Fresnel Reflection Sensor
The responses of the six individual Fresnel reflection sensors in the glass vials and the corresponding temperature data from the thermocouples during cross-linking in the oven are presented in Figures 4(a,b).
Figure 4. (a) Response of the six independent Fresnel reflection sensors and the corresponding temperature data from the thermocouples during cross-linking and cooling of the resin system in the oven. (b) Expanded view of Figure 4(a). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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With reference to Figures 4(a), the cross-linking process was monitored through channels 2–7 of the optical channel selector. Channel 1 was used to monitor a Fresnel sensor that was immersed in index-matching gel but located outside the oven under ambient conditions. Channel 8 was used to monitor the Fresnel signal emanating from a cleaved optical fiber that was immersed in neat epoxy resin (no hardener), and located in the oven along with the other vials containing the mixed epoxy/amine resin. The response of each of the Fresnel sensors was normalized to its respective signal when the temperature of the oven was between 23°C and 24°C.
The outputs from each of the Fresnel sensors and the corresponding temperature of the resin system in the vials were recorded simultaneously. The sequential switching of the array of Fresnel sensors via the optical switching unit was carried out automatically at an interval of 16 seconds.
The following section presents a discussion on the general trends observed in Figures 4(a,b).
- Magnitude of the exotherm: The magnitude of the exotherm recorded by the thermocouples is around 85–95°C; the set (desired) dwell temperature was 70°C. There are three possible reasons for this. Firstly, as the experiments were carried out in an air-circulating oven, it was not possible to compensate for the exotherm by cooling the container to maintain isothermal conditions. Secondly, the glass vials were placed on a PTFE block with holes drilled to accommodate the samples. Hence, the heat dissipation, originating from the exothermic cross-linking reactions, was not ideal. Finally, the magnitude of the exotherm is proportional to the volume of the resin used. In the current series of experiments, approximately 2 mL of the mixed resin system was dispensed manually via a syringe. However, the actual volume of the resin dispensed per vial may have varied slightly. The magnitude and the duration of the exotherm are important because it can influence the rate and the extent of the cross-linking reactions, and hence, the resultant properties of the thermosetting resin .
- Magnitude of the Fresnel signal: The general trends in the outputs from the Fresnel sensors deployed in different vials containing resin system are similar. However, the magnitude of the reflected Fresnel signal is slightly different and this may be because of one or more of the following reasons: (a) approximately 5 m of fiber was used per sensor (distance between the glass vial containing the resin in the oven and the interrogation unit). Bend-induced losses along the fiber path and losses at the fusion splices may account in part to the observed small differences in the magnitude of the Fresnel signals; (b) the optical losses across the channels in the switching unit were quantified and the variation was found to be in the range of 2–10%; (c) the quality of the cleaved optical fiber is unlikely to be a contributing factor because each sensor was inspected prior to use. However, the possibility of localized variations in the homogeneity of the resin system, in the vicinity of the core of the cleaved optical fiber (9 µm), cannot be ruled out. The channel selector used in this work has a switching reproducibility of 0.02 dB or less at a constant temperature (manufacturer's data) within the operable temperature range of 0–50°C. The variation in the temperature in the laboratory, over the duration of the cross-linking experiments (12 hours), was 20–24°C. The variation in the output signal from channel 1 (Fresnel sensor immersed in the index matching gel but located outside the oven) over the duration of the experiment was less than 1%.
- Correlation between the Fresnel reflection signal, temperature, and cross-linking: Close inspection of the data [see Figure 4(b)] during the first 20 minutes after the onset of heating shows that the Fresnel signal dropped by ∼12–20%; this is because the density of the resin system decreases as it is heated because of thermal expansion.[32, 33] In the absence of chemical reactions, degradation, or volatility of the constituent components in the resin, it can be assumed that the refractive index will decrease with increasing temperature. The refractive index is defined as the ratio of the velocity of light in a vacuum to that in the medium (resin system). With reference to Figure 4(b), the normalized refractive index decreases initially with heating because of the decrease in the density and the concomitant increase in the volume. As the temperature is increased further, at some point the cross-linking reactions proceed at an appreciable rate. In other words, as far as the refractive index of the resin system is concerned, two competing effects have to be considered: firstly, the density decreases in the initial heating period and then secondly, as evident in Figure 4(b), it increases as cross-linking reactions predominate.[34-36] However, as mentioned previously, the cross-linking reactions are exothermic and if isothermal conditions are not maintained, this temperature excursion will accelerate the rate of consumption of the reactive functional groups. This is generally referred to as the auto-acceleration of cross-linking reactions . Therefore, it is not straightforward to decouple these various factors with regard to their respective contribution to the magnitude of the Fresnel reflection signal during the initial stages of the cross-linking reactions. This issue is discussed further when considering Figure 5.
