Comparative analysis of moisture determination methods in water-based coatings

With the improvement of living standards, people's environmental protection and health awareness continue to improve, decorative coatings on human health impact has been more and more widely concerned. Countries have qualified volatile organic compounds (volatile organic compounds, abbreviated as VOCs) in coatings and have issued testing standards for total volatile organic compounds. Different countries, different regions, and even different sectors, because of different levels of production, different emphasis, so the VOC control indicators are also different requirements
China
. The
of the total volatile content, density and moisture content is involved in the determination of THE VOC content. The determination of volatile content and density has a relatively small deviation, and the determination of moisture content has the greatest influence on the results. There are many methods for determining moisture content, such as drying, distillation, Carl Fishu, gas chromatography, extraction, titration, etc. This study compares the precision and accuracy of moisture content in water-based coatings by three common methods: Carl Fishu, co-boiling distillation
and gas chromatography, and provides a theoretical basis for the appropriate selection of measurement methods and the practicality of evaluation methods.
1 Materials and methods
1. 1 Testing materials
four brands of internal wall water-based coatings sold in the market as the object of detection and analysis.
1. 2 Detection Methods
The water content determination of the purchased water-based coatings was determined by three analytical methods: Carl Fissew, co-boiling distillation and gas chromatography, and the principle of the method, the operating steps and the main instrument reagents were shown in Table 1.
2 Results and discussions
2. 1 Comparison of the accuracy of different moisture measurement methods
the determination of moisture content in water-based coatings as shown in Table 2. It can be seen from the table that there are differences in the moisture content measured by different methods of the same water-based coating. Compared with the three methods, the water content results of co-boiling distillation and gas chromatography were relatively close, and the overall content of Carl Fishu was higher than that of the other two methods. Comparing the relative standard deviation (RSD) of the three methods, it can be found that the RSD of co-boiling distillation and gas chromatography are 0. 43% to 1. 16% and 0. 29% to 1. 42%; Carl Fissofa's RSD is 5. 01% to 11. 54%, and with the increase of moisture content in the sample, the RSD value is on the rise, indicating that the method is not suitable for measuring samples with high moisture content. The reason for the large water content error of Carl Feshu method determination results may be: the inner wall coating film is a polymer emulsion, in the titration process emulsion constantly break milk, release water molecules, making the titration end point difficult to judge, bringing titration error.
2. 2 Comparison of the accuracy of different moisture measurement methods
In three water-based coatings, water was added quantitatively to determine the water recovery rate of the three methods (Table 3). As can be seen from Table 3, with Carl S. The recovery rate of moisture content in the four water-based coatings measured by the Feshu method was 87. 59% to 109. 83%, with an average recovery rate of 95. 62%; the recovery rate of co-boiling distillation was 96. 39% to 98. 68%, with an average recovery rate of 97. 54%; 44% to 101. 46%, with an average recovery rate of 99. 56%。 Overall, the accuracy of co-boiling distillation and gas chromatography can meet the test requirements, and the recovery rate of individual samples measured by Carl Fishu method exceeds (100 ±5), with relatively low accuracy.
3 Conclusion
Among the three methods commonly used to determine moisture content, the precision and accuracy of co-boiling distillation and gas chromatography are in line with the experimental requirements and are worth popularization and application, while the stability and The accuracy is relatively poor, and the relative standard deviation (RSD) of the measurement results is on the rise with the increase of moisture content, and the recovery rate of the mark-up exceeds the (100 ±5) range, which is not suitable for measuring water-based coatings with high moisture content.