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    Home > Coatings News > Paints and Coatings Market > Color measurement and control of powder coatings

    Color measurement and control of powder coatings

    • Last Update: 2020-12-05
    • Source: Internet
    • Author: User
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    0 Foreword
    powder coating development early is mainly used for metal corrosion protection, with the continuous progress of powder coating production and application technology, the world's environmental awareness and strengthening of regulations, powder coating application range continues to expand, powder coating decorative beautification role is also more and more attention, and modern enterprises assembly line processing and assembly (such as refrigerators, air conditioners, instrumentation, metal doors and windows, aluminum profiles, mechanical and electrical equipment and other manufacturers) more requires the coating products inside and outside the quality of a high degree of stability, batch coating color has become an important customer requirements.1 Color measurement
    1.1 Visual
    G B/T9761-98 "Color paint and varnish color paint visual color" and other effects are using I S O3668-1976 standard, standardized the coating color visual color method, can be in natural light, color box under the artificial standard light source detection, commonly used light source D65, A two categories. Visual color is more demanding for the observer, the observer responsible for color testing should pass the color blindness examination mirror, wearing glasses of their lenses must have a uniform spectral pass rate throughout the
    visible spectrum

    Continuous work by the observer reduces the color quality. Visual inspection is the most direct and emotional way of detection, instrument measurement in color matching can only be an important auxiliary means of quality control.
    1.2 Instrument measurement
    1.2.1 measurement principle
    The interaction of light with the material produces specular reflection, diffuse reflection, directional transmission, scattering transmission, and light absorption, while the color of the object is mainly determined by its diffuse reflection, using a spectral photometric meter to determine the spectral reflection factor of the powder coating (e.g. K/S ratio), and its three stimuli can be calculated and compared with the differences in color. Connect it to a computer and dedicated software to display, store, and output color data, color effects, K/S curves, and chromosomes.
    the scanning spectral photometer
    1.2.2 measuring instrument
    color of the scanning spectrometer using prism spectroscopy is mainly color difference meter, color meter two categories. Color difference meter cheap, easy to carry, can carry out convenient and fast color measurement ratio, can also provide color matching personnel with data reference, but the error is large, can not directly color matching operation. There are two types of color detectors, one is portable, like chromometer, but the measurement is more accurate, can be used for system data output, but not for color matching, the other can be measured and color matching operation at the same time, the price is higher, only applicable to factory use. At present, chromaometers, portable color measuring meters are widely used, and color matching color meters only a small number of large-scale powder coating enterprises, such instruments are represented by the United States X-R i t e (Aesop), Japan MINOLTA (Minolta) company.
    1.2.3 Measurement Operation
    Color detector operating procedures are: power on → into the operating interface→ black and white plate correction→ sample measurement→ data input→ color → data output→ shut down.
    general chromatometer can be used to directly compare samples, specimen measurements, and chromatosomes after power-on warm-up (around 30 m i n), or the results can be simply output. Color meter does not need special warm-up, the computer turned on, directly into the operating interface for various types of operations, and a variety of light sources under the color data, chromats, spectral curves, intuitive color effects, etc. displayed, forming a report.2 Color
    2.1 Coloring Principle
    The color scheme of conventional powder coatings is a complex decoloration blend in the law of color processing, and the K u e l l l a-M u n k theory holds that light is in the coloring layer Completely diffuse, the "two constant K u b e l l l k a-M u n k theory" uses a coated K/S spectral map (K-absorption, S-scattering) to charactertate pigments, and finally uses a spectral matching algorithm for constant matching. Color matching instrument is to use this principle, the use of color matching software, from the pigment library to screen pigments for K/S spectral compounding calculation, and finally the prediction of color difference is ideal formula for color matching personnel to choose. Because the light source set by the instrument is always limited, the computer color scheme is still a conditional match.
    2.2 Computer color matching process
    2.2.1 pigment selection
    According to the pigment manufacturer's technical instructions or product introduction, according to the basic requirements of powder coating production and application (e.g. heat resistance of 200 degrees C, acid and alkali resistance, anti-migration), the initial selection can be used pigments, and then the selected pigments are tested, generally at 200 degrees C/2 h, Q-U V250 h, and strictly required to carry out 200 degrees C/4 h, Q-U V500 h and 500 h salt spray tests. Pigments used outdoors must have good light resistance and waiting fastness. According to the test results, screening and classification, such as indoor pigments, outdoor pigments,
    pigments, corrosion-resistant pigments and so on. Coloring personnel on the pigment coloring force, oil absorption value, dispersion including price, supply, manufacturers, etc. should also pay attention to. It should be emphasized that the manufacturer's technical instructions and even quality assurance are not a substitute for self-testing.
