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Application of calcium carbonate in PVC foamed board
For PVC foamed board, whether calcium carbonate filling affects the foaming performance is very complex, which mainly depends on the size, addition amount, dispersion and water content of calcium carbonate. If these factors of calcium carbonate are used properly, it can promote foaming, otherwise it will seriously affect foaming.
There are two reasons why calcium carbonate can positively promote foaming: first, calcium carbonate can be used as nucleating agent to adsorb foaming gas to form bubble core, control the number of bubbles and make PVC bubbles finer; Second, calcium carbonate can improve the melt properties of PVC. Calcium carbonate with high rigidity can slow down the melt deformation and movement ability, so as to inhibit the rapid expansion of bubbles and control the finer size of bubbles. However, not all calcium carbonate can promote PVC foaming. Calcium carbonate must be under the following appropriate conditions to promote foaming. How to select and treat calcium carbonate properly? How to control the following factors of calcium carbonate?
01Selection of characteristics of calcium carbonate
Dimensions of calcium carbonate: When the particle size of calcium carbonate matches with the foaming agent, it can play the role of nucleating agent and play a positive role in foaming. The specific suitable size is less than 5μm. Larger than non agglomerating dimension 1.5μm。If the particle size of inorganic powder is greater than 10μm. Or it is too fine, which leads to self agglomeration and does not play the role of nucleating agent, which has a certain negative impact on foaming. I suggest 3000 mesh (4)μm) Calcium carbonate, this size can ensure that the size is not greater than 5μm. It can also ensure that there is no reunion.
Calcium carbonate addition: The appropriate amount of calcium carbonate can promote the foaming quality. Generally, it is recommended that the best filling amount is 10% - 20%.The addition of calcium carbonate is too low to form enough nucleation points in the melt, resulting in low foaming ratio; Too high addition of calcium carbonate can form many nucleation points, but too large filling and too low melt strength will lead to too much bubble breaking and also reduce the foaming ratio.
Calcium carbonate dispersion: If the inorganic powder is evenly dispersed in the resin, it can ensure that there is no agglomeration and that the size of calcium carbonate is 5μM size can be called nucleating agent, which does not affect foaming molding. The method to ensure the dispersion of calcium carbonate is calcium carbonate surface treatment. If the inorganic powder surface treatment is complete, the original hydrophilicity is completely changed to lipophilicity, and the foaming molding can not be affected. The specific surface treatment method can be parental treatment, that is, double treatment with aluminate coupling agent / polyol surfactant, or aluminate titanate coupling agent. One of the ways to ensure the dispersion of calcium carbonate is appropriate size, and the other is surface coupling agent coating. If the surface coating treatment of calcium carbonate is not complete, it will cause uneven dispersion, resulting in the agglomeration of fine powder into larger particles. Because the thermal conductivity of calcium carbonate is far greater than that of resin, it begins to absorb heat in the melt to form local hot spots, which reduces the strength of the melt near the hot spots. The foaming agent decomposes prematurely, resulting in the early foaming of the product before reaching the cooling area, resulting in no skin or poor skin on the surface of the product.
Water content of calcium carbonate: If the water content of inorganic powder is less than 0.5%, the filling basically does not affect the foaming molding.
Conclusion: select about 3000 mesh calcium carbonate and control the addition amount at 10% - 20% to ensure uniform dispersion in PVC. Even if calcium carbonate is used properly, it can improve the foaming quality of PVC.
02Selection of calcium carbonate varieties
The characteristic requirements of calcium carbonate have been selected. How to choose in the face of many calcium carbonate varieties? Among the above calcium carbonate varieties, PVC tends to choose light calcium carbonate for the following reasons.
Help stabilize: The pH value of light calcium carbonate is 9-10, while that of heavy calcium carbonate is 8-9. Therefore, light calcium carbonate is more alkaline and can absorb a large amount of HCl produced by PVC decomposition. Auxiliary heat stabilizer plays a role in thermal stability.
Granularity advantage: The particle size of light calcium carbonate is relatively fine, which can easily reach 2500 mesh. This kind of particle size is needed by PVC and can meet the performance needs of PVC pipes and profiles. The particle size of heavy calcium carbonate can only reach this size in the last 10 years, because the use habit is gradually choosing heavy calcium carbonate.
Eliminate color light: PVC products generally contain yellow light, while light carbon calcium carbonate contains blue light, which can be eliminated by adding.
Improved toughness: Light calcium carbonate is beneficial to the impact strength modification of PVC products, while heavy calcium carbonate makes an outstanding contribution to the tensile strength.
However, at present, because the particle size of heavy calcium carbonate can reach the nano level, both heavy calcium carbonate and light calcium carbonate can be selected for PVC. Considering the different measurement emphases of the two kinds of calcium carbonate in the modification contribution, the two kinds of composite use are often the best. Generally, the 3 / 1 ratio of calcite heavy calcium carbonate / light calcium carbonate is selected, which can take into account the tensile strength and impact strength of PVC.
For heavy calcium carbonate and light calcium carbonate, the formula design is slightly different. For PVC formula with light calcium carbonate: light calcium carbonate can promote the thermal stability of PVC, so the formula can reduce the content of stabilizer accordingly; Light calcium carbonate is beneficial to impact strength and can reduce the amount of toughening agent accordingly; The oil absorption value of light calcium carbonate is relatively high, so the dosage of small molecular additives such as coupling agent, lubricant and dispersant needs to be increased accordingly. For the formula of heavy calcium carbonate, the opposite is true.
