Thermogravimetric moisture testing is a high-precision analytical technique that determines the water and volatile content of materials by continuously measuring sample mass loss under a controlled heating program and atmosphere. This guide explains the working principles of thermogravimetric analysis (TGA), relevant standards such as ASTM E1131 and ISO 11358, typical sample types, test procedures, and practical applications in quality control and materials research.
Technical Overview
Thermogravimetric moisture testing, commonly performed using thermogravimetric analysis (TGA), measures the change in mass of a material as it is heated at a controlled rate under a defined gas atmosphere. Because water and other low-boiling volatiles are released first during heating, the initial mass-loss step of the TGA curve provides a reliable quantitative measure of the moisture content of the sample. Modern TGA instruments can detect mass changes at the microgram level, allowing accurate moisture determination from sample masses of only a few milligrams.
During a typical run, the specimen is placed in an inert crucible on a highly sensitive balance inside a furnace. The temperature is raised from ambient conditions through a programmed profile, often up to 600–900°C, while the instrument continuously records mass versus temperature. The resulting thermogravimetric (TG) curve and its derivative (DTG) reveal distinct mass-loss steps corresponding to moisture evolution, dehydration of bound water, volatilization of additives, decomposition of polymers or organics, and oxidation of carbonaceous residues. The final residual mass corresponds to ash or inorganic filler content.
This technique is widely applied because it combines moisture measurement with a complete compositional profile in a single experiment. It is complementary to Karl Fischer titration: while Karl Fischer is chemically specific to water, TGA provides broader information on total volatiles, thermal stability, and filler content. Accredited third-party laboratories routinely perform TGA moisture testing in accordance with recognized standards, including ASTM E1131 for compositional analysis by thermogravimetry, ISO 11358 for thermogravimetry of polymers, and related national specifications in the GB and JIS standard families.
Testing Samples
Polymer and Plastic Materials
Thermoplastic and thermoset resins in pellet, powder, film, or granule form are among the most frequently tested materials. Moisture levels in engineering plastics such as polyamides, polyesters, polycarbonates, and polyurethanes directly influence processing behavior and mechanical performance, making TGA moisture screening essential before injection molding or extrusion.
Pharmaceutical and Food Materials
Active pharmaceutical ingredients, excipients, lyophilized powders, granules, and food ingredients can be evaluated for moisture and total volatile content. Controlled moisture levels are critical for stability, shelf life, flowability, and microbial risk management in these industries.
Chemicals, Minerals, and Inorganic Materials
Fine chemicals, catalysts, ceramics, mineral powders, pigments, and hydrated salts are tested to quantify free moisture, bound water, and water of crystallization. The high-temperature capability of TGA also allows determination of loss on ignition and residual ash.
Energy and Electronic Materials
Lithium battery electrode materials, separator films, electronic encapsulants, adhesives, coatings, and composite materials are analyzed for moisture and volatile species that can degrade performance, cause outgassing, or reduce long-term reliability.
Testing Items
Typical quantitative results reported from thermogravimetric moisture testing include:
- Moisture content, defined as the mass loss occurring below approximately 105–150°C under an inert atmosphere.
- Bound water and water of hydration, released at intermediate temperatures during dehydration steps.
- Total volatile content, covering moisture plus residual solvents, monomers, and plasticizers.
- Thermal stability parameters, including onset temperature of decomposition and the temperature of maximum mass-loss rate from the DTG curve.
- Polymer or organic content, determined from the main decomposition step.
- Ash, filler, or inorganic residue content, measured as the remaining mass at the end of the program, optionally after switching to an oxidative atmosphere.
- Carbon black or carbonaceous residue, evaluated through controlled burn-off in air or oxygen.
Testing Methods
Instrumentation and Principle
A TGA instrument consists of a precision microbalance, a programmable furnace, a gas-flow control system, and data acquisition software. Sample masses of typically 2–20 mg are placed in open alumina or platinum crucibles. The balance detects mass changes with microgram resolution while the furnace executes the programmed temperature profile, and purge gases such as nitrogen, argon, air, or oxygen control the reaction environment.
