The powder coating spraying process mainly includes corona spray and tribo spray. Corona spray is widely used in China and does not have high requirements for powder coatings. However, the Faraday effect causes dead spots on complex workpieces, making it difficult to spray, i.e. it is difficult to powder some corners. The corona spray gun has been improved many times, but the Faraday effect can only be reduced. It cannot be avoided. Tribo spray can effectively solve the problem of powdering dead spots on complex workpieces, but it requires high chargeability of the powder coating. For this reason, many polyester manufacturers of powder coatings have successively launched polyester resins suitable for triboelectric spraying, such as our SJ4EDT, SJ4ETDT, SJ4866DT, SJ4C and other models, which all have very good triboelectric charging effects and have achieved ideal results in practical applications by customers at home and abroad.
2 Principle, advantages and disadvantages of tribo-gun spraying
The tribo-gun works by triboelectric charging, which means that the powder particles collide, rub, come into contact and disengage with the special polymer material (polytetrafluoroethylene or nylon) on the inner wall of the barrel, generating an electrical charge.
The advantages of the tribo-gun spraying process are
— High first-time powder application rate, which improves spraying efficiency and reduces powder recovery.
Overcomes the Faraday effect, which is particularly effective for spraying complex workpieces.
Compared with corona guns, the powder is more evenly distributed on the workpiece, and the surface of the coating film is smoother and flatter.
It can be fully and practically automated, reducing labour costs.
The disadvantages of triboelectric spraying are mainly as follows:
Triboelectric guns are expensive and have high maintenance costs.
Triboelectric spraying has high environmental and process requirements.
— Tribo-gun spraying has high quality requirements for powder coatings and must have good tribo-charging properties.
Given the many advantages of tribo-gun spraying, it is widely popular with domestic and foreign powder coating manufacturers, and powder coating manufacturers have put forward corresponding technical requirements for the tribo-charging properties of powder coatings. This paper experimentally demonstrates the factors that affect the tribo-charging of powder coatings.
3 Test part
There are differences between the different models of tribo guns supplied by different manufacturers. In order to eliminate experimental errors, this study used the Tribomatic 500 manual tribo powder spray gun from Nordson Corporation for all tests. The test conditions were room temperature 25°C, air humidity 50%, and total compressed air pressure 6MPa.
3.1 Effect of tribo aid addition
The friction rod and tube wall material in the tribo gun is a special polymer material PTFE with a dielectric constant of 2.1. Any material with a higher dielectric constant than this will acquire a positive charge after friction. The dielectric constant of the polyester resin used in powder coatings is only about 3.0. The difference between the two is too small, so the tribo charging is not good. In order to meet the needs of tribo gun spraying, a substance with a high dielectric constant can be introduced into the powder coating as a tribo charging aid. The commonly used triboelectricity enhancers are steric amine compounds, which have no effect on the properties of the powder coating. We selected triboelectricity enhancers from different manufacturers at home and abroad, marked as A (foreign liquid), B (foreign solid), C (domestic liquid) and D (domestic solid), respectively, and added them to the same type of polyester/TGIC powder coating formulation in different proportions. The powder coatings and coated film samples were prepared using the same process. The triboelectric charge test results are shown in Table 1.
Table 1: Effect of friction promoters on the tribocharging of powder coatings
Under normal circumstances, when powder coatings without friction promoters are sprayed with a tribo gun, the tribocharging is only 0.2-0.4μA, and it is difficult for the powder coating to continuously eject powder, resulting in poor powder coverage on the workpiece. As can be seen from the data in Table 1, a small amount of friction promoter can significantly increase the tribocharging of powder particles. As the amount of friction aid increases, the feedback charge value gradually increases, and when the amount increases to a certain level, the tribocharging of the powder coating will remain the same. This is because the length of the friction rod and the friction tube wall of each friction gun is fixed and has a charge saturation value. Different types of friction aids also have a certain effect on the tribocharging of powders, and liquid friction aids are generally more effective than solid friction aids.
3.2 Effect of powder particle size
A representative set of powder coatings with different particle sizes was obtained by selecting a polyester resin to which 0.2% friction promoter A had been added, cooling the extruded powder, and then sieving the powder through a sieve with different mesh sizes. The coatings were sprayed onto a plate under the same conditions to obtain the tribocharging test results in Table 2.
