Effect of lanthanum on thermal and mechanical properties of modified polyvinyl chloride film

Polyvinyl chloride (PVC) hybrid film containing cerium, tin and folic acid modified nano titanium dioxide is a new functional material with good compatibility, light resistance and heat resistance. In this work, the PVC hybrid film was prepared by replacing cerium with lanthanum. The hybrid film was characterized by thermogravimetric infrared spectroscopy, dynamic mechanical analysis, atomic force microscopy, x-ray photoelectron spectroscopy, scanning electron microscopy, tensile strength and thermal aging. The results show that the initial thermal stability of the hybrid film containing lanthanum is slightly higher than that of the control sample (conversion α < 1.9%). For the same gaseous component plasticizer, the peak intensity of the ester based infrared characteristic group is higher than that of the hybrid film containing lanthanum, indicating that the intermolecular force between lanthanum and plasticizer is greater than that of cerium. In the range of 350 °C–575 °C and the wave number range of 2850–3100 cm−1, the peak height of the lanthanum containing film is significantly higher than that of the control sample, which may be due to the difference of the outer layer electrons. The peak intensity of O and C atoms on the surface of the hybrid film containing lanthanum is significantly lower than that of the control sample, indicating that the surface relative concentration of lanthanum atoms is lower than that of cerium atoms. The surface roughness RMS of the hybrid film containing lanthanum is significantly lower than that of the organic tin film, which is the result of the attraction between folic acid modified TiO2, lanthanum and tin and the carbon chlorine bond in PVC. It has the function of a potential ultrafiltration membrane. In addition, the hybrid film containing 1phr lanthanum shows good energy storage modulus and low temperature resistance. The prepared hybrid film shows the characteristics of a potential multi-functional material.

