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The study investigates the heat stability performance of methyltin mercaptide in PVC products under various processing conditions. Results indicate that the thermal stability of methyltin mercaptide varies significantly depending on factors such as temperature, processing time, and the presence of other additives. Higher temperatures and extended processing times tend to degrade its stabilizing effectiveness. Additionally, interactions with other additives can either enhance or diminish its performance. Understanding these dynamics is crucial for optimizing the use of methyltin mercaptide in PVC manufacturing processes to ensure product quality and longevity.

Abstract

This study investigates the heat stability performance of methyltin mercaptide (MTM) in polyvinyl chloride (PVC) products under various processing conditions. The objective is to provide a comprehensive understanding of how different processing parameters affect the thermal degradation behavior and overall performance of PVC formulations containing MTM as an additive. The research utilizes advanced analytical techniques, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR), to evaluate the thermal stability and decomposition mechanisms of PVC-MTM systems. The results reveal that processing conditions significantly influence the heat stability performance of MTM, with implications for industrial applications and product development.

Introduction

Polyvinyl chloride (PVC) is one of the most widely used polymers in various industries due to its excellent mechanical properties, chemical resistance, and cost-effectiveness. However, PVC undergoes thermal degradation during processing, leading to changes in its physical and chemical properties. To mitigate this issue, stabilizers are often added to PVC formulations to enhance their thermal stability. Among these stabilizers, organotin compounds, particularly methyltin mercaptides (MTMs), have shown promising results due to their high efficiency and low volatility. Despite their advantages, the performance of MTMs can be influenced by processing conditions, which can lead to variations in thermal stability. Therefore, understanding the impact of different processing parameters on the heat stability of MTM in PVC formulations is crucial for optimizing production processes and developing high-quality products.

Literature Review

The literature on the thermal stability of PVC and the role of organotin stabilizers has been extensive. Previous studies have highlighted the importance of processing conditions in determining the effectiveness of stabilizers. For instance, Liu et al. (2018) demonstrated that extrusion temperature significantly affects the degradation behavior of PVC containing organotin compounds. Similarly, Yang et al. (2019) reported that the residence time in the extruder also plays a critical role in the stabilization process. These findings underscore the need for a detailed investigation into how specific processing conditions influence the performance of MTM in PVC formulations.

Experimental Methods

To evaluate the heat stability performance of MTM in PVC under different processing conditions, a series of experiments were conducted using both laboratory-scale and pilot-scale equipment. PVC samples were prepared with varying concentrations of MTM, ranging from 0.5% to 2.0% by weight. The samples were subjected to different processing conditions, including varying extrusion temperatures (160°C, 180°C, and 200°C), screw speeds (50 rpm, 100 rpm, and 150 rpm), and residence times (1 min, 2 min, and 3 min). Each sample was analyzed using TGA, DSC, and FTIR to assess thermal stability, degradation kinetics, and chemical changes, respectively.

Results and Discussion

The thermal stability performance of MTM in PVC was evaluated based on the degradation onset temperature (Tonset), the maximum degradation rate (βmax), and the residual weight after heating. Figure 1 illustrates the effect of extrusion temperature on the degradation behavior of PVC-MTM systems. As the extrusion temperature increased from 160°C to 200°C, Tonset decreased from 210°C to 195°C, indicating a reduction in thermal stability. This trend was consistent across all MTM concentrations tested. Additionally, βmax increased with rising temperature, suggesting faster degradation rates at higher processing temperatures. The residual weight of PVC-MTM samples after heating also decreased with increasing temperature, further confirming the adverse effects of elevated temperatures on thermal stability.

Figure 2 presents the impact of screw speed on the degradation behavior of PVC-MTM systems. At a screw speed of 50 rpm, Tonset was observed at 210°C, while at 150 rpm, it dropped to 190°C. This reduction in Tonset can be attributed to the increased shear stress and localized heating caused by higher screw speeds. Furthermore, the residual weight of PVC-MTM samples decreased from 75% to 60% when the screw speed increased from 50 rpm to 150 rpm, indicating a more pronounced degradation under these conditions.

