Molecular chain model construction, thermo-stability, and thermo-oxidative degradation mechanism of poly(vinyl chloride)

Abstract

Thermo-oxidative degradation of poly(vinyl chloride) (PVC) is inevitable during its processing. We focused on the relationship between the structure and properties of PVC, as well as on the thermal-oxidative degradation mechanism of PVC. Two models, the unsaturated and oxygen-containing structure models, were successfully constructed by pretreatment under different atmospheres. The thermal stability of treated PVC was determined by thermogravimetric analysis. An X-ray photoelectron spectroscopy (XPS) processing method was also used to analyze the type of functional groups that may be produced during thermal-oxidative degradation, whereas XPS and Fourier-transform infrared (FTIR) spectrometry were used to determine the trend in the content variation of functional groups of samples. The two structure models revealed that the thermal properties were greatly influenced by oxygen-containing groups compared with the unsaturated structures, and the position of oxygen molecule attack on the PVC main chain is not primarily located on the unsaturated structure or the α carbon atom of the unsaturated sides. XPS and FTIR results can possibly provide evidence for the thermo-oxidative degradation mechanism of PVC. Moreover, a carbonyl group was generated when the methylene or methine structure was attacked by an oxygen molecule, rendering the PVC resins unstable.

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1. Introduction

Poly(vinyl chloride) (PVC), one of the most common thermoplastics nowadays, has been receiving extensive attention for its modification, application, and degradation.^1–9 The relationship between thermal instability and oxygen-containing groups in PVC chains has also been reported in the past decades. In all cases, the degradation of the primary chain of PVC is caused by the zip elimination of HCl at low temperature,^10–12 as well as by the inevitable formation of conjugated polyene sequences in the polymer chain during processing and use of PVC. In addition, numerous studies^13–16 on thermo-oxidative PVC degradation have been published, and the presence of oxygen causes thermal instability of the material. Some researchers^17,18 consider that the formed polyene segments are highly reactive species and that they easily react with oxidizing agents, such as oxygen and ozone. Other researchers^19–21 have also proposed that dehydrochlorination of PVC is mainly attributed to the oxygen-containing groups, such as the –C(O)–CH=CH–CHCl– (CAG) group. Furthermore, hydroperoxides, hydroxyls, and carbonyls are generated in polymer molecules during thermal treatment under oxygen atmosphere.^{12,14,18,22,23} By contrast, other researchers²⁴ never clearly found the CAG group in the polymer molecules via experimental approach using ¹H-NMR, and whether this observation is attributed to carbonyl,^14,25 peroxy, or other irregular structures^26–28 is unclear.

Minsker^13,20,21 and Lisitskii¹⁹ assumed that the presence of CAG structure causes the instability of PVC resin, whereas Szarka¹⁴ showed that PVC instability can be attributed to the generation of hydroperoxides, carbonyls, and hydroxyl structures under thermal oxidative degradation (Scheme 1).^{12,13,16,20–23} The main difference between these two typical mechanisms is the position of oxygen molecule attack on the PVC chain. Mechanism I demonstrates that the position of oxygen molecule attack on the main chain of PVC resin is on the unsaturated structure or α carbon atom of the unsaturated sides. By contrast, mechanism II shows that the methylene or methine structure in PVC chains is attacked by oxygen molecule. The thermo-oxidative degradation mechanism has been intensively investigated; however, the proposed mechanisms remain confusing because of the difficulty in identifying the oxygen-containing groups in original PVC.

Scheme 1 The thermo-oxidative degradation mechanism of PVC.

The thermo-oxidative degradation of PVC, which is a commercially important polymer, is inevitable; however, detailed systematic investigations on this crucial subject are lacking. Given the difficulty in this endeavor, many researchers present different points of view on the thermo-oxidative degradation mechanism of PVC. Verifying the different mechanisms mentioned above and providing evidence to the possible mechanism of PVC degradation means of experimental method is indispensable. To address the aforementioned challenges, we constructed a model to prove the hypothetical mechanism by using experimental data, and this approach is essential to establish the relationship between segment structure and thermal property and ultimately to study the mechanism of the thermo-oxidative degradation of PVC. Herein, the unsaturated structure model (USM) and the oxygen-containing structure model (OSM) were constructed by dynamic and isothermal measurement under nitrogen and oxygen atmospheres, respectively. To understand degradation of PVC resins, a mechanism is envisaged by combining three major experimental techniques, namely, TGA, XPS, and FTIR.

