**4. Conclusions**

The obtained results confirm the effect of the addition of SS-Limonene on the improvement of rheological and mechanical properties of printed composites based on the PLA matrix. It is especially important for the FDM-based method for objects manufacturing, as usually objects prepared by such show much lower parameters than their injection-moulded counterparts, which can be explained by poor inter-layer adhesion and the presence of air gaps. On the basis of the obtained data, it can be concluded that the addition of a functionalized spherosilicate significantly improves such parameters as tensile strength, bending strength and impact strength of the samples obtained by 3D printing. SS-Limonene acted as a plasticizing additive for PLA. Additionally, the presence of SS-Limonene was found to increase hydrophobicity of the obtained composites. The addition of this modifier also facilitates the printing process itself, contributes to the improvement of rheological properties (reduces viscosity, increases MFR) and reduces production waste. It should be noted that in the conditions of processing the additive was found to undergo polymerization, leading to its secondary agglomeration observed under the microscope on increasing loading. It seems that the nature of the observed phenomena is complex, and the additive most likely exists in the system in two forms, namely a well-dispersed one and the agglomerated one, as suggested by previous research and the analysis of thermal data. The behaviour of the obtained composites is based on two types of interactions—between PLA and well-dispersed phase, as well as between PLA and the polymerized, agglomerated phase. Nevertheless, this complex interaction scheme should not be considered undesirable as it leads to a final improvement of the printing material system.

**Author Contributions:** Conceptualization, R.E.P.; Data curation, D.B. and B.S.; Formal analysis, M.F., D.P., and K.D.; Funding acquisition, D.B., B.S., and B.M.; Investigation, D.B. and B.S.; Methodology, B.S. and R.E.P.; Project administration, B.S.; Resources, D.B. and B.S.; Software, M.F., D.P., and K.D.; Supervision, B.M. and R.P.; Validation, R.K. and R.E.P.; Visualization, M.F., D.P., and K.D.; Writing—original draft, D.B., B.S., and R.E.P.; Writing—review & editing, R.K., A.B., and B.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Centre for Research and Development, Poland, gran<sup>t</sup> number LIDER/01/0001/L-10/18/NCBR/2019, and the National Science Centre, Poland, gran<sup>t</sup> number UMO-2018/29/N/ST5/00868.

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