The chemistry & properties of new thermally reversible adhesives

Most adhesives today are irreversibly cross-linked, which presents many difficulties when it comes to recycling composite materials at their end-of-life. This thesis aims to tackle such problems by suggesting an alternative; covalent adaptable networks which can debond on demand, allowing for easy separation of materials. In Chapter 2, a model system containing maleimide and furan Diels-Alder (DA) adducts was used to test the viability using such materials in adhesive applications. High bond strength was achieved, and the bond could be healed using the same material with little loss of bond strength. Then, in Chapter 3, chain extenders containing DA adducts were synthesised, for the novel application of cross-linking moisture-curing polyurethanes to add reversibility to a material that would otherwise become a thermoset. In Chapter 4, unsaturated polyesters (UPEs) were considered for use as a thermally reversible adhesive, and six materials from two polyesters and three cross-linkers were synthesised and examined. Diels-Alder bonds between the unsaturation in the polyester and the furan cross-linkers were successfully formed to generate a network. One material displayed good tensile strength, and others were improved with the addition of a comonomer. During this testing it was discovered that networks made with poly(propylene maleate) were more thermally stable than those made with poly(propylene fumarate), however synthesising PPM is difficult as the high temperatures involved in the polyesterification causes isomerisation to fumarate. Chapter 5 shows that this could be limited by the glycol choice, with rigid or long chain primary diols retaining the most maleate content. Finally, in Chapter 6, UPEs were functionalised with furan groups via the thiol-ene Michael addition onto the alkene. This gave the polyester the ability to form DA linkages with unreacted sites of unsaturation on other UPE chains in either a one-component or two-component DA system.