Application of β-diketone boron complex as an ultraviolet absorber in polyvinyl chloride film

In this paper, β-diketone boron complex (BF2-TPE) was first used as an ultraviolet (UV) absorber for polyvinyl chloride (PVC), and then a series of PVC/BF2-TPE composite films were prepared by solvent casting method. UV accelerated aging experiment was conducted to evaluate the effect of BF2-TPE on the photooxidative degradation of PVC, the structure and properties of the composite films in the UV aging process were characterized by ultraviolet-visible absorption spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, dynamic mechanical analysis and mechanical property analysis. The results show that BF2-TPE can absorb UV light in the wavelength range of 220–450 nm. The yield strength of the PVC/BF2-TPE composite film with 0.5% BF2-TPE (PVC/BF2-TPE0.5) decreases from 10.5 MPa to 7.8 MPa after 1200 h of UV irradiation, and its retention ratio is much higher compared with that of pure PVC film, indicating that BF2-TPE can inhibit the photooxidative degradation PVC. The PVC/BF2-TPE0.5 composite film shows higher temperature of onset decomposition, indicating that BF2-TPE can also enhance the thermal stability of PVC/BF2-TPE composite films.


Introduction
Polyvinyl chloride (PVC) is used in various fields because of its good chemical stability, excellent mechanical properties and low cost [1][2][3]. PVC can be divided into hard and soft products according to the content of plasticizer [4]. Hard PVC is mainly used in the production of pipes, while soft PVC is mainly used in the production of wires, cables and daily products. However, exposure of PVC products to sunlight can cause photooxidative degradation and thus reduce their outdoor service life [5]. One of the most effective methods to inhibit the photooxidative degardation of PVC is to add inorganic or organic light stabilizers into PVC matrix [6]. Unfortunately, most of the inorganic light stabilizers are pigments with strong coloring properties, making them unsuitable for application in transparent materials [7], while organic ones have the disadvantages of high mobility and easy extraction by solvents [8]. Therefore, it is necessary to synthesize new light stabilizers to improve the anti UV aging performance of PVC.
β-diketone boron complex (BF 2 -TPE) is widely used in memory chips and light-emitting devices due to aggregation induced luminescence and suppressed fluorescence discoloration properties [9]. β-diketone compounds can also be used as ultraviolet (UV) absorbers due to their optical tautomerism [10]. Therefore, the addition of BF 2 -TPE as an UV absorber to PVC is expected to improve the UV aging resistance of PVC.
In this study, BF 2 -TPE is first used as an UV absorber for PVC. BF 2 -TPE was synthesized according to the procedures described previously [9], and its chemical structure is shown in scheme 1. A series of PVC/BF 2 -TPE composite films were prepared by solvent casting method, and UV accelerated aging experiment was performed in a UV test chamber for 1200 h. Changes in the microscopic morphology and physical properties of these PVC/BF 2 -TPE composite films were characterized by ultraviolet-visible (UV-vis) absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis and dynamic mechanical analysis; Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
while changes in the macroscopic properties were characterized by scanning electron microscopy (SEM) and mechanical property analysis.

Preparation of PVC/BF 2 -TPE composite films
PVC and BF 2 -TPE with a total mass of 1 g were dissolved in 30 ml of THF, and the mass fraction of BF 2 -TPE was 0.1, 0.3, 0.5, 0.7 and 1.0 wt%, respectively. The mixed solution was subjected to ultrasonic treatment for 20-30 min, and then spread on a horizontal glass plate and evaporated at room temperature for 16 h. The resultant composite films were peeled off in cold water and dried in vacuum at 40°C for 24 h to remove excess THF. The PVC/BF 2 -TPE composite films of about 40 μm thick were obtained. Pure PVC film was prepared as a control following the same procedure.

UV accelerated aging test
UV accelerated aging test was carried out using a FUD-UV1150 accelerated aging tester (Beijing Fuyoudi Mechanical Equipment Co., Ltd, China) under the following conditions: light source: type I fluorescent UV lamp UV-A340 (40 W), average irradiation intensity: 0.76 W m −2 , irradiation and condensation temperature: 50°C, a cycle period of 240 h, including 160 h of irradiation and 80 h of condensation, irradiation time: 0, 240, 480, 720, 960 and 1200 h, respectively. In order to ensure uniform UV irradiation, samples were rearranged and shifted around the lamp array in turn.

