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The Effect of UV Exposure on the Service-life of Thermochromic Microcapsules Integrated into the Epoxy Matrix

The effect of ultraviolet (UV) exposure on the service-life of thermochromic microcapsules integrated into the epoxy matrix was investigated. The microcapsules of the formaldehyde shell contain the core of thermochromic leuco dye. Seven sets of epoxy resin samples filled with concentrations from 0 to 10 wt.% of microcapsules were investigated. The composite samples were exposed to UV for approximately 1000 h. For the quantitative evaluation of colour change under UV, a fast and simple original procedure based on samples’ image treatment was developed. With the exposure time intervals of 50 h, samples were taken out from the UV light chamber to evaluate the exposure effect on their reversible thermochromic ability and mechanical properties. Periodical evaluation of the UV light effect on mechanical properties during the exposure was performed by microhardness tests. Tensile tests of the samples till the fracture were performed every 200 h. The critical time under the exposure of the UV lamp that destroys the reversible thermochromic reaction of the microcapsules was defined as 200 h. At the same time, it has been found that the mechanical properties of the epoxy resin under the same UV source were not strongly affected after 1000 h of irradiance and changed in the frame of ~ 10 %.