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    Home > Coatings News > Paints and Coatings Market > Research on the Factors Influencing the Powder Rate of Electrostatic Powder Spraying Based on the Characteristics of Powder Coatings

    Research on the Factors Influencing the Powder Rate of Electrostatic Powder Spraying Based on the Characteristics of Powder Coatings

    • Last Update: 2021-09-28
    • Source: Internet
    • Author: User
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     Text/Chen Wenlong, Hu Yongjun, Liu Huangping, Liu Chang(1.



    Abstract: In order to study the influence of the characteristics of powder coatings on the powder rate of electrostatic spraying, scanning electron microscopy, X-ray diffractometer, inductively coupled plasma emission spectrometer and laser particle size analyzer were used to analyze the microscopic morphology and physical properties of three different commercial powder coatings.


    Keywords: powder coating characteristics; electrostatic spraying; average particle size; pigment-to-base ratio; powder loading rate


    0 Preface


    Electrostatic powder spraying technology has the characteristics of simple process, high yield, low energy consumption, and environmental protection.


    This research is based on the characteristics of powder coatings.



    1 Experimental part


    1.


    Three powder coatings (all polyester type) from two powder coating manufacturers, A and B, were selected as experimental raw materials, and the models were 7041 (sand texture dark gray), 7069 (sand texture black), and 9003 (high gloss white)


    1.


    Scanning electron microscope (JXA-8100): JEOL; energy spectrometer (Oxford-X-Max-20): Oxford, UK; X-ray diffractometer (Smartlab3): Rigaku, Japan; inductively coupled plasma emission spectrometer (Agilent7700S): United States Agilent; Laser particle size analyzer (Mastersizer3000): Malvern, UK; Aluminum alloy sample (6061 rectangular aluminum alloy plate, 12cm×7cm)): Guangzhou Aluminum Group Co.


    1.


    The test procedure of the powder rate of electrostatic powder spraying is as follows: first, the aluminum alloy test panel is subjected to spray pretreatment (water washing-degreasing and degreasing-pickling and etching-3 times of pure water washing-chromium-free passivation-drip drying), and then Use a precision electronic balance to weigh the mass (accurate to 0.


    In order to accurately measure the powder rate in the blind corner, a set of test tool for the powder rate in the blind corner is designed, as shown in Figure 1


    In the formula: 0<S<1, when the value of S is closer to 1, it indicates that the powder rate on the dead corner of electrostatic spraying is better
    .
    Conversely, the closer the value of S is to 0, the worse the powder rate on the dead corner of electrostatic spraying
    .


    2 Results and discussion


    2.
    1 Characterization of powder coating properties

    2.
    1.
    1 Microscopic morphology of powder coatings

    The micro morphology of 7041, 7069, and 9003 powder coatings is shown in Figure 2, and they are all backscattered electron images
    .

    It can be seen from Figure 2 that the three powder coatings produced by different manufacturers all show irregular shapes of different sizes, and most of the powder particles are gray in color
    .
    Use energy spectrometer to analyze the composition of the gray part.
    Its main elements are C, O, Ti.
    It can be inferred that the gray part is mainly polyester resin and pigment (titanium dioxide TiO2/carbon black).
    At the same time, each powder particle is clearly visible.
    The surface and even the inside are covered with a large number of white particles with uneven thickness.
    Analysis of the white particles composition shows that they mainly contain Ba, S, and O elements.
    Therefore, it can be inferred that the white part is mainly filler BaSO4
    .

    2.
    1.
    2 Phase structure of powder coating

    Figure 3 shows the X-ray diffraction spectrum of 7041, 7069, and 9003 powder coatings
    .

