Four Major Failure Symptoms of Heat Stabilizers and Countermeasures

Created on 04.14

Even with the right stabilizer chosen, various failures may still occur during processing. Below are the four most common failure modes in production and their solutions:

Failure 1: Initial Discoloration (Yellowish or Reddish Feed at Startup)

Phenomenon: The product turns yellowish or reddish within the first few minutes of extrusion or injection molding, and returns to normal after processing stabilizes.

Root Cause: The stabilizer has insufficient initial thermal stability and cannot quickly capture trace HCl generated in the early processing stage.

For Ca-Zn systems, it may be caused by low zinc content or poor synergy of co-stabilizers (e.g., polyols, phosphites).

Countermeasures:

· Increase the proportion of initial stabilizers, such as adding a small amount of β-diketone (e.g., dibenzoylmethane) or phosphites.

· Check the screw configuration and excessive shear heat, as physical overheating leads to decomposition.

Failure 2: Zinc Burning (Sudden Blackening)

Phenomenon: In Ca-Zn system production, the product color suddenly changes from normal or slightly yellow to dark brown or even black (commonly called "black stock") in a very short time.

Root Cause: This is a unique "sudden death" phenomenon of Ca-Zn stabilizers. When zinc soap reacts with HCl to form ZnCl₂, a strong Lewis acid, it drastically catalyzes the deep decomposition of PVC. Zinc burning occurs when co-stabilizers are used up and can no longer inhibit the catalytic activity of ZnCl₂.

Countermeasures:

· Golden rule: Keep a reasonable Ca/Zn ratio and avoid excessive zinc.

· Add "zinc inhibitors" (e.g., polyols, hydrotalcites, zeolites) to complex ZnCl₂ and extend the stabilization time.

· Strictly control the processing temperature, as large temperature fluctuations easily trigger zinc burning.

Failure 3: Plate-out and Fouling

Phenomenon: After hours of continuous production, brown or white deposits form on die openings, screws or calender rolls, causing scratches or pits on the product surface.

Root Cause:

· Low-molecular-weight substances in stabilizers (e.g., stearic acid, waxes, unreacted lubricants) separate from the melt under high temperature and pressure and deposit on metal surfaces.

· Imbalanced lubrication system, with excessive external lubricant causing plate-out.

Countermeasures:

· Use high-molecular-weight or reactive stabilizers.

· Adjust the lubrication balance: properly reduce external lubricants (paraffin wax, PE wax) and increase internal lubricants (stearic acid, oxidized polyethylene wax).

· Clean molds regularly and add a small amount of anti-plate-out additives (e.g., polyester plasticizers) to the formula.

Failure 4: Weathering Failure (Chalking and Discoloration in Outdoor Use)

Phenomenon: PVC products for outdoor use (e.g., door and window profiles, roof tiles) show severe chalking, fading or reduced impact strength within several months to a year.

Root Cause:

· Thermal stabilizers alone cannot resist ultraviolet rays. The formula lacks sufficient UV absorbers (UV-531, UV-326) or hindered amine light stabilizers (HALS).

· Poor hydrolysis resistance of the stabilizer itself leads to failure in hot and humid environments.

· Countermeasures:

· Outdoor products must adopt a dual system of "thermal stabilization + light stabilization".

· Compound rutile titanium dioxide (high shielding performance) and light stabilizers in Ca-Zn or organotin systems.

· Avoid metal soaps with poor saponification resistance (e.g., poor durability of lead salts in some acidic environments).

Future Trends: Green, Efficient and Multifunctional

With the advancement of the global carbon neutrality goal and stricter environmental regulations (e.g., EU RoHS, REACH, China GB/T 33284), the PVC heat stabilizer industry is undergoing profound changes:

1. Full lead-free transition: Ca-Zn composite stabilizers are fully replacing lead salts, which is a settled trend especially in water supply pipes and profiles. Future competition will focus on whether Ca-Zn systems can fully match lead salts in weather resistance and electrical insulation.

2. Rise of Organic Heat Stabilizers (OBS): For specific high-end fields, fully organic stabilizers (e.g., uracil derivatives, amino crotonates) are seizing the organotin market due to their complete non-toxicity, high transparency and resistance to zinc burning.

3.Hyperbranched and high-molecular-weight development: R&D of high-molecular-weight or reactive heat stabilizers to fundamentally solve the problems of migration, plate-out and food contact safety.

Conclusion

The selection of PVC heat stabilizers is not a simple direct application, but a systematic project involving chemical balance, processing technology, cost control, and environmental compliance. When selecting stabilizers, clarify the final application of the product (transparent/opaque, soft/hard, indoor/outdoor); during processing, always watch out for the three major processing hazards: initial discoloration, zinc burning, and plate-out. Only by fully understanding the properties of stabilizers can we produce high-yield, high-performance, and cost-effective products in the increasingly competitive PVC products market.

Disclaimer: The content is compiled from the Internet for study and exchange only, with no commercial purpose.

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