ASTM D7191-18

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

SIGNIFICANCE AND USE
5.1 This test method is intended for use as a control, acceptance, and assessment test.  
5.2 Moisture can seriously affect the processability of plastics. It is possible that high moisture content will cause surface imperfections (that is, splay or bubbling) or degradation by hydrolysis. Low moisture (with high temperature) has been known to cause solid phase polymerization.  
5.3 The physical properties of some plastics are greatly affected by the moisture content.
SCOPE
1.1 This test method covers the quantitative determination of water down to 20 ppm in plastics using a relative humidity sensor.  
1.2 Values stated in SI units are to be regarded as standard.  
1.3 Specimens tested in this test method can reach or exceed 250°C, use caution when handling them after testing has completed.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
Note 1: There is no known ISO equivalent to this standard.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2018
• ICS: 83.080.01 - Plastics in general
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.70 - Analytical Methods

Relations

• Replaces: ASTM D7191-10 - Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-24 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2024
• Refers: ASTM D883-23 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2023
• Refers: ASTM D883-20 - Standard Terminology Relating to Plastics
• Effective Date: 01-Jan-2020
• Refers: ASTM D883-19c - Standard Terminology Relating to Plastics
• Effective Date: 01-Aug-2019
• Refers: ASTM D883-19a - Standard Terminology Relating to Plastics
• Effective Date: 15-Apr-2019
• Refers: ASTM D883-19 - Standard Terminology Relating to Plastics
• Effective Date: 01-Feb-2019
• Refers: ASTM D883-18a - Standard Terminology Relating to Plastics
• Effective Date: 01-Dec-2018
• Refers: ASTM D883-18 - Standard Terminology Relating to Plastics
• Effective Date: 01-Nov-2018
• Refers: ASTM D1600-18 - Standard Terminology for Abbreviated Terms Relating to Plastics (Withdrawn 2024)
• Effective Date: 01-Jan-2018
• Refers: ASTM D883-17 - Standard Terminology Relating to Plastics
• Effective Date: 15-Aug-2017
• Refers: ASTM E177-14 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2014
• Refers: ASTM D1600-14 - Standard Terminology for Abbreviated Terms Relating to Plastics
• Effective Date: 01-Feb-2014
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013
• Refers: ASTM E177-13 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2013

Overview

ASTM D7191-18 is the Standard Test Method for Determination of Moisture in Plastics by Relative Humidity Sensor, developed by ASTM International. This method enables the precise quantitative measurement of water content in plastics, detecting moisture levels as low as 20 parts per million (ppm). Accurate moisture analysis in polymer materials is critical as it directly impacts processability and finished product quality, helping manufacturers avoid surface defects and material degradation. The method is suitable for use as a quality control, acceptance, and assessment test in a wide range of plastic processing and manufacturing environments.

Key Topics

• Purpose and Importance

  • Designed to determine moisture content for quality assurance and process control in plastics manufacturing.
  • Moisture can significantly affect the processability of plastics, potentially causing surface imperfections, undesirable chemical changes (e.g., hydrolysis), or unintentional polymerization.

• General Approach

  • The test involves heating a sealed vial containing the plastic sample, releasing volatiles into the headspace.
  • A dry carrier gas transports the evolved moisture to a sensitive relative humidity (RH) sensor.
  • The RH sensor measures moisture, providing a mass-based quantitative result, which is normalized to the sample mass.

• Safety Considerations

  • Samples may reach temperatures up to 250°C; operators must use appropriate safety measures for handling hot materials.
  • The method does not address all possible safety or environmental concerns - users are responsible for establishing suitable practices.

• Limitations and Interferences

  • The presence of solvents such as methanol, ethanol, or acetone may interfere, resulting in falsely high readings due to their polar nature.

Applications

Moisture determination in plastics is crucial in various industry applications, including:

• Polymer Processing: Ensuring optimal moisture content prevents surface imperfections such as splay and bubbling in injection molding, extrusion, or thermoforming.
• Material Assessment: Assessment of raw materials for moisture-sensitive plastics like polycarbonate, nylon, ABS, and PET.
• Quality Control: Routine monitoring in manufacturing plants to ensure consistent product quality and adherence to specifications.
• Research and Development: Evaluating new materials or formulations and optimizing processing parameters based on moisture performance.

Common plastics analyzed using this method include:

• Polycarbonate (PC)
• Polypropylene (PP)
• Nylon 6 (PA 6)
• Acrylonitrile Butadiene Styrene (ABS)
• Polyethylene terephthalate (PET)
• Polystyrene (PS)

Related Standards

• ASTM D1193 - Specification for Reagent Water
  Reference for water used in this and related analytical methods.

