Technical progress and application prospects of fluorocarbon powder coatings for architectural aluminum profiles

Abstract: In recent years, the application technology of fluorocarbon powder coatings in the field of aluminum profiles has made rapid progress, forming a series of products

The article systematically analyzes the technological progress, main characteristics and performance of PVDF and FEVE fluorocarbon powder coatings, as well as the application advantages and limitations in the field of architectural aluminum profiles, and puts forward the current development direction and market application prospects of fluorocarbon powder coatings.

Fluorocarbon coatings have excellent corrosion resistance and weather resistance.

The protective decorative coatings used in the architectural aluminum profile industry are mainly ordinary polyester and super weather-resistant polyester coatings, and the proportion of fluorocarbon coatings is very small

High-end architectural aluminum fluorocarbon coatings are mainly solvent-based PVDF fluorocarbon coatings, and the market has grown rapidly in recent years

The state has issued a series of laws and regulations to strictly control the VOC emissions of solvent-based coatings during the production, transportation, and construction applications

For high-end architectural aluminum profiles, there is an urgent need for an environmentally friendly fluorocarbon coating that has weather resistance and other comprehensive physical and chemical properties comparable to solvent-based PVDF fluorocarbon coatings and meets environmental protection requirements and industrial policies

Considering the practical possibility, technical feasibility, and market expectations and needs, environmentally friendly fluorocarbon coatings that have potential to replace solvent-based PVDF fluorocarbon coatings mainly include water-based PVDF and FEVE fluorocarbon coatings, powder PVDF and FEVE fluorocarbon coatings and so on

Water-based fluorocarbon coatings have problems such as high specific heat, large surface tension, and slow volatilization in the application of architectural aluminum profiles.
There is no mature supporting coating application solution technology solution, and many technical problems are faced.
These problems restrict the use of water-based fluorocarbon coatings.
Application development of aluminum profile industry
.

Compared with water-based fluorocarbon coatings, fluorocarbon powder coatings are widely used in the field of architectural aluminum profiles, with mature production and construction technology, fewer technical barriers, and rapid market development
.

1 Fluorocarbon powder coating

Powder coatings do not use or produce organic solvents in the production, construction and application process.
They are highly efficient, environmentally friendly and non-polluting.
They are the main types of coatings used in the aluminum profile industry.
The amount and proportion are increasing year by year
.

The outstanding practicability and long-term durability of fluorocarbon coatings in the application of high-end aluminum profiles have been widely recognized by design institutes and users
.

Environmentally-friendly fluorocarbon powder coatings are valued in the aluminum profile industry.
In the new version of the "Powder Coatings for Aluminum Profiles" (YS/T 680-2016) standard, fluorocarbon powder coatings are listed as the main variety
.
Table 1 shows the powder type, purpose and use environment specified in the standard
.

2 PVDF fluorocarbon powder coating

The concept of PVDF fluorocarbon powder coating has been around for a long time, but to develop a PVDF fluorocarbon powder coating that has the same performance as solvent-based PVDF fluorocarbon coatings, it is not what people imagined to remove solvent from solvent-based PVDF fluorocarbon coatings to achieve powder.
It's that simple
.

For decades, people have conducted a lot of research from many aspects such as manufacturing method, production process and formula design to find a simple production process, relatively low cost, environmental protection, and easy to achieve industrial-scale production of PVDF fluorocarbon powder coatings.
However, breakthroughs have not been made
.

In recent years, the research on PVDF fluorocarbon powder coatings is mainly reflected in two aspects: PVDF/thermoplastic acrylic fluorocarbon powder coatings and PVDF fluorocarbon modified powder coatings
.

2.
1 PVDF/thermoplastic acrylic fluorocarbon powder coating

The main agent formula of PVDF/thermoplastic acrylic fluorocarbon powder coating is the same as that of solvent-based coatings
.
One production method is to use the traditional powder coating production process, that is, the mixture is extruded and crushed by a single-screw or twin-screw extruder, and then ground and sieved with an ACM air classifying mill
.

