ASTM D7191 Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor

    What is ASTM D7191?

    ASTM D7191 describes a standard test method for quantitatively detecting water in polymers down to 20 ppm. Moisture content significantly affects the physical characteristics of different polymers, including hydrolysis-induced deterioration and surface defects. ASTM D7191 (Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor) is a commonly used technique for control, acceptance, or assessment tests. Low moisture (combined with high temperature) results in solid-phase polymerization, while high moisture concentrations cause hydrolysis-induced surface defects (splay or bubbling) or deterioration. It has a wide range of applications in the plastics industry and accurately determines polymer moisture content.

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    Get certified ASTM D7191 Testing for Reliable Determination of Moisture Content in Plastics

    The moisture content of plastic materials plays an important role in determining their processing performance, quality, and durability. Moisture testing helps manufacturers control moisture content and maintain consistent product quality. It also supports material selection, process optimization, product development, and quality control, preventing moisture-related defects. For more information about ASTM D7191 testing, consult MaTestLab.

    What is the Scope of the ASTM D7191 Test Standard?

    Moisture can significantly affect the processability and physical properties of some plastics, while excessive moisture can cause surface defects or hydrolytic degradation. ASTM D7191 is a standard test method for quantitatively determining water in plastics using a relative humidity sensor. The ASTM D7191 test method-

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    • Helps manufacturers assess potential effects on material performance and durability.
    • Measures moisture content up to 20 ppm in plastic materials via RH sensor headspace measurement.
    • Evaluates moisture content to reduce the risk of moisture-related degradation. 
    • Applies to both powder and granular plastic materials.
    • Correlates to standard gravimetric Karl Fischer results.

    What are the Uses of ASTM D7191?

    Manufacturers use ASTM D7191 in industries such as automotive, aerospace, packaging, and electronics to control moisture in plastics. It helps manufacturers control moisture content in plastics and assess its potential effects on material performance and durability. The ASTM D7191 test method-

    • Helps in routine checks during plastic processing and production. 
    • Evaluates the effects of moisture on plastic processing and material properties.
    • Reduces the risk of moisture-related defects and product failures.
    • Helps prevent manufacturing defects. 
    • Supports quality control and consistent product quality. 
    • Supports safety requirements across various industries. 

    What are the materials tested under ASTM D7191?

    The ASTM D7191 standard tests moisture-sensitive polymers such as polyamides, polyester, polyurethane, polycarbonate, and PEEK that absorb moisture and change their structure and function. The test method applies to plastics where moisture levels need to be quantitatively determined, particularly at low concentrations. Tested materials may include hygroscopic and moisture-sensitive polymers such as polyamide, polyester, polycarbonate, and polyurethane. The moisture content of plastic resins, pellets, granules, and other forms of polymer materials can also be assessed.

    ASTM D7191 Equipment and Sample Preparation Guide

    Proper sample preparation and equipment handling help obtain reliable moisture measurements.  Careful preparation of the plastic sample, controlled test conditions, and a calibrated relative humidity sensor determine moisture content. The ASTM D7191 test method involves the following equipment and sample preparation steps-

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    Specimen detailsTechnicians select plastic materials, such as pellets, granules, or other suitable forms, for moisture determination. Technicians store hygroscopic specimens in airtight containers to prevent moisture exchange.
    Specimen preparationTechnicians place the specified amount of specimen in a clean vial (25-ml vial sealed with a PTFE-coated silicone septum) and keep it less than half full. 
    Relative humidity sensorThe device measures the carrier gas relative humidity after moisture evolves from the specimen.
    Vial heaterThe device heats the specimen at the programmed temperature to release moisture.
    InstrumentationThe moisture analyzer device processes the sensor signal and determines the total moisture content. Technicians use a balance with 1-mg readability to accurately determine specimen mass.

    Testing procedure and requirements for ASTM D7191 

    The ASTM D7191 test method involves heating a plastic specimen in a sealed vial and measuring the moisture released using a relative humidity sensor. The test method uses a dry carrier gas to transport the evolved moisture to the sensor, which determines the total water content. Other ASTM standards that support similar specimen preparation/testing include ASTM D6980 for determining moisture in plastics by loss in weight and ASTM D6869 for coulometric and volumetric determination of moisture in plastics using the Karl Fischer reaction. The ASTM D7191 test method comprises the following test procedure-

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    Specimen preparationTechnicians weigh the specified specimen amount and place it in a clean, sealed vial.
    Analyzer setupTechnicians establish the carrier-gas flow and program the analyzer with appropriate test conditions.
    Moisture measurementThe analyzer device heats the vial to evolve moisture from the plastic specimen into the vial headspace. Dry carrier gas transfers the evolved moisture to the relative humidity sensor for measurement. 
    Result Determination and AnalysisThe analyzer integrates the sensor signal to determine the total water present. Technicians calculate the moisture content based on specimen mass.

    ASTM D7191 Testing Process and Data Collection

    The analyst selects a representative plastic specimen for evaluation. Technicians weigh the specimen, place it in a sealed vial, and properly position it in the moisture analyzer. The moisture analyzer heats the specimen. After heating, the dry carrier gas transfers the released moisture to the relative humidity sensor. The sensor detects and measures the moisture signal. The analyzer integrates the sensor signal to determine the total water present. Technicians calculate the moisture content based on specimen mass,  repeat the procedure for subsequent specimens, and record the displayed results. 

    The image shows the testing procedure for determining moisture content in plastics by relative humidity sensor as per ASTM D7191.
    ASTM D7191 Determination of Moisture Content in Plastics by Relative Humidity Sensor

    Common Challenges and Troubleshooting  

    A common challenge in ASTM D7191 testing is obtaining accurate moisture measurements at low concentrations. Variations in specimen preparation, sample mass, heating temperature, and carrier-gas flow can affect results. Specimen exposure to atmospheric moisture during handling can affect the measurement. Test repeatability under consistent conditions can improve result accuracy. MaTestLab follows appropriate testing procedures and equipment to provide reliable ASTM D7191 moisture measurements.

    ASTM D7191 Analysis Results and Interpretation

    The report summarizes the moisture content measurement of the plastic specimen using a relative humidity sensor. Moisture content is a critical parameter in plastics processing and manufacturing, and excessive moisture can adversely affect the material’s properties and processing characteristics. 

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    • The laboratory records the specimen mass, test temperature, carrier-gas flow, and moisture measurement during testing. 
    • The analyst compares the measured moisture content with specified or acceptable limits to assess the plastic’s suitability for its intended application. 
    • The laboratory reports the moisture content in ppm or percentage (%), based on the measured water content and specimen mass. 
    • Lower moisture content may indicate better control of moisture-sensitive plastics, while excessive moisture can increase the risk of surface defects, hydrolysis, or processing problems. 

    Link to ASTM D7191

    FAQ

    What is the primary purpose of ASTM D7191?
    The test measures the moisture content of plastic samples by analyzing the relative humidity of the headspace above a heated sample. This supports quality control and helps prevent processing defects like bubbling or hydrolytic degradation.
    Volatile polar solvents like methanol, ethanol, or acetone can interfere with the relative humidity sensor and cause inaccurate readings.
    The ASTM D7191 method is not suitable for analyzing materials containing highly heterogeneous components or certain blended polyesters and polyolefins.
    Suyog Kamble
    About Author
    Suyog Kamble
    Suyog Kamble is an Operations Associate at MaTestLab Inc. and holds a postgraduate degree in Organic Chemistry.
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