Morphology and Properties of the Rubber Toughened Nylon 6 Nanocomposites
Young-Cheol Ahn, Department of Chemical Enginering, Kyungnam University, 449 Wolyoung-dong, Masan, South Korea and Donald R. Paul, University of Texas, Department of Chemical Engineering, Austin, TX 78712.
Maleated rubbers can easily make a blend with polyamide since the grafted maleic anhydride readily reacts with amine end groups to give a graft copolymer that strengthens the interface between the two phases and controls the morphology. In a blend of these two materials, the size of rubber particles is reduced because of decrease in particle-particle coalescence rate. It was shown that the rubber particle size should be generally controlled between 1 and 0.1 Ám to give super-toughness to the polyamide matrices and these upper and lower critical particle sizes are dependent on the nylon 6 molecular weight. Polymers are usually reinforced with glass fibers to obtain synergic effects in increasing mechanical properties. Another way of reinforcing the polymeric materials is to make blends with nanometer-sized particles, that is, the nanocomposites. Fujiwara and Sakamoto of the Unitika Co. described the first organoclay hybrid polyamide nanocomposite in 1976. One decade later, a research team from Toyota disclosed improved methods for producing nylon 6-organoclay nanocomposites using in-situ polymerization similar to Unitika process. More efficient way of producing polymer nanocomposites, melt intercalation, has been introduced by Vaia et al., where polymer chains diffuse into the space between the clay layers or galleries. This idea made the conventional melt compounding process a promising new approach for forming nanocomposites since it is more economical and simpler than in-situ polymerization process so that it would greatly expand the commercial opportunities for nanocomposites. The degree of exfoliation of the intercalated organoclay is strongly affected by the hydrodynamic separating forces caused by the conditions of mixing, that is, the viscosity of matrix fluid, shear rate, and the mean residence time. And the structure of organoclay is an important factor that affects the degree of exfoliation. The modulus of nanocomposites almost doubles that of the neat nylon 6, but the Izod impact strength is decreased and the ductile-brittle transition temperature is sharply increased as the content of nano-sized particles is increased. Thus, the usage of nanocomposites in the room temperature is limited and therefore the toughening of nanocomposites becomes an interesting issue in academy and industry. In this research, toughening of nylon 6 nanocomposites has been performed for medium molecular weight polymer and selected organoclays and the morphology and properties of the rubber toughened nylon 6 nanocomposites have been discussed. The nylon 6 used in this study was chosen among the commercially available grades produced for injection molding and extrusion applications. The grade name is Capron B135WP from Honeywell (formerly AlliedSignal). The organoclay chosen for mixing with the nylon 6 is Cloisite 30B supplied by Southern Clay Products. The rubber chosen for toughening of nylon 6 nanocomposites is Exxelor 1803 supplied by Exxon Chemical, an ethylene-propylene random rubber grafted with maleic anhydride (EPR-g-MA). The organoclay are generally supplied as particles nominally 8 Ám in size. These particles consist of silicate layers and galleries filled with organic quaternary ammonium ion. The silicate layer itself has a thickness of 7.5 ┼ and the basal spacing of sodium montmorillonite is 9.6 ┼. The organic quaternary ammonium ion takes place in the sodium ion to facilitate the peeling-off of the silicate layers by increasing the gallery space and behaves like a surfactant to compatibilize the silicate with polymers. In the cases of rubber blends of neat nylon 6 most of the rubber particles were relatively small and of ellipsoidal shape. But the rubber particles in the rubber blends of nylon 6 nanocomposites were relatively large and showed an extended ellipsoidal shape. These are quite interesting phenomena since it was generally observed that a polymer with higher molecular weight brings smaller rubber particles in blends. The dimensions of the rubber particles in the longitudinal and transversal directions were analyzed using an image analyzer and the weight-average particle dimensions were obtained. This weight-average particle size is frequently referenced when evaluating the toughness of polymer composites because it is known to give a good representation of the trends reported in the literature. In neat nylon 6 composites, the weight-average dimensions of rubber particles were 0.24 Ám in longitudinal direction and 0.13~0.16 Ám in transversal direction, respectively. The dimensions lay within the particle size range of 0.1~1 Ám that is known to be a limit for rubber-toughening and the geometry was an ellipsoidal shape. On the other hand, in the nylon 6 nanocomposites the weight-average dimensions of rubber particles were 0.41~0.52 Ám in longitudinal direction and 0.19~0.23 Ám in transversal direction, respectively. The dimensions again lay within the rubber-toughening particle size range, but they were much larger than the former case and the geometry was a more extended ellipsoidal shape.