Research on Influencing Factors of Weatherability of Polyester Modified Fluorocarbon Powder

Abstract: In order to obtain a polyester modified fluorocarbon powder coating with excellent weather resistance and cost-effectiveness, this paper uses high-voltage electrostatic spraying to prepare the coating film, and tests the particle size, surface tension of the coating film, powder tap density, and Different types of flow aids are used to explore their effects on the weather resistance of polyester modified fluorocarbon powders
.

Keywords: fluorocarbon powder coating; particle size; surface tension; flow aid; weather resistance
.

Preface

Fluorocarbon powder coatings are divided into thermosetting type and thermoplastic type.
The polyester modified fluorocarbon powder prepared in this article is thermosetting powder coating coating online coatingol.
com
.
Unlike ordinary solvent-based coatings and water-based coatings, the dispersion medium of thermosetting powder coatings is not solvent and water, but air
.
It has the characteristics of no solvent pollution, 100% film formation and low energy consumption
.
With the development of powder coatings and people's understanding of powder coatings, there are more and more powder coatings with different functions, and weather resistance has always been the main concern of powder coatings
.
The weather resistance of fluorocarbon powder is the best among all powder coatings, because the chemical bond CF bond energy in the fluororesin molecule is very large, and its molecular bond is very stable, which can effectively resist UV damage to it [1]
.

Pure fluorocarbon powder coatings are popular because of their own advantages in weather resistance, but pure fluorocarbon powder coatings have poor mechanical properties and are expensive, and it is difficult to promote them in the market.
Modified fluorocarbon powders are used in this The situation arises at the historic moment
.
Modified fluorocarbon powder can not only meet customers' needs for weather resistance, but also reduce the cost of manufacturers.
It is currently an ideal super weather resistant powder coating
.
Modified fluorocarbon powder generally includes polyester modified fluorocarbon powder, polyurethane modified fluorocarbon powder, and acrylic modified fluorocarbon powder
.
This article selects polyester modified fluorocarbon powder for testing.
The polyester modified fluorocarbon powder is made by uniformly mixing polyester powder and pure fluorocarbon powder through a certain mass ratio
.
This article explores the effects of the two components of the polyester modified fluorocarbon powder on the weather resistance of the polyester modified fluorocarbon powder by the particle size, the surface tension of the coating film, the tapped density of the powder and the addition of flow aids
.

1 Experimental part

1.
1 Main raw materials

Ordinary weather-resistant polyester resin A (commercially available), fluororesin B (commercially available), fluororesin C (commercially available), blocked isocyanate B-1530, triglycidyl isocyanurate TGIC, leveling agent (Nanhai Fenghua ), brightener (Fenghua Nanhai), degassing agent (Fenghua Nanhai), nano SiO2 (commercially available), alumina C (Al2O3•C, commercially available), other additives, pigments and fillers
.

1.
2 Test items and equipment

1.
3 Experimental formula and powder coating preparation

As with other thermosetting powder coating preparation methods, the base material is weighed according to the formula, mixed uniformly, melted and dispersed by an extruder to obtain flakes, cooled and crushed, to obtain three types of resin powder coatings that need to be mixed
.
Mix components A with components B and C uniformly in proportions, spray electrostatically, and bake at 210°C/15min to obtain a coating film
.
The thickness of the coating film is maintained between 40-100μm
.

2 Results and discussion

2.
1 The influence of different particle sizes on the weather resistance of polyester modified fluorocarbon powder

Powder electrostatic spraying uses the principle of high-voltage electrostatic corona electric field
.
The metal diversion cup on the spray gun head is connected with high-voltage negative electricity, and the coated workpiece is grounded to form a positive electrode
.
A strong electrostatic field is formed between the spray gun and the workpiece
.
When the carrier gas (compressed air) transports the powder coating from the powder supply barrel to the guide cup of the spray gun through the powder pipe, a dense charge is generated around it, and the powder is negatively charged.
Under the action of electrostatic force and compressed air, The powder is uniformly adsorbed on the workpiece [1]
.

The paint with too much resistance has a low charge rate during the electrostatic spraying process, while the paint with good conductivity and low resistance is charged, the charge disappears too fast and the static voltage does not rise, which also affects the spraying effect
.
Increasing the particle size will increase the charge of the powder and increase the powder loading rate
.
But the particle size of the powder particles should not be too large.
The gravity of the large particle powder exceeds the aerodynamic and electrostatic forces.
During the flight, the powder coating falls due to gravity before reaching the surface of the workpiece, which will cause the powder to be loaded.
The rate is reduced
.
If the powder is too fine, the chargeability will decrease, and the efficiency of coating construction will decrease.
The ultra-fine powder (particle size <10μm) is basically uncharged.
At the same time, the powder is too fine to increase the difficulty of powder production [2]
.
In this paper, pure fluorocarbon powder of different particle sizes and polyester powder of fixed particle size are mixed uniformly, sprayed and cured, and then the weather resistance of the polyester modified fluorocarbon powder is judged by the UVB aging test data
.