- The dip in the temperature and the effect on Fresnel signals after ∼40 minutes: As the maximum temperature that was reached during the exotherm was approximately 95°C (25°C above the set temperature), the oven door was opened briefly to verify visually that the resin had not degraded. This accounts for the decrease in the recorded temperature to below the set value of 70°C; when the oven door was closed, the temperate equilibrated back to 70°C. A corresponding increase in the Fresnel reflection signal was observed when the temperature was lowered followed by an increase to an equilibrium value. After approximately 420 minutes, the Fresnel signals in the epoxy/amine vials had increased by ∼17–30% when compared to their respective values prior to heating. After this period, the oven was switched off and allowed to cool; after 440 minutes (∼60°C), the oven door was opened to accelerate the cooling of samples to ambient temperature.
Figure 5. Comparison of the relative outputs from the Fresnel reflection sensors and thermocouples for the neat epoxy resin (channel 8) and the cross-linking reactions (channel 3) involving the epoxy/amine resin system. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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In order to analyze the output from the sensors in detail, only the data for the Fresnel reflection signal as a function of temperature for the epoxy/amine resin system (channel 3) and the neat epoxy without the amine hardener (channel 8) are presented in Figure 5. Horizontal lines (A, B and C) have been superimposed on Figure 5 to indicate the magnitudes of the temperature and the normalized Fresnel reflection data; the vertical lines (D–G) have been superimposed to indicate the specified time during processing.
It is apparent from Figure 5 that the temperature profile for the neat epoxy resin as a function of time shows an inverse relationship with the Fresnel signal. In other words, as the temperature is increased, the Fresnel signal decreases proportionally and vice versa; thus, the time taken for an equilibrium value to be reached for the temperature and the Fresnel signal is similar. The effect of opening the oven door is once again apparent in the Fresnel reflected signal where an increase is observed as the temperature is lowered.
With reference to Figure 5, the temperatures recorded by the two thermocouples (line A) for the neat epoxy and the epoxy/amine resin system show that an equilibrium value of ∼70°C was attained after approximately 60 minutes (line F). The Fresnel signal for the neat epoxy is also seen to equilibrate at 70°C after 60 minutes. However, this was not the case for the Fresnel signal for the epoxy/amine resin system where an equilibrium value was attained after approximately 140 minutes (line G). The observed differences may be attributed to the following: (a) The magnitude of the exotherm where the peak temperature recorded by the thermocouple for the amine/epoxy resin system was 94.2°C; here the rapid increase in the temperature would have accelerated the formation of the cross-link networks. As the temperature was relatively constant after approximately 60 minutes, the gradual increase in the Fresnel signal can be attributed to the gradual increase in the cross-link density, in the diffusion-controlled phase of the cross-linking reactions, until cessation after approximately 140 minutes. (b) The temperature of the oven was lowered to approximately 65°C when the oven door was opened to inspect the samples as mentioned previously.
With reference to time-line (D) for the epoxy/amine resin, it is seen that the time for the peak temperature recorded by the thermocouple does not coincide with the minimum value for the Fresnel reflection signal. As, the synchronization between the thermocouples and the Fresnel data are reliable, the following explanation is proposed: In the case of the epoxy/amine resin system, as it is heated, the refractive index decreases. However, above a certain temperature range, the cross-linking reactions commence, and the reaction rate increases as a function of temperature. Thus, initially, the heating phase dominates the decrease in the refractive index and then the cross-linking reactions dominate. In situations where a large exotherm is observed, unlike the case where isothermal conditions prevail, it will be difficult to decouple the contribution of temperature and cross-link density to the refractive index.
On inspecting the thermocouple data for the neat epoxy and the epoxy/amine resin system, it can be observed that the two samples experience the same heating rate until about 55°C; as the temperature of the oven was preset to attain 70°C, the temperature-controller reduces the rate of power input to the heaters to prevent over-shooting the set temperature. This reduction in the rate of heating within the oven is also seen in the Fresnel reflection data from the neat epoxy resin. As mentioned previously, the effect of opening the oven door is also evident in the Fresnel data for the neat resin. After the oven door was closed, the temperature within the neat epoxy sample rose steadily (period between lines E and F) until the set temperature was reached.
