Research on Application of Low Temperature Curing Powder Coatings on Aluminum Wheels

Abstract: In order to verify the feasibility of the application of low-temperature powder on aluminum wheels, this paper selects low-temperature curing powder coatings and ordinary powder coatings for comparison, and first analyzes the realization mechanism of low-temperature curing of low-temperature powders;

Second, the baking characteristics of the two powders were tested, and the baking curve was drawn;

Third, the mechanical performance test (yield strength and reliability test) and paint film test (adhesion, gravel impact, water resistance, salt spray test and balance weight sticking test) were carried out on the painted wheels;

Finally, analyze and evaluate the application and promotion of low temperature process in aluminum wheel production

1 Introduction

Powder coating is a rapidly developing field in the entire coating technology.

Powder coatings have long been applied on automobile wheels.

However, the curing conditions of ordinary powder coatings are mostly around 180°C, and some are even as high as 200°C, and the time is about 25min.

Automobile wheels are the main load-bearing parts of automobiles, which not only require sufficient mechanical strength, but also sufficient toughness to ensure impact resistance

Reducing the curing temperature of the powder coating can increase the toughness of the wheel hub, reduce the production energy consumption of the enterprise, and achieve the purpose of energy saving and environmental protection

At present, energy-saving and environmentally-friendly low-temperature curing powders have been used in large quantities in the field of home appliances, effectively reducing energy consumption, but the traditional low-temperature curing powders used have poor leveling properties
.

The coating of aluminum wheels has higher requirements for leveling and appearance, so it is impossible to spray on aluminum wheels.
Therefore, in the automotive aluminum wheel industry, low-temperature curing powder has not been used in large quantities.
Therefore, if the aluminum wheel industry can use low-temperature curing powder, There will be a lot of room for improvement in energy conservation and environmental protection
.

To realize the popularization and application of low-temperature powder coatings in the application of automobile wheels, the premise is that the wheel products produced under the low-temperature powder process meet the current performance standards of automobile parts
.

This article is based on various performance experiments (corrosion resistance, DSC differential calorimetry, powder infrared spectrum analysis, paint film compatibility, particle size experiment, resin content, and gravel impact resistance) of low-temperature powder special for aluminum wheels by powder manufacturers.

.

Aiming at Guangzhou Kinte Powder's first development of special low-temperature curing powder for aluminum wheels, it has passed a series of standards such as the mechanical properties of aluminum wheels, the construction process of low-temperature powder coatings, the various paint film properties of the coating, and the adhesion performance of the coating balance weight.
The experiment conducted a series of feasibility studies on the application of low-temperature curing powder in the aluminum wheel industry
.

2.
Experimental materials and equipment

2.
1 Experimental materials

2.
2 Experimental equipment

(1) Mechanical performance test

(2) Paint film performance test

3.
Test method

3.
1 Spraying process

Aluminum wheel coating process: casting molding wheel blank → pretreatment → electrostatic spraying powder coating → powder curing → spraying colored paint → spraying clear paint → curing

3.
2 Test method

(1) Research method and characterization of low-temperature curing

Differential scanning calorimetry was used to study the relationship between the curing of powder coatings and the temperature
.

Through the peak temperature A, the initial reaction temperature B, the area C, and the normalization D in the figure, the curing condition of the base powder is verified
.

Among them: the size of the peak temperature A reflects the glass transition temperature of the powder, the reaction temperature range B reflects the realization of low temperature curing, the area C reflects the curing rate, and the uniformity D achieves a high leveling effect
.

(2) Test various mechanical properties of aluminum wheels through a series of impact resistance and fatigue resistance experiments
.

(3) Test the paint film performance of the coating through the corrosion resistance test and the gravel impact resistance test
.

4.
Results and discussion

4.
1 The realization mechanism of low-temperature powder low-temperature process

Generally, the powder coating from being applied to the workpiece becomes a coating, and finally solidified to form a coating film, it needs to go through three stages:

The first stage is the coalescence of individual powder particles into a continuous uneven film, that is, the coalescence stage;

In the second stage, flow from a continuous uneven surface to a relatively smooth and even surface, that is, the leveling process;

In the third stage, the viscosity of the molten coating liquid increases continuously through the cross-linking reaction, and finally solidifies to form a coating film, that is, the cooling and solidification stage
.

In the three stages, the leveling of the coating is mainly in the second stage, and the resistance that affects the flow is the viscosity of the molten coating liquid.
At this stage, as the temperature rises, the viscosity of the coating liquid becomes smaller and smaller.
When the coating liquid is solidified, the viscosity of the molten coating liquid suddenly increases
.

Generally, traditional low-temperature curing powder coatings have solved the problem of low-temperature curing through the traditional method of adding catalysts, but because the powder is too reactive during the curing process and too sensitive to temperature, the leveling performance is not good when applied to aluminum wheel spraying.
, The powder is easy to agglomerate, so the traditional low-temperature powder is only suitable for spraying on thin metal workpieces
.

The aluminum wheel workpiece is thick, and the temperature of the workpiece is slow during the heating process.
During the solidification process, after the powder is melted, because the temperature is low and the powder is not fully leveled, it will gel and solidify.
This will result in poor surface flatness of the coating film and easy formation of orange peel Shortcomings such as heavy weight and poor appearance
.

Therefore, the development of low-temperature curing powder coatings for aluminum wheels cannot use traditional low-temperature curing methods, and the difficulty is relatively high
.

