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

13 March 2025, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

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.

Supplementary materials

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Title
Supporting Information Monitoring Confined Inorganic-Polyepoxy-Inorganic Adhesive Interfacial Changes during Curing at Various Environmental Conditions
Description
S1.1 Surface configurations, nanofabrication, and FECO performance Figure S1. Comparison of Al surfaces before and after exposure to 100% relative humidity. Figure S2. Surface configurations showing fringes of equal chromatic order (FECO) Figure S3: Schematic representation of the fabrication steps of the water-oxidized Al surfaces S1.2 Surface chemical analysis Figure S4: XPS analysis of IBD alumina surfaces Figure S5: XPS analysis of Al samples initially and after exposure to DI water Figure S6: High-resolution XPS analysis of the Al2p region for aluminum surfaces Figure S7: High-resolution XPS analysis of the O1s peak for aluminum surfaces 1.3 Film surface roughness and refractive index analysis Figure S8: AFM micrographs and height profile Figure S9: Refractive indices of aluminum and alumina surfaces Figure S10: 3D mapping of intensity variation along a FECO fringe Figure S11: Cure strain as a function of time for BADGE/JeffAmine on the alumino-silicate mineral surface mica.
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