Magnesium oxide is an effective acid scavenger, which can be used as an effective and non-toxic heat stabilizer for PVC.
Examination of the Stabilizing Effect and Differences Between Magnesium Oxide and Zinc Oxide on PVC
To examine the stabilizing effects and differences of magnesium oxide and zinc oxide on PVC, formulations were designed as shown in Table 1. Analytical Reagent 2 (AR2) grade magnesium oxide and Analytical Reagent 2 (AR2) grade zinc oxide were fixed for the test, and the results are shown in Table 2.
Table 1: The Effect of the Main Stabilizers on the Heat Stabilizing Formulation
| Formulation No. | MgOMgO/Parts | ZnOZnO/Parts |
| #2 (Original Comparative Optimized Formulation) | 2 | 3 |
| #21 | – | 3 |
| #22 | 2 | – |
| #23 | – | – |
Table 2: The Effects of MgOMgO and ZnOZnO on the Properties of the Test Sample
| Formulation No. | Melt Color | Initial Discoloration Time/min | Plasticizing Time/min | Plasticizing Temp./°C | Equilibrium Torque/N·m | Equilibrium Temp./°C | Thermal Stability Time/min |
| #2 | Slight Red | 7.00 | 3.25 | 179.75 | 15.43 | 190.77 | 19.76 |
| #21 | White | 13.00 | 3.05 | 179.60 | 15.43 | 190.77 | 13.50 |
| #22 | Slight Yellow | 4.00 | 3.20 | 177.54 | 14.86 | 190.22 | 29.25 |
| #23 | Yellow | – | 1.75 | 177.00 | 16.90 | 190.08 | 13.00 |
Note: Melt color refers to the color of the material after plasticization; Initial discoloration time refers to the time when the melt changes from white to yellow, or when the color deepens significantly compared to the initial melt color (The same applies to the following tables).
Observing the initial discoloration time and thermal stability time from Table 2, when magnesium oxide was removed from the formulation (#21), the material exhibited a relatively good initial color, showing no obvious discoloration until 13 min. However, once discoloration occurred, it immediately turned black, exhibiting the “zinc burning” phenomenon. This indicates that magnesium oxide primarily plays a long-term stabilizing role in this stabilizer system. The absence of magnesium oxide leads to a significant decrease in thermal stability and an increased susceptibility to “zinc burning.” If zinc oxide is not used (#22), the material’s initial discoloration time is very short, already showing a reddish-brown color at 4 min, but it can maintain the longest period of stability thereafter. This shows that zinc oxide has a very good initial stabilizing effect, but is detrimental to long-term stability. If both magnesium oxide and zinc oxide are completely removed (#3), the material’s plasticizing time is shortened, the initial color is very poor, the thermal stability time is greatly reduced, and the material’s equilibrium torque increases. These experiments indicate that both magnesium oxide and zinc oxide have a significant effect on the thermal stability of PVC in this formulation system, and their effects on initial color and long-term stability are exactly opposite. Therefore, using them in combination can achieve better stabilization results. Furthermore, the presence of magnesium oxide or zinc oxide appears to have a certain lubricating effect, as they delay the plasticization of PVC and reduce the equilibrium torque.
Influence of Different Grades of Magnesium Oxide on PVC Thermal Stability Performance
With Analytical Reagent 2 (AR2) grade zinc oxide fixed, the dynamic thermal stability and processing performance of stabilizer systems using different grades of magnesium oxide were studied using a HAAKE rheometer. The results are shown in Table 3.
