Problems arising after replacing lead salts with PVC calcium-zinc stabilizers, cause analysis, and treatment methods

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1. What are the advantages of calcium-zinc stabilizers? 1. Environmentally friendly and free from heavy metal elements (lead, cadmium); it meets the requirements of the latest environmental standards in various countries (such as the EU’s RoHS Directive, REACH Regulation, etc.). 2. Resistant to sulfur contamination. In the field of PVC window and door profiles, the use of composite lead heat stabilizers tends to cause discoloration in some northern regions of China (where heavy use of sulfur-containing coal leads to acid rain), whereas calcium-zinc heat stabilizers are not prone to such contamination. 3. Good system switching capabilities. When switching from traditional composite lead heat stabilizers to organotin heat stabilizers, cross-contamination occurs, resulting in high costs associated with system conversion. In contrast, calcium-zinc heat stabilizers do not react with either of these two types of stabilizers; therefore, the cost of switching between them is low. 4. It has a low density, so the amount of calcium carbonate used can be increased appropriately to reduce costs. Compared to composite lead heat stabilizers, calcium-zinc heat stabilizers have a density that is approximately 40% lower. When switching to calcium-zinc heat stabilizers, the average weight per meter of PVC products decreases by about 2%. II. Mechanism of PVC calcium-zinc heat stabilizers The mechanism of action of composite calcium-zinc heat stabilizers is similar to the formula for Coca-Cola—there are many theories surrounding it. Here, we summarize some of the more credible and easy-to-understand mechanisms. 1. Factors affecting the ability of stabilizers to inhibit discoloration 1. Initial concentration of the stabilizer: The higher the concentration, the stronger the ability to inhibit discoloration ； 2. Alkalinity of the stabilizer: The stronger the alkalinity, the greater its ability to neutralize HCl. However, excessively strong alkalinity can accelerate the rate at which PVC loses HCl ； 3. Ability of the stabilizer to absorb HCl ； 4. Synergistic ability between the auxiliary stabilizer and zinc chloride. Therefore, an ideal auxiliary stabilizer should possess: 1. a high capacity to neutralize HCl without displacing zinc ions. 2. It can increase the initial effective concentration of the main stabilizer, with no degradation over time. 2. Mechanism of action and effects of several major types of auxiliary stabilizers
1. B-dioxide: It forms a complex with zinc soaps, thereby increasing the effective concentration of Zn; it also reduces alkalinity, slowing down the rate of HCl elimination. Therefore, it can effectively improve the transparency of Ca/Zn stabilizers, but lacks long-term stability. 2. Hydrotalcite: (1) Interlayer carbonate neutralizes HCl ； (2) After the carbonate is exhausted, the layered alumina and magnesium oxide neutralize HCl, but their effect is limited because they displace the primary stabilizer, zinc ions. It features neutralization and a large capacity; it surpasses Ca soap in this regard. It also has a longer duration of effect in inhibiting discoloration. 3. Polyols: (1) Neutralizing HCl in polyhydroxyl structures ； (2) Forms stable complexes with zinc chloride to suppress zinc burning. Effect: Suppresses zinc burning and improves stability. 4. Phosphites: (1) Form stable complexes with zinc chloride to suppress zinc burning ； (2) Acting as an antioxidant to absorb free radicals and terminate chain scission (primary antioxidant: bisphenol A ； Auxiliary antioxidants: triphenyl phosphite, bis(dizinc phosphite). 5. Epoxies: Promote the double-displacement reaction of zinc chloride to regenerate zinc soap. It provides post-stability, suppresses zinc burn effects, and exhibits good synergistic effects with polyols, phosphites, and primary stabilizers. III. Problems arising after replacing lead salts with calcium-zinc stabilizers and analysis of their causes 1. Unstable product color ； 2. It tends to precipitate during the production process in the die ； 3. The product is prone to yellow and black lines ； 4. The product changes color poorly over time, and it has poor aging resistance ； Analysis of the reasons for calcium and zinc deficiencies: (1) The range of processing is limited, so it is necessary to strictly adhere to the requirements of the processing procedures. Compared with traditional composite lead heat stabilizers, calcium-zinc heat stabilizers still exhibit relatively poorer long-term heat stability; they tend to yellow when exposed to longer processing times, and are also more sensitive to process temperatures. (2) The volatility content is high; it is recommended to use lubricants with high melting points and low volatility in the formulation. Compared with traditional composite lead thermal stabilizers, calcium-zinc thermal stabilizers have a higher content of organic substances, which tends to result in greater precipitation; therefore, more careful selection is required when choosing lubricants. Ester-based lubricants can effectively balance fluidity and demolding properties during plastic processing; they have a long production cycle, do not tend to precipitate, and can be widely used in formulations containing calcium-zinc stabilizers. (3) Calcium-zinc stabilizers: During the plasticization process, due to their high electronegativity, their polar groups have a certain affinity for the polar sites in PVC resin, forming bonds with strong bond energy. This reduces or eliminates the attractive forces between the ionic bonds in PVC, allowing the intertwined chain segments of PVC to diffuse more easily and reducing the boundaries between molecular groups, thereby facilitating the plasticization of PVC resin. This causes the resin to become partially plasticized in the feeding section, resulting in a rapid increase in melt pressure, a decrease in melt viscosity, an increase in temperature, and a reduction in the plasticization temperature. Resin transition plasticization occurs again. (4) Even the addition of a sufficient amount of lubricant to the calcium-zinc stabilizer system fails to prevent further plasticization of the resin within an adequate time frame; moreover, it disrupts the existing lubrication balance. In the later stages of use, the PVC melt consumes a large amount of heat stabilizer in the homogenization stage, yet it does not achieve the desired viscosity and elasticity, failing to meet the requirements for hard PVC production. Therefore, in the production process using the calcium-zinc system, it is recommended to use lubricants with high melting points and low volatility in the formulation; otherwise, various problems arise during production, which is a common issue in current applications.