## 4 Conclusions  
As demonstrated in the present work, atomization allows us to obtain micrometric structures based on ultrathin cellulose fibers. These cellulose microparticles showed a high capacity to improve the physical properties of biopolyesters, mainly in terms of mechanical strength and barrier against aroma and oxygen, without significantly altering their optical properties or reducing their thermal stability. Therefore, the atomization and subsequent incorporation of MFC into biopolyester can successfully represent a novel and sustainable solution to increase the performance and applications of green materials in the development of compostable and/or biodegradable packaging within the framework of the Circular Economy. However, it was also evidenced that the direct incorporation of these cellulose microstructures into biopolyester matrices still present problems of chemical incompatibility, which is mainly reflected in a reduction of the mechanical and barrier performance. In this regard, reactive compatibilizing agents that are based on multiple epoxide groups, such as ESAO and TGIC, have demonstrated their ability to improve the adhesion of cellulose microstructures to PHBV matrices through a reactive grafting mechanism. In particular, the combined use of TGIC with DCP, which facilitates the initiation of the reaction through the generation of free radicals, yielded the most notable improvements, resulting in PHBV/MFC composite films with improved processability and increased mechanical and barrier performances. Future studies will focus on the application of the resultant compatibilized green composite films as rigid packaging articles for food preservation applications, in particular for determining the shelf life of foodstuff whose rapid deterioration represents significant economic losses and whose market price allows the cost of innovation to be absorbed. Moreover, migration studies should also be performed in the resultant green composite films with food simulants in order to corroborate their safety and potential use in food packaging.  
**Author Contributions:** P.A.V.F.: methodology, formal analysis, writing--original draft preparation; H.B.: methodology, writing--original draft preparation; F.V.: methodology, formal analysis; D.R.: methodology, formal analysis; M.V.: supervision, funding acquisition; S.T.-G.: conceptualization, formal analysis, investigation, writing--original draft preparation, writing--review and editing, supervision. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Spanish Ministry of Science and Innovation (MICI), grant numbers PID2019-105207 RB-I00 and RTC2019-007268-2.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data are contained within the article and are also available on request.  
**Acknowledgments:** P.A.V.F. is grateful to Generalitat Valenciana (GVA) for the Grisolia P/2019/115 grant. [PERSON] acknowledges MICI for his Ramon y Cajal contract (RYC2019-027784-I). The Integrated Employment Service (SIE) of the Universitat Politecnica de Valencia (UPV) is also acknowledged for the collaboration through its Educational Cooperation Program.  
**Conflicts of Interest:** The authors declare no conflict of interest.