The problems in the development, production and construction application of thermoso-solid powder coatings are discussed

Guided reading: According to some problems in the development, production and construction application of thermosolysing powder coating, the current domestic powder coating production control and optimization formula screening, intervention in basic theoretical research and practical application, at the same time in the application of construction technology control on the quality of construction and the way to solve the discussion.
since the 1980s, powder coatings in China research and development of production applications, 30 years from production to quality has made a great leap.
Because of its resource savings, energy, low pollution and improve labor productivity, the annual growth rate of 10%, varieties from the early pure epoxy anti-corrosion type to epoxy / polyester, polyester, acrylic and other decorative advanced series .
But the overall decorative performance is not comparable to high-grade solvent-based coatings, coupled with the domestic manufacturers of different technical conditions, the production of powder coatings in the construction application of some manufacturers' products often appear such as poor leveling, low edge coverage, powder easy clumping, paint film shrink holes and other ills.
years of practical experience in production, the author put forward how to fundamentally reduce the above-mentioned ills, and strive to improve the decorative paint film is the main problem discussed in this paper.
1 The impact of line regulation on quality
1.1 Raw Material Mixing
Mixing raw materials in high-speed mixers is critical for color palettes, often with the same formulation and large color differences in small trials and production.
powder coating color after the production of non-conforming color products treatment often cause headaches for producers. One way to solve this problem is not to change pigment manufacturers and models often, and more importantly, to manipulate the production of raw materials mixing time and mixing machine speed once adjusted do not change easily.
experienced producers know that even a 10 s difference in mixing time can vary in color, with light paint having a greater color difference and dark paint having a smaller effect.
When it is found that there are more material wall in the inner wall of the mixer, it should be timely check the degree of moisture absorption of pigments, that is, whether the water content is qualified and whether the softening point of the resin used is qualified, the wall seriously has a great impact on the accuracy of the color adjustment, formulation uniformity.
1.2 Extruder interval temperature
Extruder adjusts the temperature of the heating melting zone is very important to the degree of material dispersion and the degree of resin moisture to pigment fillers, the temperature is too low then the material moisture dispersion is not good.
material wetting and dispersing is not good can be done by checking the powder coating flakes, when the paint sheet cross-section observed unscathed powder particles, can be determined as not dispersed well, it is one of the main reasons for the powder coating easy to produce shrink holes.
And the temperature regulation is too high, the external force on the paint shear force is too low, the same cause dispersion moisture is not good, because the shearing is too low powder coating in the high temperature extruder running time is too long caused by local material gel and eventually produce non-melting gum particles, this serious impact on the appearance of the coating film is difficult to remove the rubber particles are the last thing producers want to see.
extruder temperature is too high, mainly to check whether the cross-section of the pressure chip has bubbles, there are bubbles indicate that the temperature regulation is too high, if the extruder outlet material presents a flowing water is super-high temperature control.
different softening point resin on the temperature regulation will change, generally the first area 90 to 95 degrees C, two areas 125 to 130 degrees C, can be used for a variety of powder coatings, adjust the extruder control cabinet table speed of 1100 to 1200 rpm, at this time the best shear force, resin to the paint filler the best moisture.
1.3 ACM air grade grinding adjustment
whether the particle size distribution of powder coating is reasonably related to the surface performance of the paint film and construction application performance, 160 to 180 coarse powder accounted for too much paint film surface flatness is not good, more than 300 fine powder accounted for too much of the edge coverage is low, and spray construction paint film easy to produce needle holes, powder products easy to clump even can not be used.
some powder manufacturers do not pay much attention to the particle size distribution, resulting in repeated product quality problems. The principle of good particle size distribution should take into account the advantages of coarse powder moth charge and good air flow flow, and the low clearance rate of medium fine powder, which is advantageous to the characteristics of producing flat paint film.
Figure 1 lists 3 different particle size distribution powder coatings, curve (1) 160 to 180 items accounted for 10%, 180 to 250 items accounted for 60%, more than 250 items accounted for 30%, in practice (1) powder powder rate is high, the edge of the workware cover is ideal, paint film flatness is good and not easy to block the spray gun.
particle size distribution is regulated by ACM grinding, and different types of powder coatings differ slightly in different regulations due to resin softening points.
