Evaluation method for thermal stability of PVC heat stabilizer

What are the evaluation methods for heat stabilizers?
What do these evaluation methods evaluate?
What characteristics should a good stabilizer possess?

Why does PVC need heat stabilizers?

PVC plastic has a major problem during processing (for example, when heated and melted): it is heat-sensitive! At high temperatures: 1. It turns yellow and black, affecting the product’s appearance, especially for transparent packaging. 2. It decomposes to release hydrogen chloride gas (HCl), which is not only unpleasant and corrosive to machinery and equipment, but also makes the plastic brittle and degrades. Heat stabilizers specifically address this problem, preventing PVC from yellowing and decomposing.

How do you know if a heat stabilizer is good?

Three key performance indicators are considered:

• Initial stability: Also known as initial discoloration, or color retention stability, ensures that the color of the same PVC product remains stable throughout any production cycle, and that color variations between production cycles remain within acceptable limits.
• Long-term stability: Ensures that even if a production failure disrupts smooth operation, even if the PVC material decomposes and discolors, the production process does not require downtime for mold or screw cleaning.
• Residual stability: The stability required to meet the product’s requirements for use under heat. In other words, when a finished PVC product is used as a test specimen, the thermal stability evaluated is the residual thermal stability.

Three Common Methods for Evaluating Heat Stabilizers

01. Static Test Method (Basic Test, Relatively Simple)

What is Measured? This method primarily measures the speed of discoloration and hydrogen chloride release of the plastic when exposed to elevated temperatures.

Oven Color Change Method:

• How to: Place the PVC sheet in an oven set to a predetermined temperature (typically 185°C).
• What to Observe: Record the time it takes for the sheet to noticeably darken. A longer time indicates better initial discoloration resistance (important for transparent products).

Congo Red Method:

• How to: Place small PVC pellets in a test tube and place a piece of Congo Red test paper at the mouth of the test tube. Insert the test tube into a 200°C constant-temperature oil bath.
• What to Observe: Record the time it takes for the test paper to turn from red to blue. The color change is caused by the release of hydrogen chloride gas from the decomposition of the PVC. A longer retention time indicates a greater resistance to decomposition (thermal stability). This is a commonly used standard method for measuring residual stability.

The cable industry often uses this method to evaluate the thermal stability of PVC insulation and sheath materials used in wires and cables, and it serves as the primary basis for the operating temperature rating of wires and cables.

02. Dynamic Testing Method (Simulating the Real Production Process)

What is measured? Simulating the conditions of PVC in a processing machine (such as an extruder or injection molding machine), where it is simultaneously subjected to high temperatures, air, and strong agitation (shear). This is the test that most closely resembles actual processing conditions and is therefore very important.

Core equipment: Torque rheometer (which simulates the temperature and shear forces within the machine).

Key indicators can be seen in the graph:

• Plasticization time: The time required for the plastic to be completely melted and mixed after it is added. Too short or too long a time is not optimal, as it can affect production efficiency and product quality. Ideally, it should simulate complete melting (plasticization) around the 2/3 mark of the extruder screw.
• Balance torque: The amount of force (energy) required to continuously agitate the plastic after it is completely melted and mixed. A lower value indicates lower energy consumption (more energy-efficient) during processing.
• Decomposition time: The time from when the plastic is completely melted to when it begins to decompose (the curve suddenly rises). A longer time indicates that the plastic can withstand high temperatures and agitation during processing, and thus better long-term stability.

03. Residual Stability Test (for Special Uses or Recycling)

What is measured? How much heat can a plastic product withstand after it is manufactured, or during reprocessing of recycled materials.

Most commonly used method: Congo Red test.

04. Summary (Qualities of a Good Heat Stabilizer)

• Strong Color Retention: Performs well in static tests (such as the oven test) and exhibits slow initial discoloration.
• Long Heat Resistance: Describes a long decomposition time in dynamic tests (torque rheometer).
• Excellent Processability: Demonstrates low equilibrium torque (more energy-efficient) and an appropriate plasticizing time in dynamic tests.
• Suitable for Recycling: Performs well in residual stability tests (such as the Congo Red test) and maintains a certain thermal stability even after multiple processes (such as recycling).

Select a heat stabilizer based on your needs

• Developing products with high transparency and whiteness requirements? Prioritize initial stability → Test using the oven color change method.
• Developing pipes, door and window profiles, or foam products? Prioritize long-term stability → Test using a torque rheometer (focus on decomposition time).
• Developing automotive wiring harnesses/jackets, high-temperature resistant components, or products with high recyclability? Prioritize residual stability → Test using the Congo red method.
• Developing recycled plastic products? Prioritize repeated processing resistance → Residual stability is a key indicator.

FAQ

Q: What is the simplest way to test PVC heat stabilizer performance? The oven color change method is the simplest: place a PVC sheet in an oven at around 185°C and record the time until it noticeably darkens. Longer discoloration time means better initial stability.

Q: Which test best reflects real production conditions? The dynamic test with a torque rheometer, because it subjects PVC to heat, air, and shear at the same time — just like an extruder or injection molding machine. Its decomposition time is the key indicator of long-term stability.

Q: What temperature is used in the Congo red test? The standard Congo red test uses a 200°C constant-temperature oil bath. The result is the time until the test paper turns from red to blue, indicating hydrogen chloride release.

Q: Why does residual stability matter for recycled PVC? Recycled PVC has already consumed part of its stabilizer during previous processing. A residual stability test shows how much protection is left, which determines whether the material can be reprocessed safely.

Choosing the right test method is only the first step — the result depends on the stabilizer system itself. Rallychem provides PVC heat stabilizer solutions backed by full static, dynamic, and residual stability testing. If you are comparing stabilizer suppliers or troubleshooting thermal stability issues, contact our technical team with your test data.