Waste Management 121 (2021) 52–58
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Kinetics of thermal oxidative degradation of poly (vinyl chloride) containing Ca and Sn at low temperature Yi-heng Lu a,⇑, Bing Wang a, Meng-yao Xue a, Yu-wei Lu b a b
College of Chemical Engineering, Anhui University of Science and Technology, 232001 Huainan, China Laboratoire de Chimie Physique, Universite de Paris Sud, 91405 Orsay Cedex, France
a r t i c l e
i n f o
Article history: Received 27 July 2020 Revised 4 October 2020 Accepted 18 November 2020 Available online xxxx Keywords: Polyvinyl chloride Calcium Tin Thermal oxidative degradation kinetics Thermal discoloration
a b s t r a c t Calcium metal soap and polyol (dipentaerythritol) additives are replacing or partially replacing organotin in poly(vinyl chloride) (PVC) heat stabilizers due to their low cost, nontoxicity and safety. Therefore, investigating the low-temperature thermal oxidative degradation of stabilized plasticized PVC from the source is essential for recycling. This work uses isothermal thermogravimetry to investigate the thermal degradation process and isothermal discoloration of PVC/calcium metal soap/dipentaerythritol/orga notin soft products with excellent heat resistance at 453–503 K and under air atmosphere. The chemical kinetics method is used to fit a single equation model of mass loss and time during the thermal oxidation degradation of PVC, and the kinetic equation obtained is: ln(1 a) = 3.83 103exp (6834.4/T)t. When the temperature is 453–503 K, the calculation results are basically consistent with the experimental data and are independent of the heating rate and temperature changes. In addition, the isothermal discoloration of different PVC materials was tested under air atmosphere at 468 K. The results show that when the test material is PVC/calcium metal soap/dipentaerythritol/organotin, the heat aging time to become completely blackened is longer than that of the blank sample, which indicates a strong interaction occurs between Sn, Ca and dipentaerythritol complexes and PVC molecules, inhibiting the release of hydrogen chloride. At the same time, in order to recover PVC and prevent it from carbonization, if the temperature is set to 486 K, the thermal oxidation degradation time of PVC should be less than 130 min. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Modified PVC added with plasticizers, heat stabilizers, and reinforcement agents will eventually enter the environment as waste plastics (Yu et al., 2016) in large quantities, which will significantly increase the environmental pressure. Meanwhile, metal soaps and polyol additives, due to their low price, non-toxicity and safety have replaced or partially replaced organotin (OT) as the heat stabilizers of PVC. Investigating the thermal oxidative degradation pathways of stabilized plasticized PVC from the source, avoiding the generation of dioxin toxicants, and improving recycling are crucial. In this paper, the chemical kinetic method is used to study the degradation phenomenon of modified PVC, and the degradation reaction mechanism is used to determine the degradation reaction speed and the influence of various factors (concentration, temperature, etc.) on the degradation rate, thereby revealing the nature of
⇑ Corresponding author. E-mail address: [email protected]
(Y.-h. Lu). https://doi.org/10.1016/j.wasman.2020.11.019 0956-053X/Ó 2020 Elsevier Ltd. All rights reserved.
the degradation phenomenon, which is more effective to control the speed of chemical reaction. The use of chemical reaction kinetic method to study the thermal degradation of PVC to remove hydrogen chloride mainly includes isothermal method and nonisothermal thermal degradation method. The pyrolysis atmosphere is inert gas or air medium. These methods are widely used in PVC thermal degradation. (Ma et al., 2002; Xu et al., 2020; Mahmood and Qadeer, 1994; Aracil et al., 2005; Jiménez et al., 2000; Behnisch and Zimmermann, 1988). and research hotspots include degradation in inert media (Jakic´ et al., 2013; Wu et al., 2014; Nishibata et al., 2020; Castro et al., 2012; Wang et al., 2019; Masuda et al., 2006). For example, The thermal characterization and nonisothermal thermal decomposition kinetics of PVC/DOTP/ZnSt2 /Dip/OT composite materials in nitrogen were studied. The average apparent activation energy obtained in the first and second stages were 127.5 and 261.6 kJ mol1 (Xue et al., 2020), etc. But the thermal oxidative degradation of modified PVC is more complicated than pyrolysis (Boughattas et al., 2016a, Boughattas et al., 2016b; Mohammed et al., 2015; Krongauz et al., 2011; Grimes et al., 2006; Marongiu et al., 2003). Not only thermal degra-
Waste Management 121 (2021) 52–58
Yi-heng Lu, B. Wang, Meng-yao Xue et al.
