Research on the application of silicon micro-powder in powder coatings

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75em;"> KCJ-808 electrostatic spray gun (Zhejiang Yongkang Heima Electrostatic Coating Equipment Co.

, Ltd.
), tinplate with holes (120mm×50mm×0.
28mm, Ark paint instrument), DHG-9140 electric heating constant temperature blast drying oven (Shanghai Yiheng Science Instrument Co.
, Ltd.
), GT8202 coating thickness gauge (Guangzhou Guoou Electronic Technology Co.
, Ltd.
), PPH-1 pencil hardness tester, QCJ paint film impactor, WGG-60 gloss meter (Shanghai Modern Environmental Engineering Technology Co.
, Ltd.
) , WSB-3A intelligent digital whiteness meter (Hangzhou Dacheng Photoelectric Instrument Co.
, Ltd.
), STA-PTl000 thermal weight loss analyzer (German Linseis company), S-3400N scanning electron microscope (Japan Hitachi company).

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2 Preparation of powder coating

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75em;">The powder coating is prepared according to the coating formula in Table 1.

The process flow is: batching, pre-mixing, extrusion, tableting, crushing, and sieving.

75em;">The sample numbers are ST01, ST11, ST21, ST31, ST10.

ST01 means that the weight percentage (wt%) of silicon micropowder and titanium dioxide is 0:12, and ST11 means that the weight percentage (wt%) of silicon micropowder and titanium dioxide is 6 :6, and so on for samples ST21, ST31, ST10.

75em;">Then spray with an electrostatic spray gun and cure at a high temperature to obtain a coating film with a thickness of 75-90μm, and test performance.

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3 Performance testing and characterization

75em;">(1) Pencil hardness: Measure the hardness of the paint film (9B is the softest, 9H is the hardest) according to GB/T6739-1996 "Pencil Determination of Film Hardness".

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75em;">(2) Gloss: According to GB/T9754-2007 "Determination of 20°, 60° and 85° Mirror Gloss of Paint Films of Paint and Varnish without Metallic Pigments", measured with a 60° gloss meter.

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75em;">(3) Whiteness: According to GB/T5950-2008 "Measurement method for whiteness of building materials and non-metallic mineral products", it is measured by an intelligent digital whiteness meter.

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75em;">(4) Impact resistance: Measure the impact resistance according to GB/T1732-1993 "Test Method for Impact Resistance of Paint Films" (weight free fall at room temperature).

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75em;">(5) Thermal Weight Loss Analysis (TGA): Tested with a thermal weight loss analyzer (TGA), the temperature range is 50 to 800°C, and the temperature rise rate is 10°C/min.

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75em;">(6) Scanning Electron Microscope (SEM): The powder coating paint film is prepared and observed with Scanning Electron Microscope (SEM) after spraying.

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75em;">3 Results and discussion   

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1 Film forming process of powder coating

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75em;">Process conditions such as curing temperature, spraying voltage, and reaction time will have a certain impact on the performance of powder coatings.

Figure 1 shows the powder loading rate under different voltages, and Figure 2 shows the whiteness and gloss of the ST01 sample paint film under different processes.

75em;">The spraying voltage is different, the quality of the powder coating adsorbed on the tinplate sheet is different, that is, the powdering rate.

It can be seen from Figure 1 that when the voltage is between 40~50kV, the powder adsorption capacity increases rapidly.
When the voltage is 60~80kV, the adsorption capacity basically tends to be flat, but after 80kV, it drops.

75em;">During the spraying process, the main factors affecting the electric field force are the electric field strength, the size and shape of the powder particles.
Therefore, as the voltage increases, the electric field force received by the powder increases, and the powder loading rate also increases.

75em;">When the spraying voltage is too high, the charge that cannot be discharged increases the charge accumulation on the surface of the coating, leading to the rapid occurrence of reverse ionization and significantly reducing the powder loading rate.

75em;">Moreover, larger powder particles usually carry a stronger charge, which makes the attraction between the particle and its mirror image larger.
As a result, more large particles of powder deposited on the surface of the existing coating appear uneven accumulation.

75em;">The quality of the powder attached to the tinplate sheet decreased slightly for the comparative sample ST01 and other samples at 60kV, 70kV, and 80kV spraying voltages.

75em;">However, when the adhesion amount of the powder with added silicon powder is at a voltage of 50-90 kV, the powder rate of the powder coating without silicon powder changes slowly.

75em;">For samples ST01, ST11, ST21, ST31, and ST10, with the increase of the spraying voltage, the change trend of the powder loading rate of the paint is similar, that is, it increases first and then decreases.
Therefore, it is more appropriate to set the spraying voltage to 70kV.

75em;">It can be seen from Figure 2a that when the spraying voltage is selected as 70kV and the curing temperature is 150 ℃, the whiteness of the coating film for 5min, 10min, 15min, 20min, and 30min is 70.
5, 73.
5, 79.
0, 82.
4, 83.
5, respectively.

