ASTM D7133 Adhesive Lap Shear Strength Testing at Elevated Temperature
ASTM D7133-21 is used to determine the CIELAB color and Yellowness Index (YI) of liquid polyurethane raw materials using an instrument. The CIELAB and YI findings are obtained by manipulating CIE tristimulus values mathematically, according to Practices E308 and E313, respectively. The values expressed in SI units should be considered standard.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D7133-21 provides a standardized method for the instrumental measurement of tristimulus color (CIELab L*, a*, b* values) and yellowness index (YI) of liquid polyurethane raw materials, including polyols, isocyanates, prepolymers, and auxiliary components. Color is a critical quality indicator for polyurethane raw materials, as yellowing or discoloration can indicate degradation, contamination, or process variation.
This method enables suppliers and users of polyurethane raw materials to communicate and objectively control color quality, ensuring consistency in white- and light-colored finished foam, elastomer, and coating products.
Scope, Applications, and Benefits
Scope
ASTM D7133-21 covers the measurement of liquid polyurethane raw materials in transmission mode and determines:
- CIELab L*, a*, b* tristimulus color values
- Yellowness Index (YI) per ASTM E313
- APHA/Hazen color number (optional)
- Color difference (ΔE) from reference or historical data
Applications
- Polyol and isocyanate quality control
- Polyurethane foam colorant batch verification
- Prepolymer color monitoring during production
- Incoming raw material acceptance testing
- Color trend monitoring for shelf life and storage assessment
Benefits
- Objective, instrument-based color measurement eliminates visual subjectivity
- Detects subtle color changes indicative of degradation or contamination
- Enables consistent communication of color specifications across supply chains
- Applicable to a wide range of polyurethane liquid raw material types
- Aligns with ASTM E313 yellowness index standards for cross-standard compatibility
Test Process
Instrument Calibration
Spectrophotometer is calibrated against certified white and black tile standards, and appropriate transmission geometry is selected.
1Sample Cell Preparation
The liquid sample is placed in a calibrated optical cell of defined path length; air bubbles are removed.
2Measurement
Transmission spectrum is acquired; L*, a*, b* values and YI are calculated by the instrument software.
3Reporting
Values are compared to product specification limits or stored reference values; ΔE from reference is calculated if required.
4Technical Specifications
| Parameter | Details |
|---|---|
| Test Principle | Transmission spectrophotometry |
| Applicable Materials | Liquid polyurethane raw materials (polyols, isocyanates, prepolymers) |
| Color Scale | CIELab (D65/10°); YI per ASTM E313 |
| Cell Path Length | As specified (typically 10 mm) |
| Measured Outputs | L*, a*, b*, YI, optional APHA number |
Instrumentation Used for Testing
- UV-Vis spectrophotometer or dedicated colorimeter with transmission mode
- Calibrated optical cells (10 mm or specified path length)
- Reference color standards for instrument verification
- Temperature-controlled sample handling (for viscous materials)
- Data management software with CIELab calculation capability
Results and Deliverables
- CIELab L*, a*, b* values
- Yellowness Index (YI) per ASTM E313
- APHA/Hazen color number (if requested)
- ΔE from reference or specification
- Test certificate with instrument calibration traceability
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Frequently Asked Questions
Yellowing typically results from oxidative degradation of aromatic structures, thermal degradation, light exposure, or contamination; high b* and YI values indicate significant yellowing.
APHA (American Public Health Association) color is a platinum-cobalt scale used to describe the degree of yellowness in liquids; lower APHA numbers indicate lighter, more color-neutral materials.
A colorless liquid ideally has L* near 100, a* and b* near 0, and a YI near 0; commercial polyols typically have YI below 5–10 for white foam applications.
D7133 is most suitable for light-colored or near-colorless materials in transmission; for darker samples, measurement path length must be reduced or dilution methods considered.
Daily calibration with white and black standards before measurement sessions is standard practice; NIST-traceable reference standards should be verified at regular intervals.
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