Layered polymer/silicate nanocomposites

The idea of improving the properties of materials by combining two phases with different properties is not new. Throughout history, there are examples of materials that can be considered nanocomposites: some Mayan paint colorings are the result of inclusions of metallic nanoparticles and oxide in an amorphous silicate substrate. More recently, in 1917, carbon black was introduced into the composition of tires, resulting in a five-fold increase in their lifespan. This reinforcement is made up of particles ranging from 10 to 400 nm, with an average aggregate diameter of 100 to 800 nm.

In contrast to conventional polymer-matrix microcomposites, where the typical dimensions or diameters of the fillers (particles, fibers) are of the order of several micrometers, the revolution in the transition to nanometric size scales results mainly from two parameters: the considerable increase in the surface area of interfaces and the reduction, at identical reinforcement volume fraction, in the distances between particles until they reach the scale of molecular dimensions characteristic of the matrix.

The trigger for the research effort in this field was the publication in 1992 of results obtained by Toyota Research on the nanoscale dispersion of sheet silicates (mainly montmorillonite clay) in polyamide 6 by in situ polymerization. Since then, a considerable research effort has been made worldwide in the field of nanocomposite materials with thermoplastic matrix and clay platelet reinforcements, as reflected by the exponential growth in the number of publications and patents, but with only a contribution of the order of 15% at European level.

In these organic-inorganic systems, the ultra-fine dispersion and local interactions between matrix and reinforcing phase result in property levels superior to those of their micro or macro-composite equivalents, even at low reinforcing element fractions. These advantages can be seen in fire resistance and barrier properties, as well as in rigidity and ultimate mechanical properties.

In most cases, complex elaboration strategies require the clay to be organophilic, and even under these conditions, processing remains a tricky task if we are to achieve satisfactory dispersion, which is a decisive factor in controlling the final material's properties in use.