Application of the hottest laser particle sizer in

2022-08-06
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Application of laser particle sizer in coating analysis and testing application of laser particle sizer in coating analysis and testing. The large-scale instrument laser particle sizer can analyze the particle size effectively and give the test results quickly, which brings convenience to the coating researchers. Laser Particle Sizer is an instrument with precise components and complex structure. When it is used, it is necessary to accurately set the condition parameters in order to obtain effective test results. Dry dispersion method uses compressed air as dispersant to disperse the samples, which is suitable for the testing of powder samples; Wet dispersion method uses solvent as dispersant, uses ultrasonic wave to disperse, and then generates scattering signal by laser beam irradiation. It can test not only dry powder, but also emulsion and suspension. The two test methods need to be selected according to the sample conditions

1 test instrument

helos/o a s i s dry wet two in one laser particle size analyzer manufactured by Germany's simpac company, with a test range of 0.1 ~ 875 μ m. Install three lenses: R1 (0.1 ~ 35 μ m); R3(0.5~175 μ m); R5 (0.5~875 μ m)。

2 testing principle

the working principle of laser particle size analyzer is based on light scattering theory and diffraction theory. When the light beam is projected into the dispersion system of the instrument, light absorption, reflection, scattering and diffraction can occur. When the frequency of the incident light is the same as the natural frequency of the molecule, light absorption occurs; When the wavelength of the incident light is smaller than the size of the dispersed particles, light reflection and diffraction will occur; If the wavelength of the incident light is larger than the size of the dispersed phase particles, light scattering occurs. General 0.1 μ For the particles below m, the diffraction phenomenon disappears and the scattering phenomenon mainly occurs; 0.1~ 10 μ The particles of M are mainly diffracted light, and the particles will produce some refracted and scattered light; Greater than 10 μ M particles, scattering and refraction disappear, mainly diffraction. According to Fraunhofer diffraction theory, for 10 μ m,0.1~10 μ M, the size of diffraction angle is related to the size of particles, and the intensity of diffraction light is related to the number of particles, so the size and number of particles can be determined. For 0.1 μ 0.1 ~ 10 below M μ According to Mie scattering theory, not only the diffraction of light, but also the scattering and refraction of light are considered. According to the comprehensive refractive index of particles, the size and size of particles are calculated

3 several important characterization parameters related to particle size analysis

x 50: the corresponding particle size value when the cumulative distribution percentage reaches 50%, which is one of the important indicators reflecting the particle size characteristics of powder. X50 is also called median diameter or median particle size. (x 10, x 90): it means that particles smaller than x 10 account for 10% of the total particles, and particles larger than X90 account for 10% of the total volume. The two features are combined to represent the range of particle size distribution of the sample

in recent years

4 test part

4.1 determination of particle size and particle size distribution of zinc (Zn) powder by dry dispersion method

determination conditions: vibration tank frequency 70%; Pressure 0.4 m Pa; Evaluation method: H R L D; Test time: 10 s. Select R1, R3 and R5 lenses to test respectively, and the results are shown in Table 1

4.2 determination of particle size and particle size distribution of zinc (Zn) powder by wet dispersion method

dispersion medium: water; Dispersant: sodium hexametaphosphate; Mixing speed: 40%; Ultrasonic dispersion time: 30 s

5 results and discussion 5.1 discussion on the test results of Zn powder

it can be seen from the test results of the dry dispersion method of Zn powder that there are some differences in the test results of different lenses. Although the sample particles are measured in the same area of the lens, due to the different resolutions of each lens, the small lens has high resolution and the large lens has low resolution, so it is difficult for the large lens to measure the diffraction light of fine particles when measuring small particles, So that the analysis results have errors. As shown in Table 1, X50 of R1 is 4.89 μ m. X50 of R2 is 4.91 μ m. X50 of R5 is 4.10 μ m 。

because the test range of R1 lens is 0.1 ~ 35 μ m. The test range of R3 lens is 0.5 ~ 175 μ m. The test range of R5 lens is 0.5 ~ 875 μ m. The test range of each lens is divided into 31 channels, and the resolution range of the first channel of R1 lens is 0.1 ~ 0.18 μ m; The resolution range of the first channel of R3 lens is 0.50 ~ 0.90 μ m; The resolution range of the first channel of R5 lens is 0.50 ~ 4.50 μ m。 Generally, it is necessary to select a suitable lens to ensure that the cumulative particle distribution content of the first channel is less than 5%. However, the cumulative particle distribution content of the first channel of R5 lens given in Table 1 is as high as 55.61%, which is far beyond the required value of 5%. Therefore, serious deviation will occur when using this lens, which will cause many small particles of the sample to be missed and the test results to be smaller

it can also be seen from table 1 that for channel groups with a cumulative distribution content of 100%, R1 is 2 groups, R3 is 8 groups, and R5 is 20 groups. Generally, it is required that a suitable lens is also a suitable range. The number of channel groups with a cumulative distribution content of 100% of the sample should be about 4 groups. Through analysis, it can be seen that R3 lens is suitable for this Zn powder sample

from the test chart of wet dispersion method of Zn powder, the test result of wet dispersion method is larger than that of dry dispersion method, which is the test difference caused by different dispersion methods. Similarly, the test result of wet dispersion method without dispersant sodium hexametaphosphate is larger than that after adding sodium hexametaphosphate

