Mixed calcium and zinc salts of N-(3-amino-benzoic acid)terpene-maleamic acid: preparation and its application as novel thermal stabilizer for poly(vinyl chloride)

Abstract

Dipentene-maleic anhydride (DPMA) was prepared by the Diels–Alder addition of dipentene with maleic anhydride. DPMA was converted via ammonolysis with para-aminobenzoic acid (PABA) to form N-(3-amino-benzoic acid)terpene-maleamic acid (ABDPMA), which was then converted to zinc soap (ABDPMA-Zn) or calcium soap (ABDPMA-Ca). Their chemical structures were confirmed by FT-IR and ICP-AES. Thermal stabilizing effects of ABDPMA-Ca/ABDPMA-Zn were compared with ABTMA-Ca/ABTMA-Zn (ABTMA: N-(3-amino-benzoic acid)tung-maleamic acid), C36DA-Ca/C36DA-Zn (C36DA: dimer fatty acid), ZnMA/ZnO (MA: maleic acid), EFC/ZnSt₂/ESBO (EFC: calcium salt of epoxidised fatty acid; ESBO: epoxidised soybean oil, and CaSt₂/ZnSt₂ (St: stearic acid). Thermal stabilities of poly(vinyl chloride) (PVC) compounds were determined using Congo Red test, discoloration test, ultraviolet-visible spectroscopy analysis and thermogravimetric analysis. Dynamic mechanical and tensile properties of the PVC compounds were also studied. Besides better plasticization performance, ABDPMA-Ca/ABDPMA-Zn improved the long thermal stability of PVC compared with ABTMA-Ca/ABTMA-Zn, C36DA-Ca/C36DA-Zn, and CaSt₂/ZnSt₂.

---

1. Introduction

Poly(vinyl chloride) (PVC) is widely used in industrial and domestic applications, especially in construction, because of its low price and technical advantages. Despite great efforts to propose alternatives with the same technical performance as PVC, none of them is fully embraced by the market.^1–4 PVC combined with additives is applied in water pipes, window profiles, house siding, wire cable insulation, flooring and other fields.^2,5–7 However, PVC is thermally unstable at processing temperature.⁸ Appropriate stabilizers can retard PVC degradation during thermal processing.⁹ In the past decades, thermal stabilizers including organic tin/lead salts, metal soaps, and liquid-mixed metals have been investigated.^10–12 However, the application of lead salts and organic tin is limited due to their toxicity or high cost, despite the high stabilizing efficiency. The use of lead-based stabilizers in production of PVC will be banned in many countries.

Metal soaps are the most commonly-used stabilizers because of their nontoxicity. When zinc stearate (ZnSt₂) and calcium stearate (CaSt₂) are used together, ZnSt₂ first reacts with HCl briefly to form ZnCl₂, which then undergoes an exchange reaction with CaSt₂ to form CaCl₂ and ZnSt₂.¹³ Thus, the ZnSt₂ and CaSt₂ act synergistically to prolong the life of PVC at processing temperature. However, Ca/Zn stabilizers have some disadvantages in long-term stability due to their marked “zinc burning” effect.¹⁴ Furthermore, although ZnSt₂ and CaSt₂ exhibit good stabilizing effect, the efficiency of thermal stability should be improved.

Besides CaSt₂/ZnSt₂, there are some Ca/Zn stabilizers containing special groups (e.g. epoxy, aromatic and alicyclic groups). As reported, the amount of HCl release was minimized in the samples containing Zn soaps and epoxidised rubber seed oil (ERSO), which have a synergistic effect (Egbuchunam et al.). The synergism between ERSO and Zn soaps of rubber seed oil (RSO) is attributed to the esterification and etherification of the allylic chlorine atom, as the combined effect of these two reactions reduces HCl evolution.¹⁴ The thermal stability of PVC can be significantly enhanced by the addition of Zn₃(C₃N₃O₃)₂·ZnO (Zn₃Cy₂ for short) (Xu et al.). This is because the cyanurate anions in Zn₃Cy₂ strongly absorb the HCl released from PVC degradation. CaSt₂/Zn₃Cy₂ also shows excellent synergistic effects with commercial auxiliary stabilizers.¹⁵ Moreover, DPMA-Ca/DPMA-Zn and APA-Ca/APA-Zn, prepared from rosin and dipentene-derived dicarboxylic acids, are both thermal stabilizers of PVC, owing to the presence of fused-ring, bridge-ring structures and high metal ion contents (Li et al.).¹⁶ The stabilizing action of various antimicrobial maleimido phenyl urea could be attributed to a radical mechanism, which disrupts the radical chain degradation of the polymer by blocking the formation of odd electron sites on the PVC chains (Nadia A. et al.).^17,18

However, the amide-containing Ca/Zn salt which can improve the long-term thermal stability of PVC was rarely reported.^19,20 Herein we report the effective use of a radical mechanism to the Ca/Zn system embodying the long-term thermal stabilizing effect on PVC.

