the stretching vibrations of Ti-O bonds and the peak at 421 cm −1 is ascribed to the Ti-O-Ti bonds.In the infrared spectra of the modified CaCO 3 -TiO 2 , the characteristic absorption bands of -OH groups at 3414 cm −1 are different from those of CaCO 3 -TiO 2 and the modifier SS due to the reaction between the hydroxyl groups of CaCO 3 -TiO 2 and modifier SS.In addition to the characteristic bands of CaCO 3 and TiO 2 , the observed characteristic band of SS at 2927 cm −1 is the characteristic peak of H-C-H asymmetric stretching vibrations.Therefore, it can be deduced that SS was adsorbed on the surface of CaCO 3 -TiO 2 .
The characteristic peaks of COO − at 1555 and 1443 cm −1 in the spectrum of SS might shift and coincide with the peak of CO 3 2− in the spectrum of modified CaCO 3 -TiO 2 [28].The above analysis indicates that the modifier is most likely to be chemisorbed on the surface of particles through the reaction of hydroxyl groups.
To investigate the binding properties between the modifier and CaCO3-TiO2, the infrared spectra of unmodified CaCO3-TiO2, modified CaCO3-TiO2, and SS were analyzed (Figure 7).In the infrared spectra of CaCO3-TiO2, the absorption band at 3277 cm −1 is ascribed to the stretching vibration absorption band of hydroxyl groups, indicating that several hydroxyl groups or a small amount of coordination water molecules are adsorbed on the particle surface.The broadness of the peak indicates the existence of the association between hydroxyl groups.This is undoubtedly the consequence of the dehydroxylation between CaCO3 and TiO2 and the hydration of TiO2 in CaCO3-TiO2.The absorption bands at 1433 cm −1 and 874 cm −1 can be assigned to the asymmetric stretching vibration and bending vibration of CO3 2− [27].The broad band around 651 cm -1 is ascribed to the stretching vibrations of Ti-O bonds and the peak at 421 cm −1 is ascribed to the Ti-O-Ti bonds.In the infrared spectra of the modified CaCO3-TiO2, the characteristic absorption bands of -OH groups at 3414 cm −1 are different from those of CaCO3-TiO2 and the modifier SS due to the reaction between the hydroxyl groups of CaCO3-TiO2 and modifier SS.In addition to the characteristic bands of CaCO3 and TiO2, the observed characteristic band of SS at 2927 cm −1 is the characteristic peak of H-C-H asymmetric stretching vibrations.Therefore, it can be deduced that SS was adsorbed on the surface of CaCO3-TiO2.The characteristic peaks of COO − at 1555 and 1443 cm −1 in the spectrum of SS might shift and coincide with the peak of CO3 2− in the spectrum of modified CaCO3-TiO2 [28].The above analysis indicates that the modifier is most likely to be chemisorbed on the surface of particles through the reaction of hydroxyl groups.

XPS Analysis
Figure 8 shows the XPS spectra of unmodified CaCO3-TiO2 and modified CaCO3-TiO2 as well as the changes in the binding energy of Ca and Ti.In Figure 8a, the peaks appeared at 346.92 eV and 350.48 eV in the XPS spectrum of unmodified CaCO3-TiO2 and correspond to Ca2p3/2 and Ca2p1/2.It can be seen that there is no significant displacement of Ca in the binding energy after the modification

