Influence of temperature and BN nanoparticles on UV, thermal and dark curing of a cycloaliphatic epoxy resin
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Influence of temperature and BN nanoparticles on UV, thermal and dark curing of a cycloaliphatic epoxy resin C. Arribas1 · A. González‑González1 · M. G. Prolongo1 · S. G. Prolongo2 Received: 29 July 2019 / Accepted: 11 February 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract The influence of boron nitride (BN) nanoparticles on cationic photopolymerization of an epoxy resin at different temperatures was investigated employing photo-DSC. A difunctional cycloaliphatic epoxy resin was polymerized in the presence of triarylsulfonium hexafluoroantimonate salts (cationic photoinitiator). BN-epoxy dispersions containing 5 mass% of BN were obtained by sonication. The epoxy conversion during UV irradiation was determined at seven temperatures, from 30 to 90 °C. The conversion reached was strongly dependent on temperature, and lower conversions were obtained in the presence of BN. After the UV curing, the samples were thermally postcured by dynamic heating in the DSC. The thermal postcuring shows two exothermal peaks, being the main one dependent on the temperature of the previous UV curing and on the composition. The total achieved conversion (65–77%) is lower for the samples containing BN compared to neat epoxy. The UV-irradiated samples at 40 °C reached conversions between 24 and 36%; afterward, they were kept under isothermal dark curing (40 °C) up to 1 month leading to significant conversion increases, independently of the composition and reaching a final conversion between 60 and 70%. DMTA measurements of dark-cured specimens evidence a heterogeneous epoxy matrix, thus needing higher postcuring temperatures to get homogeneity. The Tg of the epoxy matrix is reduced in the presence of BN nanoparticles. Keywords Cycloaliphatic epoxy · BN nanoparticles · UV curing · Thermal postcuring · Dark curing
Introduction Epoxy resins are widely used as adhesives, surface coatings and matrices of reinforced composites for technological applications. Among epoxy resins, the cycloaliphatic ones have longer pot life, good thermal stability, excellent UV stability and excellent electrical properties due to their fully saturated molecular structure. These characteristics are decisive to fabricate components with good performance at high temperature [1, 2]. Due to their aliphatic nature, they are less reactive toward nucleophilic cross-linkers (such as primary amines) than aromatic epoxies, being cationic polymerization particularly suitable for these resins. The cationic polymerization is achieved through catalytic * M. G. Prolongo [email protected] 1
Department of Aerospace Materials and Production, Universidad Politécnica de Madrid, Madrid, Spain
Material Science and Engineering Area, Universidad Rey Juan Carlos, Madrid, Spain
2
curing agents that initiate polymerization by thermal- or photoactivation [3]. Lately, ultraviolet polymerizable epoxies have gained importance due to their lower curing times compared to thermally cured epoxies even at low temperatures, thus making them appropriate for coatings. The formu
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