Characterization and measurements
The UV-vis absorption spectra were recorded using a UV-vis spectrophotometer (UV-2550, Shimadzu Company, Japan) with a wavelength range of 200-800 nm. The FTIR spectra were characterized by an infrared spectrometer (Spectrum Two, PerkinElmer Company, USA) with a scanning range of 400-4000 cm −1 . The morphologies of PVC and PVC/BF 2 -TPE composite films before and after UV accelerated aging were observed using a JSM-6610LV field emission scanning electron microscopy (FE-SEM, Japan), and all samples were gold sprayed before observation. Thermogravimetric analysis (TGA) was conducted by a thermogravimetric analyzer (HCT-3, Beijing Hengjiu Scientific Instrument Factory, China) from room temperature to 700°C at a heating rate of 10°C min −1 in nitrogen atmosphere. Dynamic mechanical analysis (DMA) was performed by a dynamic mechanical analyzer (Q800, USA) with a scanning frequency of 1 Hz and a heating rate of 3°C min −1 in the temperature range from 40 to 120°C. The yield strength was measured by a microcomputer controlled electronic universal testing machine (104B-EX, Shenzhen Wance Test Equipment Co., Ltd, China) according to Chinese standard GB/T1040.2-2006.  Figure 1 shows the UV-vis absorption spectra of pure PVC and PVC/BF 2 -TPE composite films. A weak absorption band is observed in the wavelength range of 250-300 nm in pure PVC, which is mainly attributed to the complex formed by a trace of residual THF and O 2 during the photooxidative degradation [11]. Two strong absorption bands are observed at 306 nm and 421 nm in PVC/BF 2 -TPE composite films, indicating that BF 2 -TPE can be used as an UV absorber. The absorption band at about 306 nm may originate from the π-π * transition of C=C in enol, while that at about 421 nm may originate from the charge transfer transition from the electron donor tetrastyrene to the electron acceptor β-diketone boron [9]. The UV absorption intensity of PVC/BF 2 -TPE composite films increases gradually with the increase of BF 2 -TPE content, suggesting that BF 2 -TPE can significantly enhance the UV absorption properties of PVC. Figure 2 shows the transmittance of pure PVC and PVC/BF 2 -TPE composite films in the wavelength range of 200-800 nm. The transmittance of PVC/BF 2 -TPE composite films below 500 nm wavelength decreases greatly with the increase of BF 2 -TPE content, suggesting that BF 2 -TPE can obviously improve the UV barrier property of PVC. However, the transmittance of these PVC/BF 2 -TPE composite films is very close to that of PVC in visible wavelength range, especially in the wavelength range above 555 nm, to which the human eye is most sensitive [12]. Figure 2 shows that the transmittance of the composite films below 555 nm is higher than 80%, which is very close to that of PVC. Even for PVC/BF 2 -TPE1 composite film, its transmittance is higher than 70%, suggesting that the PVC/BF 2 -TPE composites have good transparency. Figure 3 shows the yield strength of pure PVC and PVC/BF 2 -TPE composite films at different irradiation time. The yield strength of all composite films shows an increasing trend before 480 h of UV irradiation, and the yield strength of pure PVC is increased by about 27.9% compared with that before irradiation due to the formation of a compact cross-linked structure at the beginning of UV irradiation [13,14]. However, the yield strength of all composite films exhibits a decreasing trend from 480 h to 1200 h, because the molecular chain can be broken with the increase of irradiation time, and thus a large number of defects are formed on the surface [15]. As a result, the microstructure of the polymer is destroyed, resulting in the degradation of film performance. Compared with PVC composite films with different BF 2 -TPE contents, the yield strength is decreased more obviously in pure PVC after 1200 h of irradiation. Thus, BF 2 -TPE plays a role in UV absorption. The yield strength of pure PVC decreases from 10.8 MPa to 5.6 MPa after 1200 h of UV irradiation, with a decrease of 48.1% compared to that irradiated for 480 h, while that of PVC/BF 2 -TPE0.5 composite film decreases from 10.5 MPa to 7.8 MPa after 1200 h of UV irradiation, with a decrease of 25.7% compared with that irradiated for 480 h. It is indicated that the retention ratio of the yield strength for PVC/BF 2 -TPE0.5 composite film is the highest, thus it is selected for further analysis and characterization.

