Chemical structure of ELCE
Chemical structure of ELCE characterized with FTIR and compared with linoleic acid, cardanol and linoleic acid cardanol ester. Figure 2 shows their FTIR. As seen from FT-IR of linoleic acid, the peak at 3006.2cm-1 attributed to =C-H, the strong peak at around 1720cm-1 corresponded to C=O[15]. For the FT-IR of cardanol, the peak at around 3410cm-1 attributed to -OH, and the peaks at 3007.1cm-1 corresponded to =C-H[19-20]. There is no -OH peak at around 3410cm-1 in FT-IR of linoleic acid cardanol ester, which indicated that the esterification was completed. As seen from the FT-IR of ELCE, it cannot be observed any peak attributing to =C-H at 3000cm-1 -3010cm-1[21], which indicated that the epoxidation reaction was completed and the ELCE was obtained.
Figure 3, Figure 4, Figure 5 and Figure 6 show 1H NMR of linoleic acid, cardanol, linoleic acid cardanol ester and ELCE. As seen from Figure 3, the peak at 5.2-5.6ppm attributed to protons of -CH=CH-, the peaks at 4.2-4.4ppm attributed to protons of -CH2- connecting to -CH=CH-. The peak at around 1.3-2.3ppm attributed to protons of other -CH2-. The peak at 0.9ppm corresponded to protons of -CH3[15]. As seen from Figure 4, the peaks at around 6.2-7.3ppm attributed to protons of -CH2 from benzene ring. The peak at 5.5ppm attributed to protons of -CH=CH-. The peaks at around 1.4-2.9ppm corresponded to other protons of -CH2-. The protons of -CH3 appeared at 0.9ppm[19-20]. When the esterification was finished, it can be observed that the protons of -CH2- from benzene ring of cardanol appeared at around 6.9-7.3ppm in the 1H NMR of linoleic acid cardanol ester, and the protons of -CH=CH- from linoleic acid appeared at 5.4ppm, which indicated that the linoleic acid cardanol ester was obtained. Figure 6 shows the 1H NMR of ELCE, there was no peak attributing to the protons of -CH=CH- at around 5.4ppm, and the peak corresponding to the protons of -CH2 - connecting to epoxy group appeared at 3.2ppm, which illustrated that ELCE was obtained.
Performance of PVC plasticized with ELCE
Melting behavior of PVC blends was investigated by torque variation during blends mixing in relation to the plasticizer content. Processing data were collected during mixing in the Haake chamber. The torque data were collected during thermoplastic blending process of PVC and plasticizer. The torque values relative to the mass of plasticizer in blends were shown in Figure 7. When the mass of ELCE increased from 0g to 20g, the torque of PVC blends decreased from 20N·m(P0) to 11.2N·m(P3), which corresponds to 44% decrement. The reduction of torque value means that using ELCE to replace DOP will decreased the melt viscosity and improved thermoplastic processing of PVC blends.
Figure 8 and Figure 9 shows the TGA and DTG curve of P0, P1,P2 and P3. It can be observed that there were two degradation stage for all PVC blends, which have been reported that the first degradation stage corresponding to formation and stoichiometric elimination of HCl, and the second degradation stage attributing to cross linking containing C=C bonds[22-23]. In addition, with more ELCE added in PVC blends, the degradation temperature increased, which indicated that ELCE delayed the thermal degradation of plasticized PVC materials. When PVC blends plasticized with same mass of ELCE and DOP, PVC plasticized with EPAE showed more excellent thermal degradation stability. This is mainly because the epoxy group of ELCE can react with HCl, thereby delaying thermal decomposition[24].
DMA is usually used to measure the viscoelasticity of blends, which can show the change of tan δ with temperature. The temperature at the maximum of the tan δ vs. temperature curve is defined as the glass transition temperature (Tg), which is an important indicator to measure plasticization efficiency. Figure 5 shows the tan δ vs. temperature curves of all plasticized PVC samples. All PVC materials shows one Tg, which indicated that ELCE and DOP has excellent compatibility with PVC. For P0, the Tg was 55.8oC. When the DOP was replaced with ELCE in PVC blends, Tg decreased from 55.8oC to 36.7oC, which illustrated that ELCE played more efficient plasticizing effect on PVC than DOP. Because the chlorine atom on the PVC molecular chain has an electron withdrawing effect, so the hydrogen atom shows electropositivity. The electronegative oxygen atoms in the ester group and epoxy group on ELCE form hydrogen bond interaction with the positive hydrogen atoms on the PVC molecular chain. In addition, the long non-polar alkyl chain of ELCE can play the role of lubrication on the PVC molecular chain, thereby increasing the volume of the amorphous region of the blend, which lead to a decrease in Tg.
