Study on Synthesis of Polyester Resin for Ultra Weather Resistant Powder Coatings Cured by Hydroxyalkylamide System at Low Temperature

Abstract: In order to overcome the performance defects of polyester resins and further improve the weather resistance of hydroxyalkylamide system super weather resistant powder coatings, epoxy acrylate prepolymers were prepared by solution polymerization.

Key words: polyester resin; acrylate; powder coating; low temperature curing; super weather resistance; hydroxyalkylamide

0 Preface

Compared with conventional powder coatings, ultra-weather-resistant powder coatings have longer weather resistance, and the coatings can meet the requirements of outdoor protection for more than 10a .

Conventional pirmid system ultra-weather-resistant powder coatings have a curing temperature of >180 °C.

The conventional primid system polyester for ultra-weather-resistant powder coatings is limited by the narrow range of monomer selection, the high cost of special monomers, and the ester bond structure in the molecular structure that is easily hydrolyzed and has poor aging performance.

In this study, solution polymerization was used to prepare epoxy acrylate prepolymer.

1 Experimental part

1.

Glycidyl methacrylate (GMA): Japan's Mitsubishi Gas; n-butyl acrylate (n-BA), methyl acrylate (MA): BASF; methyl methacrylate (MMA): Qilu Petrochemical; n-butyl methacrylate Esters (n-BMA): Korea LG; Propylene Glycol Methyl Ether Acetate: Dow, USA; Di-tert-Amyl Peroxide: Arkema; ​​Butyl 3-Mercaptopropionate: Guangzhou Sanwang; Neopentyl Glycol (NPG) : Yantai Wanhua; Isophthalic acid (IPA): Japan AGIC; Terephthalic acid (PTA): Zhuhai BP Chemical Co.

1.
2 Experimental equipment

3L reactor: self-assembly; electrostatic spraying equipment: HANZHE-901, Hanzhe coating; twin-screw extruder: SLJ-40, Yantai Lingyu; infrared spectrometer: Spectrum 65, PerkinElmer, USA; impact instrument: BGD 304, Guangzhou Biaogeda; Glossmeter: YJD-380, Yingchao; Colorimeter: Ci7800, X-rite, USA; Differential Scanning Calorimeter: DSC 3+, METTLER TOLEDO; Cone and Plate Viscometer: DV2THB, Brookfield, USA; UV accelerated aging machine: QUV/spray, Q-Lab, USA
.

1.
3 Synthesis process of acrylate prepolymer

The acrylate prepolymer formulation is shown in Table 1
.

Add propylene glycol methyl ether acetate in the 3L reaction kettle according to the formula amount in Table 1, feed nitrogen and turn on stirring after being rapidly heated to 142 ° C, acrylate monomer, 90% of di-tert-amyl peroxide and 3-mercapto After the butyl propionate was uniformly mixed, it was uniformly added dropwise to the reaction kettle within 3h
.
After the dropwise addition, the temperature was continued for 2 hours, and then the remaining 10% di-tert-amyl peroxide was added, and the temperature was continued for 3 hours
.
After the reaction, the nitrogen was turned off, and the temperature was rapidly raised to 160 °C to evaporate most of the solvent, and then began to evacuate, and evacuated at -0.
099MPa for 3h
.
After the vacuuming is completed, the material is discharged to obtain an acrylate prepolymer
.

1.
4 Synthesis process of acrylate-modified polyester resin

The formula of acrylate-modified polyester resin is shown in Table 2
.

Add the alcohols and acid monomers and catalysts in the raw materials into a 3L reaction kettle according to the formula amount in Table 2, heat up to 150°C under nitrogen protection and start stirring, then slowly heat up to 250°C, keep the temperature for 4h, the resin is transparent After testing the acid value, the acid value index was controlled to be 4~12mgKOH/g; after the acid value reached the standard, the temperature was lowered to 240 ° C and acrylate prepolymer was added, and the acid hydrolyzed was added after the reaction was continued for 1 hour.
50mgKOH/g; be cooled to 210 ℃, start vacuuming, keep the vacuum degree of-0.
095MPa until the acid value of the resin drops to 30～35mgKOH/g, lift the vacuum, discharge, obtain acrylate modified polyester resin
.

1.
5 Preparation of powder coatings and coatings

The powder coating formulations are shown in Table 3
.

Weigh the materials according to the formula in Table 3, and premix the raw materials.
Process to get powder coating
.
Then, the prepared powder coating was sprayed on the iron plate by electrostatic spraying, and the film thickness was controlled to be 60-70 μm
.

