Modification of the surface of cotton with supercritical carbon dioxide and water to support nanoparticles

Large pleat-like wrinkles (ca. 1 μm wide) on the surface of cotton were formed by immersing the cotton in water, and pressurized with supercritical carbon dioxide, and then reducing the pressure to the atmospheric level. The conditions to form the wrinkles were investigated by systematically changing the experimental parameters such as temperature, pressure, treating time, depressurization rate, ambient gas, with or without of water, 20 wt.% sodium hydroxide aqueous solution. The conditions favorable to form large pleat-like wrinkles are low temperature (40 °C), high pressure (20 MPa), long treating times (60 min), and fast depressurization rate (0.80 MPa min−1). Water plays an important role. Carbon dioxide (CO2) is more effective than air and helium to form wrinkles. It is assumed that the wrinkles were formed in the following sequence: water in which high-pressure CO2 is dissolved infiltrates the cotton and swells it. When the pressure is reduced, CO2 in water leaves the cotton. The degasification speeds of CO2 on the surface of cotton are different from that of the interior. The cross section of cotton has a flat and awkward shape. As a result, the cotton shrinks unevenly, and the wrinkles are formed on the surface. It is also confirmed that the wrinkles formed on the surface of cotton firmly support nanoparticles of titanium oxide (TiO2) when the cotton was immersed in water where TiO2 nanoparticles of average 35 nm in diameter were dispersed at concentrations under 2 wt.%. The process can be used to support functional agents firmly on cotton without using binders. This technique is useful for the support of functional agents on the surface of cotton.