Study on Synthesis and Performance of Polyester Resin for Hybrid Low Temperature Curing Powder Coatings

Abstract: This article introduces the synthesis of polyester resin for hybrid low-temperature curing powder coatings, which has a higher Tg and a lower melt viscosity
.

The effects of curing accelerator and its dosage, curing temperature and curing time on the curing performance of the resin were investigated

Terephthalic acid (PTA): Industrial grade; Isophthalic acid (IPA): Industrial grade; Adipic acid: Industrial grade; Hexahydrophthalic anhydride: Industrial grade; Azelaic acid: Industrial grade; Trimellitic anhydride (TMA): Industrial grade ; Neopentyl glycol: industrial grade; butanediol: industrial grade; cyclohexane dimethanol: industrial grade; ethylene glycol: industrial grade; esterification catalyst (F4101): industrial grade
.

CDR-4P Differential thermal analysis: Shanghai Balance Instrument; DV-Ⅱ type rotational viscometer: a BROOKFIELD Company; DF-4 PC softening point tester: Beijing Test Instrument Co.
datatec BHT
.

2.
2 Polyester resin synthesis

Put the raw materials diol and dibasic acid into the reactor according to the formula, and fill it with nitrogen protection.
When the temperature is raised to 190℃, the esterified water will be removed, and the temperature will continue to rise to 240℃.
Color transparent resin
.

Gradually vacuumize to 50mmHg (-0.
093Mpa), react for 15~30min under 240℃ vacuum condition, cool to 200℃, add polyacid capping agent, keep the temperature at 180~190℃ for 1h, the reaction is completed and the material is discharged
.

2.
3 Sample preparation

Put the trial-produced polyester resin and epoxy resin (E-12), pigments, fillers, leveling agents, degassing agents, curing accelerators, etc.
into the batching container according to the formula, fully stir it evenly, and melt and extrude it through the extruder Powder coating is obtained after extraction and pulverization
.
Spray the paint on the sample plate by electrostatic spraying, and solidify in the oven to obtain the coating film sample plate
.

2.
4 Performance characterization

The acid value and softening point of polyester resin are measured according to the corresponding national standards; the melt viscosity is measured by a DV-Ⅱ rotary viscometer; the glass transition temperature (Tg) and curing temperature are measured by a CDR-4P differential thermal analyzer.
The heating rate is 10°C/min
.
The performance of the coating film is determined in accordance with the corresponding national standards
.

3 Results and discussion

3.
1 Physical and chemical properties of polyester resin

The appearance of the polyester resin we developed is colorless or light yellow transparent particles; acid value 90.
0~110.
0mgKOH/g; melt viscosity (130℃) 5000~15000cP; glass transition temperature (Tg) 42~52℃; softening point ( Tf) 85~100℃
.
The thermogram of the polyester resin is shown in Figure 1
.

There are many factors that affect the surface leveling performance of powder coatings.
The melt viscosity of the main film-forming substances has a decisive effect on the melt viscosity and leveling performance of powder coatings
.

The main film-forming substances used for low-temperature curing powder coatings should have low melting viscosity characteristics at low temperatures, and the polyester resin used to ensure the storage stability of the powder coatings should also meet the glass transition temperature requirements
.

For this reason, we use a large number of flexible monomers in the molecular structure design of polyester resins to reduce the melt viscosity of the resin, and use special structural monomers (such as hexahydrophthalic anhydride, cyclohexane dimethanol) to disrupt the regularity of the resin structure.
Reduce the interaction between molecular chains to reduce the melt viscosity of the resin while ensuring that the resin has a certain glass transition temperature
.

3.
2 The effect of curing accelerator on curing temperature

The curing accelerator is an auxiliary agent that accelerates the curing reaction of the powder coating, and has a great influence on the curing performance of the powder coating
.

Correct selection of curing accelerators can reduce the curing temperature of powder coatings and shorten the curing time.
Therefore, for low-temperature curing powder coatings, it is very important to choose curing accelerators with higher reactivity at lower temperatures
.

For this reason, two curing accelerators, RB31 (as shown in Figure 2) and JG803 (as shown in Figure 3) containing cyclic amidine structure, were selected for comparative study in this experiment
.

The test results of Fig.
2 and Fig.
3 show that the Tg and the initial curing temperature of the powder coating formulated with the curing accelerator RB31 are 47.
0°C and 109.
9°C, respectively
.

The Tg and initial curing temperature of powder coatings formulated with curing accelerator JG803 are 45.
8°C and 118.
4°C, respectively.
That is, RB31 has a lower initial curing temperature and higher Tg, and is more suitable for preparing low-temperature curing powder coatings
.

It can be seen from Figure 2 that at 125~140℃, the slope of the curing curve is larger, that is, it has a faster curing rate, so we determined that 130℃ is the appropriate curing temperature
.

Note: Measured by DTA, the test conditions are: air, heating rate 10℃/min
.

3.
3 The effect of the amount of curing accelerator on the gel time

Gelation time (Gelation Time) is an important performance parameter of powder coatings.
It reflects the time required for the powder coating to start to melt to gelation
.
The effect of the amount of curing accelerator on the curing performance of powder coatings can be intuitively reflected by the gel time
.

Generally speaking, the larger the amount of curing accelerator, the shorter the gel time.
Conversely, the smaller the amount of curing accelerator, the longer the gel time
.

In this experiment, five levels of curing accelerator (RB31) are selected as 0.
5%, 1.
0%, 1.
5%, 2.
0%, and 2.
5% (based on polyester resin) for comparative study.
The test results are shown in Table 1
.

Note: The test temperature for gelation time and horizontal fluidity are both 130°C, and the amount of curing accelerator is calculated based on polyester resin
.

The test results in Table 1 show that, compared with normal temperature curing powder coatings, low-temperature curing powder coatings have poorer horizontal fluidity, and the orange peel phenomenon of the coating film is heavier.
However, when the curing accelerator dose is 1.
5% to 2.
0%, fluidity can also be obtained.
Coating film with better flatness
.

3.
4 The effect of curing time on the overall performance of the coating film

Based on our trial-produced polyester resin and epoxy resin (E-12), and RB31 as a low-temperature curing accelerator, a hybrid low-temperature curing powder coating is formulated
.
The effect of different curing times on the overall performance of the coating film is shown in Table 2 .

The test results in Table 2 show that with the extension of the curing time, the curing of the powder coating tends to be complete
.
When the curing time reaches more than 25min, a good overall performance of the coating film can be obtained
.

Note: The dosage of RB31 is 2.
0%, and the curing temperature is 130°C
.

4 Conclusion

(1) This article synthesizes a polyester resin suitable for hybrid low-temperature curing powder coatings.
The resin has a higher Tg and a lower melt viscosity
.

(2) Use synthetic polyester resin and epoxy resin (E-12) to prepare hybrid low-temperature curing powder coatings.
When the amount of curing accelerator RB31 is 2.
0%, the curing temperature is 130°C, and the curing time is 25 minutes, it can be obtained Coating film with good comprehensive performance
.