Industrial dipentene, a colorless or light yellow oily liquid, is mainly obtained from the camphor preparation and the pulp-paper industry.^21–23 It is also a volatile mixture of cyclic monoterpene hydrocarbons. The main components of dipentene are showed in Scheme 1, including isomers of camphene, p-cymene, terpinolene, limonene, α-terpinene and other menthadienes.²⁴ Industrial dipentene is an eco-friendly, high-output, low-price, inartificial and renewable feedstock for preparation of various novel bio-based products.

Scheme 1 Chemical constituents of industrial dipentene.

In this work, first, dipentene-maleic anhydride (DPMA) was prepared via Diels–Alder addition of dipentene with maleic anhydride. Then DPMA was converted to N-(3-amino-benzoic acid)terpene-maleamic acid (ABDPMA), which was then converted to zinc soap (ABDPMA-Zn) or calcium soap (ABDPMA-Ca) (Scheme 2). The thermal stabilizing effects of ABDPMA-Ca/ABDPMA-Zn on PVC were studied. Commercial CaSt₂/ZnSt₂, ZnMA/ZnO (MA: maleic acid), EFC/ZnSt₂/ESBO (EFC: calcium salt of epoxidised fatty acid; ESBO: epoxidised soybean oil), homemade ABTMA-Ca/ABTMA-Zn (ABTMA: N-(3-amino-benzoic acid)tung-maleamic acid) (Scheme 3) and C36DA-Zn/C36DA-Ca (C36DA: dimer fatty acid) (Scheme 3) stabilizers were used as controls.²⁰

Scheme 2 Synthesis routes of ABDPMA-Ca and ABDPMA-Zn.

Scheme 3 Structure of dimer fatty acid (C36DA) and N-(3-amino-benzoic acid)tung-maleamic acid (ABTMA).

2. Material and methods

2.1 Material

Industrial dipentene was purchased from Nanjing Yixin Chemical Industry Co., Ltd., China and used as received, the content of limonene, camphene, p-cymene, α-terpinolene and γ-terpinolene were 58.04, 5.98, 11.10, 17.54 and 2.78%, respectively. Methyl esters of tung oil fatty acids were obtained from the Institute of Chemical Industry of Forestry Products. Di(2-ethylhexyl)terephthalate (DOTP), maleic anhydride (99%), 1,4-dioxane, para-aminobenzoic acid and propylene glycol monomethyl ether were purchased from Shanghai Aladdin Industrial Corporation, China. Calcium stearate (CaSt₂), Zinc stearate (ZnSt₂), CaCl₂ and ZnSO₄·7H₂O were purchased from Shandong Huike Additives Co., Ltd., China. Polyvinyl chloride (PVC, S-1000) was supplied by Shandong Qilu Co., Ltd., China. Calcium dimer acids (C36DA-Ca) and Zinc dimer acids (C36DA-Zn) were homemade.¹⁶ All reagents and solvents were used as received.

2.2 Synthesis

2.2.1 Synthesis of dipentene-maleic anhydride adducts (DPMA). First, maleic anhydride (150 g) was charged to a flask equipped with a mechanical stirrer, a dropping funnel, a thermometer, and a reflux condenser. After the flask was heated to 180 °C, a mixture of iodine (0.29 g) and industrial dipentene (280 g) was added slowly. After that, the reaction continued at 200 °C for 2 h. Next, the crude product was developed through vacuum distillation (−1.05 kPa) and the fraction at 180–185 °C was collected. Finally, DPMA was obtained as a light yellow liquid (yield: 91.10%).²⁵

2.2.2 Synthesis of N-(3-amino-benzoic acid)tung-maleamic acid (ABDPMA) and its complex calcium salt (ABDPMA-Ca) and zinc salt (ABDPMA-Zn). First, DPMA (42.30 g) and 1,4-dioxane (21.15 g) were charged to a 250 mL flask equipped with a reflux condenser, a magnetic stirrer and a thermometer. After the flask was heated to 80–85 °C, a mixture of para-aminobenzoic acid (PABA, 13.71 g) and 1,4-dioxane (82.26 mL) was added dropwise through a dropping funnel. After that, the reaction continued at 95 °C for 4 h and then the 1,4-dioxane was distilled under vacuum at 80 °C. Afterwards, the solid powder was redissolved in ethyl alcohol and washed with deionized water, forming a yellow solid powder (41.28 g).²⁶

Next, ABDPMA (0.15 mol) was mixed in ethyl alcohol (10 mL) to form sodium soap (ABDPMA-Na). After the mixture was heated to 62 °C, NaOH (0.3 mol) dissolved in 3 mL was added dropwise. After vigorous stirring at 62 °C for 1 h, a 0.15 mol CaCl₂ (or ZnSO₄·7H₂O) solution (in 20 mL of ethanol and 20 mL of deionized water) was added. After the resulting mixture was kept at 70 °C for 3 h, the products were filtered, washed with deionized water and vacuum-dried to form ABDPMA-Ca or ABDPMA-Zn as an off-white solid.

