Polyvinyl chloride (PVC) is a thermoplastic general-purpose plastic. Manufacturers widely use it in building materials due to its excellent mechanical properties, good corrosion resistance, anti-aging properties, and flame retardancy. Although rigid PVC has better flame retardant properties than soft PVC materials due to the small amount of plasticizer added, it still needs improvement in terms of flame retardancy and smoke suppression. This is beacuse PVC contains chlorine, which can easily produce a large amount of harmful acidic gases during combustion. The high prices of most flame retardants and their complex preparation process make mass production difficult, so very few achieve true industrialization. Inorganic flame retardants, such as magnesium hydroxide (MH), not only play a reinforcing role but also exhibit good smoke suppression properties. The water vapor and magnesium oxide produced by its decomposition play a flame retardant and smoke suppression role in the gas phase and condensed phase, respectively.
To study the effects of a synergistic flame retardant system consisting of GY-3000, HX-3000, GY-6000 magnesium hydroxide powder, and antimony trioxide on the mechanical and flame retardant properties of rigid PVC materials, researchers have designed the formula shown in the following table.
| Formulation Table of Antimony Trioxide and Zinc Oxide Synergistic Flame Retardant System | |||||||||||
| Formulation Components | Formulation Code | ||||||||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| PVC | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 | 100 |
| Zinc Powder (GY-616) | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 | 50 |
| Zinc Oxide GY-3000 | – | – | 4 | 8 | 12 | – | – | – | – | – | – |
| Zinc Oxide HX-3000 | – | – | – | – | – | 4 | 8 | 12 | – | – | – |
| Zinc Oxide GY-6000 | – | – | – | – | – | – | – | – | 4 | 8 | 12 |
| Antimony Trioxide | – | 5 | 4 | 3 | 2 | 4 | 3 | 2 | 4 | 3 | 2 |
| Zinc-Calcium Composite Stabilizer | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 | 5.5 |
| Stearic Acid | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 | 0.6 |
| PE Wax | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 | 0.8 |
| CPE | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 |
| DOP | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 | 4 |
| Basic Physical Parameters of Zinc Oxide GY-3000, HX-3000, GY-6000 | |||||
| Brand | D50 (μm) | D97 (μm) | Specific Surface Area (m²/g) | Whiteness (°) | Oil Absorption Value (mL/100g) |
| GY-3000 | 3.538 | 11.16 | 12.566 | 92 | 34 |
| HX-3000 | 3.564 | 11.25 | 11.864 | 92 | 28 |
| GY-6000 | 1.37 | 3.596 | 20.877 | 95 | 36 |
Researchers mix the materials according to the proportions in the formula table and place them into the extruder barrel. The extruder then processes the mixture into 5mm thin sheets at 180℃-195℃. Then reseachers cut them into corresponding sizes for oxygen index (80mm×10mm×5mm), smoke density (25mm×25mm×3mm), tensile strength (150mm×10mm×5mm), and impact (80mm×10mm×5mm) samples.
Researchers measure the particle size and distribution of the powder using a laser particle size analyzer. They test the specific surface area using a BET surface area meter.
Whiteness: Tested in accordance with GB/T 5950-2008 standard.
Oil absorption: Tested in accordance with DB/T 5211.15-2014 standard.
The experimental test results are as follows:
| Formula | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| Tensile Strength (Mpa) | 27.35 | 28.28 | 24.71 | 18.84 | 25.33 | 27.88 | 26.95 | 27.10 | 26.02 | 28.21 | 28.93 |
| Impact Strength (Mpa) | 3.27 | 4.55 | 4.00 | 3.20 | 2.81 | 3.99 | 3.90 | 4.13 | 3.18 | 4.18 | 5.43 |
| Oxygen Index LOI (%) | 36.80 | 43.80 | 46.80 | 47.60 | 46.60 | 46.20 | 46.40 | 45.60 | 45.80 | 46.80 | 47.00 |
| Maximum Smoke Density (%) | 93.61 | 84.98 | 82.45 | 75.75 | 72.48 | 80.69 | 84.29 | 75.48 | 84.14 | 89.23 | 74.64 |
| Smoke Density Level | 68.25 | 64.75 | 63.52 | 61.97 | 55.31 | 62.78 | 65.48 | 61.92 | 67.24 | 64.41 | 61.74 |
Oxygen index: Tested in accordance with the GB/T 2406.2-2009 standard.
