Coating particle size analysis mainly includes particle size distribution testing of powder coatings, architectural emulsions and other coating products, as well as pigments and fillers such as titanium dioxide, iron oxide, and talcum powder. The particle size testing methods mainly include sedimentation method, laser method, sieving method, electrical resistance method, microscopic image method, electron microscope method, electrophoresis method, mass spectrometry method, scraper method, ventilation method, ultrasonic method, etc. The laser particle size analyzer test method is a new particle size test method with wide application, fast test speed and wide test range. The laser particle size analyzer is based on the scattering of laser light on the surface of the measured particle. The angle and light intensity of the scattered light will vary with the particle size. According to Mie scattering and Freund's diffraction theory, particle size analysis can be performed.
The testing methods of laser particle size analyzer can be divided into dry method and wet method. The dry method uses air as the dispersion medium and uses the principle of turbulent flow dispersion to fully disperse the sample particles, and then the dispersed samples are introduced into the optical system for testing. The wet method is to add the sample directly into a dispersion medium such as water or ethanol for dispersion, and then pass through the optical system to calculate the particle size distribution. Due to different dispersion media, the test results of dry and wet test methods will be different. At present, most particle size analyzers use the wet method for testing, but the dry method also has its advantages: fast test speed, simple operation, and can test samples dissolved in water, etc. [2]. This paper uses dry method and wet method to test the particle size of titanium dioxide, talcum powder, graphene and other pigments and fillers respectively. By analyzing the test results, the differences between the two methods and the test conditions, dispersants and test results are discussed. , and discusses the reproducibility between test results.
1 Experimental part
1. 1 Main raw materials and instruments
Titanium dioxide: R-2196, CNNC Huayuan Titanium Dioxide Co., Ltd.; talcum powder: T-777A, Utoko Minerals (Kunshan) Co., Ltd.; graphene: SE1132, Changzhou Sixth Element Materials Technology Co., Ltd. HELOS/BF wet and dry two-in-one laser particle size analyzer: German Synpatek company, the lens test range (R) is R1 (0.1-35 μm), R3 (0.5-175 μm), R5 (0 . 5～875 μm).
1.2 Test method
(1) Dry test
Weigh a certain amount of well-mixed and uniform samples, and bake in an oven at (105±2)°C for 15 minutes to remove moisture. Select the test mode as dry method. Set parameters such as dispersion pressure, vibration tank speed, etc. Sample test, the shading rate is controlled at 7% to 10% [3].
(2) wet test
Wet test samples are divided into dry powder samples and liquid samples. Dry powder samples should be mixed thoroughly before testing to ensure the homogeneity of the samples. Liquid samples are shaken and added directly to the sample reservoir. For samples that are not easy to disperse, add an appropriate amount of dispersant to the sample tank, adjust the pump speed, ultrasonic time, intensity, stirring rate, select a suitable lens, and start the test. The shading rate is between 8% and 12%.
1.3 Particle size distribution parameters
X b = a μm: indicates that the particle size smaller than a μm accounts for b% of the total volume; VMD: volume average particle size.
2 Results and discussion
2.1 Test of particle size distribution of titanium dioxide
2. 1. 1 Dry test
Test conditions: R1 lens; dispersion pressure 0.6 MPa; vibration tank rate 60%; the trigger condition is that the shading rate > 1% to start the test, and the shading rate is less than 1% to stop.
Test results (Figure 1): X 1 = 0. 20 μm; X 50 = 0. 60 μm; X 99 = 1. 80 μm; VMD is 0. 69 μm.
2. 1. 2 Wet test (without dispersant)
Test conditions: R1 lens; pump speed 40%; ultrasonic time 30 s; stirring rate 40%.
Test results (Figure 2): X 1 = 0. 11 μm; X 50 = 0. 84 μm; X 99 = 2. 52 μm; VMD is 0. 90 μm.
2.1.3 Wet test (with dispersant sodium hexametaphosphate)
Test conditions: R1 lens; pump speed 40%; ultrasonic time 30 s; stirring rate 40%.
