The pH level of calcium carbonate can actually influence the brittleness and yellowing of PVC pipes.
Calcium carbonate, serving as a crucial filler, is widely utilized in the processing and production of PVC pipeshttps://www.useencalcium.com/nano-calcium-carbonate-plastic-masterbatch-product/; however, PVC products manufactured by many companies suffer to varying degrees from issues such as yellowing and brittleness. This paper examines the impact of calcium carbonate's alkalinity on the brittleness and yellowing of PVC pipes—a factor that is frequently overlooked by many manufacturers.
The alkalinity of industrial precipitated calcium carbonate essentially refers to the presence of "free alkali" resulting from specific factors during the manufacturing process. Free alkali denotes material present in the calcium carbonate product—specifically calcium hydroxide—that failed to undergo complete conversion into calcium carbonate during production. Excessive alkalinity can react with other plasticizers within plastic formulations, leading to defects such as brittleness and yellowing. Consequently, free alkali constitutes a critical technical parameter for calcium carbonate products and must be strictly controlled during the manufacturing process.
The primary cause of high calcium carbonate alkalinity is the formation of basic calcium carbonate.
Overburnt Lime
During the lime calcination process, variations in lump size and improper control can easily lead to the phenomenon of "overburning." Overburned lime requires a greater quantity of water for hydration; furthermore, if the water temperature is too low, hydration remains incomplete, resulting in the generation of fine lime particles. During carbonation, the resulting calcium carbonate utilizes these fine particles as nucleation sites, depositing upon them to form a structure in which calcium oxide particles are encapsulated by calcium carbonate. It is well established that calcium oxide crystallizes in a cubic system, whereas calcium carbonate crystallizes in a rhombohedral system. Since the interfacial angles of these two crystal structures differ, they exhibit distinct coefficients of thermal expansion upon heating; this disparity causes the crystal grains to fracture, thereby releasing the calcium oxide and rendering the material alkaline.
Free alkali levels are elevated
Basic calcium carbonate—particularly during cold weather—forms because the low ambient temperature results in a higher solubility of calcium hydroxide. During the carbonation process, the lime slurry contains both solid calcium hydroxide particles and dissolved calcium hydroxide ions. The subsequent carbonation reaction with carbon dioxide takes place within this alkaline solution, thereby giving rise to basic calcium carbonate. The specific form of this basic calcium carbonate varies depending on the temperature of the carbonation slurry and the rate at which carbon dioxide is introduced, ultimately transforming into one of three distinct crystalline structures of calcium carbonate (calcite, vaterite, or aragonite). When the carbonation process reaches its endpoint—at a pH of 8 to 10—the solution remains slightly alkaline; consequently, the basic calcium carbonate present is not broken down. Before it has sufficient time to undergo further transformation, this basic calcium carbonate proceeds to the next stage of processing. Upon entering the rotary dryer, the rising temperature causes the basic calcium carbonate to decompose into calcium hydroxide and calcium carbonate, resulting in an elevated level of alkalinity in the final product.
This issue of elevated alkalinity is significantly more pronounced in cold weather than in hot weather. The key distinction lies in the fact that during hot weather, both the ambient temperature and the water temperature are higher; this facilitates more efficient slaking of the lime, and the temperature within the carbonation tower is also relatively elevated. Under these conditions, the solubility of calcium hydroxide is lower, which inhibits the formation of plate-like basic calcium carbonate and instead promotes its conversion directly into calcium carbonate. Thus, it is evident that the alkalinity of the product produced during hot weather is lower than that produced during cold weather.
Therefore, when processing PVC pipes and fittings, one must pay attention not only to the particle size, whiteness, moisture content, sedimentation volume, and mineral elements of the calcium carbonate, but also to its alkalinity!
