Calcium carbonate "alkalinity" can affect the brittleness and yellowing of PVC. What is the reason?
Calcium carbonate is widely used as an important filler in PVC pipe processing and production. However, many manufacturers produce PVC with different degrees of yellowing and brittleness. Today, Xiaobian mainly tells you about the effect of calcium carbonate alkalinity on the brittleness and yellowing of PVC pipes, because alkalinity is often a factor that many manufacturers can easily ignore.
Industrial precipitation of calcium carbonate alkalinity is actually the free base that is caused by some reason in our production process. Free base refers to a substance in the production of calcium carbonate in which the product is not converted into calcium carbonate but in the form of calcium hydroxide. If the alkalinity is too high, it will react with other plasticizers, causing the plastic to become brittle and yellow. It can be seen that the free base is an important technical indicator in calcium carbonate products and must be strictly controlled in production.
The main cause of high calcium carbonate alkalinity is the formation of basic calcium carbonate.
1 Lime over burning
In the process of calcination of lime, due to the different degree of blockiness, it is easy to make the lime appear to be burnt. The lime is digested by water, the water temperature is low, the digestion is incomplete, and lime particles are produced. The calcium carbonate formed during carbonization is finely nucleated and deposited thereon to form calcium carbonate-coated calcium oxide particles. We know that calcium oxide crystals are cubic and calcium carbonate crystals are rhombohedral. The crystals of the two crystals have different intergranular angles, and different expansion coefficients occur after heating, which causes crystal breakage and frees calcium oxide. Alkaline.
2 High free base
Basic calcium carbonate, especially in cold weather, due to the low temperature and low solubility of calcium hydroxide, in the process of carbonization, there is calcium hydroxide solid and water-soluble calcium hydroxide ion in the lime milk, and carbonization reaction with carbon dioxide is Occurred in an alkaline solution, thus producing basic calcium carbonate. This basic calcium carbonate changes with the temperature of the carbonizing solution and the amount of carbon dioxide introduced, and is converted into three different types of calcium carbonate (calcium type, Lingshi type, aragonite type). When carbonized to the end point, PH = 8 ~ 10, alkaline, can not destroy the basic calcium carbonate. These basic calcium carbonates have not yet had time to change, and enter the next process. When entering the rotary dryer, the temperature rises the basic calcium carbonate to decompose calcium hydroxide and calcium carbonate, so that the alkalinity is high.
The alkalinity is high, and the weather is more prominent than the hot weather. The key is that the hot weather is high, the water temperature is high, the lime is well digested, and the temperature of the carbonization tower is relatively high, while the solubility of calcium hydroxide is low, and it is not easy to produce platy basic carbonic acid. Calcium is easily converted into calcium carbonate. It can be seen that the alkalinity of hot weather is lower than that of cold weather.
Therefore, in the processing of PVC pipe fittings, not only should pay attention to the particle size, whiteness, moisture, sedimentation volume, mineral elements of calcium carbonate, but also the alkalinity of calcium carbonate!