Common Evaluation Methods for the Stability of PVC Stabilizers

Stabilizers are one of the most essential and indispensable materials in PVC products.  

Unlike the past, where the market was dominated by tribasic lead sulfate, dibasic lead stearate, lead stearate, and barium stearate, today's stabilizers have evolved.  

Except for transparent products and a few soft products that still use organotin or liquid calcium-zinc-barium-cadmium stabilizers, most of the market is now occupied by composite stabilizers. These include calcium-zinc composite stabilizers, lead-based composite stabilizers, hydrotalcite-based stabilizers, and rare-earth composite stabilizers.

Composite stabilizers simplify the formula optimization process for manufacturers and reduce application complexity. So how can we evaluate the effectiveness of a stabilizer’s thermal stability?

From the perspective of application requirements, the stability of PVC stabilizers can be classified into:

- Initial Stability: Refers to color retention. During any production cycle, from mixing to finished product, the color variation should remain within acceptable limits.

- Long-term Stability: Refers to the stabilizer’s ability to prevent discoloration caused by accidental degradation without halting production for cleanup.

- Residual Stability: Refers to the long-term stability of the final PVC product during its usage phase.

From the testing perspective, thermal stability is generally divided into:

- Static Thermal Stability: The stabilizer’s ability to prevent PVC decomposition under heat or heat plus air, without mechanical shear.

- Dynamic Thermal Stability: The stabilizer’s effectiveness under the combined effects of heat, air, and shear force during actual processing conditions.

I. Static Thermal Stability Testing Methods

1. Static Oven Test Method  

This is the simplest and fastest method for preliminary screening of many new stabilizers.  

A small piece is cut from a prepared film and placed on trays in a hot air circulation oven at 180°C. At regular intervals (e.g., every hour), one sample is removed to measure changes in whiteness and yellowness.  

To test weather resistance, UV lamps and acid water sprays can be added, and the temperature adjusted accordingly.

2. Congo Red Test Method  

This standard method assesses the static thermal stability of PVC. The oil bath is kept at 180±1°C.  

- The time when the bottom of the Congo red paper begins to turn blue indicates the stabilizer’s initial thermal stability.  

- The time when the paper turns completely blue indicates long-term thermal stability.  

This method is commonly used in the wire and cable industry to classify temperature grades. It is also used to evaluate the residual stability of recycled PVC materials.

II. Dynamic Thermal Stability Testing Methods

1. Two-Roll Mill Method  

Using a two-roll mill heated to 180°C, the PVC formulation is continuously processed. Every 5 or 10 minutes, a small sample is taken, cooled, and tested for color changes using a whiteness meter.

2. Torque Rheometer Mixing Method  

This method sets specific temperature and rotor speed conditions. The material is monitored from feeding to decomposition through torque curves and color changes.  

It is simple, efficient for comparison, and is widely favored by both stabilizer producers and downstream customers.

3. Extruder Testing Method  

Using the same PVC formulation but different stabilizers, the compound is processed through an extruder. The shear from the barrel, screw, die, and nozzle, combined with heat, is applied. If the product color darkens significantly, it indicates poor stability.  

However, this method is more production-based, and scrap generation is often unavoidable.