• ASTM D6869 - Test Method for Coulometric and Volumetric Determination of Moisture in Plastics Using the Karl Fischer Reaction
  Alternative technique for verifying or optimizing test conditions.

• ASTM E177 - Practice for Use of the Terms Precision and Bias in ASTM Test Methods
  Provides statistical guidelines relevant for reporting and interpreting test results.

• ISO Standards
  Currently, there is no known ISO equivalent for ASTM D7191-18.

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By following ASTM D7191-18, manufacturers and laboratories can reliably assess moisture in plastics, ensuring high material performance, reducing the risk of defects, and enabling stringent compliance with industry requirements. This test method is an essential tool for anyone involved in the processing, quality assurance, or research of plastic materials where moisture control is a key quality parameter.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D7191 − 18
Standard Test Method for
Determination of Moisture in Plastics by Relative Humidity
Sensor
This standard is issued under the fixed designation D7191; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
1.1 This test method covers the quantitative determination
of water down to 20 ppm in plastics using a relative humidity
3. Terminology
sensor.
3.1 Definitions—The definitions used in this test method are
1.2 Values stated in SI units are to be regarded as standard.
in accordance with Terminology D883.
1.3 Specimenstestedinthistestmethodcanreachorexceed
4. Summary of Test Method
250°C, use caution when handling them after testing has
4.1 Asample is loaded into a septum-capped glass vial that
completed.
is moved into a heater to evolve the volatiles from the sample
1.4 This standard does not purport to address all of the
into the headspace.Acoaxial needle, or two needle set, pierces
safety concerns, if any, associated with its use. It is the
the septum of the vial as it enters the heater. A dry carrier gas
responsibility of the user of this standard to establish appro-
then flows into the vial and carries the evolved volatiles in the
priate safety, health, and environmental practices and deter-
headspace into the sensor manifold. In the sensor manifold, the
mine the applicability of regulatory limitations prior to use.
carriergasiscooledtoallowhigh-boilingvolatilestocondense
NOTE 1—There is no known ISO equivalent to this standard.
on a hydrophobic filter. The filter’s hydrophobic properties
1.5 This international standard was developed in accor- allow the moisture in the carrier gas to pass through and then
dance with internationally recognized principles on standard- be measured as an increase in potential at the relative humidity
ization established in the Decision on Principles for the sensor. This sensor signal is integrated over time to provide a
Development of International Standards, Guides and Recom- measurementofthetotalmassofwaterinthesample.Thetotal
mendations issued by the World Trade Organization Technical moisture is then divided by sample mass to yield moisture
Barriers to Trade (TBT) Committee. content.
4.2 This test method utilizes a sealed, airtight flow system
2. Referenced Documents
that prevents contamination of the analyzer from water present
2.1 ASTM Standards:
in the atmosphere.
D1193 Specification for Reagent Water
5. Significance and Use
D883 Terminology Relating to Plastics
D1600 TerminologyforAbbreviatedTermsRelatingtoPlas-
5.1 This test method is intended for use as a control,
tics
acceptance, and assessment test.
D6869 Test Method for Coulometric and Volumetric Deter-
5.2 Moisture can seriously affect the processability of plas-
mination of Moisture in Plastics Using the Karl Fischer
tics. It is possible that high moisture content will cause surface
Reaction (the Reaction of Iodine with Water)
imperfections (that is, splay or bubbling) or degradation by
E177 Practice for Use of the Terms Precision and Bias in
hydrolysis. Low moisture (with high temperature) has been
ASTM Test Methods
known to cause solid phase polymerization.
5.3 The physical properties of some plastics are greatly
This test method is under the jurisdiction ofASTM Committee D20 on Plastics
affected by the moisture content.
and is the direct responsibility of Subcommittee D20.70 on Analytical Methods.
Current edition approved Dec. 1, 2018. Published December 2018. Originally
6. Interferences
approved in 2005. Last previous edition approved in 2010 as D7191 - 10.
DOI:10.1520/D7191-18.
6.1 Elevated concentrations of some common solvents such
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
as methanol, ethanol and acetone will give biased high read-
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
ings due to their polar characteristics and ability to permeate
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. the thermoset polymer layers of the relative humidity sensor.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7191 − 18
7. Apparatus 9.2 Duetothehygroscopicnatureofmanyplastics,samples
3 shall be stored in airtight containers made of glass or other
7.1 Moisture Analyzer , an apparatus that consists of:
qualified or suitable material.
7.1.1 Flow Regulator, capable of maintaining the carrier gas
