## 4 Conclusions  
The structure and mechanical properties of the investigated materials depended on three main factors: (i) processing conditions, (ii) qualitative and quantitative contribution of dispersed phase and (iii) the magnitude of the electron radiation dose. The final fibers' structures in the obtained composites were closely linked to processing conditions, which also depended on the percentage filling of these fibers in the polymer matrix. Electron radiation did not significantly change the structure of the prepared composites. Only small changes could be observed in the case of adhesion between the dispersed phase and the polymer matrix. The structure of the composites, however, depended significantly on the HF content. Mechanical properties of the study's composites were greatly influenced by electron radiation. Furthermore, as the radiation dose increased, the values of impact strength, elongation at break and in part tensile strength decreased while the flexural modulus increased. An especially interesting synergistic effect on PCL properties was observed and was related to the combined action of electron radiation and natural fibers addition. For example, depending on the used dose and fibers amount, elongation at break can be reduced from 1% to 99%, while the flexural modulus can increase from 1% to 248%.  
Hemp fibers may be used as an additive in the PCL matrix to manufacture biodegradable materials. Ecological advantages and the low price of these fibers compared to the price of PCL highlight a direction for further research in this field. Moreover, the applied fibers should additionally accelerate the biodegradation process of such materials. This statement refers, specifically, to radiation-modified biodegradable polymers that can be partially crosslinked and, as consequence of this, the biodegradation process may be slowed down. Thus, hemp fibers should lessen the adverse effect that crosslinking has on biodegradability. In this case, further studies determining the combined action of electron radiation and natural fibers on the biodegradation of these materials may be interesting. Furthermore, the post-radiation oxidation processes of both the matrix and HF as well as the protective effect of lignin in the obtained materials may also be the subject of further research. The radiation-modified composites presented in this article can be applied primarily for the production of biodegradable packaging, including rigid and flexible ones, depending on the applied radiation dose and the fibers content.  
Conceptualization, R.M. and V.K.; methodology, R.M., K.M. and W.G.; sample preparation, R.M. and W.G.; formal analysis, A.R.-K. and V.K.; investigation, R.M., A.R.-K. and K.M.; data curation, R.M.; writing--original draft preparation, R.M. and A.R.-K.; writing--review and editing, R.M. and L.W. All authors have read and agreed to the published version of the manuscript.  
The project has been financed by statutory funds (project no. 110034).  
Not applicable.  
All experimental data to support the findings of this study are available upon request by contacting the corresponding author.  
The authors of this work would like to express their special thanks to the Institute of Natural Fibers and Medicinal Plants in Poznan (Poland) for the preparation of hemp fibers.  
The authors declare no conflict of interest.