Plastic pellet yellowness index (YI) testing quantifies the degree of yellow discoloration in polymer resins, providing an objective indicator of material purity, degradation, and process stability. Performed with calibrated spectrophotometers in accordance with ASTM E313 and ISO 17223, this test supports incoming inspection, quality control, and aging evaluation across a wide range of thermoplastics. This article explains the testing principle, sample preparation methods, applicable standards, and typical applications of yellowness index measurement for plastic pellets.
Technical Overview
The yellowness index (YI) is a single-number colorimetric parameter that quantifies the tendency of a material to appear yellow rather than colorless or white. In plastic pellets (resin granules), an elevated YI typically signals polymer degradation during polymerization, drying, extrusion, or storage; thermal oxidation; contamination; or residual impurities such as monomers, catalyst residues, or degraded additives. Because many end products—especially transparent or white items such as appliance housings, films, fibers, and packaging—are judged by their visual whiteness and clarity, YI is one of the most widely monitored quality metrics in the plastics supply chain.
Yellowness index is calculated from CIE tristimulus values (X, Y, Z) measured with a spectrophotometer or colorimeter. Under ASTM E313, the general form of the calculation is:
YI = 100 × (Cx·X − Cz·Z) / Y, where Cx and Cz are coefficients that depend on the illuminant and observer angle (for example, D65/10° or C/2°).
A positive YI value indicates yellowness, a value near zero indicates a neutral appearance, and a negative value indicates blueness. The historical standard ASTM D1925 was written specifically for plastics but has been withdrawn; its function is now covered by ASTM E313 and by ISO 17223, which governs the determination of yellowness index and change in yellowness index (ΔYI) for plastic materials.
Testing Samples
Commodity Thermoplastics
Polyethylene (PE), polypropylene (PP), polystyrene (PS), and ABS resins are the most frequently tested pellet families. These materials are widely used in packaging, household appliances, and automotive interiors, where discoloration directly affects perceived product quality and market acceptance.
Engineering and High-Performance Plastics
Polycarbonate (PC), polyamide (PA), polyethylene terephthalate (PET), polyoxymethylene (POM), polymethyl methacrylate (PMMA), and polybutylene terephthalate (PBT) are sensitive to hydrolytic and thermal degradation. YI testing is used to verify drying efficiency, residence-time control, and the effectiveness of stabilization packages during compounding and molding.
Elastomers, Compounds, and Recycled Materials
TPU, PVC, TPE, masterbatch compounds, and mechanically recycled resins (such as rPET and rPP) are tested to characterize contamination levels, additive performance, and lot-to-lot consistency. Recycled pellets in particular are graded by YI because repeated processing cycles and contaminant carryover progressively raise the index.
Testing Items
- Yellowness index (YI) per ASTM E313 or ISO 17223
- Change in yellowness index (ΔYI) before and after heat aging, UV exposure, or processing
- CIE color coordinates (L*, a*, b*) and total color difference (ΔE*) per ASTM D2244 and ISO 11664
- Whiteness index for white and opaque resin grades
- Comparative color checks for transparent pellets molded into plaques
Testing Methods
Sample Preparation
Because the irregular geometry of individual pellets scatters light unpredictably, most specifications require pellets to be compression-molded into flat plaques before measurement. Following practices such as ASTM D4703, a representative sample (typically 50–200 g of pellets) is dried, melted between polished plates at the resin's specified molding temperature, and pressed into a smooth plaque approximately 2–3 mm thick. The plaque is then conditioned at 23 °C and 50% relative humidity prior to testing. For rapid comparative checks, some accredited third-party laboratories instead fill a glass or quartz cell with loose pellets and measure through the cell wall, consistent with ASTM D6290, which addresses direct color measurement of plastic pellets.
Spectrophotometric Measurement
A calibrated benchtop spectrophotometer with sphere (diffuse/8°) geometry is used. Typical settings include D65 illuminant, 10° standard observer, specular component included (SPIN), and a large measurement aperture. The instrument is verified against a white calibration tile before each session. Multiple readings—at least three to five—are taken at different positions on the plaque or pellet cell, and the average YI is reported together with the standard deviation to demonstrate measurement repeatability.
Data Processing and Reporting
The instrument software computes the tristimulus values and outputs YI under the selected formula, commonly labeled as YI E313 (D65/10°). A complete test report includes the sample description, preparation method, measurement geometry, number of readings, mean YI, ΔYI where applicable, and a pass/fail judgment against the customer's specification limit. Measurement uncertainty is typically within ±0.2 YI units for well-prepared plaques.
Applications
- Incoming quality control of resin lots at injection molders, extruders, and film converters
- Verification of polymerization endpoint and stabilizer performance at resin producers
- Evaluation of thermal aging and lightfastness after oven aging or xenon/UV weathering (for example, ISO 4892, ASTM G154, ASTM G155)
- Grading and certification of recycled pellets for quality-based pricing and material selection
- Comparative assessment of antioxidants, UV absorbers, and processing aids in formulation development
- Root-cause analysis of discoloration complaints in molded or extruded finished parts
FAQ
Can plastic pellets be measured directly without molding?
Yes. Direct pellet measurement in a cell per ASTM D6290 is fast and useful for lot-to-lot comparison, but results are influenced by pellet size, shape, and surface roughness. Molding pellets into plaques gives more reproducible and absolute YI values and is the preferred method for specification testing and dispute resolution.
What YI value is considered acceptable?
Acceptance limits depend on the application. Optical-grade PC or PMMA may require YI below 1–2, white appliance-grade ABS may allow 3–6, and recycled resins may be specified up to 10 or higher. Results should always be compared under the same illuminant and observer combination, since YI values differ between, for example, D65/10° and C/2° settings.
Why did the YI result increase after processing?
An increase in YI after extrusion or molding usually indicates thermal oxidation from excessive melt temperature or residence time, insufficient drying of hygroscopic resins (such as PET, PA, PC, and ABS), or degradation of additives. ΔYI testing before and after processing helps isolate the root cause.
How much sample is required for a test?
Approximately 50–200 g of pellets is normally sufficient to mold one or more plaques and perform replicate measurements. Accredited third-party laboratories can typically complete the test within a few working days, including plaque preparation and conditioning.
