Date of Award

8-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Committee Member

Dr. Srikanth Pilla, Committee Chair

Committee Member

Dr. Gary Lickfield, Committee Member

Committee Member

Dr. Igor Luzinov, Committee Member

Abstract

The Corporate Average Fuel Economy (CAFÉ) standards for 2025 are set to introduce a fleet-wide average of 54.5 MPG for cars and thereby, prevent emissions of 6 billion metric tons of CO2 [1]. This has propelled the automotive industry to renew their focus on lightweighting cars, particularly through the use of crude oil-based structural foams. While these foams offer a unique combination of ultra-lightweighting with adequate strength, they are practically non-renewable, non-biodegradable and contribute to the growing anthropogenic carbon footprint. An alternative paradigm to such foams is the use of biosourced polymers as they offer immense advantages due to their renewable, sustainable and biodegradable nature. Currently, polylactic acid (PLA) remains the most abundant commercially consumed biopolymer, but it suffers from two major drawbacks: its inherent brittle nature and poor melt processability. Blending PLA with an inherently toughened counterpart provides an effective mechanism to overcome both these drawbacks [2]. Additionally, foaming of PLA-based blends can provide a replacement for synthetic structural foams. However, processing of such blended foams is inhibited by challenges associated with structural foam molding with regard to controlling foam microstructure – specifically, cell size and cell density. Additionally, controlled processing of bimodal cell structure has remained elusive with currently used molding parameters and chemical blowing agents. Bimodal cellular distributions are preferred for their superior properties – enhanced toughness and compressive strength, weight reduction, and insulating properties –compared to their unimodal counterparts. This study investigates the effect of material properties and processing parameters on unique cellular distributions of polylactic acid (PLA), polybutylene succinate adipate (PBSA) and their blends processed via supercritical fluid-assisted injection molding. Cell morphology, size and density were determined via scanning electron microscopy, while their influence on mechanical properties was studied using tensile testing. Thermal stability of the blends was studied via differential scanning calorimetry and thermo-gravimetric analyzer. Effect of melt rheology and viscoelastic behavior was studied in an effort to explain the bimodal cellular structure obtained.

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