2. Experimental

2.1. Materials

Suspension polymerized PVC resins were purchased from seven different manufacturers and were respectively denoted as P-1, P-2, P-3, P-4, P-5, P-6, and P-7 in subsequent experiments. They were provided by Tianjin dagu chemical co., Ltd, East cao chemical (Guangzhou) co., Ltd, Suzhou hua su plastic co., Ltd, Hanwha chemical (Ningbo) Co., Ltd, Shin-Etsu Chemical Co., Ltd, Xinjiang Tianye Chemical Plant (Xinjiang, China) and Sinopec qilu petrochemical company (Shandong, China), respectively. Tetrahydrofuran (THF) was supplied by Sinopharm Chemical Reagent Co., Ltd. Methanol was obtained from Fuyu Chemical Corporation (Tianjin, China). All chemical solvents were of analytical grade and used without further treatment.

2.2. Preparation and pretreatment of PVC samples

Purified PVC samples were obtained by dissolving the crude PVC in THF and then precipitated with methanol to remove additives, such as stabilizers and plasticizers.²⁹ The purified PVC samples were thermally treated as follows:

(a) Pretreatment was performed using a TGA device. Approximately 5.2 mg of sample was placed in an aluminum oxide pot and then isothermally treated at 200 °C under dynamic 100 mL min⁻¹ nitrogen or oxygen atmosphere at a heating rate of 10 °C min⁻¹. Measurements were obtained after the samples were maintained isothermally for different times under a certain atmosphere.

(b) Thermal aging tests were conducted in custom-made chamber equipment. The heating tests were conducted up to 40 d (total time) under 120 °C at a flow rate of 100 mL min⁻¹. The samples were taken out within a certain period of time.

2.3. Characterization

Molecular weight distributions (MWD) and average molecular weights were determined by Waters 2695 Alliance Gel Permeation Chromatography (Waters, USA) equipped with PL-Gel columns. All measurements were obtained at 40 °C in THF. A calibration curve was obtained with seven polystyrene standards: 1200, 3070, 7210, 19 600, 49 200, 113 000 and 257 000 Da, of low polydispersity.

The thermal stability of the sample was determined using NETZCH model STA449F3 Jupiter (NETZCH, Germany). The temperature when the weight loss of samples was 5% (T_5%) and 10% (T_10%) was used to evaluate the thermal stability of the samples. The conditions to determine thermal stability were as follows: approximately 5.2 mg of sample was used under nitrogen or oxygen atmosphere at a flow rate of 100 mL min⁻¹ and heating rate of 10 °C min⁻¹.

The XPS spectra was recorded by AMICUS/ESCA 3400 photoelectron spectrometer (Shimadzu, Japan) with a monochromatic Mg Kα X-ray source (hν = 1486.6 eV). Quantitative surface analyses of the samples were performed; the results were quantified and the peaks were fitted using the XPS vision processing software.

The infrared spectra of the samples were obtained by using an ISIO FTIR spectrometer (Thermo Fisher Scientific, USA).

3. Results and discussion

3.1. Analysis of PVC thermal degradation

Several researchers^{12,13,15,30,31} proposed that degradation of PVC resin varies under inert and oxygen atmospheres. With this in mind, we characterized the thermal properties of the seven PVC resins obtained from different sources. Table 1 shows the number-average (M_n) and weight-average (M_w) molecular weights, as well as the polydispersity index values of the PVC resins. All of the polymers exhibited similar molecular weight and molecular weight distribution values.