2.2.3 Synthesis of N-(3-amino-benzoic acid)tung-maleamic acid (ABTMA) and its complex calcium salt (ABTMA-Ca) and zinc salt (ABTMA-Zn). ABTMA (yield 93.01%), ABTMA-Ca (yield: 94.27%) and ABTMA-Zn (yield: 91.93%) were synthesized in the same methods. Both calcium salt and zinc salt of ABTMA (ABTMA-Ca and ABTMA-Zn) were yellowish solid powder product.²⁰

2.3 PVC films preparation

For comparison, commercial CaSt₂/ZnSt₂, homemade C36DA-Zn/C36DA-Ca and ABTMA-Ca/ABTMA-Zn stabilizers were chosen as controls. 100 g PVC, 50 g DOPT, 3.0 g Ca/Zn thermal stabilizers (the mass ratio of Ca salts/Zn salts was 4 : 1) were milled into a homogeneous mixture using a Haake torque rheometer at 160 °C for 5 min. The formulations of thermal stabilizers are showed in Table 1.

Table 1 Formulations for four thermal stabilizers

Ingredients	Formulations
	PVC	DOTP	ABTMA-Ca + ABTMA-Zn	ABDPMA-Ca + ABDPMA-Zn	C36DA-Ca + C36DA-Zn	CaSt2 + ZnSt2
I	100 g	50 g	2.4 g + 0.6 g	0	0	0
II	100 g	50 g	0	2.4 g + 0.6 g	0	0
III	100 g	50 g	0	0	2.4 g + 0.6 g	0
IV	100 g	50 g	0	0	0	2.4 g + 0.6 g

---

2.4 Characterizations

The Fourier transform infrared (FTIR) spectra were obtained on a Nicolet iS10 FTIR (Nicolet Instrument Crop., USA) infrared spectrophotometer by KBr disc method.

The metal (Ca and Zn) content of the thermal stabilizer was measured using an Optima 7000 inductively coupled plasma atomic emission spectrometer (ICP-AES) (PerkinElmer, America).

The thermal stability of prepared PVC samples was determined by the following methods. The static thermal stability time (T_s) were measured on a heat stability tester (Dongguan xilong Quality Control instrument Co. Ltd., China) according to the standard ISO 182-1-1990.¹⁶ The thermal aging test of the stabilized PVC samples was performed according to the ISO 305:1990(E) standard.¹⁶

A ultraviolet-visible (UV-vis) spectroscopy (UV-2550, Shimadzu Co. Ltd., China) was adopted to investigate the content of double bonds in PVC samples after degradation. The optical spectra of the PVC samples were recorded at room temperature, using the UV-vis spectrometer with the slit width set at 2.0 nm over the wavelength in the range of 200–500 nm. Tetrahydrofuran was used as the solvent and the mass concentration was 0.5 g L⁻¹.

Thermal decomposition kinetics was studied using a Q600 TGA instrument (TA Instruments). Each sample was scanned from 30 to 400 °C under a nitrogen flow rate of 100 mL min⁻¹ and heating rates of 5, 10, 15, 20 and 25 °C min⁻¹. The Kissinger equation yields a simple relationship between the peak temperature on DTG (T_p) and the heating rate (β). The kinetic parameters such as activation energy (E, kJ mol⁻¹) were evaluated using Kissinger equation:

where R = gas constant (8.314 J mol⁻¹ K), . And the temperature (T_α) is the temperature when the weight loss (α) is 30 or 60. Eqn (2) indicate that lg β and 1/T_α are in linear correlation. E can be calculated from the slope (M) and intercept (N) on curves of lg β change versus 1/T_α.²⁷

Dynamic mechanical analysis (DMA) of the samples was performed on a DMA Q800 (TA Instruments) in a film stretching mode with an oscillating frequency of 1 Hz. The temperature was swept from −80 to 80 °C at 3 °C min⁻¹. For each sample, duplicated tests were performed in order to ensure the reproducibility of data. T_g was determined as the temperature at the maximum of the tan δ versus temperature curve.

Tensile properties were tested in accordance with ASTM D638-03. The tensile test region of the specimens was 0.48 mm thick, 4 mm wide and 25 mm primary scale. The tests were performed on a CMT4303 universal test machine (Sans, China) with a crosshead speed of 50 mm min⁻¹. At least six replicates for each sample were tested.

3. Results and discussion

3.1 Synthesis and characterization

The synthesis route and chemical structures of ABDPMA are outlined in Scheme 2. Because only α-terpinene takes part in the Diels–Alder reaction while other limonenes and γ-terpinene can be converted to α-terpinene through isomerization under temperatures rise, dipentene and iodine were first heated at 180 °C to completely isomerize dipentene structure to α-terpinene structure in the synthesis of DPMA.²⁸ ABDPMA was synthesized by the ammonolysis reaction of PABA with DPMA in DMF. Then dipentene-derived Ca/Zn salts were both prepared by a two-step process, including neutralization and salt metathesis reaction.

