InMat 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 filler particles, i.e. the larger the ratio of their lateral dimension to their thickness, the more effective they are in obstructing the diffusion of molecules trying to get through the film.
This concept is well known. In particular, there has been considerable work both in industry and academia over the last 5-10 years 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. Furthermore, under the correct physical and chemical conditions, one can completely separate one atomic layer from its neighbors above and below. Thus one has molecularly thin sheets that are typically about 10 Angstroms (or 1 nm) in thickness, and can be anywhere from 0.1 to 10 microns in extent. 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 Initial Innovation
Our first insight was that one could be much more effective at utilizing a nanocomposite approach to barrier films by using a coating technology. Most of the effort by other groups has focussed on thermoplastic (melt) processing, with the hope that by adding a little bit of exfoliated clay, the properties of extruded sheets and other forms could be dramatically improved. Although there has been several technical successes in this area, it does not make ideal use of the clay to improve barrier. This is because thermoplastic processing always involves a high shear-mixing step in order to disperse the filler in the polymer. If there is too much clay, or if the aspect ratio is too large, the mixing process usually degrades the aspect ratio. In addition, it is hard to get very high orientation in an extrusion process.
In a coating process, we can do all our mixing at low viscosity and therefore low shear stress. In addition, the carrier liquid (in our case water) dilutes the filler enabling us to uniformly mix polymer and filler at higher aspect ratios and filler content.
An important and novel feature of our coatings is that they are all in a mesoscopic state. This means that locally, all the filler particles are aligned. This occurs because of their aspect ratio and the concentration of filler. With an aspect ratio of 10,000 in water taking up 1% of the volume, the only way the plates will fit is to be aligned. Thus our coatings always have the plates locally aligned, and this alignment persists in the dried coating.
Our coatings start with a polymer latex that has suspended butyl spheres (about 1 micron in diameter). We then add exfoliated vermiculite. InMat has developed both patented and trade secret expertise that enables us to keep the vermiculite substantially exfoliated even in the presence of the butyl and the surfactant used to stabilize the butyl latex. This is critical to our 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. This is what enables us to make films that have substantial amounts of filler but are still very flexible. Conventional coating and thermoplastic techniques lead to extremely brittle materials when more than 5-10% by volume of a high aspect ratio filler is added. In fact, most of the thermoplastic products that have found commercial use using nanocomposite technology add only 1-5% exfoliated clay to the polymer while achieving very low reductions in permeability when compared to InMat's innovation.