# Preparation of Polyacrylate Hollow Microspheres via Facile Spray Drying
## 3 Results and Discussion
20 \(\upmu\)m. For larger droplets (>20 \(\upmu\)m), there is a large amount of moisture within the wet shell. It evaporates quickly at elevated temperatures (ca. 140-180 \({}^{\circ}\)C), which is too much to volatilize through tiny pores or capillary channels, resulting in a gradual increase in internal pressure. When the internal pressure is higher than the mechanical strength of the shell, two situations occur: (a) if the inlet temperature is relatively low (i.e., 180 \({}^{\circ}\)C or lower), it creates a drying temperature lower than the glass transition temperature of polyacrylate shell, then the microspheres rapidly burst, generating debris and/or residues; (b) if the inlet temperature is very high (i.e., 200 \({}^{\circ}\)C or higher), which generates a surrounding temperature higher than T\({}_{\text{g}}\) of the shell, then bubble punching would happen due to viscoelastic failure of the shell, instead of breaking the shells. The speed of the atomizer disk directly determines the quality of the atomization effect and size of the atomized droplets, resulting in different microscopic morphologies. Figure 4 shows the effect of atomizer speed on the particle size and size distribution of the formed polyacrylate hollow microspheres at a 220 \({}^{\circ}\)C drying temperature and an emulsion feed rate of 160 g/min. With an increasing atomizer speed from 10,000 to 28,000 rpm, the average particle diameter D(50) of the microspheres gradually decreased from 76 to 35 \(\upmu\)m. When the speed was further increased to 34,000 rpm, there was no obvious decrease in microsphere size. Overall, the higher the atomizer speed, the larger the centrifugal force and friction force subjected to the emulsion. Therefore, the emulsion is sheared and split into smaller droplets. With an improved atomization effect, the diameter of the dried microspheres is gradually reduced. Figure 3: Formation mechanism of the polyacrylate hollow microspheres during centrifugal spray drying. When the inlet temperature of the spray drying (220 \({}^{\circ}\)C) and the speed of the atomizer (28,000 rpm) are determined, the drying effect and the size and morphology of the microspheres vary with the change of the feed rate of the emulsion. The effect of feed rate on particle size, size distribution, and morphology of the formed polyacrylate hollow microspheres are shown in Figure 5. At a sufficiently high drying temperature (220 \({}^{\circ}\)C), with the feed rate increased from 82 to 190 g/min, there are only marginal changes in the size and shape of the formed microspheres. The microspheres possess a smooth surface, clear interface, and high sphericity. However, some microspheres were broken. Increasing the feed rate can slightly increase the size of the microspheres, and hence the powder fluidity of the microspheres decreases because water evaporation becomes more difficult, but a high productivity can  
Figure 4: SEM images of the microspheres prepared at different atomizer rotational speeds: (**a**) 10,000 r/min; (**b**) 16,000 r/min; (**c**) 22,000 r/min; (**d**) 28,000 r/min, and (**e**) 34,000 r/min; (**f**) Effects of atomizer rotational speed on particle size and size distribution of the formed microspheres at 220 \({}^{\circ}\)C drying temperature. be obtained. The higher the feed rate, the higher the inlet air temperature required to dry the droplets into spheres. A similar research finding has been reported [23], and if both effectiveness and efficiency are taken into consideration, the optimized feed rate is 136 g/min.