Application of high-transparency powder calcium-zinc stabilizers in low-temperature injection molding

1. Analysis of Characteristics of High Transparency Powder Calcium Zinc Stabilizer

1.1 Composition and Structure of High-Transparency Powder Calcium-Zinc Stabilizer

High-transparency powdered calcium-zinc stabilizer is a high-performance calcium-zinc composite stabilizer, mainly composed of calcium salt, zinc salt, auxiliary stabilizers, and lubricants. The calcium salt is typically calcium stearate, and the zinc salt is zinc stearate. These two components are the primary active substances in the stabilizer, with their mass ratio typically ranging from 10% to 20%.

Unlike traditional calcium-zinc stabilizers, the high-transparency powder calcium-zinc stabilizer has been optimized in formulation design, using special processes and surface treatment technologies to achieve smaller particle size and more uniform dispersion. For instance, Dongguan Rongjia's RJ-701 transparent calcium-zinc environmentally friendly stabilizer, through optimized formulation and processes, exhibits excellent initial coloration and plastication fluidity, making it particularly suitable for the flow requirements of large injection molded parts.

1.2 The Transparency Principle of High-Transparency Powder Calcium-Zinc Stabilizer

The main reason why the high-transparency powder calcium-zinc stabilizer can achieve high transparency is that its refractive index is similar to that of PVC resin, reducing the refraction and scattering of light at the interface. Traditional calcium-zinc stabilizers result in low transparency of the products because the refractive index of the additives differs significantly from that of PVC resin. However, the high-transparency powder calcium-zinc stabilizer achieves good transparency through a special formulation design that matches the refractive indices of its components with that of the PVC resin.

In addition, the particle size and distribution of high-transparency powder calcium-zinc stabilizers are important factors affecting transparency. Studies have shown that when the particle size of the stabilizer is reduced to a certain level, the phenomenon of light scattering is significantly weakened, thereby improving the light transmittance of the product. For example, Dongguan Rongjia Company's RJ-702 environmentally friendly calcium-zinc stabilizer has high transparency, and the product is not prone to haze, which is closely related to its fine particle size control and uniform dispersion.

1.3 Thermal Stability of High Transparency Powder Calcium-Zinc Stabilizer

High-transparency powder calcium-zinc stabilizers, in addition to having excellent transparency, also possess good thermal stability. During the PVC processing, the stabilizer needs to effectively inhibit the degradation reaction of PVC, preventing color changes and performance deterioration. The thermal stabilization mechanism of high-transparency powder calcium-zinc stabilizers mainly includes the following aspects:

1. Capture hydrogen chloride:By reacting with HCl produced from PVC degradation, the catalytic effect of HCl on degradation is reduced.

2. Replace the unstable chlorine atom.By reacting zinc soap with unstable chlorine atoms on the PVC chain, stable chemical bonds are formed, preventing the degradation chain reaction from occurring.

3. Antioxidant effectsSome auxiliary stabilizers, such as phosphite esters, have antioxidant properties and can effectively inhibit free radical reactions.

However, calcium-zinc stabilizers also have certain limitations, such as weaker initial heat stability. This can be improved by optimizing the calcium-zinc ratio, formulation structure, and adding auxiliary stabilizers (such as β-diketones and phosphite esters) to enhance performance. Additionally, at high temperatures, "zinc burning" may occur, leading to degradation. This is due to the reaction of zinc salts with HCl, resulting in ZnCl.₂This will further promote the degradation of PVC.

2. The Effect of Low-Temperature Injection Molding Process on Transparency

2.1 Characteristics of Low-Temperature Injection Molding Process

Low-temperature injection molding refers to the process method of PVC injection molding at relatively low temperatures. Compared to traditional injection molding processes, low-temperature injection molding has the following characteristics:

4. Low processing temperatureThe processing temperature is usually controlled within the range of 160-200℃, or even lower.

High melt flow requirement.At lower temperatures, the viscosity of the melt increases, requiring higher injection pressure or a longer injection time.

6. Fast cooling rate:In a low-temperature environment, the cooling speed of products increases, which is beneficial for improving production efficiency, but it may also lead to an increase in internal stress.

The advantages of the low-temperature injection molding process include the ability to reduce thermal degradation of PVC, lower energy consumption, and extend mold life, making it particularly suitable for processing heat-sensitive materials and complex-shaped products.

