Study on Synthesis of Acrylic Modified Hydroxy Polyester for Polyurethane Powder Coatings

Abstract: Acrylic resin intermediates containing epoxy groups are synthesized by solution polymerization.

Both the infrared and DSC test results show that the epoxy group of the acrylic acid intermediate has been successfully grafted into the polyester molecular chain;

GPC test shows that the use of low molecular weight acrylic resin intermediates for synthesis can obtain acrylic modified hydroxy polyester with moderate molecular weight and molecular weight distribution; coating performance results show that the prepared coating has excellent MEK resistance and anti-aging performance

introduction

Powder coatings have the advantages of no solvent release during the coating process, less pollution, simple spraying, powder recyclability, and simple construction.

As an important branch of powder coatings, polyurethane powder coatings have outstanding characteristics of high leveling, high hardness and excellent chemical resistance.

Polyurethane powder coatings are mainly prepared from polyester resins and isocyanate curing agents.

Acrylic resin has excellent aging resistance and chemical resistance, but the resin also has the characteristics of hot stickiness and cold brittleness, and the prepared coating film has relatively poor impact resistance;

The excellent mechanical properties of polyester can just make up for the defects of acrylic resin, so acrylic modified polyester can make full use of the advantages of acrylic resin and polyester resin [, the use of acrylic resin to modify polyester has always been the development of the coating industry One of the hot spots

At present, the preparation methods of acrylic modified polyester are divided into physical splicing method and chemical grafting method

In contrast, the chemical grafting method connects the acrylic resin molecular chain to the polyester molecular chain through chemical bonds, which solves the compatibility problem of the two, and can prepare high-gloss and high-transparency coatings
.

In this study, an epoxy group-containing acrylic resin intermediate was synthesized by a solution polymerization method.
The reaction between the epoxy group and the carboxyl group was used to graft the acrylic resin into the polyester molecular chain, which improved the compatibility of the two.
Acrylic modified hydroxy polyester resin for polyurethane powder coating was obtained
.

1 experiment

1.
1 Main raw materials

Methyl methacrylate (MMA), industrial grade, Jilin Jilin Chemical; Butyl acrylate (BA), industrial grade, BASF; Styrene (St), industrial grade, Qilu Petrochemical; Lauryl methacrylate (LMA), industrial Grade, Guangzhou Hangqin; Glycidyl methacrylate (GMA), industrial grade, DOW; Xylene, industrial grade, Maoming Petrochemical; Lauryl mercaptan, industrial grade, Guangzhou Sanwang; tert-amyl peroxide acetate Esters, industrial grade, Arkema; ​​neopentyl glycol (NPG), industrial grade, BASF; butanediol (BDO), industrial grade, BASF; Trimethylolpropane (TMP), industrial grade, Pasto; Ethyl Butyl Propylene Glycol (BEPD), Industrial Grade, Pastor; Terephthalic Acid (PTA), Industrial Grade, Zhuhai BP; Isophthalic Acid (PIA), Industrial Grade, Lotte Chemical; Adipic acid (ADA) , Industrial grade, BASF; pyromellitic anhydride (PMDA), industrial grade, Wuhan Rongcan; monobutyl tin oxide (F4100), Arkema, France; curing agent B1530, titanium dioxide, barium sulfate, benzoin, leveling Agents, commercially available industrial products
.

1.
2 Main experimental equipment

30L Reactor: Self-assembly; Twin-screw extruder: SLJ-30AF, Yantai Donghui; Electrostatic spray gun: BA-28, Nanhai Dabu Plastic Spraying Complex; Impact Tester: QCJ, Tianjin Senrida Test Equipment Co.
, Ltd.
; Rotational viscometer: DV-Ⅱ type, BROOKFIELD company; Infrared spectrometer: Spectrum 65 type, PerkinElmer, USA; Differential scanning calorimeter: DSC1 type, Mettler Toledo company; Xenon lamp weathering test box: Q-SUN Xe-3 , Q-Lab; gel permeation chromatography (GPC): HLC-8420GPC, Tosoh (Shanghai) biotechnology Co.
, Ltd.
; solvent wipe instrument: RJCS, Hebei Technology Co.
, Ltd.
Hui mining
.

1.
3 Synthesis process of acrylic resin intermediate 

The acrylic resin intermediate formula is shown in Table 1
.
Weigh the raw materials according to the formula in Table 1, put xylene into the reactor, blow in nitrogen gas and raise the temperature to 120°C, and then mix the acrylate monomer, styrene, dodecyl mercaptan and 90% t-amyl peroxyacetate.
After the ester was mixed and stirred uniformly in advance, it was slowly added dropwise to the reaction kettle within 3 hours
.

