Clearly the ability or otherwise to compound and process nanocomposite materials into various forms will be of critical importance to the utilisation of these highly promising materials. Three approaches to intercalation/exfoliation of clay minerals have been considered to date. All three involve methods of incorporating the polymeric phase within the clay interparticle galleries and thus have a major bearing on the properties of the finished product. These three principal routes involve either solution, in-situ reaction or melt intercalation/exfoliation processes.
There may also be a need to modify interparticle chemistry so as to promote organophilicity rather than the natural hydrophilic character endemic in clay materials will be a crucial prerequisite for all three intercalation/exfoliation methods.
Although all three of the above mentioned methods have been investigated, the latter, melt intercalation, has been considered the most desirable for use with thermoplastic polymers. In particular an ability to use conventional melt processing techniques such as extrusion and injection moulding, if capable of promoting vitally important intercalation/exfoliation processes, has been viewed as a particular advantage. Several organisations have investigated conventional melt processing for nancomposite fabrication.
Polymer Film Extrusion
Bayer has investigated polymer film extrusion techniques for the production of film barrier materials. Both cast film extrusion and blown film extrusion techniques have been employed to produce barrier films from organoclay – polyamide 6 formulations. Both techniques resulted in film products exhibiting isolated organoclay particles, thus confirming the ability of these techniques to promote the critically important intercalation/exfoliation processes.
It has, however, been recognised that the so-called in-situ polymerisation approach to intercalation/exfoliation leads to a more complete exfoliation of clay platelets in comparison to the melt approach. This is particularly true at the typically low loading levels generally employed for nanoclay incorporation i.e. < 5%. The enhanced swelling ability of a relatively low molecular weight monomeric precursor has been proposed as responsible for this effect. Melt compounding on the other hand relies on the shear delamination of clay aggregates in the presence of polymer melt which would be intuitively more difficult. As a result it has been suggested that melt compounding will often lead to inferior film quality and barrier property characteristics due to this incomplete platelet exfoliation effect. Melt intercalation/exfoliation does however offer prospects of lower cost processing and fabrication in comparison to solvent and reactive monomer systems.
Other Melt Processing Techniques
Other melt processing techniques such as injection moulding, blow moulding and extrusion have also been shown to be suitable for the preparation of clay nanocomposites articles. The ability of polymer compounding equipment such as twin-screw extruders to promote direct polymer intercalation from the polymer melt has been demonstrated.
Effects of Nanoclay Addition on Melt Viscosity
Although the incorporation of particulate material into polymers generally results in an increased melt viscosity, nanocomposite formulations exhibit a greater fall in viscosity with shear rate than their unfilled counterparts. This results in insignificant differences in viscosity at the shear rates frequently encountered in commercial melt fabrication processes. In addition, due to the substantially lower loading levels required to achieve the same property levels obtainable via conventional filler materials, the viscosity increases with nanoclay formulations are obviously low in comparison to their more conventionally filled counterparts. Furthermore, the lower loading levels will also be advantageous in minimizing abrasive effects on processing equipment.
Effect of Platelet Aspect Ratio
An interesting paper by van Es of DSM-Research suggests that the primary factor influencing the properties of polymer-clay nanocomposites is platelet aspect ratio rather than particle size. In such circumstances the need to avoid platelet damage during harsh conditions typically encountered during melt blending and processing would clearly be important. Evidence would suggest that platelet damage need not be a major problem with conventional melt processing techniques.
Graphite Nanotube Addition
The beneficial effects of naoparticulates as opposed to larger particulate or fibrous additives on melt flow behaviour has been further demonstrated with developments by Hyperion Catalysis International Inc. in the United States. They have developed graphite nanotubes (10-15 nm in diameter) which, in comparison to larger more conventional electrically conductive fibres, provides equivalent levels of conductivity at significantly lower loadings. This contributes to lower viscosity formulations and hence improved melt flow. This can be of particular advantage in thin-wall moulding applications.