Study on easy-dispersible aluminum powder pigments in powder coatings

Abstract: Aiming at the disadvantages of poor dispersion effect, poor voltage stability, and easy blocking of guns when aluminum powder pigments are added in powder coatings with high amounts, the key technologies for the industrial production of easily dispersible aluminum powder pigments have been studied

Keywords: easily dispersible aluminum powder pigment; active resin; surface treatment; dispersibility

0 Preface

With the increasing pressure on environmental protection, the coating process is facing new challenges, and the demand for aluminum powder pigments such as powder coatings, water-based paints, high-solid pigments, and spray-free pigments is increasing

Powder coating has the advantages of no VOC emissions, 100% film formation, thicker paint film, excellent film performance, low energy consumption, high production efficiency and automatic coating, etc.

In powder coatings, resins and pigments are important components of powder coatings.

In order to solve the above problems, this study improved the dispersion performance of the pigment by coating the surface of the aluminum powder pigment with a layer of active resin to reduce the chargeability of the pigment and the base resin [3]

1 Experimental part

1.

Polyurethane Reactive Resin (PU): Guangzhou Jiehe Polymer Material Co.

1.

After the dense silicon-coated aluminum powder pigment is uniformly dispersed in an organic solvent, active resin is added, and under certain conditions, the surface of the aluminum powder pigment is coated with a layer of resin

In this way, the surface characteristics of aluminum pigments are improved, so that the dense silica-coated aluminum powder pigments are more compatible with the base resin, and the difference in electrical conductivity between the two is shortened, so that it can be electrostatically sprayed even at high dosages.

1.

1.

Add the dense silicon-coated aluminum powder pigment and the solvent into the coating tank at a mass ratio of 1:8, stir at 80r/min, and after stirring evenly, control the temperature to 30-40℃; slowly add the same quality as the dense silicon-coated aluminum powder pigment After the active resin is evenly stirred, continue to stir at the speed and temperature.

1.

Put the blown-dried filter cake into a vacuum drying oven, adjust the baking temperature to 70~90℃, and dry it into powder
.

1.
3.
3 Decentralized processing

In this study, a mechanical dispersion process was used.
The dried pigment was added to the dispersing machine, and the product was dispersed for 8 hours before use
.

1.
3.
4 Screening

Add the dispersed powder product to the sieving machine and screen with a 150-mesh sieve.
The product under the sieve is the easily dispersible aluminum powder pigment ZPBF1030S
.

1.
4 Analysis and testing

Routine performance testing: Perform product-related testing according to HG/T2456.
2-2016
.

Particle size detection: Take a small amount (0.
5~1.
0g) of sample and place it in a beaker, add alcohol solvent, and stir evenly
.
After dispersing in an ultrasonic dispersing machine for 3 to 5 minutes, the particle size detection is carried out with an Omega laser particle size analyzer
.

Electrostatic spraying test: Take a certain amount of aluminum powder pigment, and then add resin base powder to 100g, shake it well, and perform electrostatic spraying
.
The air pressure of the electrostatic spray gun is 1.
0MPa, the electrostatic voltage is 80kV, the distance between the muzzle and the iron plate is 15cm, and the electrostatic spraying is on the tinplate (20cm×10cm), and then placed in an oven at 200°C for 15min and then taken out (coating The thickness is 60~80μm), the paint film is visually inspected and evaluated [6]
.

Gloss test: According to the standard ISO2813-2014, the paint film is tested for gloss
.
Acid and alkali resistance test: test according to the spot method in GB/T9274-1988 "Determination of Paint and Varnish Resistance to Liquid Medium"
.

2 Closing and discussion

2.
1 Selection of resin types

The type of resin coated on the surface of aluminum powder pigments will determine the application of easily dispersible aluminum powder pigments in powder coatings [7]
.
Under the same conditions, different types of active resins were used for surface treatment, and then the dried state and the spray plate after the finished product were compared.
The results are shown in Table 1
.

It can be seen from Table 1 that in the experiment, the aluminum powder pigment was coated with epoxy active resin, polyurethane active resin, acrylic active resin, and mixed active resin
.
After drying the solvent, it is found that the pigment coated with acrylic active resin is in a lump, which is difficult to powder; the pigment coated with epoxy active resin is also a block, but can be crushed into small pieces; polyurethane active resin and mixed active resin The coated pigment is loose and powdery
.
In terms of the performance of the paint film after spraying, the powdering state, flatness, clarity, weather resistance and compatibility of the mixed active resin are better than other resins.
Therefore, the mixed active resin is selected as the coating resin in this study
.

2.
2 The amount of resin

Under the same conditions, the amount of mixed active resin is changed, and after the finished product is made, its gloss is tested
.
The result is shown in Figure 2
.

It can be seen from Figure 2 that as the amount of resin coating increases, the gloss continues to increase, but considering that the resin coating amount exceeds 50%, the pigment content in the product will decrease.
In addition, the paint film with the same coating amount As a result, more products need to be added, and the manufacturing cost will increase a lot
.
Therefore, the resin coating amount is selected to be 50%, that is, the mass ratio of the dense silicon-coated aluminum powder is 1:1
.

2.
3 Response time

If the reaction time is too short, the resin will not be fully coated, and the easily dispersible aluminum powder pigment will have poor performance and relatively small particle size
.
Under the same conditions, different reaction times were changed, and after pressure filtration, the reaction time was determined by detecting the non-volatile content in the solvent after pressure filtration, product yield and powder particle size.
The results are shown in Table 2
.

It is found from Table 2 that after 4 hours of reaction, the non-volatile content in the solvent after pressure filtration is slowly decreasing
.
The increase in the powder yield and powder particle size after drying is not large, so the mixed resin reacts relatively completely when the mixed resin is reacted for 4 hours
.
Therefore 4h is the best reaction time
.

