Vitrimer-based adhesives for fully reversible joints

Abstract

The permanent nature of structural adhesive joints impairs repair, reuse, and recycling of bonded components and structures. In recent years the development of alternative, controlled de-bonding methods that aim to facilitate part disassembly and recovery has accelerated. The properties of vitrimer-based polymers can be harnessed to improve the sustainability of adhesive bonding by enabling full reversibility of structural bonded joints. In this study, the properties of two vitrimer formulations – abbreviated as VA.1 and VA.2, respectively – are studied for their application as structural adhesives. The first vitrimer is synthesized from diglycidyl 1,2-cyclohexanedicarboxylate (DCN) and internally catalyzed by the curing agent polyethylenimine (PEI), while the second formulation is commercially available, and is purchased directly from the manufacturer. Mechanical, environmental aging/durability, and reversibility tests are performed. The viscoelastic properties of the vitrimers are measured with Dynamic Mechanical Analysis (DMA) and are used to determine the glass transition temperature (Tg), which gives the adhesive’s operational temperature ceiling. Both formulations exhibit reasonable thermomechanical stability, yielding viscoelastic properties dependent on (i) constituent mix ratio, (ii) curing schedule, and (iii) environmental loading history. Bulk material specimens and aluminum alloy and multi-material (aluminum alloy + carbon fiber composite) single lap joints (SLJs) are fabricated to investigate strength and durability through lap shear testing, creep, relaxation, and hot-wet/submersion tests. Furthermore, SLJs undergo reversibility testing by means of electromagnetic excitation of the substrates for de-bonding under constant 100 N load and welding/re-bonding. Experimental results on the viability of vitrimer-based adhesives for fully reversible structural joints are presented and discussed. In Chapter 1, a broad introduction and review of the literature is included. The following areas are surveyed: the current state-of-the-art for joint de-bonding, advances in the research on covalent adaptable network (vitrimer) materials, and the use of vitrimers as de-bondable adhesives. Several research gaps are identified: as of the time of writing this thesis, vitrimer research is less than 2 decades old. At the end of the chapter, the thesis objectives are outlined. In Chapter 2, the 2 vitrimer formulations used in this thesis are described with the relevant literature studies. Joint substrate materials and test specimens geometry are defined. In Chapter 3, the experimental setup and methodology are discussed. The synthesis steps for both vitrimer formulations are outlined. The joint fabrication process and the methodology for (i) lap shear, (ii) creep, and (iii) fracture toughness testing are described. Dynamic Mechanical Analysis and environmental aging parameters for bulk material characterization are reported for the tested vitrimer configurations. Finally, the procedure for de-bonding and welding/re-bonding tests is outlined. In Chapter 4, results are presented and discussed. The lap shear strength and glass transition temperature of both formulations are comparable to commercial structural adhesives: up to 16.5 MPa and 53 C for VA.1, and 18.0 MPa and 101 C for VA.2. The DCN-PEI (VA.1) vitrimer is very hygroscopic, reaching 49 moisture uptake upon submersion in de-ionized water. VA.1 viscoelastic moduli drop by 1 order of magnitude after hot/wet storage + drying, but fully recover upon healing at high temperature. The fracture toughness of VA.1 (1.8 kJ/m2) is measured using tapered double cantilever beam specimens, and is comparable to typical toughened epoxies. De-bonding is highly repeatable for all tested variations: all joints de-bond in less than 5 min. Re-bonding and/or welding is more effective for VA.2 joints, which reach as high as 53 healing efficiency, while self-welding of VA.1 joints only yields a strength equal to 20 of the baseline lap shear strength. In Chapter 5, conclusions are discussed. Additionally, the novel findings are summarized, as well as detailed recommendations for future work. This thesis is made possible by the generous support of the National Science Foundation under Grant No. 2052658.