## 4 Discussion  
Wollastonite, being a mineral and an inexpensive filler, is widely applied in many industries for filling polymer matrixes. In general, this enables to improve both their strength properties and wear resistance, including abrasion [36; 39]. In these studies, two issues have been simultaneously solved: (1) to enhance the mechanical properties of the UHMWPE-based composites loaded with wollastonite by increasing the interfacial adhesion level and (2) to evaluate the effect of the adhesion level (and the coupling agent type) on wear resistance under the dry sliding friction conditions. The discussion of the obtained results has been presented in the same order. Loading with wollastonite in the studied content range (up to 23 wt. %) did not significantly affect the strength properties (no multiple changes occurred), since the length of individual fibers did not exceed tens of microns that was less than the size of separate spherulites. In addition, the aspect ratio of wollastonite fibers did not exceed 10. Therefore, the filler gave rise to the dispersed hardening mechanism than to the reinforcing one. It is known that a great contribution to the improvement of the strength properties of the polymer composites could be made by loading with fibers several millimeters (or more) in length. In this case, the significant increase in their both the elastic modulus and yield strength was possible due to the enhanced interfacial adhesion. The reason was that load was transferred to the greater degree (and simultaneously over the large space) from the polymer matrix to reinforcing fibers [50]. Additional features of wollastonite fibers were their needle-like shapes and rather developed surfaces. Microfibers, being tightly pressed into the UHMWPE matrix, were mechanically strongly coupled to it, although they did not possess any chemical bonds. As the obtained results showed, both their small length and additional loading with the coupling agents did not cause any drastically improvements of the strength properties. Nevertheless, certain distinctions between the applied coupling agents (in terms of yield strength) were observed. The reason was various mechanisms of their effects. From the standpoint of the action principle, each silane-containing coupling agent was characterized by the presence of two active groups. One of them was able to form new chemical bonds with the mineral filler, while the other provided adhesion to the polymer matrix. In the second case, such a relationship could be either chemical or physical in nature [51; 52; 53]. For UHMWPE (precisely because of its chemical inertness), loading with the silane-containing coupling agents could form only physical bonds (Van der Waals forces). In [54; 55], the mutual diffusion concept was applied to explain the reasons for the formation of the bonds between the silane-containing coupling agent and polyolefin (in this case, UHMWPE). Among the silane-containing coupling agents used in these studies, the \"KH-550\" one was recognized as the most effective, since it enabled the formation of new chemical bonds with the mineral filler. This was evidenced by the (Si-O) peaks in the IR spectra (Figure 5). In the previous author'sstudies of the UHMWPE-based composites [42], the functionalization of the fibrous filler with the \"KH-550\" coupling agent made it possible to improve their mechanical properties and wear resistance. According to both the obtained results and the published data, the \"KH-550\" coupling agent was characterized by the high chemical activity. Therefore, it was able to provide the maximum adhesion to the mineral filler. The \"OTS\" coupling agent possessed the longest non-polar chemical group (octadecyl). In accordance with the mutual diffusion concept, this caused the increase in the adhesion of the coupling agent to the UHMWPE matrix (since both octadecyl and UHMWPE possessed the -(CH\({}_{2}\)-CH\({}_{2}\))\({}_{n}\)-CH\({}_{3}\)) nonpolar chemical chain).