Mei Wanga,
Jianling Xiaab,
Jianchun Jianga,
Shouhai Liab and
Mei Li*a
aInstitute of Chemical Industry of Forestry Products, CAF, Key Lab. of Biomass Energy and Material, National Engineering Lab. for Biomass Chemical Utilization, Key and Lab. on Forest Chemical Engineering, SFA, Nanjing 210042, Jiangsu Province, PR China. E-mail: meiyu20032001@126.com
bInstitute of Forest New Technology, CAF, Beijing 100091, PR China
First published on 7th October 2016
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.
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.
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 3 Structure of dimer fatty acid (C36DA) and N-(3-amino-benzoic acid)tung-maleamic acid (ABTMA). |
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.
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 |
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 (Ts) 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 (Tp) and the heating rate (β). The kinetic parameters such as activation energy (E, kJ mol−1) were evaluated using Kissinger equation:
(1) |
(2) |
(3) |
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. Tg was determined as the temperature at the maximum of the tanδ 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.
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 (CO 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 CO 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.
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 (Ts) 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.
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/ESBO31 | 40 min |
PVC/ZnMA/ZnO32 | 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.
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.
α/% | β/(°C min−1) | lgβ | 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 |
α/% | β/(°C min−1) | lgβ | 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 |
α/% | β/(°C min−1) | lgβ | 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 |
α/% | β/(°C min−1) | lgβ | 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 lgβ ∼ (1000/T) for thermal degradation with different weight loss of PVC and four thermal stabilizers. |
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
As shown in Fig. 9(b), all tanδ curves display one glass transition temperature (Tg) corresponding to the curves' peak temperature, which indicates the four formulations all formed homogeneous and compatible materials.
The Tg 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 Tg 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.
This journal is © The Royal Society of Chemistry 2016 |