To epoxy / polyester powder as an example requires the door to be fully open, half-light powder coating speed 25, high-gloss powder coating speed 24, pure polyester high-gloss powder coating speed 23, epoxy fine pattern powder coating speed 26.
technical personnel should pass the laboratory graded sieve to the product particle size distribution test, in order to observe the actual construction application effect to determine a variety of ACM air grading grinding adjustment of the best way.
Figure 1 3 different particle size distribution powder coatings
2 raw material selection and ratio
2.1 resin
resin structure and performance is the main factor determining the performance of powder coatings and lacquer film, should have a suitable melting temperature, good charge, good mechanical powder coating film, but the application manufacturers do not have these project detection means, can only be determined by the actual application of a brand.
Because the resin molecular weight distribution is wide and the curing agent cross-linking curing process stress is different and produces shrink holes, additives only for the problem is not serious when one of the solutions, powder coating storage period easy clump failure is often the resin in the low molecular weight part due to too low softening point caused.
molecular weight distribution is generally not related to the resin softening point, so the choice of resin can only be applied according to the actual effect, softening point indicators are only important reference data.
2.2 Curing agent
curing agent selection range, both mutual curing agent resin type such as epoxy / polyester, acrylic / polyester, but also pure curing agent type and resin cross-cured amines, closed isocyanate, isocyanate triglycerides and so on.
the most important thing in screening curing agent is to design its gel time through the formula, powder coating gel time too fast will produce poor leveling, poor adhesion and other problems.
flow is very easy to understand, and the reason for poor adhesion is usually sprayed thick metal or large area of metal parts due to the large thermal capacity of the work piece, to make the surface of the coated work parts to reach the powder coating required melting temperature time is longer, if the production set the glue time is not enough, then the coating is not fully melting level and close contact with the metal surface.
Some sprayed into the film after the work piece slightly more force, some parts will become small pieces of shedding phenomenon, this situation in addition to the local contamination of the work piece caused by, another reason is the short gel time, powder coating corresponding to high-heat capacity work parts easy to produce the phenomenon, it has little to do with the powder coating adhesion performance.
Of course, the design glue time is too long will also produce low edge coverage and work piece corner flow ills, this paper believes that the best glue time should be set between 200 to 250 s /180 degrees C.
2.3 additives
additives have a certain effect on repairing powder paint maladies, such as polyethylene alcohol butyl to improve the paint film impact resistance, leveling to improve the edge coverage of work parts have a better effect, suitable for the selection of viscosity 35 to 40 s specifications of products.
addition of a small amount of gas-phase silica has certain effects on improving edge coverage, reducing paint film gloss and preventing product storage clumps, but not greater than 0. 5%, otherwise the powder coating is easy to produce slip difficult material in the extruder, and the powder application will produce static sparks to make the paint film produce pinholes.
a wider selection of liquid leveling agents, liquid leveling agents because of its easy to make product grouping is now less used, is now mostly using solid acrylic products. The application of sabbath-like hydropneous materials can also eliminate to a certain extent the pinholes that appear on the paint film due to the moisture in the raw material.
2.4 pigment
pigment type and proportion of the formula will directly affect the performance of the paint film, pigment should have a high temperature baking color stability, do not produce flowers, lighting and other ills.
So pigments should be good heat resistance, the selected pigments and resins have good wet dispersion, wet dispersion of good or bad to paint film gloss brought a significant impact, which depends on additives have little effect.
pigment particle size has a great influence on the surface performance of the paint film. In the process of powder coating curing process, resin, curing agent, additive melt flow, but not melt flow of pigment is the main body of the final formation of paint film skeleton, it can be seen that the geometry and particle size of pigment is one of the paint film surface performance factors.
in terms of particle size, the reasonable mating of different particle size pigments can effectively reduce the gap in the paint film during melting, which is conducive to the formation of smooth paint film surface.
according to the principle of pigment filler, the production practice is summarized that the coarse pigment filler is matched in a reasonable proportion with the minimum powder volume clearance rate. According to this principle in the production practice also summarized in the appropriate range in order to ensure the surface performance of the paint film filler maximum addition ratio. Figure
2 shows that the proportion of pigments in the formulation from 35% to 45% gloss changes little, the principle is that the pigment in addition to the amount of oil absorption and its chemical properties, geometry on the paint film, the gap between its particles is quite important.
powder coating in the melting flow process due to the lack of clearance rate of pigment filler skeleton formation, so that part of the melted resin to the surface of the paint film and form the effect expressed above, this principle to ensure the quality of the premise of reducing costs is very effective.
the theory of pigment stacking we can also make an in-depth understanding of the principles of geometric and physical properties such as particle shape, crystalline state, density, relative density, surface area, etc.
