Chemical Knowledge 09:The mechanism by which the 'alkalinity' of calcium carbonate can affect the brittleness and yellowing of PVC pipes

Update time: 26-03-23 Views: 26

The mechanism by which the 'alkalinity' of calcium carbonate can affect the brittleness and yellowing of PVC pipes

Calcium carbonate, as an important filler, is widely used in the processing and production of PVC pipes. However, many manufacturers' PVC products to varying degrees have issues such as yellowing and brittleness. So what causes this? Today, let's take a closer look at the impact of the alkalinity index of calcium carbonate on pipes.

Calcium carbonate alkalinity is actually the free alkali of calcium carbonate in our production. It refers to the substance that exists in the form of calcium hydroxide rather than being converted into calcium carbonate during calcium carbonate production. If the alkalinity pH value is too high, it will react with other plasticizers in the plastic, causing the plastic to become brittle and yellow.

1. The main mechanistic causes are as follows:

1.Mechanism of brittleness: An alkaline environment may interfere with the regularity and arrangement of PVC molecular chains. During processing, PVC molecular chains need to form a stable structure through appropriate plasticization and cooling processes. Excessive alkalinity may disrupt the orderly arrangement of the molecular chains, weaken the bonding forces between chains, reduce the mechanical properties of the material, and manifest as brittleness;
2. Yellowing Mechanism

2.1 Accelerated PVC Degradation: PVC is prone to thermal degradation and oxidative degradation during processing and use, releasing hydrogen chloride (HCl) during the degradation process. When the alkalinity of calcium carbonate is too high, alkaline substances react with the HCl generated from degradation to form salts such as calcium chloride. This reaction disrupts the balance of the PVC degradation process, promoting the degradation of more PVC molecular chains and forming conjugated double bond structures. Conjugated double bonds can absorb blue-violet light in the visible spectrum and reflect complementary colors (yellow), causing yellowing on the surface of the pipes.

2.2 Interaction with Stabilizers: Stabilizers are usually added during PVC processing to inhibit degradation and yellowing. However, excessive alkalinity in calcium carbonate may interact with stabilizers, reducing their effectiveness. For example, some stabilizers may lose activity in an alkaline environment or form complexes with alkaline substances, failing to effectively prevent PVC degradation and yellowing, thus causing the pipes to turn yellow. Therefore, free alkali is an important technical indicator in calcium carbonate products and must be strictly controlled during production. The main reason for high alkalinity in calcium carbonate is the formation of basic calcium carbonate.

2. Causes of Alkalinity

1. Overburnt lime

During the calcination of lime, due to uneven lump sizes, if not properly controlled, it is easy for the lime to become overburned. Overburned lime requires more water for hydration, and if the water temperature is low, it will not hydrate completely, producing lime fines. During carbonation, the calcium carbonate formed acts as fine particles serving as crystal nuclei, depositing on the surface and forming a calcium carbonate coating around the calcium oxide particles. We know that calcium oxide crystallizes in a cubic form, while calcium carbonate crystallizes in an orthorhombic form; the interfacial angles of these two crystal forms are different, and upon heating, they expand at different rates, causing the crystals to fracture and releasing free calcium oxide, which exhibits alkalinity.

2. Free alkali is slightly high

Basic calcium carbonate, especially in cold weather, due to low temperatures and the high solubility of calcium hydroxide, contains both solid calcium hydroxide and water-soluble calcium hydroxide ions in the lime slurry during the carbonation process. These then react with carbon dioxide in a carbonation reaction that occurs in an alkaline solution, resulting in basic calcium carbonate. This basic calcium carbonate changes with the temperature of the carbonation solution and the amount of carbon dioxide introduced, transforming into three different structures of calcium carbonate (calcite, aragonite, vaterite). When carbonation is complete, the pH is 8–10, slightly alkaline, and the basic calcium carbonate is not destroyed. These basic calcium carbonates have not yet had time to change and enter the next process. When they enter the rotary dryer, the temperature rises and the basic calcium carbonate decomposes into calcium hydroxide and calcium carbonate, causing higher alkalinity.
High alkalinity is more pronounced in cold weather than in hot weather. The key is that in hot weather, the temperature is high, the water temperature is also high, and lime digestion is better. Meanwhile, the temperature in the carbonation tower is relatively high, and the solubility of calcium hydroxide is low, making it difficult to generate plate-shaped basic calcium carbonate, which easily transforms into calcium carbonate. Therefore, the alkalinity in hot weather is lower than in cold weather.
Thus, when processing PVC pipes and fittings, attention should be paid not only to the particle size, whiteness, moisture content, sediment volume, and mineral elements of calcium carbonate, but also to the alkalinity of calcium carbonate.