# Preparation of Polyacrylate Hollow Microspheres via Facile Spray Drying
## 3 Results and Discussion  
During the entire process, three key parameters affect the drying result and therefore the morphology of the formed polyacrylate hollow microspheres: (1) temperature, including inlet air temperature (i.e., the temperature of the hot air enters the tower) and outlet air temperature (i.e., the temperature of the cyclone separator above the receiver); (2) size of the droplets, which is mainly decided by the speed of the atomizer; and (3) feed rate of the emulsion. Properly tuning the inlet and the outlet air temperatures is particularly critical to the formation of the polyacrylate hollow microspheres. These temperatures not only affect the water evaporation rate, moisture content, and drying efficiency, but also directly determine the microsphere nucleation mode, thereby affecting the microsphere size and morphology. The effect of inlet and outlet temperatures on the microsphere bulk density is illustrated in Table 1. At a 28,000 rpm atomizer rotational speed and an emulsion feed rate of 160 g/min, the bulk density of the hollow microspheres decreased with an increasing inlet/outlet temperature, especially when the inlet air temperature was at 200-220 \({}^{\circ}\)C. As shown in Figure 2, inlet air temperature has an effect on the size, size distribution, and morphology of the prepared polyacrylate microspheres. The microsphere size increased very marginally when the inlet temperature was increased from 140 to 200 \({}^{\circ}\)C. However, a slight size increase occurred in the range of 200-220 \({}^{\circ}\)C. For example, the D(50) of the microspheres increased from 34 to 43 \(\upmu\)m when the inlet temperature was increased from 200 to 220 \({}^{\circ}\)C. The SEM images in Figure 2 show the morphology and surface structure of the microspheres prepared at various temperatures. One can observe that most of particles show a spherical morphology and a large portion of them are hollow, as shown in Figure 2a. Meanwhile, a small amount of broken fragments of microspheres are observed in Figure 2a-c, which is consistent with the literature [22]. Figure 1: Schematic of a high-speed centrifugal spray dryer. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Inlet** & **Outlet** & **bulk** & **D(10)/\(\mu\)m** & **D(50)/\(\mu\)m** & **D(90)/\(\mu\)m** \\
**Temperature/\({}^{\circ}\)C** & **Temperature/\({}^{\circ}\)C** & **Density/(g/cm\({}^{3}\))** & **D(10)/\(\mu\)m** & **D(50)/\(\mu\)m** & **D(90)/\(\mu\)m** \\ \hline
140 & 61 & 0.52 & 16 & 31 & 58 \\
160 & 66 & 0.50 & 18 & 33 & 58 \\
180 & 73 & 0.49 & 18 & 33 & 57 \\
200 & 84 & 0.44 & 18 & 34 & 60 \\
220 & 98 & 0.37 & 24 & 43 & 74 \\ \hline \hline \end{tabular} D(10), D(50), and D(90) particle sizes are the equivalent diameters of the largest particles with a cumulative volume distribution of 10\(\%\), 50\(\%\), and 90\(\%\) in the distribution curve, respectively.  
\end{table} 
Table 1: Effect of inlet/outlet temperature on the bulk density and dimensions of the microspheres. Figure 2: SEM images of the polyacrylate hollow microspheres prepared at different inlet temperatures: (a) 140 \({}^{\circ}\)C; (b) 160 \({}^{\circ}\)C, (c) 180 \({}^{\circ}\)C, (d) 200 \({}^{\circ}\)C, and (e) 220 \({}^{\circ}\)C; (f) Particle size distribution as a function of inlet temperature. Based on the above results, the mechanism of microsphere formation is proposed and briefly illustrated in Figure 3. First, small droplets form when the emulsion goes through the atomizer. Due to the evaporation of water, the droplets shrink and the latex particles aggregate, leading to the initial formation of the wet shells of the microspheres. Wet shell microspheres are gradually dried and maintain the spherical morphology until the droplets completely solidify. The residual water within the spheres subsequently evaporates through the tiny pores and capillary channels between the latex particles. Finally, the hollow microspheres are generated. According to SEM characterization, this mechanism is applicable to the droplets with a size lower than ca.