The Effect of Annealing on the Structure and Electric Performance of Polypropylene Films

. Biaxially oriented polypropylene (BOPP) and uniaxially oriented polypropylene (UOPP) ﬁ lms were annealed. The e ﬀ ect of annealing temperature ( T a ) on dielectric strength was studied. The electric breakdown strength ( E b ) of BOPP and UOPP ﬁ lms changes in a quite di ﬀ erent trend with the annealing process. E b of BOPP ﬁ lms decreases with the increase in T a , whereas E b of UOPP ﬁ lms increases ﬁ rst and then decreases with T a . The structural changes during annealing were investigated. The crystallinity rises with T a , while the orientation degree and E b show a similar trend with T a . Although the crystallinity and crystal structure can a ﬀ ect E b of polypropylene ﬁ lms, the orientation of chain segments has a much larger correlation with E b . Our results indicate that the deterioration of the metallized BOPP ﬁ lm capacitor may originate from the orientation degree decrease of chain segments after experiencing high temperature.


Introduction
Biaxially oriented polypropylene (BOPP) thin films are widely used in metallized film capacitors due to their extremely low dielectric loss (0.02%) and high dielectric strength (720 kV mm −1 ) [1][2][3]. Furthermore, metallized film capacitors have the properties of self-healing, which have promoted a wide application of metallized BOPP film capacitors in ultrahigh voltage flexible direct current (DC) power grid, high-speed railway, and new energy electric vehicles [4,5]. Increasing the energy density and service temperature of metallized film capacitors is the endless pursuit of producers and customers [6,7]. The energy density of the metallized film capacitor is determined by the dielectric constant and the electric breakdown strength (E b ) [8,9]. The dielectric constant at a specific frequency and temperature is hard to be improved since the dipole or polarization is the intrinsic properties of polypropylene, while E b can be affected by the processing parameters during BOPP manufacturing. E b can be modified by changing the thickness, crystallinity, and tensile strength of BOPP films [10]. E b can also be improved with a higher biaxially stretching ratio [11].
When the metallized BOPP film capacitor experiences excessive voltage above E b , a point defect short circuit between metallized electrodes of the capacitor can occur. The metallic layer of electrodes evaporates rapidly due to arc discharge together with localized high temperature. The elevated temperature will considerably reduce the lifetime expectancy of metallized polymer film capacitors [12]. Thermal aging is one of the main failure mechanisms in polymeric film capacitors [13,14]. Most studies concentrated on the aging of metallized film capacitors at a high temperature by monitoring the changes in their basic parameters [15,16]. Capacitance and equivalent series resistance were monitored as the parameters that reflect the aging process [17,18]. Basically, some of the parametric changes can be traced from the structure variation of the BOPP film [19]. However, although the performance of polypropylene (PP) capacitors during aging has been studied carefully, the structure evolution of the BOPP film during aging is not clearly investigated, which can be studied by annealing the BOPP film separately [20].
Annealing is an important process to modify the performance of plastics. During annealing, the oriented chain relaxes accompanied with the re-arrangement of chain segments, resulting in a variation of the structure and performance. A uniaxially oriented polypropylene (UOPP) film is a base film for preparing microporous films by stretching. To prepare microporous film with uniform pore size, the UOPP film should be annealed first to modify the crystallinity and orientation degree before stretching. Some studies showed that both the orientation degree and crystallinity increased with annealing temperature (T a ) [21,22]. After annealing, the content of defects in crystals was decreased, the orientation of the crystalline phase was increased, and microcrystals formed in the amorphous between primary lamellae [23]. While some proposed that the amorphous part could be divided into mobile amorphous fraction and rigid amorphous fraction. Annealing promoted the formation of looser mobile amorphous fraction and more rigid amorphous fraction by the microstructural re-arrangement [24]. Although there were some studies investigating the structure evolution of annealing on UOPP film, few studies concentrated on the annealing behavior of produced BOPP film. While the structure change after annealing is vital to understand the performance deterioration of metallized BOPP film capacitor. Also, no one has ever studied the dielectric and electric breakdown properties of UOPP film. Although both BOPP and UOPP are oriented films, various manufacturing art has endowed the two kinds of films with different crystal structures. The influence of crystal structures on the dielectric properties of polypropylene has been investigated a lot. However, the differences of oriented crystal structures between BOPP and UOPP, as well as the primary factor of the oriented crystal structures on impacting the breakdown strength have not been well studied.