The tables have been translated into English below:
Table 1: The Effect of the Main Stabilizers on the Heat Stabilizing Formulation
| Formulation No. | MgOMgO/Parts | ZnOZnO/Parts |
| #2 (Original Comparative Optimized Formulation) | 2 | 3 |
| #21 | – | 3 |
| #22 | 2 | – |
| #23 | – | – |
Table 2: The Effects of MgOMgO and ZnOZnO on the Properties of the Test Sample
| Formulation No. | Melt Color | Initial Discoloration Time/min | Plasticizing Time/min | Plasticizing Temp./°C | Equilibrium Torque/N·m | Equilibrium Temp./°C | Thermal Stability Time/min |
| #2 | Slight Red | 7.00 | 3.25 | 179.75 | 15.43 | 190.77 | 19.76 |
| #21 | White | 13.00 | 3.05 | 179.60 | 15.43 | 190.77 | 13.50 |
| #22 | Slight Yellow | 4.00 | 3.20 | 177.54 | 14.86 | 190.22 | 29.25 |
| #23 | Yellow | – | 1.75 | 177.00 | 16.90 | 190.08 | 13.00 |
Note: Melt color refers to the color of the material after plasticization; Initial discoloration time refers to the time when the melt changes from white to yellow, or when the color deepens significantly compared to the initial melt color (The same applies to the following tables).
Table 3: The Dynamic Heat Stability and Processing Features of Systems Containing MgOMgO with Different Specifications
| Formulation No. | MgOMgO Grade | Melt Color | Initial Discoloration Time/min | Plasticizing Time/min | Thermal Stability Time/min | Equilibrium Torque/N·m | Plasticizing Temp./°C | Equilibrium Temp./°C |
| #1 | Analytical Reagent 1 (AR1AR1) | White | 15.00 | 3.45 | 18.63 | 15.70 | 175.62 | 190.08 |
| #2 | Analytical Reagent 2 (AR2AR2) | Slight Red | 7.00 | 3.25 | 19.76 | 15.43 | 179.75 | 190.77 |
| #3 | Active 1 | Slight Red | 7.00 | 3.36 | 18.80 | 15.18 | 177.00 | 190.77 |
| #4 | Active 2 | Slight Red | 7.00 | 2.66 | 20.01 | 15.50 | 177.00 | 190.77 |
| #5 | Industrial Grade | Pink | 7.00 | 2.68 | 19.25 | 16.09 | 177.00 | 192.15 |
As seen in Table 3, the processing performance parameters such as plasticizing time, equilibrium torque, plasticizing temperature, and equilibrium temperature show little difference across the formulations. This indicates that different grades of magnesium oxide do not have a significant impact on processing performance. However, when using different grades of magnesium oxide, there are differences in the initial thermal stability of the system. The initial discoloration time and color depth are highly related to the grade, while the long-term stability is relatively similar (thermal stability time is around 19 min). Analytical Reagent 1 (AR1) grade showed the best initial color and a long duration, but black spots began to appear at 15 min. Although the color change was slight afterwards, the number of black spots increased. Therefore, in terms of initial color, only AR1 grade magnesium oxide exhibited a relatively good stabilizing effect. Interestingly, although both are Analytical Reagent grades, the performance of AR2 grade was similar to the stabilizing performance of several other grades of magnesium oxide, suggesting that purity is not the sole determining factor affecting the initial color.
Influence of Different Grades of Zinc Oxide on Dynamic Thermal Stability Performance
With Analytical Reagent 2 (AR2) grade magnesium oxide fixed, the dynamic thermal stability and processing performance of different grades of zinc oxide were studied using a HAAKE rheometer. The results are shown in Table 4.
