## 3 Results and Discussion
### The Influence of the Studied Parameters on the Responses  
The unpredictable effects of the processing parameters on POA particle size are shown in Figure 6. According to the main effect of the S/N ratio presented in Figure 6, the effects on the production of POA nanoparticles increase with higher milling time. Despite the milling time considered as an influential parameter in this process, nevertheless with longer milling time, the POA particles tend to agglomerate due to the rapid reduction of POA size and would be challenging to analyze by particle size analyzer. This is also supported by [PERSON] [26], where nanoparticles tend to agglomerate and are challenging to disperse even though using the ultrasonic treatment. Moreover, there is a possibility of powder contamination due to chipping or breakage from the stainless steel ball and the inner wall of the milling jar. As tested by [PERSON] and [PERSON] [27], an increase of milling time will affect mineralogy and physicochemical of POA due to the creation of new surface and particle breakage, which undergoes structural changes and mechanical activation. Lower milling speed, as illustrated in Figure 6, showed a higher impact on the production of POA nanoparticles. In the ball milling process, rotational at higher speed could affect the particle size distribution and particle characteristics. Findings from [16] revealed that heat would be produced due to impact and abrasion between balls and milling jar walls. Hence, the energy input into the powders will increase and could deform the properties of particles. Additionally, the high temperatures generated may also contaminate the transformations of powders. As reported by [PERSON] [28], higher milling intensities could increase the average crystal size due to the enhanced dynamical recrystallization. However, there are certain limitations to the maximum speed that could be engaged depending on the design of the mill and milling jar. In fact, almost all researchers applied more than 100 rpm even higher up to 2000 rpm to produce nanoparticles [29]. Though, the ball milling practice \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Level** & **Milling Time** & **Milling Speed** & **Size of Balls** \\ \hline
1 & 0.34422 & 1.72604 * & 1.83909 \\
2 & 1.01416 & 1.53138 & \(-\)0.09304 \\
3 & 2.3507 * & 0.45170 & 1.96308 * \\ Delta \(\Delta\) & 2.00652 & 1.27434 & 2.05612 \\ Rank & 2 & 3 & 1 \\ \hline \hline \end{tabular}  
* Optimum parameter level.  
\end{table} 
Table 4: Signal to noise ratio for each parameter and level. Figure 6: Main effects of S/N ratio for POA particles size versus the level of input parameters. beyond the maximum speed limit will cause the balls to pin to the inner wall of the milling jar and could not produce any impact force. In the meantime, an increase in the size of balls in the ball milling process, as illustrated in Figure 6, showed a significant effect in producing POA nanoparticles. The ball size is a critical parameter influencing the performance of a ball milling process. It is well known that to produce effective breakage of large particles, larger balls are needed, whereas smaller balls are needed for the effective breakage of fine particles. The optimization of balls size was very limited due to variables data obtained from the previous experiment. Nevertheless, research by [PERSON] and [PERSON] [30] found out that using a larger size of balls would contribute minimal impact on the rate production of particles. Based on the effect of product fineness study by [PERSON], [PERSON] [31], to produce the finer particle distribution, the higher quantity of smaller balls are needed. This was due to the total expended energy between balls and particles that assist the deagglomeration of the fine particles.