In this article, two kinds of oriented PP films were annealed at various temperatures, and studies were carried out to reveal the relationship between structure and dielectric strength. It is expected that our results can provide guidance for understanding the performance variation of metallized film capacitor.

Experimental Section
BOPP films with a thickness of 8 μm were obtained from Tongfeng Electronics (Tongling, China). The BOPP films were commercial products for metalized PP film capacitors. UOPP films with a thickness of 16 μm were obtained from Cangzhou Mingzhu Plastic Co. Ltd. (Cangzhou, China). The UOPP films were prepared by melt casting at a large die draw ratio with the properties of hard elastic, which could be used as base films for preparing uniaxially stretched PP microporous films. BOPP and UOPP films were rolled to anneal at 100-150°C for 1 hour before further characterized, which simulated the state when PP films were rolled in the capacitors.
The dielectric properties were measured using an impedance analyzer (Concept 40, Novocontrol GmbH, Montabaur, Germany) at room temperature from 10 7 to 10 2 Hz. E b was conducted on the Dielectric Withstand Voltage Test System (Beiguang Jingyi Instrument Equipment Co., Ltd, Beijing, China). The leakage current was measured by a combination of power supply (Keithley, 2290-10) and System SourceMeter™ (Keithley, 2635B). The films were tested in insulating oil at room temperature. The two-parameter Weibull distribution was employed to analyze the DC breakdown strength of samples from the experimental data.
Differential scanning calorimetry (DSC) measurements were taken with a DSC (PerkinElmer, DSC 8000) at 10°C minute −1 . Indium and tin were employed for the temperature calibration. Samples of~3.0 mg were sealed in aluminum pans and a nitrogen gas purge with a flux of~20 mL minute −1 was used to prevent oxidative degradation of samples during the heating run. The crystallinity was calculated using the following equation: where ΔH f is the melting enthalpy of α form crystals, is the standard melt enthalpy of α form crystals.
Wide-angel X-ray diffraction [WAXD; Phillips X'pert Pro MPD diffractometer with Cu-Kα (λ = 0.15418 nm)] was further used to characterize the crystal form and crystallinity. The 2θ scanning range was 10-30°with the rate of 3 min −1 . The overall crystallinity, X c , was calculated according to the following equation [25]: where A cryst and A amorp are the fitted areas of crystal and amorphous regions, respectively. Small-angle X-ray scattering (SAXS) was performed at 4B9A, Beijing Synchrotron Radiation Facility (BSRF), using a Mar345 detector. The sample was mounted onto a stage at the beamline at a sample to detector distance of 1750 mm. The one-dimensional correlation function K(z) can be derived from the scattering intensity distribution profile as follows [26][27][28]: where z denotes the drawing direction. It must be noted that no multiplication q 2 to I(q) is performed because of the highly anisotropic orientation of the lamellar crystals in the sample investigated and no extension of I(q) to a higher angle is carried out.
Fourier transform infrared spectroscopy (FTIR) was applied to characterize the orientation degree of the PP films International Journal of Polymer Science according to the dichroic ratio D [29].
where A ∥ is the absorption parallel and A ⊥ is the absorption perpendicular to a specific reference axis at a specific wavenumber. The Herman orientation function of this vibration is obtained according to [29]: where α is the angle between transition moment and polymer chain axis, and the α value of 18 o was used. The orientation of the crystalline phase, f c , can be determined from the D at 998 cm −1 . While the average orientation function, f av , is obtained from the D at 972 cm −1 . The orientation of amorphous phase, f a , is calculated according to: where X c is the degree of crystallinity determined by WAXD.