    It can be seen from Figure 3 that the diffraction patterns of the three powder coatings produced by different manufacturers have the diffraction peaks of BaSO4 (filler) and TiO2 (titanium dioxide), and the three powder coatings all contain BaSO4 and TiO2 phases
    .
    It can be seen that the previous inference on Figure 2 is correct
    .
    It is known that the 9003 powder coating is white, so the TiO2 phase diffraction peak of the 9003 powder coating is higher than that of the 7041 and 7069 powder coatings, as shown in Figure 3(c)
    .
    7041 and 7069 powder coatings are similar to black, but there is no diffraction peak of crystalline carbon in the diffraction patterns of 7041 and 7069 powder coatings, indicating that the pigment carbon black added in 7041 and 7069 powder coatings is amorphous carbon
    .
    In addition, from Figure 3 (a) and (b), it can be seen that a small amount of impurity CaCO3 phase diffraction peaks appear in B-7041 and B-7069 powder coatings, which may be mixed or added by manufacturer B during the preparation of 7041 and 7069 powder coatings.
    A small amount of impurities are mixed in the filler
    .

    2.
    1.
    3 Powder coating composition (color to base ratio)

    In order to accurately measure the content of the main components in the three powder coatings, inductively coupled plasma emission spectroscopy was used to measure the mass fraction of the main components in the powder coating, and the pigment-to-base ratio of each powder coating was calculated at the same time, as shown in Table 2
    .

    It should be noted in Table 2 that since the filler BaSO4 is white, which is an extender pigment, the "pigment" in the measured pigment-to-base ratio includes pigments and fillers; "base" refers to the base material, mainly resin
    .
    It can be seen from Table 2 that the BaSO4 filler content of A-7041 and A-7069 powder coatings is significantly higher than that of B-7041 and B-7069 powder coatings.
    Therefore, the calculated pigment base of A-7041 and A-7069 powder coatings The ratio is relatively higher than that of B-7041 and B-7069 powder coatings
    .
    The filler, pigment and resin content of A-9003 and B-9003 powder coatings are similar, so their pigment-to-base ratio is not much different
    .
    Comparing the content of the three powder coating components produced by manufacturer A, it can be seen that the BaSO4 content of A-7069 powder coating is the highest, reaching 41.
    05%, and the calculated pigment-to-base ratio is also the highest, which is close to 1:1
    .
    Since the resin content of A-7041 and A-9003 powder coatings is similar, the pigment-to-base ratio of the two is equivalent, close to 3:4
    .
    Comparing the content of the three powder coating components produced by manufacturer B, it can be seen that the filler, pigment and resin content of B-7041 and B[1]7069 powder coatings are similar.
    2:5
    .
    Because the sum of TiO2 and BaSO4 content of B[1]9003 powder coating is higher than that of B-7041 and B-7069 powder coating, the pigment base of B-9003 is relatively high, which is close to 2:3
    .

    2.
    1.
    4 Particle size distribution of powder coatings

    A laser particle size analyzer was used to measure the particle size distribution of the powder coating, and the results are shown in Table 3
    .

    Comparing the Dr(50) of the average particle size of the three powder coatings in Table 3, it can be seen that the average particle size of A-7041 powder coating and B-7041 powder coating is similar, about 34μm; while the average particle size of A-7069 and A-9003 powder coatings The particle size is higher than the corresponding B-7069 and B-9003 powder coatings
    .
    Comparing the particle size distribution Dr(10) of the three powder coatings in Table 3, it can be seen that only the ultrafine powder (≤10μm) content in B-7069 powder coating reaches 10%, and the other powder coatings have the same ultrafine powder content.
    <10%
    .
    Comparing the particle size distribution of the powder coating produced by manufacturer A, it can be seen that the average particle size of the three powder coatings is the same.
    The difference is that the content of coarse powder (≥75μm) in A-7041 powder is higher, while the powder coating in A-7069 The fine powder content is higher
    .
    Comparing the particle size distribution of powder coatings produced by manufacturer B, it can be seen that the average particle size of B-7041 powder coatings is significantly larger than that of B[1]7069 and B-9003 powder coatings, but B-9003 powder coatings have the highest ultrafine powder content, and The coarse powder content is the lowest
    .

    2.
    2 Comparison of powder rate of electrostatic powder spraying and discussion of influencing factors

    2.
    2.
    1 Comparison of powder application rate of electrostatic powder spraying

    The average value of mass increase per unit area is used to characterize the primary powder application rate of electrostatic powder spraying, as shown in Figure 4
    .

