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    Home > Coatings News > Paints and Coatings Market > The particle size theory and the choice of particle size meter in powder coating are discussed briefly

    The particle size theory and the choice of particle size meter in powder coating are discussed briefly

    • Last Update: 2020-11-25
    • Source: Internet
    • Author: User
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    Introduction: This paper introduces the basic theory, basic concepts and commonly used granularity meters, puts forward the suggestions of how coating enterprises choose granularity meters, and explains the importance of particle size control of powder coatings.
    1, foreword
    in the process of powder coating manufacturing, granularity is one of the important technical indicators of powder coating products, the properties of coating film has a great impact.
    is most common in traditional granularity measurements, but it is difficult to give detailed particle size distribution

    .
    especially the production of some high-grade powder, in which the middle and final products are particle-like conditions, the quality of particulate matter inspection, particle size measurement is particularly important. With the development of science and technology, the appearance of granular measurement instruments provides reliable data for production and service.
    2, particle size measurement theory
    2.1 particle size (also known as particle size)
    the vast majority of powder coating particles are irregular in shape, that is, non-spular particles, the diameter size is determined by the particle center of gravity connected to the particle surface between the two points of the straight segment.
    In this way, the determined diameter is not a single, but a distribution that continuously changes from an upper-limit Dmax to a lower-limit Dmin, at which point the granularity value can only be a statistical average of all these diameters, which is related to the calculation of the average.
    three ways of calculating the average particle size: geometric mean diameter, arithmetic average diameter, and reconcile average diameter.
    2.2 particle size distribution and notation method
    powder coating is a multi-dispersed particle system, its particle size has a certain distribution range, the particle groups in it are divided into groups, each group of particles and the entire particle group of the ratio, called frequency distribution;
    generally speaking, frequency distribution and cumulative distribution are collectively referred to as granularity distribution. Frequency distribution and cumulative distribution will vary greatly due to the different benchmarks set.
    Therefore, the particle size measurement work is faced with a large number of particles statistical research, that is, to measure the particle size distribution of the particle system, measured particle size distribution data have three ways of representing: list, illustration, function.
    2.2.1 Granularity Distribution Table
    A table is used to list the percentages or granularity composition of each granularity range, which is the simplest form of particle size distribution, as shown in Table 1. It should be noted that table 1, such as 27.0 m, should not include 27. μ 0 μ μ m;
    2.2.2 particle size distribution curve
    In order to study the particle size distribution of particle systems for analysis, understanding and interpretation, the graph form is easier to understand than the table form.
    types of illustrations are many, but in granularity analysis, there are two main types: histograms and line charts, and the axes used are arithmetic, semi-contummetic, and count-probability coordinates. The general particle size distribution curve is shown in Figure 1.
    2.2.3 Function
    Theoretically, particle size distribution can be represented by parsing mathematics, and the particle size distribution function is usually represented by three kinds of normal distribution, number of normal distributions, and dozin-Ramla distributions. In most cases, the use of functions to represent particle size distributions is only commonly used in theoretical studies.
    2.3 Particle size distribution of powder coatings
    The particle size distribution of powder coatings depends on the setting, type and system of equipment and production process parameters manufactured, and the particle size distribution controlled by powder coating varieties is different.
    general, when the particle size powder ultra-fine powder too much (<10 μ m), due to the powder coating at this time the dry powder flow is slightly worse, will cause spraying problems, sticky gun head and other spray construction problems;
    When the proportion of excessive coarse powder (> 75 μ m) in the particle size distribution is too large, it will cause poor leveling of the coating film, reduce the spray area per kg of powder coating, the static effect of powder coating during construction is poor, easy to cause de-powdering.
    3, particle size profile
    3.1 laser particle size meter
    laser particle size meter inherited laser technology, modern photoelectronic technology, electronic technology, precision machinery and computer technology, with fast measurement speed, large dynamic range, easy to operate, good repeatability and other advantages, has become the most popular particle size testing equipment.
    the particle size is measured according to the scattering phenomenon of light, the structure principle is shown in Figure 2. Its main advantages:
    First of all, the dynamic range of the measurement is large, dynamic range refers to the instrument can measure the minimum particles at the same time compared to the maximum particles, the greater the dynamic range is more convenient to use.
    Second, the measurement speed is fast, starting from the completion of the dispersed post-in sample, to the display output test report only needs about 1 min, is one of the fastest measuring instruments of all kinds of granular instruments;
    3.2 sedation particle size meter
    seeding particle size meter is by measuring the rate of particles in the liquid to detect the particle size distribution, generally commonly used with light-permeable gravity particle size meter or light-permeable centrifugal seuration meter.
    the measurement principle shown in Figure 3, the sink is filled with a mixture of sedation fluid and powder under test, and at a certain position below the liquid surface, a beam of detecting light parallel to the liquid surface passes through the mixture.
    the distance from the liquid surface to the detecting beam is called the sethering height H, and the light intensity of the mixture is measured by a photodetector, whose output of the light signal is sent to subsequent computers for processing.
    Over time, the particles in the pool gradually sink, and the coarse particles sink faster than the fine particles, when the largest particles on the surface of the mixture sink below the detection beam, the particle concentration in the area around the detection path gradually decreases, the light transmission rate gradually increases, the light strength becomes stronger.
    main advantage of the sembed particle size meter is that the price is cheap, but the operation is more troublesome, the results of the measurement are easily affected by the ambient temperature, operating methods and other factors, the actual repeat accuracy is low.
    3.3 granularity instrument variety model
    At present, the market appears in the particle size meter is roughly the following companies and products:
    United States: Kudrett particle size meter, the use of Froome and Meishi theory, dual-lens patented technology, polarization light intensity differential patented technology, 132 detectors, circulation tube for silicone tube, range (0.04 to 2,000) sm.
    France: Silas granular meter, the use of semiconductor laser tubes and optical fiber laser light source, range (0.04 to 2,500) μ m, built-in peristaltic pump, low replacement costs, which is important for centrifugal pumps.
    Germany: The new Partek granular meter, using a helium light source, uses the Fraunhofer principle, the range (0.1 to 3,500) sm, built-in 31 sensors. Dry testing is its unique technology, sample dosage is small, repetition is good, wet test, the sealing ring of the circulating tube and centrifugal pump can be replaced according to the solvent requirements used.
    UK: Malvern particle size meter, using He-Ne light source full-range meter theory, circulation tube for ordinary plastic tube, built-in non-uniform cross-arranged 3D fan detection system, very high resolution, no signal blind spot, range (0.02 to 2,000) sm.
    chinese granular meter manufacturers about seven or eight, quite influential such as "European and American grams", "fine new", "Bait", "micro-sodium" and so on.
    the performance of domestic granular meters and imported products are narrowing, but European and American products are the mainstream market system, in a period of time will not change.
    4, particle size meter selection
    powder coating manufacturers how to choose cost-effective products, it is recommended that we give priority to the following points:
    (1) repeatability. That is, reproducible or accurate. Refers to the repeated error of the results of the instrument's multiple measurements of the same sample, the smaller the error, indicating the better the repeatability.
    (2) range and dynamic range. The range of most instruments is in gear, and the particle size range that can be measured in the first gear is called dynamic range, i.e. the ratio of the maximum particle size to the smallest particle size that can be measured in this gear. In general, the middle segment of the dynamic range has the highest accuracy, and the closer you get to the edge, the lower the accuracy.
    if the particle size distribution width of the general powder coating is measured and the dynamic range of the instrument is less than the particle size distribution range of the sample, it is impossible to measure reliable results.
    (3) ease of operation. Refers to the ease of use of the instrument.
    (4) powder coating industry habits. Refers to the standard of powder coating industry or industry usage habits and their development trends.
    (5) price. Whether domestic or imported, the price of granular meter is very expensive, so there are times when the price is also a very important factor, in the use of the same effect, try to choose low-cost products, try to buy domestic instruments.
    (6) after-sales quality service.
    5, conclusion
    With the rapid growth of powder coating use, the study of powder coating more and more in-depth, particle size theory on the performance of powder coating is very important, especially in some high-grade powder coatings, such as high-grade furniture, high-grade lighting, automotive components.
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