Figure 13 shows the XRD diffractograms of plasticized PVC blends, which was used to determine the structure, complexation and crystallization of the PVC matrix. From the corresponding angle of the characteristic peaks and the applying Bragg’s Law (nλ=2dsinθ), for λ of 1.542 Å, the interlamellar basal distances (d) could be calculated. The diffractograms showed two characteristic peaks at 2theta=17.9°and 24.8°. According the XRD diffractograms, there was no difference in the crystalline of PVC blends plasticized with ELCE and DOP, which illustrated that both ELCE and DOP did not change the crystallinity of PVC blends.
Microstructure of PVC blends was investigated with optical microscope to reveal the plasticizing mechanism. Figure 12 shows the microstructure of all plasticized PVC blends. For P0, which shows the rough and irregular microstructure and there are many agglomerates on the surface of P0. With more ELCE added into the PVC blends, the microstructure of plasticized PVC materials, as seen from Figure 12 (P1-P3), the rough and irregular microstructure, and agglomerates disappeared gradually. P3 shows homogeneous and smooth surface microstructures, indicating excellent compatibility between PVC and ELCE[25], because the strong interaction of the polar groups of ELCE(ester group, benzene ring and epoxy groups ) with polar groups of PVC.
Figure 12 Resistance to volatilization and migration of plasticized PVC blends
Figure 12 shows the mass loss of all plasticized PVC blends after the volatilization resistance test and solvent extraction resistance test. The volatility loss of the plasticized samples P0, P1, P2 and P3 were 5.94%, 3.76%, 1.92% and 0.42%, respectively. Obviously, P3 shows better volatilization resistance than other samples, which indicated that ELCE has excellent migration resistance in PVC, and is greatly superior to DOP. Figure 12 shows the extraction loss of P0, P1,P2 and P3 in extraction tests were according to ASTMD 1239-98. The PVC blend was in petroleum ether at 23 ± 1 ℃ and 50 ± 5% relative humidity. For P0, the weight loss was 15.56% of mass, while for P3, the weight loss was 6.74%, which illustrated that ELCE showed excellent solvent extraction resistance than DOP. The polarity and structure of the plasticizer have a decisive influence on the migration resistance and solvent extraction resistance of the plasticizer in PVC. The abundant epoxy groups and higher molecular weight give ELCE stronger intermolecular interactions and higher compatibility with PVC than that of DOP[26-28], thus obtaining more excellent migration stability and volatility in PVC blends.
Table 2 shows tensile strength and elongation at break of all PVC materials. It can be observed from Table 2 that with the increase of the ELCE in PVC blends, the elongation at break of the PVC blends increased from 122.12±6.51% to 250.43±7.12%, while the tensile strength and modulus of elasticity gradually decreased. This indicated that the addition of ELCE increased the flexibility and movement ability of the PVC molecular chain to a certain extent, and the effect becomes more obvious as the content increases. This is because that chemical structure of ELCE is similar to DOP. In addition, the epoxy group in the long aliphatic chain of ELCE has a strong electrostatic interaction with the PVC molecular chain, which can inserted PVC molecular chains and increased free volume of PVC chains[29-30].
Table 2 Tensile data of plasticized PVC materials
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PVC materials
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Tensile strength (MPa)
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Elongation at break (%)
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Modulus of elasticity (MPa)
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P0
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34.12±0.78
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122.12±6.51
|
193.45±2.18
|
P1
|
29.64±0.98
|
180.89±7.18
|
156.87±6.12
|
P2
|
25.34±0.89
|
210.45±9.87
|
143.45±7.01
|
P3
|
22.16±0.68
|
250.43±7.12
|
129.21±5.12
|
The plasticizing mechanism can be explained according to formation of hydrogen bonds between PVC and plasticizers, as seen from Figure 13, the ester groups and epoxy groups of ELCE interacted with α-hydrogen of PVC chains to form hydrogen bonds, which decreased interaction of PVC chains and increased the distance between themselves[31-32]. The interaction promoted the movement of PVC chains to make plasticized PVC flexible and malleable.