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Investigation of the Dissolution Behavior of Waste Polymers In Biodiesel

Ying Zhang, Surya K. Mallapragada, and Balaji Narasimhan. Chemical & Biological Engineering, Iowa State University, Ames, IA 50011

The dissolution of polymers is an important phenomenon in polymer science and engineering. It has found significant applications in a variety of areas such as drug delivery, microlithography, and membrane science. Polymer dissolution also plays a major role in the treatment of unsorted plastics for recycling. In the present study, investigation of the dissolution behavior of common waste plastics in biodiesel was carried out. The polymers of interest are polystyrene (PS) and low-density polyethylene (LDPE), which are commonly found in waste. The solvent of interest in this work was biodiesel (i.e., composed of methyl esters in this study). Biodiesel is a non-petroleum-based diesel fuel consisting of short chain alkyl (methyl or ethyl) esters, typically made by transesterification of vegetable oils, recycled cooking greases or oils, or animal fats, which can be used (alone, or blended with conventional petroleum-based diesel) in unmodified diesel-engine vehicles. Since biodiesel is produced from domestic, renewable resources, it can act as a clean burning alternative fuel. The objective of our research is to provide experimental data for dissolution of waste plastics in biodiesel, which combined with engine performance testing, will help obtain optimal conditions for use of the polymer solutions for energy applications.

Our experiments demonstrate that PS is completely soluble in methyl esters over the range of molecular weights studied (80,000-980,000 Da). LDPE is also soluble in biodiesel, but only at elevated temperatures and at low polymer molecular weight (below 6500 Da). The dissolution kinetics of PS and LDPE in methyl esters was monitored by gravimetric methods and FTIR imaging. Dissolution rates of the two polymers as a function of polymer molecular weight and temperature were extracted from the experiments. From the data, parameters such as activation energies and scaling indices (of dissolution rate vs. polymer MW) were estimated. Other properties of the PS (LDPE)/biodiesel solution such as shear viscosity and particle size in solution were also characterized. Together, the dissolution and the shear viscosity experiments provided conditions in which these solutions could be used as fuel in engines. To complement the experimental investigations, molecular dynamics simulations were employed to construct models of methyl esters, and parameters such as cohesive energy densities and solubility parameters were calculated. These properties were used to predict thermodynamic polymer/solvent interaction parameters and based on these calculations, phase diagrams were constructed. The modeling work also provides a predictive toolbox to design biodiesels with different compositions (e.g., mixed solvents, and additivies) that will dissolve other common plastics.