After dripping, add the remaining 10% tert-amyl peroxyacetate and keep it warm for 4h
.
The temperature was raised to 150°C, and the solvent and unreacted monomers in the reactor were removed by vacuuming, and vacuuming was performed at -0.
098MPa for 3h
.
Finally, the vacuum is released and the acrylic resin intermediate is obtained
.

1.
4 Synthesis process of acrylic modified hydroxy polyester

The formula of acrylic modified hydroxy polyester is shown in Table 2
.
Weigh the raw materials according to the formula in Table 2, add the alcohol monomers to the reactor, pour nitrogen into the reactor, turn on the stirring, raise the temperature to 160°C and wait for the alcohol monomers to melt completely, then add the acid monomers and catalysts;

Slowly increase the temperature to 250℃, keep it at 250℃ for 2h, test the acid value after the resin is clear, control the acid value index to 12-16mgKOH/g; then lower the temperature to 210℃, and add the acrylic resin intermediate;

Continue the reaction at 210°C for 2 hours, take samples to test the acid value, and control the acid value index to 7-10 mgKOH/g
.
Begin vacuuming and maintain a vacuum of -0.
098MPa until the resin sample has an acid value index of 1~4 mgKOH/g and a hydroxyl value of 20~25mgKOH/g, release the vacuum, discharge the material, and obtain acrylic modified hydroxy polyester
.

1.
5 Preparation of powder coating

The reference formula of powder coating is shown in Table 3
.
The raw materials are weighed according to the formula in Table 3, and the raw materials are pre-mixed.
The pre-mixed material is extruded through a twin-screw extruder, and the powder coating is obtained through processes such as tableting, crushing, and sieving
.

Then, the prepared polyurethane powder coating was sprayed on the iron plate by electrostatic spraying.
The thickness of the coating was controlled at 60~70 μm.
The sample was placed in an electric oven and cured at 200°C for 10 minutes.
After cooling, the relevant properties of the coating were tested.

.

1.
6 Testing and Characterization

The acid value is measured according to GB/T 6743-2008; the hydroxyl value is measured according to GB/T 12008.
3-2009; the coating gloss is measured according to GB/T 9754-2007;

The viscosity of the resin is measured at 200°C using the American Boller fly cone and plate viscometer DV2THB; the impact resistance is tested according to GB/T 1732-1993;

The resistance to butanone wiping is tested according to GB/T 23989-2009; the resistance to salt spray is tested according to GB/T 1771-2007; the adhesion is tested according to GB/T 9286-1998;

The glass transition temperature is measured in accordance with GB/T 19466.
2-2004, using a nitrogen atmosphere, and the heating rate is 10°C/min
.

2 Results and discussion

2.
1 Infrared test analysis  

The infrared spectrum of acrylic resin intermediate and acrylic modified hydroxy polyester is shown in Figure 1
.

Figure 1 Infrared spectrum of acrylic resin intermediate, the peak at 910 cm -1 is the characteristic absorption peak of the epoxy group, while the peak at 910 cm -1 in the acrylic modified hydroxy polyester basically disappears, indicating that the reaction degree of the epoxy group is relatively higher.
High, the acrylic resin intermediate has been successfully grafted to the polyester resin;

The peak around 3500cm -1 is the stretching vibration absorption peak of the hydroxyl group.
This peak is relatively weak in the infrared spectrum of acrylic modified hydroxy polyester, which may be caused by the low hydroxyl value of the synthetic acrylic modified hydroxy polyester; in addition, 1700cm The peak around -1 is the characteristic absorption peak of the carbonyl group
.

2.
2 Test and analysis of resin Tg

Because acrylic resin and polyester resin are incompatible, physical blending of acrylic resin and polyester will show two independent glass transition temperatures when using DSC to test the T g of the resin
.

In order to verify that the acrylic resin intermediate in the synthesized acrylic modified hydroxy polyester has been successfully grafted into the polyester molecular chain, this paper uses DSC to test the T g of the synthesized acrylic resin intermediate and the acrylic modified hydroxy polyester.
The result is shown in Figure 2
.

From the resin Tg test results in Figure 2, it can be seen that the synthesized acrylic resin intermediate Tg is 54.
07°C, while the DSC test result of acrylic modified hydroxy polyester only has one glass transition at 56.
55°C, and there are no two independent glass transitions.
Tg
.

Therefore, it can be proved that the acrylic resin intermediate has been successfully grafted to the polyester molecular chain, and the compatibility of the two is good after grafting
.