Standard Test Procedure
A representative test generally follows these steps:
- Homogenize the sample and, if necessary, condition it under defined humidity to ensure representative moisture content.
- Weigh an appropriate quantity into a tared crucible and load it into the instrument.
- Purge the furnace with high-purity nitrogen to establish an inert atmosphere.
- Execute the temperature program, for example heating from ambient temperature to 110–150°C and holding isothermally until the moisture step is complete, then continuing to 600°C or higher at 10–20°C/min for full compositional analysis.
- Where required, switch the atmosphere to air or oxygen at high temperature to oxidize carbonaceous residue and quantify ash.
- Evaluate the TG and DTG curves, assign mass-loss steps, and calculate each component as a percentage of the original sample mass.
Relevant Standards
Testing is typically performed in conformance with ASTM E1131 (compositional analysis by thermogravimetry), ISO 11358 (thermogravimetry of polymers), and applicable GB/T and JIS methods for thermal analysis. For water-specific verification, results may be cross-checked against Karl Fischer titration methods such as ASTM E203 and ISO 760.
Data Interpretation and Quality Control
Moisture content is calculated from the first mass-loss step as the percentage of mass lost between the start of the run and the completion of the low-temperature plateau. Careful method design, including slow heating rates or isothermal holds in the moisture region, improves separation between moisture and other volatiles. Blanks, replicate runs, and certified reference materials are used to verify balance stability, temperature calibration, and result reproducibility. Reportable precision for moisture determination is typically at the level of a few hundredths of a percent by weight, depending on sample homogeneity and moisture level.
Applications
Thermogravimetric moisture testing supports a wide range of industrial and scientific activities:
- Incoming material inspection and supplier qualification for resins, additives, and chemicals.
- Quality control of moisture-sensitive polymers before processing to prevent hydrolysis, foaming, or degradation during molding.
- Battery materials qualification, where residual moisture affects electrolyte stability and cell life.
- Verification of drying process effectiveness and optimization of drying temperature and residence time.
- Shelf-life and stability studies for pharmaceuticals, foods, and hygroscopic powders.
- Failure analysis of molded parts, coatings, adhesives, and electronic assemblies suspected of moisture-related defects.
- Compositional characterization of compounds, including polymer, filler, and reinforcement fractions.
- Regulatory compliance and specification testing performed by accredited third-party laboratories.
FAQ
How does TGA moisture testing differ from Karl Fischer titration?
Karl Fischer titration is chemically selective for water and provides excellent accuracy for free water at low levels. TGA measures mass loss and therefore reports all species volatilized in the selected temperature window, including moisture and other volatiles, while also delivering thermal stability and ash data in the same run. The two methods are often used together for cross-validation.
What temperature window is used to define moisture?
Moisture is conventionally taken as the mass lost from ambient temperature up to about 105–150°C under an inert purge gas, frequently assisted by an isothermal hold to ensure complete water removal. The exact limits are defined in the test method agreed with the client or specified in the applicable standard.
How much sample is required?
Typical sample quantities range from 2 to 20 mg. Because such small amounts are analyzed, careful sampling and homogenization are essential, particularly for heterogeneous materials.
Can TGA distinguish free moisture from bound water?
Yes, in many materials. Free moisture is released at lower temperatures, whereas bound water and water of crystallization evolve at higher, characteristic temperatures that appear as separate steps on the DTG curve, allowing stepwise quantification.
What accuracy can be expected?
Modern TGA balances resolve microgram-level mass changes, so moisture results are commonly reproducible to within approximately ±0.02–0.05% by weight under well-controlled conditions, with actual precision depending on sample homogeneity, moisture level, and method design.
Which industries most commonly request this test?
Plastics and composites, battery and electronics manufacturing, pharmaceuticals, food processing, chemicals, coatings, and advanced ceramics all rely on thermogravimetric moisture testing for specification control and research and development support.