Synthesis of Polyester Resin for β-Hydroxyalkylamide Low Temperature Curing Powder Coatings

0 Preface

Powder coatings have developed vigorously around the world with their own advantages of pollution-free, energy-saving, and low VOC

At present, low-temperature curing powder coatings include UV-curing powder coatings, indoor low-temperature curing powder coatings and outdoor low-temperature curing powder coatings.

In this paper, by studying the formulation and technology of polyester resin for low-temperature curing powder coatings, introducing monomers with different reactivity and flexibility, changing the molecular structure of the polyester resin, and synthesizing a carboxyl-terminated polymer suitable for low-temperature curing of β-HAA curing agent Ester resin

1 Test part

1.

Neopentyl glycol (NPG), 2-methyl-1,3-propanediol (MPD), 2-ethyl-2-butyl-1,3-propanediol (BEPD), ethylene glycol (EG), p-benzene Dicarboxylic acid, isophthalic acid, 1,4-cyclohexanedicarboxylic acid (CHDA), adipic acid, F4100 (monobutyl tin oxide), T105 (β-HAA), titanium dioxide, barium sulfate, leveling agent , Benzoin, brightener, etc.

1.

8L small stainless steel reactor; φ30 twin screw extruder and other small powder making equipment; small electrostatic spraying equipment; impact tester and other coating performance testing equipment

1.

Add polyol, polyacid, and catalyst (F4100) into an 8L reactor according to the ratio.

1.

The powder coating is prepared according to the basic formula in Table 1.

1.

The resin and curing agent are mixed in proportion, then crushed and sieved.

1.
6 Gel time detection

Place the prepared powder coating on a copper plate at 180°C and stir continuously.
The time from the beginning of melting to the generation of gel is the gel time, which is calculated in s or min
.

1.
7 Tg determination

Rigaku TG/DTA thermal analyzer, heating rate 10℃/min
.

2 Results and discussion

2.
1 The effect of raw material monomer on the Tg of resin

The relationship between the Tg of the polyester resin and the structure of the monomer: The Tg of the resin depends on its composition and structure, such as the ratio of rigid and flexible monomers in the main chain, the presence or absence of branches, and the size of steric hindrance
.
From the perspective of the monomers used in the synthesis, different monomers and their ratios have a great influence on the Tg of the resin
.
The Tg of the resin has a great influence on the storage stability of the powder coating
.
The formulation monomers studied in this paper are mainly selected from NPG, MPD, terephthalic acid, isophthalic acid, adipic acid, CHDA, etc.
, and are synthesized by a two-step method.
The first step is excess alcohol, and acidolytic agent is added after the reaction reaches a clear point.
Carry out acid hydrolysis, end-capping, and finally vacuum polycondensation and adding additives to obtain sample resin
.
The following research ratios are all the ratios of materials to alcohol or acid substances
.

The polyol is a mixture of NPG and MPD, and the polyacid is terephthalic acid and isophthalic acid
.
It can be seen from Figure 1 that as the amount of MPD increases, the Tg of the resin basically decreases in a linear trend.
 

Polyol is a mixture of NPG and EG, and polyacid is a mixture of isophthalic acid and CHDA.
It can be seen from Figure 2 that as the content of CHDA increases, the Tg of the resin also shows a downward trend, but CHDA has no effect on the Tg of the resin.
The difference between the Tg of the resin synthesized with isophthalic acid and CHDA is only 2℃
.

Generally speaking, the main chain of a molecule is composed of saturated single bonds.
Since the molecule easily rotates around the single bond, the activity of the chain segment increases and the Tg decreases
.
If rigid groups such as phenyl, biphenyl or heterocyclic compounds are introduced into the main chain to hinder the rotation of the single bond, the Tg will be increased; the presence of branched chains also has a certain influence on the Tg, and bulky branched groups produce Space obstacles hinder the rotation of single bonds, thereby increasing Tg; polar group branches hinder the activity of molecular segments due to their interaction forces, which will also lead to an increase in Tg
.
Therefore, the Tg of the polyester resin can be adjusted by selecting the raw material monomers and changing the ratio of the components
.
 

