ASTM D6869 Moisture in Plastics
ASTM D6869-03 method determines the amount of moisture in a polymer sample using the reaction of Iodine with water (Karl Fischer Reaction). Coulometric titration and Volumetric Titration instruments are used for moisture calculating.
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ASTM D6869 Moisture in Plastics
- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
ASTM D6869 Moisture in Plastics Overview
ASTM D6869 is the standard test method for determining moisture content in plastic materials using Karl Fischer titration -both the coulometric and volumetric variants. Moisture content in plastics matters more than most engineers initially assume. Many engineering thermoplastics -nylons, polyesters, polycarbonate, ABS, and acrylic among them -are hygroscopic and absorb moisture from the atmosphere during storage and handling. When these materials are processed at elevated temperatures without adequate drying, the absorbed moisture hydrolyzes polymer chains and degrades molecular weight, leading to splay, bubbles, brittleness, and mechanical property loss in the final part. Knowing the actual moisture content, not just estimating it from storage time, is the right way to make the drying decision.
Karl Fischer titration is the gold standard for moisture determination in plastics. It is highly specific to water -it does not respond to other volatiles like residual solvents or monomer -and it is far more accurate at low moisture levels than loss-on-drying (LOD) methods, which cannot distinguish water from other volatiles. Coulometric KF is preferred for low moisture levels (below approximately 1 percent) and is the typical choice for dried engineering resins. Volumetric KF is used for higher moisture levels and wet samples.
The test is performed by dissolving or extracting the moisture from the plastic specimen and introducing it into the Karl Fischer reagent, where water reacts quantitatively with iodine, sulfur dioxide, and a base in the reagent. The amount of water is determined by the electrical charge consumed (coulometric) or the volume of reagent consumed (volumetric) in the reaction. The result is reported as parts per million (ppm) or percent moisture by weight.
ASTM D6869 Moisture in Plastics Scope, Applications, and Benefits
Scope
ASTM D6869 covers coulometric and volumetric Karl Fischer moisture analysis of plastic materials including pellets, powders, films, molded parts, and other plastic forms. Sample introduction is performed by dissolution in an anhydrous solvent (for soluble plastics), by extraction using a drying oven or heated extraction vessel (oven method), or by direct titration for liquid or semi-solid samples. Coulometric Karl Fischer is appropriate for moisture levels from approximately 10 ppm to 10,000 ppm. Volumetric Karl Fischer is used for higher moisture levels. Conditioning before testing must be controlled to prevent moisture pick-up or loss during sample handling. The standard falls under ASTM Committee D20 on Plastics.
Applications
- Pre-drying moisture verification for nylon (PA), PET, PBT, polycarbonate, ABS, and other hygroscopic resins before injection molding or extrusion
- Dryer performance validation -confirming resin is dried to specification moisture level before processing
- Incoming resin lot moisture testing for materials received in packaging that may have been compromised
- Moisture content specification compliance testing for resin suppliers and compounders
- Film and sheet moisture analysis for packaging materials and barrier films
- Effect of moisture on mechanical properties -correlation of KF moisture with tensile strength, impact, and molecular weight
- Pharmaceutical packaging plastic moisture testing where moisture levels affect drug stability
- Moisture content determination in composite prepreg and adhesive film materials
Benefits
- Specific to water -does not respond to other volatiles, residual solvents, or processing aids unlike LOD methods
- Accurate at low moisture levels -detects moisture down to 10 ppm where LOD methods are impractical
- Both coulometric (low moisture) and volumetric (higher moisture) covered under one standard
- Results in ppm or percent weight directly comparable to resin supplier specifications and dryer recommendations
- Fast analysis -typical test time 5 to 15 minutes per sample depending on method
- Applicable to all major hygroscopic engineering thermoplastics and specialty resins
- Results directly drive processing decisions -no more guessing on drying time
ASTM D6869 Moisture in Plastics Process
Sample Preparation
Plastic samples -pellets, granules, ground material, or cut film sections -are weighed to the nearest 0.1 mg.
1Moisture Extraction
For coulometric KF analysis of solid plastics, the oven extraction method is used
2Karl Fischer Titration
In coulometric KF, the water entering the titration cell reacts with iodine generated electrochemically at the anode.
3Calculation and Reporting
Moisture content is calculated from the mass of water determined and the initial sample mass.
4ASTM D6869 Moisture in Plastics Technical Specifications
| Parameter | Details |
|---|---|
| Standard | ASTM D6869-17 |
| ASTM Committee | D20 – Plastics |
| Methods Covered | Coulometric Karl Fischer (low moisture), Volumetric Karl Fischer (higher moisture) |
| Sample Types | Pellets, powders, film, molded parts, ground or dissolved plastic material |
| Extraction Methods | Oven extraction with dry gas sweep, solvent dissolution, direct injection |
| Typical Detection Range | Coulometric: 10 ppm to 10,000 ppm; Volumetric: 0.01% to several percent |
Instrumentation Used for ASTM D6869 Moisture in Plastics
- Coulometric Karl Fischer titrator with electrochemical iodine generation and amperometric endpoint detection
- Volumetric Karl Fischer titrator with burette and endpoint detection for higher moisture samples
- Oven extraction unit connected to the KF titrator by dry gas sweep for solid sample analysis
- Analytical balance accurate to 0.1 mg for sample weighing
- Anhydrous KF solvent and reagent -methanol-based or methanol-free as appropriate to the plastic
- Dry gas (nitrogen or dry air) supply for oven extraction sweep and cell blanket
- Temperature-controlled drying oven with sealed sample boat holder
ASTM D6869 Moisture in Plastics Results and Deliverables
- Moisture content in ppm or percent by weight for each sample tested
- Sample mass and extraction temperature or dissolution method used
- Blank correction value applied
- Replicate results and average where multiple aliquots are tested
- Comparison to the resin supplier’s specified maximum moisture level for processing or incoming acceptance
- Material identification -resin type, grade, lot number
- Full test report formatted for incoming quality control, process control, or material specification documentation
Frequently Asked Questions
ASTM D6869 determines the precise moisture content in plastics using Karl Fischer titration methods. It is used to quantify both surface and absorbed water that can affect processing and performance.
Even small amounts of moisture can cause defects like voids, hydrolysis, or reduced mechanical strength during molding or extrusion. Measuring moisture helps ensure consistent material quality and process stability.
Coulometric KF is used for very low moisture levels (trace analysis), while volumetric KF is suitable for higher moisture content. The selection depends on expected water concentration in the plastic sample.
The method is widely used for engineering plastics, resins, polyethylene, polycarbonate, nylon, and other hygroscopic polymers where moisture sensitivity impacts performance.
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