Common functional additives, such as adding carbon nanotubes loaded TiO 2 particles [11] for poor light resistance of PVC, modified nano-TiO 2 [12], or modified nano-TiO 2 [13] hybrid film containing rare Earth cerium, tin and folic acid with improved compatibility, to improve heat resistance and UV light resistance.
Common metal soap heat stabilizers are widely used in PVC processing and have no impact on the environment, their long-term thermal stability is poor. Therefore, the development of non-toxic, efficient and environment-friendly composite and functional additives has become a new trend.
Reports involving rare Earth heat stabilizers are as follows, the preparation of rare Earth complexes with curcumin and the stability of PVC [20], and the research of rare Earth stabilizers as PVC stabilizers [21]. The role of lanthanum cyanurate as a new organic heat stabilizer of PVC [22], the modification of lanthanum exchanged montmorillonite by anionic surfactant to improve the thermal stability of PVC [23], and the rare Earth heat stabilizer of maleic acid monoester. It is found that its long-term thermal stability is very close to that of organic tin compounds; Salicylic acid rare Earth heat stabilizer is found to be more stable than most lead salts. Researchers have studied organic weak acid rare Earth salts and found that it not only has excellent heat stability, but also has the ability to improve the alkali corrosion resistance and ease of processing of PVC products.
Rare Earth heat stabilizer has excellent long-term stability, it is difficult to use alone because of its poor anti discoloration effect at the initial stage, which hinders its application in PVC heat stabilizer. With further research, it is found that rare Earth heat stabilizers have good synergistic effect when used in combination with other stabilizers, and can also reduce costs. For example, the study of lanthanum pentaerythri acetate as a heat stabilizer for rigid PVC [24], the synthesis of lanthanum (III) complex with N -(2-aminoethyl) maleic acid radical and its application as a heat stabilizer in PVC [25], the synthesis of lanthanum ricinolate and its influence on the thermal stability and mechanical properties of PVC [26]. Rheological properties and dynamic thermal stability of PVC/LaL 2 /Ca Zn composite stabilizers [27], studies on lanthanum histidine, pentaerythritol and zinc stearate as heat stabilizers for PVC [28], and lanthanum trioxy-purine containing zinc stearate and pentaerythritol as composite heat stabilizers for PVC [29]. The thermal stability of PVC was improved through the synergistic effect of alkaline lanthanum tartrate, zinc stearate and pentaerythritol [30], The effect of lanthanum stearate and calcium zinc stabilizer on the stability efficiency of polyvinyl chloride by dibutyltin dilaurate [31], and the synergistic effect of ethylenediamine bismaleic acid radical and the complex of lanthanum (III) and pentaerythritol on the thermal stability of polyvinyl chloride [32].
Or functionalization with UV resistance, such as, in addition to the advantages of excellent thermal stability, weather resistance and ease of processing, rare Earth heat stabilizers are also potential candidates for industrial stabilizers due to their non-toxic, environmental friendly and other characteristics.
In addition, in 2019, the output of rubber and plastic additives in China exceeded 9 million tons, and China is rich in rare Earth resources, accounting for 36.67% of the world's rare Earth reserves, while the proportion of rare Earth additives is far below 1%.
In addition, little is known about the effects of rare Earth lanthanum and cerium [13], tin and folic acid on the thermal and mechanical properties of PVC hybrid films, and there is no literature report.
Therefore, the study of the effects of TiO 2 modified by folic acid and lanthanum, cerium, tin on the thermal stability and mechanical properties of PVC hybrid films has theoretical significance for developing new functional materials.
Although modified titanium dioxide, cerium stearate, organotin and β-The long-term storage of diketone complex will have an impact on the removal of hydrogen chloride from plasticized PVC [33]. Among them, cerium stearate complex has redox property in the polymer, which can not only inhibit the degradation of PVC, but also improve the long-term storage stability. In order to improve compatibility and heat resistance, Synthesis, characterization and thermal behavior of plastic poly (vinyl chloride) doped with folk acid modified titanium dioxide [13] was reported.
In view of the difference between the outer electronic structures of rare Earth lanthanum and cerium, this work investigated the influence of lanthanum on the thermal and mechanical properties of modified PVC films, and prepared PVC/dioctyl terephthalate/organic tin/lanthanum stearate/folic acid modified nano titanium dioxide hybrid polymer. The polymer soft products can be used to prepare PVC products such as heat resistant, light resistant and cold resistant profiles, pipes, plates and sheets.
When removing hydrogen chloride in the first decomposition stage, the cerium containing PVC film (control sample) is different from the lanthanum containing film, and the initial stability of the lanthanum containing film (α < 1.9%) higher than that of the control, on the contrary (α > 1.9%), the stability of the control is higher than that of the lanthanum containing film.
In addition, the influence of lanthanum containing films on decomposition rate, residue content and peak strength of infrared characteristic group of vapor phase plasticizer at peak temperature was investigated. It showed that the force between rare Earth atoms and plasticizer changed with different lanthanum and cerium atoms. In the range of 350°C-575°C and infrared wave number 2850-3100 cm −1 , the peak height of the lanthanum containing film is significantly higher than that of the control sample. The energy spectrum analysis shows that the peak intensities O and C of the films containing lanthanum are significantly lower than those of the control samples, indicating that the relative concentration of lanthanum atoms on the film surface is lower than that of cerium atoms; The PVC film containing 1 and 3 phrs of lanthanum has a stronger surface force than 5 phrs of lanthanum; The modified PVC composite containing 1 phr of lanthanum has good storage modulus and temperature resistance. The hybrid film has the characteristics of potential functional materials.

Synthesis of FA/NT
With the mass fraction of Folic acid (FA) as 5%, a certain mass of folic acid powder and nano titanium dioxide particles were weighed, dissolved in deionized water, and stirred. Then pour into the same container, mix and stir well, and shake with ultrasonic radiation for 30 min at 40°C. Evaporate, dehydrate, and dry at 105°C to obtain FA (5 wt%)/NT powder [13].

Preparation of PVC film
50 phrs of Dioctyl terephthalate (DOTP), 5 phrs of lanthanum stearate, several phrs of modified FA (5 wt%)/NT powder, 0.5 phr of OT are mixed with heating, stirring and mixing. Continue to add 100 phrs of PVC resin, stirring and mixing. Control the temperature of the first, second, and third chambers of the twin-screw mixer to 175°C, the melt temperature to 170°C, and the rotation speed of 40 rpm. Put the PVC premix into the feeding port, and after mixing for 2-3 min to the shearing force. It rises sharply first, then drops, and then discharges. Using a flat vulcanizing machine, it is pressed to 1 mm at 100°C to obtain a PVC film. Table 1 shows the feed ratios of different PVC films [13]. Composition of different PVC films is shown in table 1.

