Thermogravimetric Analyzer for Determining the Solid Content of Varnish

The principle of thermogravimetric analysis for determining the solid content of varnish is as follows: When the sample is heated, volatile solvents and moisture evaporate, leading to a reduction in mass, while the remaining solid mass stabilizes. The solid content is calculated by comparing the initial and final mass ratios. The procedure includes instrument calibration, sample preparation, setting the heating program, data acquisition, and analysis. Key influencing factors include heating rate, endpoint temperature, sample quantity, and atmosphere control. This method offers advantages such as minimal sample usage, high automation, and precise results, making it superior to traditional oven methods. In practical applications, it is essential to validate conditions based on the type of varnish, consider the risk of component decomposition, and integrate results with other analytical techniques for comprehensive analysis.

Principle

Thermogravimetric analysis is a technique that measures the relationship between the mass of a substance and temperature or time under programmed temperature control. When applied to the determination of varnish solid content, the core principle is that during the heating process, the volatile solvents and moisture in the varnish sample will evaporate with the increase of temperature, resulting in a decrease in sample quality, while the remaining non-volatile components such as solid resins, pigments and fillers remain relatively stable. By monitoring the percentage of mass loss from the initial mass of the sample to the high-temperature constant weight stage, the solid content of the varnish can be accurately calculated. The process follows the law of conservation of mass, and its basic calculation formula can be expressed as:

Solids content (%) = (m_f / m_i) × 100%

Where, m_i is the initial mass of the sample, m_f The final residue mass after the end of the program warm-up. Compared with traditional oven methods, this method has a higher degree of automation, more accurate temperature and quality control, and can provide a continuous relationship curve between mass change and temperature, which helps to better understand the thermal behavior of components.

Assay method

To ensure the accuracy and repeatability of the measurement results, standardized operating procedures must be followed. The following are recommended steps based on a universal thermogravimetric method:

1. Instrument Preparation and Calibration:Turn on the thermogravimetric analyzer to bring it to a steady state. Quality calibration using standard weights and temperature calibration according to instrument requirements is usually recommended by relevant national or international standards such as ASTM E1131.

2. Sample preparation:Using a clean microbalance, accurately weigh an appropriate amount (typically 5-20 mg) of a uniform varnish sample into an open alumina crucible. The sample should be spread in a thin layer to facilitate adequate escape of volatile components.

3. Test Parameter Setting:Set up the test program in the instrument software. Typical procedures include: raising from room temperature to a set end temperature (e.g. 150°C or based on varnish characteristics) at a constant heating rate (e.g. 10°C/min) in an inert atmosphere (e.g. nitrogen) and maintaining the end temperature for a period of time until the mass is constant. The atmosphere flow rate needs to be stable.

4. Test Run and Data Collection:The crucible containing the sample is placed in the instrument furnace body and the test procedure is initiated. The instrument will automatically record the curve of the sample mass with temperature and time (thermogravimetric curve).

5. Data Analysis:After the test, the initial mass point and the mass point of the platform area after the end of the volatilization process are determined from the thermogravimetric curve. Use the above formula to calculate the solids content. Modern instrument software often has automatic labeling and calculation capabilities.

Influencing factors

The accuracy of assay results is influenced by a variety of experimental conditions, and understanding and controlling these factors is crucial.

Influencing factors	Description and optimization suggestions
Rate of warming	Too fast a rate may lead to thermal hysteresis and high volatilization end temperature. A moderate rate (e.g., 5-20°C/min) is recommended.
End temperature and constant temperature time	The temperature should ensure that the volatiles completely escape but avoid the decomposition of solid components. It needs to be determined by pre-experiment or material knowledge, and maintain a sufficient constant temperature time to constant weight.
Sample volume and morphology	Excessive or thick sample volume can lead to uneven heat and mass transfer. It is recommended to use a small, thin-layer sample.
Atmosphere type and flow rate	The inert atmosphere prevents the oxidation reaction from interfering. A constant flow rate helps stabilize the thermal environment and remove volatiles.
Instrument baseline drift	The instrument itself may have small mass changes at high temperatures. Baseline correction can be performed by running a blank crucible experiment.

By systematically optimizing these parameters, repeatable and accurate assay results can be obtained. Varnishes with different formulations may require adjustments to specific test conditions.

Notes:

Thermogravimetric analysis showed significant characteristics in determining the solid content of varnish. Its advantages include the low sample volume required, high automation, continuous and accurate data on quality changes, and simultaneous evaluation of the thermal stability of the sample. In addition, this method avoids the problem of solvent residue caused by surface conjunctiva in the traditional oven method.

In practical application, it should be noted that: First, the method establishment stage needs to be conditionally verified for specific varnish types (such as acrylic paint, alkyd paint, etc.) to ensure that the end temperature is set reasonably. Secondly, for components that may decompose or undergo chemical cross-linking at low temperatures, the interpretation of the results should be cautious, and if necessary, it should be combined with other techniques such as differential scanning calorimetry for comprehensive analysis. Finally, the laboratory environment (temperature and humidity) and the standardization of operators are also important aspects to ensure the consistency of results.

Conclusion

Thermogravimetric analyzers provide an efficient, accurate, and informative technical means for determining the solid content of varnishes. By understanding how they work, following standardized operating procedures, and tightly controlling key parameters such as temperature rise rate, temperature, and atmosphere, laboratory personnel can obtain reliable data on non-volatile content content. This data has reference value for product quality control, formulation research and development and cost calculation of varnishes. With the advancement of instrument technology and the improvement of standard methods, the application of thermogravimetric analysis in this field will become more in-depth and widespread.

References

1. ASTM International. ASTM E1131-20, Standard Test Method for Compositional Analysis by Thermogravimetry.

2. International Organization for Standardization. ISO 11358-1:2022, Plastics — Thermogravimetry (TG) of polymers — Part 1: General principles.

3. Compilation of basic standards for coatings and varnishes related test methods. China Standard Press.