1.
6 Performance Test

The acid value of resin is tested according to GB/T 6743-2008; the gloss of coating is tested according to GB/T9754-2007; the viscosity of resin is tested by cone and plate viscometer at 200℃; the impact resistance is tested according to GB/T1732-1993; the color difference is tested according to GB/T 7921-2008; salt spray resistance according to GB/T1771-2007; adhesion according to GB/T9286-1998; horizontal flow according to GB6551-1986; storage stability according to GB/T 21782.
8-2008 for testing; glass transition temperature is determined according to GB/T 19466.
2-2004, using nitrogen atmosphere, heating rate 10 ℃/min; artificial aging test using UV accelerated aging machine to test with reference to GB/T14522-2008, wavelength 313nm, illumination temperature 60°C, condensation temperature 50°C, radiation intensity 0.
71 W (/m2·nm), test time 550h
.

2 Results and discussion

2.
1 Infrared characterization of polymers

Acrylic prepolymer with epoxy value of 0.
4 mol/kg was prepared; acrylic prepolymer with epoxy value of 0.
4 mol/kg was added with a mass fraction of 20% of modified polyester to synthesize acrylate-modified polyester.
The spectrum is shown in Figure 1
.

It can be seen from Figure 1 that 2800~3000cm-1 is the characteristic absorption peak of the methylene group of the polymer molecular chain, 1700cm-1 is the characteristic peak of the C=O bond, and 1150cm-1 is the stretching vibration peak of C—O—C
.
The characteristic absorption peak of the epoxy group of the acrylate prepolymer is at 910cm-1, but it can be clearly found from the infrared spectrum of the acrylate-modified polyester that the characteristic absorption peak of the epoxy group at 910cm-1 disappears, indicating that the epoxy group The reaction was completed, and the acrylate prepolymer was successfully grafted into the polyester molecule
.

2.
2 Influence of epoxy value of acrylate prepolymer on the properties of modified polyester and powder coating

Acrylate prepolymers have epoxy groups, and the acrylate prepolymers are grafted into the polyester molecular chain through the reaction of epoxy groups and carboxyl groups.
The epoxy value of acrylate prepolymers will directly affect the modified polyester.
The degree of branching has an important impact on the properties of the modified polyester
.
The effects of different epoxy values ​​of acrylate prepolymers on the properties of modified polyester and powder coatings were investigated experimentally.
The results are shown in Table 4
.

It can be seen from Table 4 that with the increase of the epoxy value of the acrylate prepolymer, the viscosity of the modified polyester increases sharply, the leveling of the coating becomes worse, and the impact resistance of the coating first increases and then decreases.
When the epoxy value of the acrylate prepolymer is 0.
4mol/kg, the synthesized modified polyester has the best comprehensive performance
.
This is because the epoxy value of the acrylate prepolymer determines the branching degree of the modified polyester.
Higher polyester viscosity, on the other hand, this also causes the appearance of the cured coating to deteriorate, and the orange peel on the board surface becomes heavier; at the same time, due to the increase of branching degree, the crosslinking density of the cured coating is also relatively increased, It helps to improve the impact resistance of the coating, but when the crosslinking density of the coating is too high, it will cause stress concentration and the coating is too brittle, so the impact resistance of the coating shows a trend of first increasing and then decreasing
.

2.
3 Influence of the amount of acrylate prepolymer on the properties of modified polyester and powder coating

According to the above experiments, the epoxy value of the acrylate prepolymer was selected to be 0.
4 mol/kg, and the effect of different amounts of the acrylate prepolymer on the properties of the modified polyester and powder coating was studied.
The experimental results are shown in Table 5
.

It can be seen from Table 5 that with the increase of the mass fraction of acrylate prepolymer, the viscosity of the modified polyester gradually increases, the horizontal flow gradually decreases, the leveling appearance of the cured coating becomes worse, and the impact resistance also shows The trend of increasing first and then decreasing, the gloss retention during the aging process of the resin coating increases with the increase of the mass fraction of acrylate prepolymer
.
This is because the increase in the mass fraction of epoxy acrylate prepolymer will give the modified polyester a higher degree of branching, resulting in a gradual increase in the viscosity of the modified polyester and a worsening of the leveling appearance of the coating; in addition, The increase of the branching degree of the modified polyester makes the cured coating have a higher crosslinking density, resulting in improved impact resistance of the coating, but when the crosslinking density of the coating is too high, the coating becomes brittle and the impact resistance deteriorates
.

Due to the presence of a large number of ester bonds and benzene rings in the main chain of polyester resin molecules, polyester is prone to hydrolysis and photoaging of the main chain during the aging process, and its weather resistance is limited; while the main chain of acrylate resin molecules does not contain ester bonds and benzene rings.
, the resin has a strong light aging resistance, so with the increase of the mass fraction of the acrylate prepolymer, the weather resistance of the modified polyester is significantly improved
.
When the mass fraction of acrylate prepolymer is 20%, the synthetically obtained modified polyester is ideal in comprehensive performance
.

