New Thermally Reversible Thermoplastic Polyurethanes

Traditional reactive polyurethane adhesives are versatile materials with a range of mechanical properties. However, they are irreversibly crosslinked preventing separation of substrates at end-of-life for reuse or recycling, causing negative environmental impacts. Therefore, reversible adhesives are of interest in the drive towards a circular economy. Thermoplastic polyurethanes (TPUs) display a physical response to temperature (solid at ambient, fluid with heat), but the strong intermolecular hydrogen bonding persists in the melt phase causing highly viscous fluids and processing challenges. To overcome these issues, the structure-property relationships of new TPUs were investigated with the aim of exploring potential copolymers as reversible adhesives.
Initially, model copolymers were made without chain extenders using semi-crystalline
polyols. Polyol molecular weight influenced the crystallinity, degree of physical interactions
and molecular weight of the TPU. Branching in chain extenders hinders hard segment
association leading to mechanically poor copolymers, whereas urea functionality affords
enhanced mechanical performance but lower sensitivity to thermal stimulus, retaining high
melt viscosity. Therefore, physical interactions alone do not seem viable for reversible
adhesion.
Reversible covalent bonds were incorporated along TPU copolymer backbones via Diels-Alder
(DA) cycloadducts from the copolymerisation of functional PU prepolymers made from semi-crystalline and amorphous polyols. All copolymers displayed substantial change in physical state as a response to temperature with dynamic and reversible Mn (typically changing from 6 to > 25 kDa). Material properties were revealed as heavily dependent on constituent prepolymers and starting polyols, with semi-crystalline materials showing greater stiffness and amorphous materials greater flexibility. Therefore, final studies were conducted using blends of different prepolymers, making linear TPUs that contain DA cycloadducts. TPUs were synthesised showing phase separation without chain extender and highly tuneable properties. Several materials throughout these studies show promise in the application of thermally reversible adhesives differentiating between high mechanical properties and low melt viscosity.