2.
3 The influence of acrylic resin intermediate molecular weight on resin performance

In this paper, a series of acrylic resin intermediates with different molecular weights were synthesized, and the influence of the molecular weight of acrylic resin intermediates on the molecular weight of acrylic modified hydroxy polyester was tested.
The results and data are shown in Figure 3 and Table 4
.

Among them, acrylic modified hydroxy polyester A2 is synthesized using acrylic resin intermediate A1, acrylic modified hydroxy polyester B2 is synthesized using acrylic resin intermediate B1, and acrylic modified hydroxy polyester C2 is synthesized using acrylic resin intermediate C1
.

From the elution time data in Figure 3, it can be seen that the acrylic resin intermediate has been successfully grafted to the polyester molecular chain, so only a single elution peak appears in the GPC graph
.

The relative molecular mass and molecular weight distribution data of the resin in Table 4 show that as the molecular weight of the acrylic resin intermediate increases, the molecular weight of the synthesized acrylic modified hydroxy polyester also gradually increases, and the molecular weight distribution becomes wider
.

This may be because as the molecular weight of the acrylic resin intermediate increases, it is more difficult to unfold the acrylic molecular chain due to the incompatibility of the acrylic resin and polyester and the entanglement of the acrylic resin molecular chain;

Part of the epoxy groups on the acrylic molecular chain are embedded in the molecular chain and are not easy to participate in the reaction, resulting in a broadened molecular weight distribution of the synthesized modified polyester
.

Therefore, the use of low molecular weight acrylic resin intermediates for polyester modification is easier to obtain acrylic modified hydroxy polyester with moderate molecular weight and molecular weight distribution
.

Table 5 shows the performance test results of acrylic modified hydroxy polyester synthesized from acrylic resin intermediates of different molecular weights
.

The test results in Table 5 show that as the molecular weight of the acrylic resin intermediate increases, the viscosity and Tg of the synthesized acrylic modified hydroxy polyester increase, and the leveling effect and impact resistance of the prepared powder coating become worse
.

This may be because as the molecular weight of the acrylic resin intermediate increases, its compatibility with the polyester resin molecular chain becomes worse;

In addition, the molecular chain of the acrylic modified hydroxy polyester synthesized by grafting to the polyester molecular chain also becomes larger, and the molecular chain is more difficult to move, so the viscosity and Tg of the synthesized modified polyester increase;

As the viscosity of the modified polyester increases, the fluidity of the powder coating becomes worse during the curing process, so the leveling appearance of the cured coating is relatively worse
.

Among the molecular segments of the modified polyester, the acrylic resin molecular segment is a brittle segment, and its impact resistance is not as good as that of the polyester molecular segment.
The modified polyester synthesized by using low-molecular-weight acrylic resin is used;

The molecular chain of acrylic resin is more evenly distributed in the polyester, and the toughness of the polyester molecular chain can make up for the brittleness of the acrylic resin molecular chain.
Therefore, the modified polyester synthesized by the low molecular weight acrylic resin intermediate shows better impact resistance.

.

2.
4 Performance comparison of powder coatings

Table 6 shows the performance comparison of polyurethane powder coatings prepared from self-synthesized acrylic modified hydroxy polyester and conventional hydroxy polyester
.

It can be seen that the conventional properties of powder coatings prepared from self-synthesized acrylic modified hydroxy polyesters are basically the same as those of conventional hydroxy polyesters, but they are significantly better than those prepared from conventional hydroxy polyesters in terms of methyl ethyl ketone wiping and aging properties.
Powder coating
.

3 Conclusion

Through the formulation design, the acrylic modified hydroxy polyester for polyurethane powder coating was successfully synthesized
.
By synthesizing an acrylic resin intermediate containing epoxy groups, the reaction between epoxy groups and carboxyl groups is used to graft the acrylic resin into the polyester molecular chain, which improves the compatibility of the two and significantly improves the powder coating’s performance.
Resistance to butanone wiping performance and weather resistance
.

Both the infrared and DSC test results show that the epoxy group of the acrylic intermediate has successfully reacted with the polyester, and the acrylic resin and polyester have good compatibility; GPC test shows that the low molecular weight acrylic resin intermediate can be used to synthesize the molecular weight and molecular weight.
A moderately distributed acrylic modified hydroxy polyester
.

Source: Li Yong, Ma Zhiping, Xie Jing, Lin Xien, Zhang Enci (China National Electrical Research Institute Co.
, Ltd.
, Guangzhou, Guangdong 510300; Kinte Material Technology Co.
, Ltd.
, Dongguan, Guangdong 523981)