Thesis

The environmental resistance of glass fibre vinyl ester composites and their interface for use in structural applications

Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2022
Thesis identifier
  • T16910
Person Identifier (Local)
  • 201886963
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • The use of fibre reinforced polymers (FRPs) in large infrastructure applications, such as the renewable energy sector has notably increased over the past decade. During their lifetime FRP composites can undergo significant changes upon their inevitable contact with environmental degradation agents, such as moisture. Such interactions may trigger irreversible degradation mechanisms in FRPs and in turn impair their properties. The state of the fibre/matrix interface is a crucial region in defining the reliability, and therefore the durability, of composite materials, or structures, during their service life. Previous reports suggest that potential improvements in the interfacial properties of FRPs do often correlate with improvements in composite macroscale properties. FRP interface can be particularly affected by environmental degradation, as it forms a passageway for moisture ingress into the material. This thesis is part of the larger EU Horizon 2020 funded DACOMAT (Damaged Controlled Composite Materials) project. The scope of this work is to produce an accelerated ageing study which allows the examination of two different DACOMAT glass fibre/vinyl ester systems in wet environments. A major part of this thesis involves the use of vinyl ester specimens on the microscale. Although information is available on the cure kinetics of vinyl ester matrices on a bulk scale, arising scalability effects when employing microscopic matrix volumes remains an area in need of significantly improved understanding. The use of thermal analysis techniques revealed that microscopic vinyl ester specimens may exhibit a dual glass transition temperature (Tg), indicative of the diphasic nature of the resin. Polymerisation kinetics of microscopic scale samples of vinyl esters were found to be impeded by excess styrene loss promoted by the high surface-to-volume ratio of the employed specimens. Oxygen inhibition was also identified and deemed critical for the curing of vinyl esters. Methods to counteract such effects are proposed. The microbond test was utilised for the characterisation of the glass fibre/vinyl ester interface. Several vinyl ester matrix systems and vinyl ester-compatible fibres have been examined. Scanning Electron Microscopy (SEM) was also utilised for the study of two DACOMAT vinyl ester micro-composite systems. A novel discovery of the interaction of the cyanoacrylate adhesive often used in the microbond test to “fix” single fibre micro-composites on a mounting card with vinyl ester-compatible sizings and (or) vinyl ester resins is presented. A macroscale characterisation of the water uptake kinetics, mechanical and thermal properties of two DACOMAT composite systems and their matrix constituents as a function of their environmental history is enabled. The studied reinforcing fibres were two vinyl ester-compatible glass fibres coated with the same sizing but of a different glass formulation (HiPertex vs Advantex). The studied matrices were a commercially available bisphenolepoxy-based vinyl ester resin, and a newly developed methyl methacrylate (MMA), styrenebased modified vinyl ester resin. An interesting observation was made; the latter was found to be more hydrophilic than the former, but it formed more moisture-resilient interfacial bonds and showed greater mechanical property retention than the former, when these were used as a composite reinforcement in wet environments. The stress transfer capability at the glass fibre/vinyl ester interface of the aforementioned composite systems as a function of their environmental history was also assessed by means of microbond testing of IFSS. An epoxy micro-composite system was also employed for comparative purposes. A correlation between the moisture uptake of a microbond vinyl ester droplet and its effect on the resulting IFSS was attempted. Since the monitoring of the moisture gain of microbond droplets was deemed impractical, experimental thin film models, as well as simple mathematical diffusion models were employed. Fourier-transform Infrared Spectroscopy (FTIR) was utilised for the evaluation of hydrolytic changes in vinyl ester films, while an attempt of the estimation of the moisture uptake of films using two different FTIR interfaces is also presented. Micro-mechanical testing was found to be useful for the coupling of the hydrothermal response of the bulk vinyl ester-based composites. Ageing-induced degradation in single fibre micro-composite structures was identified by SEM in the forms of micro-cracking and moisture wicking along the fibre/matrix droplet interface, matrix blistering, as well as the appearance of a two-phase structure, hich was made visible by ageing in a wet environment.
Advisor / supervisor
  • Yang, Liu
  • Thomason, James
Resource Type
DOI

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