Could tomorrow’s silk be self-cleaning and sun-proof?

Textile researchers have found a simple process for coating silk with ZnO

Silk fibre being spun. Image taken from a YouTube video of Sara Lamb

There’s nothing quite like the feeling of cool, smooth silk on your skin. It’s perhaps one reason why the luxury fabric has retained its popularity many thousands of years after it was first developed. It does have some limits though – silk’s porous nature means that it absorbs stains easily, and it degrades under UV. There have been many successful attempts to confer hydrophobic and UV-blocking properties onto other textiles, mainly through the addition of nanoparticles like titanium dioxide. But silk has long proven to be a more challenging prospect.

Writing in Materials Letters [DOI: 10.1016/j.matlet.2018.11.035], researchers from the Wuhan Textile University report on their latest development – a simple, in-situ synthesis of zinc oxide (ZnO) nanorods, on natural silk fabrics. Without using any pre-deposition processes, or the addition of seed particles, the team have managed to produce superhydrophobic silk that withstand photodegradation, whilst maintaining many of the silk’s mechanical properties.

They started by immersing samples of natural silk in a solution of zinc nitrate hexahydrate. The solution was heated for two hours, before being cooled to room temperature. The silks were then removed and dried for a further two hours. The Zn-infused silks were added to a solution of n-octadecanethiol (ODT) for 12 hours, before being dried and baked in an oven. SEM imaging of the treated silk revealed the presence of high-density elongated features on the fibre surfaces, producing a hierarchical micro-nanoscale topography. The diameter of each nanorod was between 70 and 160 nm and with an average length of 0.9–1.3 µm. X-ray diffraction results showed that these structures were pure, highly-crystalline ZnO, and XPS confirmed that they contained no metallic Zn inclusions.

To test the fabric’s wetting behaviour, water droplets were added to the surface of both pristine and treated silk. The former wetted completely, while the water contact angle of the treated silk was measured as 151.93°, making it a super-hydrophobic material. Dry methylene blue powder was placed on the fabric surface to examine its self-cleaning performance. The addition of water removed the powder entirely, leaving the treated silk unstained. To examine the durability of the coating, the fabric was treated with alkaline and acidic solutions, as well as being washed repeatedly – it retained good water-repellency (contact angle = 147.44°) even after 25 washes.

The fabric’s UV protection performance was tested by examining the transmittance of UVA (320–400 nm) and UVB (280–320 nm) through it. As a result, they could determine its ‘ultraviolet protection factor’ (UPF); a metric that determines how long someone wearing a particular fabric can safely stay out in the sun. For pristine silk, the UPF was just 9.10, whereas that of the ODT/ZnO silk was 73.28, putting it in the range of some of the best UV protective clothing on the market.
The coating did have some impact on the silk’s ‘comfort performances’ – it took on a slight yellow hue, and slightly increased in rigidity. Any effect on air permeability was very minor (11.6 ± 0.2 cm/s for the treated silk, versus 12.1 ± 0.2 cm/s for pristine silk). In conclusion, the authors seem to suggest that silk retains most of its attractive qualities after treatment, while also gaining other, very useful functionality.

This story was written for Materials Today, and it originally appeared here:

Original research paper ($): Jingjing Huang, Yingying Yang, Ling Yang, Yiming Bu, Tian Xia, Shaojin Gu, Hongjun Yang, Dezhan Ye, Weilin Xu. “Fabrication of multifunctional silk fabrics via one step in-situ synthesis of ZnO” Materials Letters, 237 (2019) 149–151. DOI: 10.1016/j.matlet.2018.11.035