title: Stimuli-responsive debondable adhesives
Creators: name: Babra, Tahkur Singh
Publisher: name: University of Reading
Year: 2023

description: Polymeric adhesives are becoming an increasingly important industrial product. They are routinely used in a wide range of high value and disposable products ranging from bonding large sections of the interior of new cars to wound dressings that replace stiches; from single use packaging to safety critical components in the aeronautical industry. Recently, the introduction of debondable adhesives that break down with an external stimulus has opened up new markets and applications in the adhesive industry. For the first time, components can be securely bonded for the working lifetime of the product, then disassembled on demand to allow for efficient recycling and disposal at the end of the product’s usable life-cycle. The objective of this study was to design and synthesise new, debondable polymeric adhesives that can breakdown or depolymerise in response to an external stimulus, such as chemicals, light or heat. This would ultimately lead to a new class of debond-on-demand adhesive. The work towards this goal is summarised below. Chapter 2 reports the design, synthesis and evaluation of a fluoride degradable unit which can be incorporated within a linear polyurethane (PU) thermoplastic adhesive. Detailed solution state studies carried out by 1H NMR spectroscopy and gel permeation chromatography before and after depolymerisation confirmed the efficient degradation of the material in response to the addition of fluoride ions. Mechanical strength testing on homogenous films together with rheometric and differential scanning calorimetry studies over multiple heat-cool cycles confirmed the reversibility of the supramolecular network within the PU. Lastly, adhesion testing with different material substrates before and after degradation showed the debond-ondemand nature of this novel material. Chapter 3 further explores the use of the fluoride degradable unit within two series of linear polyurethanes that vary in chemical structure as part of efforts to increase the adhesive and thermal properties over the polyurethane reported in Chapter 2. The first series of adhesives explores the effect on bonding properties that varying nature of the diisocyanate linkers in the PU has on the adhesive. The second series of adhesives varies by the nature of the soft segment within the PU. It was found that introducing low-melting point crystalline regions into the material results in a polymer that can provide excellent adhesion at lower bonding temperatures (ca. 60 °C compared to 120 °C). The thermal response and morphology of the two series of polymers were analysed by variable temperature rheometric analysis, small angle x-ray spectroscopy and wide angle x-ray spectroscopy. Finally, the adhesives were tested at AWE Preface vi Aldermaston following international standards at different temperatures before and after degradation with a fluoride source. Chapters 4 and 5 explore the possibility of producing a fluoride responsive crosslinked material using two different approaches. The first system was a variant of a reactive adhesive which was produced by mixing a trifunctional degradable group with a bis(isocyanate) terminated prepolymer (Chapter 4). Adhesion testing displayed a 28 % strength increase over the linear polymers, and a 55 % strength loss after treatment with fluoride ions. The second method explored the possibility of creating star polymers incorporating the degradable groups at the core. The resulting alcohol terminated branched polymer was then reacted with a commercially available aromatic diisocyanate to form a crosslinked adhesive. Adhesive testing was carried out before and after degradation with fluoride ions showed that fluoride ions could penetrate the crosslinked network resulting in a measurable reduction in bonding strength (approx. 23 %). Finally, Chapter 6 reports the design and realisation of a novel UV responsive degradable group. Model compound studies were carried out before and after degradation with a UV light source (36 W) using 1H NMR and UV/visible spectroscopies and showed rapid degradation after only 5 minutes irradiation. The UV group was incorporated into a polyurethane adhesive, which also showed rapid degradation in solution on exposure to UV light. The linear PU proved to be an excellent hot melt adhesive for glass substrates (bonds strength = 0.43 MPa), which weakened by up to 86 % after 5 minutes irradiation at 365 nm. However, the mechanism by which debonding occurs in the solid state in still an area of active investigation.