Reference : UMR5223-SEBLIV-004
Workplace : VILLEURBANNE
Date of publication : Thursday, June 23, 2022
Scientific Responsible name : Sébastien Livi & Nathalie Lazaric
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2022
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
Due to their outstanding properties such as high optical transparency, high electrical resistance, high thermal insulation, good thermo-mechanical properties, and dimensional stability, epoxy networks represent important thermosetting polymer materials estimated to $10.3 billion for 2021. In fact, they are widely used in Industry as adhesives, paints and coatings, electrical, and electronic applications as well as composite matrices for automotive, wind turbines, and aerospace applications [1-2]. However, the End-of Life of structural composite materials is a major issue due to the crosslinked of the thermoset matrices, i.e. non reprocessable by melting and non-soluble polymer networks . Thus, landfilling and incineration are the most commonly used methods for these materials causing significant negative impacts on ecosystems including the production of microplastics. Recently, the European Union (EU) and the socio-technical pressure foster the Industry to minimize the environmental impact of the products by designing sustainable composite recycling solutions. The EU Directive denoted 99/31/EC has introduced some regulations that prohibit large composite parts landfilling, such as wind turbine blades. Moreover, a large part of the European countries imposed a landfill taxes or totally ban landfill and incineration for composites such as Germany, Finland and Netherlands [4-5]. The recycling of fiber reinforced composite materials will play also a crucial role in the near future due to the rapid evolution of aircraft, automotive and marine production, but also due to the increasing share of wind energy in global electricity production .
Different routes have been investigated in the literature through the development of polymer networks reaching their End-of-Life by the concept « design to degrade », mainly by chemical reclycling (degradation or depolymerization into monomers or oligomers) [7-8]. Various authors reported a recycling approach for anhydride-cured epoxy networks by using alcohol solvent with the presence of catalyst inducing a transesterification reaction [7-9]. This procedure is also sometimes called vitrimerization. After soaking and swelling of the resulting networks in solvent with the transesterification catalyst, the epoxy network can be decomposed after many hours at higher temperature (> 150°C) and re-polymerized, extruded, injection molded or compression molded (See Figure 1) . Other authors introduced dynamic covalent bonding into the thermoset epoxy resin, i.e. a reversible character under specific stimuli (temperature, UV-light exposure, pH)(Covalent Adaptable Networks CAN) For example, Odriozola et al have developed epoxy resin with exchangeable disulfide crosslinks to obtain reprocessable, repairable and recyclable fiber-reinforced thermoset composites . In this work, they have used an amine hardener containing disulfide bonds named 4,4ʹ-dithiodianiline (DTDA) with diglycidyl ether of bisphenol A (DGEBA) in order to produce carbon fiber reinforced plastics (CFRP). They succeeded to completely dissolve the CFRP into a mixture of 2-mercaptoethanol/dimethylformamide at room temperature after 24 hours. No traces of contaminants, i.e. epoxy prepolymer, were found on the fibers and the epoxy residues were not reused as recycled resource .
Nevertheless, the different strategies require the use of volatile and sometimes toxic organic solvents, high temperatures and long periods of time. In addition, the reuse of depolymerized or degraded compounds has rarely been studied to develop recycled materials. As previously mentioned, new regulations impose responsability on manufacturers concerning their products after their End-of-Life. Thus, they are clearly pushed to design and operate a closed loop supply chain. In a closed loop supply chain or circularity of materials, the returned flows from the user should be incorporated in addition to a re-processing stage of the End-of-Life product to a useable one without down-cycling .
Very recently, Ionic Liquids (ILs) have generated a growing interest in the field of thermosetting polymers representing a real opportunity to design new (multi)functional-dedicated polymer materials with enhanced properties such as thermal stabilities, mechanical performances, gas or water barrier properties, shape memory, and self-healing properties [13-16]. Moreover, ILs have unique set of physico-chemical properties as well as a multitude of possible chemical structures representing a new class of industrial solvents combined with their recycling ability. Based on the large expertise of our laboratory in the field of polymer matrix composite materials combined with complementary skills of the IMP on reaction-based polymerizations (epoxy/amine and epoxy-IL/amine polyaddition), the objectives of this project will be to design innovative epoxy polymer materials by controlling their End-of-Life. In order to ensure the success of the project, the Synthesis and characterization of reference epoxy networks and their ternary recycling (depolymerization). Reference networks having different glass transition temperatures, Tg, will be prepared and characterized. Then, various ILs or alcohol/IL mixtures will be investigated and used to catalyze the depolymerization (transesterification, aminolysis) and/or degradation of the networks. Thus, different parameters such as the revelant choice of cation/anion combination, temperature, and duration can be optimized and determined as a function of the chemical nature of the thermosetting networks.
IMP team will be involved for their expertise on the processing and characterization on epoxy systems and nanomaterials from Ionic Liquids. The research activities of the Polymer Materials Engineering Laboratory (IMP) are ranging from synthesis of new macromolecular architectures and formulation of polymers to the processing of complex materials (blends, (nano)composites, nanostructured networks…) and establishment of polymer architecture-morphology-physical properties relationships.
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