Test results (Figure 3): X 1 = 0. 11 μm; X 50 = 0. 66 μm; X 99 = 2. 08 μm; VMD is 0. 74 μm.
2.1.4 The difference between the two test methods of particle size distribution of titanium dioxide
From the test results of titanium dioxide dry method and wet method, it can be seen that the test results of the two methods are similar, and the test results of the dry method are smaller than those of the wet method. Compared with the wet test with dispersant, the difference of X 1 value of the two methods is 0.09 μm, the difference of X 50 value is 0.06 μm, the difference of X 99 value is 0.28 μm, and the difference of VMD is 0.05 μm . If no dispersant is added in the wet test, the sample cannot be fully dispersed in the dispersion medium, and double peaks will appear in the particle size distribution diagram of the sample (see Figure 2). It can be seen that the dispersant has a great influence on the dispersion effect of the sample, and a suitable dispersant is conducive to the dispersion of the sample in the dispersion medium to ensure the accuracy of the test.
2.2 Test of particle size distribution of talcum powder
2. 2. 1 Dry test
测试条件:Ｒ1 镜头;分散压力 0. 3 MPa;震动槽速率 65%;触发条件为遮光率＞1%开始测试，遮光率小于 1% 停止。
测试结果(图 4):X 1 = 0. 57 μm;X 50 = 4. 35 μm;X 99 =19. 19 μm;VMD 为 5. 41 μm。
2. 2. 2 湿法测试(未加分散剂)
测试条件:Ｒ1 镜头;泵速 40%;超声时间 30 s;搅拌速率 40%。
测试结果(图 5):X 1 = 0. 61 μm;X 50 = 6. 21 μm;X 99 =22. 01 μm;VMD 为 7. 03 μm。
2. 2. 3 湿法测试(加分散剂六偏磷酸钠)
测试条件:Ｒ1 镜头;泵速 40%;超声时间 30 s;搅拌速率 40%。
测试结果(图 6):X 1 = 0. 60 μm;X 50 = 5. 73 μm;X 99 =23. 63 μm;VMD 为 7. 03 μm。
2. 2. 4 滑石粉粒径分布 2 种测试方法之间的差异
比较滑石粉干法测试和湿法测试的粒径分布图
可以看出，湿法比干法测试结果偏大。滑石粉密度较大，在干法测试的过程中，选择了 0. 3 MPa 的分散压力。湿法测试中，加入分散剂和未加分散剂的测试结果相近，可以看出添加分散剂对滑石粉的测试结果影响不大。滑石粉能够较好地分散在水中。
2. 3 石墨烯粒度分布的测试
2. 3. 1 干法测试
测试条件:Ｒ1 镜头;分散压力 0. 1 MPa;震动槽速率 65%;触发条件为遮光率＞1%开始测试，遮光率小于 1%停止。
测试结果(图 7):X 1 = 0. 62 μm;X 50 = 3. 86 μm;X 99 =8. 10 μm;VMD 为 3. 89 μm。
2. 3. 2 湿法测试(不加分散剂)
测试条件:Ｒ1 镜头;泵速 40%;超声时间 30 s;搅拌速率 40%。
测试结果(图 8):X 1 = 1. 94 μm;X 50 = 9. 69 μm;X 99 =20. 37 μm;VMD 为 10. 19 μm。
2. 3. 3 湿法测试 (加分散剂)
测试条件:Ｒ1 镜头;泵速 40%;超声时间 30 s;搅拌速率 40%。
2. 3. 4 石墨烯 2 种测试方法之间的差异
从石墨烯 2 种方法的测试结果可以看出，干法的测试结果偏小，湿法的测试结果较大(加入分散剂测试)。这是因为石墨烯样品密度较小，会浮在分散介质上，样品的分散效果较差。2 种方法 X 1 值相差0. 72 μm，X 50 值相差 3. 59 μm，X 99 值相差 9. 94 μm，VMD 相差 4. 06 μm，说明石墨烯样品难于在水中较好地分散，干法测试更适合石墨烯。湿法测试中，添加分散剂和不加分散剂的粒径分布结果相差也较大，说明使用分散剂六偏磷酸钠可以较好地分散石墨烯。而分散剂的浓度和用量对样品分散效果的影响则需要通过另外的实验来确定。
2. 4 涂料粒径分析干法和湿法之间的差异
干法和湿法虽然测试的结果比较接近，但是由于两者的分散介质的折射指数不一样，两者的测试结果之间会有一些差异。进行粒径分析，重要的是要保证样品在各自使用的介质中的分散效果。干法的进样速率、压力等分散条件的选择要合适，在保证可以分散好样品的情况下，尽量选择较小的压力，减少对样品颗粒的冲击，避免颗粒的二次破碎。对于一些难于分散的样品，比如氧化铁，密度较大，需要选择较大的分散压力，否则无法取得好的分散效果，或者改变进样量来改变样品的分散效果。湿法进样要通过改变搅拌速率、超声时间来进行调整，同时使用合适的分散剂来对样品进行分散。对于一些较轻，可漂浮在分散介质上的样品，要延长样品的测试时间，以利于样品的充分分散。同时湿法测试应该使用超声波去除气泡，否则会在结果中形成拖尾峰 。
2. 5 干法和湿法测试的重复性比较
2. 5. 1 干法测试重复性
重复性指标是衡量粒径分布测试结果好坏的重要指标，是指同一个样品多次测量结果之间的偏差，通常用 X 50 之间的偏差表示。粒径分布的重复性测试与样品的分散程度有较大的关系，样品分散的好，则测试的重复性也较高。选取 2 种常用的颜填料钛白粉和滑石粉进行干法重复性试验。结果见表 1。
2. 5. 2 湿法测试重复性
选取乳液和钛白粉分别进行了 2 次湿法重复测量。测试结果见表 2。