Fig. 1 shows the FTIR spectra of amic acids and Ca/Zn salts. The spectra of amic acids (ABDPMA and ABTMA) shows significant bands typical of amide group at 3461 or 3333 cm⁻¹ (NH stretching vibration), 1286 or 1260 cm⁻¹ (C–N stretching vibration), 1523 cm⁻¹ (NH bending vibration), 1635 cm⁻¹ (C=O stretching vibration). Compared to TMA and DPMA, amic acids do not show the peaks of anhydride group at 1865, 1836, 1786 and 1776 cm⁻¹. Moreover, the peaks at 1607, 1603, ∼843, and 3000 cm⁻¹ are due to benzene ring. The FTIR spectrum of neither Ca salts nor Zn salts shows the characteristic peak at 1711 or 1692 cm⁻¹ ascribed to the C=O of carboxylic acid. Peaks at 1413, 1409, 1520, and 1530 cm⁻¹ are due to the symmetric and asymmetrical stretching vibrations of carboxylic soap, respectively. Ca and Zn contents in ABTMA-Ca (6.87% Ca), ABTMA-Zn (10.51% Zn), ABDPMA-Ca (8.48% Ca), and ABDPMA-Zn (12.89% Zn) were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). FTIR and ICP-AES suggest the successful conversion of amic acids to the corresponding calcium or zinc soaps.

Fig. 1 The FTIR spectra of ABDPMA-Ca, ABDPMA-Zn, ABTMA-Ca, ABTMA-Zn and its intermediates.

3.2 Stabilization of thermally-degraded PVC using various thermal stabilizers

Generally, in the heating process, the PVC degradation primarily involves a progressive unzipping of neighboring labile chloride atoms, with the release of HCl. The amounts of HCl and other acidic products were determined via Congo Red test (at 180 °C), while the color stability of PVC added with each stabilizer after heating at 185 °C was assessed through thermal aging test. PVC stabilized with ABDPMA-Ca/ABDPMA-Zn was tested, with C36DA-Ca/C36DA-Zn, CaSt₂/ZnSt₂ and ABTMA-Ca/ABTMA-Zn as controls. These three controls were selected because they represent the two major classes of frequently-used Ca/Zn stabilizers: basic monoacid and diacid salts. DA is a mixture of C36 aliphatic dibasic acids. Possible structures include a linear dimer acid and alicyclic with two alkyl side chains as shown in Scheme 3.^29,30

The Congo Red test results of PVC strips containing different stabilizers (the mass ratio of Ca salt/Zn salt was 4 : 1) are shown in Table 2. The data of the static stability time (T_s) of PVC samples stabilized by CaSt₂/ZnSt₂, ZnMA/ZnO, EFC/ZnSt₂/ESBO, C36DA-Ca/C36DA-Zn, ABTMA-Ca/ABTMA-Zn and ABDPMA-Ca/ABDPMA-Zn are 21 min 06 s, 35 min, 40 min, 50 min 42 s, 66 min 24 s and 71 min 48 s, respectively (Table 2). The results indicated that ABDPMA-Ca and ABDPMA-Zn exhibited excellent synergistic effect.

Table 2 Stabilization time of the PVC/stabilizer samples^a

Formulations	The static stability time (Ts, heat at 180 °C)
a m(PVC) : m(EFC) : m(ZnSt2) : m(ESBO) = 100 : 2.7 : 0.3 : 1; m(PVC) : m(ZnMA) : m(ZnO) = 100 : 1.38 : 0.62.
PVC/ABDPMA-Ca/ABDPMA-Zn	71 min 48 s
PVC/ABTMA-Ca/ABTMA-Zn	66 min 24 s
PVC/C36DA-Ca/C36DA-Zn	50 min 42 s
PVC/EFC/ZnSt2/ESBO^31	40 min
PVC/ZnMA/ZnO^32	35 min
PVC/CaSt2/ZnSt2	21 min 06 s

---

To comprehensively investigate the thermal stability of PVC samples, we determined the color stability through thermal aging test. The thermal aging test results of PVC strips containing different stabilizers (the mass ratio of Ca salt/Zn salt was 4 : 1) are shown in Table 3. The PVC strips containing CaSt₂/ZnSt₂ and C36DA-Ca/C36DA-Zn both exhibit excellent initial color, but totally turn black within 40 min. This result is consistent the widely accepted view that reaction between Zn salts and PVC chains would produce ZnCl₂, which catalyzes the degradation of PVC chains. The PVC strips stabilized by ABTMA-Ca/ABTMA-Zn and ABDPMA-Ca/ABDPMA-Zn which are faint yellow and exhibited excellent long-term stability are both initially yellow. These results are almost consistent with the PVC strip Congo Red tests.

