Study on Preparation of Low Temperature Curing Powder Coating

Text/Jiang Rong, Zou Lingling, Bi Haipeng, Zhang Qi

(1.

Abstract: Trimethylolpropane (TMP) is used as the main raw material to prepare low-temperature curing powder coatings

Keywords: polyester resin; low temperature curing; powder coating

1 Introduction

Powder coating is a new type of solvent-free, solid powder coating composition online coatingol.

The curing of powder coatings is divided into 4 stages: melting, flow, gelation and curing, among which the curing process determines the properties of the coating film [2]

The curing temperature of general-purpose powder coatings is generally 180℃-200℃, and the curing temperature of low-temperature curing powder coatings can be lower than 160℃.

Based on the existing formula, this article introduces trimethylolpropane monomer to replace part of neopentyl glycol.

2 Experimental part

2.

Raw materials: neopentyl glycol, trimethylolpropane (TMP), terephthalic acid, isophthalic acid, adipic acid, monobutyl tin oxide, antioxidant, TGIC, titanium dioxide, barium sulfate, leveling Agent

Instruments: Magnetically stirred stainless steel reactor, gapless twin-screw extruder, electric thermostatic drying oven, rotary vane vacuum pump, film thickness gauge, gloss meter, electrostatic spraying equipment, paint film impactor

2.

2.

Add neopentyl glycol, TMP, terephthalic acid, catalyst monobutyl tin oxide, etc.

2.

Add antioxidant at about 225-228°C, cool down to 210°C, add adipic acid and isophthalic acid, maintain the temperature, carry out acidification, heat up to 240°C at 10°C/h to keep the acid value, the viscosity is qualified, and vacuum Reduce pressure for 2h, while tracking acid value and viscosity, take out the resin to cool

2.
3 Preparation of powder coating

(1) Weigh polyester resin, TGIC, titanium dioxide, barium sulfate and leveling agent according to the formula in Table 1, and mix them evenly.
When the gapless twin-screw extruder reaches the set temperature, open the cooling water pipeline and set the feeding speed Adjust to 30Hz and screw to 40Hz
.
Then pour the sample into a twin-screw extruder and extrude into a tablet
.

(2) Cool the extruded sample into a pulverizer and pulverize it into powder.
Use a 180-mesh sieve to sieve out the powder
.
Turn on the electrostatic spraying equipment, take a spare metal plate and blow it with an air gun, and hang it on the iron frame of the electrostatic spraying equipment.
Pour the powder into the spray gun and start spraying.
Uniformity
.
Go back and forth about 5 times, then remove the sprayed board and put it in an electric heating constant temperature blast drying oven for baking and curing
.

(3) After curing, take out the board to cool, measure the thickness of the board with a coating thickness gauge, measure the gloss with a gloss meter, and use a paint film impactor to impact the board (positive and recoil), and finally get the board Face conclusion
.

3 Results and discussion

3.
1 Single factor experiment

Using TMP content, curing time and curing temperature as the influencing factors, and gloss as the index, single-factor experiments were carried out
.
Using gloss as an indicator can effectively reflect the degree of influence and facilitate observation
.

3.
1.
1 The influence of TMP on the gloss of the coating film

Add polyester with different TMP content, make powder according to the formula, spray it, and cure at 160°C for 10 minutes
.
Investigate the influence of TMP content on the gloss of the coating film, the result

It can be seen from Table 2 that the gloss of the coating film is significantly reduced after adding TMP, which improves its performance, but when 25% is added, the gloss is reduced, just because excessive TMP increases the viscosity of the powder coating system , Causing orange peel on the smeared surface, so the gloss is also significantly reduced
.

3.
1.
2 The influence of temperature on the gloss of the coating film

Add TMP10% polyester resin and spray it into powder according to the formula.
The curing time is 15min.
The effect of temperature on the gloss of the coating film is observed.
The results are shown in Table 3
.