the calculation results are basically consistent with the experimental data and are independent of the heating rate and temperature changes. In addition, the isothermal discoloration of different PVC materials was tested under air atmosphere at 468 K. When the test material is PVC/calcium metal soap / dipentaerythritol/ organotin, the heat aging time to become completely blackened is longer than that of the blank sample, which shows that there is a strong relationship between organotin, calcium metal soap and dipentaerythritol complex and PVC molecules. To recover PVC and prevent its carbonization, if the temperature is set to 468 K, the thermal oxidation degradation time of PVC was measured.
dation must be considered but also the oxidation of conjugated polyenes. Because it is difficult to identify the main products of degradation, the mechanism of thermal degradation of PVC should include free radicals. The formation of peroxy radicals can oxidize the normal structure, and the inhibition of free radicals by additives should also be considered. The hydrogen chloride elimination reaction is usually accelerated in an oxygen medium (Paciorek et al., 1974; Vrandecˇic´ et al., 2004) The heat stabilizer of calcium/ zinc ricinoleate containing phosphate groups has been investigated. PVC/DOTP/CaSt2/ZnSt2 film, PVC/DOTP/EFC(epoxy fatty acid calcium)/ZnSt2/ESBO (epoxy soybean oil) film, PVC/ DOTP/LRA-Ca/ LRA-Zn film and PVC/DOTP/LPPRA–Ca /LPPRA-Zn film were subjected to thermal oxidative aging experiments, and the time to complete blackening (carbonization) was 40, 70, 80, and 120 min (Wang et al., 2018). The isothermal discoloration of PVC/DOP/ CaCO3 / stabilizer (50/50/15/3phr) composite film at 180 °C and in air was investigated. When the stabilizer CaSt2/Zn3Ur2/DBM is 1.8/1.2/0.9 parts, the time to be completely blackened is 170 min (Ye et al., 2018). The similarities with this study are 180 °C, air atmosphere, and isothermal discoloration; the difference is Zn3Ur2, no kinetic study, and no PVC recovery as the goal. The isothermal thermal degradation kinetics of composite films with different molar ratios of PVC/CPE at 240, 250, 260, 270 °C and in air was studied. The activation energy obtained is 57–153.0 kJ mol 1 , and the degradation reaction conforms to the PT mechanism model (Vrandecˇic´ et al., 2005). The isothermal discoloration of PVC/DOP/amitrole/ZnSt2/ESBO (100/30/1.0/1.0/4phr) composite film at 180 °C and in air was investigated, and the complete blackening time is 120 min (Chen et al., 2019). Despite the fact that there are many reports in the literature, metal soaps (such as calcium and zinc) and polyols (such as pentaerythritol, etc.) are currently used as candidate additives for improving the thermal stability of PVC. Their role is mainly to absorb the decomposition and release of PVC. HCl or replacing the unstable parts of partially decomposed PVC macromolecules plays a stabilizing effect. Among them, metal soap has a certain impact on the thermal degradation performance of PVC. It is generally believed that defects such as tertiary chlorine, allyl chloride and the head structure in PVC destroy its thermal stability. Due to organotin and metal soap (Balköse et al., 2001), rare earth stabilizers or polyol additives alone are not effective. To overcome the impact of organotin on the environment, metal soaps, rare earths, polyhydroxy compounds (such as pentaerythritol) and nanometal particles are mainly used to form polymer composites (Wang et al., 2020; Mohammad et al., 2020; Shen et al., 2007); partial or complete replacement of organotin (OT) has become a future trend. All stabilizers as inhibitors of hydrogen chloride release will