Table 3 Color evolutions of the PVC/stabilizer samples treated at 185 °C for different periods of time

---

To confirm the thermal stabilization of amic-acid-based Ca/Zn salts, we investigated the degradation of stabilized PVC samples at 185 °C for 40 min by ultraviolet-visible (UV-vis) spectroscopy. The UV-vis spectrum shows the length of conjugated carbon chains, whereas the absorption intensity is related to the concentration of conjugated double bonds.³³ Fig. 2 presents the UV-vis spectra of stabilized PVC heated at 185 °C for 40 min. Each curve shows the maximum peak (between 240 nm and 300 nm), which also belongs to trienes and tetraenes formed in dehydrochlorination (DHC) of PVC. All the absorptions occur in a region characteristic of n < 6.³⁴ The absorbance (A) of the PVC/ABDPMA-Ca/ABDPMA-Zn is the lowest among the 4 samples. Since free HCl can prolong the length of polyenes formed during PVC thermolysis, efficient HCl scavenging might be responsible for the polyene shortening, indicating that ABDPMA-Ca/ABDPMA-Zn might be good at absorbing HCl and has a better thermal stabilizing effect on PVC compared with the other three stabilizers. These results are consistent with the Congo Red test and thermal aging test.

Fig. 2 UV-vis spectra of the PVC stabilized with various stabilizers and treated for 40 min at 185 °C.

In the Congo Red test, thermal aging test and UV-Vis spectroscopy, better thermal stabilization was also found in the presence of ABDPMA-Ca/ABDPMA-Zn relative to ABTMA-Ca/ABTMA-Zn. Compared with other systems, the ABDPMA system and ABTMA system both have better thermal stability for the radical mechanism. As shown in the thermal stability test, ABDPMA system has better thermal stability than ABTMA system. This is probably because the introduction of bridge-ring molecules into the structures of thermal stabilizers offered better thermal stabilization than fatty acid in PVC application.¹⁶ It looks bridged structure and radical mechanism may both display comparable function on improved the thermal stability of PVC. Another reason may be the dispersion of heat stabilizer in PVC. Fig. 3 presents the SEM microphotographs of the fracture surface and flat surface of different locations within the film. Clearly, PVC/ABDPMA-Ca/ABDPMA-Zn show smaller particles compared with other control stabilizers. That means the dispersion of ABDPMA-Ca/ABDPMA-Zn in PVC is superior to the other compared stabilizers. A better dispersion of thermal stabilizer in PVC helps to prevent the extraction of the stabilizer, which contributes to the high thermal stability. The above observations further prove that ABDPMA-Ca/ABDPMA-Zn was a better thermal stabilizer compared with other control stabilizers.

Fig. 3 SEM microphotographs of PVC films with different stabilizers before heating test.

3.3 Thermal degradation kinetics of the stabilized PVC by Kissinger method

For the kinetic study of thermal degradation, the activation energy (E, kJ mol⁻¹) for the PVC thermal decomposition was calculated using the thermogravimetric analysis (TGA) data obtained at different heating rates using Kissinger's method. Generally PVC-degradation is considered to be a two-stage process (Fig. 4). The first stage (185–375 °C) is DHC, where HCl is eliminated, leaving the unsaturated hydrocarbons containing long conjugated double bonds. The second stage (375–500 °C) mainly involves the cyclization of conjugated polyene sequences to form aromatic compounds,^35–37 which inherently results in a color change. The main thermal degradation of PVC is known as a zip type of DHC that involves the heat sensitive structures such as internal allylic or tertiary chlorine substituents.³⁸ The decomposition is accelerated by the presence of hydrochloric acid.³⁹ Thus, the kinetic parameters of the degradation of stabilized PVC samples during the first stage (α = 30% and α = 60%) were study by TGA and DTG. Fig. 5 shows the TGA curves of PVC compounds stabilized by different thermal stabilizers, while Tables 4–7 show the characteristics. Fig. 6 displays the kinetic parameters. According to Kissinger's equation, the activation energy of thermal degradation (E_α, kJ mol⁻¹) of four formulations at α = 30% and 60% ranks in the same order: PVC/ABDPMA-Ca/ABDPMA-Zn (129.25 vs. 131.44) > PVC/ABTMA-Ca/ABTMA-Zn (117.60 vs. 119.06) > PVC/C36DA-Ca/C36DA-Zn (109.41 vs. 110.50) > PVC/CaSt₂/ZnSt₂ (98.99 vs. 103.58). The PVC/ABDPMA-Ca/ABDPMA-Zn has the highest E_α in both α = 30% and 60%. Generally, a higher E_α suggests a higher resistance against thermal degradation. Therefore, the TGA results indicate that ABDPMA-Ca/ABDPMA-Zn is more effective in stabilizing PVC than the other three thermal stabilizers.

Fig. 4 Two-stage degradation during thermal decomposition of PVC.

Fig. 5 TG curves of the stabilized PVC samples at various heating rates.