PVDF resin has high hardness and strong toughness.
Thermoplastic acrylic resin also has a certain degree of toughness at room temperature.
It is difficult for ACM air classifying mill to pulverize it ultrafinely.
In foreign countries, liquid nitrogen cryogenic pulverization is usually used, and the equipment is expensive and the cost is high
.

This method is difficult to produce PVDF/thermoplastic acrylic fluorocarbon powder coatings, and the production efficiency is extremely low.
It is not suitable for large-scale industrial production, and its production capacity and cost are difficult to meet the market requirements of the aluminum profile field
.

Another production method is divided into three steps: the first step is to prepare liquid PVDF fluorocarbon coatings according to a solvent-based process: the second step is to remove the solvent through a similar spray drying process to directly obtain various monochromatic PVDF fluorocarbon powder coatings: third Step by step, dry mix and match a variety of single-color paints to meet customer demand for paint colors
.

When the spray drying process prepares PVDF/thermoplastic acrylic fluorocarbon powder coatings, it is first necessary to solve the environmental problems caused by production safety and solvents
.

In order to better meet the requirements of the spray drying process for heat-sensitive materials
.
Need to choose a suitable thermosetting acrylic resin to replace the thermoplastic acrylic resin
.

2.
1.
1 Performance characteristics of PVDF/thermoplastic acrylic fluorocarbon powder coating

With the same formula composition, the PVDF fluorocarbon powder coating produced by the melt extrusion method has lower gloss than the liquid PVDF fluorocarbon coating, and the appearance of the coating film is also very different
.

The QUVB artificial accelerated aging test showed that the gloss retention rate of PVDF fluorocarbon powder coating was 81.
9% after 500h, and the gloss continued to decrease with the increase of aging time after 500h, which was significantly different from the excellent gloss retention of liquid PVDF fluorocarbon coating
.

Table 2 shows the main performance comparison of PVDF fluorocarbon powder coating and liquid PVDF fluorocarbon coating produced by melt extrusion method
.

This may be because PVDF resin is a high-hardness and high-toughness material.
In the process of extrusion, melting and mixing, the pigment wetting and dispersing is poor, especially organic pigments with large oil absorption are difficult to disperse, resulting in the preparation of PVDF fluorocarbon powder coatings as a whole The performance is not the same as the type of liquid PVDF fluorocarbon coating
.

2.
2 PVDF fluorocarbon modified powder coating

The production of PVDF fluorocarbon powder coatings according to the traditional process has many potential safety hazards and low production efficiency, which cannot meet the diversified needs of the market
.

In recent years, people have made new technological developments and attempts on PVDF fluorocarbon powder coatings, and developed a series of PVDF fluorocarbon modified powder coatings.
Considering the practicality of the product, PVDF fluorocarbon modified powder coatings are based on PVDF fluorocarbon modified poly Mainly ester powder coatings
.

Three key elements need to be considered in the formulation design of PVDF fluorocarbon modified powder coatings: First, in order not to change the production process of ordinary polyester powder coatings, low melting point, low molecular weight, high fluidity PVDF resin (the resin melting point is Around 178°C);

The second is to choose a polyester resin that has good compatibility with PVDF resin; the third is to increase the amount of PVDF resin in the formula as much as possible while maintaining the appearance, leveling, and surface effects of the polyester powder coating;

In theory, the more PVDF resin content, the more obvious the improvement of the weather resistance of the formulation.
However, when the proportion of PVDF resin in the coating formulation is higher than a certain value, the appearance and leveling performance of the coating will deteriorate
.

PVDF fluorocarbon modified polyester powder coatings currently on the market are divided into two categories, one type of PVDF resin addition amount is less than 5% (the mass ratio of PVDF resin to film-forming substance, the same below), and the other type of PVDF resin addition amount is less than 5% Around 15%~25%
.

Table 3 shows the QUVB artificial accelerated aging performance comparison of these two types of PVDF fluorocarbon modified polyester powder coatings, PVDF/thermoplastic acrylic fluorocarbon powder coatings, and ordinary polyester powder coatings
.