    2.2.2 Database
    color matching software describes in detail how the database was built, mainly blackboards, whiteboards, masking forces, and pigment data. In addition to the pigments in the formula to establish data, other raw materials should choose the best quality varieties or the most commonly used varieties of enterprises and maintain a fixed, to ensure reliable data and benefit from future production of color mixing, avoid poor quality (such as noise, instability), non-frequently used varieties. Name a database, with the selection of pigments, the general amount is 20 to 25 kinds (such as red 4, blue 2, green 2, orange and yellow 2, orange red 1, purple 2, black 3, white 2, other 3), its data into, that is, to complete the establishment of a single database, coloring as long as the name of the database, the system randomly filter the pigments in the database to match. The establishment of an accurate database of information can guide color matching work effectively in the long run, which is good for color matching personnel to realize expectations quickly. However, due to objective factors such as equipment (e.g. extruder aging), customer pigment quality fluctuations, raw material adjustment, etc., the deviation between sample and computer prediction results will continue to increase, with the introduction of new pigments, database pigment replenishment, update and correction work should continue.
    2.2.3 standard input
    this refers to domestic and foreign color standards, customer samples or self-selected sample data input, wherein color standards are generally paper printing, customer samples are mostly test boards or samples. Some of the above standards are flat, some are surfaces, have highlights or half light or even no light (e.g. German color standard R A L 8 4 1 - G L and R A L 8 4 0 - H R R), are flat and textured, have new and wear, and these color gauges are not recognizable. Therefore, the standard measurement should try to select the coating representative measuring point (area), such as cleaning, flat, wear-free, brand-new, the sample abnormal state in the data storage should also do a good job of marking, conducive to the subsequent color reference. In the case of worn old plate, coating gloss or texture change, surface leakage, etc., the measured data can not be used for the final color match of the instrument, only as a technical reference in the color matching process.
    2.2.4 formula selection
    in the color matching interface, select the standard color preparation, according to the process specified by the software to operate the selected color varieties, can let the computer automatically filter (can be set in advance quantity), can also directly ask the computer to choose 1 to 9 pigments to match, set more varieties, computer matching is easier to achieve, but the slower the calculation speed, the longer the time, in order to facilitate the production of applications generally choose 2 to 4 (including white paint). The best formula given automatically by computer can be arranged by color difference or by cost, but the actual selection is mainly based on K/S spectral matching, color difference size, pigment varieties and economy. Color meter to the main light source listed all kinds of optical data, secondary light source only to excellent poor, its chroma results similar to conditional matching, and enterprises commonly used pigment varieties are significantly less than the database, so color matching personnel to be considered comprehensively before selecting a more ideal formula for testing;
    2.2.5 Color adjustment
    formula selection, generally no ready-made product formula is exactly the same, color matching personnel must be selected according to the formula for laboratory powder, spray plate, color measurement, and then the pigment dosing and even the increase or decrease of the variety to adjust the correction (the correction can be manually or by the computer after providing recommendations) re-powdering, plate making, measurement, etc., until the conditions match, and finally determine the best color adjustment in the laboratory.
    manual color adjustment, generally not simply red and red, lack of blue and blue, to calculate the proportion of each pigment before and after the adjustment of the total amount of pigment, combined with saturation, phase, according to the comprehensive situation of L, a, b for careful adjustment. When coloring, any pigment is not a single color with 100% saturation, the formula pigments mix with each other there is still a significant compound effect, in fine-tuning should be carefully analyzed to ensure the best color matching effect. Color adjustment is generally accompanied by coating cover, mechanical properties, surface effects (e.g. gloss, freshness, texture) and formulation costs, powder performance (e.g. live, powder rate) adjustment, and sometimes even at the same time coating resistance to high temperature, salt spray or artificial aging testing, so the color adjustment process is also the product formulation of a comprehensive mature and reasonable process.