Table 4 The characteristics of the thermogravimetric degradation analysis of PVC/ABDPMA-Ca/ABDPMA-Zn

α/%	β/(°C min^−1)	lg β	T/°C	T/K	1000/T
30	5	0.70	280.29	553.44	1.81
	10	1.00	296.56	569.71	1.76
	15	1.17	302.17	575.32	1.74
	20	1.30	308.28	581.43	1.72
	25	1.40	312.34	585.49	1.71
60	5	0.70	303.70	576.85	1.73
	10	1.00	320.48	593.63	1.69
	15	1.18	326.60	599.75	1.67
	20	1.30	333.21	606.36	1.65
	25	1.40	338.30	611.45	1.64

---

Table 5 The characteristics of the dynamic thermogravimetric degradation analysis of PVC/ABTMA-Ca/ABTMA-Zn

α/%	β/(°C min^−1)	lg β	T/°C	T/K	1000/T
30	5	0.70	282.31	555.46	1.80
	10	1.00	296.56	569.71	1.76
	15	1.18	305.21	578.36	1.73
	20	1.30	312.34	585.49	1.71
	25	1.40	317.95	591.10	1.69
60	5	0.70	306.23	579.38	1.73
	10	1.00	320.48	593.63	1.69
	15	1.18	329.64	602.79	1.66
	20	1.30	338.29	611.44	1.64
	25	1.40	344.41	617.56	1.62

---

Table 6 The characteristics of the dynamic thermogravimetric degradation analysis of PVC/C36DA-Ca/C36DA-Zn

α/%	β/(°C min^−1)	lg β	T/°C	T/K	1000/T
30	5	0.70	247.73	520.88	1.92
	10	1.00	262.99	536.14	1.87
	15	1.18	269.59	542.74	1.84
	20	1.30	277.21	550.36	1.82
	25	1.40	281.29	554.44	1.80
60	5	0.70	280.30	553.45	1.81
	10	1.00	297.08	570.23	1.75
	15	1.18	306.23	579.38	1.73
	20	1.30	315.88	589.03	1.70
	25	1.40	319.96	593.11	1.69

---

Table 7 The characteristics of the dynamic thermogravimetric degradation analysis of PVC/CaSt₂/ZnSt₂

α/%	β/(°C min^−1)	lg β	T/°C	T/K	1000/T
30	5	0.70	249.75	522.90	1.91
	10	1.00	266.04	539.19	1.86
	15	1.18	273.67	546.82	1.83
	20	1.30	281.80	554.95	1.80
	25	1.40	287.92	561.07	1.78
60	5	0.70	289.96	563.11	1.78
	10	1.00	307.26	580.41	1.72
	15	1.18	316.42	589.57	1.70
	20	1.30	323.53	596.68	1.68
	25	1.40	329.14	602.29	1.66

---

Fig. 6 Curves of lg β ∼ (1000/T) for thermal degradation with different weight loss of PVC and four thermal stabilizers.

3.4 Thermal stabilization mechanism of ABDPMA-Ca/ABDPMA-Zn

ABDPMA-Ca/ABDPMA-Zn could more significantly enhance the long-term thermal stability of PVC, as indicated in the Congo Red test, thermal aging test, UV-vis spectroscopy and thermogravimetric analysis. The reason may be attributed to the Ca/Zn stabilizing mechanism (Fig. 7) and radical mechanism (Fig. 8).

Fig. 7 Ca/Zn stabilizing mechanism of PVC with ABDPMA-Ca/ABDPMA-Zn.

Fig. 8 Radical mechanism of PVC with ABDPMA-Ca/ABDPMA-Zn.

The generally accepted Ca/Zn stabilizing mechanism is that the ABDPMA group belonged to ABDPMA-Zn displaces the labile chlorine on the PVC chains. However, ZnCl₂ that acts as a catalyst for the degradation process is formed during the above-mentioned action and can enhance the degradation rate. ABDPMA-Ca decreases the activity of ZnCl₂ by producing CaCl₂ and regenerating ABDPMA-Zn. Besides, ABDPMA-Ca can also effectively neutralizes HCl. Thus, ABDPMA-Zn and ABDPMA-Ca act synergistically to prolong the life of PVC at processing temperature.¹⁶

According to the multistep mechanism based on the radical trapping potency between amide stabilizers and PVC: a stabilizer radical intermediate [eqn (2)], which is immediately formed after the labile chlorine atom detaching from the PVC chain [eqn (1)], is directly trapped by the ethylenic C–C double bond of the stabilizer. The once generated intermediate most probably hinders the odd-electron sites created on the PVC chain, resulting in the disruption of the radical chain degradation of PVC [eqn (3)]. The process quit only if the aromatic nucleus is completely detached from the stabilizer molecules [eqn (4)–(6)]. The greater efficiency of the amide is most probably due to their various reactive centres which can act as traps for radical species produced during the degradation process, and the ability of their fragmentation products (aromatic amines) to react with the evolved HCl.^17,18,20