UV absorption properties of PVC/BF 2 -TPE composite films
The UV-vis absorption spectra can be used for the detection of unsaturated conjugated double bonds [16]. In order to gain more insight into the reactions involved in the UV accelerated aging process, the UV absorption properties of PVC and PVC/BF 2 -TPE0.5 composite films before and after irradiation are analyzed, as shown in figure 4. The UV absorption intensity of all composite films increases with the increase of irradiation time, indicating that unsaturated conjugated double bonds are formed in the photooxidation process [17]. The characteristic absorption peaks of PVC/BF 2 -TPE0.5 composite film disappear in the UV region after irradiation, which may be related to the interconversion between keto and enol in β-diketone molecules. βdiketone can absorb UV light to destroy the intramolecular hydrogen bond of enol and leads to the shift of isomerization equilibrium towards keto under long-term UV irradiation. Thus, the number of enol isomers is decreased and that of keto isomers is increased under long-term UV irradiation [18,19]. Figure 5 shows the FTIR spectra of PVC and PVC/BF 2 -TPE0.5 composite film at different irradiation time. As can be seen from figure 5(a), the sharp peak at 1429 cm −1 belongs to the C-H bending vibration in the R-CH 2 -R′ structure of PVC molecular chain. After 480 h of UV irradiation, several weak peaks can be found at 1606 and  1700-1800 cm −1 . The weak peak at 1606 cm −1 belongs to the characteristic absorption of C=C, while that at 1700-1800 cm −1 belongs to the stretching vibration of C=O [20,21]. The absorption peak intensities of C=O and C=C of the pure PVC film increase gradually at 480-1200 h compared with that before irradiation. These results show that UV irradiation can lead to photodegradation of PVC and formation of the unsaturated bonds. As shown in figure 5(b), a new absorption peak is observed at 1540 cm −1 in the FTIR spectra of PVC/BF 2 -TPE0.5 composite film, which corresponds to the stretching vibration of C=C in the β-diketene alcohol structure [22]. The characteristic absorption peaks of C=O and C=C can also be observed in the PVC/BF 2 -TPE0.5 composite film after UV irradiation for 480 h, and the absorption peak intensity of C=O increases significantly at 480-1200 h compared with that before irradiation. Thus, UV irradiation can accelerate the photooxidative degradation of PVC composite films.

FTIR spectra of PVC/BF 2 -TPE composite films
Because the UV aging reaction occurs initially on the surface, the thickness of samples is an important parameter affecting the distribution of photodegradation products [23]. The carbonyl index (CI) is introduced here to quantify the photodegradation degree. The absorption peak area at 1429 cm −1 is taken as the internal standard [24], CI is the ratio of the peak area in the carbonyl region (1700-1800 cm −1 ) to the internal standard peak area. CI can be calculated according to formula (1): 1700 1800 cm 1380 1480 cm 1 1 Where A 1700-1800 cm−1 is the absorption peak area in the range of 1700-1800 cm −1 , and A 1380-1480 cm−1 is the absorption peak area in the range of 1380-1480 cm −1 . The variation of CI of pure PVC and PVC/BF 2 -TPE0.5 composite film as a function of irradiation time is shown in figure 6. It shows that the CI value increases with the UV irradiation time for both pure PVC and PVC/BF 2 -TPE0.5 composite film, indicating that both of them undergo photodegradation to varying degrees under UV irradiation. The CI of pure PVC film reaches a  maximum of 3.0 after 1200 h of UV irradiation. However, the CI of PVC/BF 2 -TPE0.5 composite film is 2.6 after 1200 h of UV irradiation, and it is much lower than that of pure PVC film, indicating that BF 2 -TPE can effectively inhibit photooxidative degradation of PVC. This is because BF 2 -TPE absorbs UV light and causes molecular thermal movement, which destroys the intramolecular hydrogen bonds, opens chelate ring and releases incoming radiation in the form of thermal dissipation. Figure 7 shows the surface SEM images of PVC composite films before and after UV irradiation. The surfaces of pure PVC and PVC/BF 2 -TPE0.5 composite film are smooth before irradiation (figures 7(a) and (c)), but obvious cracks are observed after irradiation (figures 7(b) and (d)). This is mainly due to the chain breakage and cross-linking of PVC in the photooxidation aging, which lead to the degradation of films and affect their morphology [25]. However, the surface cracking of PVC/BF 2 -TPE0.5 composite film is weaker than that of pure PVC, suggesting that BF 2 -TPE can inhibit photooxidative degradation of PVC. The molar contents of C and Cl elements in pure PVC and PVC/BF 2 -TPE0.5 composite film before and after irradiation were measured by EDS, as shown in table 1. It can be seen that the molar content of Cl in pure PVC is decreased by 16.3% after 1200 h of UV irradiation, while that of PVC/BF 2 -TPE0.5 composite film is decreased by only 4.4%, which further illustrates that BF 2 -TPE can prevent the dehydrochlorination of PVC and thus may play a role as an UV absorber. Figure 8 shows the TG and derivative TG (DTG) curves of PVC/BF 2 -TPE composite films in nitrogen atmosphere. Pure PVC exhibits a slight weight loss (7.0%) in the temperature range of 70-260°C, which is due to the volatilization of the solvent in the PVC film rather than the degradation of PVC [26]. Therefore, the decomposition of PVC and its composite films consists of two stages. In the first stage, a large weight loss (68.2%) occurs in the temperature range of 260-390°C, which is due to the chain dehydrogenation of PVC and the formation of a large number of conjugated polyene sequences [27]. A lower weight loss (24.3%) occurs in the temperature range of 390-700°C due to the chain breaking and cross-linking reactions of PVC [28].

