In situ study of the epoxy cure process using a fibre-optic sensor

Real-time, in situ monitoring for quality control of the polymer cure process is of great interest. In fact, high-performance composites, made with polymer matrixes, are used extensively in high-tech areas, such as the aircraft, space and automobile industries. In particular, epoxy resin reinforced with fibre is a system with good mechanical properties and low density. In this paper, a fibre-optic sensor to monitor the cure of an epoxy resin is studied. Optical fibres are, in fact, compatible with the reinforcing fibre in laminate composites. This sensor is based on the measurement of the angular distribution of light transmitted through an optical fibre inside the cured polymer. The light guiding properties of this sensor are linked to the difference between the refractive index of the core and that of the cladding. So, by partially removing the cladding and placing a sample of curing epoxy around the stripped region, it is possible to monitor the extent of the cure. In fact, the refractive index of the DGEBA/amine system varies from 1.54 to 1.58, mainly due to the increase in density during the cure. For manipulation, an experimental fibre made of special glass was prepared so as to have a high refractive index core (n = 1.62). The sensitivity to the refractive index was tested with aniline (n = 1.58) mixed with various concentrations of toluene (n = 1.50). The response of the sensor to this liquid test showed that it would be possible to measure the refractive index in the index range of epoxy during cure. Tests with an epoxy system were also performed. In order to understand the angular distribution of the transmitted light, a model based on geometrical optics was developed using the reflection coefficient of the optical ray on a boundary between two dielectrics. This allowed us to obtain real and imaginary parts of the cladding refractive index, thus the cross-link density during the epoxy cure can be monitored with a single fibre-optic sensor.