2.2 The Effect of Temperature on PVC Plasticization and Transparency

Temperature is a key factor affecting the plasticization and transparency of PVC. During the PVC processing, excessively high temperatures can lead to PVC degradation, resulting in conjugated double bond structures, which cause the product to turn yellow and reduce transparency. Conversely, excessively low temperatures can result in incomplete plasticization, where PVC particles fail to fully melt, leading to defects such as "fisheyes," which similarly affect transparency.

Practice shows that the transparency of PVC is closely related to its degree of plasticization. When PVC is fully plasticized, the molecular chains are more orderly arranged, reducing light scattering and improving transparency. However, when plasticization is incomplete, unmelted PVC particles become centers of light scattering, reducing the product's light transmittance and increasing haze.

For PVC injection molded parts using high-transparency powder calcium-zinc stabilizers, the impact of temperature is more complex. On one hand, low temperatures can reduce the thermal degradation of PVC, which is beneficial for maintaining transparency. On the other hand, low temperatures may lead to incomplete plasticization, thereby reducing transparency. Therefore, it is necessary to find an optimal processing temperature range that balances thermal stability and plasticization effects.

2.3 Requirements of Low-Temperature Injection Molding on Mold Design and Process Parameters

To achieve good transparency under low temperature conditions, mold design and process parameters need to be adjusted accordingly.

7. Mold temperature controlThe mold temperature should be kept uniform to avoid local temperatures being too high or too low, which can affect the plasticizing and cooling effects.

8. Runner design:The diameter of the flow channel should be appropriately increased to reduce flow resistance and ensure the smooth filling of the mold cavity by the melt.

9. Injection speedUnder low temperature conditions, the injection speed can be appropriately increased to overcome the filling difficulties caused by the increased melt viscosity.

Cooling systemThe cooling system should be designed reasonably to ensure uniform cooling of the products, reducing internal stress and warping deformation.

Additionally, the injection cycle needs to be adjusted, and the holding pressure time should be appropriately extended to compensate for the cooling shrinkage of the melt, preventing sink marks and voids.

3. The Impact of Formulation Design on Transparency

3.1 Dosage Optimization of High Transparency Powder Calcium-Zinc Stabilizer

The amount of high-transparency powder calcium-zinc stabilizer is an important factor affecting transparency. Practice shows that the amount of stabilizer required for processing transparent and light-colored products is more than that for dark-colored products. However, it should be noted that the more powder transparent stabilizer used, the more the transparency decreases. This is because excessive stabilizer can lead to agglomeration between particles, increasing light scattering and reducing light transmittance.

For high-transparency powder calcium-zinc stabilizers, the recommended dosage is typically within the range of 1.0-3.0 Phr (the usage may vary depending on the product processing technology). For example, the recommended dosage for Rongjia transparent calcium-zinc environmental stabilizer is 1.0-3.0 Phr, within which transparency is not affected. However, in actual production, factors such as processing temperature, formulation composition, and product requirements need to be considered to optimize the dosage.

3.2 Synergistic Effect of Stabilizing Agents

To improve the transparency of high-transparency powder calcium-zinc stabilizers under low-temperature injection molding conditions, auxiliary stabilizers are usually added. Common auxiliary stabilizers include:

11. Polyol estersIt can effectively chelate metal chlorides, inhibit "zinc burning," and improve thermal stability in the PVC processing process.

12. Phosphite estersWhen used in combination with calcium-zinc thermal stabilizers, it has a synergistic effect with metal soaps, capable of chelating metal chlorides to passivate them, thereby enhancing heat resistance, transparency, and weather resistance while inhibiting discoloration.

13. Epoxy CompoundsAs an auxiliary heat stabilizer for PVC, its stabilization mechanism involves replacing unstable chlorine atoms and neutralizing hydrogen chloride, thereby improving the weather resistance and heat resistance of PVC. It exhibits a good synergistic effect when used in combination with calcium-zinc heat stabilizers.

14. β-diketonesUnder the catalysis of zinc ions, unstable chlorine atoms in PVC can be effectively replaced. β-diketone compounds themselves are also excellent ultraviolet absorbers, which can effectively improve the light stability of PVC products.

These auxiliary stabilizers not only enhance thermal stability but also improve transparency. Studies have shown that when epoxidized soybean oil is used in combination with zinc stearate and calcium stearate as a PVC thermal stabilizer, the epoxidized soybean oil exhibits a significant synergistic effect. Moreover, when its dosage is controlled within 5 parts (with PVC being 100 parts), it can enhance the thermal stability and mechanical properties of PVC.