Mix polyester component A with pure fluorocarbon components B and C in a certain proportion, spray at 1, 3 air pressure at a voltage of 60KV, and bake at 210°C/15min to obtain a coating film and test the gloss (measurement angle 60°)
.
The recorded data is shown in Table 6 below
.

If the pure fluorocarbon powder on the surface of the mixed coating film accounts for a large proportion, the overall gloss of the coating film will be low, and vice versa
.
The degree of gloss can be used as the basis for judging the proportion of pure fluorocarbon powder on the surface of the coating film
.
From the test data of AB1-AB8 in Table 6, it can be seen that the larger the median particle size D50 of the pure fluorocarbon component, the less the proportion of the pure fluorocarbon powder on the surface of the coating film after mixing.
The bit size D50 is large to a certain extent, and the proportion of pure fluorocarbon powder on the surface of the coating film remains basically unchanged after mixing
.

As a result, the mixed coating film appears different gloss, which may be due to the poor compatibility of the two mixtures

Figure 1 shows the relationship between the weather resistance of the two mixed powders and the difference in powder particle size.
The larger the particle size of the fluorocarbon component, the overall weather resistance of the polyester-modified fluorocarbon powder coating will decrease
.
Because the weather resistance of fluorocarbon powder is much higher than that of ordinary polyester, the larger the proportion of fluorocarbon powder on the coating surface, the better the weather resistance of the coating
.
Although the smaller the median particle size D50 of the pure fluorocarbon component, the better the weather resistance of the coating is realized, but because the particle size of the pure fluorocarbon powder is too fine, it will bring certain difficulties to the manufacturing process, and the particle size of the powder The thinner, the easier it is to absorb moisture
.
Slight moisture absorption will affect the electrification of the powder, reduce the powder loading rate, affect the fluidity of the powder, film-forming performance, etc.
, and even difficult to adsorb on the workpiece.
The coating film will produce bubbles, pinholes, and spray gun clogging.
The production cost becomes higher, so the powder particle size should not be too fine
.
When the median particle size D50 of the pure fluorocarbon powder component is between 32-39μm (such as AB4, AB5, AB6), the gloss change of the coating film is relatively stable, indicating that the particle size of the polyester powder is 35μm, and the pure fluorocarbon powder particles When the diameter is 32-40μm, the proportion of pure fluorocarbon powder in the polyester modified fluorocarbon powder coating film is relatively stable
.
That is to say, it is more suitable when the difference in particle size of the two powders in the polyester modified fluorocarbon powder is kept within ±5μm
.

2.
2 The influence of surface tension and density on the weather resistance of polyester modified fluorocarbon powder

Using a surface tension test pen, you can easily analyze the surface ability, hydrophilicity, wettability and other small changes of different solids.
When testing, you should choose an intermediate value as the starting point, such as 38mN/m.
If you test within 2 seconds If the pen ink wets the test surface, it means that the surface tension of the test surface is greater or just right than the selected value.
Then a test pen with a larger value must be selected for the second test, and so on, until the test result is changed within 2 seconds.
Shrinked into water droplets (spherical), the value before this test is regarded as the surface tension of the test surface
.
And use this for comparison and analysis.
If the first test shrinks into water droplets (spherical), replace the test pen with a smaller value for the second test until the surface is wet[4]
.
This method can accurately measure the surface tension and surface wetness of the substrate and determine whether the surface factors of the substrate meet the requirements before work so that it can be adjusted to the needs of the work
.

Next, the surface tension of the three resins listed in the article is tested by the ACCU Dyne pen in the United States, and then the tap density is tested with a powder comprehensive characteristic tester
.
The surface energy of a solid determines its wettability.
The basic condition for the spontaneous spread of a liquid on a solid surface is that the surface energy of the liquid is smaller than the surface energy of the solid.
The lower the surface energy of the liquid (the higher the surface energy of the solid), the more Conducive to spreading, on the contrary is not conducive to spreading
.

From Table 8 in conjunction with Figure 2, it can be seen that the surface tension of the fluororesin C coating film is the smallest.
When mixed with ordinary weather-resistant polyester A, the powder with low surface tension and low tap density will be more likely to float on the surface tension in the melting stage.
, Tap the dense powder on top
.
Therefore, fluororesin powder will float on top polyester powder, fluororesin powder C and the relative degree of fluorine resin powders floating more B
.
From the test data of AB4 (blank) and AC (blank) in Table 9 below, it can also be seen that the AC (blank) group has significantly better resistance to B lamp aging than the AB4 (blank) group, which proves that the fluororesin powder C is relatively The fluororesin powder B will rise to a greater degree
.

