Abstract
Pressure-sensitive adhesives (PSAs), as the name implies, form a bond with nearly any solid surface under the application of light pressure. With the increasing use of PSAs in various industries, there is a need for greater sustainability, particularly in developing materials from renewable resources, as well as the reuse and recycling of materials to reduce their environmental impact, reduce waste, or extend their life.
This thesis investigates two distinct strategies for developing polymer materials for more sustainable PSAs. The first strategy is debonding on demand in response to ultraviolet (UV) radiation, and the second strategy is degrading polymer networks via a chemical treatment. Both approaches could potentially enhance the reuse and recycling of bonded materials, as well as address the ‘stickies’ issue, thus making the recycling processes more effective. In the first part of this work, perfluorophenylazide chemistry was explored and employed to control precisely a polyacrylate network for application as a PSA. Upon UV irradiation, the highly reactive nitrene from the azide moiety reacts with nearby molecules through a C–H insertion reaction, resulting in cross-linking via covalent bonding. This approach offers three benefits: (1) a means to optimise adhesive properties without the addition of an external photo-initiator; (2) the ability to switch off the tack adhesion on demand via a high cross-linking density; and (3) a platform for additional chemical modification. This first demonstration of using azide functionality has enormous potential for functional PSA design.
In the second part of this work, degradable PSAs were developed that provide the required performance in use but have networks that can be degraded after use. A series of copolymers were prepared from n-butyl acrylate, 4-acryloyloxy benzophenone (ABP) photoinitiator, and dibenzo[c,e]oxepin-5(7H)-thione (DOT) to provide a cleavable thioester. The optimum tack and peel strengths were found for molar contents of 0.05 mol% ABP and 0.25 mol% DOT. Degradation of the backbone thioesters through aminolysis or thiolysis led to the full dissolution of the networks, loss of adhesive properties of films, and the quick detachment of model labels from a substrate. The inclusion of DOT into PSAs offers a viable route toward degradable and recyclable packaging labels.