the selection of dry dispersion method or wet dispersion method shall be determined according to the specific sample conditions. The dry dispersion method is convenient and fast. However, the air flow is used in the dispersion system of the dry dispersion method to disperse the sample particles. Due to the electrostatic force generated by the friction between the molecules of the sample, the sample particles will agglomerate. If the selected pressure is low, the sample will be unevenly dispersed in the dispersion tube, resulting in test errors. At the same time, the consumption of the sample in the dry dispersion method is relatively large. Compared with the dry dispersion method, the wet dispersion method can use ultrasonic wave and appropriate dispersant to disperse samples. The methods are flexible and diverse, and the amount of samples is less. The disadvantage is that the dispersion medium must be found accurately

in addition, the state of the sample to be tested also determines which test method should be selected. For example, some easily soluble pigment particles must be tested by the dry dispersion method, while the resin lotion must be tested by the wet dispersion method. For many samples, dispersant must be added to the dispersion medium, otherwise they cannot be fully dispersed. After a small amount of dispersant is added to the dispersion medium, the positive or negative ions of the dispersant will be adsorbed on the surface of the sample particles, resulting in hydration and mutual repulsion between the particles. When the repulsion force is greater than the van der Waals attraction between particles, the particles will be kept in a good dispersion state [4]. Therefore, after adding surfactants such as sodium hexametaphosphate, the particles will be well dispersed, so that the test results are lower than those before adding surfactants

5.2 discussion on test methods

5.2.1 discussion on dry dispersion test methods

the steps of dry dispersion test are generally to set the test conditions of the sample on the software operation interface of the instrument, and then start the background test button. The system will test the optical state of the instrument lens and the cleanliness of the pipeline without adding samples, and then give the background value, which is usually not higher than 5. If the test background value is large, you can use a cotton ball dipped in alcohol to wipe the lens or start the exhaust system device to remove the residual dirt particles in the pipeline. After the background test is normal, you can inject samples for testing

the dry dispersion method is to use compressed air to disperse the sample. It is necessary to ensure that the sample particles will not be broken and can be well dispersed. At the same time, the amount of sample taken is representative. Therefore, for the selection of the test conditions of the dry dispersion method, special attention should be paid to the key parameters such as the air pressure and the shading rate. At the same time, according to the characteristics of the sample, the sampling rate, the height of the vibrating tank, and the starting conditions of the test should be set. The shading rate reflects the number of samples in the laser beam during each test. Its value is not only related to the shape and composition of the samples, but also proportional to the number of samples. Therefore, the sampling amount of the sample required for the test needs to be adjusted according to the shading value. The required amount should not only avoid multiple scattering, but also avoid signal shortage. Generally, the opacity value is controlled below 15%, and the best is 7% ~ 10%

the pigment particles with large density and fine particle size such as titanium dioxide and iron oxide used in the coating are easy to agglomerate due to the effect of electrostatic force. Generally, it is necessary to use the pressure above 0.3 MPa to effectively disperse the samples, while the pressure below 0.3 MPa cannot effectively disperse the samples. When the red pigment iron oxide was tested at a pressure of 0.1 MPa and a rate of 60%, a double peak appeared on the distribution diagram, with a high tail peak, indicating that a large number of particles were agglomerated and failed to disperse, as shown in Figure 7. The white pigment titanium white, with a pressure of 0.1 M PA, also shows the phenomenon of large particle aggregation at a rate of 60%, as shown in Figure 8. Therefore, for different samples, the set dispersion pressure is also different because the density and particle size are different. Usually, the minimum dispersion pressure is used on the premise of good dispersion, which can not only disperse the sample, but also protect the dispersion tube of the instrument and prolong the service life of the dispersion tube

5.2.2 discussion on wet dispersion test method

when using the wet dispersion method for testing, first set the wet dispersion method as the current test method, and then the equipment will automatically switch from the dry dispersion method to the wet dispersion method. Then set the test conditions of wet dispersion method, such as peristaltic pump speed, ultrasonic speed, time, mixing speed, etc., and then start the start button for background test. If the background is very high, it is necessary to clean the lens, flush the pipeline, etc. to ensure that the background value is relatively low, and then add the sample. Control the amount of sample added to keep the shading rate within a certain range. These tubes often have standard shapes and sizes. The range of shading rate can be 5% ~ 30%, preferably 20% ~ 30% [5]. After the sample test, the software will give the results in the form of charts. The general conditions for selecting the dispersion medium in the wet dispersion method are as follows:

① it is transparent so that the laser can pass through

② it can be used together with the material of the instrument without dissolving or changing the particle size of the sample

③ insoluble in air bubbles or other particles

④ the sample particles can be easily and stably dispersed

⑤ the refractive index is quite different from that of the sample particles

⑥ there is a suitable viscosity for circulation. In general, water is used as the dispersion medium in the test, in which low foam surfactant is added to reduce the surface tension of water. Ethanol, isopropanol, hexane or isooctane can also be used as the dispersion medium according to the sample and instrument conditions [6]

the pigment used in the coating is also the reason why Tesla, the most advanced technology in the research and development of new energy vehicles, has not used graphene batteries in one product. Titanium dioxide, iron oxide, calcium carbonate, kaolin, etc. its refractive index is generally 2.50, 2.60, 2.42, 1.52, 1.61, etc., while the refractive index of water is about 1.33. Usually after adding surfactant,

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