ASTM D6979 Unwind Force Testing for Pressure-Sensitive Tape
The essential components in polyols that are soluble in glacial acetic acid and reactive with perchloric acid are measured using this ASTM D6979-18 test procedure. This approach has been used to assess samples containing 0.3 to 10% nitrogen. The values indicated in SI units are to be regarded as standard.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM D6979-18 describes a test method for determining the isocyanate (–NCO) content of polyurethane raw materials and prepolymers using potentiometric back-titration. The isocyanate content directly controls the stoichiometry of polyurethane reactions and is a critical quality parameter for foam, elastomer, coating, and adhesive manufacturers.
Accurate NCO determination ensures proper formulation ratios, which are fundamental to achieving target mechanical properties, cure rates, and product performance in polyurethane systems.
Scope, Applications, and Benefits
Scope
ASTM D6979-18 covers the determination of free isocyanate group (–NCO) content in:
- Polyisocyanates (MDI, TDI, HDI, IPDI)
- Isocyanate prepolymers
- NCO-terminated semiprepolymers
- Other isocyanate-functional raw materials
Applications
- Polyurethane foam and elastomer manufacturing QC
- Isocyanate raw material incoming inspection
- Prepolymer synthesis and quality verification
- Coatings and adhesive formulation development
- Batch release and specification compliance testing
Benefits
- Provides highly accurate NCO content measurement
- Enables precise stoichiometric formulation calculation
- Detects changes in NCO content from moisture ingress or side reactions
- Supports consistency in polyurethane product performance
- Faster than older colorimetric titration methods with better endpoint detection
Test Process
Sample Dissolution
A weighed isocyanate sample is dissolved in dry solvent and reacted with a known excess of di-n-butylamine.
1Back Titration
The unreacted amine is back-titrated with standardized hydrochloric acid using a potentiometric endpoint detector.
2NCO Calculation
The consumed amine is calculated from the volume of HCl used, and NCO content is expressed as weight percent.
3Reporting
Results are compared to specification limits; multiple replicates are averaged for final reporting.
4Technical Specifications
| Parameter | Details |
|---|---|
| Test Principle | Potentiometric back-titration of excess amine |
| Applicable Materials | MDI, TDI, HDI, IPDI, and isocyanate prepolymers |
| Reagent | Di-n-butylamine; standardized HCl |
| Measured Output | NCO content (% by weight) |
| Endpoint Detection | Potentiometric (pH/mV break) |
Instrumentation Used for Testing
- Potentiometric titrator with a combined pH or glass electrode
- Analytical balance (0.1 mg resolution)
- Burette or automated titrant dispenser
- Dry solvent preparation and storage
- Reference buffer solutions for electrode calibration
Results and Deliverables
- NCO content expressed as weight percent
- Titration curve and equivalence point data
- Replicate values and standard deviation
- Comparison to specification tolerance
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Frequently Asked Questions
NCO content is the weight percentage of –NCO groups in the isocyanate material; it determines how much isocyanate reacts with polyol, directly affecting the polyurethane's crosslink density and properties.
D6979 uses potentiometric endpoint detection instead of visual indicators, providing more accurate and reproducible endpoint determination, especially for colored or dark samples.
Moisture reacts with –NCO groups to form carbamic acid, which decomposes to release CO2 — this reduces the measured NCO content and causes urea formation, degrading product quality.
Pure MDI has approximately 33.5% NCO, TDI 80/20 approximately 48%, and prepolymers may range from 3–25% depending on formulation.
At least two or three replicates within an acceptable precision range are typically required; the average is reported as the final NCO content.
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