*3.4. Aggregation of Ground Talc*

Laser diffraction particle size analysis and BET-specific surface area analysis are general methods to analyze the changes in particle size upon grinding. Though the calculated ESD based on the BET-specific surface area is much smaller than the results of particle size analysis because the shape of phyllosilicate talc and the increase in specific surface area generally indicates the decrease in particle size. According to the laser diffraction particle size analysis, the particle size of talc ground using 2 mm and 1 mm balls significantly decreased until 60 min of grinding, and further size reduction was not subsequently observed. On the contrary, BET-specific surface area continuously increases after 60 min of grinding. In addition, the significant difference in a specific surface area between talc powders ground using 2 mm and 1 mm balls is observed while the result of the particle size analysis shows only a trivial difference. These differences between the laser diffraction particle size and surface area analysis can appear when the ultrafine talc particles are aggregated by grinding [12]. In the BET-specific surface area analysis, there is a rare effect of aggregation because the N2 gas is adsorbed to the particle surface. However, in the particle size analysis using laser diffraction, the size of aggregates can be measured rather than the individual particles. These results indicate that aggregation of ultrafine talc powder occurred as a result of grinding.

The aggregation of ground talc particles can be observed in SEM images of the talc particles grounded using 1 mm balls (Figure 8).

Before grinding, a unique laminar structure of talc is clearly observed with a size of tens of micrometers. With increasing grinding time, the particle size decreases to approximately 5 μm after 30 min, less than approximately 1 μm after 240 min, and approximately 200–300 nm after 360 min of grinding. The aggregation of ultrafine particles is clearly observed after 240 min of grinding, which is consistent with results of the laser diffraction particle size and BET-specific surface area analysis.

**Figure 8.** SEM images of ground talc for (**a**) 0 min, (**b**) 30 min, (**c**) 240 min, and (**d**) 360 min using 1 mm balls for high-energy ball milling.

In order to understand the effect of ball size on the aggregation of ground talc, Figure 9 compares the particle size analysis results of talc powders with similar specific surface areas. As described above, we obtain the ground talc series that have a similar specific surface area of talc particles that are approximately 85 m2/g and 170 m2/g. The laser diffraction particle size of the ground talc using 2 mm and 1 mm balls showed similar particle size distribution when compared with the case of using 0.1 mm balls. In both series, the intensity of the sub-microscale group was larger when using 2 mm and 1 mm balls than when using 0.1 mm balls, which indicates a larger particle size when using 0.1 mm balls.

The different particle size distributions with similar surface area could be observed if the aggregation of ultrafine particles occurred as a result of grinding. The larger particle size distribution indicates the more aggregated status of talc powder. In this study, the ground talc powder using 0.1 mm balls shows a larger particle size than other talc powders with similar surface areas, which indicates that it is easily aggregated due to grinding.

**Figure 9.** Comparison of particle size analysis results for ground talc powder having similar specific surface area. The BET specific surface areas are (**a**) 81–88 m2/g and (**b**) 165–172 m2/g.

The increased aggregation of ground talc with 0.1 mm balls than with 1 mm or 2 mm balls could be related to the degree of dispersion in the aqueous solution used in the particle size analysis. The aggregation of ultrafine particles generally occurs in order to reduce the high surface energy [34,35]. The talc powders with a similar surface area and different aggregation behavior indicate that these ground talc powders have a different surface energy and/or property. The results of XRD analysis showed that different crystallinity of ground talc powder depends on ball size. The crystallinity of ground talc is higher when using 0.1 mm balls than when using 1 mm and 2 mm balls. Crystalline talc is hydrophobic due to the siloxane sheet of the T-O-T layer. Hence, it aggregates well in aqueous solution [1,36]. On the other hand, disordered talc with lower crystallinity due to grinding is delaminated. Breakage of the layers exposes the hydroxyl groups in the octahedral layer. As a result, the talc is less hydrophobic due to the loss of crystallinity [8,37,38]. In an aqueous solution, the more hydrophilic talc is better dispersed.

As shown in Figure 3, the crystallinity of ground talc prepared using 0.1 mm balls during grinding is higher than that using 1 mm and 2 mm balls even though these talc powders have similar surface areas. The ground talc with relatively higher crystallinity (i.e., when using 0.1 mm balls for grinding) has higher surface energy than other ground talc powders (i.e., when using 1 mm and 2 mm balls for grinding). Thus, increased aggregation could occur in ground talc using 0.1 mm balls due to the loss of crystallinity and hydrophobicity.

#### **4. Discussion**

As described above, the grinding of talc using a high-energy ball mill results in differences in crystallinity, particle size, specific surface area, and aggregation behavior. Figure 10 shows the changes in the crystallinity, particle size, and specific surface area of talc, according to the ball size used in milling.

**Figure 10.** Effect of ball size for high-energy ball milling on (**a**) relative intensities of (001) XRD peak, (**b**) particle size (D50), and (**c**) specific surface area.

#### *4.1. E*ff*ects on Crystallinity of Ground Talc*

The crystallinity of talc decreased upon grinding, and the degree of decrease varied depending on the ball size (Figure 10a). When 1 mm and 2 mm balls were used in grinding, the crystallinity decreased rapidly within 60 min of grinding, and delamination of (*00l*) planes mainly occurred. After 120 min of grinding, almost all XRD peaks disappeared. In contrast, when grinding was performed using 0.1 mm balls, not only was there a reduction in the crystallinity decrease, but also crystal surfaces other than the (*00l*) surface were fractured together. The crystallinity decreased continuously even after 120 min, but it was still much higher when compared with the case of using 1 mm and 2 mm balls. Compared with it prior to grinding, the (001) peak heights of XRD decreased to 91.7%, 23.9%, and 14.3% of the original size after 30 min of grinding when 0.1 mm, 1 mm, and 2 mm balls were used, respectively. After 120 min of grinding, it decreased to 85.1%, 4.7%, and 1.3%, respectively, and, after

360 min of grinding, it decreased to 43.9%, 2.2%, and 0.1%, respectively. When comparing the XRD patterns of ground talc with a similar specific surface area, the smaller the ball size used, the higher the final crystallinity of the talc is. These results indicate that the smaller ball size is appropriate to grind the talc while retaining the crystalline structure.