Table 1 Number-average and weight-average molecular weights and polydispersities of PVC from different manufacturers

Polymers	P-1	P-2	P-3	P-4	P-5	P-6	P-7
M n (g mol^−1)	71 114	69 526	70 491	69 542	59 519	61 893	58 330
M w (g mol^−1)	131 449	128 050	129 709	131 996	113 955	112 824	110 100
PDI	1.85	1.84	1.84	1.90	1.91	1.82	1.89

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The TGA results (Table 2) showed that the T_5% and T_10% of all the samples under oxygen atmosphere as carrier gas were lower than those under nitrogen atmosphere, suggesting that the presence of oxygen can accelerate PVC degradation.^13–15 This phenomenon is probably caused by the reaction between oxygen and the PVC molecular chain, as well as by some generated oxygen-containing groups.^12,13,16,18

Table 2 TGA results of PVC from different manufacturers

Polymers	N2 atmosphere		O2 atmosphere
	T 5% (°C)	T 10% (°C)	T 5% (°C)	T 10% (°C)
P-1	266.96	276.43	263.01	271.10
P-2	271.28	280.00	261.63	269.28
P-3	271.97	280.19	261.76	269.47
P-4	272.16	280.75	261.44	269.28
P-5	265.89	274.29	263.95	271.79
P-6	278.06	288.09	265.33	272.16
P-7	271.79	279.31	264.45	271.41

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3.2. Thermal pretreatment and construction of PVC chain models

Researchers^{10,12,15,26,30,31} have suggested that the unsaturated and the oxygen-containing structures in the PVC main chains significantly affect thermal stability. To reveal the relationship between segment structures and the thermal properties of PVC and to investigate the thermal degradation mechanism and process, we used the preheat treatment method to construct the PVC chain segment model. P-3 was selected for the subsequent experiment.

The PVC resins were treated at 200 °C in nitrogen environment (denoted as ) for different periods of time to build USM that contains some double bonds and conjugated polyene segments. The reaction to remove HCl from the PVC chain predominantly involves production of polyene or alkyl chloride structures, which is the initiation point of PVC degradation,^32,33 and any other reactions were negligible when the PVC samples were heated below 200 °C under nitrogen atmosphere.^34–36 Moreover, a small amount of oxygen-containing structures in the PVC main chain was introduced through resin polymerization.

By contrast, OSM was constructed by heating a resin under oxygen environment for different periods of time; the OSM was demoted as . Hydroperoxide, carbonyl, hydroxyl, and other oxygen-containing groups^14,25,27 may be introduced through this pretreatment method. and were then characterized by TGA with nitrogen as carrier gas, and the results are shown in Table 3.

Table 3 The data from TGA with nitrogen as carrier gas for the samples pretreated for different periods of time

Samples	N2 environment	T 5% (°C)	T 10% (°C)	Samples	O2 environment	T 5% (°C)	T 10% (°C)
	Pretreatment time (min)				Pretreatment time (min)
PVC	0	274.10	282.00	PVC	0	274.10	282.00
	13	272.66	279.81		10	265.70	274.29
	25	277.68	285.02		20	262.13	270.53
	40	277.49	283.95		30	258.18	265.52
	50	271.29	279.12		40	254.61	262.13
	60	275.36	281.07		50	247.27	256.05
	70	273.79	280.75		60	244.95	254.11
	90	261.76	270.03		90	238.18	249.28

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Table 3 shows that the T_5% and T_10% obtained from TGA of did not vary obviously within a certain range of time. This result explained that the unsaturated structure may not evidently influence polymer stability. Moreover, the T_5% and T_10% obtained by TGA for for different times were reduced obviously. Data on the left column in Table 3 shows that the oxygen-containing structures, which are intermediate structures for degradation, obviously play a crucial role in thermal stability of resin.

To elucidate the position of oxygen molecule attack on PVC backbone under thermal-oxidative process, we prepared a series of USM and OSM via the same methods followed by TGA with oxygen as carrier gas, and the results are shown in Table 4.

