Nanofillers

Abstract

Nanofillers, especially those based on clays and carbon nanotubes, have received a great deal of attention recently. The accepted definition of nanoparticles is that they should have at least one dimension in the range 1–100 nm and that the others should be greater than 100 nm. This can be further divided; when three dimensions are in the nano range we have nanoparticles, when it is two then nanofibers, and when one then nanoplates.

The first thing to notice is that, despite all the recent publicity, nanofillers are nothing new, and nanoparticle fillers already have a very significant market presence. These include the carbon blacks, precipitated and fumed silicas, and precipitated calcium carbonates. More recently interest in using synthetic polymer nanoparticles has been revived.

Where the principal novelty lies today is with the nanoplates and nanofibers, which are more recent arrivals on the scene. Mineral derived nanoplate fillers are the more promising for volume applications, due to a relatively low cost compared to the fibers. Color is also a limiting factor for the nanocarbon fillers.

While a large number of layer minerals exist with the potential for delamination and dispersion as nanoplates, clays and especially those of the montmorillonite family have shown most promise and have been focused on commercially. Nanoclays offer the potential for high stiffness at low loadings, excellent barrier properties, and some useful flame retardant properties. However, after more than two decades of effort and some limited successes, their market penetration is well short of initial hopes and commercial interest is declining. This is due to the practical difficulties in fully delaminating and dispersing them, and thus realizing their potential benefits, in all but a few cases.

Among other nanoplates, graphite and ultimately graphene have promise for some advanced composite applications. Cost (and color) are likely to be limiting factors. The same factors will affect graphene fiber analogues, such as carbon nanofibers and tubes.

Extraction of nanocrystals such as starch and cellulose from abundant plant materials is also of interest, especially from the sustainability angle. Despite some promise, the current production processes are costly and far from truly “green.” They also face similar difficulties in redispersion into polymer matrices as the nanoclays.