Table 4: The Dynamic Heat Stability and Processing Features of System Containing ZnOZnO with Different Specifications
| Formulation No. | ZnOZnO Grade | Melt Color | Initial Discoloration Time/min | Plasticizing Time/min | Thermal Stability Time/min | Equilibrium Torque/N·m | Plasticizing Temp./°C | Equilibrium Temp./°C |
| #2 | Analytical Reagent 2 (AR2AR2) | Slight Red | 7.00 | 3.25 | 19.76 | 15.43 | 179.75 | 190.77 |
| #6 | 99.7% | Slight Red | 8.50 | 3.88 | 17.00 | 15.22 | 178.37 | 190.77 |
| #7 | 99.9% | White | 12.00 | 3.12 | 22.77 | 15.43 | 178.37 | 190.77 |
| #8 | Active | Slight Yellow | – | 3.90 | 19.80 | 15.30 | 179.06 | 192.15 |
| #9 | Industrial Grade | Deep Yellow | – | 3.25 | 19.96 | 15.70 | 177.00 | 192.15 |
| #10 | Analytical Reagent 1 (AR1AR1) | White | 10.00 | 2.42 | 19.10 | 16.09 | 177.00 | 192.84 |
The processing performance parameters in Table 4, such as plasticizing time, equilibrium torque, plasticizing temperature, and equilibrium temperature, show little difference, indicating that different grades of zinc oxide, like different grades of magnesium oxide, do not significantly affect the processing performance of PVC. Table 4 also shows that when different grades of zinc oxide are used, the thermal stability of the system varies significantly. Purity has a clear impact on both initial stabilization and thermal stability time. The most obvious comparison is between the two high-purity zinc oxides: the 99.9% ZnO extended both the initial discoloration time and the thermal stability time by about 5 min compared to the 99.7% ZnO. Industrial grade zinc oxide caused the material’s initial color to be too deep, although its thermal stability time was still close to 20 min. The two analytical reagent grades of zinc oxide also showed relatively good thermal stability performance.
Influence of Temperature on the Magnesium Oxide/Zinc Oxide Stabilizer System
The purpose of the experiment was twofold: first, to determine the most suitable temperature for comparison and observation in thermal stability tests; and second, to understand the stable time duration of this stabilizer system under higher processing temperature conditions. For the experiment, Analytical Reagent 2 (AR2) grade magnesium oxide and Analytical Reagent 2 (AR2) grade zinc oxide were fixed. The materials were mixed in a high-speed mixer for 10 min according to the formulation. Dynamic thermal stability tests were then performed using a HAAKE rheometer at different temperatures. The results are shown in Table 5.
Table 5: The Dynamic Heat Stability and Processing Features in Different Temperatures of Cylinder
| Formulation No. | Set Temperature/°C | Melt Color | Initial Discoloration Time/min | Plasticizing Time/min | Thermal Stability Time/min | Equilibrium Torque/N·m | Plasticizing Temp./°C | Equilibrium Temp./°C |
| #11 | 170 | Slight Red | 7.50 | 4.50 | 21.55 | 14.75 | 179.75 | 190.80 |
| #12 | 175 | Slight Red | 6.00 | 3.09 | 17.10 | 13.18 | 178.37 | 194.21 |
| #13 | 180 | Slight Red | 5.00 | 2.10 | 13.50 | 12.26 | 179.06 | 197.66 |
| #14 | 185 | Slight Red | 3.00 | 1.59 | 10.90 | 11.21 | 181.82 | 201.10 |
| #15 | 190 | Slight Red | 3.00 | 1.25 | 8.57 | 10.61 | 179.75 | 205.25 |
As shown in Table 5, the initial discoloration time, plasticizing time, thermal stability time, and equilibrium torque all decrease as the set temperature increases, while the equilibrium temperature increases. When the set temperature is above 185°C, the initial discoloration time becomes very short, and the thermal stability time drops below 11 min. This duration usually cannot meet the processing requirements for methods such as extrusion or injection molding. Based on a comprehensive analysis, for this stabilizer system, the equipment temperature should preferably not exceed 180°C. This meets both the convenience requirements for experimental observation and comparison, as well as the heat exposure time requirements of common processing methods.
Conclusion
(1) Zinc oxide exhibits a good initial stabilizing effect, but is detrimental to long-term thermal stability; the effect of magnesium oxide is the opposite.
(2) Different grades of magnesium oxide and zinc oxide have a clear impact on thermal stability. Different grades of magnesium oxide mainly affect the initial color of PVC, while different grades of zinc oxide affect both the initial color and the long-term stability. For zinc oxide, the primary factor influencing its effect is likely purity. However, the main factor influencing the effect of magnesium oxide is not yet clear, and further research is needed for both.
(3) Tests under different temperature conditions indicate that when this stabilizer system is used, the equipment temperature should preferably not exceed 180°C.
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