There are three ways to achieve low-temperature curing of powder coatings on aluminum wheels:

One is the improvement of the resin, including the type of functional groups, the number of functional groups, the distribution of functional groups, molecular weight, etc.
The purpose is to reduce the melt viscosity, softening point and increase the degree of crosslinking of the resin itself;

The second is the characteristics of the curing agent, including the type of functional groups, number of functional groups, molecular weight, miscibility with resin, etc.
;

The third is the application of curing accelerators, including the type and quantity of accelerators
.

Therefore, to achieve low-temperature curing of powder coatings on aluminum wheels, it is first necessary to introduce reactive groups that can be cured at lower temperatures (140-160°C) and use them to modify polyester resins to reduce the activation energy of the reaction and increase Cross-linking density, a hybrid polyester resin with low-temperature curing groups is obtained;

Secondly, on the basis of choosing a low-viscosity resin, by selecting the type of curing accelerator, the appropriate reactivity of the curing system is adjusted
.

Using reasonable formula design to solve the relationship between curing rate and curing time, thereby solving the balance between leveling and temperature, developed a high-leveling and low-temperature curing base powder coating for automotive aluminum wheels
.

Low-temperature curing aluminum wheel base powder belongs to a hybrid system.
Through the synthesis of saturated carboxyl-terminated polyester resin with appropriate reactivity and viscosity, and at the same time, bisphenol A epoxy resin with appropriate softening point and epoxy value is selected, and each other is cured as a film.
The body is made by adding various additives, the reaction mechanism:

4.
2 Detection and analysis of test powder

Main technical parameters of low temperature powder:

Through the measurement and analysis of the various technical parameters of Guangzhou Qingtian low-temperature powder, the technical parameters of the low-temperature powder meet the standard and are equivalent to the high-temperature powder, indicating that the low-temperature powder can be fully cured at a lower temperature
.

4.
3 Comparison table of baking temperature curve between ordinary flour and low-temperature flour

As shown in Figure-1 and Figure-2, the baking temperature of low-temperature powder is 150℃/20min, and the baking temperature of ordinary powder is 180℃/20min.
The curing temperature of low-temperature powder is about 30℃ lower than that of ordinary powder, which can effectively save energy.
Consumption, reduce carbon dioxide emissions, play a role in energy saving and environmental protection
.

4.
4 Powder workability

The construction conditions of the low-temperature powder were verified from the aspects of the powder loading rate, stickiness, cutting and chipping, and grinding amount of the two powders.
According to the workshop production operation feedback, the construction performance of the low-temperature powder meets the requirements and is equivalent to the high-temperature powder
.

4.
5 Test results of mechanical properties of two kinds of process wheels

Mechanical properties of aluminum wheels:

In the process of wheel casting, in order to increase the strength and toughness of the wheel, it is usually necessary to heat the wheel, that is, quenching and tempering.
Tempering is the most effective way to increase the toughness of the wheel.
The quality of the tempering process directly affects the wheel.
The toughness is strong and weak, generally the tempering temperature of the wheel hub is controlled at about 150℃;

When the baking temperature of the wheel hub is higher than 150℃ during the coating process, the effect of tempering will be weakened, which will affect the toughness and impact strength of the wheel.
Because the baking temperature of the high-temperature powder is 180~200℃, the toughness of the wheel There will be losses during the painting process;

The higher the baking temperature, the more the loss, while the baking temperature of low-temperature powder is 150℃, which is equivalent to the tempering temperature in wheel casting, and the toughness loss of the wheel is smaller
.

In general, the strength of the wheel after using the low-temperature powder process decreases by 15-20MPa, and the elongation increases by about 2; its impact resistance has improved, and the 13° limit impact height has increased by about 25mm, reaching the product standard
.

4.
6 Test results of paint film of two kinds of aluminum wheels

The first test result:

As a result of the first low-temperature powder test, there are problems with gravel impact and insufficient cohesive force of the balance block
.

The powder manufacturer reduces the brittleness of the powder coating by adjusting the resin molecular weight, viscosity and reactivity in the formula, ensuring the complete curing degree, and improving the impact resistance of the paint film; adjusting the auxiliary agent to enhance the bonding force between the balance weight and the wheel hub, and the original powder Improve
.

The second test result:

From the comparison of Fig.
6 and Fig.
7, it can be seen that the corrosion resistance test of Guangzhou Kinte's low-temperature curing powder is passed, and is equivalent to that of high-temperature powder, that is, it realizes low-temperature curing while ensuring the density of the coating and the curing rate of the coating system
.

As shown in Table -7, the balance weight adhesion force adjusted by the powder additives has been qualified, and the balance weight adhesion force of Guangzhou Qingtian low temperature foundation powder is equivalent to that of high temperature foundation powder.
The curing temperature is achieved while maintaining the powder coating's adhesion.
Adhesive force of balance weight
.

5 Conclusion

In the application of low-temperature powder on aluminum wheels, the mechanical properties of aluminum wheels and the various paint film properties of the coatings can meet the standards.
At the same time, they can take advantage of low temperature, reduce production energy consumption, and improve toughness and impact resistance, which meets the expectations of the test
.

The strength of wheels using low-temperature solidification powder has decreased, and the heat treatment process after casting needs to be adjusted to meet the requirements in the application
.

Low-temperature curing powder technology is not mature enough in application, and there may be occasional problems of gravel impact or insufficient adhesion of the balance weight, but the problem can be solved by improvement
.

In short, with the development of powder coating technology, the application of low-temperature curing powder for aluminum wheels is a major trend in the development of automotive aluminum wheel coating technology, and it is an inevitable choice for human health, energy saving and emission reduction, and building green wheels
.
It has obvious economic and ecological advantages and good market prospects
.