    It can be seen from Figure 4 that the primary powdering rate of A-7041, A-7069, and A-9003 powder coatings is higher than the corresponding B-7041, B-7069, B-9003 powder coatings
    .
    Especially for A-7069 powder coating, the one-time powder coating rate is nearly 40% higher than that of B-7069 powder coating
    .
    It can be known from the principle of electrostatic powder spraying that powder coatings are mainly adsorbed on the surface of the sample by electrostatic force and then solidified to form a film.
    Therefore, the primary powder coating rate of powder coatings should be positively correlated with the amount of charge of the powder particles
    .
    According to Coulomb's law, the relationship between powder particle charge and powder particle size can be expressed by equation (2) [6]
    .

    In formula (2): Qs—charge amount of powder particles, C; ε—dielectric constant of powder particles, F/m; d—particle size of powder particles, μm; E—applied electric field strength, N/C; εo—vacuum Dielectric constant, εo=(8.
    9×10-11)F/m
    .
    It can be seen from formula (2) that the charge amount of the powder particles is positively correlated with the square of the particle size of the powder particles
    .
    It can be seen from Table 3 that the average particle size of A-7069 and A-9003 is higher than that of B-7069 and B-9003.
    It can be inferred that the average particle size of powder coatings is one of the main factors affecting the primary powdering rate
    .
    However, the average particle size of A-7041 and B-7041 in Table 3 are the same, but the primary powdering rate is different, indicating that in addition to the average particle size of the powder coating, there are other factors that affect the primary powdering rate
    .
    Comparing the particle size distributions of A-7041 and B[1]7041 in Table 3, it can be seen that although the average particle size of the two is the same, the content of coarse powder in A[1]7041 is significantly higher than that of B-7041
    .
    It can be seen from formula 2 that as the particle size of the powder particles increases, the charge amount of the powder particles increases, but at the same time the mass of the powder particles will increase accordingly, which will cause the overweight powder particles to fall due to excessive gravity before reaching the surface of the workpiece.
    , Thereby reducing the one-time powder rate
    .
    However, the actual measurement of A-7041's primary powdering rate is higher than that of B-7041, which shows that there are other important factors that affect the powder coating's primary powdering rate
    .
    It can be seen from Table 2 that the BaSO4 content of the filler in the A-7041 component is significantly higher than that of B-7041, resulting in a higher pigment-to-base ratio of A-7041 than B-7041.
    At the same time, by comparing the characteristics of B-7041 and A-9003, it is found that the two The particle size distribution is almost the same, but the pigment-to-base ratio of A-9003 is higher than that of B-7041, and the primary powdering rate of A-9003 in Figure 4 is also higher than that of B-7041, so it can be inferred that the pigment-to-base ratio of powder coatings affects the powder The main factors of paint
    .
    In addition, comparing the characteristics of A-7041 and A-9003, it is found that the two have the same pigment-to-base ratio and the same average particle size, but the content of coarse powder in A-7041 is higher, which results in a lower powdering rate than A-9003; Comparing the characteristics of A-7069 and A-9003, the particle size distribution of the two is equivalent, and the pigment-to-base ratio of A-7069 is higher than that of A-9003, but the first-time powdering rate is lower than that of A-9003, which shows that it is not the pigment-to-base ratio.
    The higher the one-time fan rate, the better
    .

    In summary, the main factors that affect the primary powder application rate of electrostatic powder spraying are the average particle size and the pigment-to-base ratio of the powder, the secondary factor is the content of coarse powder in the powder coating, and the primary powder coating rate and average particle size of the electrostatic powder spraying The relationship between diameter and color-to-base ratio is not a simple linear relationship, that is, the larger the particle size of the powder, the higher the color-to-base ratio, and the first-time powdering rate is not necessarily better
    .
    Based on the analysis of the experimental data, the characteristics of powder coatings with higher primary powdering rate should satisfy the average particle size of 35μm≤Dr(50)≤45μm, Dr(95)≤75μm, and the pigment-to-base ratio close to 3:4
    .

    2.
    2.
    2 Comparison of powder rate in dead corners of electrostatic powder spraying

    Using the formula in formula (1), the powder rate on the dead corner of electrostatic powder spraying is calculated, as shown in Figure 5
    .

