Color difference testing of plastics after aging quantifies how exposure to light, heat and moisture changes the appearance of polymer materials over time. By combining artificial weathering techniques such as xenon arc and fluorescent UV exposure with instrumental color measurement using the CIELAB color space, manufacturers can express discoloration as a single ΔE value and set objective pass/fail criteria. This article explains the sample requirements, testing items, standard methods and typical applications of color difference experiments on aged plastics, as performed by accredited third-party laboratories and in-house quality departments.
Technical Overview
Plastic materials discolor when ultraviolet radiation, heat, oxygen and moisture act on the polymer matrix, pigments, fillers and additives. Common degradation mechanisms include chain scission and oxidation of the resin, depletion of antioxidants and UV stabilizers, and photobleaching or darkening of organic colorants. The visible result is yellowing, fading, browning or graying of the material surface, which is often the first sign of aging even before mechanical properties deteriorate.
A color difference experiment after aging is designed to measure this change objectively. Specimens are first conditioned and measured to establish a baseline color, then subjected to a controlled aging regime, and finally re-measured under identical instrument settings. The change is expressed in the CIELAB color space, where L* represents lightness, a* represents the red-green axis and b* represents the yellow-blue axis. The total color difference ΔE*ab combines these three components into a single number that can be compared directly against product specifications.
This experiment is a core tool for material qualification, formulation comparison, stabilizer package evaluation and long-term quality assurance in industries where appearance retention is a contractual requirement. Because the results are quantitative and reproducible, they are widely accepted in supplier audits, customer complaints analysis and certification submissions to accredited third-party laboratories.
Testing Samples
Injection-Molded Plaques and Standard Panels
The preferred specimen is a flat, opaque plaque with a minimum thickness of 2 mm, produced under controlled molding conditions so that surface texture, gloss and internal stress are uniform. Standard panel sizes of 50 mm × 100 mm or 75 mm × 150 mm fit most specimen holders in xenon arc and fluorescent UV chambers. Plaques should be molded from the same resin lot and color masterbatch as the production material to ensure representativeness.
Finished Parts and Components
Complete products such as automotive trim, appliance housings, pipe fittings and electronic enclosures can be exposed directly. Curved, textured or grained surfaces require special attention: measurement geometry and location must be fixed and marked, because curvature and texture scatter light and inflate color difference values. Where possible, a flat area of at least the instrument port diameter should be selected for measurement.
Films, Sheets and Coated Specimens
Thin films and translucent sheets must be backed with a defined substrate, typically multiple layers of the same film or a standardized white or black backing, so that background effects do not distort the readings. Coated or painted plastic parts are tested with the coating system intact, since the aging behavior of the coating, primer and substrate together determines the final color shift.
Prior to testing, all specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618, normally for a minimum of 24 hours. Each specimen is labeled on the reverse side, and measurement positions are marked so that baseline and post-aging readings are taken at exactly the same points. A minimum of three replicate specimens per condition is recommended to account for statistical variation across the exposed surface.
Testing Items
The core deliverable of the experiment is the color difference between the aged and unaged states, together with the component changes that reveal the direction of the shift. Typical reporting items include:
- Total color difference ΔE*ab (or ΔE*00 under CIEDE2000), the headline acceptance criterion
- Lightness difference ΔL*, indicating fading or darkening
- Red-green difference Δa* and yellow-blue difference Δb*, where a positive Δb* indicates yellowing
- Yellowness index (YI) and whiteness index, calculated in accordance with ASTM E313 for white and near-white materials
- Gloss retention at 60° or 20° geometry per ISO 2813 or ASTM D523, since gloss loss affects perceived color
- Chroma difference ΔC* and hue difference ΔH*, useful for pigment-specific failure analysis
- Grey scale rating per ISO 105-A02 as a complementary visual assessment method
Acceptance limits are set by the product specification. In automotive and consumer electronics, a common requirement is ΔE*ab ≤ 1.0 to 1.5 for interior components after a defined exposure, while outdoor construction products may allow ΔE*ab ≤ 3.0 to 5.0 depending on the color family and market expectation.
Testing Methods
Artificial Weathering with Xenon Arc Lamps
Xenon arc exposure per ISO 4892-2, ISO 11341 or ASTM G155 is the most widely used method because its filtered spectrum closely matches terrestrial sunlight. Typical cycles combine radiant exposure at specified irradiance, such as 0.35 W/m² at 340 nm, with water spray and dark humid phases. Black panel or black standard temperature, chamber temperature and relative humidity are controlled parameters. Exposure durations range from several hundred to several thousand hours depending on the intended service life of the product.