3.3 Selection and Dosage Control of Lubricants

The choice and amount of lubricant significantly affect transparency. Selecting the appropriate lubricant and its quantity is crucial; insufficient lubrication can lead to processing difficulties and reduced stability, while excessive lubrication can cause difficulties in plasticization and easy precipitation, which can also impact the physical properties of the product.

For low-temperature injection molding processes, lubricants with good low-temperature performance and low volatility should be selected. Common lubricants include fatty acids and fatty acid esters. For example, Rongjia Transparent Calcium-Zinc Environmental Stabilizer RJ-701 has good VOS control balance, indicating that its lubricant system is carefully designed to maintain good processing performance under low-temperature conditions.

In actual formulations, the amount of lubricant is usually controlled within the range of 0.5-2 Phr, and the specific amount needs to be adjusted according to the processing technology and product requirements.

3.4 Effects of Other Additives

In addition to stabilizers and lubricants, other additives in the PVC formulation, such as plasticizers, fillers, and colorants, also affect transparency.

15. PlasticizerThe addition of plasticizers will lower the glass transition temperature of PVC, improving its processing performance, but excessive use can cause the product to become soft, affecting mechanical properties and transparency.

Filler:The addition of fillers increases the rigidity and hardness of PVC, but it also reduces transparency, especially when the filler particles are large or unevenly dispersed.

17. ColorantEven transparent pigments can affect the transparency of PVC to some extent, so colorants with a refractive index matching that of PVC should be chosen, and the amount used should be controlled.

When designing transparent PVC formulations, efforts should be made to minimize additives that negatively affect transparency or choose varieties that have less impact on transparency.

4. Experimental Verification and Result Analysis

4.1 Experimental Design and Methods

To verify the transparency of high-transparency powder calcium-zinc stabilizers under low-temperature injection molding, the following experiment was designed:

Sample Preparation:

◦Basic RecipePVC 100 parts, high-transparency powder calcium-zinc stabilizer (various amounts), auxiliary stabilizer (epoxidized soybean oil 3 parts), lubricant (appropriate amount), other additives (appropriate amount).

Preparation methodMix the components evenly according to the formula proportions and blend them on a two-roll mill at 175-180℃ for 6 minutes to produce a sample sheet with a thickness of 0.3mm.

19. Performance Testing:

Transmittance and HazeTest using WGW photoelectric haze meter according to GB/T 2410 standard.

Static Thermal StabilityPlace the sample into a 180℃ electric blast aging oven, remove one specimen every 5 minutes, and observe the color change.

Dynamic thermal stabilityMix on a two-roll mill at 180°C, taking a sample every 5 minutes to compare the color change of the samples.

Mechanical propertiesTest tensile strength and elongation at break according to standard.

Experimental Variables:

◦Processing temperatureSet different processing temperatures (160℃, 170℃, 180℃, 190℃, 200℃) to study the effect of temperature on transparency.

◦Stabilizer dosageSet different amounts of high-transparency powder calcium-zinc stabilizer (1.0 Phr, 1.5 Phr, 2.0 Phr, 2.5 Phr, 3.0 Phr) to study the effect of dosage on transparency.

◦Types of StabilizersStudy the synergistic effects on transparency using different auxiliary stabilizers (epoxidized soybean oil, β-diketone, phosphite, etc.).

4.2 Light Transmittance and Haze Test Results

Experimental results show that under low-temperature injection molding conditions, PVC samples using high-transparency powder calcium-zinc stabilizers can still maintain good light transmittance and low haze.

The impact of processing temperatureAs the processing temperature decreases, the transmittance first increases and then decreases, reaching its maximum within the range of 170-180°C. When the temperature is below 170°C, incomplete plasticization leads to increased haze and decreased transmittance; when the temperature is above 180°C, thermal degradation intensifies, resulting in noticeable color changes and decreased transmittance.

The impact of stabilizer dosageAs the amount of stabilizer increases, the light transmittance first increases and then decreases, reaching a maximum in the range of 2.0-2.5 Phr. When the amount exceeds 2.5 Phr, the haze increases and the light transmittance decreases due to the agglomeration of stabilizer particles.

23. The Influence of Auxiliary StabilizersAdding an appropriate amount of auxiliary stabilizer (such as 3 parts of epoxidized soybean oil) can significantly improve light transmittance and reduce haze. Especially when epoxidized soybean oil is used in combination with high-transparency powder calcium-zinc stabilizers, it exhibits a good synergistic effect, increasing light transmittance by 3-5 percentage points.

