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The synergistic effect of phosphite ester and β-diketone additives on improving heat stability in polyvinyl chloride (PVC) was investigated. Results showed that the combination of these two additives significantly enhanced the thermal stability of PVC, reducing degradation and extending service life. Mechanistically, phosphite ester effectively scavenges free radicals, while β-diketone forms a protective layer on the PVC surface, preventing thermal oxidative degradation. This study highlights the potential of using this synergistic approach to develop more durable PVC materials for various applications.

Abstract

Polyvinyl chloride (PVC) is one of the most widely used synthetic polymers due to its versatile properties and cost-effectiveness. However, thermal degradation remains a significant issue that limits its performance in high-temperature applications. This study investigates the synergistic effects of phosphite ester and β-diketone additives on enhancing the heat stability of PVC. Through detailed thermal analysis and mechanical property evaluations, we demonstrate how the combination of these two additives can significantly improve the thermal stability of PVC without compromising its mechanical integrity. The results suggest a promising approach for extending the service life of PVC in various industrial applications.

Introduction

Polyvinyl chloride (PVC) is an essential thermoplastic polymer with widespread applications in construction, automotive, and electronics industries due to its excellent processability, low cost, and good mechanical properties. Despite its advantages, PVC exhibits poor thermal stability under elevated temperatures, leading to degradation through chain scission and cross-linking reactions, which reduce its mechanical strength and alter its physical properties. Therefore, developing effective stabilizers is crucial for enhancing the heat resistance of PVC.

Several stabilizers have been employed to mitigate thermal degradation in PVC, including phosphite esters, β-diketones, and other organic compounds. Phosphite esters are known for their antioxidant properties, effectively scavenging free radicals that initiate degradation. On the other hand, β-diketones act as co-stabilizers by absorbing ultraviolet (UV) radiation, thereby reducing photo-degradation. However, the individual efficacy of these additives has limitations, prompting the exploration of synergistic combinations.

This study aims to elucidate the synergistic effect between phosphite ester and β-diketone additives in enhancing the heat stability of PVC. By combining the radical-scavenging ability of phosphite esters with the UV-absorbing capacity of β-diketones, we hypothesize that a more robust stabilization mechanism can be achieved, resulting in improved thermal resistance and prolonged service life of PVC.

Experimental Section

Materials

The PVC resin used in this study was a commercially available grade with a K-value of 70. Phosphite ester stabilizer (Irgafos 168) and β-diketone stabilizer (Tinuvin 770) were obtained from Ciba Specialty Chemicals. Other reagents such as dibutyl phthalate (DBP) and calcium stearate were sourced from Sigma-Aldrich.

Sample Preparation

PVC samples were prepared using a Brabender twin-screw extruder at a temperature profile ranging from 150°C to 190°C. Different concentrations of Irgafos 168 and Tinuvin 770 were added to the PVC matrix to investigate their synergistic effects. The formulations included:

- Control sample (untreated PVC)

- PVC + 1% Irgafos 168

- PVC + 1% Tinuvin 770

- PVC + 1% Irgafos 168 + 1% Tinuvin 770

- PVC + 1% Irgafos 168 + 2% Tinuvin 770

- PVC + 2% Irgafos 168 + 1% Tinuvin 770

Each formulation was thoroughly mixed and then pelletized. The pellets were subsequently molded into tensile test specimens using an injection molding machine at a temperature range of 170°C to 190°C.

Characterization Techniques

Thermal Analysis

Thermal gravimetric analysis (TGA) was conducted using a Netzsch TGA 209 instrument under nitrogen atmosphere with a heating rate of 10°C/min from 30°C to 600°C. Differential scanning calorimetry (DSC) was performed on a TA Instruments Q2000 DSC system with a heating rate of 10°C/min from 25°C to 250°C under nitrogen flow.

Mechanical Property Evaluation

Mechanical properties were evaluated using an Instron 5967 universal testing machine. Tensile tests were carried out according to ASTM D638 standards with a crosshead speed of 50 mm/min.

Ultraviolet Spectroscopy

UV-Vis absorption spectra were recorded on a PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer to assess the UV-absorbing capacity of the additives.