Results and Discussion
3.1. Dielectric Properties. The dielectric properties of BOPP and UOPP films are shown in Figure 1. It can be observed that the dielectric constants of BOPP and UOPP films are almost constant (~2.25 and 2.45, respectively) when the frequency is lower than 1 × 10 6 Hz, and the losses are <0.02%, suggesting that PP films are superior stable dielectrics. The permittivity and dielectric loss of homogeneous materials are mainly related to the dipole moment of materials [30, . Therefore, it is reasonable that the manufacturing method and annealing have no obvious effect on the dielectric constant and loss.

Electric Breakdown Strength.
The electric breakdown strength of PP films was determined and fitted with a twoparameter Weibull distribution function: is the cumulative probability of the electric breakdown failure, E is the experimental electrical strength, E b is the scale parameter, which means the breakdown strength at a cumulative failure probability of 63.2% and can be referred to as the characteristic breakdown strength of the dielectrics. The shape parameter β shows the dispersion of E data and the quality of film dielectrics [32,33]. Figures 2(a) and 2(b) show the Weibull distribution of the PP films. The shape parameters β are about 10.0 and 8.5 for the BOPP films and UOPP films, respectively, suggesting a narrow dispersion of E data. Figures 2(c) and 2(d) show the E b of PP films at different T a . The E b of initial BOPP film is 724.9 kV mm −1 . After annealed at 100°C for 1 hour, there is no obvious change in the E b . When T a is increased to 110°C, E b decreases to about 712.5 kV mm −1 . Further increasing T a leads to a gradual decrease of E b . When T a is higher than 140°C, an obvious decrease in the E b can be observed. It can be concluded that E b decreases with T a for the BOPP films. For the UOPP films, quite different trend is observed. The E b of initial UOPP film is 478.4 kV mm −1 , much lower than that of initial BOPP film. After annealed at 100°C for 1hour, E b increases to about 480.8 kV mm −1 . Further increasing T a leads to a gradual increase of E b to 578.7 kV mm −1 . When the T a is higher than 140°C, an obvious decrease in the E b can also be observed. The obvious decrease of E b in both kinds of PP films can be attributed to the partial melting of PP crystals at temperature higher than 140°C. When T a is lower than 140°C, E b of the BOPP film decreases with the increase in T a , while that of UOPP film shows a quite different trend. The different effects of annealing on E b should originate from different microstructure evolutions.
3.3. Thermal Analysis. The PP films were characterized with DSC as shown in Figure 3. The initial BOPP film has two melting peaks, one at 164.5°C and the other at 170.9°C as  Figure 3(a). As the BOPP film was annealed at 100°C, a shoulder peak appears at 161.1°C, which gradually shifts to higher temperature and merges with the peak at 164.5°C at higher T a . The shoulder peaks at about 161.1°C can be attributed to the melting of crystals formed at T a , since they increase with T a . While the peak at 164.5°C can be assigned to the melting of crystals from recrystallization. The peak at 170.9°C is much higher than the melting point of α form PP crystals (~163-167°C), which does not shift with T a . Therefore, it is supposed that this melting peak should originate from the crystals with special microstructure. The industrially produced BOPP films have been stretched at large ratios (5:0 × 8:0) with the heat setting process to control the film shrinkage ratio. Therefore, there should exist a more stable microstructure that cannot be affected during the DSC heat scan. When polymers were stretched at a large ratio, fibril crystals may form. Different from the folding chain crystals, the fibril crystals have the properties of extend chain crystals with a melting point approaching the equilibrium melting point. The equilibrium melting point of PP α form crystal is~186°C [34]. As a result, it is deduced that the peak at 170.9°C may be attrib-uted to the melting of fibril crystals with partially extended chain segments [35]. Estimated from the proportion of the peak at 170.9°C, it is supposed that a large number of fibril crystals exist in BOPP films due to high ratio stretching in the manufacturing process. The UOPP films show quite different thermal behavior from that of BOPP films as shown in Figure 3(b). The initial UOPP film has a melting peak at 166.3°C. After the UOPP film was annealed at 100°C, another melting