By UVA and UVB irradiating, the performance of poly-p-phenylene benzobisoxazole (PBO) fiber is changed with an increase in irradiation time, and the surface morphology, chemical structure, mechanical performance and thermal properties of the fiber are poorer. After 320-h irradiation, the properties of the PBO fiber irradiated by UVB are stronger than those irradiated by UVA. The strength of PBO fiber irradiated by UVB and UVA declined by 40% and 20%, respectively, than that of non-irradiated fiber. The roughness of fiber surface improved, and there was corrosion on the surface of PBO fiber irradiated by UVB. After the UV irradiation, the degree of macromolecule crystallinity and orientation changed, the macromolecule chain is broken, new groups (-NO2) are produced in the PBO fiber, and the temperature (660°C) of the thermolysis is decreased by 100°C from that of fibril.
Poly-p-phenylene benzobisoxazole (PBO) fiber is widely used in resin composite with its high strength, high modulus and other great properties. However, the structure and properties of PBO fiber is changed by solar irradiation, and the safety and reliability of PBO fiber will be affected.
Photodegradation of PBO fiber and light resistance of modified PBO have been reported (1–3); Zhang C et al. and Zhang H et al. (4, 5) reported that the mechanical strength property of PBO fiber is decreased by exposure to visible light (400~500 nm), and the photodegradations can be accelerated by UV light (200~400 nm). Fu et al.’s research found that fiber strength of PBO declined 65% by irradiation with xenon lamp for 6 months (6, 7).
Presently, the study on PBO fiber is focused on thermal degradation, hydrothermal aging and light aging (8–10); thermal degradation and hydrothermal degradation have been mostly researched, the regulation about which is clear, but there are few researches on light aging degradation. The research on the change of the microstructure of PBO fiber by light aging has little detailed data at present. The specific topics on UV aging of PBO fiber that have been studied are the influence of UVA and UVB on the tensile strength of PBO fiber, the change of physical and chemical structures by UVA and UVB, and analysis of the procedure for light aging degradation of PBO fiber.
The material used was PBO fiber with 1.5 dtex.
The ultraviolet light test chamber, which we made by ourselves, has ultraviolet lamps (Model: F20T8, 20W, produced by ZHONGTENGBOYI-TECH CO, LTD, Beijing, China) of UVA (368 nm) and UVB (308 nm). The temperature is 22.5°C, and the relative humidity is 50~60% in this experiment.
3 Results and discussion
For the influence of the time that the UV-light irradiates on PBO fiber, the experiment discusses the effect on fiber tensile property by intensity of ultraviolet (UVA, UVB). The scanning electron microscope (SEM) is used to analyze whether there are any impurities and corrosion on the fiber surface after UV irradiation and determine the surface damage properties of the fiber. The fiber macromolecular structure changes can be analyzed by X-ray diffraction (XRD) spectroscopy, the infrared (IR) spectrum is used as a method to analyze the variation of molecular chemical structure, and the fiber thermal decomposition by ultraviolet irradiation can be shown by differential scanning calorimetry (DSC) thermogravimetric analysis. The change of mechanics and chemical structure performance of fibers under the UVA and UVB irradiation can be determined, and theoretical basis for improving the fiber performance can be provided.
3.1 Influence of irradiation time on PBO fiber strength
Figure 1 shows the change of PBO fiber strength irradiated by UVA and UVB. The changes of the fiber strength are tested by single-fiber tensile strength tester, 50 fibers per group, which was shown on a single-fiber tensile strength sketch map (Figure 2). The single-fiber tensile strength (in centinewtons) was calculated by Weibull statistic distribution, and the tensile strength retention rate was computed by Equation , with the results shown in Table 1.
