For the development of metal substrate epoxy powder coating

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EP resin, industrial grade (grades GESR903H, GESR912, GESR924, epoxy equivalent 750, 612, 695, 720~770), Hongchang Electronic Materials Co.

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BT125D electronic analytical balance, Doris Scientific Instrument Co.

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The basic formula of EP powder coating is shown in Table 1.

Weigh the EP resin, curing agent, pigments, fillers, and additives according to the dosage of the formula, premix them with a high-speed mixer, melt and extrude them through a twin-screw extruder, cool the tablet, and then pulverize them to the required particle size with a high-speed pulverizer.
Get paint powder.
Melt extrusion conditions are: screw length to diameter ratio 16:1, extrusion frequency 20 Hz, extrusion temperature 110 ℃.

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Using a high-voltage electrostatic powder spray gun, spray the sample on the electrostatic powder spraying equipment, and then cure it in a 200 ℃ oven for 10-15 min to obtain a 35-50 μm thick coating.

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(1) Gloss: Measured in accordance with the GB/T 8754-1988 standard (the final result is shown as the average value of 5 sets of samples).

(2) Impact strength: Measured in accordance with the GB/T 1732-1993 standard (the final result is shown as the average value of the 3 sets of samples).

(3) Adhesion: Measured in accordance with the GB/T 9286-1998 standard.

(4) Leveling: Measured in accordance with the GB/T 21782.

(5) Thermal performance: Measured by differential scanning calorimetry (DSC) method (nitrogen atmosphere, flow rate of 50 mL/min, heating rate of 5, 10, 15, 20, 25 K/min).

(6) Glass transition temperature (Tg): Characterized by DSC method (heating rate is 20 K/min, nitrogen atmosphere).

2 Results and discussion

2.
1 The effect of matting curing agent on coating film performance

Since the matting curing agent will make the prepared coatings have good mechanical properties and obvious matting, the matting agents are widely used in EP resin powder coatings.
As a kind of auxiliary agent, the matting curing agent is cured by chemical reaction while achieving matting.
The principle is to use the difference in reaction rate and compatibility between the two powder film forming materials to form a coating film with a rough surface and poor reflection.
.
The most commonly used matting curing agents on the market are B68 and XGP603-1A.
B68 is a matting curing agent developed earlier successfully abroad.
It is a hybrid curing agent that is self-contained with cyclic imidic acid and polybasic acid through salt formation; XGP603 series is a matting curing agent made by Ningbo Nanhai Chemical Company to graft this type of matting curing agent with acrylate.
Into the matting curing leveling agent.

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1 The effect of the amount of matting curing agent on the performance of the coating film

In order to determine the best amount of matting curing agent in the EP powder coating system, change the amount of matting curing agent (relative to the quality of the matrix resin), and the content of other substances remains unchanged, then the effect of different B68 content on the coating film performance is shown in Table 2.
Show.

It can be seen from Table 2: On different metal substrates, as the amount of curing agent increases, the gloss shows a gradually decreasing trend, that is, the matting effect becomes better and better; however, as the amount of curing agent increases, the impact strength It shows a trend of increasing first and then decreasing.
This is because when the amount of curing agent is small, it is not enough to initiate the cross-linking and curing reaction of the EP group, and the EP resin is not cured completely; when the amount of curing agent is too much, the curing rate is too fast, resulting in a large number of small molecular structures that cannot be formed.
Long-chain cross-linked cured products result in lower strength of cured products.
In summary, this study chooses w (matting curing agent) = 5.
97% is more appropriate.

2.
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2 The effect of the type of matting curing agent on the performance of the coating film

The types of matting curing agents are different, and the properties of the coating film formed by electrostatic spraying of EP powder coatings are also different.
When the matting curing agent is B68 and XGP603-1A, the effects on the properties of the coating film are shown in Table 3 and Table 4.

It can be seen from Table 3 and Table 4 that under the conditions of all other components being the same, with XGP603-1A as the matting curing agent, the gloss of EP resins GESR902, GESR912, E-12 and GESR903H is significantly lower than that of B68 for matting The gloss of the coating film of the curing agent.
The impact strength and leveling are both high and low, and the adhesion is 0 grade.
On the whole, XGP603-1A has better coating performance than B68 when it is used as a matting curing agent.

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2 The influence of EP resin on coating film performance

EP resin is an important part of EP powder coatings.
The various indexes of EP resin have an important influence on the performance of powder coatings.
In order to compare the effects of different types of EP resins on the coating properties of EP powder coatings, 65 g of EP resin GESR924 was selected in this experiment with EP resins GESR903H, GESR902, GESR912, DER663U, S-903H, CYD-014U and E-12 500 each.
g is used as the matrix resin, and a series of EP resin powder coatings are prepared using the formula in Table 1 (the amount of B68 is 50 g).

It can be seen from Table 3: The order of the matting effect of various EP resins is E-12, GESR902, CYD-014U, GESR912, GESR903H, DER663U and S-903H, among which the effects of CYD-014U, GESR912 and GESR903H are different Not much, and E-12 and GESR902 are relatively good, but S-903H is relatively poor.
The order obtained from Table 4 is GESR902, E-12, GESR912, GESR903H, and in Tables 3 and 4, the gloss levels below 5% are E-12 (B68), GESR902 (XGP603-1A), GESR912 (XGP603- 1A) and E-12 (XGP603-1A).

