Microwave curing carbon fiber composites automobile rearview mirror

2.4.1 Conventional oven curing

The sample was put into the oven and heated from room temperature (25 °C) to 120 °C at a rate of 3 °C/min and kept for 2 h to obtain the sample. At the end of the holding time, the instrument was turned off and the sample was allowed to cool down naturally to room temperature within the furnace.

2.4.2 Curing by microwave heating

The prepared sample and mold were placed inside the microwave equipment, the optical fiber was affixed at the position shown outside the vacuum bag to observe the temperature change, and thermal insulation cotton was laid on. The following Fig. 4 shows the position of the optical fiber. All six magnetrons were then turned on simultaneously, and the output power of each magnetron was set to 100 W. The reciprocating linear movement of the mold started after the microwave output was turned on for 2 s, the mold displaced at a rate of 0.01 m/s for 200 mm. After 23 min of microwave irradiation, the output power of six magnetrons was increased to 150 W and the samples were irradiated for another 19 min. The samples were then kept warm for 2 h without microwave irradiation. The heating process is shown in Fig. 5. At the end of the holding time, the instrument was turned off and the sample cooled down to room temperature naturally. In the heating process, as shown in the Fig. 5, the temperature will not exceed 120 °C because when the temperature inside the microwave instrument reaches the specified limit, the magnetron will stop radiating, and without a heat source, the temperature will not continue to rise.

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Fig. 4

The arrangement of the optical fiber temperature measuring probe when the rearview mirror of the car is cured.

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Fig. 5

Temperature time power curve.

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2.4.3 Microwave heating process

The same sample was used to repeat the above-mentioned heating process without holding for 2 h to obtain incompletely cured samples. At the end of the heating process, the magnetron was turned off and the sample naturally cooled down to room temperature within the microwave oven.

3.2.1 Differential scanning calorimeter analysis

Differential scanning calorimetry (DSC) is used to monitor the thermal behavior of the sample cured through different methods during the heating process, and the heat flow result is shown in Fig. 8. The step-like curve at a temperature above 100 °C implies the occurrence of the glass transition. The obtained glass transition temperature ( ${\mathrm{T}}_{\mathrm{g}}$ ) and specific heat change after glass transition are listed in Table 1.

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Fig. 8

Heat flow curves for samples after different curing processes.

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${\mathrm{T}}_{\mathrm{g}}$ of the sample through microwave curing is about 8 °C higher than that of conventional curing, and ${\mathrm{T}}_{\mathrm{g}}$ for an incompletely cured sample through microwave curing is moderate. The gradual enhancement of the ${\mathrm{T}}_{\mathrm{g}}$ indicates an increase in the curing degree (Zhou et al., 2019). Besides, the specific heat change of the sample after microwave curing is 17.2% greater than that of conventional curing. The larger specific heat change reflects an enlargement of absorbed thermal energy for a specific mass of the sample to rise one degree in temperature (Baghad and El Mabrouk, 2022). Since the resin releases heat during the curing process, the more complete the curing, the more heat is released. Consequently, in the reverse process for glass transition of the sample with a higher curing degree, the freezing polymer chains at a glassy state need to absorb more heat to overcome the higher energy barrier after curing and become movable (Strobl, 2007). Therefore, it absorbs much more heat. At the same time, we found that the glass transition process for the sample under incomplete curing is not as obvious as it is for the other two samples in the heat flow curve. This could be explained by the inhomogeneous internal structure of the material; only part of the chemical structure formed cross-links during curing, while others remained uncured. Therefore, the flow curve reflects both the cured and uncured transition processes of the whole material, and the distinguishable step-like heat flow curve implies a homogeneous structure of samples through completed curing. It is worth noting that a lot of experiments have been done with the microwave curing method mentioned in this paper and that several DSC tests have been conducted. The test results in this section are typical.

3.2.2 Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) can measure the relationship between the mechanical properties of viscoelastic materials and time, temperature, or frequency. It is widely used in the study of the viscoelastic properties of material and can obtain the dynamic storage modulus, loss modulus, and loss tangent of materials. The energy storage modulus (E') reflects the elastic characteristics of the sample, the degree to which the sample can recover from deformation, and the capacity of the sample to store energy under the influence of stress. The loss modulus (E“) reflects the viscous characteristics of the sample, the degree of heat consumption (loss) of the sample during the deformation process, and the energy dissipation capacity of the material. The loss angle ( $\tan \mathrm{\delta }$ ) is the ratio of the loss modulus to the energy storage modulus, DMA results of three samples treated with incomplete curing, microwave curing, and conventional curing are shown in Fig. 9. The glass transition temperature in DMA is determined by the position of $\tan \mathrm{\delta }$ maximum, and the occurrence of glass transition leads to a one-magnitude drop in the storage modulus (E'). For the incompletely cured sample and conventionally cured sample, the peak of maximum loss appears at about 132.8 °C, while in the microwave-cured sample, a sub-peak appears at 155 °C after the main peak at 127.25 °C, as Fig. 9(b) shows. We deduce that the main peak derives from the transition process, which is the same as the glass transition occurring in the other two samples, while the sub-peak indicates that microwave curing forms a small range of stacked chains. With microwave curing, the flexibility of a part of the molecules becomes worse, and a higher temperature is needed to overcome the energy barrier, leading to a sub-peak occurrence. It can be seen from the data that the glass transition temperature from the conventional curing method is almost the same as that of incomplete curing, and microwave curing reduces the temperature of the main transition process while causing a sub-transition temperature, and the loss tangent of the sub-transition is lower than that of the main transition. The combination of two transition peaks implies that the overall transition process is not completed until the temperature reaches 155 ℃. It can be concluded that microwave curing results in a higher curing degree.

