## 1 Introduction and Scope
### Chemical Surface Modification of Polymer Fibers--Selected Methods  
The scientific and technical literature contains descriptions of numerous attempts to improve polymer fiber quality and durability by chemical modification. For instance, the authors themselves successfully performed the surface modification of fibers made from recycled PET with ethylene-vinyl acetate copolymer (EVA) [6] to prevent hydrolysis of ester linkages of poly(ethylene terephthalate) in the highly alkaline environment of fresh cement paste (for detailed information on PET degradation in alkaline environments, see also: [PERSON] et al. [10], [PERSON] [11] and [PERSON] et al. in [12]). In the case of polypropylene fibers, [PERSON] et al. [8] modified the fibers using chrome acid to roughen their surface, and they concluded that non-polar polymer surface oxidation could be an efficient way to improve the fiber-reinforced concrete mechanical properties. In the case of polyethylene fibers, [PERSON] et al. [7] applied the approach used earlier, among others, by [PERSON] et al. [4], [PERSON] et al. [13] or [PERSON] et al. [14], for other non-polar polymers to test the possibility of improving the adhesion between polyethylene fibers and cementitious matrix by modifying the surface of the fiber with polydopamine. Using dopamine through oxidative self-polymerization reactions applied by a simple water  
Figure 2: Examples of macrofibers with various geometries that are commercially available in Poland: (**a**) crimped polypropylene macrofibers (length of 25 mm); (**b**) polyolefin copolymer fibers twisted in bundles (length of 24 mm). borne deposition process, which forms a thin reactive polymeric layer comprising hydroxyl and amino groups on diverse inorganic and organic substances [13; 15], enabled imparting a hydrophilic nature of the modified polymer, thus increasing its surface polarity and hydrophilicity [16]. They concluded that by dopamine surface modification, it was possible to increase the tensile strength of polyethylene fibers, pull-out strength and interfacial shear strength, thus enhancing the bonds between the initially hydrophobic polymer fibers and cementitious matrix. Recently, [PERSON] et al. [9] researched the possibility of improving the adhesion between PE fibers and cementitious matrix using tannic acid. They obtained promising results, showing that this method allowed them to chemically roughen the fiber surface (Figure 3) and significantly increase the energy required to pull the modified fibers out of the matrix compared to unmodified fibers. They experimentally scaled the modification method, using different sequences of adding chemical reagents to the modifying solution and using different exposure times of the fibers to the modifying medium (compare Figure 3b-d), to finally indicate the most effective procedure for modifying polyethylene fibers assuming 3-h immersion of fibers in tannic acid supplemented with selected additional modifiers. The research by [PERSON] et al. [9] included only tests of individual fibers placed straight in a pure cement paste of high water/cement mass ratio of 2.0, i.e., a value several times higher than the ratio of actual scale composites used in construction. However, the authors decided to use the same chemical roughening mechanism in the context of polypropylene fibers available on the local market and check whe  
Figure 3: ESEM images of pristine PE fibers and various tannic acid-modified PE fibers: (**a**) pristine fibers; (**b**) fibers treated with tannic acid for 30 min; (**c**) fibers treated with tannic acid for 1 h; (**d**) fibers treated with tannic acid for 3 h (based on [9]).