Development of Low Temperature Curing Polyester Resin for Outdoor Powder Coatings

introduction

Solvent-based products still occupy a large proportion in China's industrial coatings.

In terms of protecting environmental resources, improving people's quality of life, and protecting human health and safety, the development of environmentally friendly coatings will become an inevitable trend

As a solvent-free solid coating, powder coatings have almost zero VOC emissions and greatly reduce environmental pollution.

The curing temperature of most systems in powder coatings is 180-200 °C, which is higher than other coatings.

Low temperature curing can reduce more energy consumption and greatly reduce the cost of powder coating application

(1) The film forming temperature is low, which can save a lot of energy.

(2) The low curing temperature can further expand the application field of powder coatings

In 2010, triglycidyl isocyanurate (TGIC) was listed as an outdated product by the Ministry of Industry and Information Technology, which promoted the rapid development of the polyester/HAA system.

However, we believe that the polyester/HAA system has a promising future through continuous improvement, and it still has greater advantages than the polyester/TGIC system in terms of toxicity, reactivity, and storage stability

In this paper, carboxyl-terminated polyester resins for low temperature curing powder coatings in polyester/HAA system were synthesized, and the system was studied
.

2 Experimental part

2.
1 Experimental materials

Neopentyl glycol (NPG), ethylene glycol (EG), trimethylolpropane (TMP), 1,4-cyclohexanedimethanol (CHDM), terephthalic acid (PTA), isophthalic acid ( IPA), adipic acid (ADA), esterification catalyst, acid hydrolysis agent, antioxidant, HAA curing agent, titanium dioxide, leveling agent, benzoin, etc.
are all industrial products
.

2.
2 Synthesis of polyester resin

Put the polyol, polyacid and catalyst into the 2L reactor equipped with distillation column, stirrer and thermometer according to the formula quantity, and stir evenly
.
Under nitrogen protection, gradually heat up to 180-250 °C for a certain period of time for polycondensation reaction;

Add acid hydrolyzing agent to adjust the acid value, then vacuumize, control the polyester end point, add antioxidant, cool down, and obtain samples with acid value and viscosity meeting the requirements, which are resins A, B, C, and D respectively
.

2.
3 Powder coating and coating preparation

The basic formula of powder coating is shown in Table 1
.

Make powder according to the basic formula in Table 1, and its technological process is: batching, mixing, extrusion, crushing and sieving
.
The powder coating was electrostatically sprayed and cured at 160 °C for 15 min to obtain a coating, and the coating and coating properties were tested
.

2.
4 Performance Characterization

The determination of polyester resin acid value, softening point, gel time of powder coating and coating film performance were carried out according to the corresponding national standards; the melt viscosity was measured by ICI laminar viscometer at 175 ℃;

The glass transition temperature and curing curve were tested by German NETZSCH DSC 200PC, the heating rate was 10 K/min, and the nitrogen atmosphere was used;

The aging resistance of the coating film is tested according to ASTM G 154, using the QUV/Spray ultraviolet accelerated aging tester of Q-Lab Company in the United States, UVB-313 fluorescent ultraviolet lamp, wavelength 310nm, irradiation intensity 0.
71 W/m2, 4 h UV/60 °C, 4 h condensation/50 °C, 240 h
.

3 Results and Discussion

3.
1 Properties of polyester resin P9250PR and its powder coating coating

The properties of polyester and its powder coating are shown in Table 2
.

3.
2.
Effect of EG dosage on artificial aging properties

The effect of EG content on the properties of polyester resin is shown in Table 3, and the effect of EG content on the weather resistance of the coating is shown in Figure 2
.

The polyol in Table 3 is a mixture of neopentyl glycol, ethylene glycol, and 1,4-cyclohexanedimethanol
.
Without ethylene glycol, the gloss retention rate was 70% after 240 h of QUVB313 artificial accelerated aging
.

As the amount of ethylene glycol in the polyol increases, the amount of neopentyl glycol decreases.
After artificially accelerated aging, the gloss retention rate drops sharply.
When 10% ethylene glycol is used, the gloss retention rate of the coating is only 43%.
When the ethylene glycol content increased to 30% to 50%, the gloss retention rate dropped to only 30%
.

It shows that with the increase of ethylene glycol content, the weather resistance of the coating film decreases greatly
.
The weather resistance decreases, and the coating is easy to age, which makes the material brittle, the mechanical properties decrease, and the service life is shortened
.

When synthesizing polyester, ethylene glycol that is not conducive to weather resistance should be used as little as possible
.
At the same time, it can be seen from Table 3 that with the increase of the amount of ethylene glycol, the glass transition temperature of polyester decreases
.

3.
3 Reactivity at different temperatures

The reactivity at different temperatures is shown in Figure 3
.

The reaction temperature directly affects the reactivity, thereby affecting the curing speed.
Figure 3 shows the gelation time of powder coatings measured at different temperatures
.

From the relationship between gelation time and temperature, it can be seen that at low temperature, the reactivity is low, the reaction time is long, and the gelation time is long; as the temperature increases, the reactivity increases, the reaction is accelerated, and the gelation time is shortened
.
As the curing temperature increases, the time required for curing of powder coatings decreases gradually
.

The esterification reaction between the hydroxyl group in the β-hydroxyalkylamide and the carboxyl group of the polyester will generate water molecules, so if the curing rate is too fast, the melt viscosity of the coating will rise rapidly, and the water molecules generated in the powder coating reaction process are not easy to Spill from the coating, causing pinholes in the coating film
.

