# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 4 Discussion  
Considering the major role played by fibre length in determining the toxicological action of asbestos, controlling this parameter is of paramount importance in targeted studies. Preparing batches of chrysotile fibres only differing in length generally involve size reduction of pristine fibres followed by separation. The first step is crucial, as alternation of the fibre properties must be avoided. The scientific literature has discarded traditional milling by way of reducing fibre length, as the mechanical action leads to plastic deformation of the chrysotile structure (Table 1). In the present study, cryogenic milling was systematically explored as a potential size reduction technique. More precisely, the structural integrity was verified following various  
Figure 5: (**a**) FTIR spectra (600–4000 cm\({}^{-1}\)) of raw Russian chrysotile (CHR), chrysotile after 5 min of milling (CHR5), chrysotile after 10 min of milling (CHR10), chrysotile after 20 min milling (CHR20), chrysotile after 30 min of milling (CHR30) and chrysotile after 40 min of milling (CHR40). (**b**) The OH-stretching region. milling durations, using XRPD and FTIR, which are powerful techniques in evidencing structural changes on a molecular level. The summary graphs in Figure 6 show the percentage of long (L > 5 \(\upmu\)m) and short (L < 5 \(\upmu\)m) Russian chrysotile fibres as a function of milling time (0 to 40 min). According to Figure 6, increased duration of cryogenic milling resulted in a successive reduction in fibre length. The longest milling duration investigated in this work (i.e., 40 min) resulted in 95% of the fibres having a length < 5 \(\upmu\)m (Figure 6). XRPD data (Figure 3) and FTIR spectra (Figure 5) did not evidence any structural changes imposed by milling, as the data remained substantially invariant. Instead, the contrary was observed in earlier studies dealing with traditional milling methods. For example, [PERSON] [31] studied the effect of dry milling on the crystal structure of chrysotile. The author observed important changes in the diffraction pattern following milling in terms of intensity loss and peak broadening [31]. IR analyses showed that dry milling led to important changes in the lattice vibrations, indicating a disrupted chrysotile structure and the formation of amorphous silica [31]. The IR data indicated the presence of adsorbed CO\({}_{2}\) and H\({}_{2}\)O on the surface of milled chrysotile, which, according to the author, was explained by newly formed reactive surface sites [31]. [PERSON] et al. [30] performed an FTIR investigation of asbestos-cement containing chrysotile before and after prolonged high-energy ball milling. In particular, changes in the intensity and position of the chrysotile bands provoked by milling were evaluated. The authors observed a gradual disappearance of the characteristic bands as the milling time increased, being virtually extinct following 3 h of ball milling [30]. [PERSON] et al. [37] conducted dry milling of chrysotile using a ring and puck mill and the XRPD data showed a continuous amorphization of the fibres, which, following 10 min of mechanical action, was near complete. The temperature during milling is a key factor determining the different outcome obtained by cryo-milling in comparison to traditional milling. Lowering the temperature to cryogenic conditions changes the mechanical behaviour of the material from ductile to brittle. Hence, the stress imposed by the milling medium results in brittle fracture rather than structural changes. In addition, temperature-induced dehydroxylation of chrysotile observed during traditional milling is avoided [37]. Even though the macroscopic temperature of the system generally is moderate, the local microscopic temperature can be even higher than 1000 \({}^{\circ}\)C [53]. The reaction mechanism by which chrysotile transforms into an amorphous phase following temperature-induced dehydroxylation has been elucidated in previous work [54].