10/24/2023
By Danielle Fretwell

The Francis College of Engineering, Department of Plastics Engineering, invites you to attend a Doctoral Dissertation
defense by Akanksha Patel, on "Understanding the effects of polymer structure and melt processing on enzymatic depolymerization for polyester recycling."

Candidate Name: Akanksha Patel
Degree: Doctoral
Defense Date: Nov. 6, 2023
Time: 2-4 p.m.
Location: ETIC 445

Committee: 

  • Advisor: Margaret SobkowiczKline, Professor, Plastics Engineering, University of Massachusetts Lowell
  • Ramaswamy Nagarajan, Professor, Plastics Engineering, University of Massachusetts Lowell
  • Stephen P. Johnston, Professor, Plastics Engineering, University of Massachusetts Lowell
  • Dongming Xie, Associate Professor, Chemical Engineering, University of Massachusetts Lowell

Brief Abstract:

Poly(ethylene terephthalate) (PET), commonly known as polyester, is an omnipresent material used in numerous applications such as bottles, textiles, food packaging and automotive parts, etc. Its unique properties like high transparency, strength-to-weight ratio, and chemical resistance make it a desirable material for many household applications. High demand for PET results in more waste, leading to a need for more efficient recycling strategies. Enzymatic deconstruction of PET is a new recycling method that uses enzymes to depolymerize polyester waste to recover monomers. It offers several benefits including the involvement of less harmful chemicals and lower energy costs compared to other commonly used recycling technologies such as mechanical, chemical, or thermal recycling.

Enzyme efficiency and effectiveness are the main limiting factors, but straightforward improvements can be made by altering the physical properties of the materials including glass transition temperature, crystallinity, molecular weight, and specific surface area. This dissertation explores various pretreatment techniques including single and twin-screw extrusion to improve the enzymatic deconstruction efficiency of polyesters. The first part of this work investigated the effect of thermomechanical shear and throughput on the structural properties of PET material. The main aim of these pretreatments was to disrupt the molecular packing of PET substrate via quenching from the melt, which increases access for enzymes to initiate hydrolysis of ester bonds. These pretreated samples showed a decrease in molecular weight, glass transition temperature, and crystallinity. Leaf-branch compost cutinase (LCC-ICCG) enzyme was used for the enzymatic depolymerization reaction, and higher depolymerization was observed in samples with low crystallinity and high specific surface area.

The second part of this work investigates a comparative study using both single- and twin-screw extrusion to understand the influence of thermo-mechanical shear, residence time, and throughput on the structural properties of copolyesters. For this work, five copolyesters with diverse chemical structures were pretreated, and enzymatic depolymerization of the pretreated feedstocks was investigated using purified LCC-ICCG enzymes. Samples with low crystallinity and low Tg showed improved monomer recovery, whereas negligible improvement in monomer recovery was observed when Tg and crystallinity remained high after pretreatment.

In the third part of this work, the effect of improved specific surface area (SSA) was explored to increase the number of binding sites for LCC-ICCG enzymes. Chemical foaming was utilized to improve the SSA using a chemical foaming agent (CFA), containing citric acid and sodium bicarbonate (tradename Hydrocerol). The CFA was added (1 wt.% and 2 wt.%) in the twin-screw extruder to foam polyesters with and without comonomer cyclohexanedimethanol (as a crystallinity modifier). Depolymerization using LCC-ICCG enzyme was performed on melt-processed and foamed polyester substrates to investigate the direct effects of enhanced specific surface area on the monomer conversion. Results showed an improved specific surface area provides more binding sites to enzymes, yielding faster depolymerization kinetics.

The pretreatment performed in this dissertation work demonstrated a significantly faster and low energy-intensity method for polyester depolymerization using LCC-ICCG enzymes, which can be directly integrated into the current supply chain system for efficient recycling of mixed and contaminated polyester waste streams.