**5. Conclusions**

An efficient process has been developed to manufacture a new composite biomaterial made from the two SPC and BP biopolymers, and urea. The composite material has been fabricated in the forms of extruded pellets and then injection-molded pieces, although it contains components having no film forming properties or not processable through extrusion. First of all, the twin-screw extrusion process comprises two main phases, carried out continuously in the extruder depicted in Figure 1. First, SPC is destructured and rendered thermo-plastic by adding aqueous sodium sulfite. In the second phase, BP and urea are added into the thermoplastic SPC matrix and homogeneized under pressure as the blend proceeds to the exit of the extruder. Additionally, the extruded pellets generated can be transformed into denser pieces through injection-molding.

The injection-molded composites, in the form of dense pellets and characterized for their mechanical and nitrogen release properties, have evidenced a number of specific benefits contributed by BP. These allow for concluding that the manufacture process and the formulation of the composite make possible producing new biomaterials in the form of dense pellets from the renewable SPC and BP biopolymers, and urea. These have all mechanical and nutrient release properties for being tested as eco-friendly CRFs for the cultivation of specific plants.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2079-6 412/11/1/43/s1, Table S1: Calculated amount of N from each component (NSPC, NBP, NU) in the blended SPC-U, SPC-BP, SPC-BP-U pellets compared to the total N amount released in solution from the pellets. Calculations based on the 4 g sample weight (see Materials and Methods), on the composite formulations, and on Table 1 data.

**Author Contributions:** C.V.-G. and E.M. developed the concept, acquired the funds for the work dedicated to the manufacture of the composite pellets and the characterization of their properties, and supervised it. P.E., L.L., and C.V.-G. developed the process, manufactured, and characterized the mechanical properties of the composite pellets. E.P., V.B., and M.N. performed the characterization, and developed the analytical procedure for the release properties of the composite pellets. E.M. developed the process to manufacture BP, and its analytical protocol. E.M. wrote the original draft of the paper. P.E. edited the final version of the paper. C.V.-G. and M.N. administered the funds at their institution, respectively, to carry out the work. All authors reviewed and completed the paper before submission and agreed to the published version of the manuscript.

**Funding:** This research was supported partly by endowed funds of the authors' institutions, and partly funded by the European Commission in order to support the implementation of the actions pursued in the LIFE16 ENV/IT/000179-LIFECAB and the LIFE19 ENV/IT/000004-LIFEEBP projects.

**Data Availability Statement:** Data is contained within the article or supplementary material.

**Acknowledgments:** The authors acknowledge the kind assistance of Claudio Spitaleri (Istituto Nazionale di Fisica Nucleare, LNS, Catania, Italy) for the mathematical elaboration of the kinetics data.

**Conflicts of Interest:** The authors declare no conflict of interest.
