Room Temperature, Force-Activated Cross-Linked Coatings from Reactive Core–Shell Particles

Abstract

Conventional powder coatings eliminate volatile organic compound emissions, but most systems still require elevated temperature curing to achieve sufficient cross-linking. To address this gap, we developed shelf-stable, reactive core–shell particles (CSPs) that formed robust, on demand coatings at room temperature. Two particle types were designed with mutually reactive core chemistries and protective shells that isolate the cores during storage but fracture under applied pressure to enable film formation. Suspension polymerization was used to synthesize (i) amine CSPs, consisting of a phenalkamine core protected by a poly(methyl methacrylate) (PMMA) shell, and (ii) epoxy CSPs with a polyurea (PUR) shell. Chemical compositions of the CSPs were confirmed using proton nuclear magnetic resonance and thermogravimetric analysis. Dry mixtures of CSPs at three amine-to-epoxy ratios (1:1, 1:2, and 2:1) were prepared and tested for film formation. Films were formed via force-activated compression at room temperature and cured for 24 h at 20 °C or with a mild post-cure at 70 °C. Scanning electron microscopy was used to verify and visualize the formed films. Gel content analysis demonstrated network formation in all cases, with the 1:2 amine-to-epoxy formulation achieving the highest gel contents of 28.2 ± 1.4 wt % after 24 h at 20 °C and 44.6 ± 1.5 wt % at 70 °C, corresponding to 60.5 ± 3.0% and 95.6 ± 3.2% of its theoretical maximum, respectively. Uniaxial tensile testing demonstrated that coatings formed from any of the amine-to-epoxy ratios had an equivalent Young’s modulus of ∼37 MPa at 20 °C, which increased to ∼107 MPa when cured at 70 °C. Local atomic force microscopy mapping revealed that the 1:2 formulation had the highest Young’s modulus, reaching 8111 MPa. These results demonstrate a freestanding cross-linked powder coating formed at room temperature and provide design guidelines for next-generation ambient-curable industrial coatings.