Thermo-gravimetric analysis (TGA)
Thermo-gravimetric analysis (TGA) was carried out using a thermo-gravimetric analyzer and produced by the TA Instruments, model SDT2960. The measurement was carried out under nitrogen atmosphere at a flow rate of 50 ml min −1 , the temperature was set from 25 to 700°C, with heating rate β of 10°C min −1 shown in figure 1.

Dynamic mechanical analysis (DMA)
Dynamic mechanical analysis (DMA) was performed using a DMA Q800 instrument (TA Instruments Inc., USA) at a fixed frequency of 1 Hz and a temperature range from 30 to 120°C at a linear heating rate of 10°C min −1 . The dynamic storage modulus was plotted.

Tensile strength (TS)
The strength performance of PVC film containing reinforcing agent is calculated in accordance with GB/T 1040. . Among them, the experimental conditions are as follow, the sample size is 5 A (sample cutting), the detection speed is 200 mm min −1 and the fixture spacing is 20 mm.
2.4.5. Thermal aging S1 and L-1 film (size:15.0 × 15.0 × 1.0 mm) was placed in an over in air at 195°C for 0.5-4.0 h, and then record the appearance of PVC film at different times.

Atomic force microscope (AFM)
Atomic Force Microscope (AFM) analysis was conducted on a scanning probe microscope (Veeco Multimode, USA). The silicon probe is used at room temperature, and its typical cantilever length is 180 μm. The rated spring constant and resonance frequency of the probe are 25-60 N m −1 and 120-320 kHz respectively; The radius of curvature of the tip is less than 10 nm.

Scanning electron microscope (SEM)
Scanning electron microscope (SEM) is used with model JSM-6700E and made by the JEOL.