have a greater impact on the thermal degradation kinetics and recycling of the final waste plastics. At present, we have found that adding dipentaerythritol can change the degradation pathway of PVC (Wang et al., 2020), by increasing the methane content in the degradation products and avoiding the formation of aromatic hydrocarbons and dioxins. The thermal oxidation degradation kinetics of PVC with CaSt2 and dipentaerythritol additives has never been reported in the literature. Plastic additives are becoming more nontoxic, efficient and environmentally friendly, and it is increasingly important for the PVC industry to solve the chemical cycle from the source. This work uses isothermal thermogravimetry to investigate the thermal degradation process and isothermal discoloration of PVC/calcium metal soap / dipentaerythritol/organotin soft products with excellent heat resistance at 453–503 K and under air atmosphere. The chemical kinetic method was used to fit a single equation model of mass loss and time in the process of PVC thermal oxidation degradation. The kinetic equation obtained is: ln(1 a) = 3. 83 103exp (6834.4/T)t. When the temperature is 453–503 K,
2. Experimental section 2.1. Materials Industry qualified PVC resin, i.e., DG-700, was purchased from Dagu Chemical Co, Tianjin, China. Dioctyl phthalate (DOTP) of AR grade was obtained from BASF Chemical Co, Tianjin, China. Methyl tin mercaptan (OT), i.e., DX-181, was used as the heat stabilizer and purchased from Hangzhou Dongxu Additives Co. Industry qualified calcium stearate (CaSt2) was obtained from Zibo Luchuan Rubber Additives Co. Dipentaerythritol (Dip) (content of 90%, reagent grade) was purchased from Shanghai Maclean Biochemical Technology Co. 2.2. Preparation of PVC films First, PVC, DOTP, OT, CaSt2 and Dip were measured respectively, and their feed ratio was 100:50: 0.5–1.0: 0.5–1.0:0.5–2.0 (phrs), respectively. They were dissolved and mixed, and kneaded in a mixer under the condition of 443–448 K. The screw speed was 40rmin1 during mixing for 3 ~ 5 min, and when the torque reached a minimum value, it was increased rapidly for approximately 0.5 min, and then shut down and pressed into a film (1.0 mm) at 373 K. After removal, PVC0, PVC1, PVC2, PVC3, PVC4, PVC5, PVC6 and PVC7 composite films were obtained respectively, and the formulation of the PVC films is shown in Table 1. 2.3. Isothermal TG Isothermal TG analysis was carried out by using a thermogravimetric analyzer (TA Instruments, SDT2960). The investigations were carried out under air atmosphere at a flow rate of 50 mlmin1. The temperature was set at 453 K, 493 K, and 503 K, respectively. 2.4. Thermal aging test The discoloration test was carried out in an oven maintained at 468 K in air. The samples were subjected to static thermal aging according to the standard ASTM D2115-10. Twelve time periods (0–150 min) were chosen to investigate the degradation process. 3. Results and discussion 3.1. Integration mechanism function g(a) From the perspective of plasticization (Jiménez et al., 2000) and the effect of heat and oxygen on thermally stabilized PVC materials, low-temperature removal of hydrogen chloride, exploring the possibility of recovering hydrocarbons, can lead to thermal degradation and recycling of plasticized PVC materials from the PVC formulation or at the source. The plasticized and stabilized polymer 53
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Waste Management 121 (2021) 52–58
Table 1 The formulation of PVC films. Additives
Polymer Plasticizer Costabilizer Auxiliary Stabilizer
PVC DOTP CaSt2 Dip OT
100 50 0.5
100 50 1.0
100 50 0.5 1.0 0.5
100 50 0.5 2.0 0.5
Note: Blank sample-PVC0, Control sample-PVC1.