Postcure Thermal Cycling
The cured resins and the neat epoxy (no amine hardener) samples were subjected to three sequential heating and cooling regimes, and the responses of the Fresnel sensors and the thermocouples were recorded. During the first two cycles, the oven was heated from ambient to 70°C and cooled back to room temperature with the oven door closed. After the third heating/cooling cycle, the samples were heated to 78°C and held for 1–2 minutes and then ramped to approximately 84°C to dwell for ∼30 minutes. Subsequently, it was ramped to 94°C after which the power to the oven was switched off and allowed to cool naturally with the oven door closed.
The output data for the Fresnel sensors for the epoxy/amine resin system and the corresponding thermocouples are presented in Figure 6.
Figure 6. Postcure thermal cycling and the responses of the Fresnel sensors for the cured epoxy/amine resin system and the corresponding data from the thermocouples. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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In Figure 6, consideration is given initially to the thermocouple data where linear regression lines have been superimposed on the linear portions of the heating (A1–A4) and cooling (B1–B3) regimes; the criterion for performing the linear regression was to obtain the rates of heating and cooling. The coefficient of determination (R2) was 0.99. A summary of the slopes for the temperature channel 3 is presented in Table 2 where it can be seen that slopes A1 and A2 are similar. However, the rate for A3 is relatively higher and is probably because of the higher set temperature. The slope for A4 is lower because the set temperature of 94°C is relatively close to the preceding dwell temperature of 84°C. The same conclusion can be reached for the cooling rates (slopes B1–B3) from Table 2.
Table 2. Heating and Cooling Rates from Figure 6 for the Cured Resin (CH 3) During the Post-Cure Heating/Cooling Regimes
| ||Slopes for specified regression lines|
As stated previously, the temperature controller adjusts the heating rate at approximately 25°C below the set isothermal temperature. This is evident on inspecting the slopes and the data from the thermocouples for the first and second temperature cycles. The heating regime for the third cycle, which accounts for the short dwell of 1–2 minutes at 78°C followed by a 30 minute dwell at 84°C, shows the modulations imposed by the temperature controller. The third heating regime was used to investigate if: (i) any additional cross-linking had taken place; and (ii) the glass transition temperature (Tg) could be detected by the Fresnel sensors.
On inspecting the Fresnel data in Figure 6, the overall trends for channels 2–8 look similar but some anomalies are apparent. For example, (i) Figure 7(a) (expanded view of Figure 6) shows an uncharacteristic fluctuation in the data are observed for channel 6 within the region “A”. As the resin in the vial was already cross-linked, the anomaly may have been caused by some unintentional perturbation of the optical fiber connectors that were located outside the oven. Moreover, this feature in channel 6 is not observed in the two subsequent heating and cooling cycles. However, three further possible contributing reasons for the anomaly need to be considered. Firstly, considering the situation prior to cross-linking but during the first heating regime (to cross-link the resin), the coefficient of thermal expansion of the liquid epoxy/amine resin is higher than that of the optical fiber. This difference in the thermal expansion is “locked” when the resin cross-links to an infusible solid. When the assembly is cooled to room temperature, residual stresses will develop. Hence, during the cooling cycle, debonding between the optical fiber and the matrix can occur. This may not be a repeatable process because it will also depend on the magnitude of interfacial bond strength. Secondly, it is also worth noting that the resin shrinks as it cross-links and this too will contribute to the magnitude of the residual stresses. Finally, the Tg needs to be considered when the postcured resin is subjected to further heating. If the heating regime transgresses the Tg of the cross-linked polymer, properties such as the heat capacity, specific volume, and stiffness undergo a reversible change.[34, 38, 39] The unfortunate complication in the current experiment is the fact that the Tg is in the same temperature range over which the temperature controller on the oven adjusts the power input as the preset isothermal temperature is approached. (ii) Channels 3, 5, and 7 show a distinct inflection region over the period marked “A”.
Figure 7. (a) An expanded view of Figure 6 showing the first postcured heating and cooling cycle. (b) An expanded view of the second heating and cooling cycle for the postcured resins where the temperature and Fresnel reflection data are presented. (c) An expanded view of Figure 6 where the third heating and cooling cycle is shown for the Fresnel sensor and thermocouple data. (d) A magnified view of Figure 6 illustrating the inflection points in the Fresnel sensor data during the third heating cycle. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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Figure 7(b) shows an expanded view of the second postcure heating cycle. The anomaly observed previously for channel 6 is not apparent but the inflection point is present for channel 5 within zone-A.
