How to solve the application shortcomings of ultrafine powder coatings?

Powder coatings and coatings are rapidly developing powder coatings with outstanding advantages of environmental protection, economy, efficiency, and energy conservation. They are 100% solid components and do not contain a large amount of solvent emissions like liquid paints. They are environmentally friendly coating products that are pollution-free; Powder that has not been utilized in powder coating spraying can be recycled and reused. The spraying process is simple and stable, with low energy consumption, and compared to liquid coatings, it has the characteristics of low cost and good performance.

The development speed of powder coatings is very fast, and in China, it is much faster than other countries. The main reason is that China's economic development speed is very fast, and a large number of newly built coating lines prefer to use powder coatings. China has become a major country in the use and production of powder coatings, but there is still a certain gap compared to developed countries in the production of high-end raw materials and the development and development of high-end products.

How to target cutting-edge topics with great practical application significance, utilize domestic and foreign high-tech for research, and achieve industrialization is an important decision faced by Chinese scientists and industry insiders.

Powder coatings have shown strong competitiveness in the past decade, but there are also shortcomings and limitations that need to be overcome in their development process. The four recognized drawbacks of powder coating are that the coating is too thick, the apparent effect of the coating is poor, it cannot be cured at low temperature, and it is difficult to change colors.

In response to these limitations and shortcomings, scientists and engineering technicians from various countries have conducted extensive research and achieved considerable results in many fields.

The development and research of low-temperature curing powder coatings have already produced commercial products that can be baked and cured at 120 ℃ for the coating of wood and composite boards; The research on UV curing of powder coatings has also made breakthrough progress and has industrial applications.

Shanghai Xinxing Electrostatic Spraying Equipment Co., Ltd., an old factory of powder coating equipment in China, has collaborated with UCB Company in Belgium to successfully develop powder coating UV curing equipment, which has brought beneficial development and huge potential to this emerging field in China.

In the current situation, in order to make industrial users accept powder coatings, it can be considered that the role of high quality and economy is the greatest contribution compared to compliance with environmental regulations.

The market share of powder coatings is not significant compared to other types of coatings. However, in the relatively mature industrial coatings field, powder coatings are one of the few products that can maintain a high growth rate over the years.

Introduction to Ultrafine Powder Coating Technology

Powder coating has developed rapidly due to its outstanding advantages of environmental protection, economic efficiency, and energy conservation. However, powder coating also has shortcomings and limitations that need to be overcome urgently. The two recognized drawbacks of powder coating are: too thick coating and poor surface flatness of the coating.

The reason is that the particle size of powder coatings is relatively large, far exceeding the thickness of ordinary paint films. Not only does it cause material waste, but in many cases, too thick a coating can also cause a decrease in the performance of the paint film.

For example, the coating is prone to detachment and the hardness of the paint film decreases. To overcome the aforementioned shortcomings, scientists from various countries have conducted extensive research and developed ultrafine powder coatings. The particle size of this powder becomes finer, and the coating surface effect is very good, which can achieve thin coating coating.

Scientists from Canada's Huixu Micropowder Technology Company have successfully overcome the molecular attraction between ultrafine powder particles using special techniques. Avoiding the phenomenon of agglomeration, a powder coating with a particle size of 10-20pm and excellent fluidization performance was obtained.

This coating can form a very flat coating surface and can also be applied with a thin coating. The surface effect of ultrafine powder coatings has been greatly improved. Salt spray experiments have shown that very thin ultrafine powder coatings have excellent corrosion resistance, mainly because the thickness of the ultrafine powder coating in the thinnest part of the coating is higher than that of the coarse powder coating.

Fine Pulverization and Development of Powder Coatings

Among the four main drawbacks of powder coatings, the coating thickness and poor apparent effect are the most significant.

The coating thickness of powder coatings usually ranges from 60 to 100pm, far exceeding the thickness of ordinary paint films. This not only brings unnecessary waste, but in some cases, the coating is too thick, which can also cause a decrease in film performance, such as easy detachment of the coating. The poor apparent performance of the coating reduces the decorative properties of the powder coating, thereby limiting its application and development, especially for some high-end products (such as cars), which cannot be coated with powder coating.

The coating thickness and poor apparent effect of powder coatings are mainly caused by the larger particle size of the powder coatings. The particle size of ordinary powder coatings is usually 30-40mm, which is difficult to achieve a flat surface and good surface effect after electrostatic spraying.