It can be seen from Table 3
.
When the PVDF resin content in the coating formulation is less than 5%, compared with ordinary polyester powder coatings, the QUVB artificially accelerated weather resistance of the modified coating is not significantly improved;

When the content of PVDF resin in the coating formulation is close to 20%, the addition of PVDF resin can significantly improve the overall performance of the polyester powder coating.
It has a good gloss retention rate after the aging test for 300 hours.
After the aging test for 300 hours, the coating begins to lose gloss sharply.
Discoloration, 500h coating began to chalk
.

2.
2.
1 Performance characteristics of PVDF fluorocarbon modified powder coating

The weather resistance of PVDF fluorocarbon modified polyester powder coating is higher than that of super weather resistant polyester powder coating, but there is a big gap compared with PVDF/thermoplastic acrylic fluorocarbon powder coating
.

In order to analyze the reasons for the large difference in weather resistance of different types of PVDF fluorocarbon powder coatings, 20% PVDF fluorocarbon modified polyester powder coatings and PVDF/thermoplastic acrylic fluorocarbon powder coatings were prepared on the aluminum alloy plate surface that was simply degreased.
Coating
.

Through X-ray energy spectrum analysis (EDS), three points were taken on the air surface and the metal surface of the coating to test the fluorine content of the coating section
.
Table 4 shows the fluorine content test results of PVDF fluorocarbon modified polyester powder coating and PVDF/thermoplastic acrylic fluorocarbon powder coating
.

It can be seen from Table 4
.
The fluorine content on the air surface of the PVDF fluorocarbon modified polyester powder coating is close to 19%, and the fluorine content on the metal surface is less than 1%.
Self-layering effect
.

PVDF/thermoplastic acrylic fluorocarbon powder coating, the metal surface of the coating has the same fluorine content as the air surface, and the PVDF resin does not show obvious self-delamination or upward migration tendency during the film forming process
.

Although the PVDF fluorocarbon modified polyester powder coating produces a good self-layering effect during the film formation process, the air surface of the coating has a high fluorine content, but the improvement of the weather resistance of the coating formulation is limited
.

The fluorine content in the PVDF/thermoplastic acrylic fluorocarbon powder coating coating is evenly distributed, and the coating has comprehensive properties such as excellent weather resistance
.

This may be because PVDF resin and thermoplastic acrylic resin form an interpenetrating network structure polymer alloy during the film forming process of PVDF/thermoplastic acrylic fluorocarbon powder coating
.

In the film forming process of PVDF fluorocarbon modified polyester powder coating, PVDF resin is a semi-crystalline fluororesin, and the polyester resin has a linear straight chain structure
.

Polyester powder coatings melt, level, and gel much faster than PVDF resins.
PVDF resins have crystallization enthalpy and poor fluidity, before the polyester powder coatings are gelled
.

PVDF resin melts, but does not form a homogeneous film on the coating surface, and the coating surface lacks a dense film protective layer, which leads to the artificial accelerated aging data of PVDF fluorocarbon modified polyester powder coating QUVB and PVDF/thermoplastic acrylic fluorocarbon powder There is a big gap in the coating
.

3 FEVE fluorocarbon powder coating

FEVE fluorocarbon powder coatings entered the architectural aluminum profile market later than liquid PVDF fluorocarbon coatings
.
It is earlier than PVDF fluorocarbon powder coating and has been widely used in engineering since the early 1990s
.
Because FEVE fluorocarbon powder resin and coating technology is basically monopolized by foreign companies, its application abroad is better than that in China
.

In 2013, the first domestic FEVE fluorocarbon powder resin production line was completed and successfully put into operation in Dalian
.
In recent years, the development speed, influence and market consumption of FEVE fluorocarbon powder coatings in the country have grown rapidly year by year, which has injected a new vitality into the development of the domestic high-end architectural aluminum industry
.