gð0:15Þ ¼ AeEa=RT t 0:15
usually undergoes the same degradation reaction as the original polymer. However, in most cases, the degradation rate will vary according to the nature of the additive, the degree of compatibility with the polymer, and the interaction with the degradation product. Schema 1 is the low-temperature thermal degradation reaction of stabilized PVC6 soft products in air: Thermal oxidative degradation under isothermal conditions, due to many influencing factors, such as heat flow, air, PVC resin, DOTP and additives, CaSt2, Dip and OT degradation inhibitors, leads to a very complicated thermal degradation reaction. As the temperature is constant, the degradation becomes a heterogeneous solid-state reaction. Since pure air with a constant flow rate is used as the carrier gas in the experiment, and the gasification thermal degradation reaction is carried out under normal pressure, the reaction rate (Vyazovkin et al., 2011) can be expressed as:
da ¼ kðTÞf ðaÞ dt
Dividing both sides of Eq. (4) and Eq. (5), we can obtain Eq. (6):
gðaÞ AeEa=RT t ¼ gð0:15Þ AeEa=RT t 0:15
In the formula, the mass conversion rate a = (m0 m)/ (m0 m1), where m, m0, and m1 are the mass of the sample at t, the initial mass and the mass after the test is completed, respectively. f(a) is the reaction mechanism function in differential form and k(T) is the Arrhenius rate equation. From Eqs. (1), (2) and (3) can be obtained:
da ¼ 0 f ðaÞ
gðaÞ t ¼ gð0:15Þ t0:15
gðaÞ ¼ AeEa=RT t
da 0 f ðaÞ
A series of a values is taken and a standard curve of a ~ t/t0.15 is made. These curves have nothing to do with the kinetic parameters and heating rate and are only related to the integral mechanism function g(a). A theoretical curve of a ~ t/t0.15 of different reaction mechanism functions g(a) is drawn and then compared with the experimental curve. The simulated calculated value is mostly consistent with the experimental curve, and this is exactly the reaction mechanism function in line with the degradation reaction.
Table 2 is derived from geometric factors, diffusion factors or a combination of the two, and the common differential and integral forms of solid-phase reaction mechanism functions f(a) and g(a) (Vyazovkin et al., 2011). Since the products generated during the thermal oxidative degradation of PVC soft products are not only related to heat and oxygen but also closely related to the composition and additives of PVC, the reaction is extremely complicated and it is difficult to describe with a clear physical and chemical process. To this end, this paper establishes a macroscopic chemical kinetic model. For an isothermal solid-phase reaction, assuming that the pre-exponential factor A and activation energy Ea remain unchanged with the progress of the reaction, Eq. (7) can be obtained from Eq. (6):
da Ea ¼ AeRT dt f ðaÞ
¼ g ðaÞ into Eq. (3), we obtain Eq. (4): 3.2. Calculation of kinetic parameter
In the formula, g(a) is the integral mechanism function. From the frequency factor A of Eq. (4) and the expressions of apparent activation energy Ea and g(a), the relationship of a-t can be established. To solve g(a) of the solid isothermal oxidative degradation reaction, the isothermal model fitting method can be used (Gotor et al., 2000; Koga, 1998; Koga et al., 1998), a series of preparation curves can be designed, and the differential thermogravimetric data can be used to judge the followed solid thermal oxidative degradation reaction form. Specifically, for a fixed value of a, usually a = 0.5 because the conversion rate of PVC6 in air and at 453– 493 K in this test is a less than 0.20, the fixed value a = 0.15 is selected, and Eq. (5) can be obtained from Eq. (4):