peak appears at 163.6°C. Meanwhile, the melting peak at 163.6°C shifts to higher temperature and merges with the peak at 166.3°C, forming a shoulder peak on the left. According to the mechanism of lamellar thickening during high temperature annealing, the higher the T a , the thicker the lamellae, the higher the melting temperature. The shoulder peak on the left side should originate from melting crystals formed at T a , which may be attributed to the microcrystal form in the amorphous between the primary crystals [23]. For all UOPP films, the melting peak at 166.3°C does not change with the T a . Since melting and recrystallization can easily take place during heating, the peak at 166.3°C may be attributed to the melting of crystals from recrystallization [36]. To 5 International Journal of Polymer Science make sure that the different melting behavior is due to various manufacturing processes but not the raw material, the second DSC heating scan for both BOPP and UOPP were carried out. As shown in Figure S1, after eliminating thermal history, both PP resins show two melting peaks, that is, one at 148.7°C, another at 163.3°C, which indicates that PP resin with similar chain structure and composition has been used, and the different melting behavior of BOPP and UOPP films is originated from the difference in the manufacturing process.
The crystallinity was calculated using the melting enthalpy. The crystallinity of the initial BOPP film is 57.2%, which does not change when annealed at 100°C. As T a is higher than 100°C, a gradual increase in crystallinity with T a is observed, which changes greatly as the temperature is higher than 140°C. The highest crystallinity reaches 64.7% when annealed at 150°C. The crystallinity of initial UOPP film is about 52.5%. A large increase is observed as T a is 110°C, and the highest crystallinity is 62.0% when annealed at 150°C. The crystallinity of the UOPP films is low, and it increases in a different manner compared with the BOPP films, which can be attributed to different manufacturing processes.

Wide-Angel X-Ray Diffraction. Figures 4(a) and 4(b)
show the WAXD profiles of the PP films. The peaks at 2θ angles of 14.1, 16.9, 18.6, and 25.7°can be assigned to the reflection peaks of (110), (040), (130), and (060) of α-form, respectively. The crystallinity was calculated after separating the amorphous phase, and the results are shown in Figures 4(c) and 4(d). The crystallinity of both kinds of films increases with T a . For BOPP films, the crystallinity of the initial film is about 65.5%. The highest crystallinity of 70.0% is obtained at 150°C. While for UOPP films, the crystallinity of the initial film is about 64.7%, lower than that of BOPP film. The values increase sharply with T a . The highest crystallinity of 75.8% is obtained at 150°C. The crystallinity obtained by DSC and WAXD shows the same trend of increasing with T a , but in a different manner. Melting and recrystallization during the DSC heating scan may be the factor leading to the difference. When comparing Figure 2(c) with Figure 4(c), it can be observed that the crystallization and E b of BOPP films change in an opposite trend. The initial BOPP film has the highest E b , while the crystallinity is the lowest. After annealing at 100°C, slightly decreased E b with a slightly increased crystallinity is  International Journal of Polymer Science observed. The slightly increase in crystallinity may be due to the relaxation and crystallization of low-molecular-weight fraction. As T a is in the range of 110-140°C, E b declines gradually, while the crystallinity increases slowly. The increase in T a may have resulted in the partial melting and recrystallization, leading to the re-organization of crystals. As T a is 150°C, a temperature near the melting point, the melting recrystallization and crystal re-organization should be more intense during annealing, and E b decreases sharply. The comparison implies that the crystal structure change due to annealing will weaken the E b of BOPP film. When comparing Figure 2(d) with Figure 4(d), both E b and crystallinity of UOPP films increase with T a , as T a is lower than 140°C, which suggests that annealing favors strengthening the E b of the UOPP film. As T a is 150°C, the crystallinity reaches the highest, while E b also decreases sharply, indicating the collapse of oriented crystal structure. The E b and crystallinity of BOPP and UOPP films change in different ways with T a , which suggests that crystallinity is not the determining factor affecting E b .