Figure 2 The gloss effect of different pigment ratios
3 construction applications
3.1 powder rate
powder coating construction applications generally use electrostectrectecturic spraying, the principle is that the metal work and gun electrodes between the application of 30 to 90 kV high-voltage current, with compressed air powder coating from the spray gun to space.
particles are electrocuted by the electrode halo discharge of the spray gun to form an electric field and the ionization of the particles collides to form charged powder particles adsorbed on the surface of the grounded metal work piece.
the powder rate is mainly spray equipment debugging, powder coating charge, powder coating particle size distribution, coating ambient temperature and humidity, etc.
On the powder rate is fatal to the application manufacturers, because in general, recycling powder has the environment to bring impurities and these impurities are difficult to remove through the screen, while the recycling powder particle size distribution secondary differentiation is too thick and too fine powder accounted for an important proportion, these powder powder rate is very low, so recycling powder manufacturers are generally waste treatment.
General normal powder coating application rate of 85% to 95%, the solution to the low powder rate is mainly powder paint manufacturers to control the optimal particle size distribution of products, as well as improve the charge of powder coatings such as the addition of conductive aids or a small amount of gas phase silica.
other application manufacturers can improve the voltage and reduce the powder pressure in the construction will receive a significant increase in the effect of powder rate. Corresponding to the powder rate is the thickness uniformity of the coating film, especially pay attention to the uniformity of the walking gun for large flat work pieces.
Wet weather and low temperature of the spring in particular to pay attention to, generally speaking, the coating film is too thick easy to produce pinholes and orange peel disease, too thin part of the color uneven and easy to dew bottom, especially light gray, light red and other transition colors are most likely to see color unevenness, appropriately improve the voltage reduction of low pressure can be more ideal effect.
of course, it is also important to adjust the spray gun to the optimal powder atomization state, the above adjustment can also solve the problem of poor edge coverage of the work piece.
3.2 Baking line temperature regulation
powder coating applied to the work piece after heating baking, powder began to melt and the air between the powder particles discharged, melted powder gradually flattened and gradually lost leveling, curing into a film, the most important time area is the gel time, that is, the coating began to enter the curing crosslinking phase of the start time.
From Figure 3, it can be seen that the coating has a melting leveling time zone before the gel point, which, if reasonably adjusted for the temperature of the first baking area (by 5 min walking time meter per zone) at the time of production, will play a decisive role in the appearance of the final product.
figure 3 epoxy/polyester powder coating application as an example (curing conditions 180 degrees C /20 min), one zone to adjust 175 degrees C, the other three regions 185 degrees C;
because the second end of the production line and the outside connection actual temperature will be lower, so the curing conditions should actually be 150 to 170 degrees C / 10 min 185 degrees C 10 min;
can fully meet the curing conditions in the case of powder in the first zone of 150 to 170 degrees C can have 5 min adhesive time, so that it is fully leveled, if the first zone is set at 180 degrees C then the gelting time is only 3 to 4 min is not conducive to the paint film full leveling.
epoxy/polyester powder coating melting temperature of 110 to 120 degrees C, 180 degrees C gluing time of 4 min, 150 to 170 degrees C up to 5 min, production through temperature control adjustment to extend adhesion time, so that the paint film full leveling not only to the paint film to bring flat and plump effect
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Figure 3 epoxy/ polyester powder curing curve
4 Outlook
the current domestic powder coating to pure epoxy, epoxy/ polyester, pure polyester-based, through the development of China's powder coating (refers to the thermo-cured cross-link powder coating) is still lagging behind the advanced level of foreign countries.
fluorine resin powder coatings with an outdoor service life of up to 20 to 30 years have been introduced abroad. From China's powder coatings, the development direction should first of all vigorously develop excellent weather resistance acrylic powder, while focusing on the development of fluorine resin powder coatings, the second focus on the development of low-temperature curing powder coatings.
with China's increasing attention to environmental protection and the introduction of various national environmental regulations, will become China's powder coating research and production applications to catch up with the world's advanced level of power.
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