Nanoparticle treatment adds antimicrobial coatings to leather

Franz will describe their work at the AVS 63rd International Symposium and Exhibition held Nov. 6-11, 2016 in Nashville, Tennessee during the Advanced Surface Engineering and Tribology Focus Topic session.

Along with their partners in Romania and Portugal, Franz and his colleagues tested a mix of nanoparticles made of titanium and silver for their antimicrobial properties. When materials treated with titanium dioxide were exposed to UV light, the metal set off a catalytic reaction that destroyed organic matter adhered to the surface. Silver nanoparticles -- well-known for their antimicrobial properties -- are already used in textiles to prevent sweaty odors caused by microbial activity.

The team relied on a process known as magnetron spattering to create the nanoparticles in the lab. Within a vacuum chamber, where the air has been removed, they ignited spattering plasma to erode a solid titanium dioxide or silver target into nanoparticles. They added oxygen to the plasma, which is reactive, so they could grow thin films of the materials. The process resulted in a fine titanium-dioxide layer embedded with silver nanoparticles. Although these titanium-silver nanoparticles are not commercially available, the process to manufacture them will likely prove easy to scale up, according to Franz.

"For other materials these processes are already widely used in industry," Franz said. They tested two ways of applying nanoparticles to leather. In one method, they mixed the titanium-silver nanoparticles with film-forming polymers and coated the leather surface. A comparison of leathers treated in both ways will yield the most likely method to be successfully introduced to commercial applications.

The team tested the durability of leather samples that were either untreated, pre-treated with traditional chemicals, or coated with the titanium dioxide-silver film. They chose different counterpart materials, including rubber and various polymers with defined pore sizes, and pressed spheres of these mixtures against the leather samples. "As the leather rotates along the other material, we can measure the friction that develops, and later analyze the wear tracks formed where the two surfaces were in contact," Franz said.

They then turned to Raman spectroscopy, optical microscopy and scanning electron microscopy to examine the wear patterns. They also measured how much of the nanoparticles was retained on the leather versus transferred to the other surfaces.

While low friction materials such as the polymer polytetrafluoroethylene may not cause a lot of wear, rubber -- which causes more friction -- could cause nanoparticles to be transferred from the leather to itself, which could cause the leather to lose its antimicrobial properties.

Verifying durability is a crucial step. "We frequently work with thin films and coatings on other materials like steel, and there we have very well established techniques," Franz said. "But for a substrate like leather, these nanoparticle coatings have not been tested."

In the long term, such technology could help industries such as footwear manufacturing adopt greener practices. "We would reduce pollution a lot, and could reach the same antimicrobial functionality with environmentally friendly processes," Franz said.