Cotton fabrics modified with tannic acid/long-chain alkylamine grafting for oil/water separation

Cotton fabrics were modified on surface wettability for efficient oil/water separation. The modification adopted in previous literature are commonly complicated in steps, expensive in chemicals applied, and not environmentally begien in solvents used. This article applied a novel one-step modification protocol to modified cotton fabrics using tannic acid (TA) grafted via Michael addition/Schiff base reactions with long-chain alkylamines, including 1-tetradecylamine (TDA), 1-hexadecylamine (HDA), and 1-octadecylamine (ODA) with 14, 16 and 18 methyl groups, respectively, were applied in this study for cotton fabrics modification. The TA, an inexpensive natural polyphenol, was adopted for providing excess catechol groups for linking the cotton surface and the alkylamines. The grafting reactions via Michael addition/Schiff base reaction were performed at room temperature and pH 8.5 with catalysts CuSO4/H2O2 for significantly reducing the reaction time needed. The chemical analysis confirmed the modification of the cotton fabrics to possess hydrophobic surface, with contact angles for water droplets on the modified surface as 133.3o, 136.0o and 142.9° for TA-TDA, TA-HDA, and TA-ODA cotton fabrics, respectively; and diiodinemethane droplets on the modified surface being 56.0o, 58.8o and 59.3o, respectively. The modified cotton surface has intermediate dispersive component of surface energy (29.0–31.2 mJ/m2) and low polar component (4.2–6.2 mJ/m2), yielding high affinity to oil but low affinity to water. Parametric study revealed that the pH value and reaction time are the most important parameters for the alkylamine grafting reactions, while the reduction of catalyst amount used but still with sufficient modification on cotton fabrics can be achieved. The oil absorption tests with cotton fabrics contacting hexane (top)-water (bottom) layers revealed high hexane adsorption capacity: 0.96, 1.22, 1.25 g hexane/g cotton for TA-TDA, TA-HAD and TA-ODA cotton fabrics, respectively. The absorption tests with cotton fabrics contacting water first for 30 s then hexane for another 30 s had much lower absorption capability of hexane: 0.52, 0.85, 0.87 g hexane/g cotton for TA-TDA, TA-HAD and TA-ODA cotton fabrics, respectively, since water would penetrate the major pores for inhibiting further oil absorption.