Date of Graduation
Statler College of Engineering and Mineral Resources
Chemical and Biomedical Engineering
Rakesh K Gupta
Robin S Hissam
Charter D Stinespring
Environmental concerns have led to research interest in biodegradable plastics, especially polylactic acid (PLA). PLA, a bio-derived and biodegradable polymer which is readily available, is easy to process, and it can be a good substitute for conventional non-biodegradable polymers in food packaging applications. However, its poor gas barrier property has to be improved to make it competitive with more widely-used materials such as polyethylene terephthalate (PET) and polystyrene (PS). This can potentially be accomplished by dispersing nanoplatelets in the polymer as these additives act as impermeable barriers around which the diffusing molecules are forced to take a longer, tortuous path. The increased path length results in a reduction in the concentration gradient with a simultaneous reduction in the mass flux. A similar situation arises upon annealing the PLA which leads to the formation of crystals that again act as barriers to mass transfer. A combination of the two approaches may lead to further reductions in permeability.;To accomplish the goal of reducing permeability through PLA, an internal mixer was used to melt-mix nanoclay into the polymer matrix, and thin films were made using compression molding. Thermal measurements showed that these films were amorphous. Since the extent of hydrophobicity or hydrophilicity of the nanoclay surface influences the compatibility between the filler and the matrix, water vapor transmission rate experiments were conducted on nanocomposite films containing different commercially-available organically-modified nanoclays; a MOCON PERMATRAN W3/33 instrument was employed for this purpose. Based on the results obtained, it was found that Cloisite 30B was the most compatible clay with PLA. In other words, PLA containing Cloisite 30B had the largest reduction in water vapor permeability at a given clay loading level. Internal mixer operating conditions were then optimized to determine processing conditions that resulted in the best barrier properties for nanocomposite films made using Cloisite 30B. These were 200°C and 80 rpm for 5 mins. Under these conditions, it was found that, with an addition of 5.3 vol% (10 wt%) of Cloisite 30B, the water vapor permeability was reduced by 69% compared to neat PLA. The corresponding absolute value compares favorably to moisture permeability through PS. The solubility of moisture in the nanocomposites was measured in a separate set of experiments and was found to increase as the nanoclay loading increased. This was due to moisture adsorption on the nanofiller surface and not due to enhanced solubility within the polymer. Thus, the fractional reduction in water vapor diffusivity was the same as the fractional reduction in permeability.;The nanocomposite films were annealed to promote polymer crystallization, and the crystallinity level could be increased to 40% by annealing at 115°C for 40 hours. Under these conditions, the water vapor permeability was reduced by 45% compared to unannealed and unfilled PLA. When a nanocomposite film containing 2.6 vol% (5 wt) of Cloisite 30B was annealed under these same conditions, the water vapor permeability was reduced by a total of 66% compared to neat PLA. Annealed films containing more than 2.6 vol% clay were brittle, but the addition of acetyl butyl citrate (ATBC) which is a plasticizer enhanced ductility. When 5 wt% of ATBC was added to PLA, an annealed sample containing 5.3 vol% (10 wt%) of Cloisite 30B exhibited a 74% reduction in water vapor permeability compared to unannealed and unfilled PLA.;A new model that builds on the tortuous path concept was developed based on the reduction in both the mass flux and area for mass transfer when nanoplatelets are dispersed in a polymer matrix. According to this theory, the ratio of the permeability in the absence of filler to that in the presence of filler is given by (1 + [h/2t]&phis;) where h/t is the aspect ratio of the nanoplatelets, and &phis; is the filler volume fraction. When this theory was applied to moisture permeability results presented in this dissertation using an average aspect ratio determined from TEM pictures, there was quantitative agreement between the model prediction and the measured permeability value on PLA/clay nanocomposites at each filler loading level. By contrast, other theories available in the literature overpredicted the permeability values by a significant amount. Additionally, the theory predicts that the relative reduction in permeability is independent of the temperature of measurement and the concentration driving force, and this is again borne out by experimental results.
Tang, Man Chio, "A Model for Permeability Reduction in Polymer Nanocomposites and Its Experimental Validation" (2017). Graduate Theses, Dissertations, and Problem Reports. 6771.