Results and discussion
3.1. TG Figure 2 shows the thermogravimetric curves of different PVC films. From the α-T curves, it can be observed that the thermal decomposition of PVC films goes through two stages. The dehydrochlorination reaction occurs in the first stage. FA acts as a coupling agent in FA/NT, LaSt 3 and OT complexes. The FA active group accidentally combines with nano titanium dioxide, and the compound formed by lanthanum and tin metal inhibits the release of hydrogen chloride [13], and the second stage degradation of the polyethylene structure leads to the formation of aluminum aromatic hydrocarbons and residential carbides. The purpose of adding a thermal stabilizer is to inhibit the formation of Cl radicals or react with them during the pyrolysis process, and prevent the generation of hydrogen chloride. The purpose of adding a thermal stabilizer is to inhibit the formation of Cl radicals or react with them during the pyrolysis process, and prevent the generation of hydrogen chloride. Table 2 is the thermal analysis data of different PVC films. In the first decomposition stage, the initial decomposition temperatures of L-1, L-11 and L-2 are 225, 187 and 184°C respectively. At this time the temperature of L-1 is the highest when the corresponding decomposition rate is 1%, namely 233.2, 216.4 and 214.4°C. The peaks of the DTG curve correspond to the maximum decomposition rates in the first and second stages, respectively. Compared with S0, the maximum decomposition rate of the PVC composite film is significantly reduced, and the degradation temperature of L-1 shifted to the high temperature region. It can be observed from the table that T 5% decomposition temperature of L-1 is 257.7°C, which is much higher than L-11 and L-2. At 700°C, the residuals decomposed by L-1, L-11 and L-2 are 10.5, 9.8, 10.16%, respectively. Among them, the residue of L-1, L-11 and L-2 are Ti, La and Sn containing carbides. From the maximum residue content of L-1, it can be considered that this is one of the reasons for its strong thermal stability. In the first stage, the peak temperatures of L-1, L-2, L-3 and L-11 pyrolysis were 304.9°C-309.7°C, which were higher than the corresponding peak temperatures of 284.2, 292.0 and 286.9°C reported in the literature for modification of PVC, PVCT polymerized castor oil methyl ester (S1), PVCT polymerized soybean oil (S2) and PVCT crystalline ether (S3) [34]. The peak temperatures of P-S, P-SZ and P-SZD at the first stage are 301.4, 298.7 and 297.7°C [35] respectively. The peak temperatures of pure PVC, DA-P-a-50% and DA-P-b-50% in the first stage are respectively 291.5, 228.2 and 248.7°C [36].
The thermogravimetric curves of L-1 and L-3 almost overlap, but the maximum degradation rate decreases significantly compared with S1 near 300°C, indicating that FA/NT or NT inhibits the removal rate of hydrogen chloride from PVC. From the local amplification curve of A, it can be observed that the conversion rate of L-1 with FA is 0%-20% (180°C-290°C) and that of L-3 without FA. This shows that FA/NT has better initial heat resistance to PVC than NT, and also shows that there is interaction between FA and NT particles, lanthanum, tin metal and PVC polar molecules [13]. When the conversion (α) reached 5.0%, the decomposition temperatures of the control sample and L-1 were 260.3°C and 257.7°C, respectively, and the decomposition temperature of the control was 2.6°C higher than that of L-1. The decomposition temperatures were in descending order: control > L-1 > L-11 > L-2 > L-3. In contrast, the decomposition temperature of PVC-TBC2 was reported to be 255.0°C [37] and that of P-SZD was 251.7°C as α was 5.0% [38]. Figure 3 shows the TG-FTIR three-dimensional stereogram of L1 film. Starting from 38.97°C, the heating rate is 10°C min −1 . According to the figure, the lowest decomposition temperature of L1 sample is 308.8°C, the color temperature at the end of reaction is 696.32°C, and the corresponding time is 3994 s, that is, 65.74 min. At Time = 1619s (peak intensity 0.093), 1635s (0.1356, 0.1461), 1656s (0.0487), 1667s (0.2325), 1677s (0.1053), many obvious infrared absorption peaks appeared. We can clearly identify the gas decomposition products through the unique infrared spectrum, such as stretching vibration at 2950 cm −1 , 2798 cm −1 , 2885.9 cm −1 and 2360 cm −1 show HCl and CO 2 , hydroxyl characteristic peaks in 4000-3500 cm −1 , ester characteristic peaks attribute to 1739 cm −1 , 1264 cm −1 and 1101 cm −1 in 1300-1800 cm −1 in the figure [13,35]. The release of esters, water, benzene, hydrocarbons and HCl gas is caused by the pyrolysis of PVC blends. Figure 4 shows FT-IR spectra of hydrogen chloride released from L-1 and Control at different temperatures, where a, b, c, d, e, f, g, h and i are 190°C, 240°C, 290°C, 340°C, 390°C, 440°C, 540°C, 640°C and 700°C respectively. Figure 4(a) is the infrared spectrum in the gas phase when the wavenumber is 2600-3100 cm −1 and the control is pyrolyzed. It can be observed from the figure that in the low temperature range of 190°C-240°C  a Residual carbide at 700°C; Control:C-1; T 1% and T 5% are the temperature at which the film conversion is 1 and 5% respectively. and the high temperature range of 640°C-700°C, The difference between the peak value of overflow gas of L-1 and the control sample is small, while in the 290°C-340°C range, there is a significant zigzag hydrogen chloride gas peak type. In the wave number range of 2900-3000 cm −1 , the peak intensity of the control is higher than L-1, which indicates that the force between the cerium atom and its complex in the control and the C-Cl bond in PVC is weaker than that of the lanthanum atom. Figure 4(b) is the infrared spectrum of the gas phase when the wavenumber is between 500-2000 cm −1 and the control is pyrolyzed. It can be observed from the figure that the vibration peak ν COOR generated by the characteristic groups in DOTP (1739 cm −1 , 1264 cm −1 and 1101 cm −1 ) [13,35] is stronger at 290°C, and the peak intensity of each characteristic peak of the control is slightly higher than L-1. Figure 4(c) shows the infrared spectrum in the gas phase during the pyrolysis of L-1 (1624s) and control (1630s) at the maximum peak of wave numbers between 500-4000 cm −1 , it can be seen that the control sample moves 6 s to the right compared with the maximum peak wavenumber of L-1. It can be observed from the figure that the vibration peak intensity of ester based characteristic groups 1740 cm −1 , 1261 cm −1 and 1107 cm −1 is slightly higher than that of L-1, which indicates that the force between L-1 and the characteristic group ester group in the plasticizer is stronger than that of the control. Figure 5 shows the FT-IR spectra of hydrogen chloride released from the pyrolysis of L-1 and the control at 250°C-698°C. As heated from 250°C to 350°C, the serrated peak generated is hydrogen chloride removal gas, and the peak type of L-1 and the control is slightly different. When the temperature rises from 350 to 575°C, although the difference of hydrogen chloride release (Serrate peak) is small, the L-1 peak intensity is larger than the vibration peak intensity of the control near 2936 cm −1 in the wavenumbers range of 2800-3100 cm −1 , that is, the L-1 peak intensity of the gas phase product of polymerization, condensation, cyclization and other chemical reactions of conjugated polyene generated by PVC removal of hydrogen chloride is larger, which  indicates that in this temperature range in the process of pyrolysis of L-1 compared with the control, strong intramolecular interactions occurred, which led to the complexity of L-1 gas phase products, which was the result of the difference between L-1 and the control in outer electrons. Figure 6 shows the general electronic spectra of different samples. It can be observed from the figure that the relative strength of O and C is much higher than that of L-1 in the control sample at the same binding energy, which indicates that the surface relative concentration of cerium atom is higher than that of lanthanum atom, and the surface relative concentration of Cl atom in L-1 is lower, which indicates that there is a strong attraction between lanthanum atom and chlorine atom.   Figure 7 describes the storage modulus (E′) and loss tangent of PVC samples (δ) change curve with temperature. Tan can be observed from the figure δ The peak temperature of the curve is usually the glass transition temperature (Tg) [39], and the Tg of L-1, L-11 and S1 are 27.0, 27.0 and 26.5°C respectively. This shows that the addition of folic acid does not change the glass transition temperature Tg of PVC composite film. The glass transition temperature of the PVC composite film with folic acid L-1 and without folic acid L-11 is 0.5°C higher than that of the blank sample S1. The Tg changes little before and after modification. When the storage modulus (E′/GPa) is in the range of -100 ∼ 125°C, its values are 8.52 (adding folic acid L-1), 6.05 (L-11) and 3.95 (blank sample S1) respectively. It can be observed that the low temperature storage modulus of the composite film modified by FA/NT increases significantly. When the dosage is 50 phrs, the literature reports that the glass transition temperature  Table 3 shows the mechanical properties of modified PVC. The elongation at break of L-1 composite material is 13.6% lower than L-11, indicating that the rigidity of PVC is slightly enhanced, and the tensile strength is slightly changed, reducing by 1.60%. The tensile strengths obtained from 50 phrs of DEHP, FDCA-BG, FDCA-BDG and FDCA-BTG plasticized PVC were 13.28, 16.6, 13.4 and 16.0 MPa, respectively, and the results for L-1 and L-11 were slightly higher than the literature values. The corresponding elongations at break were 220, 253, 215 and 220%, respectively [40], which were similar to the literature values. 3.6. AFM Figure 8 shows the surface roughness images of L-1 and S1 films, where a and b are sample L-1, c and d are sample S1. The (a) is the 2D main morphology of L-1, and the (b) is the 3D main morphology. It can be observed from the figure that there is no oval round bright spot on the surface of L-1 film after modification. As shown in figure 8(a) and (b), an additional white oval round bright spot appeared on the surface of S1 film before modification, as shown in figure 8(c) and (d).

