Powder Precipitation in Rigid PVC Die? 3 Tips to Solve Stabilizer-Related Issues

Created on 10.10

In the production of rigid PVC, the problem of powder accumulation at the die lip and "white spots" on product surfaces is closely related to stabilizers in 90% of cases. Though seemingly a minor issue, it can lead to production halts and waste materials. Today, we will explain in detail why stabilizers cause precipitation, its hazards, and solutions.

I. Why Do Stabilizers Cause Powder Precipitation in Rigid PVC?

Rigid PVC tends to decompose and release hydrogen chloride (HCl) during high-temperature processing (e.g., pipe extrusion). Metal soap stabilizers (calcium-zinc, lead salt, barium-cadmium-zinc types) must be added to neutralize HCl. However, improper use can turn stabilizers into the "culprit" of precipitation:

1.Core Cause: Excessive Reaction Produces "Surplus Products" Metal ions in stabilizers (e.g., Ca²+, Zn²+, Pb²+) react with HCl to form metal chlorides (calcium chloride, zinc chloride, etc.). These inorganic substances have limited solubility in PVC melts. If the reaction is excessive (due to overused stabilizers or excessively high processing temperatures), they will exceed the solubility limit, precipitate out, and form powder that accumulates at the die after cooling.

2.Easily Overlooked Factor: Wrong Dosage and Compatibility The misconception that "more stabilizers mean greater safety" is incorrect: excess stabilizers cannot be dispersed and will precipitate directly. Additionally, if stabilizers have poor compatibility with fillers or processing aids, they will be "squeezed out" of the melt and form powder.

II. Three Major Hazards of Powder Precipitation

Appearance Rejection and Sharp Yield Drop Powder adheres to product surfaces, forming "white spots." Products sensitive to appearance (e.g., construction profiles, home appliance parts) will be directly rejected. The yield rate can drop from over 95% to below 60%, turning large amounts of raw materials into waste.

1.Reduced Performance

Rigid PVC Becomes "Brittle" Precipitated powder contains unreacted stabilizers or reaction by-products, leading to insufficient effective stabilizers in PVC.

2. During subsequent use

PVC decomposes and discolors easily at high temperatures (e.g., pipes exposed to summer sun); its impact resistance decreases at low temperatures (e.g., profiles cracking easily in northern winters), potentially causing engineering quality issues.

3. Production Interruption and Schedule Delays

Accumulated powder clogs the flow channel, resulting in uneven melt flow or even material breakage. Production often needs to be halted for cleaning every hour, taking over 20 minutes each time. This leads to a reduction in daily output by hundreds of meters of products.

III. 3 Tips to Solve Stabilizer-Related Precipitation

1. Choose Composite Stabilizers Instead of Single-Component Ones

If precipitation is caused by metal chlorides, prioritize replacing with "composite metal soap stabilizers" (e.g., "calcium-zinc + epoxy/phosphite auxiliary components"). Such stabilizers inhibit the formation of metal chlorides and ensure uniform dispersion. After a profile factory switched to this type, metal chloride production decreased by 40%, and white spots disappeared.

2.Control Stabilizer Dosage

Add Precisely According to Formulas More stabilizers are not better: when producing PVC pipes, the addition amount of calcium-zinc stabilizers is usually 1.5%-2.5% of PVC resin. If the processing temperature is high (180-190℃), it can be increased by 0.2%-0.3%, but should not exceed 3%. Before changing formulas, small-scale tests are recommended: add different doses of stabilizers to a small amount of raw materials, extrude, and observe precipitation.

3.Test Additive Compatibility to Avoid "Conflict"

Precipitation may also occur if stabilizers are incompatible with fillers (e.g., calcium carbonate) or other additives. Request "compatibility reports" from suppliers when purchasing, or conduct self-tests: mix stabilizers and fillers in formula proportions, heat to processing temperature, and replace with special stabilizers (e.g., calcium-zinc stabilizers for high-filler formulas) if powder appears.

IV. 3 Prevention Tips to Reduce Precipitation at the Source

1. Choose Regular Stabilizers and Reject Inferior Products

Inferior stabilizers contain impurities (e.g., unreacted metal salts), which easily precipitate directly. Select products from regular manufacturers and request "composition test reports," focusing on metal ion ratios (e.g., Ca/Zn ratio in calcium-zinc stabilizers) and impurity content.

2.Stabilize Processing Temperature: Avoid Fluctuations

Excessively high temperatures accelerate the reaction between stabilizers and HCl; excessively low temperatures cause poor stabilizer dispersion—both lead to precipitation. Control the extrusion temperature of rigid PVC at 170-185℃, record the temperature hourly, and calibrate equipment promptly if fluctuations occur.

3.Proper Storage: Prevent Moisture and Mixed Placement

Stabilizers absorb moisture and clump (losing dispersibility); mixing with chemicals causes deterioration. Store in a dry, well-ventilated area, seal with airtight bags, and reseal promptly after use.

Final Notes

The key to solving powder precipitation in rigid PVC dies lies in "using stabilizers correctly." Remember: select the right type, control the dosage, ensure compatibility, and match with stable processes—this will fundamentally resolve the issue.

If you encounter problems such as persistent precipitation after switching to composite stabilizers or uncertainty about dosage calculation, feel free to leave comments in the comment section to discuss!

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