Study on the Preparation of Coatings Based on Michael Addition Principle

Existing research shows that using Real Michael Addition reaction (RMA) as a curing technology is expected to solve the inherent defects of high curing temperature and high energy consumption of traditional coatings

At present, many research institutes and enterprises have successively reported coatings cured based on the Michael addition principle

In this study, two kinds of solid dicarbonyl resins with different structures, acetoacetate resin and malonate resin, were synthesized by melt polycondensation method, and a series of coatings were obtained by Michael addition reaction with multifunctional acrylate

1 Experimental part

1.

Terephthalic acid: Industrial grade, Zhuhai BP Chemical Co.

Fourier transform infrared spectrometer (FT-IR): iS50R, Thermo Fisher Scientific, USA; nuclear magnetic resonance spectrometer (NMR): AVANCEIIIHD400, Bruker, Germany; differential scanning calorimeter (DSC): DSC3, METTLER TOLEDO; thermal Gravity analyzer (TGA): STA409PC, NETZSCH, Germany; dynamic mechanical analyzer (DMA): DMA+300, France Matt Weber; electronic universal testing machine: CMT-5105GD, Sansitaijie

1.

The basic formula of malonate resin/acetoacetate resin is shown in Table 1

Table 1 Resin base formula

1.
3 Preparation of the coating

Add dicarbonyl resin and acrylate derivative (curing agent) according to the theoretical group ratio of dicarbonyl resin and acrylate at 1:1, mix them in dichloromethane, control the solid content to be 15%~20%, and add 2% of the total mass The catalyst DBU was uniformly dispersed and coated on the alcohol-wiped dried tinplate with a 60 μm wire rod
.
Place the iron plate in a 70°C oven for 1~3h to form a film, and after cooling, the coating is naturally peeled off for relevant tests
.
The coating sample numbers are shown in Table 2
.

Table 2 Coating sample number

1.
4 Testing and Characterization

Nuclear magnetic resonance spectroscopy was used for NMR characterization, CDCl3 was used as solvent, and the frequency was 400MHz; Fourier transform infrared spectroscopy was used for infrared characterization, resin and acrylate were pressed with KBr, and the coating was directly tested; Thermogravimetric analyzer (TGA) was used for thermal analysis.
Performance analysis, temperature range: 30~600°C, heating rate 10°C/min, nitrogen as protective gas; DSC curve was tested by differential scanning calorimeter, heating rate 10°C/min, nitrogen atmosphere, resin and coating The glass transition temperature is the midpoint of the thermal transition in the secondary heating curve; the dynamic thermomechanical properties of the coatings were tested in the film stretching mode with a dynamic thermomechanical analyzer (DMA) at a frequency of 1 Hz, and the heating rate was 5 °C/min.
, nitrogen atmosphere, the coating size is 30mm×6.
5mm×0.
1mm
.
The tensile test is carried out according to GB/T1040.
3-2006
.
The crosslinking density Ve of the coating is calculated by formula (2)
.

where E and T are the storage modulus and temperature at 50°C above Tg, respectively; R is the gas constant, 8.
314J/(K·mol)
.

2 Results and discussion

2.
1 H NMR spectrum of dicarbonyl resin

Both the malonate resin and the acetoacetate resin are dicarbonyl resins, and Figure 1 shows the H NMR spectra of the two, respectively
.

Fig.
1 1H NMR spectra of malonate resin and acetoacetate resin

It can be seen from Figure 1(a) that δ=8.
0 is the proton peak on the benzene ring, δ=4.
0~4.
2 is the proton peak of methylene in neopentyl glycol, and the proton peak of active C—H is at δ=3.
4, δ= 0.
78~1.
23 are the proton peaks of methyl group in neopentyl glycol
.
In Figure 1(b), the proton peak of active C—H also exists at δ=3.
4, and the proton peak of acetyl group also appears at δ=2.
2
.
The above results indicated that malonate resin and acetoacetate resin were successfully prepared
.

2.
2 Tg of dicarbonyl resin

The DSC curve of the dicarbonyl resin is shown in Figure 2, and the midpoint of the thermal transition in the second heating curve is taken as the glass transition temperature of the resin
.

Fig.
2 DSC curve of resin

It can be seen from Figure 2 that the Tg of the malonate resin and the acetoacetate resin are 51.
0°C and 31.
3°C, respectively
.
This difference is because tert-butyl acetoacetate acts as an end cap on the resin, reducing the relative molecular weight of the resin, resulting in a decrease in Tg, while diethyl malonate has two reactive ester groups, which increases the relative molecular weight of the resin.
, Tg is relatively high
.

2.
3 Infrared characterization of coatings

Acrylate derivatives are used as cross-linking agents to prepare coatings by reacting with dicarbonyl resin under the action of Lewis base.
Due to the high activity of Michael addition reaction, the reaction can be carried out at relatively low temperature, and the cured coating is insoluble in conventional organic solvents
.
Taking the infrared spectra of malonate resin, TMPTA and its coating P2 as examples to analyze the structure of reactants and products, the results are shown in Figure 3
.