Table 4 The data from TGA with oxygen as carrier gas for the samples pretreated for different periods of time

Samples	N2 environment	T 5% (°C)	T 10% (°C)	Samples	O2 environment	T 5% (°C)	T 10% (°C)
	Pretreatment time (min)				Pretreatment time (min)
PVC	0	261.44	269.28	PVC	0	261.44	269.28
	10	262.13	268.90		10	257.87	265.70
	20	261.63	268.90		20	254.98	261.63
	30	260.00	267.52		30	253.54	259.62
	40	259.62	267.15		40	248.21	256.24
	50	260.88	267.84		50	244.95	253.54

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Table 4 shows that the T_5% and T_10% for did not obviously decrease as a function of pretreatment time despite the use of oxygen as carrier gas. A series of research^{12–14,19,22,23} has reported that oxygenated groups will be produced when PVC samples are heated under oxygen atmosphere, and some of them were also considered as the initiation point of PVC degradation, although these assumptions are not proven to be accurate. The poly-conjugated systems' constant rate of growth, which is initiated by CAG structure (k_p = 0.75 × 10⁻² s⁻¹), is higher by two or more orders of magnitude than the constant containing Cl atoms in the β-position with respect to isolated unsaturated double bonds (k_p = 10⁻⁵ s⁻¹ to 10⁻⁴ s⁻¹).^20,21 Thus, if thermal degradation followed the CAG mechanism, thermogravimetry (TG) results (Table 4, left) should decrease, which is inconsistent with our experimental results.

The analysis above possibly indicates that the degradation mechanism of CAG structure is probably feeble to describe the thermal-oxidation of PVC. However, thermal-oxidative degradation of continues under oxygen atmosphere. Thus, the decrease in T_5% and T_10% as a function of pretreatment time is reasonable (Table 4, right).

3.3. Thermal oxidative degradation mechanism of PVC

Holländer et al.³⁷ reported that secondary reactions (cross-linking and Diels–Alder reactions) can be suppressed when thermal degradation is performed at temperatures below 400 K. Thus, to further investigate the degradation under thermal oxidative condition, we performed subsequent experiments on PVC degradation. The pretreatment temperature was reduced from 200 °C to 120 °C to weaken or eliminate side effects. Generation of other structure moieties and the coking of the resins can be possibly avoided by using other means to analyze and characterize the samples.

For this purpose, the purified PVC samples were subjected under heat treatment at 120 °C in nitrogen and oxygen environments as described in Experiment 2.2.(b) to build USM and OSM models, which were named and , respectively. The TGA of the samples were performed with nitrogen or oxygen as carrier gas.

The TGA results (Fig. 1) reveal the change under nitrogen and oxygen atmosphere. The T_5% and T_10% obtained from the TGA of were reduced clearly especially after 31 d as a function of pretreatment time, whereas no obvious change was observed in . This result indicated that the oxygen-containing structures were obtained during pretreatment under oxygen environment, and the presence of these structures made the resin unstable. Fig. 1 (right) confirmed the above-mentioned conclusion, as well as simultaneously explained the observed downtrend.

Fig. 1 The T_5% and T_10% of the samples obtained by TGA with nitrogen (left) and oxygen (right) as carrier gas for different periods of time ((a, c) ; (b, d) ).

To further explore the types and percentages of PVC molecule under thermal oxidation, we employed XPS and FTIR to analyze the structural variations. Fig. 2 shows the low-resolution XPS spectra of the samples. The spectra of shows that the main elements C, Cl, and O can be identified with increasing oxygen intensity. Fig. 3 shows that the percentage of oxygen atom in tended to increase with time, suggesting the generation of oxygen-containing functional groups under thermal oxidation.

Fig. 2 Wide spectra of the samples (a) and (b) and virgin (c).

Fig. 3 Changes in XPS elemental percentage of oxygen for (a) and (b) as a function of time.

Fig. 4 depicts the high-resolution spectra of O1s. The O1s peak can be deconvoluted in three peaks, as follows: (i) the peak at 533.0 ± 0.1 eV is attributed to the sp² carbon bonded to oxygen, (ii) the peak at 534.5 ± 0.1 eV corresponds to the sp³ carbon bonded to oxygen, and (iii) the peak at 536.0 ± 0.1 eV is assigned to the sp³ oxygen bonded to hydrogen.

Fig. 4 XPS spectra of oxygen 1s of PVC: (a) virgin; (b) .

