The Development of Superhydrophobic Textiles and Polymer Surfaces

Abstract

The lotus leaf (Nelumbo nucifera) and the Stenocara beetle represent two classic examples in nature where the structural patterning and chemical nature of a surface has led to very specific properties. Superhydrophobicity and superhydrophilicity are terms used in a field that centres on the understanding and replication of those surface characteristics. Inspired by both examples, this research focuses on the understanding, design and development of those properties to create new materials that possess a wide range of consumer applications in, for example, the textile, upholstery and plastics industries. In this manner, inherent hierarchical or single-level morphologies may be created on both natural wool fibres and synthetic polymers, that in combination with chemical surface modifications have led to materials that amongst other characteristics, impart superhydrophobicity.

The hierarchical surface structure in the micro- and nano-range was created on knitted wool fabric and nylon 6,6 substrates, through the use of functionalised silica nanospheres of different diameters. These were either commercial or produced via a modified Stöber process. Static contact angle measurements below 10° for the superhydrophilic surfaces and contact angles around 150° for the superhydrophobic surfaces were achieved by lowering the surface energy of the substrates using Dynasylan® (F8815). High advancing and receding contact angles as well as roll-off angles below 20° were also measured for the superhydrophobic surfaces.

Through the use of conventional textile fabrication techniques knitted, woven, knotted and wet-felted single-level surface structures were investigated using different wool fibre diameters (16, 20, 34 μm). Subsequent chemical surface alterations induced either superhydrophilicity or superhydrophobicity with static contact angles of 0° and as high as 160°, respectively. Theoretical calculations were also conducted to predict and verify the measured contact angles.

The effects of fibre consolidation and chemical surface alterations, using perfluorohexylamine, perfluorooctylamine or Dynasylan® on wet-felted 16 μm merino wool fibres with regards to the contact angles of three different test liquids (water, sunflower oil and hexadecane) were studied. Static water contact angles exceeding 150° with corresponding contact angle hysteresis values below 10° were obtained. A high oleophobicity of 138° for hexadecane was achieved, and some antimicrobial activity was observed when using Dynasylan® as a treatment. The knowledge gained from these samples was then used to create superhydrophobic and superhydrophilic surfaces on photopolymers.

The capabilities of 3D printing technology have made major advances and this technology has become much more accessible for a range of different applications. In the present case it was used to create the required surface roughness. The parameters (width, height and spacing) of 3D printed square pillar surface patterns using photopolymers were varied to determine the effects on both the static and dynamic contact angles. The untreated surface structures created showed superhydrophilic properties and, upon lowering of the surface energy, superhydrophobicity and oleophobicity with contact angles as high as 152° and 116° respectively were achieved.

In two separate industrial scale trials, a state of the art, fully automated, commercial needle-felting process was used to fabricate nonwoven textile surfaces using 16 μm merino wool fibres. In the first trial, the consequence of varying the fibre consolidation on the static and dynamic water contact angles was studied. Subsequent chemical modifications via Dynasylan® allowed the formation of nonwoven textile surfaces with superhydrophobic and oleophobic properties. These textiles were also breathable with a water vapour transmission rate of 75-80 g m-2 24 h-1. Good wash-fastness was achieved with static water contact angles as high as 155° even after several hours of washing.

In the second industrial trial all the surface properties obtained (oleophobicity, superhydrophobicity, superhydrophilicity) were combined into one novel 3-layered surface structure consisting of pre-treated 16 μm merino wool fibres. Each individual wool layer imparts particular surface properties. This novel sandwich structure allowed the successful separation of oil water emulsions. Water vapour condensation experiments showed promising results, that could be useful in respiratory equipment for example.

As such, the materials created within this research allow for a broad range of consumer applications, especially in the upholstery, carpet and textile industries where surface properties such as water-repellency yet breathable, self-cleaning and stain resistance are desired. The multifunctional material opens up an entirely new array of different applications in filtration systems, respiratory equipment or liquid separations.