ASTM D7191: Detecting Hidden Moisture in Plastics

Introduction

The ASTM D7191 test method quantitatively determines the water content in plastics down to 20 ppm. Moisture in plastics directly impacts the physical qualities and processability of various plastics. The warm-up of ASTM D7191 marked a considerable improvement in plastic material testing. Before this method, time-honored procedures for moisture resolution in plastics often involved laborious and time-consuming strategies like Karl Fischer titration. These methods also required larger sample sizes and potentially involved harsh chemicals, introducing limitations and safety concerns.

Scope 

ASTM D7191 was mainly developed to quantitatively (down to 20 parts per million, ppm), measuring moisture content in plastics or eclectic plastic materials by manipulating a relative humidity sensor. This test technique applies to various plastic types, including liquids, powders, pellets, and ground materials.

Test Procedure

The ASTM D7191 procedure involves several key steps:

1. Sample Preparation: The test standard specifies that a representative sample of the plastic material be chosen. The sample size is typically negligible; the material can be liquids, powders, pellets, or ground material.
2. Sample Loading: The prepared sample is carefully weighed and placed inside a septum-capped glass vial. This vial serves as the container for the sample during the testing process.
3. Heating and Volatiles Evolution: The sample’s sealed vial is placed inside a controlled heating device. This heating process induces the evolution of volatile components from the plastic material, including any present moisture.
4. Carrier Gas Flow and Moisture Capture: A dry carrier gas (an inert gas like nitrogen) continuously passes through the vial. This gas stream carries the evolved volatiles, including water vapor, into the analysis system. However, high-boiling volatiles condensate on a hydrophobic filter within the system.
5. Moisture Detection and Measurement: The hydrophobic filter selectively permits water vapor to pass through. A relative humidity sensor downstream of the filter gauges the water content in the carrier gas established on the modification in relative humidity. This cadenced shift in comparative humidity is then connected to the water content in the unblemished sample.
6. Data Acquisition and Calculation: The data from the comparable humidity sensor is jotted down and used to estimate the total mass of water evolved from the instance. This significance is then spread out by the initial specimen weight to differentiate the moisture content as a share.

Data Analysis

The immediate data disseminated from the ASTM D7191 test is the portion of steam content in the plastic specimen. This knowledge is compulsory for material quality management and guaranteeing the plastic meets its intended enactment specifications.

Here are some essential reflections when deciphering the test results:

Specimen Size

The ASTM D7191 definitive specifies that the specimen size should be diminutive and symbolic of the entire plastic material. The precise size can vary depending on the distinct material and the test kit. However, it’s naturally in the spectrum of a few milligrams to a few hundred milligrams.

Conclusion

Though moisture is harmless, it can devastate the enactment and quality of plastic materials. ASTM D7191 benefits manufacturers, engineers, and quality management personnel to ensure their plastics meet the desired condensation content specifications. This method ensures informed decision-making throughout the plastic’s lifecycle by apprehending the test technique, data estimation, and potential impediments.

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