The development and application of domestic FEVE fluorocarbon powder coatings in the field of aluminum profiles are mainly concentrated in two aspects: one is pure FEVE fluorocarbon powder coatings, and the other is FEVE modified polyester powder coatings
.

Affected by many factors such as environmental pressure and the limitation of trivalent chromium surface treatment of aluminum profiles, domestic companies are devoting more energy to the development and application of FEVE modified polyester powder coatings
.

3.
1 Pure FEVE fluorocarbon powder coating

The formula design principle of pure FEVE fluorocarbon powder coating is similar to that of polyurethane powder coating
.
The curing agent is selected from internally blocked or blocked polyisocyanates, ultra-weather resistant pigments, and inorganic pigments or ceramic pigments with small oil absorption and easy wetting and dispersing
.

FEVE fluorocarbon powder coating formulas of foreign companies and joint ventures are basically finalized, and they are all based on pure FEVE fluorocarbon powder coatings
.

In order to ensure good adhesion between the coating and the substrate, the application of pure FEVE fluorocarbon powder coating requires a strict quality control yellow chromization treatment on the aluminum profile, and spray application of the fluorocarbon powder coating during the effective period of the chromization
.

3.
1.
1 Performance characteristics of pure FEVE fluorocarbon powder coating

In addition to the excellent weather resistance and chemical resistance of pure FEVE fluorocarbon powder coatings, the coating has some congenital defects in physical and mechanical properties
.

include:

(1) FEVE fluorocarbon powder resin has strong rigidity, and the prepared coating film has poor impact resistance and poor adhesion to the aluminum alloy substrate;

(2) FEVE fluorocarbon powder resin is easy to absorb moisture, resulting in high moisture content of the coating, and the coating film is prone to yellowing during the curing process;

(3) The curing agent produces small molecular compounds during the curing and crosslinking process
.
When the spraying is too thick, pinholes are easy to appear on the surface of the coating film;

(4) Many years of practical application experience show that only the surface of the aluminum alloy substrate that has been yellow-chromized can solve the problem of adhesion between the fluorocarbon coating and the surface of the aluminum alloy
.

The yellow chromium pretreatment technology on the surface of aluminum profiles brings harm to the ecological environment, and 13 benefits have attracted the attention of the industry and related enterprises
.
EU regulations have banned the use of the yellow chromizing process in 2016
.

At present, various chromium-free pre-treatment technologies and new products launched in the domestic market are emerging one after another
.
However, no suitable technology to replace the yellow chromizing process has been found, and it is difficult to truly solve the long-term adhesion of the FEVE fluorocarbon powder coating to the surface of the aluminum alloy substrate
.

Pure FEVE fluorocarbon powder coatings have not reached the best balance of mechanical, physical and chemical properties, and do not have the most reasonable cost performance
.
Although there are certain defects in the physical and mechanical properties, many foreign companies have not made major adjustments or changes after the initial formulation of pure FEVE fluorocarbon powder coatings was finalized based on the long-term weather resistance quality assurance provided to customers
.

3.
1.
2 New development of pure FEVE fluorocarbon powder coating formulation technology

With technological progress and increasing diversification of people’s needs, powder coating formulation technology is changing with each passing day.
In recent years, people have made many technical improvements and improvements to the pure FEVE fluorocarbon powder coating formulation to improve the overall performance of the coating and meet relevant standards.
Suitable for aluminum alloy substrates
.

FEVE fluorocarbon powder coating film has poor impact resistance (generally <30 cm)
.
Adding toughening resin or low molecular weight high elastic rubber to the coating formulation can effectively improve the impact resistance of the FEVE fluorocarbon powder coating
.

FEVE fluorocarbon powder coatings are prone to yellowing during the baking and curing process
.

Benzoin is the main factor that causes yellowing of the coating film during baking and curing.
By choosing a reasonable combination of wax defoaming and degassing agents and benzoin, the amount of benzoin in the coating formulation is reduced as much as possible to reduce the yellowing
.

FEVE fluorocarbon powder coating is easy to absorb moisture during production, transportation and use.
During the high temperature baking process of the coating, the water and polyisocyanate curing agent react to make the coating film yellow
.