It can be seen from Table 3 that as the temperature increases, although the gloss of the coating film has little effect, it still drops
.
This is because the temperature rises, the surface of the coating film is prone to yellowing, which affects the gloss
.

3.
1.
3 The effect of time on the gloss of the coating film

Add TMP10% polyester resin and spray it into powder according to the formula.
The curing temperature is 150°C.
The effect of time on the gloss of the coating film is investigated.
The results are shown in Table 4
.

It can be seen from Table 4 that time has little effect on the gloss of the coating film, but as the baking time increases, the surface of the coating film is continuously oxidized and its gloss also decreases
.

3.
2 Orthogonal experimental design

3.
2.
1 Choice of factors affecting orthogonal test

Take trimethylolpropane (TMP) content (A), curing temperature (B), curing time (C) as the inspection factors, and use gloss as an indicator to conduct a three-factor and three-level experiment.
The influencing factors and levels are shown in Table 5.
, The orthogonal test results are shown in Table 6
.

From the range analysis results in Table 6, it can be seen that the three factors that have the greatest influence on gloss are the trimethylolpropane content, and the curing time and curing temperature have the same influence
.
The gloss of experiment 1 to experiment 6 are all greater than 90, which meets normal requirements
.
The optimal film-forming conditions of the orthogonal test are A2B1C2, that is, the content of trimethylolpropane is 10%, the curing temperature is 150℃, and the curing time is 15min.
The film gloss performance reaches 98, which is the optimal condition
.

3.
2.
2 Analysis of variance

From the analysis of variance in Table 7, it can be concluded that the variance ratio F value of factor A (the content of TMP) is 27.
40, which is much higher than the other two groups, while the P value of 0.
03 is much smaller than the other two groups
.
This proves that the influence of A factor on the experimental results is very significant, while the B factor (curing temperature) and C factor (curing time) have no significant influence on the experiment
.

3.
3 Impact resistance of coating

Taking the impact strength of 50kg.
cm as the benchmark, 9 groups of experimental samples were subjected to coating impact resistance experiments, and the results are shown in Table 8
.

It can be seen from Table 8 that because the content of TMP in the first three groups of experiments is 0, the impact resistance is poor and cannot reach the standard.
With the addition of TMP, the degree of branching of the system increases, and the reaction activity increases, even if the curing conditions are reduced.
It is completely cured, and its crosslinking density is high, so the impact performance is good
.
Combined with the results of the orthogonal experiment, the best film-forming conditions are A2B1C2, that is, the content of trimethylolpropane is 10%, the curing temperature is 150℃, and the curing time is 15min.
The gloss of the coating film is 98.
The results of the impact resistance test are positive and negative.
All passed
.

4 Conclusion

In this paper, by designing a three-factor and three-level orthogonal experiment to find the low-temperature curing study of powder coatings under the condition of meeting the specified indicators, the following conclusions can be drawn
.

1.
Adding TMP will reduce the gloss of the coating film;

2.
The curing time of powder coatings is different at different temperatures, the higher the curing temperature, the shorter the curing time;

3.
When the TMP content is increased by 10%, the curing temperature can be shortened to 150°C, effectively reducing the temperature to about 20°C;

4.
When the TMP content is zero, although the gloss of the coating film is high, the flexibility of the coating film is poor, but the recoil is not good, and the gloss performance of the coating film is good when the TMP content is 10%;

5.
Under the premise of ensuring the alkyd ratio, appropriately increasing the content of trimethylolpropane can effectively improve the flexibility of the polyester resin system and improve the impact resistance of the coating;

6.
After experimental research, the coating formula has the best performance when the content of trimethylolpropane is 10%, the curing temperature is 150℃, and the curing time is 15min
.

Based on the above conclusions, a polyester resin coating formulation with low-temperature curing properties was selected by changing the raw material components, which not only has the excellent performance of the original coating formulation, but also greatly reduces energy consumption, and has good application value
.

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