|Tensile strength retention rate (%)||100||96.11||94.58||91.89||85.92||80.64|
|Tensile strength retention rate (%)||100||63.37||86.54||77.48||67.56||61.35|
TFr=Tensile strength retention rate %
TF0=Tensile strength of single fiber before irradiation (cN)
TFn=Tensile strength of single fiber after n hours irradiation (cN)
Figure 1 shows, with the increase of the UV irradiation time, that the PBO fiber strength tends to decrease. The stability of PBO fiber is considered to have changed under UV irradiation. The fiber absorbs the photon, the fiber macromolecule is activated, the photodegradation is triggered, the macromolecule bonds break, the fiber orientation degree is lowered, and the fiber structure is damaged. Hence, the declining trends of fiber strength presents obvious downtrend when the UV irradiation time is longer than 200 h; the PBO fiber strength drops by 20% under UVA irradiation and 40% under UVB irradiation. The damage on irradiated PBO fiber by UVB is worse than that by UVA; at that time, the application performance of the fiber is reduced.
Table 1 shows that the tensile strength retention rate of PBO fiber after irradiating by UVA and UVB was decreased. And the tensile strength retention rate of the fiber irradiated by UVB decreased more than that by UVA. The reason might be that the UVB penetrated the fiber surface rapidly and caused more damage on the fiber than did the UVA.
3.2 PBO fiber surface characterization by SEM analysis
Using SEM to observe the PBO surface morphology which is irradiated by UVA and UVB, the influence of the ultraviolet brands is shown in Figure 3.
The fiber’s surface morphology has significant change (Figure 3) after the irradiation by UVA and UVB. In Figure 3A, the surface of PBO fiber is smooth and normalized, whereas in other figures, the surface with small particles and some bumps is rough, and there is corrosion on the axis of the fiber. The reason is that after the UVA and UVB irradiation, the macromolecule of the fiber is activated, and the particles are on the fiber surface; after some time, the fiber is degraded, the macromolecular chains are fractured, and the fiber structure is changed.
3.3 Influence on molecular structure of PBO fiber by ultraviolet irradiation
3.3.1 Analysis on the change of PBO structure by XRD spectroscopy
The change of the structure of PBO fiber which is irradiated by UVA and UVB is analyzed through the XRD pattern, which is shown in Figure 4. (Figure 4A shows the comparison between the UVA irradiated fiber and non-irradiated fiber, and Figure 4B shows the comparison between the UVB irradiated fiber and non-irradiated fiber.)
After irradiating PBO fiber for some time, a diagram of the large change in XRD diffraction is shown in Figure 4.
Generally speaking, the molecular arrangement of PBO structure (11) and the molecular chain of lattice structure are both oriented along the C-axis. In a crystal cell, PBO is not exactly coplanar; the molecular structure of PBO is composed of two molecular chains through a monoclinic system crystal cell (12). In Figure 4, PBO fibrils have two obvious diffraction peaks, which indicates that the fibril has high crystallinity and orientation. Irradiated by UVA and UVB for some time, according to the relational expression between diffraction ray spatial orientation and crystalline structure, two characteristic diffraction intensities are reduced, which is called Bragg equation
where “α” represents diffraction plane spacing, “θ” is diffraction angle, and “λ” is the wavelength of X-ray.
For the PBO fibril, 2θ=15.34°, 2θ=27.23°, 2θ=38.2°, and 2θ=58.4°; for the PBO fiber irradiated by UVA, 2θ=14.98° and 2θ=27.1°; and for the PBO fiber irradiated by UVB, 2θ=14.8° and 2θ=27.03°.
In this experiment, the wavelength of X-ray (λ) is 0.154056 nm, the temperature (T) is 299 K, and the calculated results are shown in Table 1.
Table 2 shows that the diffraction plane spacing (α) has a decreasing trend; with the decrease of the crystal lattice spacing, the crystal structure would be destroyed.
|Ultraviolet brands||2 diffraction angle (°)||λ (nm)||α diffraction plane spacing (nm)|
In Figure 4, it can be seen that the diffraction intensity in the irradiated PBO fiber has dropped greatly. It is analyzed that the precise crystal structure of PBO fibril is destroyed, and after increased irradiation time, the orientation of the fiber macromolecule changes, which leads to its double bond opening from the gamma rays exposure, and the macromolecule chain breaks.
