The first commercial application of a pioneering nanotechnology is among one of a number of material innovations which have recently been introduced by manufacturers of the humble tennis ball.
The quality of a tennis ball depends on both the felt and the core. Poor quality felt means the ball will give low performance and go bald very quickly, causing irregular bounce. If the core is poorly made then pressurised balls will lose pressure very quickly.
Wilson's Double Core premium tennis ball, officially selected for the 2002 Davis Cup, is said to retain its original air pressure and bounce two times longer than other tennis balls.
Its inner core is coated with a butyl-based barrier called Air D-Fense by InMat, which inhibits air permeation by 200%.
InMat's technology is based on the principle that the diffusion of molecules through a polymeric film can be significantly reduced if impermeable platelet fillers are aligned in such a way as to significantly increase the distance a molecule must travel. The larger the aspect ratio of the platelet (ratio of their lateral dimensions to thickness) the more effective they are.
Over the last 5-10 years there has been considerable work both in industry and academia on the use of exfoliated clay as a filler.
Exfoliated clay differs from other flat particles such as mica or aluminum flakes in its thickness. Clay belongs to a family of minerals that have a layered structure. The atoms within a single layer are tightly bound together, but the forces between layers are relatively weak.
It is therefore possible to insert molecules between the layers, spreading them apart. Under the correct physical and chemical conditions, one atomic layer can be completely separated from its neighbours. The use of such particles to reduce the permeability of polymers, as well as to improve mechanical properties, is an important part of nanocomposite technology.
InMat's first insight was that utilising a nanocomposite approach to barrier films could be much more effective if a coating technology was employed.
Most of the effort by other groups had focused on thermoplastic (melt) processing. Although there have been several technical successes in this area, it does not make ideal use of the clay and it is difficult to get a high orientation of particles.
In a coating process, InMat can do all its mixing at low viscosity and therefore low shear stress. In addition, the carrier liquid (in InMat's case water) dilutes the filler enabling a uniform mix of polymer and filler at higher aspect ratios and filler content.
An important and novel feature of InMat's coatings is that they are all in a mesoscopic state. This means that locally, all the filler particles are aligned.
The coatings start with a polymer latex that has suspended butyl spheres (about 1 micron in diameter). Exfoliated vermiculite is added. InMat has developed both patented and trade secret expertise that enables the vermiculite to be kept substantially exfoliated even in the presence of the butyl and the surfactant used to stabilize the butyl latex. This is critical to achieving low permeability films.
The second area of technology that is proprietary to InMat includes techniques to control the interaction between the vermiculite and the butyl matrix.
InMat says that a 10-50 micron coating of its product provides same or better air barrier as a 0.75 mm butyl rubber sheet. In addition it has good adhesion to rubber substrates; is compatible with standard rubber vulcanisation conditions; remains flexible at temperatures as low as -40°C; is applied by an aqueous coating; is non-hazardous to use; and can be optimised for compatibility with both the manufacturing processes (spray, dip, etc.) and the barrier and flexibility requirements of the application.
Future Materials - 07/11/02