flow rate within the manufacturer’s specified conditions. 9.3 Samples that have been heated to remove moisture prior
7.1.2 Flow Meter, capable of measuring the carrier gas flow to processing and testing shall be allowed to cool to room
rate in accordance with the manufacturer’s specified condi- temperature in a sealed container prior to determination.
tions.
9.4 Testspecimensintheformofliquid,powders,pellets,or
7.1.3 Manifold, which provides:
ground material.
7.1.3.1 A thermally stable port for mounting and operation
10. Calibration and Standardization
of the relative humidity sensor.
7.1.3.2 Inlet and outlet ports for the carrier gas.
10.1 To maintain the integrity of the test results, the vial
7.1.3.3 A cold trap filter loop which filters out particulates
heater shall be calibrated using a NIST-traceable temperature
and re-condensed high boiling volatiles.
calibration interface, and the RH sensor shall be verified and
7.1.3.4 A port for mounting the coaxial needle, or two
calibrated using NIST-traceable capillary tubes. Other suitable
needle set.
instrument calibration methods and standards shall be used as
7.1.4 Coaxial Needle, a needle which has a dual flow path
specified by the instrument manufacturer.
allowing the carrier gas to flow into the sample vial and then
10.2 Perform the vial heater calibration in accordance with
back into the manifold, or a two needle set configured to allow
the manufacturer’s instructions.
inflow into the sample vial through one needle and outflow
from the sample vial through the second needle. 10.3 If the results are not within the acceptable range,
7.1.5 Relative Humidity (RH) Sensor, a sensing element that contact the analyzer manufacturer.
measures the relative humidity of the carrier gas coming into
10.4 Perform the RH sensor verification daily in accordance
the manifold.
with the manufacturer’s instructions by way of using a capil-
7.1.6 Sample Vial Heater,capableofmaintainingthesample
lary tube (see 8.1) filled with water (see 8.4) or other method
vial temperature within 1°C of the programmed temperature
as specified by the instrument manufacturer.
between 25°C and 275°C.
10.5 If the RH sensor verification result is not within the
7.1.7 Microcontroller, which provides:
acceptable range, perform the RH sensor calibration in accor-
7.1.7.1 Capability of integrating and converting the RH
dance with the manufacturer’s instructions using a capillary
sensor signal.
tube (see 8.1) filled with water (see 8.4) or other method as
7.1.7.2 Capability of controlling the temperature of the
specified by the instrument manufacturer.
sample vial heater and sensor manifold.
10.6 Repeat step 10.4 to verify RH sensor calibration.
7.2 Balance, external, with 1-mg readability.
10.7 If results are not within the acceptable range, contact
8. Reagents and Materials
the analyzer manufacturer.
8.1 Capillary Tubes—NIST-traceable at the desired total
11. Procedure
mass of water, typically 1000 µg, or other qualified water
11.1 Sample Analysis:
standard devices.
11.1.1 Turn on the analyzer.
8.2 Sample Vials—70-mm tall and 25-mL capacity.
11.1.2 Establish carrier gas flow in accordance with the
8.3 Sample Vial Septa—polytetrafluoroethylene (PTFE)- manufacturer’s specifications.
coated silicone. 11.1.3 Program the analyzer with the appropriate test con-
ditions.
8.4 Water—Unless otherwise indicated, references to water
shall be understood to mean reagent water as defined by Type
NOTE 2—Suggested test conditions are listed in Appendix X1. If test
II of Specification D1193. conditions for a specific material are not list
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7191 − 10 D7191 − 18
Standard Test Method for
Determination of Moisture in Plastics by Relative Humidity
Sensor
This standard is issued under the fixed designation D7191; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the quantitative determination of water down to 20 ppm in plastics using a relative humidity sensor.
1.2 Values stated in SI units are to be regarded as standard.
1.3 Specimens tested in this test method can reach or exceed 250°C, use caution when handling them after testing has
completed.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
NOTE 1—There is no known ISO equivalent to this standard.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D883 Terminology Relating to Plastics
D1600 Terminology for Abbreviated Terms Relating to Plastics
D6869 Test Method for Coulometric and Volumetric Determination of Moisture in Plastics Using the Karl Fischer Reaction (the
Reaction of Iodine with Water)
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions—The definitions used in this test method are in accordance with Terminology D883.
4. Summary of Test Method
4.1 A sample is loaded into a septum-capped glass vial that is moved into a heater to evolve the volatiles from the sample into
the headspace. A coaxial needle, or two needle set, pierces the septum of the vial as it enters the heater. A dry carrier gas then flows
into the vial and carries the evolved volatiles in the headspace into the sensor manifold. In the sensor manifold, the carrier gas is
cooled to allow high-boiling volatiles to condense on a hydrophobic filter. The filter’s hydrophobic properties allow the moisture
