Tailoring Epoxy Network Properties with Gradient Siloxane Hardener Substitution: From Aliphatic Amine Systems to Hybrid Architectures

Abstract

This study explores the synthesis and application of a novel multiamino-functional siloxane resin (SR-T-NH2) as a sustainable and high-performance curing agent for epoxy-siloxane hybrid materials. The siloxane hardener was synthesized via controlled hydrolysis and condensation of 3-aminopropyltriethoxysilane, with 1H, 13C, and 29Si NMR spectroscopy confirming a high degree of ethoxy group conversion (∼88%) and a structure dominated by fully condensed T3-type units. The SR-T-NH2 resin was subsequently used to gradually replace a commercial amine hardener (Z-1) in epoxy compositions. The use of this oligomeric siloxane architecture allows for a significant reduction in volatile organic compounds compared to traditional amine-cured systems, aligning with modern green chemistry requirements for high-solids coatings. FT-IR analysis of the curing process revealed near-quantitative conversion of epoxy groups, confirming the high reactivity of the synthesized oligomeric system. Non-isothermal DSC curing kinetics analysis revealed a well-defined exothermic cross-linking peak, showing that despite the steric hindrance of the oligomeric structure, the system maintains a highly competitive compared to standard amine-cured systems. EDS mapping confirmed the uniform distribution of the hardener throughout the mass without the formation of heterogeneous domains. The incorporation of the siloxane-based hardener significantly altered the optical and surface properties of the composites, inducing a pronounced matting effect (gloss reduction from 63.9 to 12.7 GU) and a shift in chromaticity from yellowish to cooler, bluish tones (b* decrease from 1.44 to −1.69). Despite a reduction in macro-scale surface roughness, a dramatic increase in the water contact angle (from 8.0° to 84.4°) was observed, indicating a shift toward a hydrophobic surface state. While DSC measurements showed a moderate decrease in the glass transition temperature (Tg) due to the flexible Si–O–Si linkages, the modified materials exhibited enhanced surface hardness (up to 81.8 Shore D) and improved thermal stability. Pyrolysis combustion flow calorimetry demonstrated a significant reduction in both the peak heat release rate and total heat release for the siloxane-modified networks. Furthermore, the limiting oxygen index increased systematically with higher siloxane content, transitioning the material toward low-burning behavior. Vertical burning tests showed that the flammability of epoxy materials cured by SR-T-NH2 was not significantly reduced but confirmed the formation of an ablative, silicon-rich char layer throughout the material during the combustion process, which acts as an effective mass and heat transfer barrier. These results demonstrate that the synthesized siloxane resin is an effective multifunctional hardener capable of tailoring the reaction kinetics, aesthetic, thermal, and fire-retardant properties of epoxy systems for advanced coating applications.