Wednesday, November 27, 2013

Nanocrystals bond silicone to PTFE

Tetrapod shape aids bonding (CAU, Xin Jin)
The potential for silicone in medical applications keeps growing.
In the newest development, researchers in Germany have discovered a way to join silicone and polytetrafluoroethylene (PTFE) using nano-scaled crystal linkers as internal staples. A major side benefit is that it's a purely mechanical process, ensuring no change in the chemical structure of the polymers.
Potential applications include breathing masks, implants or sensors.
"If the nano staples make even extreme polymers like Teflon (PTFE) and silicone stick to each other, they can join all kinds of other plastic materials", says Professor Rainer Adelung, who runs the functional nano materials group at the Institute of Materials Science in Kiel that participated in the announcement of the discovery.
The key to the approach is the use of very tiny crystals made of zinc oxide that are shaped like tetrapods with four legs protruding from the point of origin. They interlock and form strong bonds, and have been used in larger forms in coastal protection.
Here's how it works:  zinc oxide crystals are distributed carefully on a heated layer of PTFE, kind of like sprinkling sugar on partially baked cookies. After silicone is poured on top, the polymer sandwich is heated to 100°C for less than sixty minutes.
"It's like stapling two non-sticky materials from the inside with the crystals: When they are heated up, the nano tetrapods in between the polymer layers pierce the materials, sink into them, and get anchored", says Xin Jin, the first author of the publication, who is currently working on her PhD thesis. Her supervisor, Yogendra Kumar Mishra, adds: "If you try to pull out a tetrapod on one arm from a polymer layer, the shape of the tetrapod will simply cause three arms to dig in deeper and to hold on even firmer."
The peel strength of the composite structure is 200 Newtons per meter, which is described as similar to peeling sticky tape off glass. "The stickiness we have achieved with the nano tetrapods is remarkable, because as far as we could verify, no one has ever made silicone and Teflon stick to each other at all," says co-author Lars Heepe, PhD student from the Zoological Institute of Kiel University. 
Adelung said fundamental research on the project will continue as practical applications are pursued. One of the business partners, nanoproofed GmbH, is currently developing a product for painting on top of silicone.
The work was conducted within the German Research Foundation (DFG)-funded Collaborative Research Center 677"Function by Switching".