<title>Optical fiber evanescent-wave cure monitoring of epoxy resins</title>

Abstract

OPTICAL FIBRE EVANESCENT WAVE CURE MONITORING OF EPOXY RESINSG. R. Powell, P. A. Crosby, G. F. Fernando#, C. M. France, R. C. Spooncer', D. N. Waters*.Brunel University, Department of Materials Technology, Kingston Lane, Uxbridge, Middlesex, UB8 3PH, UK.*Department of Manufacturing and Engineering Systems, Department of Chemistry.#To whom correspondence should be addressed.ABSTRACTTwo types optical fibre sensors (OFS) were investigated for use in monitoring the cure of an epoxy-amine resin system: (i)an evanescent wave sensor and (ii) a refractive index sensor. The evanescent wave sensor was used to detect changes inconcentration of the active chemical species involved in the cure reaction via evanescent wave near-infrared spectroscopy.By using the optical fibre as an attenuated total reflection waveguide, spectra were collected over the range 1490-1570 nmat regular time intervals during the cure. This technique enabled the depletion of amine to be monitored. Results obtainedvia this method were fitted to kinetic models which allowed prediction of the reaction rate at different cure temperatures andconversions. The optical fibre evanescent wave sensor results were compared with data obtained using an established curemonitoring technique (FT-IR spectroscopy).A theoretical model of the evanescent sensor has been used which describes the relationship between evanescent absorptionas a function of absorber concentration and refractive index. Predictions of sensor response were undertaken using absorptiondata from FT-JR spectroscopy and refractive index results as a function of cure time. The predicted sensor response was thencompared with experimentally obtained sensor data.An optical fibre sensor which monitored the cure process via refractive index change was also investigated. Sensors wereset up to allow simultaneous collection of data during cure from the OFS, together with data from transmission near-infraredspectroscopy and Abbe refractometry. In this way the response of the sensor to changes in the cure state of the resin,refractive index and temperature were compared.Keywords: optical fiber sensors, cure monitoring, epoxy resins, evanescent wave, near-infrared spectroscopy, refractometry.1. INTRODUCTIONEpoxy resin and amine hardener systems are used extensively in advanced fibre reinforced composites (AFRC). Themechanical properties of AFRC are generally dominated by the properties of the reinforcing fibres, however, the matrix-dominated properties can be affected by the crosslink density of the resin system used. The crosslink density is in turninfluenced by the chemical state of the resin before cure (including the moisture content) and the processing conditions usedto cure the resin'. It is therefore attractive to have a cure sensor which can determine the state of cure of a compositecomponent at locations remote from the composite surface and which will not affect the integrity of the finished componentafter processing.To date, many in-situ techniques have been employed to monitor the state of cure of a composite or resin system. Theseinclude electrical resistance and dielectric analysis2, ultrasonic wave propagation3 ,