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Fig. 9

Dynamic thermal mechanical analyzing results of the sample under (a) incomplete curing, (b) microwave curing, and (c) conventional curing. (d) Comparison of $T_g$ and storage modulus at glassy state after different curing methods.

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A comparison of ${\mathrm{T}}_{\mathrm{g}}$ from both the DSC and DMA tests as well as the storage modulus at glassy state for three samples is shown in Fig. 9(d). Here we found that the storage modulus of the sample with conventional curing is much lower than that of microwave curing, by about one-magnitude. The storage modulus of the completed microwave-cured sample and the incomplete microwave-cured sample are at the same degree. This modulus difference indicates the material cured through microwaves has better mechanical strength, which is very important for application (Li et al., 2017). The ${\mathrm{T}}_{\mathrm{g}}$ obtained from the DSC and DMA tests demonstrates a similar result: that the sample completely cured by microwave has the highest glass transition temperature and indicates the highest degree of curing. To note, the difference in ${\mathrm{T}}_{\mathrm{g}}$ obtained through DSC and DMA is due to the different measuring mechanisms. The ${\mathrm{T}}_{\mathrm{g}}$ determined from DSC is a result of thermal behavior change during the transition, while the ${\mathrm{T}}_{\mathrm{g}}$ determined from DMA is a direct result of the modulus change when the polymer chain is movable.

3.2.3 FTIR analysis

To understand the influence of different curing methods on chemical reactions, the Fourier transform infrared spectrum (FTIR) is used, and the results of the uncured, incompletely cured, microwave-cured, and conventionally cured specimens are shown in Fig. 10.

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Fig. 10

FITR spectrum for samples with (a) no curing, (b) incomplete curing, (c) microwave curing, and (d) conventional curing.

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The vibration of epoxy ring molecules causes a peak at a wavenumber of 908 cm⁻¹, and the intensity becomes weaker along the curing process, which implies the original chemical bonds in the epoxy ring are broken and new cross-links are formed in molecules; this is also the nature of curing (Xu et al., 2016). This chemical reaction also leads to the reduction of peak intensity at 1630 cm⁻¹, which presents the N–H in-plane bending vibration. The chemical group vibrations shown in the FTIR spectrum are listed in Table 2.

The same positions of peak appearance under microwave curing and conventional curing indicate the same products after the chemical reaction; therefore, the formed cross-link networks are also the same. However, the peak intensity shows a small difference and means a divergence in the curing degree. The relative curing ratio between the two curing methods is therefore calculated through the relative peak height between epoxy ring vibration and C–H vibration on the benzene ring, following the equation. $$\mathit{Relativecuringratio}=\frac{O-A_{\mathit{microwave}}}{O-A_{\mathit{conventional}}}$$

Where O, A_microwave and A_conventional are the relative peak heights for uncured material, microwave-cured material, and conventionally cured material, respectively. According to the relative peak height, the curing degree of the microwave-cured sample is 1.918 times that of the conventionally cured sample. Notably, due to the superposition of neighboring peaks, the approximate result calculated from peak height might deviate from the real ratio.

3.2.4 SEM analysis

According to the DMA analysis, the microwave-cured composite showed a higher storage modulus than the conventionally cured specimens, which is mainly influenced by the reinforced resin matrix. SEM micrographs of the composite composed of the resin matrix and local fiber, as displayed in Fig. 11, show the direct influence of different curing methods on materials.

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Fig. 11

SEM micrographs of samples experiencing incomplete curing, microwave curing, and conventional curing from left to right. (a)-(c) composites; (c)-(d) local carbon fiber.

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Compared to incomplete curing and conventional curing, it is obvious that the specimen under microwave curing shows a homogenous matrix that is uniform and compact, as Fig. 11(b) exhibits, while the resin matrix in Fig. 11(c), under conventional curing, is inhomogeneous. The matrix at the bottom of Fig. 11(c) is well-cured, while the matrix in other areas shows a loose state. The incomplete curing matrix in Fig. 11(a) is uniform, but to a lower degree than in Fig. 11(b). The different homogeneity of the results is due to the different heating mechanisms. Microwave curing generates heat through dielectric loss throughout the material (Mgbemena, et al., 2018). When an alternating electromagnetic field is applied, all dipoles tend to align along the direction of the electromagnetic field the violent movement and collision of molecules would generate heat; therefore, the total material can be cured uniformly (Chen, 2018). However, the conventional method conducts heat from an external source to the inside through radiation, leading to a large temperature gradient inside the material, resulting in uneven curing of resin along the thickness direction, and making it easy to produce large internal stresses (Mgbemena, et al., 2018). The unevenly cured resin consequently can not support the whole structure and results in a lower modulus. Furthermore, the loose structure of the conventionally cured samples contains more voids than the compact sample, which would lead to a reduction in the mechanical properties of composites, especially the shear stress and compressive modulus (Baghad et al., 2021). As for the carbon fiber, the local morphology can be observed in Fig. 11(c-d d-f). There is no distinguishable difference in the morphology from different curing processes, and the high curing degree of microwave curing does not lead to defects in the carbon fiber composites. At the same time, the process of microwave curing involves heating the resin in the carbon fiber prepreg to produce a crosslinking reaction. The reaction does not affect the carbon fiber in the prepreg because the carbon fiber does not participate in the reaction; hence, the scanning electron microscope reveals no change in the carbon fiber.