Energy Dispersive Spectrometer (EDS, Energy Dispersive Spectrometer) is used in conjunction with scanning electron microscope and transmission electron microscope to analyze the types and contents of elements in the micro-region of the material
.
Because various elements have their own characteristic X-ray wavelength, the size of the characteristic wavelength depends on the characteristic energy △E released during the energy level transition.
The energy spectrometer uses the characteristic energy of different X-ray photons of different elements to perform the process.
Component analysis
.

Next, we use an energy spectrometer to analyze the elements and determine the element content of the AB4 coatings
.
The powder coating is electrostatically sprayed onto the substrate.
After baking and curing, we usually call the coating on the side contacting the metal substrate the metal surface, and the coating that is in contact with the air as the air surface
.
In other words, the fluorocarbon component will migrate upward from the metal surface to the air surface.
This phenomenon can be characterized by an energy spectrometer, as shown in Figure 3 and Figure 4 below
.

It can be seen from Figure 3 and Figure 4 that after fluorocarbon powder and polyester powder are dry mixed in a certain ratio, the fluorocarbon component with lower surface tension migrates up to the coating surface (the air surface in Figure 3), while the metal The content of fluorine elements on the surface is significantly lower than that of the air surface, so the polyester modified fluorocarbon powder coating made by dry blending has the weather resistance close to that of the pure fluorocarbon powder coating, and avoids the effect of the pure fluorocarbon powder coating.
The disadvantage of poor mechanical properties also reduces the cost of fluorocarbon powder coatings and has a very high cost performance
.

2.
3 The influence of external flow aids on the weather resistance of polyester modified fluorocarbon powder

The shelf life of powder coatings is generally around 3-12 months
.
If the powder coating is stored under high temperature and humidity conditions, the powder coating will agglomerate due to the low glass transition temperature, which will affect the atomization spraying, and even lead to poor coating film leveling and weather resistance.
Be affected
.
We found that after spraying polyester modified fluorocarbon powder with flow aid, the surface gloss of the coating film fluctuates
.
This problem may be caused by the instability of the proportion of pure fluorocarbon powder on the surface of the coating film in the polyester modified fluorocarbon powder
.
Now, pure fluorocarbon powders B4 and C with similar particle size (D50≈32μm) are uniformly mixed with polyester powder A in proportion, then appropriate flow aids SiO2, Al2O3•C are added, and the board is sprayed
.
By observing the surface color and comparing the aging data of the B lamp to judge the weather resistance of the mixed coating film
.

In Table 9, comparing the gloss of the coating film under 1 and 3 atmospheres, although different types of flow aids from different manufacturers can make the gloss stability of the mixed powder coating film better, it will also cause the pure fluorocarbon powder to change in the polyester.
The proportion of fluorocarbon powder in the fluorocarbon powder changes to different degrees, which affects the gloss change of the coating film surface
.
Among them, SiO2 causes the most obvious increase in the gloss of the coating film, and Al2O3•C also has the phenomenon of higher gloss
.
In Figure 5 below, from the surface color of the coating film, Al2O3•C disperses pure fluorocarbon powder and polyester powder more evenly, which looks darker but actually has relatively weather resistance.
The coating film without flow aid has become Worse
.

2.

In polyester modified fluorocarbon powder coatings, pure fluorocarbon powder has low surface tension and poor wetting and dispersing ability with pigments.

The results in Table 10 indicate that in the polyester modified fluorocarbon modified powder, as the amount of fluorocarbon powder C added increases, the durability of the coating film becomes significantly better

3 Conclusion

1) For polyester-modified fluorocarbon powder coatings produced by the dry blending process, the difference in median particle size of the two powders that need to be mixed should be controlled at about 5 microns to obtain better stability

2) When two powder coatings with different surface tensions are mixed, the surface tension and density will float above the high surface tension when melting.

3) The addition of flow additives to powder coatings is conducive to storage, but the polyester-modified fluorocarbon powder coatings produced by the dry blending process are not suitable for adding flow additives, otherwise the weather resistance of the coating film will be reduced

4) In the polyester modified fluorocarbon powder coating, the larger the proportion of fluorocarbon powder, the better the overall weather resistance of the coating film

references

[1]Ni Yude.

[2] Liu Hong.

[3]He Zhenjuan, Wang Luquan, Liu Chaohong.

[4] Wang Qingguo.

[5] Gong Yongzhong, Tao Ye.

About the author: Lian Fuquan (1992-), male, bachelor, assistant engineer, mainly engaged in the development of new products such as fluorocarbon powder coatings
.