The intensity of PBO fiber by UVA exposure for some time, which is shown in Figure 5, is obviously lower than that of PBO fibril. This is the same to exposure by UVB. Therefore, the fiber structure is severely damaged because of exposure for a long time.
3.3.2 The change of PBO chemical structure by IR spectroscopy analysis
The changes of chemical structure of PBO fibers which is irradiated by UVA and UVB at different times are shown in Figure 5A and B.
It can be seen in Figure 5 that the macromolecular structure of PBO fiber has changed distinctly after the UVA and UVB irradiation. The characteristic peaks show that the peak areas (the C=N- at 1561 cm-1, -C-OH at 1059 cm-1, -CO- at 1273 cm-1 and so on) of PBO fibril are reduced. It can be inferred that the macromolecular structure is oxidized, and the bond breaks. In Figure 5B, there is a new absorbing peak at 1350 cm-1. It may be the stretching vibration of nitro, which shows a new group (nitro) in the fiber structure after UV irradiation.
It can be determined that the macromolecular structure of PBO fiber
3.4 The change of the thermal performance of PBO fiber by DSC thermo-analysis
The changes of thermal performance of the PBO fiber irradiated by UVA and UVB are shown in Figure 7.
According to DSC thermo-analysis (Figure 7), different ultraviolet irradiated PBO fibers have different thermal performance. In Figure 7A, the temperature at the first endothermic peak in the DSC curve of PBO fibril is 150~200°C. This step is the volatilization of material moisture and decomposition of micromolecule (or melting), and the weight loss ratio is about 2%. When the temperature increases to over 700°C with about 1.0 mW/mg of decalescence, the weight loss ratio is about 12%. When the temperature exceeds 760°C, the fibril decomposes rapidly, and the weight loss ratio is about 34%. The PBO fibril (Figure 7A) and the fiber irradiated by UVA (shown in Figure 7B) have different thermal decomposition temperatures. The first endothermic peak of the UVA-irradiating fiber at temperature 140~160°C is lower than that of PBO fibril in DSC curve. At this point, this state is still the volatilization of material moisture and decomposition of micromolecule (or melting), and the weight loss ratio is about 2%. As the temperature increases continually to 350~450°C, the weight loss ratio of the fiber is about 6%, and the fiber is decomposed. When the temperature is over 660°C, the fiber rapidly decomposes, and the weight loss ratio is about 15.5%. The first endothermic peak of the UVB-irradiating fiber (Figure 7C) is shown at temperature 100~120°C. The initial decomposition temperature of the fiber is 400~500°C, and the weight loss ratio of the fiber is about 8%. The fiber decomposes quickly at temperature of over 660°C, and the weight loss ratio is around 17.5%.
The following are the findings of this study:
After UVA and UVB irradiation for 320 h, the PBO fibers are seriously damaged. The strength of the fiber which is irradiated by UVB reduced 20%, which is much lower (about 20%) than that irradiated by UVA.
The structure of PBO fibers irradiated by UVA and UVB has changed. The XRD spectrum indicates that the crystal structure of the PBO fiber has been broken and the orientation has been changed. And it can be indicated from the IR spectra that oxidation and light of the PBO fiber by UV irradiation cause NO2, the new group are produced.
Compared with PBO fibril, the thermal properties of PBO fiber have been changed by ultraviolet irradiation, and the temperature of thermal decomposition(660°C) declines by 100°C.
This work was financially supported by the Shaanxi Province Textile Sciences and Engineering Leading Academic Discipline Project.
Funding: Xi’an Polytechnic University Shaanxi Construction of Key Disciplines of Shaanxi Province.
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