Low-Temperature Curable Bisphenol A-Free Epoxy-Amine Network Enabled by Trifunctional Epoxide Cross-Linking and Benzyl Alcohol for Strong Adhesion, Sweat Durability, and Skin Compatibility

Low-temperature molding materials for skin-contact electronics must reconcile mutually competing requirements: rapid/complete cure with low exotherm, high hardness, durable adhesion to metallic substrates, chemical stability against perspiration, and biocompatibility. Here we report a BPA-free epoxy-amine system derived from a bisphenol-F (BPF) diepoxide, a trifunctional epoxide precursor (TEP), and benzyl alcohol (BA) that together enable a highly cross-linked architecture combining a covalent epoxy-amine network, multipoint cross-linking provided by TEP, and hydrogen-bond-rich interfacial interactions promoted by BA. A fixed 10 wt % TEP content was selected based on preliminary hardness screening to provide a rigid network backbone, while BA was tuned to maximize adhesion without sacrificing cure completeness. Differential scanning calorimetry (DSC) revealed that BA contents up to 20 wt % showed no residual curing exotherm after 5 h at 60 °C, whereas 30 wt % BA produced an uncured exotherm, indicating incomplete conversion. On plasma-activated titanium substrates, the optimized formulation increased ultimate lap-shear load from 3.8 kgf to 8.5 kgf. In an aggressive artificial sweat solution at 60 °C for 48 h, the optimized network exhibited no visible cracks or voids, in contrast to a BPF-only control that degraded under identical conditions. Independent ISO 10993-guided biological evaluation of the optimized formulation further showed noncytotoxicity in the extract test (reactivity grade 1; RCC 94.69%), no skin sensitization in the guinea pig maximization test for either saline or cottonseed-oil extracts (0/10 responders for both), and negligible primary skin irritation with P.I.I. = 0.0 for both extract media. These results demonstrate a practical and scalable route to low-temperature curable epoxy molding materials that integrate high hardness, strong adhesion, sweat durability, and a favorable preliminary biological profile for skin-contact applications.