Theoretically, the EP group content (ie EP value) in EP resin is inversely proportional to Mr, that is, the smaller the EP value, the larger the average molecular weight of the EP resin, the higher the softening point, and the cured product has better bending and impact resistance.
Performance, the storage performance of powder coatings is also better.
But only when the EP resin and matting curing agent are in the equivalent ratio range, the EP powder coating can have qualified physical and chemical properties.
From this perspective, the higher the average molecular weight of the EP resin of the same quality, the less the amount of matting agent that needs to be proportioned.
That is, the higher the molecular weight, the more cost-effective.

It can be seen from Table 2 and Table 3 that when the actual amount of matting curing agent (3.
97%) is much smaller than the equivalent ratio of matting curing agent and EP resin, its impact performance will be significantly reduced, so it is necessary to achieve a balance of EP equivalent.
GESR903H, GESR912, CYD-014U, and DER663U have similar extinction effects.
Among them, GESR903H, CYD-014U, and GESR663U have better impact performance, while GESR912 has a poorer effect, which is also due to the improper ratio.
E-12 has good matting performance, but poor impact performance, mainly because the resin contains too much hydrolyzable chlorine and contains more impurities.

Therefore, only pursuing the matting effect can choose EP resin with a smaller molecular weight.
However, EP resin with larger molecular weight should be used when the overall performance is better.
The extinction performance of GESR903H is smaller than that of DER663U, CYD-014U, and S-903H and can be substituted for each other.
GESR912 and GESR902 can further improve the extinction effect.

The film properties of powder coatings obtained from different grades of EP resins have certain differences, and the reason for this effect is that there are certain differences in the manufacturing process and internal technical indicators of different specifications of EP resins from different manufacturers, and the different EP resins used There are also differences in the composition of raw materials, resulting in different structures of EP resins; in addition, the catalysts and accelerators used in the synthesis of various specifications of EP resins are also different.
These differences cause differences in the relative molecular mass, melt viscosity, and reactivity of the synthesized EP resin, which will ultimately affect the gelation time and melt horizontal fluidity of the EP powder coating obtained, as well as the appearance of the coating film and other physical properties.
Mechanical behavior.

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3 Selection of additives

The amount of additives used in powder coating formulations is small, but their effects cannot be ignored.
Commonly used additives include curing accelerators, matting agents, leveling agents, corner covering modifiers, coating film brighteners and powder loosening agents, etc.
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After comparison and selection, this formula uses B68 as the matting curing agent and benzoin as the curing accelerator.
The addition of these additives accelerates the reaction between the resin and the curing agent, reduces the curing temperature, shortens the curing time, and reduces the amount of curing agent.
; At the same time, the appearance, gloss, mechanical properties and chemical properties of the resin have also been correspondingly improved.

The DSC method is used for analysis.
When the matting curing agent is B68 and XGP603-1A, the characteristic data of the DSC curve is shown in Table 5 and Table 6, respectively.

It can be seen from Table 5 and Table 6 that the extinction performance of EP resin is related to the average heat output per minute and peak top temperature Tp on the DSC chart.
The greater the average heat output per minute, the lower the peak top temperature Tp, and the lower the gloss of the resulting coating film.
However, the peak onset temperature To, exothermic time and gloss do not have a strong correlation.
Therefore, it can be considered that the reactivity of the EP resin and the matting curing agent has an important influence on the matting effect.
The better the reactivity, the better the matting effect.
This may be because the better the reactivity, the more violent the reaction between the cyclic imidase and the resin, so that when the remaining resin reacts with the acid anhydride, the greater the difference between the two reactions, the greater the damage to the microscopic flatness, and the gloss of the resulting coating.
The lower the degree.
Among the three different grades of EP resins GESR902, GESR912 and GESR903H, GESR902 has the smallest molecular weight and the largest reactivity; GESR903H has the largest molecular weight and the smallest reactivity.
Therefore, the extinction effect of GESR902 is the best, and the extinction effect of GESR903H is the worst.

3 Conclusion

Using EP resin as the matrix, XGP603-1A as the matting curing agent, benzoin as the curing accelerator, carbon black as the pigment and barium sulfate as the filler, EP powder coatings with excellent performance can be prepared by the melt blending method.

(1) When w (GESR924) = 6.
47%, w (benzoin) = 0.
30%, w (barium sulfate) = 18.
30%, w (carbon black) = 4.
99% and w (matting curing agent XGP603-1A) = 5.
97% When the system is cured most completely, the coating film has lower gloss (3.
3%), higher impact strength (147.
05 J/m), excellent adhesion (grade 0) and leveling.

(2) When EP is GESR902 and E-12, the gloss of the coating film (up to 3.
3%) is relatively low.

(3) The reactivity of EP resin and matting curing agent is very important to the matting effect.
The better the reactivity, the better the matting effect.