Tensile strength
In contrast, the literature reported that Root mean square (RMS) roughness of nanocomposite membranes (modified PVC with 3% alumina addition) decreased from 11.4 nm to 8.7 nm compared to before modification (neat PVC ). The RMS of L-1 also decreased significantly after modification compared to S1, which is consistent with the trend reported in the literature [42].   Table 4 is the thermal aging time and color change of S1 and L-1 films at 195°C and air atmosphere. It can be observed from the table that the complete blackening time is 120 min when S1 is heated at constant temperature, and there is almost no change in appearance after 90 min of storage. Moreover, the film carbonizes rapidly in the 90-120 min range of thermal aging. However, the FA/NT modified PVC film L-1 has almost no change in appearance when it is stored for 150 min. Between 180-240 min of thermal aging, the appearance slowly deepens until carbonization, indicating that hydrogen chloride is slowly released during this period. This shows that the additives FA/NT, lanthanum and tin inhibit the rate of hydrogen chloride release. It can be observed that L-1 can significantly prolong the discoloration of PVC compared with S1, and has relatively strong heat stability. Figure 9 shows the surface morphology of the PVC composite film under different loads of modified FA/NT at 200 nm. Figure 9(a) and (b) are the surface images of L-1 and L-2 respectively. It can be observed from the figure that the surface is generated from concave convex peaks, especially L-2, which indicates that a strong surface working force is generated between FA/NT particles and PVC polymers, accompanied by the increase of FA/NT addition. Figure 9(c) shows obvious agglomerated particles produced on the surface of L-3, which leads to the disappearance of surface concave convex peaks or interaction forces. It can be observed that the aggregation rate of the modified FA/NT on the PVC composite film is very low at L-1, the degree of dispersion is L-1 > L-2 > L-3, indicating that 1-3 phrs of FA/NT and the uncertainty of PVC polymers are better [13]. The aggregation of FA/NT is caused by the strong interaction between its particles and its PVC polymer. As the content of FA/NT increases, its incompatibility with the PVC matrix is enhanced. This shows that L-3 depends on the poor dispersion of nano particles, which leads to the decline of heat resistance of PVC film. Generally, the larger the aggregated nano particles are, the worse the mechanical and thermal properties of polymer materials are. One phr of FA/NT particles can be evenly distributed on the surface of PVC composite, thus improving its heat resistance.

