Problems and treatment methods of PVC calcium and zinc stabilizers after replacing lead stabilizer

Problems and and treatment methods of PVC calcium and zinc stabilizers after replacing lead salts

1. What are the advantages of calcium and zinc stabilizers?

1. Green and environmental protection
Without heavy metal elements (lead, separator), it can meet the requirements of the latest environmental protection standards of various countries (such as the EU RHOS directive, REACH directive, etc.).

2. Resist sulfidation pollution.
In the field of PVC door and window profiles, the use of composite lead heat stabilizers is prone to blackening in some parts of northern my country (a large amount of sulfur-containing coal causes acid rain), while calcium and zinc heat stabilizers will not be polluted.

3. Good system switching characteristics.
When the traditional composite lead heat stabilizer is converted into an organotin heat stabilizer, cross-contamination will occur, resulting in high system switching costs, while the calcium-zinc heat stabilizer will not react with these two heat stabilizers, and the cost of mutual switching is low.

4. The density is low, and the amount of calcium carbonate can be appropriately increased to reduce the cost.
Compared with the composite lead heat stabilizer, the density of calcium zinc heat stabilizer itself is about 40% lower, and the average weight of PVC products is reduced by about 2% only when the heat stabilizer is switched.
Second, the mechanism of PVC calcium zinc heat stabilizer

The mechanism of action of the complex calcium-zinc heat stabilizer is like the formula of Coca-Cola. There are different opinions. Now, some more convincing and easy-to-understand mechanisms are summarized.

1. Factors affecting the ability of stabilizers to inhibit discoloration
1. Initial concentration of stabilizer: the higher the concentration, the stronger the ability to inhibit discoloration;
2. Stabilizer alkalinity: the stronger the alkalinity, the stronger the ability to neutralize HCl, but the strong alkalinity will promote the speed of PVC removal of HCl;
3. The ability of the stabilizer to absorb HCl;
4. Compatibility of auxiliary stabilizer and zinc chloride.

Therefore, the ideal auxiliary stabilizer should have:
1. The ability to neutralize HCl is large, but it does not replace zinc ions.
2. It can increase the initial effective concentration of the main stabilizer, and it can not decay for a long time.
2. Action mechanism and effect of several types of auxiliary stabilizers
1. B-diketone: It forms a complex with zinc soap to increase the effective concentration of Zn, and reduces the alkalinity to slow down the speed of HCl removal. Therefore, the transparency of the Ca/Zn stabilizer can be effectively improved, but there is no long-term stability.
2. Hydrotalcite: (1) Interlayer carbonates neutralize HCl; (2) After the carbonates are exhausted, aluminum oxide and magnesium oxide in the layers neutralize HCl, but the effect is limited because they will replace the main stabilizer zinc ions. Features: The neutralization capacity is larger than that of Ca soap, and the effect of inhibiting discoloration for a long time is better.
3. Polyol: (1) The polyhydroxyl structure neutralizes HCl; (2) It forms a stable complex with zinc chloride to inhibit zinc burning. Effect: inhibit zinc burning and improve stability.
4. Phosphite: (1) form a stable complex with zinc chloride to inhibit zinc sintering; (2) as an antioxidant to absorb free radicals to terminate chain breakage (main antioxidant: bisphenol A; auxiliary antioxidant: bisphenol A) Triphenyl Phosphate, Benzyl Diisozinc Phosphite).
5. Epoxy: Promote the metathesis of zinc chloride to regenerate zinc soap. It has late-stage stability, and has the effect of inhibiting zinc sintering, and has good synergistic effect with polyols, phosphites and main stabilizers.

3. Analysis of the problems and causes after the replacement of lead salts stabilizers by calcium-zinc stabilizers

1. The color of the product is unstable;
2. It is easy to separate out during the die production process;
3. The product is prone to yellow and black lines;
4. The product has poor discoloration in the later stage and poor aging resistance;

Analysis of the reasons for the deficiency of calcium and zinc:

(1) The processing range is narrow, and the processing technology requirements must be strictly implemented. Compared with the traditional composite lead heat stabilizer, the long-term heat resistance stability of calcium zinc heat stabilizer is still relatively poor, the residence time is long during processing, it is easy to yellow, and the sensitivity to process temperature is also high.

(2) The volatile content is high, and it is recommended to use lubricants with high melting point and low volatility in the formula.
Compared with the traditional composite lead heat stabilizer, the calcium-zinc heat stabilizer has a high organic content, which is easy to cause excessive precipitation, and the choice of lubricant needs to be more cautious. Ester lubricants can effectively balance the fluidity and release properties of plastics during processing, with long production cycle and not easy to precipitate, and can be widely used in the formulation of calcium and zinc stabilizers.

(3) Calcium-zinc stabilizer, in the process of plasticization, due to the large electronegativity, the polar group has a certain affinity with the polar point of the PVC resin, forming a bond with strong bond energy, thereby weakening or It eliminates the attraction of ionic bonds in each layer of PVC, and the intertwined segments of PVC are easy to diffuse, and the boundaries between molecular groups are easy to be small, which promotes the plasticization of PVC resin. This causes the resin to be partially plasticized in the feeding section, the melt pressure increases rapidly, the melt viscosity decreases, the temperature increases, and the plasticizing temperature decreases. Resin transition plasticization occurs again.

(4) Adding a sufficient amount of lubricant to the calcium-zinc stabilizer system cannot prevent the resin from being further plasticized in a sufficient time, but also destroys the original lubrication balance, and the consumption of PVC melt in the homogenization section in the later period of use At the same time, it can not achieve the ideal viscosity and elasticity, and can not meet the needs of rigid PVC production.
 
  Therefore, in the production process of calcium-zinc system, it is recommended to use lubricants with high melting point and low volatility in the formula, otherwise there will be continuous problems in production, which is a common problem in the current use process.