At present, GB/T 21782.13-2009 stipulates that the repeatability requirement of powder coating particle size test is that the deviation between any particle size fraction of the two test results is not more than 1% [5]. From the test results of the above samples, the repeatability of the dry test and the wet test both meet the standard requirements.
The main factor affecting the repeatability test is the degree of dispersion of the sample, so the sample should be taken before the test to ensure the uniformity of the sample. For the sample that is easy to agglomerate, the repeatability is poor, so whether it is a dry test or a wet test, it must be done. Good sample preprocessing work. For dry powder samples, pay attention to dehydration and drying. For some dry powder samples that are not well dispersed in water, it is necessary to add a dispersant to the dispersion medium, and set the auxiliary dispersion conditions such as ultrasonic time and stirring speed of the instrument. Liquid samples are used for wet testing, and the samples need to be stirred evenly. For emulsion and water dispersion samples, since the particles to be tested have been dispersed in the sample to form a stable system, the repeatability of the test results is good. The dispersion medium of the wet test has a great influence on the sample, and it is easy to react with the dispersion medium (water), or the sample whose refractive index is not much different from water is not suitable for the wet test. For samples with higher density such as iron oxide, the dispersibility of the dry test is poor, and the wet test can be used. By adding a dispersant, prolonging the ultrasonic time and increasing the stirring rate, the sample can be fully dispersed, thereby improving the test repeatability of the sample
3 Conclusion
The particle size distribution of titanium dioxide, talcum powder, graphene and building emulsion used in the dry and wet testing of coatings by laser particle size analyzer is discussed. For the laser particle size analyzer test method, due to the difference in the dispersion principle between the dry test and the wet test, for the same sample, the test results will also be different. The wet test results are larger than the dry test results. In the process of testing samples with low density, the samples will float on the dispersion medium. Surfactants such as sodium hexametaphosphate should be added to reduce the surface tension of the dispersion medium and improve the dispersion of the samples, so as to ensure that the samples are in the dispersion medium. fully dispersed. Under the condition of ensuring accurate instrument settings, the repeatability of the laser particle size analyzer test is good, and the deviation of the results of the dry test of titanium dioxide, talcum powder and other powders is less than 1%. In the wet test, the test repeatability of the emulsion is better than that of the dry powder, and the deviation of the results of the wet test twice is less than 1%.
