Polymer materials may deteriorate during synthesis, storage, processing, or end-use, manifesting as performance degradation such as yellowing, reduced molecular weight, surface cracking, loss of gloss, or severe declines in mechanical properties (e.g., impact strength, flexural strength, tensile strength, and elongation). This deterioration, termed "chemical aging" or simply "aging," compromises material functionality. Chemically, polymer materials—whether natural or synthetic—possess molecular structures with inherent weak bonds that act as reaction sites.
Aging is essentially a chemical process initiated by reactions at these weak bonds (e.g., oxidation), triggering cascading reactions. Causes include heat, ultraviolet (UV) light, mechanical stress, high-energy radiation, electric fields, or combinations thereof, altering molecular structures, reducing molecular weight, or inducing crosslinking. These changes degrade material performance, leading to failure.
Common Aging Factors
Heat and UV light are predominant aging triggers, as polymers are frequently exposed to these during production, storage, processing, and use. Studying heat- and UV-induced aging is particularly critical for practical applications.
Why Conduct Aging Tests?
- Material/formulation screening
- Competitor benchmarking
- Failure mechanism identification
- Anti-aging performance enhancement
- Service life prediction
Outdoor Exposure Testing: Advantages and Limitations
Direct outdoor exposure evaluates weather resistance under natural sunlight and climate conditions.
Advantages
- Highly representative of real-world conditions
- Simple and low-cost to implement
Disadvantages
- Prolonged testing duration
- Variability in global climates
- Inconsistent sample sensitivity across regions
Aging Test Methods
1. Carbon-Arc Light Aging Test
An older technology, carbon-arc lamps were initially used to assess lightfastness in dyed textiles. Available in enclosed or open configurations, their spectral output differs significantly from sunlight. Despite being superseded by newer methods, carbon-arc testing remains in historical standards, particularly early Japanese protocols.
2. Xenon-Arc Aging Test
Xenon lamps replicate full solar spectra, including UV, visible, and infrared light. Filtered xenon arcs are ideal for testing light-sensitive pigments, dyes, and inks. Spectral distribution is adjustable to simulate diverse conditions, from extraterrestrial sunlight to daylight through glass. By controlling irradiance, temperature, and humidity, xenon devices can replicate environments like automotive interiors/exteriors. The figure compares xenon lamp spectra (e.g., 0.55 W/m² irradiance) to natural sunlight. Xenon aging tests are now a primary method, with protocols standardized by ISO, ASTM, SAE J, and GM.
3. Fluorescent UV Lamp Aging Test
Fluorescent UV lamps are low-pressure mercury lamps (254 nm) modified with phosphors to emit longer wavelengths. Classified as UVA or UVB, lamp selection depends on application requirements. The table below summarizes UV lamp types and applications.
4. Metal Halide Aging Test
Metal halide lamps, gas-discharge devices using metal halides, produce spectral distributions closely resembling direct and diffused sunlight. Their large size suits accelerated aging tests for vehicles, components, and electrical/electronic products.
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