04/05/2024
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation Proposal defense by Saeed Alanazi on: Effects of Organic Nucleating Agents on Thermal and Shear-Induced Crystallization Kinetics and Mechanical Performance of PLA.

Candidate Name: Saeed Alanazi
Degree: Doctoral
Defense Date: Friday, April 19, 2024
Time: 1 to 3 p.m.
Location: Perry Hall 415

Committee:

  • Advisor: Davide Masato, Assistant Professor, Plastics Engineering, University of Massachusetts Lowell
  • Carol Barry, Professor and Chair, Department of Plastics Engineering, University of Massachusetts Lowell
  • Amir Ameli, Assistant Professor, Department of Plastics Engineering .University of Massachusetts Lowell
  • Peng Gao, Assistant Professor, Department of Engineering and Design, Western Washington University

Brief Abstract:

Poly (Lactic Acid) (PLA) is one of the most researched bio-based polymers. However, regarding processing and injection molding specifically, PLA has some challenges that limit its applications. The slow crystallization rates limit the achievable product performance and require lengthy post-processing operations (i.e., annealing). In injection molding, the non-isothermal conditions and the high cooling rates promote rapid material solidification, consequently not allowing time for crystalline structure growth. The resulting mechanical properties are scarce, and most bio-based polymers are used for low-performance applications. Overcoming these challenges could enable significant opportunities for increased use of bio-based polymers as a substitute for conventional fossil-based ones.

Several approaches to address these challenges have been explored in the literature. One of the main strategies to increase the crystallinity of PLA is the addition of nucleating agents, which promote crystallinity through the formation of nuclei. However, most studies do not focus on the characterization of crystalline structures formed under processing conditions but instead rely on offline characterization using rheometers. Hence, limitations are evident when considering high shear rates typical for injection molding. Indeed, crystallinity is not only affected by thermal effects and dynamics. The shear rate can also play a significant role in the formation of nuclei and the development of shear-induced crystalline structures.
This work investigates the development of crystalline structures in PLA under injection molding conditions, considering thermal and shear-induced effects. The interaction between these phenomena and organic nucleating agents is evaluated. The effects of different polymer grades and processing conditions are also explored. The ultimate goal is to map the correlations between material characteristics, processing, morphology, and mechanical properties. I anticipate that the results will support designers in product and process development for bio-based polymers.

To isolate the thermal effects of nucleating agents on crystallinity, the first chapter focuses on the study of quiescent crystallization with Orotic Acid (OA) and Ethylene Bis-Stearamide (EBS). The novelty of this work lies in the study of crystallinity under thermal conditions only, without the introduction of any shear. The goal is to establish the baseline effect of nucleating agents on crystalline structure dynamics and polymorphism. In-depth characterization is carried out using Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The findings indicate that both nucleating agents promote a substantial acceleration of the crystallization dynamics, leading to a reduction in incubation time and crystallization half-time. The most notable outcomes were observed with 1% EBS at 110°C, which displayed the swiftest crystallization. Considering the results of the experiments with different nucleating agents, preliminary injection molding experiments have been focused on EBS-based compounds. The nucleating agent was compounded to PLA using twin-screw extrusion and pelletized for injection molding. Current experiments are focused on injection molding and characterization of compounds. Different experimental plans are being developed to evaluate the effect of rapid heat cycle molding and low-pressure injection molding. The work aims to generate different shear conditions during processing, morphological characterization, and mechanical testing.