Toughening of epoxy-anhydride thermoset and epoxy-diacid vitrimer: From binary to multi-component composites

Abstract

Inherent brittleness due to the highly crosslinked structure of epoxy thermosets and epoxy vitrimers limits their high performance application. To enhance fracture toughness (KIc) without losing the stiffness of crosslinked epoxy, two toughening approaches were applied in this present study, including the incorporation of rigid toughening agents and the modification of network characteristics. Toughening mechanisms related to fracture toughness (KIc) enhancement were comprehensively investigated. Carbon fibre reinforced polymer (CFRP) composites based on toughened epoxy-based polymers were further studied with a particular focus on interlaminar fracture toughness (GIc). Epoxy-anhydride thermoset was toughened by individual and hybrid systems of tougheners, including polyethersulfone (PESU), graphene nanoplatelets (GNPs), and multi-walled carbon nanotubes (MWCNTs). Among those tougheners, PESU and GNPs pronounced more positive toughening effects than MWCNTs. The greatest enhancement of KIc was observed in a hybrid system of 5 wt% PESU/0.5 wt% GNPs modified epoxy, revealing 60% increase compared to virgin epoxy. The synergistic toughening effects from PESU (crack deflection) and GNPs (crack deviation and crack bifurcation) were accounted for the improvement. In the study of CFRP composites based on toughened epoxy thermoset, a small amount of MWCNTs can promote matrix-fibre adhesion and GNPs dispersion in the laminate. CFRP with hybrid GNP/MWCNT at 0.4/0.1 wt%/wt% with 5 wt% PESU modified matrix provided the optimum interlaminar properties, showing GIc improvement of 80% and 26% at crack initiation and crack propagation, respectively. This indicates that toughening mechanisms and good interfacial adhesion are essential for enhancing interlaminar properties. For toughened epoxy-dicarboxylic vitrimer, the understanding of dynamic networks formation from the reaction between diglycidyl ether of bisphenol A (DGEBA) and 1, 4 cyclohexane dicarboxylic acid (CHDA) was developed by considering the influence of organotin catalyst content and epoxy:acyl ratio. The presence of sufficient catalyst (5% meq to epoxide) allowed for the fully crosslinked network while the excess ratio of epoxy was crucial for tuning glass transition temperature (Tg) and mechanical properties without losing self-healing ability. In particular, KIc remarkably improved from 0.93 to 1.59 MPa.m1/2 when epoxy:acyl ratio changed from 1:1 to 1:0.5. It is probably due to strain localisation of the heterogeneous network which is obtained from epoxy homopolymerisation. By incorporating GNPs or MWCNTs, the bond exchange reaction of off-stoichiometric vitrimer (epoxy:acyl ratio at 1:0.5) was accelerated, resulting in the reduction of activated energy (Ea) and freezing topology temperature (Tv). Furthermore, KIc of off-stoichiometric vitrimer was promoted with the presence of 0.1 wt% MWCNTs (9 % increment) due to the pull-out mechanism of nanotubes. To highlight the successful development of epoxy-CHDA vitrimer, interlaminar fracture toughness of stoichiometric and off-stoichiometric vitrimer (epoxy:acyl ratio at 1:0.5) CFRP composites was compared to epoxy-anhydride laminate. Stoichiometric vitrimer laminate revealed the greatest GIc at crack initiation (268 J/m2) and crack propagation (873 J/m2) regarding to matrix toughness and better interfacial adhesion between matrix and fibre. The present research has improved the understanding of the concept of hybridisation of tougheners and heterogeneous networks for the toughness improvement of epoxy-based matrices and their fibre composites. This has the potential to apply those concepts to other brittle polymer systems with a good expectation of toughness properties.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alberto Saiani (Supervisor) & Arthur Wilkinson (Supervisor)