75em;">As the curing temperature rises from 150°C to 190°C, the whiteness curve of the film forming time of 5min, 10min, and 15min increases; the whiteness curve of the film forming time of 20min, 30min first increases and then decreases.
The whiteness of the coating film with a curing temperature of 180°C and a film-forming time of 20 minutes is 88.
5 at the highest.

75em;">It can be seen from Figure 2b that when the spraying voltage is selected as 70kV and the curing temperature is 150 ℃, the film gloss of 5min, 10min, 15min, 20min, and 30min is 14.
1, 18.
7, 60.
5, 70.
0, 72.
0, respectively.

75em;">As the curing temperature rises from 150°C to 190°C, the gloss curve of the film forming time of 5min, 10min, 15min shows an increase, and the gloss value of the coating film is low and matt; the film forming time is 20min, 30min The gloss curve appears to increase first and then decrease.

75em;">The gloss of the coating film with a curing temperature of 180°C and a film-forming time of 20 minutes is 78 at the highest.
The curing temperature is too low, the film forming time is too short, the powder coating is partially cured, the leveling effect is not good, and the surface of the coating film is uneven, which causes low whiteness and gloss.

75em;">Since the powder coating components did not have enough time and temperature to undergo a complete cross-linking reaction, some components did not reach the melting point to fuse.
When the curing temperature is too high, the film forming time is too long, and the whiteness of the coating film decreases.

75em;">This is due to the aging of the coating film after curing, which indirectly causes the whiteness of the coating film to decrease.
It can be seen from Figure 2 that the surface of the coating film is rough when the temperature is relatively low and the film forming time is short, indicating that the powder coating has not fully cured and crosslinked at a lower temperature, and there are certain gaps, resulting in the surface of the coating film.
The inhomogeneity.

75em;">In addition, during the film formation process, some additives are decomposed into small molecules, which will generate pores and make the surface of the coating film uneven.
When the temperature is higher and the film forming time is longer, the whiteness of the coating film will decrease and the performance of the coating film will decrease.
Therefore, the film-forming conditions with a curing temperature of 180°C and a reaction time of 20 minutes are better.

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2 Research on the performance of composite powders on powder coatings

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75em;">In order to determine the better replacement amount of silicon powder, 5 different ratios of silicon powder and titanium dioxide composite powder (0:1, 1:1, 2:1, 3:1, 1:0) were added to the powder coating In, the performance of the corresponding powder coating film was tested.

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2.
1 Hardness of Paint Film Pencil

75em;">Pencil hardness directly affects the application of powder coatings.
Table 2 shows the pencil hardness values ​​of different sample paint films.

75em;">It can be seen from Table 2 that the hardness of the coating film increases with the increase of the silicon powder content in the composite powder.
The hardness of the coating is related to the main film-forming materials, pigments and fillers.
When the main film-forming materials are constant, the pigments and fillers play a dominant role in the hardness of the coating film.

75em;">Silicon micropowder has high hardness, which helps to improve the hardness of the coating film when the powder coating system is cured to form a film.
When all the titanium dioxide is replaced by silicon micropowder, the pencil hardness of the coating film is greatly improved from 3H of the sample ST01 to 5H of ST10.
The abrasion resistance of the coating film.

75em;">During the curing of the powder coating into a film, the fine silicon powder particles migrate to the surface of the powder coating film under the action of the auxiliary agent, and cure at the same time.
Then one end of the particle is on the surface, and the other end is compatible with the film-forming material resin and pigment.
membrane.

75em;">When a high-hardness pencil scratches the coating film, the more flat silicon powder on the surface of the coating film can resist the pencil's scratches more effectively.

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2.
2 Impact resistance of coating film

75em;">The impact resistance results of different samples are shown in Table 3.

75em;">It can be seen from Table 3 that as the proportion of silicon micropowder in the composite powder increases, the impact resistance of the powder coating film shows a trend of rising first and then falling.

75em;">When the ratio of silicon micropowder and titanium dioxide is 1:1, the composite powder formed by silicon micropowder and titanium dioxide is more fully compatible in the powder coating system and produces a synergistic effect.

75em;">This is because silicon powder has relatively large surface activity, and there are many opportunities to physically or chemically bond with polymer chains, and the contact area with the matrix increases.
The force of silicon powder with epoxy groups at the interface is much greater than the van der Waals force.
, The formation of an ideal interface is conducive to the stress transfer between the silicon powder and the epoxy resin, and the ability to bear the load is improved.

75em;">When the composite material is subjected to an external impact, the epoxy resin will transfer the impact force to the silicon powder, so that the silicon powder can absorb more energy, so that the composite material can withstand greater impact, then the impact resistance of the sample ST11 powder coating The force is increased to more than 50kg·cm.

75em;">When the content of silicon micro-powder gradually increases, and the ratio with titanium dioxide is higher than 1:1, the strength of the composite material is reduced, thereby reducing the impact strength.

