To give silk hydrophobic and UV-blocking properties, researchers from Wuhan Textile University have experimented with in-situ synthesis of zinc oxide (ZnO) nanorods on fabrics made of natural silk. A Materials Today article says that the team’s report, which appeared in Materials Letters, says they’ve produced super-hydrophobic silk without the addition of seed particles or the use of pre-deposition processes.
The process the researchers used involved immersing the fabric samples in a zinc nitrate hexahydrate solution, then heating the solution for two hours. When the silks were room temperature, they were removed from the solution and dried for two hours. The Zn-infused silks were placed in a solution of n-octadecanethiol (ODT) for 12 hours then dried and baked in an oven.
SEM (scanning electron microscopy) imaging of the treated silk revealed the presence of high-density elongated features on the fiber surfaces, producing a hierarchical micro-nanoscale topography. X-ray diffraction results showed that these structures were highly crystalline ZnO, and XPS confirmed that they contained no metallic Zn inclusions, the article says.
Testing of the material included adding water droplets to untreated silk and the Zn-infused silk. The untreated silk “wetted completely.” The treated silk rendered a water contact angle of 151.93 degrees, “making it a super-hydrophobic material.” The fabric was treated with acidic and alkaline solutions, as well as undergoing repeated washing, and it retained good water-repellency (contact angle = 147.44 degrees) after 25 washes.
To test self-cleaning properties, dry methylene blue powder was placed on the fabric surface. With the addition of water, the report says the powder was removed and no stain was left behind. Zinc infusion in silk appears to provide UV protection as well. After testing UVA and UVB transmittance, the result was an ultraviolet protection factor of 73.28 compared to 9.10 for the untreated silk.