Melt processed polymer-organoclay nanocomposites
Polymer nanocomposites with organoclay fillers offer improved properties and performance, providing opportunities for commercial applications. The key to significant property enhancement is to exfoliate the individual organoclay platelets into the polymer matrix to utilize their high aspect ratio and modulus. The affinity between the polymer matrix and the organoclay is one of the most important factors for determining the exfoliation level. Although polar polymers, such as nylon 6, exfoliate the organoclay well, hydrophobic matrices, such as polyolefins, generally do not effectively exfoliate the organoclay. Thus, a significant part of this work investigates various routes to improve polyolefin-organoclay interactions and organoclay exfoliation in these systems. Nanocomposites formed from organoclay and blends of high density polyethylene and maleic anhydride-grafted high density polyethylene over the entire range of compositions were melt processed to obtain further insights into the 'compatiblizing' role of maleated polyolefins. The organoclay particle aspect ratio was found to initially increase drastically, reach a maximum, and slightly decrease with increased maleation. As the maleation level increases, the relative modulus increases initially and then levels off at higher loadings To a certain extent, the affinity between the polymer and the organoclay can be enhanced by optimizing the organoclay structure for a given polymer matrix. A silanized organoclay was investigated to determine if reduced agglomeration, improved exfoliation, and matrix reinforcement could be achieved in a polypropylene matrix without using a more costly compatibilizer. The silanized organoclay was found to be superior to the non-silanized precursor, but did not achieve the benefits obtained with a compatibilized matrix. Ionomer matrices have also been used as a means of improving organoclay exfoliation. This study examined the effects of ion type (K⁺, Na⁺), neutralization level, and melt index on the nanocomposite morphology and properties. The Na⁺ ionomers appear to have more favorable interactions with the organoclay. Exfoliation and matrix reinforcement tend to increase with decreased melt index and with increased neutralization, except at high levels. In these cases, it is possible that the additional exfoliation results in particles with lower aspect ratios. Composite properties are highly dependent on the particle aspect ratio. Several theories were used to predict the modulus and the thermal expansion coefficient of composites based on the filler aspect ratio. Novel two-population approaches were applied to enable the modeling of nanocomposites containing organoclay tactoids and single platelets, organoclay particles and glass fibers, or organoclay and elastomer particles. The quantitative agreement between the values predicted using experimentally determined particle aspect ratios and experimental modulus and thermal expansion was vastly improved using these methods.