in the carrier gas to pass through and then be measured as an increase in potential at the relative humidity sensor. This sensor signal
is integrated over time to provide a measurement of the total mass of water in the sample. The total moisture is then divided by
sample mass to yield moisture content.
4.2 This test method utilizes a sealed, airtight flow system that prevents contamination of the analyzer from water present in
the atmosphere.
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.70 on Analytical Methods.
Current edition approved April 1, 2010Dec. 1, 2018. Published June 2010December 2018. Originally approved in 2005. Last previous edition approved in 20052010 as
D7191 - 05.D7191 - 10. DOI:10.1520/D7191-10.DOI:10.1520/D7191-18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7191 − 18
5. Significance and Use
5.1 This test method is intended for use as a control, acceptance, and assessment test.
5.2 Moisture can seriously affect the processability of plastics. It is possible that high moisture content will cause surface
imperfections (that is, splay or bubbling) or degradation by hydrolysis. Low moisture (with high temperature) has been known to
cause solid phase polymerization.
5.3 The physical properties of some plastics are greatly affected by the moisture content.
6. Interferences
6.1 Elevated concentrations of some common solvents such as methanol, ethanol and acetone will give biased high readings due
to their polar characteristics and ability to permeate the thermoset polymer layers of the relative humidity sensor.
7. Apparatus
7.1 Moisture Analyzer , an apparatus that consists of:
7.1.1 Flow Regulator, capable of maintaining the carrier gas flow rate within the manufacturer’s specified conditions.
7.1.2 Flow Meter, capable of measuring the carrier gas flow rate in accordance with the manufacturer’s specified conditions.
7.1.3 Manifold, which provides:
7.1.3.1 A thermally stable port for mounting and operation of the relative humidity sensor.
7.1.3.2 Inlet and outlet ports for the carrier gas.
7.1.3.3 A cold trap filter loop which filters out particulates and re-condensed high boiling volatiles.
7.1.3.4 A port for mounting the coaxial needle, or two needle set.
7.1.4 Coaxial Needle, a needle which has a dual flow path allowing the carrier gas to flow into the sample vial and then back
into the manifold, or a two needle set configured to allow inflow into the sample vial through one needle and outflow from the
sample vial through the second needle.
7.1.5 Relative Humidity (RH) Sensor, a capacitive sensing element that measures the relative humidity of the carrier gas coming
into the manifold.
7.1.6 Sample Vial Heater, capable of maintaining the sample vial temperature within 1°C of the programmed temperature
between 25°C and 275°C.
7.1.7 Microcontroller, which provides:
7.1.7.1 Capability of integrating and converting the RH sensor signal.
7.1.7.2 Capability of controlling the temperature of the sample vial heater and sensor manifold.
7.2 Balance, external, with 1-mg readability.
8. Reagents and Materials
8.1 Capillary Tubes—NIST-traceable at the desired total mass of water, typically 1000 μg, or other qualified water standard
devices.
8.2 Sample Vials—70-mm tall and 25-mL capacity.
8.3 Sample Vial Septa—polytetrafluoroethylene (PTFE)-coated silicone.
8.4 Water—Unless otherwise indicated, references to water shall be understood to mean reagent water as defined by Type II of
Specification D1193.
8.5 Carrier Gas—Any dry gas with water content less than 3 ppm including but not limited to dry air, nitrogen, helium or argon.
9. Sampling and Test Specs and Units
9.1 Due to the small specimen size, exercise care to ensure that the specimen is representative of the sample.
9.2 Due to the hygroscopic nature of many plastics, samples shall be stored in airtight containers made of glass or other qualified
or suitable material.
9.3 Samples that have been heated to remove moisture prior to processing and testing shall be allowed to cool to room
temperature in a sealed container prior to determination.
9.4 Test specimens in the form of liquid, powders, pellets, or ground material.
th
The sole source of supply of the apparatus known to the committee at this time is Arizona Instrument, 1912 West 4 Street, Tempe, AZ 85281. If you are aware of
alternative suppliers, please provide this information to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend.
D7191 − 18
10. Calibration and Standardization
10.1 To maintain the integrity of the test results, the vial heater shall be calibrated using a NIST-traceable temperature
calibration interface, and the RH sensor shall be verified and calibrated using NIST-traceable capillary tubes. Alternatively, other
Other suitable instrument calibration methods and standards canshall be used as specified by the instrument manufacturer.
10.2 Perform the vial heater calibration in accordance with the manufacturer’s instructions.
10.3 If the results are not within the acceptable range, contact the analyzer manufacturer.
10.4 Perform the RH sensor verification daily in accordance with the manufacturer’s instructions by way of using a capillary
tube (see 8.1) filled with water (see 8.4) or other method as specified by the instrument manufact
...