3.5. Small-Angle X-Ray Scattering. SAXS was used to explore the effect of annealing on the microstructure of PP films. The typical 2D SAXS pattern of the BOPP and UOPP initial films are displayed in Figure 5. The initial BOPP film has an oblate pattern, indicating that the orientation in the traverse direction is larger than the machine direction (MD). After annealing at 130°C, there is no significant change in the shape of the SAXS pattern when compared with the diagrams of initial films, whereas the UOPP film shows a typical uniaxial orientation pattern. The weak meridional streak in SAXS patterns is attributed to the shish-like crystals such as extended chain crystal structures and row nuclei in MD, while the appearance of strong equatorial maxima is attributed to kebab-like lamellar stacks, oriented perpendicularly to the MD [37]. After annealing at 130°C, the scattering intensity of both the shish-like and kebab-like crystals increases obviously. The morphologies of both BOPP and UOPP films after annealed at 130°C were observed with atomic force microscopy as shown in Figure S2. Crystals with no obvious orientation are observed in the BOPP film [ Figure S2(a)], while parallel lamellar stacks are seen in the UOPP film [ Figure S2(b)], which accords well with the 2D SAXS pattern. The one-dimensional scattering intensity distributions of the BOPP films along the MD direction are shown in Figure 6(a). A much stronger increase in intensity is observed as T a is higher than 130°C, indicating crystal perfection with the increase of the electron density difference between the lamellar crystals and amorphous layers. The one-dimensional correlation function derived from the scattering intensity distribution profile is shown in Figure 6(c). When the crystallinity is larger than 50%, the average thickness of the amorphous layer (d a ) and long period of lamellar stacks (d ac ) can be directly obtained from the correlation function [ Figure 6(c)] [27]. The average d a , d ac , and average thickness of crystals (d c ) values of BOPP films are shown in Figure 6(e). The initial d a , d c , and d ac values of the initial BOPP film are 4.2, 13.9 and 18.1 nm, respectively, which keeps almost stable as T a is lower than 120°C. Obvious lamellar thickening appears as T a is higher than 120°C, which can be attributed to the partial melting of unstable crystals followed by the structural re-organization during annealing at high temperatures. The one-dimensional scattering intensity distributions of UOPP films along the MD direction are shown in Figure 6(b). It can be observed that the scattering peak shifts to smaller scattering vector q values with stronger intensity with the increase in T a , which is the evidence of crystal perfection with the re-arrangement of lamellae and/or amorphous parts, resulting in an increase of the long periodicity and the electron density difference between the lamellar crystals and amorphous layers. The resultant correlation functions are shown in Figure 6(d). The average d a , d c , and d ac values of UOPP films are shown in Figure 6(f). Compared with the initial UOPP film, all the thicknesses increase with the increase in T a . The increase in thickness can be assigned to volume expansion, the crystallization of amorphous part and re-organization of lamellar crystals by lamellar thickening. Compared with d a , d c and d ac increase in a much higher rate, especially when T a is higher than 130°C, suggesting the lamellar thickening with the rise of T a . The average d a values of both the initial UOPP and BOPP films are at the same level, while the d c values are quite different, which can be attributed to the different film manufacturing process. The initial UOPP film is prepared using highspeed melt elongation, followed by quenching, resulting in imperfect crystals with small d c (~8.1 nm). While the BOPP film is made by biaxially stretching followed by heat setting near the melting temperature, high temperature heat setting leading to more stable crystals with much higher d c (~13.9 nm). The initial size and stability of the crystals in the initial films are quite different, so annealing has different effects on the structure evolution of two kinds of PP films. The d c increases with T a , while E b varies in a different trend [Figures 2(c) and 2(d) and 6(e) and 6(f)], which implies that long period of lamellar stacks or lamellar thickness variation cannot be directly correlated with E b .