2.
4 Effect of acid hydrolyzing agent on the properties of modified polyester and powder coating

As the curing accelerator of the system, the acid hydrolyzer directly affects the reactivity of the terminal carboxyl groups of the polyester resin and the curing degree of the coating, and has an important impact on the low temperature curing performance of the polyester resin
.
Since the water generated during the curing of the primid system powder coating will absorb heat during the vaporization process [5], it is impossible to directly use DSC to monitor the enthalpy during the curing process of the primid system powder coating to judge the degree of curing of the powder coating.
The glass transition temperature (Tg) reflects the degree of curing of the coating
.
On the basis of adding 20% ​​acrylate prepolymer with an epoxy value of 0.
4mol/kg, the powder coatings prepared by using different types of acidolysis agents (IPA, BDA, ADA, CHDA) on modified polyester resin were investigated.
The effect of Tg is shown in Figure 2
.

It can be seen from Figure 2 that the difference in Tg of the coatings cured at 160 °C and 180 °C is small, indicating that the modified polyester shows higher reactivity under low temperature curing, and the coating has higher curing To a certain extent, the modified polyester can meet the application of low temperature curing occasions
.
Compared with BDA and ADA, the modified polyester synthesized by using IPA and CHDA as acid hydrolyzing agent has less difference in Tg of coatings cured at 160℃ and 180℃, and the coating curing degree is higher, indicating that when it is used as acid hydrolyzing agent The prepared modified polyester can show higher reactivity
.

The curing conditions were selected at 160°C and 15min, and the effect of acidolyzing agent on the properties of modified polyester and powder coatings was investigated.
The results are shown in Table 6 and Figure 3
.

It can be seen from Table 6 that the impact resistance, leveling effect and storage stability of the powder coatings prepared by using CHDA as the acid hydrolyzing agent are better
.
Compared with the aromatic dibasic acid IPA, the six carbon atoms in the alicyclic structure of CHDA are not in the same plane, and the six-membered ring can change between the chair structure and the boat structure when it is impacted to offset part of the impact energy.
Therefore, Good impact resistance, and IPA is a meta-position benzene ring structure, the synthetic polyester molecular segment has strong rigidity, so the synthetic polyester Tg is high, and the prepared coating has poor impact resistance
.
Compared with the aliphatic dibasic acids BDA and ADA, the structure of CHDA contains a saturated six-membered ring, which shows a certain rigidity.
The synthesized polyester has a higher Tg, and the prepared powder coating has better storage stability.
Although the polyester prepared from aliphatic dibasic acid has better molecular chain flexibility and appropriately improves the impact resistance of the coating, it will significantly reduce the Tg of the polyester, resulting in decreased storage stability
.

As can be seen from Figure 3, the powder coating prepared by the modified polyester synthesized using the aromatic dibasic acid IPA as the acid hydrolyzing agent has the most obvious decrease in gloss retention during the artificial accelerated aging test, while the aliphatic or cycloaliphatic The powder coating prepared by the modified polyester synthesized with dibasic acid as acid hydrolyzing agent showed better aging resistance, and the gloss retention and color difference during aging were better than those synthesized by IPA as acid hydrolyzing agent.
Among them, the powder coating prepared by modified polyester synthesized by CHDA as acidolysis agent has the highest gloss retention and the lowest color difference in the artificial accelerated aging process
.
This is because IPA contains a benzene ring structure, which is more prone to photoaging, resulting in the degradation of the polyester molecular chain and the color change of the coating.
Therefore, in the aging process, the polyester molecular chain is not prone to aging and fracture, showing better aging resistance
.

Based on the above experimental results, it is most suitable to use CHDA as the acid hydrolyzing agent of the new modified polyester
.

2.
5 Comparative analysis of powder coating performance

The powder coatings prepared by the new low-temperature curing super-weather-resistant polyester resin were synthesized using the above optimal conditions, and the performance was compared with the powder coatings prepared by conventional super-weather-resistant polyester.
The results are shown in Table 7
.

As can be seen from Table 7, the conventional properties of the super-weather-resistant powder coating prepared from the self-synthesized new low-temperature curing super-weather-resistant polyester cured at 160 °C are basically the same as those of the conventional super-weather-resistant polyester powder coating cured at 180 °C.
, but the powder coating prepared by the new polyester has obvious improvement in impact resistance, heat resistance and weather resistance, and the weather resistance is better than that of conventional ultra-weather-resistant powder coatings
.

3 Conclusion

In this study, an acrylate prepolymer with an epoxy value of 0.
4 mol/kg and an addition amount of 20% was used to synthesize modified polyester, and CHDA was used as an acid hydrolyzing agent.
The compatibility problem of acrylic resin overcomes the defects of polyester resin in weather resistance and can achieve low temperature curing, and the prepared powder coating shows better comprehensive performance in impact resistance, leveling effect, aging resistance and storage stability.
It has broad application prospects in the field of low-temperature curing ultra-weather-resistant coatings
.