If the curing rate is gentler and the melt viscosity of the coating rises slowly, the water generated in the early stage of the reaction can be easily removed, which can increase the thickness of the coating film when pinholes occur and improve the compactness of the coating film
.

We chose to cure the coating film at 160 ℃ for 15 min with better surface leveling, and the maximum film thickness of pinholes can reach 110 μm
.

3.
4 Curing reaction

β-Hydroxyalkylamide has 4 functional groups and has high activity.
The esterification reaction of conventional aliphatic alcohol needs to be higher than 225 ℃ or the catalyst must be used
.

On the other hand, β-hydroxyalkylamides can react with carboxyl groups from 150 °C at relatively low temperatures, and so far there is no effective curing accelerator that can adjust the curing speed
.

Activation energy is the minimum energy required in a chemical reaction from reactant molecules to activate molecules
.
Only when the energy of the colliding molecules equals or exceeds this activation energy, an effective collision is possible and the reaction can proceed
.

The rate of chemical reaction is closely related to the size of its activation energy.
A low activation energy can make more reactant molecules become activated molecules, and the reaction rate will be accelerated.
Therefore, reducing the activation energy will effectively promote the reaction
.

In addition, increasing the concentration of reactants can increase the total number of activated molecules and speed up the reaction rate
.

Low temperature curing can reduce the reaction activation energy by adding catalysts or using raw materials with higher reactive functional groups
.

However, there is no suitable catalyst for the polyester/HAA system to adjust the activation energy of the reaction, so it can only be achieved by improving the reactivity of the polyester
.

Shortening the curing time can be achieved by increasing the concentration of the reactants
.
The acid value is an indicator of the content of reactive groups—carboxyl groups in the resin, and the higher the acid value, the larger the carboxyl group content
.

That is to say, there are many carboxyl groups that can participate in the reaction on average, and the total number of molecules participating in the reaction per unit volume increases, and the number of activated molecules per unit volume will increase, which can speed up the chemical reaction rate
.
When the acid value is high, the carboxyl group content increases, the amount of curing agent is large, the crosslinking density is large, and the viscosity rises rapidly, which will affect the fluidity of the coating film
.

In order to make the coating film have good appearance and mechanical properties, on the one hand, an acid hydrolyzing agent with suitable reactivity should be selected, and on the other hand, the viscosity of the polyester needs to be reduced to achieve sufficient melt flow before polymer crosslinking
.

However, when reducing the viscosity, it is necessary to ensure that the polyester resin has a sufficiently high glass transition temperature to make the storage stability of the powder coating good
.

Figure 4 shows the non-isothermal curing curves of the powder coatings of three different ratios of polyester/HAA system heated from 0 ℃ to 250 ℃ at 10 K/min
.

Table 4 shows the properties of the three samples
.
Before the powder coating is cured, there is an endothermic peak at 50-70 °C, which is the glass transition region of the powder coating.
Since HAA is insoluble in polyester, it has little effect on the glass transition temperature (Tg) [2], The glass transition temperature here varies with the glass transition temperature of the polyester resin
.

The temperature continued to increase, and a step appeared between 90 and 100 °C, which was the beginning of the powder melting, resulting in a change in the heat capacity of the material
.
Sample 1 started melting at only 91.
9 °C, so it could melt and level well before solidifying
.

In the powder curve of sample 1, there is an obvious endothermic peak at about 120 ℃, which is the melting endothermic peak of HAA.
Because the amount of HAA in sample 1 is relatively large, the peak is more obvious.
In 3, there is almost no obvious peak, and the absorption peak weakens as the amount of HAA decreases
.

As the temperature increases, the powder coating begins to cure
.
There is a broad endothermic peak in the figure, which is due to the small molecule product produced in the reaction, water, which absorbs a lot of heat when it escapes (usually an exothermic peak here in TGIC and hybrid powder coating systems)
.

The three powder coatings of polyester/HAA system with different ratios have no catalyst added, and the activation energy of the system is the same, so the initial curing temperature is basically the same, all at 124 ℃, and the reaction proceeds gradually
.

The maximum reaction rate is reached at the top of the peak, where the most water is produced and the most heat is removed by escaping
.
The reaction rate then gradually slows down as less unreacted material is available to react
.

4 Conclusion

The synthesized HAA system low-temperature curing carboxyl-terminated polyester resin (P9250PR) and HAA were used to prepare powder coatings at a ratio of 92.
5: 7.
5.
Although the acid value was high, the prepared powder coatings produced pinholes with a maximum coating thickness of 110 μm.
, cured at 160 ℃/15 min, and the obtained mechanical properties were excellent
.

Because no curing accelerator is added, the activation energy of the powder coating is not reduced, the storage stability is good, and the pre-reaction is not caused due to the low activation energy during the extrusion process, and the resulting coating film has good leveling
.

At the same time, because ethylene glycol is not used as a polyol component, it has good weather resistance
.
The polyester resin has excellent comprehensive properties and high application value
.

Energy saving and emission reduction are the outstanding advantages of low temperature curing powder coatings, and HAA system low temperature curing powder coatings will have more market prospects because of its non-toxicity
.

With the enhancement of people's awareness of environmental protection and the strengthening of ecological civilization construction, the environmental protection and energy saving advantages of low temperature curing powder coatings are more prominent, which will promote its development
.