Thermal stability of PVC/BF 2 -TPE composite films
Relevant important parameters such as the temperature of onset decomposition (T Onset ), the temperature of maximum loss rate (T max ) and residual yield are summarized in table 2. The T Onset and T max of PVC/BF 2 -TPE0.5 composite film in the first stage are higher than that of PVC matrix before UV irradiation, indicating that the addition of BF 2 -TPE can improve the thermal stability of PVC. However, after UV irradiation for 1200 h, the T Onset of pure PVC and its composite films in the two stages shows a decreasing trend. The T Onset of pure PVC is decreased by 13.8°C from 257.5°C to 243.7°C in the first stage, while that of PVC/BF 2 -TPE0.5 composite film is decreased by 6.5°C from 261.0°C to 254.5°C. This is due to the thermal degradation of PVC. On the other hand, PVC photodegradation can also promote the autocatalytic reaction of dehydrochlorination of PVC to produce a large number of conjugated polyene sequences, which accelerates the degradation rate of PVC [13,29]. The T Onset of pure PVC is decreased by 10.8°C from 408.3°C to 397.5°C in the second stage, while that of PVC/BF 2 -TPE0.5 composite film is decreased by 4.8°C from 403.6°C to 398.8°C. This further confirms that UV irradiation can reduce the T Onset of polymer. It is noteworthy that the addition of UV absorber BF 2 -TPE to PVC results in decrease in the degradation rate of PVC/BF 2 -TPE0.5 composite film, suggesting that BF 2 -TPE can inhibit the photodegradation of PVC and thus slow down the thermal degradation rate.    Figure 9 shows the relationship of the storage modulus (E′) and loss factor (tan δ) of pure PVC and PVC/BF 2 -TPE composite films with temperature. As shown in figure 9(a), the E′ of PVC/BF 2 -TPE composite films is higher than that of pure PVC below the glass transition temperature (T g ) of PVC, and it increases with the BF 2 -TPE content. Compared with pure PVC, the E′ of PVC/BF 2 -TPE0.5 and E′ of PVC/BF 2 -TPE1 at 40°C are increased by 51.5% and 56.0%, respectively. BF 2 -TPE is a rigid molecule with a conjugated structure, and F atoms in BF 2 -TPE can form hydrogen bond interaction with tertiary H atoms in PVC (figure 10), which can improve the miscibility between BF 2 -TPE and PVC. Thus, the rigid BF 2 -TPE molecule can reinforce PVC matrix. As depicted in figure 9(b), the similar phenomenon is also observed for PVC/BF 2 -TPE composite films irradiated for 1200 h. The T g of PVC obtained from tan δ peak position is shown in table 3. The T g values of all composite films, irradiated or not irradiated alike, are lower than that of pure PVC. This may be because BF 2 -TPE is a small organic molecule with fast molecular motion, and its thermal movement speeds up with the increase of temperature, which can increase the free volume of PVC matrix. On the other hand, the strength of hydrogen bond between BF 2 -TPE and PVC decreases with the increase of temperature. Both of them can increase the flexibility of the PVC molecular chain, and thus results in lower T g . However, there is no significant difference in  T g between PVC/BF 2 -TPE composite films with different content of BF 2 -TPE, suggesting that BF 2 -TPE has no obvious effect on the glass transition behavior of PVC due to its low content [30]. After irradiation for 1200 h, the T g of PVC and PVC/BF 2 -TPE composite films increases compared with that without irradiation. This is due to the formation of cross-linking and conjugated structures in the UV aging process [13,14], which hinder the movement of chain segments of PVC and reduce the flexibility of the molecular chain.

Conclusions
In this study, a series of PVC/BF 2 -TPE composite films were prepared by solvent casting method, and their UV absorption properties, mechanical properties, thermal stability and the morphology during UV accelerated aging were studied. The results indicate that the UV light in the wavelength range from 220 to 450 nm can be absorbed by BF 2 -TPE, and the UV resistance of PVC/BF 2 -TPE composite films is improved with the addition of BF 2 -TPE. After irradiation for 1200 h, the yield strength of pure PVC film and PVC/BF 2 -TPE0.5 composite film is decreased by 48.1% and 25.7%, respectively. BF 2 -TPE can effectively inhibit the photodegradation of PVC, increase the thermal degradation temperature and reduce the thermal degradation rate of PVC/BF 2 -TPE composite films.