Introduction to Yellowness Index Testing
Yellowness Index (YI) testing is a method of determining how a material turns yellow under certain conditions. It is frequently used to evaluate plastics, paints, textiles, papers, adhesives, and clear film for color uniformity. The most commonly accepted standard for testing the performance of the yellowness index is ASTM E313 (Standard Practice for Calculating Yellowness and Whiteness Indices from Instrumentally Measured Color Coordinates).
The standard provides procedures for calculating yellowness values from instrumentally measured color coordinates obtained with a spectrophotometer or colorimeter. The yellowing of materials can be caused by ultraviolet, thermal degradation, oxidation, and/or chemical aging. Thus, manufacturers rely on ASTM E313 to determine the magnitude of these variations and to reproduce a product’s appearance and quality. The standard helps control product quality, compare products with one another, support studies, and guide accelerated aging tests across various industries.
Scope of ASTM E313
- Develops methods to measure yellowness and whiteness indices.
- Works successfully with both white and near-white materials.
- Works with color coordinates from the instrument measurements
- Assesses discoloration from aging, heat, and light
- Supports quality control and product comparison processes
- Applies to most plastics, coatings, textiles, films, and paper products
- Describes the various color stability methods and analyzes their pros and cons
Principle of ASTM E313
The principle of ASTM E313 is to determine the degree of yellow discoloration in a material using numerical color measurements obtained from a spectrophotometer or colorimeter. The instrument records values of reflected or transmitted light under controlled illumination and observer conditions. The standard then uses mathematical calculations to transform these color coordinates to a value for the Yellowness Index.
The higher the Yellowness Index, the more likely that the material is yellowed or degraded. This method is suitable for manufacturers to compare a fresh or control sample with an aged sample with high accuracy and to check for any changes resulting from environmental exposure, processing, or long-term storage.
Test Procedure Overview of ASTM E313
ASTM E313 specifies that the specimen must be properly prepared and have smooth, clean surfaces in order to obtain accurate optical measurements. Samples must be free from scratches, dust, fingerprints, or surface contamination and must represent the production material. Specimens are conditioned before being tested in a laboratory under controlled environmental conditions.
| Instrumentation | A spectrophotometer or colorimeter measures reflected or transmitted light values under specified illuminant and observer settings. |
| Sample Preparation | Samples must remain clean, flat, and representative of the material batch. Transparent materials may require transmission measurements. |
| Measurement Process | The instrument records color coordinates, and the standard formula calculates the Yellowness Index value for comparison and evaluation. |
| Result Expression | Results are reported as numerical Yellowness Index values that indicate the degree of yellow discoloration. |
| Environmental Exposure | Some evaluations include UV aging, thermal aging, or humidity exposure prior to measurement to assess long-term color stability. |
Significance of ASTM E313
ASTM E313 is an important test for assessing the visual quality and durability of materials susceptible to discoloration. A yellowish tint can impair the acceptability of a product in consumer products, medical products, packaging materials, and optical applications, even in minor quantities. So, manufacturers rely on the standard to ensure consistency of appearance and to catch early signs of degradation.
The standard also assists researchers in comparing material formulations and assessing the effectiveness of stabilizers, antioxidants, and UV-resistant additives. The procedure yields numerical values, allowing laboratories to closely track color changes throughout product development, production, and accelerated aging studies.
Applications of Yellowness Index Testing
- ASTM E313 is used in many industries to ensure that the color stability and appearance of finished products and raw materials are maintained.
- This testing is used to assess the thermal stability, UV resistance, and oxidation resistance of polymers, resins, and molded products for use by plastic manufacturers.
- Sunlight exposure, curing temperatures, and environmental aging are examples of variables monitored to ensure the decorative and protective performance of the coating.
- YI is used to evaluate transparent films, containers, and food packaging materials for visual clarity and appearance stability over time.
- The standard is used to measure the retention of brightness and the discoloration of textiles and paper during storage, processing, and environmental exposure.
- Low yellowness values are important in medical devices, lenses, and optical components due to the need for transparency, appearance, and long-term functional performance.