Smoke density: Tested in accordance with the GB/T 8627-2007 standard.
Mechanical properties: Plastic tensile properties and cantilever beam impact strength tests were carried out in accordance with the GB/T 1040.1-2006 standard and GB/T 1843-2008 standard.
As seen from the table, the tensile strength without any flame retardant is 27.3 MPa, and the tensile strength of PVC with Sb₂O₃ added alone is slightly improved to 28.3 MPa. Adding MH to GY-3000 results in a slight decrease in the tensile strength of the product. The tensile strength of HX-3000 does not decrease, and the tensile strength of formula No. 5 (which replaces 1 part of Sb₂O₃ with 4 parts of MH) is 27.8 MPa. This indicates that the compatibility of HX-3000 with PVC is improved after surface treatment, thereby enhancing the mechanical properties.
When added 4 parts of GY-6000 to the MH composite material, the tensile strength decreases, but as the amount of MH added increases, the tensile strength gradually increases, reaching a maximum of 28.9 MPa. This is significantly higher than in other formulas, suggesting that the smaller particle size of MH increases its contact area with PVC, leading to enhanced tensile performance.
As seen from the table, the impact strength without any flame retardant is 3.27 MPa, and the impact strength of PVC with Sb₂O₃ added alone significantly becomes to 4.55 MPa. Adding 4 parts of GY-3000 to the MH composite material greatly increases the impact strength to 4 MPa. However, as the content continues to increase, the impact strength of the composite material decreases. The impact strength of active HX-3000 is significantly increased, reaching 4.13 MPa, showing that surface treatment effectively improves the mechanical properties. The impact strength of the GY-6000 MH composite material shows the greatest increase. With more MH added, the impact strength increases rapidly, reaching a maximum of 5.42 MPa, which is significantly higher than other formulas. This suggests that the finer particle size leads to an enhanced microsphere toughening effect, greatly improving the impact toughness.
The oxygen index data in the table show that adding magnesium hydroxide significantly improves the oxygen index of the PVC composite material. Adding 8 parts of GY-3000 increases the oxygen index to a maximum of 47.6%. The oxygen index of HX-3000 was slightly lower. This may be due to the effect of the surfactant on the outer surface, but it is still higher than the PVC without any flame retardant. Adding more GY-6000, the oxygen index increases, reaching a maximum of 47%.
The smoke suppression data show that adding flame retardants significantly reduces the smoke density level of the PVC composite material. Using Sb₂O₃ alone lowers the maximum smoke density to 85%, while GY-3000 provides the best smoke suppression effect. As the amount of GY-3000 increases, its smoke suppression effect continues to improve, reducing the minimum smoke density to 72.5%. The smoke suppression effects of HX-3000 and GY-6000 are slightly lower than that of GY-3000, with the minimum maximum smoke density values being 75.48% and 74.64%, respectively.
Conclusion
By studying the flame retardant, smoke suppression, and mechanical properties of magnesium hydroxide composite materials with different types and components, the conclusions are as follows:
The oxygen index of PVC composite materials with the addition of magnesium hydroxide is significantly improved. When added 8 parts of GY-3000, the oxygen index reaches a maximum of 47.6%. The more added GY-6000, the greater the oxygen index, with the maximum oxygen index reaching 47%.
GY-3000 has the best smoke suppression effect. As the amount of GY-3000 added increases, the smoke suppression effect continues to improve, with the minimum smoke density dropping to 72.5%. The smoke suppression effects of HX-3000 and GY-6000 are slightly lower than that of GY-3000, with minimum maximum smoke densities of 75.5% and 74.6%, respectively.
The tensile strength and impact strength of the composite material with 12 parts of GY-6000 MH added are the highest, reaching 28.9 MPa and 5.4 MPa, respectively.
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