Fig. 1 shows the initial mass loss and time curves of isothermal oxidation degradation of PVC6 to remove hydrogen chloride. Among them, the maximum mass loss a at 453, 493 and 503 K is 0.055, 0.173 and 0.237 respectively. The corresponding degradation times are 58.47, 58.7 and 59.0 min, respectively. Affected by the oxygen in the air, the plasticizer DOTP in the polymer can easily react with oxygen to generate water and then undergo degradation reaction in addition to gasification. In addition, the carbon, hydrogen and chlorine in the polymer also undergo the action of oxygen molecules. Usually, in the air, it participates in the thermal degradation reaction as a double-radical oxygen, increasing the initial degradation rate. However, oxidation in the later stage of degradation promotes the carbonization reaction,
Schema 1. Schematic diagram of the thermal degradation process of PVC6. 54
Waste Management 121 (2021) 52–58
Yi-heng Lu, B. Wang, Meng-yao Xue et al. Table 2 Kinetic mechanism function of common solid phase decomposition reaction. No
1 2 3 4 5 6 7 8 9
First-order reaction One-dimensional diffusion Two-dimensional diffusion Valensi equation Three-dimensional diffusion Jander equation Phase-boundary controlled reaction Phase-boundary controlled reaction Avrami-Erofeev Avrami-Erofeev Avrami-Erofeev
F1 D1 D2 D3 R2 R3 A2 A3 A4
(1 a) 1/(2a) [ln(1 a)]-1 1.5[1 (1 a)1/3]-1(1 a)2/3 2(1 a)1/2 3(1 a)2/3 2(1 a)[ln(1 a)]1/2 3(1 a)[ln(1 a)]2/3 4(1 a)[ln(1 a)]3/4
a2 a+(1 a)ln(1 a) [1 (1 a)1/3]2 1 (1 a)1/2 1 (1 a)1/3 [ln(1 a)]1/2 [ln(1 a)]1/3 [ln(1 a)]1/4
gðaÞ ¼ kðT Þt
In Eq. (8), g(a) is the integral mechanism function of F1, R2 and R3 respectively, that is, g(a) = ln(1 a), 1 (1 a)1/2 and 1 (1 a)1/3 respectively. Substituting the isothermal conversion rate and time data at different temperatures into the above model and drawing g(a) ~ t as a straight line, its slope is k(T). Ea
kðTÞ ¼ AeRT
As the temperature is 453–503 K, for R2 and R3 according to Eq. (8), 1 (1 a)1/2 = k(T)t and 1 (1 a)1/3 = k (T)t curves are made in the relationship of 1 (1 a)1/2 vs. t and 1 (1 a)1/3 vs. t. Furthermore, the slopes of the straight lines of the R2 model at 453, 493, and 503 K can be obtained, and k(T) is 0.00053, 0.00164, and 0.00235 min1, respectively, and the corresponding linear coefficients R2 are 0.9999, 0.9905, and 0.9945, respectively. According to the R3 model, the slopes of the straight lines obtained at 453, 493 and 503 K can be obtained as k(T) = 0.00036, 0.00111 and 0.0016 min1, and the corresponding linear coefficients R2 are 0.9999, 0.9913 and 0.9952 respectively. Assuming Ea is constant, according to the Arrhenius equation, ln k(T) = ln A – Ea/RT, and in view of the R2 and R3 models, the rate constants lnk and 1/T are not linear. Therefore, the possibility of R2 and R3 mechanisms in the thermal oxidative degradation of PVC6 material is excluded. Substituting a series of different values of a and t into Eq. (8) and since g(a) = ln(1-a), g(a) ~ t curves are made and the slope k(T) is obtained by the least square method. The rate constants of the thermal oxidative degradation reaction of PVC at 453 K, 493 K and 503 K are obtained. The k(T) values are 0.00108, 0.00345 and 0.00501 min1, and the corresponding linear coefficients R2 are 0.9999, 0.9928 and 0.9966. Taking the logarithm of both sides of Eq. (9), Eq. (10) is obtained:
Fig. 1. Curves of initial mass loss vs time.
prevents the penetration of oxygen, and slows down the release of hydrogen chloride. Fig. 2 shows the a ~ t/t0.15 curves of PVC6 thermal degradation at different temperatures. The a ~ t/t0.15 experimental curves of degradation at different temperatures were drawn by Eq. (7), and the theoretical curves of a ~ t/t0.15 calculated from the 9 common mechanism functions in Table 1 are also listed in Fig. 2. It can be seen from the figure that when the temperature is 493 and 503 K, the experimental curves of a ~ t/t0.15 as PVC6 degraded are more consistent with the theoretical curves of F1, R2 and R3 in Table 2. It shows that in this temperature range, the possible integral mechanism functions g(a) of the initial thermal degradation reaction of PVC6 are ln(1 a), 1 (1 a)1/2 and 1 (1 a)1/3, respectively. Eq. (8) can be obtained from Eq. (4), where k(T) is the Arrhenius equation as shown in Eq. (9):
lnkðTÞ ¼ lnA
The Eq. (11) can be obtained according to Eq. (10) .