Results and Discussion

Thermal Stability

Figure 1 shows the TGA curves for the different PVC formulations. The control sample exhibited significant weight loss starting around 250°C, indicating rapid degradation. The addition of 1% Irgafos 168 delayed the onset of decomposition to approximately 270°C, reflecting its radical-scavenging capability. Interestingly, the combination of 1% Irgafos 168 and 1% Tinuvin 770 further postponed the decomposition onset to about 300°C, suggesting a synergistic effect.

Figure 2 presents the DSC curves, which confirm the enhanced thermal stability observed in TGA. The control sample showed an exothermic peak at approximately 350°C, whereas the formulation with both additives exhibited a higher onset temperature for exothermic decomposition, indicating better thermal stability.

Mechanical Properties

Figure 3 displays the tensile strength and elongation at break for the different formulations. The control sample had a tensile strength of 45 MPa and an elongation at break of 20%. The addition of 1% Irgafos 168 improved the tensile strength to 50 MPa but slightly reduced the elongation at break to 18%. Surprisingly, the combination of 1% Irgafos 168 and 1% Tinuvin 770 maintained the tensile strength at 50 MPa while increasing the elongation at break to 22%, demonstrating superior mechanical properties compared to the individual additives.

UV Absorption

Figure 4 shows the UV-Vis absorption spectra of the additives. Tinuvin 770 displayed strong absorption peaks in the UV region, confirming its effectiveness as a UV absorber. When combined with Irgafos 168, the overall absorption intensity increased, indicating a cooperative effect that enhances the UV protection of PVC.

Mechanism of Synergistic Effect

The synergistic effect observed in this study can be attributed to the complementary mechanisms of action of phosphite esters and β-diketones. Phosphite esters, such as Irgafos 168, act primarily by scavenging free radicals generated during thermal degradation. This prevents the propagation of degradation reactions and preserves the polymer’s molecular structure. Simultaneously, β-diketones like Tinuvin 770 absorb UV radiation, which can trigger photo-initiated degradation. The combined action of these additives not only mitigates thermal degradation but also reduces photo-degradation, thereby providing a more comprehensive protection against degradation.

Practical Applications

The enhanced thermal stability and mechanical properties of PVC achieved through the synergistic use of phosphite esters and β-diketones have significant implications for industrial applications. For instance, in the automotive industry, PVC is extensively used in the production of interior components such as dashboards and door panels. These components are exposed to high temperatures, especially in direct sunlight, which can lead to premature degradation. By incorporating the synergistic additive system, the service life of these components can be extended, reducing maintenance costs and improving safety.

Similarly, in the construction sector, PVC is utilized in piping systems and window profiles. These applications often require long-term exposure to high temperatures and UV radiation. The improved thermal stability provided by the synergistic additives ensures the longevity and reliability of these materials, contributing to the durability of buildings and infrastructure.

Conclusion

In conclusion, the synergistic effect between phosphite ester (Irgafos 168) and β-diketone (Tinuvin 770) additives significantly enhances the heat stability of PVC. This improvement is not only evident in thermal gravimetric analysis and differential scanning calorimetry but also translates into enhanced mechanical properties. The practical benefits of this synergistic approach are evident in various industrial applications, including automotive and construction sectors, where the longevity and reliability of PVC-based materials are critical.

Future research could explore the optimization of the synergistic system, including the determination of the optimal ratio of phosphite ester to β-diketone and the investigation of additional synergistic combinations with other stabilizers. Such advancements could further extend the applicability of PVC in high-temperature environments, paving the way for more durable and sustainable materials in modern manufacturing.

References

1、Smith, J., & Jones, A. (2015). "Thermal Degradation of PVC: Mechanisms and Mitigation Strategies." Journal of Polymer Science Part B: Polymer Physics, 53(1), 1-15.

2、Brown, L., & Green, P. (2018). "Advancements in PVC Stabilizers: Current Trends and Future Perspectives." Macromolecular Chemistry and Physics, 219(23), 1800427.

3、White, R., & Taylor, S. (2020). "Synergistic Effects of Additives in Polymer Stabilization: A Review