Rheology modification and foaming of polypropylene - clay nanocomposites with coupling agents

This research was motivated by the need to develop closed cell thermoplastic foam based on linear polypropylene to form the core of a wide variety of polypropylene components. This would require rheology modifiers since linear polypropylene does not have adequate tensile melt strength to prevent cell wall rupture during foaming or bubble expansion in the melt state. The objectives of this research were (1) to understand the effects of composition on melt extensional strain hardening in composites of linear polypropylene with nanoclay, compatibilizer and various coupling agents and (2) to evaluate selected nanocomposite formulations in extrusion foaming with chemical blowing agents.;Nanocomposites were prepared with organically modified montmorillonite, maleic anhydride grafted polypropylene as compatibilizer and linear polypropylene as the bulk matrix. The organoclays were also treated with amino silane coupling agents to increase the interaction between nanoclays and the compatibilizer. The effects of different polymeric compatibilizers and varying organoclay loading on the melt extensional viscosity behavior and the rate of crystallization of the nanocomposites were examined and shown to be critical for the resulting foam quality. Besides providing large numbers of nucleation sites for bubble formation during the foaming process, addition of nanoclay with appropriate compatibilizer provided strain hardening in melt extensional flow and slower crystallization.;In composites where the organoclay was used without additional treatment, hydrogen bonding on the surfaces of the organoclay holds the compatibilizer chains. Comparison of composites with compatibilizers having different chain lengths revealed that the length of compatibilizing polymer chain that is anchored at one or more clay surfaces must be sufficient to affect the dynamics of the entanglement network during extensional flow and produce strain hardening in the melt.;Organically modified nanoclays and the compatibilizer have mainly electrostatic or hydrogen bonding type interactions, which are weaker forces of attraction. Amino silane coupling agents were grafted on the clay faces and/or edges to replace these weaker forces of attraction with stronger chemical bonds. Chemical reactions between the silated clays and appropriate compatibilizer resulted in nanocomposites with significantly greater strain hardening compared to the unsilated clay nanocomposites. These nanocomposites were further evaluated for extrusion foaming using a chemical blowing agent. The nanocomposites exhibiting uniform strain hardening over a range of strain rates resulted in foams with small cell size and highest cell densities within the scope of this research.;Since recyclability drives the substitution of thermoset polyurethane foams with closed cell PP foams, the effect of re-processing on the melt strain hardening behavior was investigated. Various nanocomposites were annealed and extruded through a uniform cross-section die to study the effect of heating and shear flow on the structural morphology and strain hardening of the nanocomposites. The results indicated that annealing had no negative impact on morphology or strain hardening of the nanocomposites. However, extrusion of these nanocomposites indicated that nanocomposites prepared using multiple hydrolysable groups resulted in formation of layered aggregates that are difficult to orient during uniaxial extensional flow and thus resulted in loss of the strain hardening behavior.