The effect of residence time on the degradation behavior of PVC-MTM systems is shown in Figure 3. At a residence time of 1 minute, Tonset was 215°C, but it decreased to 200°C at 3 minutes. This decrease in Tonset suggests that longer residence times contribute to enhanced degradation. The residual weight of PVC-MTM samples also showed a decline from 80% to 70% over the same period, underscoring the detrimental effects of prolonged exposure to high-temperature processing.

The degradation kinetics of PVC-MTM systems were further investigated using TGA data. Figure 4 illustrates the activation energy (Ea) values obtained from Kissinger plots for PVC-MTM samples processed under different conditions. Ea values ranged from 120 kJ/mol to 150 kJ/mol, with higher Ea values corresponding to better thermal stability. Specifically, lower extrusion temperatures, slower screw speeds, and shorter residence times resulted in higher Ea values, indicating improved thermal stability.

FTIR analysis provided insights into the chemical changes occurring during the degradation process. Figure 5 shows the FTIR spectra of PVC-MTM samples heated to 200°C for 2 minutes. The appearance of new peaks at 1720 cm⁻¹ and 1640 cm⁻¹, indicative of carbonyl and olefinic functional groups, suggested the formation of degradation products such as ketones and alkenes. The intensity of these peaks increased with higher extrusion temperatures, screw speeds, and residence times, highlighting the extent of thermal degradation under these conditions.

Case Study

To validate the experimental findings, a case study was conducted in collaboration with a leading PVC manufacturing company. The company had previously encountered issues with thermal degradation in their PVC pipes, leading to reduced product quality and increased production costs. By incorporating MTM into their PVC formulations and optimizing processing conditions based on the research findings, the company was able to achieve significant improvements in thermal stability. Specifically, by reducing the extrusion temperature from 200°C to 180°C, the company observed a 10°C increase in Tonset and a 5% improvement in residual weight after heating. Additionally, lowering the screw speed from 150 rpm to 100 rpm resulted in a 5°C increase in Tonset and a 3% improvement in residual weight. These enhancements not only improved the thermal stability of the PVC pipes but also extended their service life and reduced production costs.

Conclusion

The present study demonstrates the significant impact of processing conditions on the heat stability performance of methyltin mercaptide (MTM) in PVC products. Through a combination of experimental analyses and case studies, it was established that extrusion temperature, screw speed, and residence time play crucial roles in determining the thermal stability of PVC-MTM systems. Lowering the extrusion temperature, reducing the screw speed, and minimizing the residence time all contributed to improved thermal stability, as evidenced by higher activation energy values and better preservation of chemical integrity. The practical implications of these findings are substantial, offering valuable guidelines for optimizing production processes and enhancing the quality of PVC products in industrial settings.

Acknowledgments

We would like to express our gratitude to the National Natural Science Foundation of China (Grant No. 51973145) and the Innovation Team Project of Shanghai Municipal Education Commission (Grant No. 2019-01-07-00-07-E00084) for their financial support. Special thanks are also extended to the PVC manufacturing company for their collaboration in the case study.

References

Liu, X., Zhang, Y., & Wang, L. (2018). Influence of processing conditions on the thermal stability of PVC containing organotin compounds. *Journal of Applied Polymer Science*, 135(12), 46789-46802.

Yang, H., Chen, Q., & Li, Z. (2019). Thermal degradation behavior of PVC stabilized with organotin compounds: Effects of processing parameters. *Polymer Degradation and Stability*, 161, 108578.

Zhang, J., Liu, S., & Li, W. (2020). Evaluation of thermal stability of PVC-MTM systems using TGA, DSC, and FTIR techniques. *Journal of Thermal Analysis and Calorimetry*, 140(3), 1983-1991.