Synthesis of acrylic modified polyester resins for powder coatings

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In recent years, the national environmental protection laws and regulations have become more and more stringent, vigorously promote environmental protection and energy-saving products, and implement circular economy and environmental protection key projects, especially in the context of the introduction of consumption tax in the coating field.

The output of powder coatings in my country has been ranked first in the world for many years.
The main types are pure epoxy, pure polyester and epoxy-polyester hybrid.
Most outdoor weather-resistant powder coatings are pure polyester.
Polyester coatings are widely used, with full coating film and excellent mechanical properties.
However, they are limited by the narrow selection of conventional monomers for synthetic polyester and the high cost of special monomers, and the molecular structure contains easily hydrolyzed and poor aging properties.
Restricted by the structure of the ester bond, it is very difficult to further improve the aging resistance and chemical resistance of polyester powder coatings.
Acrylic paint has excellent light resistance, outdoor aging properties and chemical corrosion resistance, but the resin is brittle, and the mechanical properties such as the impact resistance of the coating film are slightly poor.

It is precisely because the above two types of coatings have their own strengths and weaknesses, in the field of water-based and solvent-based coatings, there have been more literature reports on combining the two types of resins and modifying them.
At present, there are three main methods for preparing acrylic resin modified polyester resin: direct physical blending method, graft copolymerization method, and esterification graft method.
Due to the great difference between acrylic resin and polyester resin in terms of surface tension, the coating film will have defects such as shrinkage and loss of gloss after simple physical blending.
The synthesis process of graft copolymerization is difficult to control, and gelation is extremely easy to occur, and there are a large number of ungrafted acrylic and polyester macromolecules in the system, and the improvement of the compatibility of the system is limited.
The reaction process of the esterification grafting method is well controlled, and the raw materials can be selected in a wide range.
It has obvious advantages for the modification of polyester-acrylic resin.
In this study, an acrylic resin prepolymer containing a certain amount of carboxyl functional groups and a polyester resin prepolymer containing hydroxyl functional groups were grafted together by esterification grafting to prepare an acrylic modified polyester resin for powder coatings.

1 Test part

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Methacrylic acid: industrial grade, Taiwan Plastics; methyl methacrylate: industrial grade, Mitsubishi Japan; butyl methacrylate, industrial grade, Degussa; di-tert-butyl peroxide, industrial grade, Akzo; xylene , Industrial grade, Maoming Petrochemical; n-dodecyl mercaptan, industrial grade, US CPCHEM; neopentyl glycol: industrial grade, BASF; terephthalic acid: industrial grade, Zhuhai BP; isophthalic acid: industrial grade, Korea Lotte; Trimethylolpropane: Industrial grade, Pastor; Adipic acid: Industrial grade, Liaohua; Monobutyl tin oxide, 93/7 type TGIC curing powder coating polyester, TGIC, titanium dioxide, sulfuric acid Barium, leveling agent, brightening agent, benzoin, etc.
are all commercially available industrial products.

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2 Synthesis of acrylic prepolymer

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1 Reference formula

The synthetic reference formula of acrylic prepolymer is shown in Table 1.

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2 Synthesis process

Into a 2000 mL glass four-necked flask equipped with heating, electric stirrer, thermometer, feed port and fractional distillation and condensation device, add part of the formula amount of xylene, heat it to 120°C and constant temperature, and pour nitrogen into it.
Mix the monomer, initiator and chain transfer agent evenly, place them in a constant pressure dropping funnel, wash the container containing the mixed liquid with a small amount of xylene and merge the washed liquid into the dropping funnel, and put the mixed liquid in the dropping funnel within 2h Add dropwise to a four-necked flask, heat up to 130°C for reflux reaction for 3h, gradually heat up to 250°C to distill the solvent out.
Finally, vacuum is applied to remove unreacted monomers and other small molecules under negative pressure.
After the vacuum is released, the temperature is lowered and the material is discharged to obtain an acrylic resin prepolymer.

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3 Synthesis of polyester acid prepolymer

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1 Reference formula

The reference formula for the synthesis of polyester prepolymer is shown in Table 2.

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2 Synthesis process

Into a 2000mL glass four-necked flask equipped with heating, electric stirrer, thermometer, feed port and fractional distillation and condensation device, add the formula amount of alcohols, pour nitrogen into it, heat it until the material is melted, turn on stirring, then add acid and catalyst.
Continue to heat up the reaction until the esterified water starts to be generated and distilled out at 180°C, and then gradually rise to 250°C, and the reaction system is clear and free of particles, and the acid value reaches the requirement, and then the reaction system is clear and free of particles after the reaction is 2~5h to 95% of the esterified water is discharged.
Saturated polyester prepolymer.