75em;">Although silicon powder cannot completely replace titanium dioxide, it can produce a synergistic effect, increase the effective utilization rate of titanium dioxide, reduce the amount of titanium dioxide, and correspondingly improve the impact strength of powder coatings.

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2.
3 Film gloss

75em;">The gloss depends on the specular reflection ability of the surface of the object, and has nothing to do with the color.
Table 4 shows the gloss of different samples.

75em;">It can be seen from Table 4 that as the content of silicon powder in the composite powder increases, the gloss of the coating film gradually decreases.
Because the reflective ability of silicon powder in the powder coating system is lower than that of titanium dioxide in the powder coating system.

75em;">Silicon powder is used as a filler.
After the curing reaction of the powder coating system, more silicon powder particles appear on the surface of the coating to form a film.
Because the particle size of the silicon powder is relatively coarse, and the weather resistance of the silicon powder is good, the distribution in the powder coating reduces the flatness of the coating film.

75em;">At the same time, the silica powder has an auxiliary matting effect on the powder coating, thereby reducing the gloss of the coating film.

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2.
4 Whiteness of coating film

75em;">Whiteness indicates the degree of whiteness on the surface of a substance, expressed as a percentage of the white content.
Table 5 shows the whiteness of the paint film of different samples, and Figure 3 shows the actual pictures of different samples.

75em;">It can be seen from Table 5 that the whiteness of the powder coating film first slowly decreases with the increase of the silicon powder content, and then greatly decreases.
Corresponding to the actual figure 3 can also see the reduction of whiteness, especially the sample ST10 coating film has appeared gray.

75em;">As a white pigment, titanium dioxide acts as a fluorescent whitening agent.
Since the reflectance of titanium dioxide is higher than that of silicon powder, and the whiteness of silicon powder is lower than that of titanium dioxide, as the content of silicon powder increases, the whiteness of the coating film decreases.

75em;">Although silicon powder cannot completely replace titanium white, it can produce a synergistic effect, increase the effective utilization rate of titanium white, and reduce the amount of titanium white.

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3 TGA of powder coatings

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75em;">Figure 4 shows the thermal weight loss curves of different sample paint films.
The temperature at the point of maximum weight loss rate is represented by Tp; the temperature at 5% weight loss is represented by T5.

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75em;">Combining Figure 4 and Table 6, it can be seen that with the increase of the proportion of silicon powder in the composite powder, the heat resistance of the paint film has been improved, with T5 rising from 372.
23°C to 402.
46°C, and Tp from 433.
19°C to 440.
76°C.

75em;">This is related to the degradation of the polymer matrix, initially the water produced by the elimination of unsaturated carbon-carbon bonds.
The carbon-oxygen bond at the β-position shows thermal brittleness in epoxy and polyester resin systems, and tends to destroy the cross-linked structure produced by molecular fragments during the heating process.

75em;">As the proportion of silicon micropowder in the powder coating increases, the maximum decomposition rate increases.
At the same time, it can be seen from the retained mass fraction that the degree of cross-linking reaction of each component first decreases and then increases.

75em;">Due to the increased tortuosity of the composite powder particles dispersed in the polymer matrix with a high aspect ratio, the molecules in the gas or liquid are prevented from diffusing into the material.

75em;">Moreover, the strong interaction between silicon powder and epoxy and polyester resins forms a polymer network and restricts the mobility of molecules.
Therefore, the addition of silicon powder improves the thermal stability of the powder coating.

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4 SEM of powder coating

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75em;">The morphology of the cross-section of the coating film formed by some types of powder coatings is shown in Figure 5 after magnification of 10000 times.

75em;">It can be seen from Figure 5 that under different film forming conditions, the morphology of the coating film is different.
Under the film forming conditions of sample ST01 at 160°C for 20 minutes, some of the powder particles were not completely melted, and the titanium dioxide powder was unevenly distributed in the coating film (Figure 5a);

75em;">Samples ST11 and ST10 under the film forming conditions of 180℃ and 20min (Figure 5b, c), the composite powder distribution is relatively uniform, the melting and solidification is relatively complete, the resin crosslinking is relatively complete, and the overall cross-section is relatively level, but there is still Part of the pores may be small pores formed by air entering.

75em;">In general, the cross-sectional morphology of the powder coating film obtained at the film-forming temperature of 180°C and the curing time of 20 minutes is better.

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75em;">4 Conclusion  

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75em;">(1) Optimal process conditions for silicon-rich powder coatings: spraying voltage is 70kV, curing temperature is 180°C, curing time is 20min;

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75em;">(2) For powder coatings containing silicon powder, the whiteness and gloss of the paint film have decreased, but the hardness of the paint film has increased.
When the ratio of silicon powder to titanium dioxide is 1:1, the performance of the powder coating film is better;

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75em;">(3) For powder coatings containing silicon micropowder, both T5 and Tp have been improved by 30°C and 7°C respectively.