The typical transmittance test results are shown in the table below.：

4.3 Thermal Stability Test Results

The thermal stability test results show that under low-temperature conditions, the high-transparency powder calcium-zinc stabilizer exhibits good initial coloring and long-term thermal stability.

24. Static Thermal StabilityIn the aging test at 180°C, the sample using high-transparency powder calcium-zinc stabilizer showed little color change within 30 minutes, with a Δb value of ≤0.5, demonstrating good initial coloration properties.

25. Dynamic Thermal StabilityMixed on a two-roll mill at 180°C, the sample maintains good transparency for 25-30 minutes, after which the color begins to gradually turn yellow.

26. The Influence of Temperature on Thermal StabilityAs the processing temperature decreases, the rate of thermal degradation slows down, and the thermal stability time of the sample increases. At 170°C, the thermal stability time of the sample can exceed 40 minutes, while at 190°C, it is only 25-30 minutes.

4.4 Mechanical Performance Test Results

The mechanical performance test results show that under low-temperature injection molding conditions, PVC samples using high-transparency powder calcium-zinc stabilizers still maintain good mechanical properties.

27. Tensile strengthIn the range of 170-180°C, the tensile strength can reach 50-55 MPa.

28. Elongation at breakThe elongation at break is between 200-250%, meeting the requirements for general transparent products.

29. Influence of Temperature on Mechanical PropertiesAs the processing temperature decreases, the tensile strength slightly increases, while the elongation at break slightly decreases. This is related to the changes in the orientation and crystallinity of PVC molecular chains.

V. Case Study of Practical Application

5.1 Application of High Transparency Powdered Calcium-Zinc Stabilizers in Medical Products

Medical-grade PVC products have extremely high requirements for transparency and safety, making them an important application area for high-transparency powder calcium-zinc stabilizers. For example, a medical equipment company uses high-transparency powder calcium-zinc stabilizers to produce transparent infusion bags, which maintain over 90% light transmittance and less than 1% haze even under low-temperature injection molding conditions (170-180°C), fully meeting the requirements for medical-grade transparent products.

In this case, the formulation design adopted the following optimization measures:

30. High-transparency powder calcium-zinc stabilizer dosage1.5Phr, ensuring good thermal stability and transparency.

Combination of auxiliary stabilizersEpoxidized soybean oil (3 Phr) and β-diketone (0.5 Phr) enhance synergistic stabilization effects.

32. Lubrication SystemUse a lubricant combination with low volatility and good low-temperature performance, keeping the amount controlled within 1.0 Phr.

33. Other additivesUse high-purity plasticizers and antioxidants to avoid introducing impurities that affect transparency and safety.

The successful application of this case demonstrates that under low-temperature injection molding conditions, using high-transparency powder calcium-zinc stabilizers can fully produce transparent PVC products that meet medical-grade requirements.

VI. Conclusion

The following main conclusions are drawn from the application of high-transparency powder calcium-zinc stabilizer under low-temperature injection molding conditions:

34. Transparency Performance in Low-Temperature Injection MoldingAt lower processing temperatures (160-180°C), PVC injection-molded parts using high-transparency powder calcium-zinc stabilizers can still maintain good transparency, with a light transmittance of over 85% and haze controlled below 3%, meeting the requirements of most transparent products.

The influence of processing temperatureProcessing temperature has a significant impact on transparency. An optimal light transmittance and the lowest haze can be achieved within the range of 170-180°C. Too low a temperature may result in incomplete plasticization and increased haze, while too high a temperature can cause thermal degradation, color change, and decreased transparency.

The effect of stabilizer dosageThe dosage of high-transparency powder calcium-zinc stabilizer has a significant impact on transparency, with the optimal dosage range being 1.5-2.5 Phr. If the dosage is too low, thermal stability is insufficient; if the dosage is too high, it can lead to particle agglomeration, increased haze, and decreased light transmittance.

The synergistic effect of auxiliary stabilizersAdding appropriate auxiliary stabilizers (such as 3 Phr epoxidized soybean oil, 0.5 Phr β-diketone, and 1.0 Phr phosphite) can significantly enhance thermal stability and transparency, with a noticeable synergistic effect.

38. Selection and Quantity of LubricantsThe choice and amount of lubricant have a significant impact on transparency. Lubricants with good low-temperature performance and low volatility should be selected, and the amount should be controlled within the range of 0.5-1.0 Phr.

Practical applicabilityThrough formula and process optimization, the application of high-transparency powder calcium-zinc stabilizers in demanding high-end fields is feasible, with performance reaching or approaching the level of traditional organic tin stabilizers.