Abstract: in this paper, the influence of polyester resin on the compatibility of powder coating is analyzed simply. The distribution of polyester resin is verified by experiments from the aspects of acid value, viscosity, sheet distribution and reaction activity.
1, foreword
With the further improvement of environmental protection awareness, powder coating, as an environmentally friendly coating, has been widely used. In recent years, the quality of powder coating has also been widely used, but there are still many problems to be solved.
For example, the most common problem is the compatibility of high gloss powder coating, which is a headache. Powder coating can be divided into high gloss powder coating, flat gloss powder coating, matte powder coating and matte powder coating.
In the practical application of these powder coatings, due to the low gloss of matte and matte powder coatings, the coating shows that it is not smooth on the micro level, so the compatibility problem is not very obvious;
However, the compatibility between batches of high gloss and flat gloss powder coating is often poor, which leads to pinholes, shrinkage holes, fog shadow and other problems on the surface of the workpiece after spraying, which affects the smoothness and luster of the surface. Therefore, the compatibility of powder coating is worthy of further study.
2. Compatibility judgment method
The simplest method to test the compatibility of powder coating between batches is visual inspection, that is, the powder between batches is mixed evenly according to a certain proportion, usually the proportion is 1:99,; 10:90; 50:50; 90:10; 99:1. After baking, observe the surface condition of the sample plate, such as pinhole, shrinkage cavity, fog shadow, etc.
However, it is easy to judge pinholes and shrinkage holes by visual inspection, and it is not easy to judge the grade of fog shadow by naked eyes, and there is no clear concept, especially for some matte coatings.
So the instrument method can get the real data well and give more intuitive judgment and comparison. Fog shadow instrument is a good instrument to detect the fog shadow degree of the coating film. It is a double beam gloss instrument, in which the reference beam can eliminate the influence of temperature on the gloss and color on the fog shadow value.
The main receiver of the instrument receives the gloss of the coating, and the secondary receiver receives the fog shadow around the reflected gloss. The 20? Method is used when the fog shadow value is measured by the fog shadow instrument. The haze value can reach up to 1000, while the haze value of normal powder coating is usually below 250. Therefore, the following will be based on the fog shadow value measured by the fog shadow meter.
3. Experimental demonstration
There are many problems affecting the compatibility of powder coating, but in the final analysis, it is mainly because of the stability of the two batches of powder. Of course, stability is only relative, because chemical polymer materials cannot be completely consistent.
There are many raw materials of powder coating, such as resin, curing agent, pigment, filler, assistant, etc. as the most important film-forming material of powder coating, the stability of resin batch is directly related to the compatibility and stability of the powder coating.
Therefore, taking resin as an example and test data as a reference, what are the main factors that affect the compatibility between resin batches?
3.1 acid value
Acid value is an important index of resin. It is not only an index to judge the reaction degree when resin is synthesized, but also an important data to calculate the ratio between resin and curing agent.
Although the powder coating is usually produced according to the curing ratio recommended by the resin manufacturer or curing agent manufacturer, we will find that the acid value of the resin has a certain range, such as the outdoor pure polyester 93:7, the acid value range is 30-36mgkoh / g, so whether the acid value of the resin in this range has an impact on the compatibility?
Therefore, we select a resin P1 of the same model and select ten batches of P1 resin, as shown in Table 1. According to the proportion of 50:50, the mixing test is carried out between different batches, and the data is shown in Table 2 below.
From the data in Table 1, it can be seen that the viscosity of the selected 7 batches of resin P1 is basically the same, and the acid value is between 30-36mgkoh / g. from the data in Table 2, it can be seen that the greater the acid value difference, the greater the haze value compared with the haze value of the single board, indicating the worse the compatibility.
3.2 viscosity
Viscosity is also an important index in the process of pig synthesis. It indirectly reflects the molecular weight of the resin. In addition, it also affects the mixing effect and the flow performance of the coating in the melting leveling stage.
For this reason, we select a resin P2 and try to select batches with different viscosities, as shown in Table 3. After pulverizing, the compatibility test shall be carried out among different batches according to the ratio of 50:50, and the test results are shown in Table 4.
It can be seen from table 3 and table 4 that under the condition of relatively stable acid value, the greater the viscosity difference, the greater the haze value, that is, the worse the compatibility. It can be seen that viscosity is also one of the important factors affecting the compatibility between batches of powder coatings.
3.3 sheet distribution
The resin is a liquid product in the high-temperature reactor. After filtering impurities, it is delivered to the cooling steel belt, evenly distributed through the steel belt distributor, cooled and broken into uniform particles, and then packaged.
In the whole feeding process, if the distributor is not evenly distributed or the crusher is not evenly broken, the size and thickness of resin flakes will be inconsistent, which will not only cause trouble to the production packaging, but also lead to uneven dispersion in the pulverizing process.
Select the same batch of resin P3 here, change the thickness and size of resin particles by adjusting the density of the distributor and crushing teeth, and get the following distribution of sheet materials after standard screening, as shown in Table 5. Finally, the influence of the size and thickness of the sheet on the compatibility is verified by pulverizing, and the data is shown in Table 6.
It can be seen from table 6 that if the resin sheet is too small or too large, there will be obvious fog shadow on the surface of the veneer, and the greater the sheet distribution gap, the worse the compatibility.
Therefore, it can be seen that the sheet distribution of resin also affects the compatibility between batches.
3.4 reactivity
The reactive activity of powder coatings is generally indirectly reflected by the gelation time or the temperature of exothermic reaction peak through DSC.
Polyester resin is the main film-forming material in powder coating. The activity of the resin itself has a direct impact on the activity of powder coating. Therefore, the greater the difference of resin reaction activity between batches, the direct impact on the compatibility effect of powder coating.
As an example, P4 resin was adjusted by adding a small amount of accelerant, and the results of compatibility were compared. See Table 7 and table 8 respectively.
It can be seen from table 8 that the greater the difference of reaction activity, the greater the haze value, indicating the worse the compatibility. It can be seen that the stability of the reaction activity is important to the compatibility.
Of course, it is not only the resin that affects the reactive activity of the powder, but also the curing agent, the materials involved in the reaction in the formula, and the promotive auxiliaries also affect the reactive activity of the powder coating, which will not be demonstrated in this paper.
4. Results and discussion
According to the above test results, the main reasons that polyester resin affects the compatibility between batches of powder coating are acid value, viscosity, reaction activity and resin sheet distribution. Because it is used as experimental demonstration, some extreme data are produced specially.
As a mature resin, its synthetic raw materials and process conditions should always be stable, because only stable raw materials and process conditions can produce stable resin.
In addition, as mentioned above, in addition to polyester resin, there are many factors that affect the compatibility between batches of powder coating, such as the crushing effect of the mixer, the mixing effect of the extruder, the volatility of other raw materials, etc., which will not be discussed in this paper. In short, "stability" is the best way to solve compatibility. Source: Anhui Shenjian New Material Co., Ltd
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