    Comparing the powder coating rate of the powder coatings of manufacturers A and B in Figure 5, it can be seen that the powder coating rates of A-7041 and B-7041 and A-9003 and B-9003 are equivalent (S≈0.
    11), but the powder coating rate of A-7069 The powder rate in the dead corner is about 28% higher than that of the B-7069 powder coating.
    This can also be seen from the macro photos of the sprayed samples in the dead corner area of ​​A-7069 and B-7069 in Figure 6
    .
    Moreover, the surface color of the sample in the dead area sprayed with A-7069 is darker, indicating that more powder coating is deposited on the surface, that is, the powder rate of A-7069 in the dead area is better
    .

    Comparing and analyzing the characteristics of B-7041 and A-9003, the powder particle size distribution is almost the same.
    The pigment-to-base ratio of A-9003 is higher than that of B-7041, but the powder rate on the dead corner is the same.
    Therefore, the pigment-to-base ratio does not affect the electrostatic powder spraying on the dead corner.
    The main factor of fan rate
    .
    Comparing A-7041 and B-7041, in addition to the difference in pigment-to-base ratio, the content of coarse powder in the paint is also different, but there is no difference in the powder rate of the dead corner between the two, so the coarse powder content in the powder coating does not affect the powder on the dead corner of the electrostatic powder.
    The main factor of the rate
    .
    Comparing A-9003 and B[1]9003, both have the same face-to-base ratio and the same coarse powder content in the powder.
    The only difference is that the average particle size of A-9003>B-9003, but the powder rate in the dead corner of the two The difference is not large, so the average particle size of the powder coating is not the main factor affecting the powder rate in the dead corner
    .
    Comparing the characteristics of A-7069 with other powder coatings, it can be seen that in addition to the difference in the pigment-to-base ratio, average particle size and coarse powder content, the biggest difference is that the ultrafine powder content in A-7069 is significantly higher than other powder coatings.
    It can be seen from 3 that D(10)=10.
    1μm of A-7069, that is, the content of ultrafine powder (≤10μm) in powder coatings is 10%, while the content of ultrafine powder in other powder coatings is >10%.
    It is known that electrostatic spraying The formation of the dead zone is caused by the Faraday cage effect, which causes the electric field intensity inside the groove to decrease or even no electric field, which makes it difficult for the charged powder particles to deposit [7]
    .
    Although ultra-fine powder has a small charge, its weight is very light.
    Under the action of the powder carrier gas, the disturbed air flow is easier to reach the inside of the groove, and is deposited on the bottom of the groove under the action of a weak electric field, thereby affecting the electrostatic powder The powder rate of spraying dead corners
    .
    In the actual production process, under the condition of ensuring that the powder coating rate of electrostatic powder spraying is good, the proportion of ultrafine powder in the powder coating should be appropriately increased, that is, Dr(10)≤10μm, which can effectively improve the powder coating rate of dead corners and reduce artificial compensation.
    Powder cost
    .


    3 Conclusion


    This research is based on the comparative analysis of the characteristics of three commercial powder coatings, and the comparison test of the primary powder coating rate and the powder coating rate of the three powder coatings.
    The main factors affecting the powder coating rate of electrostatic powder spraying are mainly studied, and the improvement of electrostatic powder is proposed.
    The method of coating powder rate
    .
    The research results show that the average particle size and pigment-to-base ratio of powder coatings are the main factors affecting the primary powder application rate of electrostatic powder spraying, but the relationship between the primary powder rate of electrostatic powder coating and the average particle size and pigment-to-base ratio is not a simple linear relationship
    .
    In addition, the ultra-fine powder content in powder coatings is an important factor affecting the powder rate of electrostatic powder spraying dead corners
    .
    In order to obtain a higher electrostatic powder spraying rate, the powder coating should meet the following characteristics: 35μm≤Dr(50)≤45μm, Dr(95)≤75μm, and the pigment-to-base ratio is close to 3:4
    .


    references


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    [3] Liu Wei.
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    [4] Cui Zhiming, Yang Yajiang.
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    [5] Gao Qingfu, Shi Zhongping.
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