XPS Analysis
Figure 8 shows the XPS spectra of unmodified CaCO 3 -TiO 2 and modified CaCO 3 -TiO 2 as well as the changes in the binding energy of Ca and Ti.In Figure 8a, the peaks appeared at 346.92 eV and 350.48 eV in the XPS spectrum of unmodified CaCO 3 -TiO 2 and correspond to Ca2p 3/2 and Ca2p 1/2 .It can be seen that there is no significant displacement of Ca in the binding energy after the modification of CaCO 3 -TiO 2 .In Figure 8b, the peaks at 458.41 eV and 464.10 eV in the XPS spectra of unmodified CaCO 3 -TiO 2 correspond to Ti2p 3/2 and Ti2p 1/2 [29].In the XPS spectrum of modified CaCO 3 -TiO 2 , the corresponding peaks of Ti2p at 458.35 eV and 464.04 eV show no obvious displacement compared with those of unmodified CaCO 3 -TiO 2 , indicating that the chemical environment of the Ti element is unchanged after modification.In other words, the modifier does not chemically bond with Ti 4+ directly.Due to the strong hydration of TiO 2 , the surface of TiO 2 is covered by a large number of hydroxyl groups [30], and the change in the outermost hydroxyl groups has little effect on the binding energy of Ti atoms.The results of the IR spectrum and XPS spectra indicate that CaCO 3 -TiO 2 may bond with SS through the dehydroxylation between hydroxyl groups on the surface of TiO 2 and SS.
with those of unmodified CaCO3-TiO2, indicating that the chemical environment of the Ti element is unchanged after modification.In other words, the modifier does not chemically bond with Ti 4+ directly.Due to the strong hydration of TiO2, the surface of TiO2 is covered by a large number of hydroxyl groups [30], and the change in the outermost hydroxyl groups has little effect on the binding energy of Ti atoms.The results of the IR spectrum and XPS spectra indicate that CaCO3-TiO2 may bond with SS through the dehydroxylation between hydroxyl groups on the surface of TiO2 and SS.According to the above analysis, SS was chemisorbed on the surface of CaCO3-TiO2 and this adsorption occurred mainly in the TiO2 region on the surface of the composite particles.
There is a small number of uncovered CaCO3 and a large number of coating TiO2 on the surface of CaCO3-TiO2 [18].For CaCO3, its outermost surfaces are coated by TiO2 and there are only a small number of unsaturated Ca 2+ and CO3 2− on its surface.In the aqueous medium, Ti 4+ on the surface of TiO2 is strongly hydrolyzed to form hydrolyzates (mainly including Ti 4+ hydroxylate), which finally form a surface morphology of TiO2 dominated by hydroxyl groups [31].Therefore, the hydroxyl in TiO2 is the group involved in the chemical reactions between TiO2 and the modifier.The modifier SS will undergo the following hydrolysis reactions in aqueous medium: RCOO  + H2O  RCOOH + OH − (5) Therefore, RCOOH should be the main group involved in the chemical reactions between the modifier and CaCO3-TiO2.
Based on the above analysis, the surface organic modification model of CaCO3-TiO2 by SS was established (Figure 9).According to the above analysis, SS was chemisorbed on the surface of CaCO 3 -TiO 2 and this adsorption occurred mainly in the TiO 2 region on the surface of the composite particles.
There is a small number of uncovered CaCO 3 and a large number of coating TiO 2 on the surface of CaCO 3 -TiO 2 [18].For CaCO 3 , its outermost surfaces are coated by TiO 2 and there are only a small number of unsaturated Ca 2+ and CO 3 2− on its surface.In the aqueous medium, Ti 4+ on the surface of TiO 2 is strongly hydrolyzed to form hydrolyzates (mainly including Ti 4+ hydroxylate), which finally form a surface morphology of TiO 2 dominated by hydroxyl groups [31].Therefore, the hydroxyl in TiO 2 is the group involved in the chemical reactions between TiO 2 and the modifier.The modifier SS will undergo the following hydrolysis reactions in aqueous medium: RCOONa → RCOO − + Na + , (4) Therefore, RCOOH should be the main group involved in the chemical reactions between the modifier and CaCO 3 -TiO 2 .
Based on the above analysis, the surface organic modification model of CaCO 3 -TiO 2 by SS was established (Figure 9).

Figure 1 .
Figure 1.Measurement device of the sedimentation rate of powder samples.

Figure 1 .
Figure 1.Measurement device of the sedimentation rate of powder samples.

Figure 2 .
Figure 2. Influences of the modification temperature on the activation index and sedimentation rate of CaCO3-TiO2.

Figure 2 .
Figure 2. Influences of the modification temperature on the activation index and sedimentation rate of CaCO 3 -TiO 2 .

Figure 3 .
Figure 3. Influences of the modifier dosage on the activation index and sedimentation rate of CaCO3-TiO2.

Figure 3 .
Figure 3. Influences of the modifier dosage on the activation index and sedimentation rate of CaCO 3 -TiO 2 .

Figure 4 .
Figure 4. Influences of the grinding speed (a) and grinding time (b) on the activation index and sedimentation rate of CaCO3-TiO2.
Note: X% in the modified CaCO 3 -TiO 2 -X% represents the dosage of SS.

Table 4 .
Surface free energy parameters of different solvents (mJ/m 2 ).

Table 5 .
Surface free energy and its components of CaCO 3 -TiO 2 (mJ/m 2 ).: X% in the modified CaCO 3 -TiO 2 -X represents the dosage of SS. Note