## 4 Conclusions  
In summary, poly(St-_co_-MMA)/FA composite microspheres were successfully synthesized via an in situ suspension polymerization. The properties of the poly(St-_co_-MMA)/FA were characterized by various techniques and the results demonstrated that FA was successfully introduced into the microspheres of poly(St-_co_-MMA). All of the obtained composite microspheres had an apparent density in the range of 1-1.3 g/cm\({}^{3}\), with a good sphericity >0.9, which is very suitable for hydraulic fracturing. With the incorporation of FA (FA ratio of 30%), the crushing rate decreased to 0.92% at 52 MPa and the maximum weight loss temperature increased from 404 to 414 \({}^{\circ}\)C. The studied proppant thus has a comparable performance to that of conventional proppants. Moreover, it also allows improvements in conductivity that are not achieved by conventional proppants because it has an ultra-low density. The preparation approach is facile and controllable, which is suitable for large-scale production. The as-prepared cross-linked poly(St-_co_-MMA)/FA composite microsphere are a promising material for application as ULW proppants in the shale oil or gas fracturing industry.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Proppant** & **Sphericity** & \begin{tabular}{c} **Apparent Density** \\ **(g/cm\({}^{3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Bulk Density** \\ **(g/cm\({}^{3}\))** \\ \end{tabular} &
\begin{tabular}{c} **Crushing Rate** \\ **(\%)** \\ \end{tabular} & **Reference** \\ \hline Poly(St-_co_-MMA)/FA & \textgreater{}0.9 & 1.26 & 0.68 & 0.95 (52 MPa) & This work \\ Polymeric materials & \textgreater{}0.9 & 1.08 & 0.66 & 2.00 (52 MPa) & 34 \\ Walnut shells & 0.65 & 1.25 & 0.77 & 1.50 (52 MPa) & 34 \\ Resin-coated porous ceramic & 0.8 & 1.75 & 1.19 & 7.00 (52 MPa) & 34 \\ Quartz sand & 0.68 & 2.65 & 1.60 & 17.50 (52 MPa) & 4 \\ Resin-coated sand & 0.70 & 2.55 & 1.56 & 5.50 (52 MPa) & 4 \\ Ceramic & 0.8 & 3.27 & 1.84 & 0.20 (52 MPa) & 35 \\ Porous proppant & 0.8 & 1.25 & & 6.95 (52 MPa) & 36 \\ PS/MWCNT & 0.9 & 1.05 & 0.695 & 1 \\ Inorganic polymer proppant & 1 & 1.91 & 1.28 & 21 (52 MPa) & 37 \\ Epoxy resin coated ceramic & 0.9 & 2.27 & 1.32 & 1.16 (69 MPa) & 38 \\ Second grade bauxite ceramic & 1 & 3.43 & 1.89 & 3.6 (52 MPa) & 5 \\ Resin-wrapped proppant & 0.9 & 1.29 & 0.86 & 9.0 (52 MPa) & 39 \\ PMMA/AG & \textgreater{}0.9 & 1.099 & & 0.5 (69 MPa) & 19 \\ PMMA/FA & \textgreater{}0.9 & 1.135 & & 3 (69 MPa) & 22 \\ PS/AG proppant & 0.9 & 1.06 & 0.614 & 1.2 (53 MPa) & 23 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of different types of proppants.  
**Author Contributions:** Conceptualization, T.C.; methodology, J.G.; and T.C.; validation, Y.S., Y.Z. ([PERSON]) and X.H.; formal analysis, Y.S., Y.Z. ([PERSON]) and P.M.; investigation, Y.S., Y.Z. ([PERSON]) and P.H.; data curation, T.C.; writing-original draft preparation, T.C.; writing-review and editing, J.G., Y.Z. ([PERSON]) and L.J.; supervision, J.G.; project administration, Y.Z. ([PERSON]); funding acquisition, T.C. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was supported by grants from the National Natural Science Foundation of China (51903080, 62001159), Foundation of Hubei Provincial Department of Education (D20212801, B2021223), and Scientific Research Foundation of Hubei University of Science and Technology (2019-21 GP10, 2019-20 KZ06, BK202003, H2019006).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.