| With the development of industrial technology, poly(vinyl chloride)(PVC) resin has been widely used in all walks of life because of its low cost, easy processing and other prominent features. However, due to its structural flaws, PVC will degenerate its mechanical properties and color changes, which are induced by heat, oxygen, and light, constraining its utility consequently. Up to now, researchers have tried many ways to overcome this problem, and the most effective method is to add thermal stabilizer. The commonly used thermal stabilizers on the market include the following categories: lead salts, organic tin, rare-earth, metal soap and organic compounds. Among them, the lead salts have the best effect both on the initial color stability and long-term thermal stability. Nevertheless, it can do nothing but withdraw from the market, because it is heavy metal salt and can cause great harm to the environment and human body. Though the effect of organic tin and rare earth thermal stabilizer is relatively good, the cost is pretty high. Therefore, the research of efficient and non-toxic metal soap thermal stabilizer is of great value. Nowadays, the most widely used metal soap thermal stabilizers is calcium zinc thermal stabilizer(stearate), but this kind of thermal stabilizer occur ―zinc burning‖ phenomenon, and the long-term thermal stability is not that ideal. Therefore, the way to inhibit ―zinc burning‖ and extend long-term thermal stability of calcium zinc heat stabilizer is the focus and difficulty in current research.Pentaerythritol is an excellent auxiliary heat stabilizer; they can cooperate with Ca/Zn metal soap heat stabilizers and inhibit ―zinc burning‖, and possess better long-term thermal stability. However, the pentaerythritol has highe melting point, so its compatibility with PVC is the disappointment. In this paper, we esterify pentaerythritol with adipic acid, butyric acid, and acetic acid, creating the polybasic acid pentaerythritol ester which has better thermal stability and processability. The thermal stability of polybasic acid pentaerythritol complexed with Ca/Zn thermal stabilizer was tested by Congo red test, thermal gravimetric analysis(TGA), conductivity test, thermal aging test and UV-visible spectroscopy, and oven thermal aging test. Furthermore, we also discussed their thermal stability mechanism. The experimental results are as follows:1. Polybasic acid and pentaerythritol were mixed in a certain proportion with concentrated sulfuric acid which served as the catalyst. The cyclohexane was used as water-carrying agent. This mixture was placed in a three-necked flask connected with a water separator and stirred by a magnetic stirrer. The esterification reaction was carried out under 110~150 oC. The water-carrying agent was removed by vacuum distillation. The melting point of polybasic acid pentaerythritol ester(less than 150 oC) was lower than that of pentaerythritol(261 oC), and thus it had better compatibility with PVC resin.2. In our parallel experiments,the results of esterification reaction showed that the best amount of catalyst was 2.5% of the acid quality, the best amount of water-carrying agent was 20% of the reactants, the optimum reaction temperature was 100~150 oC.3. Testing and characterization of polybasic acid pentaerythritol ester: FTIR results showed that the product had ester carbonyl and alcoholic hydroxyl group because of there was a larger peak in the vicinity of 1750 cm-1 and 3200 cm-1. The Congo red test and conductivity test showed that the long-term thermal stability of PVC was greatly improved when polybasic acid pentaerythritol ester was complexed with Ca/Zn thermal stabilizer. Oven thermal aging test demonstrated that polybasic acid pentaerythritol ester could constrain ―zinc burning‖, and the long-term thermal stability time was 30 min, but the initial whiteness was slightly lower than that of Ca/Zn metal soap only. Thermal gravimetric analysis(TGA)showed that the temperature of initial decomposition of PVC compouneded with polybasic acid pentaerythritol ester was above 220 oC, indicating the degradation was more difficult to happen. UV–visible spectroscopy test showed that polybasic acid pentaerythritol ester could decrease the concentration of the conjugated bonds of PVC; activation energy analysis showed that with the addition of polybasic acid pentaerythritol ester PVC samples had increased activation energy. When pentaerythritol was complexed with Ca/Zn thermal stabilizer, the long-term thermal stability could be improved, but the effect was weaker than that of polybasic acid pentaerythritol ester. Meanwhile, the addition of polybasic acid pentaerythritol ester had certain lubricating effect on PVC.4. Through the study of the mechanism of polybasic acid pentaerythritol esters as an auxiliary heat stabilizer, we found that the reason of auxiliary heat stabilizer had better long-term thermal stability was that they could form a complex with Zn Cl2 and reacted with active chlorine of PVC chains. |
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