2.
4 Comparison before and after aluminum powder pigment coating

2.
4.
1 The color contrast of the spray plate

Take 2.
5g, 5g, 10g, 15g of ZPS1030 to make powder coating spray panels, respectively take 5g, 10g, 20g, 30g of ZPBF1030S to make powder coating spray panels, visually inspect the paint film, the results are shown in Figure 3 and Figure 4 ( Because the powder coating made of 15g of ZPS1030 is very serious, there is no spray plate)
.

It can be seen from Figure 3 that with the increase in the amount of ZPS1030, the hue change is not obvious, and with the increase in the amount of ZPBF1030S, the hiding power becomes better and the hue becomes whiter and whiter
.

It can be seen from Figure 4 that when the aluminum content is 2.
5%, the particle size of ZPBF1030S increases after the resin is coated, so the hiding power of ZPBF1030S is worse than that of ZPS1030, but as the aluminum content increases With continuous increase, the hiding power of ZPBF1030S is more and more obviously better than that of ZPS1030
.

2.
4.
2 Dispersion comparison

Record the phenomena in the electrostatic spraying process and visually compare the sample, and take any point on the same board as the standard, and make the color difference on the different points of the sample.
The measurement is performed 5 times, and the largest color difference is the recorded value
.
The detailed results are shown in Table 3
.

It can be seen from Table 3 that when the addition amount of the untreated raw materials is 5%, the aluminum powder pigment starts to have a gun.
As the amount of addition continues to increase, the gun polymerization phenomenon becomes more and more serious.
When it reaches 15%, it even appears.
The phenomenon of blocking the gun
.
With the addition of 30% of the easy-dispersible aluminum powder pigment, the pigment can also be sprayed stably by dry mixing, without blocking the muzzle, with small color difference, uniform hue and no yin and yang colors, so the pigment is dispersible in the paint film Good
.

2.
4.
3 Electrostatic spraying under different voltages

ZPS1030 and ZPBF1030S were prepared into 3 parts of 100g powder coating according to the added amount, and electrostatic spraying was carried out at different voltages (40KV, 60KV, 80KV) to prepare samples, and to detect the color difference with 40KV spray panels.
The results are shown in Table 4
.

It can be seen from Table 4 that under different voltages, ZPBF1030S has small chromatic aberrations in the samples prepared by electrostatic spraying, while ZPS1030 samples have larger chromatic aberrations, so ZPBF1030S has better voltage stability
.

2.
4.
4 Chemical resistance test

ZPS1030 and ZPBF1030S were mixed evenly with the base powder by bonding and dry blending methods (the aluminum content is guaranteed to be 5%), made into samples, and tested for acid and alkali resistance.
The results are shown in Table 5
.

It can be seen from Table 5 that the acid resistance of aluminum powder pigments has not changed significantly.
When the alkali-resistant aluminum powder pigments are dry mixed, the coated aluminum powder pigments are obviously improved; while the aluminum powder pigments have some alkali resistance after bonding.
Down
.
After bonding, the easy-dispersible aluminum powder has higher alkali resistance than the dense silica-coated aluminum powder pigment
.
The above shows that ZPS1030 is coated with a thick and dense silica protective layer on the surface of the aluminum sheet, which has excellent chemical resistance; while the easy-dispersible aluminum powder pigment ZPBF1030S is coated with a layer of active resin on the surface of the aluminum powder pigment.
, The chemical resistance is slightly improved; compared with the bonding process, the mechanical shearing damages the coating layer of the pigment surface during the bonding process of the pigment, which causes the chemical resistance of the paint film to decrease; but after the easy dispersion treatment , The paint film prepared by the easy-dispersible aluminum powder pigment with bonding is also slightly better than the paint film prepared by directly bonding the powder, indicating that a layer of resin can improve the shear resistance of the pigment
.

3 Conclusion

Through the type of coating resin, the amount of coating resin, and the reaction time, it is determined that the mixed active resin is used as the coating resin and the mass ratio of the dense silica-coated aluminum powder pigment is 1:1.
After 4 hours of reaction , The prepared easily dispersible aluminum powder pigment has good voltage resistance and acid and alkali resistance, and still has good dispersibility in the case of high addition
.
 

references

[1] Nan Renzhi.
Powder Coating and Coating Technology [M].
3rd edition.
Beijing: Chemical Industry Press, 2014.

[2] Chen Ying.
Double-layer coating of aluminum pigment and its corrosion resistance and mechanical properties [D].
Hefei: Hefei University of Technology, 2019.

[3] Gao Aihuan, Pi Pihui, Lu Yang, et al.
Double coating modification of aluminum pigment with polyacrylate and SiO2[J].
Journal of South China University of Technology (Natural Science Edition), 2010, 38(2): 1- 5.

[4] Zhou Weiming, Qu Chunhai, Wu Deqing.
Synthesis and application of polyester resin for outdoor hydroxyalkylamide-cured dry-blended matting powder coatings[J].
Coatings Industry, 2018, 28(10); 25-29.

[5] Liu Zhifang, Liu Xinhong, Huang Yalei, et al.
Research progress in surface coating modification of aluminum powder [J].
Materials Guide A (review), 2017, 31 (6): 73-79, 85.

[6] An Qinglei.
Study on formulation optimization and performance of epoxy resin-based powder coatings [D].
Jinan: Shandong University, 2012.

[7] MA LP, ZHOU SS,DU A, et al.
Progressof surface modilfication of alminum pigments [J].
Applied Mechanics andMaterials,2015,731:528-534.