Monitoring Confined Inorganic-Polyepoxy-Inorganic Adhesive Interfacial Changes during Curing at Various Environmental Conditions

Polyepoxides are becoming a preferred alternative to heavier materials in structural applications, functioning as bonding agents and matrices for high-performance composites. The durability of polyepoxy-inorganic substrate interfaces depends on surface treatments (physical and chemical) and environmental curing conditions such as temperature, humidity, and mechanical stress. Detecting real-time changes at these interfaces is crucial to prevent failures. This study used a Surface Forces Apparatus (SFA) with multiple beam interferometry to investigate interface degradation during curing and aging. A stoichiometric mixture of bisphenol-A diglycidyl ether (BADGE) and amine-terminated polyoxypropylene glycol (JeffAmine D-230) was confined between smooth alumina substrates under varying humidity. The SFA and fringes of equal chromatic order (FECO) technique enabled real-time imaging and refractive index measurements of the confined polyepoxy. Curing strains and stresses were monitored over 48 hours. Key findings include: (1) low tensile forces promote complete curing across different humidity levels, dominated by molecular interactions; (2) high tensile forces prevent full curing unless compressive forces are applied; (3) high humidity rapidly induces water diffusion, increasing plasticization and preventing cure. Additional micro-tensile tests and failure analysis imaging revealed primarily cohesive (polymer-polymer) rather than adhesive (polymer-substrate) failure modes. This study introduces a novel method for assessing interface quality in polyepoxy-inorganic substrate systems, which has implications for various technological applications requiring durable bonding and adhesion performance.