Structural Properties and Antibacterial Effects of Hydrophobic and Oleophobic Sol−Gel Coatings for Cotton Fabrics

Abstract

In a continuation of previous studies, the wetting properties of the hydrophobic diureapropyltriethoxysilane [bis(aminopropyl)-terminated polydimethylsiloxane (1000)] (PDMSU) sol−gel hybrid, which forms washing-resistant water-repellent finishes on cotton fabrics, were further investigated. The addition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) to PDMSU resulted in a highly apolar low-energy surface on aluminum with γ_S^total equal to 14.5 mJ/m² and a ΔG_iwi value of −82 mJ/m². Mixed PFOTES−PDMSU finishes applied on cotton fabrics increased the water contact angles (θ_w) from ∼130° (PDMSU) to 147°, also imparting oleophobicity (θ_{diiodomethane} = 130°, θ_n-hexadecane = 120°) to the finished cotton fabrics. Washing caused breakage of the coating's integrity as established from SEM, which was attributed to the partial removal of PFOTES from the composite films, also shown by subtractive IR attenuated total reflectance (ATR) and XPS spectral measurements made on washed and unwashed fabrics. The antibacterial properties of the PFOTES−PDMSU-finished fabrics were assessed with the transfer method (EN ISO 20743:2007), revealing that the reduction of Escherichia coli bacteria on unwashed cotton fabrics was nearly 100%. Moreover, for washed (10 times) cotton fabrics a much higher bacterial reduction was noted for the PFOTES−PDMSU finishes (60.6 ± 10.8%), surpassing PDMSU (30.4 ± 6.1%) and commercial fluoroalkoxysilane (FAS) (21.9 ± 5.7%) finishes. The structure of PFOTES−PDMSU gels, xerogels, and the corresponding coatings was investigated by analyzing the ²⁹Si NMR and IR ATR spectra and comparing them with the spectra of PFOTES and octameric (T₈) PFOTES based polyhedra. The results revealed the tendency of PFOTES to condense in octameric silsesquioxane polyhedra (T₈), coexisting in the PDMSU sol−gel network with cyclic tetramers (T₄(OH)₄) and open cube-like species (T₇(OH)₃). The presence of −OH-functionalized PFOTES silsesquioxanes, established even in coatings heat-treated at 140 °C (15 min), also explained the excellent washing fastness of PFOTES finishes on cotton fabrics. The regenerative nature of the water- and oil-repellent properties of washed PFOTES−PDMSU-finished cotton fabrics was attributed to the surface mobility of the T₈ PFOTES based polyhedra, ousted from the coating interior during consecutive washings.