lnA ¼ lnkðTÞ þ
In a certain temperature range, if the reaction mechanism remains unchanged, the activation energy Ea remains unchanged, and the logarithmic value of the reaction rate constant ln k(T) is plotted against 1/T as a straight line. The slope is -E/R, and the ln A can be obtained according to Eq. (11). The kinetic parameters are solved according to the above method, and the logarithmic value of the reaction rate constant k(T) is plotted at different reaction temperatures versus 1/T to obtain the results Ea and R2. Fig. 3 shows the lnk(T) ~ 1/T curve of the thermal degradation reaction of PVC6 at different temperatures. Using the least square method, the pre-exponential factor A obtained is 3.83 103 min1 and the average apparent activation energy Ea obtained is 56.82
Fig. 2. Curves of a vs t/t0.15 (Note: Curves-theoretical model; Circle and squareexperimental value, where ( ) 503 K; ( ) 493 K). 55
Yi-heng Lu, B. Wang, Meng-yao Xue et al.
Waste Management 121 (2021) 52–58 Table 3 Comparison of activation energy values of PVC pyrolysis obtained by previous studies.
Fig. 3. Plot linear curve of k(T) vs 1/T.
6834:4 Þt T
Activation energy/kJ mol1 First stage
This work Arkıxs and Balkose (2005) Gupta and Viswanath (1998) Ma et al (2002) Vrandecˇic´ et al. (2005) Shen et al (2007) Tüzöm Demir and Ülütan (2015) Krongauz et al (2011) Li et al. (2017a,b) Wang et al.(2017) Wei et al.(2012)
56.82 (180–230 °C) 58.0 (70–250 °C) 90.1(60–400 °C) 130 (180–350 °C) 52–136 (240–270 °C) 139 (150–180 °C) 23.1–159.1 (140–160 °C)
Li et al. (2017a,b) Gamage et al.(2009) Wang et al.(2018) Li et al.(2014) Ye et al.(2019)
kJmol1, and the linear coefficient R2 is 0.9919. The literature reported that the activation energy Ea obtained by thermal degradation of the film produced by the PVC/DOP/LSN117 component is 58.0 kJmol1 (Arkıxs and Balkose, 2005), and the Ea values of the two are similar. The activation energy of the initial zone of thermal degradation of PVC containing PVC/DOTP plasticizer is 79.6 kJmol1 (Tüzöm Demir and Ülütan, 2015). Gupta and Viswanath (1998) found the Ea as 90.1–366.5 kJ mol1, Jimenez et al. (1996)) found it as 90–170 kJ mol1, Audouin et al. (1992) found it as 103–151 kJ mol1, and Xue et al. (2020) found it as 127.5 kJ mol1. Therefore, the kinetic equation between the mass loss a and the time t during the thermal oxidation degradation of PVC6 in the range of 453–503 K is shown in Eq. (12):
ln ð1 aÞ ¼ 3:83 103 expð
59.1–152.6 (170–220 °C) 139.1 (Kissinger) 115–128 (a = 0.30) 98.46 (a = 0.30–0.37), 111.24(a = 0.50– 0.55) 101.4–115.0 (Kissinger) 49–83.0 (100–130 °C) 99–116 (a = 0.30) 101.4–137 (Kissinger) 118.2–129.0 (Kissinger)
in the results. The PVC samples, media, test conditions and kinetic methods may be responsible for these significant differences.