After a lot of experiments, it is found that the selection of high-efficiency and high-activity antioxidants can effectively inhibit the yellowing of the coating film during the baking and curing process
.

After curing, FEVE fluorocarbon powder coating has insufficient film density and poor water penetration resistance
.
When designing the coating formulation, under the condition that the content of FEVE fluorocarbon resin remains unchanged, by increasing the degree of cross-linking of the coating film, the compactness of the coating film is increased, thereby improving the water resistance
.

The experiment design adds an appropriate amount of acid anhydride, such as succinic anhydride, succinic anhydride, etc.
to the coating formulation
.
The acid anhydride quickly chemically reacts with the hydroxyl group in the resin during the extrusion, melting and mixing process of the FEVE fluorocarbon powder coating dry blend to form a carboxyl group.
The production of an appropriate amount of carboxyl group can play the following roles:

(1) An appropriate amount of TGIC curing agent can be added to the coating formulation, and the carboxyl group can undergo curing reaction with TGIC or polyisocyanate to improve the crosslinking degree and compactness of the coating;

(2) The presence of an appropriate amount of carboxyl groups can improve the wetting and dispersing of FEVE resin pigments, and at the same time can improve the adhesion of the coating to the aluminum alloy substrate;

(3) The addition of acid anhydride increases the compactness of the coating and improves the water resistance of the coating film
.
Under the same formula conditions, while improving the adhesion of the coating to the aluminum alloy substrate, the artificial accelerated aging performance of QUVB is also significantly improved
.

3.
2 FEVE fluorocarbon modified powder coating

In order to reduce the cost of fluorocarbon powder coatings
.
Reduce the technical requirements of fluorocarbon powder coating on the surface pretreatment of aluminum alloy substrates
.

In recent years, many domestic enterprises to focus on developing a series FEVE fluorocarbon-modified powder coatings, including: FEVE fluorocarbon-modified polyurethane powder coatings, FEVE fluorocarbon modified acrylic powder coatings, FEVE fluorocarbon-modified polyester powder coatings
.

FEVE fluorocarbon modified powder coating formula design base point: In the process of coating baking and film formation, FEVE fluorocarbon powder resin has lower surface tension and poor pigment wetting and dispersing ability compared with other film forming materials
.

It is easy to migrate to the surface during the curing process.
If the amount of FEVE resin that migrates to the surface is sufficient to form a homogeneous coating film, it will protect the underlying coating film.
The design of FEVE fluorocarbon modified powder coating has better Weather resistance
.
Generally speaking, the addition of FEVE fluorocarbon powder resin accounts for 5% to 50% of the mass of the film-forming material
.

From the analysis of the migration mechanism on the FEVE fluorocarbon powder coating film, it is easier to achieve the purpose of migration by choosing coating systems with different curing mechanism systems and different compatibility
.

The curing system and curing mechanism of thermosetting polyurethane powder coatings and thermosetting hydroxy acrylic powder coatings are similar to those of FEVE fluorocarbon powder coatings.
In theory, they are not the preferred resin system for FEVE fluorocarbon modified powder coatings
.

There are many varieties of polyester powder resins, wide selection, and the curing mechanism is different from FEVE fluorocarbon powder coatings.
The domestic research on FEVE fluorocarbon modified polyester powder coatings is the most
.

3.
2.
1 FEVE fluorocarbon modified polyester powder coating

In order to ensure the quality stability of FEVE fluorocarbon modified polyester powder coatings, super weather resistant polyester with higher isophthalic acid content is generally selected
.

The selection principle of super weatherproof polyester: when the mass ratio of FEVE fluorocarbon powder resin to polyester resin in the coating formulation film is 1:1, the coating is smooth and flat without shrinkage, pinholes, yellowing and other obvious paint films Ills
.

FEVE fluorocarbon modified polyester powder coating formulation development process mainly includes melt co-extrusion method and dry blending method
.
The melt co-extrusion method is to blend the FEVE fluorocarbon modified polyester powder coating according to the formula materials together, melt co-extrusion, crush, and grind
.

