ASTM D6869 Coulometric and Volumetric Determination of Moisture in Plastics

Scope:

This ASTM D6869 test technique is designed to determine moisture content in most polymers. To quantify the amount of moisture in a polymer sample, this approach employs the interaction of iodine (I₂) with water (Karl Fischer Reaction). Plastics having volatile components, such as residual monomers and plasticizers, release substances that interfere with the I₂/water reaction.  

As per ASTM D6869, the processability of various plastics will be influenced by moisture.  Surface flaws (such as splay or bubbling) or hydrolysis deterioration are caused by high moisture content. Polymerization is caused by a combination of low moisture and high heat. This method  can be used to measure moisture in the range of 0.005% to 100%.

Procedure:

For ASTM D6869, the samples are heated to release water, which is then carried to the titration cell by a nitrogen carrier gas. The reaction of water with I₂ is used to determine the moisture collected in the solution within the titration cell. Instrumented approaches are used to detect the endpoints. The interaction of I₂ with water is used to determine the amount of moisture present. With 10.71 Coulombs of current produced equating to 1 mg of water, coulometric instruments employ Faraday’s equation to quantify the moisture present. Whereas, volumetric devices measure the volume of the I₂ solution necessary to maintain a constant current.

Video 01: Coulometric vs. Volumetric Karl Fischer instruments

Specimen size:

For ASTM D6869, specimens in a variety of forms are allowed, including molded powder, molded shapes, and re-grind. Prior to testing, it is recommended that molded specimens be chopped into smaller portions (recommended maximum size 4 by 4 by 3 mm).

Data:

As per ASTM D6869, automatic calculations are performed by commercial coulometric devices depending on the micrograms of water measured. The sample type, sample weight, and % moisture are reported.

Conclusion:

The ASTM D6869 test technique is designed to determine moisture content in most polymers. To quantify the amount of moisture in a polymer sample, this approach employs the interaction of iodine (I₂) with water (Karl Fischer Reaction).