An expanded view of the third heating and cooling cycle is presented in Figure 7(c) where three time zones of interest (A, B, and C) have been highlighted. Zones A and B show the inflection points mentioned previously. The third heating cycle involved ramping the samples from room temperature to 78°C with a short dwell of 1–2 minutes and then a temperature ramp to 84°C with a dwell for ∼30 minutes; this is clearly observable in the Fresnel and thermocouple data in zone B. An expanded section of zone-A is presented in Figure 7(d) where the inflection points are clearly visible.
With regard to the cooling curves for channels 2–7 [zone C of Figure 7(c)], the trends for the Fresnel sensors seem to reflect the temperature profile inside the oven. In other words, the inflection regions are not detected for any of the samples.
The final part of this discussion is concerned with the relative outputs from the neat epoxy sample (channel 8) and one of the cured samples that was subjected to three heating/cooling cycles (channel 3). The Fresnel sensor and thermocouple data from channels 3 and 8 is presented in Figure 8. The trends in the thermocouple data were discussed previously; therefore the current discussion is focused on the Fresnel sensor outputs for these two datasets.
Figure 8. Responses of Fresnel sensors for the cured epoxy/amine resin (channel 3) and the neat epoxy resin (channel 8). The data from the corresponding thermocouples from each vial are also presented. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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With reference to the neat epoxy Fresnel sensor data presented in Figure 8, linear regression lines (A1–A3) were calculated as described previously for the three sequential heating cycles. A summary of the slopes for these regression lines is presented in Table 3; it is apparent that the slopes are similar and the deviation from linearity starts at about ∼50°C for the first two cycles and approximately 55°C for the third cycle. The characteristics of the Fresnel signals (B1–B3) during the cooling phase of the neat resin also show a similar dependence on temperature. In this instance, deviation from linearity is observed at approximately 45°C for the three cooling cycles; this corresponds to the time when the oven was switched off and permitted to cool.
Table 3. Slopes of the Fresnel Signals from Channels 3 and 8 During the Post-Cure Thermal Cycles as Shown in Figure 8
|Slopes for specified regression lines|
|Fresnel signal (Neat resin – CH8)||A1||A2||A3||B1||B2||B3|
|Fresnel signal (Cured resin – CH3)||C1||C2||C3||D1||D2||D3|
The corresponding Fresnel sensor output for the cured epoxy/amine sample during heating (C1–C3) show a similar dependency on temperature. However, in the case of the third heating cycle (where the sample was heated from ambient to 80°C), a distinct change in the slope is observed. This is concurrent with the trends observed for the neat epoxy sample. The trends during the cooling phase are represented by lines (D1-D3). A linear behavior is observed from 93°C to 65°C; the change in slope for the cured resin after 65°C is mirrored in that observed for the temperature profile and the neat epoxy resin.
A further analysis of Figure 8 is presented in Figures 9(a and b) representing the time derivatives of normalized Fresnel reflection (NFR) and the corresponding temperature for channels 8 (neat resin) and 3 (epoxy/amine cure) respectively. It is readily apparent from Figure 9(a), that the plots of the derivatives of NFR and temperature show mirror-image-like trends for the neat epoxy. In Figure 9(a), the perturbation observed in the Fresnel signal may have been because of mechanically induced perturbations caused by opening/closing of the door of the oven. Figure 9(b) shows the time-derivative data for channel 3 (epoxy/amine resin system). Peak-splitting is seen in the time-derivative of the NFR data as the cross-linked resin is heated to the set temperature. As this feature was not observed in for the neat epoxy resin, it is concluded that the peak-splitting may be a manifestation of Tg the resin system. The Tg obtained via a differential scanning calorimeter is generally defined as the mid-point of the inflection in the thermogram. With the current resin system, the Tg was measured to be in the region of 75°C. It is worth noting that the glass transition is a second-order transition (Tg) and its value will depend on the technique (dynamic mechanical thermal analysis, differential thermal analysis, torsional braid, etc) that is used to obtain it.[32-39]
Figure 9. (a) Time derivatives from Figure 8 for the NFR and temperature data for channel 8 (neat resin). (b) Time derivatives from Figure 8 for the NFR and temperature data for channel 3 (cross-linked resin).
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