The production of FEVE fluorocarbon modified polyester powder coating by melt co-extrusion method does not change the traditional production process and method of polyester powder coating
.
The appearance of the prepared coating film is smooth and flat, and the coating effect is close to that of polyester powder coating .

Most domestic powder coating manufacturers use this method to produce FEVE modified polyester powder coatings
.
The dry blending method is to first produce pure FEVE fluorocarbon powder coating and polyester powder coating according to their respective processes, and then mix the two powder coatings thoroughly according to the formula design ratio
.

3.
2.
2 Performance characteristics of FEVE fluorocarbon modified polyester powder coating

For the same FEVE fluorocarbon modified polyester powder coating
.
Using two production processes, melt extrusion and dry mixing, the artificially accelerated weather resistance data of the prepared coatings vary greatly
.

Table 5 shows when the mass ratio of FEVE fluorocarbon powder resin and super weather resistant polyester resin in the coating formulation is 3:7
.
The artificial accelerated aging test results of QUVB coating prepared by melt extrusion method and dry blending method
.

It can be seen from the test results in Table 5 that the QUVB artificial accelerated aging performance of the coating prepared by the dry blending method far exceeds that of the coating produced by the co-extrusion method
.

Melt co-extrusion method produces FEVE fluorocarbon modified polyester powder coating.
With the different proportion of FEVE fluorocarbon resin in the formula, QUVB artificially accelerated weather resistance is improved to different degrees, but the best coating formulation is designed, QUVB artificial accelerated aging test 1000 At h, the gloss retention rate is less than 50%
.

The dry blending method produces FEVE fluorocarbon modified polyester powder coatings.
The ratio of fluorocarbon powder coatings and polyester powder coatings is selected between 1:5 and 5:5 according to the requirements of the formulation design.
As the proportion of fluorocarbon powder coating increases, the weather resistance of the coating becomes better
.

When the dry blending ratio is between 3:7 and 5:52, if you choose a suitable super weather-resistant polyester, the gloss retention rate of the QUVB artificial accelerated aging test for 3000h is greater than 50%, which is close to the level of pure FEVE fluorocarbon powder coating
.

During the experiment, it was found that under the same formula conditions, the coating produced by the melt co-extrusion method is a homogeneous coating film, and the migration tendency of the FEVE fluorocarbon powder resin is not as good as that of the dry blending method
.

Figure 1 shows the dry blending of gray pure FEVE fluorocarbon powder coating and white polyester powder coating at mass ratios of 1:9 (14), 2:8 (24), and 5:5 (34) to prepare coating films
.

By comparing the air surface and metal surface coating film in Figure 1, it is found that the gray FEVE fluorocarbon powder coating has an obvious upward migration tendency during the curing process.
When the FEVE fluorocarbon powder coating reaches an appropriate ratio, the coating surface is basically A layer of homogeneous FEVE fluorocarbon protective film is formed
.

4 Conclusion

PVDF fluorocarbon powder coatings and PVDF fluorocarbon modified powder coatings are used in coating formulation design, manufacturing process, and coating weather resistance mechanism
.

There are still many technical problems that need to be developed and resolved
.
The market for pure FEVE fluorocarbon powder coatings should continue to develop.
How to develop long-term and high-reliability alternatives to yellow chromium pretreatment technology is an important topic in future development
.

The dry blending method FEVE fluorocarbon modified polyester powder coating has excellent weather resistance at the same time
.
The processing requirements for architectural aluminum profiles are the same as those of ordinary super weather-resistant polyester powder coatings, which greatly expands the application field of fluorocarbon powder coatings
.

With the successful development of new personalized fluorocarbon powder coating products such as wood grain transfer fluorocarbon powder coatings, 3D wood grain transfer fluorocarbon powder coatings, sand grain fluorocarbon powder coatings, fluorocarbon powder coatings are in the high-end The market development prospects in the field of architectural aluminum profiles are broad
.