2.
2 The influence of different polyols on the performance of resins and powder coatings

The influence of different polyols on the properties of resins and powder coatings is shown in Table 2
.
The polybasic acid used in the synthesis is terephthalic acid: isophthalic acid = 10:1
.

It can be seen that the resin synthesized with NPG has the smallest viscosity, and the resin synthesized with MPD has the largest viscosity.
At the same time, 4 different polyols have little effect on the reactivity of the resin, and the gel time has a certain difference.
It is prepared with the polyester resin synthesized by MPD.
The powder coating has good appearance, gloss and thick film pinhole resistance, excellent impact resistance, and can pass both positive and negative impact
.
 

2.
3 The influence of different polybasic acids on the performance of resins and powder coatings

The effect of different polybasic acids as acidolytic agents on the performance of resin and powder coatings is shown in Table 3
.
The materials in the first step are NPG and terephthalic acid
.

It can be seen that the polyester resin synthesized with adipic acid has the lowest viscosity.
This is because adipic acid is a long-chain aliphatic monomer, and its molecular chain is relatively flexible and easy to reduce its viscosity
.
At the same time comparing the reactivity and gel time, the resin synthesized with CHDA is the shortest, and the resin synthesized with adipic acid is the longest.
The addition of CHDA improves the flexibility of the molecular chain, improves the reactivity of the resin, and significantly shortens the gelation.
Time, the coating has good leveling and excellent mechanical properties
.
 

2.
4 Research on the mechanical properties of low-temperature curing powder coatings

Table 4 is the comparative data of the mechanical properties of the synthesized sample resin and the resin cured under normal conditions (180°C/15min) under different curing temperature conditions
.
Figure 3 shows the gel time of powder coatings at different temperatures
.

It can be seen that the gelation time of the two resins at different temperatures is different.
The gelation time of powder coatings decreases rapidly with the increase of temperature.
Generally speaking, the temperature increases, the faster the reaction rate increases, that is, the gelation of powder coatings.
The faster the reaction time changes with the increase in temperature, the higher the activation energy required for the reaction, and the greater the energy required
.
As the temperature increases, the rate of decline of the conventional resin is significantly faster than that of the sample resin, indicating that the sample resin can be reacted and cured at a lower temperature than the conventional resin
.
It can be seen from Figure 3 that the gelation time of the powder coating prepared by the sample resin measured at 160°C is not much different from the gelation time of the powder coating prepared by the conventional resin at 180°C.
This can indicate that the sample resin is in The reaction speed at 160°C is equivalent to that of conventional resin at 180°C
.
At the same time, compare the impact resistance of the sample resin at 160℃/15min and the conventional resin at 180℃/15min.
The positive and recoil of both can pass.
It can be seen that the curing degree of the coating film should be the same when the two are cured for 15min under different temperature conditions.
Big
.

In addition, in the resin synthesis process or powder making process, you can also add a curing accelerator to reduce the curing temperature, but due to the different curing mechanisms of β-HAA and TGIC [3], the selected curing accelerator is also different from the curing accelerator of the TGIC category , This work is not discussed in this article for the time being
.

3 Conclusion

In this paper, a carboxy-terminated polyester resin suitable for low-temperature curing of β-HAA system is synthesized by selecting appropriate polyols and polyacids.
The powder coatings prepared have excellent mechanical properties and good overall performance at lower curing temperatures, which are comparable to conventional resins.
The performance of the coating cured at normal temperature is equivalent, which can replace conventional temperature curing and has high application value
.

At present, the whole world is facing the crisis of energy shortage.
Therefore, low-temperature curing powder coatings have better development prospects.
In particular, β-HAA series powder coatings will be vigorously developed due to their low curing temperature and low toxicity
.