Thermosetting resins are used extensively as adhesives, coatings, and matrices for fiber reinforced composites. In general, thermosets consist of two primary components namely; the resin and hardener. In a typical manufacturing operation, the resin and hardener are weighed in the required stoichiometric ratio and mixed thoroughly prior to use. Depending on the chemical reactivity of the hardener, the cross-linking reaction can be initiated at room-temperature or by the application of heat. The transformation of the mixed resin and hardener from a liquid (or semi-solid) state to a highly cross-linked structure is commonly referred to as “curing”.
Generalized cross-linking reaction schemes between an epoxy resin and an amine hardener are illustrated in Figures 1(a,b). Figure 1(a) illustrates the reaction between an epoxy and amine functional group that leads to the formation of a covalent bond between the two reagents, along with the formation of a hydroxyl group. In addition, it is seen that the primary amine is converted to a secondary amine. The secondary amine in turn can react with another epoxy group with the formation of a tertiary amine and another hydroxyl group; this is illustrated in Figure 1(b).
With reference to cross-linking of thermosets illustrated in Figures 1(a, b), the following additional comments can be made.
- The resin and the hardener are generally mixed manually prior to initiating the cross-linking reaction via the application of heat. The issue here is the homogeneity of the mixed resin system and the possibility of entrapped and dissolved gasses. Hence, it is a common practice to de-gas the mixed resin in a vacuum chamber prior to use. A facility to track the progression of the cross-linking reaction in real-time, against a previously determined profiles, will offer a means for appropriate remedial action to be taken in the event that a deviation is observed.
- The molecular weight of the resin system increases over the course of the reaction in proportion to the cross-link density. As a consequence, the density of the resin system and the refractive index increase during the cross-linking reactions. Therefore, there is significant interest in developing techniques to monitor the evolution of the molecular weight and the refractive index in real-time.
- The ring-opening reactions are exothermic. Therefore, the volume of the resin system used can have a major bearing on the temperature attained within the reaction vessel; in other words, the actual peak temperature experienced by the resin can be significantly higher than the set or desired isothermal condition. Hence, efficient thermal management and accurate monitoring of the temperature is important if data on the cross-linking kinetics are required.
- In a typical cure schedule, the resin system is heated from ambient to the desired isothermal value and it is held at this point for a specified time (dwell). The viscosity of the resin system initially reduces as the temperature is increased from ambient to the desired isothermal temperature. At some stage during the temperature-ramp, the resin system starts to cross-link and consequently, the viscosity and the refractive index increase rapidly. This increase in the viscosity reaches a point where the liquid resin system is converted to a gel; this is generally referred to as the gel-point. The other important stage in the processing of thermosets is vitrification; this is the conversion of the gel to a glassy solid. Tools to identify these transitions in the resin system during processing will enable process optimization routines to be specified.
- The formation of covalent bonds between the resin and hardener leads to shrinkage in typical high-performance thermosetting resins. The shrinkage stresses can be high enough to cause debonding from the reinforcement or the container/substrate. There is significant interest developing technique to quantify the magnitude of the shrinkage associated with thermosetting resin systems.
- During the heating phase of the mixed resin system and the reinforcement, the constitutive components expand. It is important to appreciate that the peak temperature experienced will be a function of the set isothermal value, the thermal management and the magnitude of the exotherm. At some stage in the cross-linking process, “bonding” between the surface of the fiber reinforcement and the matrix will occur and this may be a combination of chemically and/or mechanically induced processes. Cooling the material from the processing temperature to ambient will result in the generation of residual fabrication stresses. Residual stresses can initiate debonding and cracking in composites materials even without the application of mechanical load. Furthermore, they can also lead to warping and loss of dimensional stability.
- The glass transition temperature of the cross-linked resin system represents a second-order thermodynamic transition where the properties such as the heat capacity, thermal expansion, and the stiffness undergo reversible changes over a specified temperature range. Therefore, there is significant merit in developing techniques to monitor the occurrence of the glass transition temperature.
With reference to the above-mentioned issues, the ability to monitor progression of the cross-linking reaction at multiple points in the preform may be necessary in some instances. For example, in situations where the cross-section of the preform changes or when thick laminated preforms are being processed. A number of the above-mentioned topics and parameters can be accessed and/or monitored using optical fiber-based sensor systems.
For example, extrinsic and intrinsic fiber Fabry-Perot interferometric sensors have been used for in situ strain monitoring. The extrinsic fiber Fabry-Perot interferometric sensor has also been adapted for monitoring temperature.[4, 5] Intensity-based optical fiber sensors have been used for logging strain. Fiber Bragg gratings continue to be used extensively for monitoring strain and temperature during the processing of fiber reinforced composites.[7, 8] The feasibility of using combined strain and temperature monitoring via optical fibers has also been demonstrated. Long-period gratings have been used for cure monitoring. More recently, the feasibility of monitoring multiple parameters (strain, temperature, refractive index, and cross-linking) using a single sensor and interrogation system was demonstrated. Sensor designs for the quantitative monitoring of the relative concentrations of specified functional groups are predominantly based on transmission and reflection and evanescent wave near-infrared spectroscopy.[12, 13]
As this article is concerned with Fresnel sensors, the following section provides a brief review of this intensity-based sensor design.