3.3. Thermal oxidation discoloration To better observe the surface color change during thermal oxidation degradation of modified PVC under air atmosphere at a low temperature of 468 K, we investigated the effect of the combination of additives, such as plasticizer DOTP, CaSt2, Dip and OT, on the surface discoloration (further reaction of conjugated polyene) in the removal of hydrogen chloride from PVC. Table 4 shows the surface discoloration test of different PVC films after isothermal aging. It can be seen from the table that PVC0 is not added with heat stabilizer. When the aging time is 10 min, the appearance of the sample is brown, and it quickly changes to black (carbonization) at 30 min. After adding 1 phr of OT stabilizer, the PVC1 sample completely turned black (carbonized) after 110 min. Comparing PVC2 and PVC3, it can be seen that 0.5–1.0 phr of organic additive Dip cannot replace 0.5 phr of OT stabilizer. As the amount of Dip increases from 0.5 phr to 1.0 phr, the stability is slightly improved, but the stabilizing effect is not as good as PVC1. Compared with PVC1, the discoloration of PVC4 and PVC5 shows that the addition of 0.5 phr of CaSt2 can replace 0.5 phr of OT, that is, PVC4 and PVC1 have the same heat resistance and similar effects. If CaSt2 is increased by 0.5 phr, the heat aging time for PVC5 to become completely black is 130 min, which is 20 min longer than that for PVC4 and PVC1. From the heat aging times of PVC5, PVC6 and PVC7, it can be seen that adding 0.5 phr of CaSt2 can significantly improve the stabilizing effect of PVC4 (0.5 phr of OT + 0.5 phr of CaSt2) and inhibit the formation of a PVC conjugated polyene structure, but the addition of organic additives, such as Dip, does not increase the stabilizing effect even at 2.0 phr, e.g., PVC7 compared to PVC6. Therefore, it can be seen from the above thermal oxygen aging test that PVC0 has difficulty recovering hydrocarbons due to the rapid carbonization. The carbonization sequence is: PVC6 > PVC7 > PVC5 > PVC4 = PVC1 > PVC3 > PVC2 > PVC0. It can be seen that when subject to 468 K thermal oxidation treatment, the time for PVC6 to avoid carbonization is less than 130 min.
To visually show whether the obtained kinetic equation is reasonable, Fig. 4 is a comparison between the calculated values of the thermal oxidation degradation (a) and (t) of PVC6 calculated according to Eq. (12) and the experimental curves. The curve is the experimental value, and the data point is the calculated value. It can be seen from the figure that the calculated values and experimental data are in good agreement at 453, 493 and 503 K. As an isothermal kinetic equation, Eq. (12) can provide a theoretical basis for the engineering scale-up of PVC6 polymer thermal degradation and reactor design. Table 3 shows the comparison between the current data and the data obtained in previous studies and there are obvious differences
Fig. 4. Comparison of calculated and experimental values (Note: Curves-experimental values; ( )503 K; ( ) 493 K; ( ) 453 K -calculated values). 56
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Yi-heng Lu, B. Wang, Meng-yao Xue et al. Table 4 Thermal aging test of different PVC films. Sample No
Composition Feeding ratio
OT/ Dip 0.5/0.5
Thermal aging time/min 0
Appendix A. Supplementary material
The thermal degradation behavior of PVC soft products containing dioctyl terephthalate (DOTP)/calcium stearate (CaSt2)/dipentaerythritol (Dip)/organic tin (OT) in air atmosphere were investigated by using isothermal thermal analysis methods. The chemical kinetics method was used to fit a single equation model of mass loss and time in the process of PVC thermal oxidation degradation. The results show that if PVC6 is degraded at 453, 493 K and 503 K, the appropriate kinetic equation is: ln(1 a) = 3.83 103exp (6834.4/T) t. When the temperature is in the range of 453–503 K, the calculation results are in good agreement with the experimental data and are basically independent of the heating rate and temperature changes. In addition, the influence of different compositions on the isothermal discoloration of PVC was tested under air atmosphere at 468 K. The results show that when the test materials are PVC5, PVC6 and PVC7, the heat aging time to become completely blackened (carbonization) is better than the control sample PVC1. This shows that OT, CaSt2 and Dip complexes have strong interactions with PVC molecules, which inhibit the release of hydrogen chloride gas. At the same time, in order to recover PVC and prevent its carbonization, if the temperature is set to 468 K, the thermal oxidative degradation time of PVC should be less than 130 min. In a word, the discoloration and carbonization of PVC material seriously hinder the recovery and utilization of PVC. In essence, it is the rapid reaction of conjugated polyenes generated by hydrogen chloride removal under high temperature, such as the formation of macrocyclic aromatic hydrocarbons and coking. In the presence of OT, CaSt2 and Dip, low temperature oxidation degradation of plasticized PVC may inhibit the generation of macrocyclic aromatic hydrocarbons, improve the recovery rate of hydrocarbons, and reduce environmental hazards. In addition, the thermal oxidation kinetics and thermal discoloration data obtained in this research are expected to provide basic data for the scale-up of the recovery of PVC.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements Financial support from the National Natural Science Foundation of China (grant number: 21775002) are gratefully acknowledged. 57
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