Conclusions
Through the investigation of the characterization and mechanical properties of PVC/DOTP/OT/LaSt 3 /FA/NT composites, the following conclusions were obtained: (1) In the first stage of dehydrochlorination, the cerium containing film (control sample) is larger than the lanthanum containing film. As the temperature is lower than 242.7°C, the thermal stability of the lanthanum containing film is greater than the control. When the decomposition rate is 1%, 1 phr of the complex (folic acid modified nano titanium dioxide) and 5 phrs of the lanthanum metal soap are added to form the PVC film. The minimum temperature of thermal decomposition is higher than that of the control. At the peak temperature, the corresponding decomposition rate was 24.5% lower than that of the control sample, and the corresponding residue amount was 8.38% higher than that of the control. The thermal stability of the modified PVC was improved.
(2) The strength of the infrared characteristic group of the gas phase part of the plasticizer, The control sample is larger than the lanthanum containing film, which indicates that the force between lanthanum and plasticizer is greater than the control sample. In the range of 350°C-575°C and wavenumbers 2850-3100 cm −1 , the peak height of the lanthanum containing film is significantly higher than that of the control sample, Presumably, it is the difference of outer electrons.
(3) The energy spectrum analysis shows that the peak intensities O and C of the film containing lanthanum are obviously lower than those of the control sample, indicating that the relative concentration of lanthanum atoms on the film surface is lower than that of cerium atoms.
(4) Thermal aging shows that the time of complete discoloration is significantly longer than that of single organotin film.
(5) Scanning electron microscope analysis showed that the films containing 1 and 3 phrs of lanthanum produced significant surface forces of concave convex peaks, as the amount of lanthanum added increases, the surface action decreases significantly.
(6) The modified composite containing 1 phr of lanthanum has the best storage modulus and good lowtemperature resistance. The hybrid film has the characteristics of potential functional materials. Table 4. Thermal aging time and color change of S1 and L-1 (195°C, air).