3.5 Dynamic mechanical properties and tensile properties

Fig. 9 shows the temperature-variable changes of storage modulus (E′) and damping (tan δ). As shown in Fig. 9(a), the storage modulus curves all display a similar trend: under −80 °C, E′ remains at a high level from 3000 to 4100 MPa; E′ decreases sharply from −40 to 40 °C; with further increase from −40 to 40 °C, the frozen segmental structure ever in glass state is relaxed suddenly, and above 40 °C, all E′ values are close to a constant level below 10 MPa. Fig. 9(a) shows that PVC/ABDPMA-Ca/ABDPMA-Zn exhibits the higher E′ than does PVC/ABTMA-Ca/ABTMA-Zn, PVC/C36DA-Ca/C36DA-Zn and PVC/CaSt₂/ZnSt₂ up to 25 °C, in particular, nears the −80 °C. The increased E′ value means under a given load the network will deform elastically to a decreased extent, and therefore the ABDPMA-Ca/ABDPMA-Zn can give the PVC resin with higher stiffness than ABTMA-Ca/ABTMA-Zn can when used as hard plastics near room temperatures.⁴⁰ This is probably owing to the rigid bridge-ring structure from dipentene moiety, which endow the PVC resin with certain high rigidity.

Fig. 9 Dynamic mechanical properties of PVC compounds mixed with different additives.

As shown in Fig. 9(b), all tan δ curves display one glass transition temperature (T_g) corresponding to the curves' peak temperature, which indicates the four formulations all formed homogeneous and compatible materials.

The T_g of the PVC/CaSt₂/ZnSt₂, PVC/C36DA-Ca/C36DA-Zn, PVC/ABTMA-Ca/ABTMA-Zn, and PVC/ABDPMA-Ca/ABDPMA-Zn are 29.47, 30.25, 27.16, and 25.87 °C, respectively. This, however, was not the case. It has been rationalized that T_g is probably due to a combination of molecular structure, crosslinking density, etc.⁴¹ That only means ABDPMA-Ca/ABDPMA-Zn has better plasticization performance for PVC than that of the other compared stabilizers.

Fig. 10 presents the tensile properties of PVC stabilized by different additives. PVC/ABDPMA-Ca/ABDPMA-Zn exhibit lower breaking elongation compared with both controls, but the elongations at break is maintained at a high level within 320–390% for all the PVC compounds.

Fig. 10 Tensile properties of PVC compounds mixed with different additives.

4. Conclusions

Zinc soap (ABDPMA-Zn) and calcium soap (ABDPMA-Ca) of N-(3-amino-benzoic acid)terpene-maleamic acid (ABDPMA) were successfully prepared. Thermal stability of poly(vinyl chloride) compounds was determined using Congo Red test, discoloration test, ultraviolet-visible spectroscopy analysis and thermogravimetric analysis. ABDPMA-Ca/ABDPMA-Zn exhibits higher long thermal stabilizing effect than ABTMA-Ca/ABTMA-Zn, C36DA-Ca/C36DA-Zn and CaSt₂/ZnSt₂. The static stability time (Congo Red test), discoloration time (discoloration test) and activation energy (thermogravimetric analysis) of the stabilized PVC compositions indicate the stabilizing effects of the four mixed Ca/Zn stabilizers follow the order of ABDPMA-Ca/ABDPMA-Zn > ABTMA-Ca/ABTMA-Zn > C36DA-Ca/C36DA-Zn > CaSt₂/ZnSt₂, suggesting that higher thermal stability of PVC/ABDPMA-Ca/ABDPMA-Zn is due to the presence of Ca/Zn stabilizing mechanism and radical mechanism. DMA and tensile properties results reveal that PVC/ABTMA-Ca/ABTMA-Zn displayed the lowest T_g and storage modulus among the four stabilized PVC compounds, respectively. That means ABDPMA-Ca/ABDPMA-Zn has better plasticization performance for PVC than that of the ABTMA-Ca/ABTMA-Zn stabilizer. The results from this study demonstrate that dipentene can be a very promising alternative potential feedstock for PVC thermal stabilizers.

Acknowledgements

The authors are grateful for the financial support from the National Nonprofit Institute Research Grant of CAFINT2015C04, the Basic research funding earmarked for the Key Lab. of Biomass Energy and Material of Jiangsu Province, China (grant number: JSBEM-S-201508), National Natural Science Foundation of China (No. 31600468), and the industrialization project of forestry intellectual property of China (grant number: Forestry patent 2015-2).

