Anyone working with PVC knows that yellowing is a major problem that cannot be avoided.

Yellowing of pipes, dirtiness of sheets, graying of cables…not only affects appearance but can also impair performance.

But do you know why PVC yellows? And how can it be effectively prevented?

Today, we’ll provide a comprehensive analysis of the yellowing mechanism, prevention and control measures, and testing methods.

Why does PVC yellow?

1. Polymer chain breakage ➤ Formation of conjugated double bonds ➤ Darker yellowing

▶ Root cause: High-temperature processing and repeated heating break C-Cl bonds, generating HCl and forming a conjugated polyene structure (a chain of conjugated double bonds).

▶ Color development mechanism: The more conjugated double bonds there are, the stronger the π electron’s light absorption ability, causing the color to deepen from light yellow to orange-yellow to brown-red.

• 4 double bonds: Yellowing begins (Δb ≈ 2-4)
• 6 or more: Rapid yellowing begins (Δb > 5)

▶ Detection method: UV-Vis absorption spectroscopy: Detect the absorption peak between 250-450nm. Colorimeter: The higher the yellowing index Δb*, the darker the color.

2. Thermal/photooxidation ➤ Carbonyl compounds are generated ➤ Yellowish-brown discoloration

▶ Reaction mechanism:

• High temperature or UV light → Oxidation of PVC molecules → Formation of carbonyl groups, conjugated ketones, and aldehydes
• Conjugated carbonyl structures absorb light (λ max ≈ 300-400nm) → Initiates yellowish or brownish discoloration

▶ Accelerated photooxidation: Chain reaction of reactive oxygen species (·O, ROOH) in the high-energy UV-A/B bands of UV light

3. Decomposition of nitrogen-containing additives ➤ Nitrosamines/azo compounds ➤ Darkening of color

▶ Cause:

• Under the influence of light or heat, amine stabilizers and antistatic agents decompose, generating chromophores.
• These chromophores form yellow complexes with metal ions (Fe³⁺/Cu²⁺), significantly darkening the color.

▶ Additional Note: Some low-molecular-weight HALS are also susceptible to decomposition, especially in the presence of acidic substances (such as stearic acid), causing them to discolor more easily.

How to effectively prevent PVC from yellowing?

Monitoring + Verification
Key to Ensuring Stable Results

▶ Accelerated Weathering Test (QUV)

• Aging using UVA-340 lamps, alternating heating/light cycles
• Regularly measure color difference Δb* to determine if yellowing exceeds the specified limit

▶ Molecular Structure Characterization

• FTIR: Detects absorption peaks at 1700 cm⁻¹ (carbonyl) and 1600 cm⁻¹ (conjugated double bonds)
• HPLC-MS: Identifies color degradation products such as quinones and nitrosamines

▶ Process Control

• Online temperature measurement and temperature control stability (±5°C)
• Prevents localized overheating or stagnation of the melt

PVC yellowing is a “chemical storm” caused by the combined effects of heat, light, and oxygen. To truly control it, we can’t rely solely on a single additive. Instead, we must develop a multi-faceted closed-loop approach encompassing formulation design, additive selection, processing control, and testing mechanisms. Only in this way can we achieve long-term anti-yellowing and stable appearance quality.