Fresnel Reflection Sensors
Fresnel sensors are essentially based on the measurement of light reflected at the interface of two dielectric media owing to the discontinuity in the refractive index. A schematic illustration of the Fresnel reflection sensor is presented in Figure 2 where the cleaved fiber-end is shown to be surrounded in the resin system. The reflection at normal incidence is also indicated.
Light undergoing Fresnel reflection is governed by the refractive index contrast between the core of the optical fiber and the medium surrounding the cleaved-end. When light illuminates the interface of two dielectric media of refractive indices n1 and n2 at an angle of incidence of θ1, the coefficients of the reflected field amplitudes parallel (r||) and perpendicular (r⊥) to the plane of incidence can be expressed using the Fresnel equations:
where θ2 is the angle of refraction into the second medium. During normal incidence (θ1=θ2=0), the magnitudes of the field amplitudes are retained and the reflectivity “R” at the interface can be expressed as:
Optical fiber-based Fresnel reflection sensors have been used to track the changes in the refractive index of thermosetting resin systems during cross-linking.[15-18] For example, Crosby et al. reported on an optical fiber-based refractometer for monitoring the cross-linking of an epoxy/amine resin system. The reflection from the cleaved-end of the fiber was calibrated using reference refractive index liquids. The response of the Fresnel reflection sensor showed a good correlation with the quantitative cross-linking kinetic data obtained using a conventional Fourier transform near-infrared spectrometer. Liu et al. reported on a multipoint cure monitoring system that employed Fresnel reflection sensors in conjunction with optical time domain reflectometry. The degree of conversions calculated from the calibrated Fresnel sensors were found to be in good agreement with data obtained from a differential scanning calorimeter (DSC). Huang et al. demonstrated a parallel multipoint Fresnel-based temperature sensor. Chen et al. used the same sensor design concept to monitor the temperature and liquid concentration.[22, 23]
Cusano et al. demonstrated a Fresnel reflection sensor using a modulated light source and a lock-in amplifier. The degree of conversion obtained via the sensor was said to correlate with the DSC data.
It is generally accepted that intensity-based sensing techniques can suffer from fluctuations in the light source. This issue was addressed by Chang et al. and Kim and Su. They used a 2 × 2 fiber coupler with a splitting ratio of 50 : 50, where one of the output arms (one of the cleaved fiber-ends) served as the sensing element and the other was spliced to a fiber spool and this cleaved-end served as a reference element. A pulse generator was used to modulate the laser source. The sensing arm of the coupler was placed in the liquid of interest whilst the other longer section of the reference arm was left in air. The reflected light from the cleaved fiber-ends were detected by a de-coupled detector. As the light source was modulated and because the lengths of the sensing and reference arms of the coupler were significantly different, it was possible for the two signals to be separated temporally. The dual-coupler-based sensor interrogation design of Buggy et al. enabled monitoring of the refractive index during the cure reaction of epoxy resin via UV-light irradiation. The reflection from air served as the reference. Giordano et al. demonstrated an optical fiber sensing system for the simultaneous measurement of refractive index via Fresnel reflection, and strain via a fiber Bragg grating. More recently, Mahendran et al. demonstrated a multifunctional sensor system where a conventional fiber-coupled FTIR near-infrared spectrometer was used to monitor four independent parameters simultaneously: strain, temperature, refractive index, and cross-linking kinetics of an epoxy/amine resin system. Fiber-optic Fresnel sensors have also been used for cure monitoring of thermoset resins containing fillers and during resin infusion.
With reference to the routine manufacture of fiber reinforced composites in industry, in general, the same resin system is used over long periods for a given product range. This may be because of end-user specifications or to comply with certification requirements. In such circumstances, once the resin system has been characterized thoroughly in terms of its mechanical, thermal, optical, and chemical properties, on-line quality assessment can be provided by intensity-based sensor systems. Multipoint monitoring during processing can provide valuable information on the relative rates of reactions at specified locations within large preform or where there is a change in the cross-section. The advantage of intensity-based sensors is that they offer a low-cost option to infer the relative rates of reactions of thermosetting resin systems. As the sensors are embedded in the composites, they can be used in-service to monitor parameters that influence the refractive index of the cross-linked resin. For example, temperature and ingress and egress of fluids. The majority of the previous publications on the use of the Fresnel reflection sensor have been associated with primarily single sensors. With reference to the limited previous publications on multiplexed Fresnel reflection sensor designs, the device reported in the current publication is significantly simpler because it merely involves cleaved optical fibers. The rationale for the current work was to study the cross-linking behavior of a thermosetting resin system using a low-cost multiplexed Fresnel reflection sensor system. The evolution of the refractive index and the output from multiple Fresnel reflection sensors during the processing of an epoxy/amine resin system was monitored and analyzed in detail.