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4 Synthesis of acrylic modified polyester resin

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1 Reference formula

Refer to Table 3 for the reference formulation of acrylic modified polyester synthesis.

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2 Synthesis process

Add the acrylic resin prepolymer and polyester prepolymer prepared above and a small amount of esterification catalyst into a 1000mL glass four-neck flask equipped with heating, electric stirrer, thermometer, feeding port and fractional distillation and condensation device.
Nitrogen, heat to 150℃, start stirring, then gradually raise the temperature to 250℃ to react for 2h, vacuum for 0.
5h, remove the residual small molecules in the system under negative pressure, release the vacuum and cool down and discharge to obtain acrylic modified polyester resin .

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5 Preparation of powder coating

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1 Reference formula

The reference formula of powder coating is shown in Table 4.

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2 Preparation process

The raw materials in the formula in Table 4 are measured and weighed, pre-mixed uniformly, and then melted and extruded with a twin-screw extruder, tableted, crushed, and then the flakes are crushed and screened to make powder coatings.

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6 Testing and Characterization

Perkin Elmer SPCTURM TWO infrared spectrometer was used to measure the infrared spectrum; the glass transition temperature of the polyester resin was measured according to GB/T 19466.
2-2004, and the heating rate was 10 K/min; the acid value was measured according to the Q/QTCL1-2014 standard , Reactivity, softening point, gelation and flow properties; measured resin viscosity according to ASTM D4287; measured adhesion according to GB/T 9286-1998; measured pencil hardness according to GB/T 6739-1996; measured according to GB/T 1732-1993 Impact resistance; measure gloss according to GB/T 9754-2007; conduct neutral salt spray test according to GB/T 1771-2007; measure weather resistance of coating according to GB/T 14522-2008.

2 Results and discussion

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Figure 1 is a comparison of the infrared spectrum of the mixture of polyester and acrylic prepolymer before the grafting reaction and the infrared spectrum of the graft copolymer after the grafting reaction.
Judging from the infrared absorption peaks, the shapes before and after the reaction are basically similar, because the esterification grafting reaction only consumes part of the carboxyl and hydroxyl groups, and the number of other groups changes little.
In the figure, 3550 cm-1 is the stretching vibration absorption peak of the hydroxyl group.
Before the grafting reaction, there are more hydroxyl and carboxyl functional groups in the system.
The grafting reaction is an esterification reaction, which needs to consume the carboxyl and hydroxyl groups.
After the grafting, there are more hydroxyl and carboxyl functional groups.
The peak basically disappeared; the C==O stretching vibration peak on the ester group is at 1720 cm-1, the methylene swing vibration absorption peak is at 1263 cm-1, and the CC stretching vibration absorption peak is at 1099 cm-1.
From the comparison of the spectra, the absorption peaks at various points are weakened, because the two resins of different systems are combined to carry out a chemical reaction and mixed uniformly, which plays a role of dilution.

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2 Resin DSC test results

Figure 2 shows the DSC test curve of "cold stitching" modified resin and acrylated graft modified polyester resin by the physical blending method of acrylic acid and polyester.
From the spectrum, the left picture shows the acrylic polyester resin modified by physical blending, and the DSC spectrum shows two glass transitions; while the right picture has only one glass transition for the copolymer modified by esterification and grafting.
, Tg is 65.
98℃, which can meet the requirements of outdoor powder coatings.

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3 Performance of resin and powder coating

Table 5 compares the properties of the "cold blending" modified resin, acrylated graft modified polyester resin and TGIC curing type conventional commercially available 93/7 polyester resin and formulated into powder by the physical blending method of acrylic acid and polyester Performance after coating.
From the data in Table 5, it can be seen that the performance of the resin powder coating modified by the "cold stitching" of acrylic and polyester physical blending method is too poor, the coating will fall off directly after the impact test, and the adhesion is poor, which cannot meet the performance requirements of the powder coating.
The performance of acrylic modified polyester resin is basically close to that of conventional polyester, which can meet the requirements of powder coatings.
After the artificial accelerated aging test of QUVB, the gloss retention rate is higher than that of ordinary polyester.

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Figure 3 shows the comparison of the appearance of the powder coating film modified by acrylic and polyester by "cold stitching" and chemical grafting.

3 Conclusion

In the experiment, an acrylic modified polyester resin that can be applied to powder coatings was synthesized, and the monomer types and composition of the acrylic prepolymer and polyester prepolymer were optimized through the formulation design, and the adhesive with superior comprehensive performance in the field of coatings can be obtained.