References

1. D. Braun, J. Vinyl Addit. Technol., 2001, 7, 168–176.
2. D. Braun, Prog. Polym. Sci., 2002, 27, 2171–2195.
3. I. C. McNeill, L. Memetea and W. J. Cole, Polym. Degrad. Stab., 1995, 49, 181–191.
4. R. Bacaloglu and M. Fisch, Polym. Degrad. Stab., 1994, 45, 301–313.
5. W. H. Starnes Jr, Prog. Polym. Sci., 2002, 27, 2133–2170.
6. M. Beltran, J. C. Garcia and A. Marcilla, Eur. Polym. J., 1997, 33, 453–462.
7. M. A. Keane and J. Chem, J. Chem. Technol. Biotechnol., 2007, 82, 787–795.
8. G. Yuan, D. Chen, L. Yin, Z. Wang, L. Zhao and J. Y. Wang, Waste Manage., 2014, 34, 1045–1050.
9. X. P. Xu, S. Chen, W. Tang, Y. J. Qu and X. Wang, Polym. Degrad. Stab., 2014, 99, 211–218.
10. M. K. Naqvi, P. A. Unnikrishnan, Y. N. Sharma and I. S. Bhardwaj, Eur. Polym. J., 1984, 20, 95–98.
11. G. M. Anthony, Polym. Degrad. Stab., 1999, 64, 353–357.
12. H. IsmetGokcel, D. Balkose and U. Kokturk, Eur. Polym. J., 1999, 35, 1501–1508.
13. N. Bensemra, T. V. Hoang, A. Michel, M. Bartholin and A. Guyot, Polym. Degrad. Stab., 1989, 24, 33–50.
14. T. O. Egbuchunam, D. Balkose and F. E. Okieimen, Polym. Degrad. Stab., 2007, 92, 1572–1582.
15. X. P. Xu, S. Chen, W. Tang, Y. J. Qu and X. Wang, Polym. Degrad. Stab., 2014, 99, 211–218.
16. M. Li, J. W. Zhang, K. Huang, S. H. Li, J. C. Jiang and J. L. Xia, RSC Adv., 2014, 4, 63576–63585.
17. N. A. Mohamed and W. M. Al-Magribi, Polym. Degrad. Stab., 2003, 80, 275–291.
18. N. A. Mohamed and W. M. Al-Magribi, Polym. Degrad. Stab., 2002, 78, 49–165.
19. M. Li, Y. D. Liang, X. B. Wang, K. S. Li and X. R. Liu, Polym. Degrad. Stab., 2016, 124, 88–91.
20. M. Wang, J. L. Xia, J. C. Jiang, S. H. Li, K. H. W. Mao and M. Li, Polym. Degrad. Stab., 2016, 133, 136–143.
21. H. Pakdel, C. Roy, H. Aubin, G. Jean and S. Coulombe, Environ. Sci. Technol., 1991, 25, 1646–1649.
22. H. Pakdel, D. M. Pantea and C. Roy, J. Anal. Appl. Pyrolysis, 2001, 57, 91–107.
23. H. Pakdel and C. Roy, J. Chromatogr. A., 1994, 683, 203–214.
24. Q. G. Zhang, L. W. Bi, Z. D. Zhao, Y. P. Chen, D. M. Li, Y. Gu, J. Wang, Y. X. Chen, C. Y. Bo and X. Z. Liu, Chem. Eng. J., 2010, 159, 190–194.
25. J. N. Xin, P. Zhang, K. Huang and J. W. Zhang, RSC Adv., 2014, 4, 8525–8532.
26. J. Correa-Basurto, I. V. Alcántara, L. M. Espinoza-Fonseca and J. G. Trujillo-Ferrara, Eur. J. Med. Chem., 2005, 40, 732–735.
27. N. S. Vrandecic, I. Klaric and T. Kovacic, J. Therm. Anal. Calorim., 2003, 74, 171–180.
28. W. W. Wang, M. J. Liu, J. W. Gu, Q. Y. Zhang and J. W. Mays, J. Polym. Res., 2014, 21, 502–504.
29. D. H. Mcmahon and E. P. Crowell, J. Am. Oil Chem. Soc., 1974, 51, 522–527.
30. J. Zhao and S. V. Olesik, Anal. Chim. Acta, 2001, 449, 221–236.
31. G. X. Zheng. Study on Synthesis and Application of Epoxidized Calcium Soap and Modified Pentaerythritol Esters [D], Fujian Normal University, Fujian, 2007, pp. 42–51.
32. G. X. Li, M. Wang, X. L. Huang, H. X. Li and H. He, J. Appl. Polym. Sci., 2015, 132, 41467.
33. Q. Zhang and H. C. Li, Spectrochim. Acta, Part A, 2008, 69, 62.
34. C. Y. Wan, Y. Zhang and Y. X. Zhang, Polym. Test., 2004, 23, 299.
35. T. Yoshioka, T. Akama, M. Uchida and A. Okuwaki, Chem. Lett., 2000, 29, 322.
36. A. Marcilla and M. Beltran, Polym. Degrad. Stab., 1995, 48, 219.
37. Y. Ning and S. Y. Guo, J. Appl. Polym. Sci., 2000, 77, 3119.
38. T. Hjertberg and E. M. Sorvik, Polymer, 1983, 24, 673–684.
39. J. Světlý, R. Lukáš and M. Kolínský, Macromol. Chem. Phys., 2003, 180, 1363–1366.
40. J. T. Wan, B. Gan, C. Li, J. Molina-Aldareguia, E. N. Kalali, X. Wang and D. Y. Wang, Chem. Eng. J., 2016, 284, 1088–1089.
41. P. Y. Jia, M. Zhang, C. G. Liu, L. H. Hu, G. D. Feng, C. Y. Bo and Y. H. Zhou, RSC Adv., 2015, 5, 41169–41178.

---