As can be seen from the data in Table 2, the smaller the particle size, the greater the triboelectric charge of the powder coating, but a particle size that is too small is not conducive to improving the powder coating rate. The reason for this is that the smaller the particle size, the more friction there is between the powder and the friction bar and the walls of the barrel during the friction process, and therefore the greater the triboelectric charge. However, after the powder leaves the friction gun, the fine powder particles are easily affected by the airflow in the spray booth, which reduces the powder coating rate. Similarly, coarse particles are also easily affected by air flow and gravity, as they are not as easily charged by friction as fine particles. They are not easily in contact with the workpiece and tend to bounce off. Therefore, the particle size distribution of the powder coating sprayed by the tribo gun should be appropriate. It is generally controlled at 35-45μm, and the finer or coarser powder particles should be as few as possible.
Table 2: Relationship between particle size and tribo charging of powder coatings
3.3 Selectivity of polyester
A hybrid polyurethane (50:50), a TGIC-cured pure polyester (93:7), a HAA-cured pure polyester (95:5), and an isocyanate-cured polyester (80:20) were selected, respectively, to prepare powder coatings with the same filler ratio, and the coatings were sprayed under the same process conditions to obtain the test results of triboelectric charge as shown in Table 3.
Table 3: Triboelectric charge test results for different types of polyester resins
Figure 1: Triboelectric charge of different types of polyester resins
Analysis of Table 3 shows that
there are significant differences in the triboelectric charging properties of different types of polyester, with hybrid polyester having the worst triboelectric charging properties. However, adding a very small amount of triboelectric charge aid can significantly improve the charging properties:
the triboelectric charging properties of HAA-cured polyester are significantly higher than those of other types of polyester;
Without the addition of friction promoters, the order of the chargeability of the different curing types of polyester is as follows: HAA type > TGIC type polyester > isocyanate curing polyester > hybrid polyester.
The American ‘PCI’ coatings magazine also gives similar data analysis, and Figure 1 further verifies the difference in the triboelectric performance of different types of polyester.
3.4 Effect of air pressure
Powder coatings with 0.2% friction promoter were selected, and the test results of the effect of spraying air pressure on the triboelectric charge of the coating were obtained by adjusting the spraying air pressure of the triboelectric gun (Table 4).
As can be seen from the data in Table 4, as the air pressure increases, the chance of collision between the powder and the tribo gun increases. The tribo charge of the powder particles increases. However, as the air pressure continues to increase, the flight speed of the powder particles is too fast, which intensifies the floating and bouncing of the powder in space, resulting in a decrease in the powder transfer rate. Therefore, although the tribo static electricity reading increases, it does not guarantee a high powder transfer rate. Adjusting the appropriate air pressure is particularly important for the tribo gun spraying process.
Table 4 Effect of spraying air pressure on powder charge
3.5 Other influencing factors
There are many other factors that affect the triboelectric charge of powder coatings and the powder transfer rate on the workpiece, such as air humidity, compressed air dew point temperature, workpiece grounding, powder fluidity, etc. Triboelectric spraying has high requirements for the air humidity in the workshop. Excessively high or low air humidity directly affects the powder transfer rate on the workpiece. Excessively high air humidity also causes greater wear on the friction rod and tube wall of the triboelectric gun, shortening the service life of the triboelectric gun. Other influencing factors will not be described in detail here.
The above test analysis shows that the main factors affecting the triboelectric charge of powder on the tribo gun are the friction aid, the particle size of the powder coating, the type of powder coating, the spraying air pressure and the spraying environment.
Tribo gun spraying of complex workpieces has an excellent powder charging rate and a more perfect coating film quality, so tribo gun spraying is becoming more and more popular. It is particularly important for powder coating suppliers to understand the triboelectric charging properties of powder coatings. Therefore, by selecting the correct tribo-type resin or adding tribo-assistants, and by pulverising and spraying under reasonable process conditions, satisfactory coating results and economic benefits can be achieved.
The above test data was obtained under specific conditions. Different tribo-gun tests were used under different spraying conditions, and the data inevitably differed. However, the statistics can reflect the impact of various factors on the tribo-charge of powder coatings. If you have different opinions, please feel free to correct and discuss.
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