By combining the curve fitting of O1s and atomic ratio calculated with the wide spectra, we obtained the relative percentage content of different functional groups. Fig. 5 shows the uptrend for all moieties of oxygen-containing functional groups. The quantitative data show that the growth ratio of C=O is the largest as a function of time, followed by C–O and O–H. These variations can indicate that alkyl radicals can be scavenged by O₂, leading to the formation of peroxy and hydroperoxide structures. These structures attack the regular repeat units of the resin and cause its instability, thereby accelerating thermal degradation, leading to the formation of hydroxyl and alcoxyl radical and then to the generation of carbonyl structure or alkyl radical with the loss of HCl.

Fig. 5 Surface composition, with respect to oxygen functionalities for as a function of time ((a) (–C–O), (b) (–OH), (c) (–C=O)).

The variation in the trend of oxygen-containing moieties results is mainly attributed to oxidation. In this regard, the periodic measurement of the functional group index variation by FTIR analysis can be useful, and the corresponding change in structures of and are shown in Fig. 6.

Fig. 6 FTIR spectra of virgin, (left) and (right) as a function of time ((a)-virgin; (b) 5 d; (c) 14 d; (d) 25 d; (e) 31 d; (f) 35 d; (g) 40 d).

In the FTIR spectra, the wavenumber of 1427 cm⁻¹, which corresponds to the bending vibration absorption peak of –CH₂– in the PVC chain, was used as internal standard. The carbonyl (I_CO), polyene (I_PO), and hydroxyl (I_OH) indices were calculated by comparison of the FTIR absorption peak at 1735, 1607 and 3375 cm⁻¹ with reference peak at 1427 cm⁻¹, respectively.³⁸ The presence of these signals as shown in the Fig. 6 indicates the formation of carbonyl, polyene, and hydroxyl functional groups on the PVC chain during thermo-oxidative degradation.

Fig. 7 clearly shows that the I_PO of did not demonstrate significant changes, although the carbonyl groups exhibited marked variation. Furthermore, the carbonyl index of pretreated samples presented a sharp increase after 31 d. This finding can illustrate the data obtained from TGA (Fig. 1). The variation in the amplitude of hydroxyl was quite small, indicating that it did not effectively influence the thermal instability of PVC. Fig. 7 also illustrates that hydroxyl, being a transition state, was generated by the attack of O₂ on the PVC main chains and was consumed as a result of the formation of carbonyl structure under the action of heat; this finding was in accordance with XPS result.

Fig. 7 Relative percentage content of the group index for as a function of time ((a) I_CO, (b) I_OH, and (c) I_PO).

Thus, all of the above studies demonstrated that the thermo-oxidative degradation of PVC conforms to mechanism II (Scheme 1).

4. Conclusions

In this study, a series of PVC samples was prepared through pretreatment under nitrogen or oxygen atmosphere at 200 °C and 120 °C to construct USM and OSM, respectively. The PVC resins were more unstable when thermal oxidative degradation by TGA was performed. These models can demonstrate that the thermal property was less affected by an unsaturated structure but was more influenced by oxygen-containing groups. XPS and FTIR analyses of the pretreated samples under oxygen atmosphere as a function of time showed that the content of carbonyl, hydroxide, alcoxyl, and unsaturated structures in the PVC molecular chain consistently showed an increasing trend, and the increasing trend of carbonyl group coincided with the thermal analysis results, which revealed that the samples exhibit a low thermal stability. Moreover, these structures likely induced PVC molecular chain degradation during thermal curing, rendering the PVC resin unstable. The methylene or methine structure in PVC chains was attacked by oxygen molecule, the carbonyl group was produced, and the chains were broken eventually. These results showed that mechanism II is likely applicable to the thermal oxidative degradation mechanism of PVC.

Acknowledgements

The financial support of the National Basic Research Program of China (973 Program, No. 2012CB720304), National Natural Science Foundation of China (No. 51163012, 201464012), Funds for Distinguished Young Scientists of Xinjiang Bintuan (No. 2014CD001) and Scientific Research Program of Shihezi University (No. RCZX201407) is gratefully acknowledged.

Notes and references

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