*4.2. E*ff*ects on Comminution Rate and Limitation*

The ball size in ball milling also affects the comminution rate and the limitation in particle size reduction. The increase in the comminution rate is clearly observed in the variation of particle size and BET-specific surface area, according to the ball size (Figure 10b,c). The BET-specific surface area of ground talc powders for 360 min shows approximately 419.1, 365.1, and 171.7 m2/g using 2 mm, 1 mm, and 0.1 mm sized balls, respectively. The ESD for those talc powders correspond to 5.1, 5.9, and 12.5 nm, respectively. The particle size measured by the laser diffraction method also corresponds to the submicroscale when using 1 mm and 2 mm balls, but to the microscale when using 0.1 mm balls after grinding for 360 min. According to the variation of BET-specific surface area, the comminution rate increased with increasing ball size. At the early stage of ball milling, the comminution rate difference between using 1 mm and 2 mm balls is insignificant. The comminution rate when using 1 mm balls is slightly faster than when using 2 mm balls at the early stage of grinding, but both the comminution rate and the limitation in particle size reduction are more effective when using 2 mm balls in prolonged grinding.

#### *4.3. E*ff*ects on Aggregation*

The ball size used in ball milling also affects the aggregation properties of talc. The larger the ball size used for grinding, the less aggregation occurred for talc powders with similar specific surface areas. When 1 mm and 2 mm balls were used, aggregates of submicron particles were formed in most cases, as observed in laser diffraction particle size analysis. However, when 0.1 mm balls were used, aggregates of several micron particles were formed, as shown in Figure 9. The differences between the particle size analysis and the specific surface area analysis would be caused by the formation of aggregates of the sample due to ongoing grinding. This means that, as the ball size increases, the aggregates are formed more easily. When talc powders with similar specific surface areas were compared, the smaller the ball size is, the larger the aggregates formed are. The aggregation behavior of ground talc varies with the ball size used during milling, which is the result of different talc crystallinities. The loss of talc crystallinity results in a loss of hydrophobicity [8,37,38] and, consequently, better dispersion in an aqueous solution. The ground talc exhibits more hydrophilic properties when larger balls are used during milling.

### **5. Conclusions**

In this study, talc was ground at a fast rotation speed of 2000 rpm using a high-energy ball mill, and ultrafine talc particles were obtained. Furthermore, the effects of ball size on the grinding efficiency and crystallinity of talc were investigated. When a larger ball size was used in ball milling, the crystallinity decreased faster, and the grinding efficiency and the specific surface area of the talc increased. In the case of crystallinity, the smaller the ball size was, the smaller the decrease in crystallinity was. Therefore, using a small ball size can produce talc nanoparticles with higher crystallinity. Upon the progress of milling, the aggregation of talc particles occurs and the degree of aggregation shows the relations with crystallinity of talc. Comparing the ground talc with similar specific surface areas, the more disordered the structure of the ground talc is, the fewer aggregation and more hydrophilic properties are observed.

**Author Contributions:** Conceptualization, H.N.K. Validation, J.W.K. and M.S.K. Investigation, H.N.K., J.W.K., M.S.K., B.H.L. and J.C.K. Writing—original draft preparation, H.N.K. and J.W.K. Writing—review and editing, H.N.K. Visualization, J.W.K. and M.S.K. Project administration, H.N.K. Funding acquisition, H.N.K. Please turn to the CRediT taxonomy for the term explanation.

**Funding:** The National Research Foundation of Korea (grant number NRF-2016R1C1B1010108) supported this work and the "Human Resources Program in Energy Technology" of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (grant number 20194010201730).

**Conflicts of Interest:** The authors declare no conflict of interest.