    2.2.6 Workshop amplification
    Due to the laboratory small equipment and workshop large production facilities in the mixing effect there is a certain degree of difference, can not guarantee that the laboratory formula in the workshop has good reproducible, generally need to be batch amplification to further adjust, according to the mixing effect difference of the estimated combined product production volume and production line minimum production volume to determine the test amplification batch (e.g. 30%, 50% or 100% of the conventional batch). After the sample measurement, according to the color difference effect of the enlarged formula for further adjustment, and then select the amount of production sample measurement, until a satisfactory matching effect of
    fruit, to determine the final production formula. As with laboratory regulation, the workshop amplification process, according to the sample quality testing situation, in addition to pigments, but also fillers, a variety of additives fine-tuning, to ensure the expected quality of the product and ideal production costs. It should be noted that the enlargement process will produce a large number of non-conforming products (color matching difference), need to be digested as far as possible in the production, more difficult should also be used in the next batch of production, if feasible, can also be used in the production of other products.
    2.2.7 Mass Production
    Shop floor amplification determines that the production formula is not required or allowed to change arbitrarily under normal circumstances, but in those small and medium-sized powder coating enterprises, the raw materials used by various factors often appear batch-to-batch quality fluctuations or even varieties And the replacement of manufacturers, in order to ensure timely delivery, technical personnel have to often make formula adjustments to complete production, as raw material prices continue to rise, many enterprises also downgraded the use of preferential prices of raw materials, which bring challenges to the stability of product quality between batches, especially the control of inter-batch color difference.
    2.3 Color quality control points
    2.3.1 samples
    the initial powder coating manufacturers for coating customers to provide standard models (color cards) are mostly experimental production, when customers specify the purchase of a color card products, production products With the original test color card is very easy to have a significant color difference, the U.S. standard A A M A2603,2604,2605 pairs of color difference are clearly pointed out that the actual production line prepared film products, rather than laboratory-prepared models. Later, powder coating color cards were basically made from mass-produced product spraying, ensuring that powder coating products that were subsequently available to customer satisfaction. Customer standards to powder samples the best, followed by new slabs, samples, old spades are not conducive to the accurate setting of standards, and even have customers use language to describe their product requirements (such as approximate or similar to a certain color), in order to avoid quality disputes at the time of delivery, these circumstances before the standard agreement with the customer to fully communicate and reach a consensus.
    2.3.2 Isochromat spectrum
    The different colors of the spectral reflection curve match under a set of observations and light sources, but do not match under another condition, this phenomenon is called isochromat spectrum, due to the change of light source caused by mismatches called lighting body conditions isochromats, the error of observer changes is called observer isochromats. Color palette can show a variety of light sources under the K/S spectral matching effect, the same color spectrum at a glance, but when the production of pigments lack, many color matching personnel know that the same color spectrum also have to be sampled, and expect a better matching effect. To avoid the problem of the same color spectrum, we must strengthen the construction of pigment database, the general enterprise pigment library is less than a hundred pigments, commonly used varieties only twenty or thirty kinds, while large enterprises have thousands of pigment data, hundreds of common pigments, the market with various colors with pigments almost exhausted storage, color matching staff work easily, there is basically no same color spectrum problem.
    2.3.3 standard
    after the same product many times or long-term production, quality control personnel often will be the product produced in front of the product only with the recent production issued by the color difference comparison, as long as the color is close to the original standard, easy to ignore the original standard, because the adjacent batch of products color difference is small, most customers are not easy to detect, the product can be successfully accepted. However, the principle of the quality fluctuation control chart tells us that it is very easy to cause product quality fluctuations beyond the control line. The author saw an example, powder coating personnel continued above batch of products for standard control production, resulting in chroma gradually deviated from the original standard, and the customer for a multinational processing enterprises, a variety of assembly materials have always been the original sample as a unified acceptance standard, resulting in serious quality accidents. Therefore, in color control, especially in the multi-batch and long-term continuous production process, not only to control the chroma between batches, but also to always compared with the customer's historical samples and original standards, control color deviation, to avoid excessive deviation to the customer batch product assembly difficulties.
    2.3.4 Cover Force
    Although coating cover is associated with coating thickness, pigment quality, and dispersion performance of powder production equipment, the variety, individual dosing and total dosing of pigments in the product are the factors that influence coating cover. Because pigment prices are generally higher,
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