Fig.
3 Infrared spectra of malonate resin, TMPTA and its coating P2

It can be seen from Figure 3 that the absorption peak intensities of coating P2 at around 1640cm-1 (stretching vibration peak of CH=CH2) and around 808cm-1 (out-of-plane vibration peak of C=C) basically disappear, indicating that dicarbonyl The resin and acrylate derivatives undergo Michael addition reaction
.

2.
4 Thermal properties of coatings

The thermal performance test results of the coatings are shown in Table 3 and Figure 4
.

Table 3 Thermal data of coatings

Figure 4 Thermal performance curve of the coating

It can be seen from the DMA curve in Figure 4(a) that with the increase of temperature, the storage modulus of the coating decreases continuously.
When the temperature reaches Tg, the storage modulus of the coating decreases sharply, and the loss factor reaches the maximum value at this time.
The Tg of the coating measured by DMA is between 18.
8 and 24.
4 °C, and the small peak of the loss factor around 35 °C is caused by the switching of the heating program
.
It can be seen from the DSC curve in Figure 4(b) that the Tg of the coating ranges from 8.
3 to 18.
2 °C, which is different from the DMA test results, mainly due to the different test methods.
Polymer enthalpy change
.
The Michael addition coating prepared by the same acrylate curing agent in the literature has a maximum Tg of -14 °C measured by DMA, which is much lower than the coating prepared in this study, which may be because the high Tg solid resin significantly improves the Heat resistance of the coating
.

Combining Figure 2 and Table 3, it can be seen that the Tg of the cured coating is lower than that of the dicarbonyl resin to a certain extent, which may be because the acrylate component is a more flexible structure, which reduces the rigidity of the cross-linked network
.
It can be seen from Table 3 that P1 has lower crosslinking density than P3 and P2 than P4
.
This difference is mainly due to the higher concentration of reactive groups in the prepared acetoacetate resin, which requires more acrylate curing agent to be consumed, resulting in an increase in the crosslinking density of the coating; secondly, the reactive group in the malonate resin The hydrogen atom is mainly located in the molecular chain, and the reaction has a large steric hindrance, while the active hydrogen atom in the acetoacetate resin is only located at the end of the chain, which is prone to group collision, and the cross-linking reaction is more sufficient
.

It can be seen from the TG curve in Figure 4(c) that the coating decomposes less before 200 °C, mainly due to the volatilization of the adsorbed water vapor and catalyst.
After 400 °C, the polymer begins to break down and decompose.
The specific thermal performance data See Table 3
.
The 5% mass loss temperature and carbon residue rate of P1 and P2 are lower than those of P3 and P4, respectively.
This may be because the malonate structure mainly exists in the middle of the molecular chain.
Due to steric hindrance and other factors, some active hydrogen atoms remain after curing.
, which is easily decomposed by heat, resulting in a decrease in thermal stability.
In addition, a relatively low crosslinking density will also lead to a decrease in the thermal stability of the coating
.

2.
5 Mechanical properties of coatings

Figure 5 is a stress-strain curve of the coating
.

Fig.
5 Stress-strain curve of the coating

It can be seen from Figure 5 that the cured coating has good mechanical properties and is not easy to be brittle during the tensile test
.
The tensile strength of the coating is 6.
0~10.
7MPa, the Young's modulus is 30.
1~129.
8MPa, and the elongation at break is 135.
2%~172.
3%
.
Although P3 and P4 have higher crosslinking density, they have lower tensile strength and modulus compared with P1 and P2, which may be because acetoacetate resin needs to consume a large amount of acrylate curing agent , which increases the flexibility of the coating molecular chain
.
The good mechanical properties are mainly due to the rigid molecular chain structures such as ester groups and benzene rings, and secondly, the moderate crosslinking density also gives the coating a certain toughness
.

3 Conclusion

Through formula design, malonate resin and acetoacetate solid resin were prepared with Tg of 51.
0℃ and 31.
3℃, respectively, and their structures were verified by NMR spectroscopy
.
The performance of the cured coating was characterized, and the following conclusions were drawn: the solid resin with high Tg significantly improved the heat resistance of the coating, and the Tg of the coating measured by DMA was 18.
8~24.
4℃; The ester resin coating has higher Tg and thermal stability, which may be due to higher crosslinking density; in terms of mechanical properties, the tensile strength and modulus of the acetoacetate resin coating are relatively lower, which may be due to consumption Large amounts of curing agent lead to a decrease in chain rigidity
.
However, the experimentally prepared coatings still emit VOCs, and the heat resistance and mechanical properties of the coatings are still inferior to those of traditional liquid coatings and powder coatings
.