Excessive residual monomers in the emulsion will bring stability and odor problems to the emulsion, and affect environmental protection, so it is a quality control index of polymer emulsions. There are three types of testing methods, physical method, chemical method and instrumental analysis method, among which chemical method and instrumental analysis method are the most used.
(1) Physical method Add 20~25g of the polymer emulsion sample to be tested, 100g of water, a small amount of polymerization inhibitor and defoamer into the Deen-Seark moisture test tube, and carry out distillation in it. After the distillate is separated, the volume of the monomer can be read directly.
This method is only suitable for polystyrene, acrylic acid; 1L and other monomers with extremely low solubility in water. For copolymer emulsions, after reading the volume of monomers in the distillate, other methods are needed to determine the relative content of each monomer.
(2) Chemical method There are three commonly used chemical methods for the determination of residual monomer content in polymer emulsions: bromination method, mercury salt method and iodine-alcohol method. For different polymer emulsions, the determination procedures are slightly different. For example, for vinyl acetate emulsion, a simple bromination method or an iodine-alcohol method can be used for direct determination, and satisfactory results can be obtained. But for volatile monomers, the monomers need to be distilled out first, and then measured. For polystyrene emulsion, since the emulsion precipitate will also absorb bromine and iodine, it will bring great errors to the test results. In this case, bromination method and iodine-alcohol method are not suitable, and it is better to use Mercury salt method.
(3) Gas chromatography Gas chromatography is an effective method to quickly and accurately determine the residual monomer content in polymer emulsions. It takes less than 20 minutes to measure a sample, and the minimum detection amount is 5X10-5. Some can reach 5><10-6. Both thermal conductivity cell chromatograph and hydrogen flame chromatograph can be used, but more accurate results can be obtained with hydrogen flame chromatograph. For gas chromatographic analysis, it is first necessary to use an emulsion with known monomer concentration and measure it on a chromatograph. The peak position, height and area were used to make a standard curve. During the determination, the injection volume is the same as that of the standard curve, and the peak height is measured, and the amount of various monomers can be obtained by using the standard curve.
The draft for approval of the national standard "Emulsion for Architectural Coatings" stipulates that the total amount of residual monomers should not exceed 0.10% by gas chromatography.