Fluorescent UV-Condensation Exposure
Fluorescent UV lamps per ISO 4892-3 or ASTM G154 emphasize the ultraviolet portion of the spectrum and are particularly effective for screening UV-sensitive materials and reproducing condensation damage. The standard cycle alternates between UV irradiation at 60 °C and condensation at 50 °C. This method is economical and reproducible, making it popular for routine quality control of polymers, coatings and masterbatches.
Thermal and Long-Term Heat Aging
For materials used at elevated temperatures, oven aging per ASTM D573 or ISO 2578 evaluates color stability under dry heat. Specimens are held at a defined temperature, for example 100 °C to 150 °C, for fixed intervals and measured at each interval to build a color-versus-time curve. This approach is essential for engine bay components, electrical enclosures and other heat-loaded applications.
Color Measurement and ΔE Calculation
Color is measured with a calibrated spectrophotometer using a standard illuminant D65 and the 10° standard observer, with either d/8° sphere geometry or 45°/0° geometry selected according to the surface finish. Instrument operation follows ISO 11664-4, ISO 7724-3, ASTM E308 and ASTM D2244. The total color difference is calculated as:
ΔE*ab = √[(ΔL*)² + (Δa*)² + (Δb*)²]
The general test sequence is: condition and measure baseline color on all specimens, expose replicate sets for the specified duration, recondition to standard atmosphere, remeasure at the marked positions, and calculate the average ΔE for each exposure interval. Where perceptibility weighting is required, particularly for mismatch complaints, the CIEDE2000 formula (ISO/CIE 11664-6) may be applied instead of the classical ΔE*ab equation. A measurement uncertainty statement and instrument calibration traceability should accompany the report for third-party submissions.
Applications
- Automotive interior and exterior trim, where OEM specifications limit color shift after simulated sunlight exposure of 500 to 3000 hours
- PVC window profiles, siding, geomembranes and wood-plastic composites for outdoor construction and building certification
- Appliance housings, consumer electronics enclosures and white goods panels requiring appearance matching across assembled parts
- Packaging films and rigid containers where yellowing affects brand perception and food-contact compliance messaging
- Formulation development, including the comparison of stabilizer packages, pigment grades, recycled content levels and processing conditions
- Warranty claims, customer complaint investigation and incoming material acceptance for masterbatch and compound suppliers
- Aerospace, rail and solar energy components exposed to intense UV at altitude or in outdoor service
FAQ
What ΔE value is considered acceptable after aging?
There is no universal limit; the specification depends on the industry and application. As a guideline, a ΔE*ab below 1.0 is barely perceptible to a trained observer, values between 1.0 and 3.0 are noticeable on close inspection, and values above 3.0 are visible to most observers. Automotive interiors typically require ΔE ≤ 1.0 to 1.5, while outdoor building products commonly allow ΔE ≤ 3.0 to 5.0.
How long should the aging exposure last?
The exposure duration should simulate the intended service life. Typical screening protocols use 300 to 1000 hours of xenon arc or UV exposure, while qualification programs for outdoor products may extend to 2000 to 4000 hours or more. Interval measurements, for example every 250 hours, are recommended so that the discoloration trend over time can be plotted and extrapolated.
Xenon arc or fluorescent UV, which method should be chosen?
Xenon arc best reproduces full-spectrum daylight including visible light effects on color, and is preferred for final qualification and color-matching studies. Fluorescent UV is more aggressive in the UV region, faster and cheaper, and suits comparative screening of UV stability. When a correlation with natural weathering is required, ISO 465 or ASTM G7 outdoor exposure data should be used to validate the accelerated protocol.
Can finished products be tested directly instead of plaques?
Yes. Finished parts can be exposed in chambers equipped with suitable holders, provided that flat measurement areas are available or a suitable measurement jig is used. Testing the actual product captures the influence of processing, texture and assembly, and is often required for type approval, although results may be less reproducible than molded plaques.
How many specimens are required for a valid experiment?
A minimum of three specimens per material per exposure condition is standard practice, with color measured at multiple marked points on each specimen. A set of unexposed control specimens should be retained in the dark under standard conditions so that any instrument drift or baseline change can be corrected during the final calculation.
Is ΔE alone sufficient to judge discoloration?
ΔE summarizes the magnitude of change but not its direction. For failure analysis, the component values ΔL*, Δa* and Δb*, together with yellowness index and gloss retention, should always be reviewed. Two samples with identical ΔE can appear very different to the eye, which is why some specifications also require visual assessment against a grey scale or a physical limit sample under a standardized light booth.
