**1. Introduction**

Talc (Mg3Si4O10(OH)2) is a phyllosilicate mineral with a T-O-T layer composed of tetrahedral silicon and octahedral magnesium, which share oxygen and are strongly bonded with each other. The weak van der Waals bonds between the T-O-T layers is the origin of softness of talc. The silicon in the siloxane sheet renders talc hydrophobic, since it cannot easily be substituted with aluminum [1]. Talc is physically easy to handle due to low hardness, does not react with acid due to its chemical stability, and has high adsorptivity, low plasticity, and low thermal/electrical conductivity. Because of these characteristics, talc is used as a coating, refractory, and additive in various industrial fields such as paper, paint, rubber, ceramic, refractory material, and polymer manufacturing. Chemical and pharmaceutical industries also require high-grade talc powders with high purity and uniform particle sizes. The grade and usage of talc is classified considering the purity, whiteness, particle size, and more. For example, talc with a particle size of 44 μm is used in ceramics and paints, 8–12 μm in paper, 7 μm in cosmetics, and 2 μm or smaller in rubber [2]. In addition, the nanoscale talc recently received attention for its application in areas such as improving the heat resistance of nanocomposites, and for use in waste water filter materials [3–5].

Given the importance of the above, various attempts have been made to produce ultrafine talc powders and better understand their physicochemical properties [6–11]. Various processes, such as planetary ball milling, tumbling milling, stirred ball milling, and disk milling, have been used to grind talc [10,12–17]. A reduction in particle size and the collapse of the crystalline structure of talc have been observed in mechanical grinding studies [8,10,12,13,15,18–20]. Table 1 summarize the particle size and/or surface area of ground talc with processed various milling methods. Delamination of the layered structure in the direction of the (00*l*) surface is the predominant mechanism of grinding, which, consequently, accompanies the breaking of individual plates and disordering in the talc crystal [8,10,15,21]. Further excessive grinding generally causes the collapse of the crystalline structure into the amorphous phase and the aggregation of ultrafine particles [13,14]. The degree of particle size reduction varies depending on the type of milling machine and milling conditions such as duration and rotating speed [11,21–26]. Besides the collapse of the crystalline structure, the physicochemical properties of talc such as thermal and dispersion behaviors, cation exchange capacity, wettability, and whiteness are also changed by mechanical grinding [6,10,12,14,27–29]. In accordance with the increasing demand for high-quality and ultrafine talc powder, the importance of comminution technology, capable of controlling crystallinity, particle shape, and particle size, is increasing.

**Table 1.** Specific surface area and equivalent spherical diameter of talc powder depending on the milling condition.


† S.S.A. = specific surface area. ‡ ESD = equivalent spherical diameter.

Planetary ball milling, which is one of the most frequently used lab-scale milling tools, has been applied to increase the grinding efficiency of talc. The collision impact and friction between powder samples and balls in a rotating jar grind the microscale talc into sub-microscale particles. The grinding efficiency of ball milling is greatly affected by the jar rotation speed, ball size, and ratio of balls and sample talc [22,24,25]. A faster rotation speed generally produces talc with a smaller particle size and lower crystallinity [11]. Recently developed high-energy ball mills reach rotation speeds of up to 2000 rpm and provide an opportunity to reduce the size of talc powder to the nanoscale level. The high-energy ball mill system consists of turn discs and jars. The oppositely rotating jar and turning disc produce highly energetic impact energy (Figure 1) [30]. However, while the ball size is an important factor in ball milling, the effect of the ball size on the comminution of talc has not yet been fully understood [31,32]. While the particle size is known to generally increase with an increasing ball size, in 2013, Shin et al. showed that particle size decreased and then increased as the ball size increased from 1 to 10 mm during the comminution of Al2O3 powder at low rotation speeds (50–153 rpm) [32]. The optimum ball size is dependent on the milling conditions and the properties of the mineral. Thus, to obtain ultrafine talc powder while minimizing the loss of crystallinity, a systematic approach is necessary to understand the effect of ball size in high-energy ball milling on the comminution of talc.

In this study, high-energy ball milling with high rotation speed of 2000 rpm is applied to obtain the nanoscale talc powder. The effect of the ball size for planetary ball milling on the grinding rate and behavior of talc is investigated with a varying ball size (2, 1, and 0.1 mm). Changes in the agglomeration characteristics of mechanochemical talc and the relationship with the ball size is also investigated.

**Figure 1.** Schematic view of the motion of the balls and powder in a jar during high energy ball milling.
