Self clean only - materials for dirt-free fibres and clothing
Advances in nanotechnologies and photocatalytic materials could make dirt-free fibres and clothing a reality. Dr Walid Daoud, Lecturer at Monash University, Australia, describes the concepts, recent developments and potential applications.
With increasing demand for hygienic, self-disinfecting, and contamination-free surfaces, interest in developing efficient self-cleaning, protective materials has grown.
Self-cleaning surfaces can be classified as hydrophobic or hydrophilic. The former, commonly referred to as the lotus effect, is where rolling water droplets remove surface-attached dirt and fine debris. Such a surface could be achieved through chemical or geometrical surface modification by mimicking the natural features of lotus leaves or butterfly wings.
Hydrophilic self-cleaning, also known as the photocatalytic effect, is a chemical surface modification that uses photoactive substances such as titanium dioxide (TiO2) to induce photocatalytic purification functions on a substrate surface. These materials chemically decompose surface-attached dirt or contaminants through oxidation reactions in the presence of light.
Titanium dioxide nanoparticles have been deposited on various materials using the sol-gel process, chemical vapour deposition or ionised cluster beaming. Although various substrate materials have been identified, such as glass, silica, paper, fibres and textiles, the technologies of grafting TiO2 onto flexible substrates are limited due to the low thermal and chemical resistance of these materials.
With bottom-up nanotechnology, nanostructured anatase TiO2 coatings on organic fibrous materials have been achieved. Since then, attempts have been made to widen the application to include other fibrous materials.
Photocatalytic self-cleaning fibres are formed by imparting photocatalytic function by surface treatment using bottom-up nanotechnology. These fibres can clean themselves when exposed to incident light. The mechanism of self-cleaning fibres is shown in the image top.
These fibres are multifunctional. They possess antimicrobial, deodorising and self-cleaning properties. Although many photosensitive substances, such as SrTiO3, SnO2, ZrO2, ZnO, WO3 and Fe2O3, have small band gap and/or good light absorption, research shows that TiO2 has several advantages. In addition, titanium is one of the most abundant elements in earth, making TiO2 inexpensive.
The first self-cleaning cotton has been developed by dip coating cotton fibres in TiO2 nanocolloid. The cotton fibres are then pressed at a nip pressure of 2.75kgcm-2 using a roller rotation speed of 7.5rpm. The pressed samples are dried and cured. An anatase coating is obtained by hydrothermally treating the fibres in boiling water.
Further work has demonstrated that anatase-coated wool fibres possess good UV protection and reproducible photocatalytic ability. Keratin fibres contain fewer anchoring functional groups such as carboxyl and hydroxyl groups than cellulose fibres, so to enable stable bonding between TiO2 and the keratin fibres, acylation with acid anhydride is introduced. This enriches the fibre surface with carboxyl groups, enhancing the anatase coating and self-cleaning functionality. Self-cleaning keratins are being optimised for potential commercialisation.
Many factors which influence the efficiency of photocatalysts have yet to be studied. One vital concern is the limit of light absorbability. Current photocatalysts inherently require UV light irradiation to trigger advanced oxidation effect. Ultraviolet light, however, constitutes only 0.1% of indoor lighting. Therefore, it is essential to widen the effective absorption wavelength range to include other regions such as visible light or even infrared.
Uneven distribution of the photocatalyst and non-sustainable coating are also common problems. This may limit the efficiency and long-term stability of the self-cleaning function.
Self-cleaning may one day become a standard feature of textiles and clothing. This would improve the environment through reduced use of water, petrochemical-derived detergents and energy.
There would also be a benefit to the quality of life if cleaning fibers was as simple as hanging them on the line. Work to optimise such fibres is ongoing.