3.6. Fourier Transform Infrared Spectroscopy. The polarized FTIR spectra of polypropylene are shown in Figures 7(a)  and 7(b). The absorption at the wavenumber of 998 cm −1 is attributed to the crystalline phase (c-axis) while absorption at the wavenumber of 972 cm −1 is due to the contribution of both crystalline and amorphous phases [38]. The Herman orientation function was calculated according to the dichroic

9
International Journal of Polymer Science ratio as shown in Figures 7(c) and 7(d). For the BOPP films, f c decreases slowly while f a decreases fast with the rise of T a first, which suggests that the amorphous chain segments relax faster than those in the crystal. As T a is higher than 130°C, f c reduces in a much rapid rate, indicating reorganization of crystals. For the UOPP films, f c and f a almost keep stable below 110°C. As T a is further increased, the orientations of the amorphous part and the crystal increase. Since the UOPP film consists of shish-kebab lamellae, that is, minor shish-like crystals growing along MD and a large number of kebab-like crystals oriented along the traverse direction, the increase in the orientation degree indicates that chain segments pack along MD during the lamellar reorganization. As T a is higher than 140°C, the orientation degree falls down suggesting the collapse of the uniaxial orientation structure, which should be the upper limit application temperature for the UOPP film. When comparing Figure 2(c) with Figure 7(c), it can be observed that the initial BOPP film has the highest E b and orientation degree. As T a is increased to a temperature lower than 120°C, f a declines faster than, which may suggest that the decrease of E b may mainly originate from the decrease of f a . As T a is further increased to temperature lower than 140°C, f c declines faster than f a , implying the decrease of E b may be due to the relaxation of f c as a result of melting and recrystallization. As T a is 150°C, f c declines sharply, indicating obvious melting has led to the structure collapse of BOPP film, so E b has the lowest value. For the UOPP film, both f c and E b increase with T a , as T a is lower than 140°C. As T a is 150°C, both f c and E b fall. The orientation degree of both BOPP and UOPP films can correlate with E b very well, suggesting that the orientation degree is highly correlated with the E b of PP films. But linear correlation is not observed (Figure S1), since the crystallinity also has great influence on E b [39]. Uniaxial orientation endows larger anisotropy to the films than biaxial orientation ( Figure 5), so it seems that orientation degree is not feasible for comparison between different types of PP films. BOPP film has been stretched at large ratios (5:0 × 8:0) at both MD and traverse direction. Although the orientation degree of BOPP film is smaller, the chain segments orientation should be much larger than those of the UOPP film. E b of BOPP film is much larger than that of UOPP film [Figures 2(c) and 2(d)], suggesting that microscopic chain segment orientation plays a more important role in E b than the macroscopic orientation of lamellar stacks. Due to much higher E b , BOPP film is the best choice for metallized PP film capacitor. When metallized PP film capacitor is thermally aged or self-healing after locally electric breakdown, the performance deterioration may result from chain segment re-organization, especially the decrease in orientation degree after experiencing high temperature.

Conclusion
BOPP and UOPP films were annealed, and the electric performance and structure evolution had been investigated. The BOPP films have high initial E b , which decreases with T a . The UOPP films have low initial E b , which increases first and then decreases with T a . In BOPP films, there exist a large number of fibril crystals with larger lamellar thickness and higher melting temperature. While in UOPP films, folded chain crystals are the main crystal species. The crystallinity of both films rises with T a , and obvious lamellar thickening takes place after 130°C. The orientation degree and E b show similar trend with T a for both BOPP and UOPP films, suggesting that orientation degree is highly correlated with the E b . Higher E b of BOPP films can be attributed to larger lamellar thickness and higher orientation of chain segments. The performance deterioration of metallized PP film capacitors after aging or experiencing local high temperature may result from the decrease in the orientation degree of the BOPP film.

Data Availability
Data are presented in the supplementary information files.

Conflicts of Interest
The authors declare that they have no conflicts of interest.