Inherent Differences in Molecular Structure

Not all plastics behave the same in identical environments. Some materials yellow or become brittle quickly under sunlight, while others remain stable for a long time. The reason lies in the “inherent differences” within their molecular structures.

Plastics Easily Affected by Light and Heat

• PVC (Polyvinyl Chloride)
Its molecular chains contain chlorine atoms, which easily undergo dehydrochlorination when exposed to light or heat. This reaction produces conjugated polyene structures (long-chain double bonds that absorb visible light), causing color changes from pale yellow to brown and eventually black.
• ABS (Acrylonitrile–Butadiene–Styrene)
The PB (polybutadiene) rubber phase contains double bonds that are highly sensitive to light and oxygen. As a result, ABS turns yellow and becomes brittle after sunlight exposure—its natural “weak point.”
• PC (Polycarbonate)
The carbonate groups in the polymer chain are sensitive to UV and high temperatures, causing chain scission that generates carbonyl groups. This leads to decreased transparency and yellowing—one reason PC headlamp covers require additional UV protection.
• PET (Polyethylene Terephthalate)
When moisture is present, hydrolysis may occur during processing, resulting in chain scission, darker color, and reduced toughness. Long-term light exposure also leads to oxidation and new carbonyl groups, reducing gloss in transparent parts.
• PA (Nylon)
Containing amine and amide groups, PA is sensitive to heat and oxygen and easily yellows. Its high water absorption also means inadequate drying will worsen performance and appearance.
• PP/PE (Polypropylene / Polyethylene)
Although common and inexpensive, their C–C backbone is still vulnerable to free radical attack under UV or heat. Without stabilizers, long-term outdoor exposure causes yellowing followed by chalking and embrittlement.

👉 A simple analogy: Some people naturally tan or burn easily due to sensitive skin, while others resist sunlight better. Likewise, different plastics have different “molecular genetics,” resulting in varying levels of weatherability.

🔍 Testing Insight: How to Determine Which Materials Resist Aging Better?

• QUV / Xenon Accelerated Weathering Tests: Used to compare color changes under identical light and temperature conditions—commonly applied to evaluate weatherability levels.
• FTIR (Infrared Spectroscopy): Observes changes in carbonyl peaks or double-bond structures to determine oxidation tendency.
• Mechanical Tests (Tensile, Impact): Yellowing often accompanies performance degradation. Mechanical data reveals both “appearance + performance” aging.
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👉 Summary: Plastic yellowing is not only caused by external conditions—it also reflects the material’s “genetic makeup.” Some plastics are inherently more sensitive to light and heat, while others are more resistant. Therefore, proper material selection and the right additives are essential during product design.
 

Read more –

Six Causes of Plastic Yellowing Part 1: Photo-oxidation

Six Causes of Plastic Yellowing Part 2: Thermal Oxidation

Six Causes of Plastic Yellowing Part 3: The “Side Effect” of Antioxidants

Six Causes of Plastic Yellowing Part 4: Additives and Impurities

Six Causes of Plastic Yellowing Part 5: Gas Fading

The Science of Stability: How UV Absorbers Extend Material Performance

🔹Chitec Technology | The Expert in Anti-Aging Solutions for Materials