If the particle size of powder coating can be reduced, a coating with good surface effect can be obtained, and thin coating coating can be achieved. The fine pulverization of powder coatings can not only achieve significant cost savings for thin coatings, but also achieve excellent coating surfaces, making it one of the most important topics in the research and development of powder coatings.

Scientists from various countries and major powder coating companies around the world have invested heavily in this project, and it has also attracted the attention of other industries and some governments.

Due to market demand and people's attention, there have been many breakthroughs in the research of fine powder coating.

The most common method is to add some lubricants to powder coatings to make them less prone to clumping, thus achieving the goal of appropriately reducing powder particle size;

Some large companies also adopt the production of products with narrow particle size distribution, and choose products with high fluidity and lubricant adjustment to further reduce the particle size of powder coatings and preliminarily achieve thin coating coating;

Ferro Company in the United States has developed a new process for preparing powder coatings using supercritical carbon dioxide, which can obtain powder coating products with uniform dispersion and narrow particle size distribution;

In China, many companies produce products with high flatness and claim to have achieved thin coating coating. In fact, the powder particle size has not decreased, but due to the ability to obtain thinner coatings, it is indeed very popular in the market.

The above methods are only some ways to achieve fine powder coating, and they have indeed brought many benefits to the powder coating industry, but they have not truly achieved fine powder coating.

Fine pulverization of powder coatings refers to the particle size of powder coatings reaching below 20mm. Usually, when the coating thickness is 2.5 times the powder particle size, the surface effect is better, while ultrafine powders have their own characteristics, namely poor gas fluidization performance. This is because as the particle size decreases, the mass of the powder decreases exponentially, and the surface area of the powder increases exponentially. As a result, the molecular force is greatly enhanced, making the ultrafine powder clump and unable to undergo normal fluidization.

Normal fluidization is a prerequisite for achieving electrostatic spraying of powder coatings, therefore, it has become the main technical reason for the difficulty in achieving fine powdering of powder coatings. Fine powder agglomeration is a natural characteristic of ultrafine powders. To achieve fine powder formation, it is necessary to first overcome the molecular forces between ultrafine powders.

Progress in Preparation Technology of Ultrafine Powder

1. Mechanical and Chemical Methods

The preparation of ultrafine powders mainly involves mechanical crushing and chemical synthesis methods. Mechanical crushing is the process of ultrafine conventional block or powder materials through mechanical force.

The chemical synthesis method generates the basic particles of a substance - molecules, atoms, and ions - through chemical reactions of the substance, which undergo nucleation, growth, and condensation to form ultrafine powders. This method has three major advantages:

Firstly, versatility. Capable of preparing ultrafine powders with various compositions, morphologies, and particle sizes.

Secondly, product quality can be controlled at the molecular or atomic scale.

Thirdly, the process can achieve precise control and adjustment, making it easy to achieve industrial production.

From the perspective of ultrafine powder preparation and application, the chemical synthesis method of ultrafine powder represents the development direction of ultrafine powder preparation technology and has become a research and development focus in various countries.

2. Engineering issues in the preparation of ultrafine powders

The production process of ultrafine powder materials has its own unique industrial reaction process, with the main difference being that the proportion of raw material costs is relatively reduced. The function of the material determines the high added value of the product, which largely depends on the morphology of the product (morphology distribution, crystal composition and shape, etc.).

The morphology of powder materials is the key to industrial production, and the solution of engineering problems in material preparation is a prerequisite for process control and amplification. Mastering the laws of ultrafine powder process is the foundation for solving engineering problems.

Requirements for ultrafine powder coatings

Ultrafine powder coatings can achieve flat thin coatings, which means that powder coatings characterized by ultra-fine powder thin coatings may have special technical requirements for both coatings and coatings.

Usually, powder coatings require a low melting viscosity of the resin, but the glass transition temperature of the resin cannot be low; Pigments have good dispersibility and strong covering power; The particle size of the powder should be small and the distribution should be narrow, which requires the crushing and grading effects of the crushing equipment to be good.

Powder coatings also need to have good dry powder fluidity and electrostatic properties. To comprehensively solve these problems, it requires the joint efforts of raw material manufacturers, powder mills, equipment manufacturers, and users.

This article studies ultrafine powder coatings with good flowability. The focus of the study is that users can apply them normally without changing any equipment. Below, we will list a series of technical problems encountered in the actual development and application process.

1. Covering power

It is difficult for ordinary powder to apply thin coatings, with a film thickness generally ranging from 60 to 90mm, and there is usually no problem with covering power. In application, it was quickly discovered that the ultrafine powder obtained with ordinary formulas does not have sufficient covering power when the coating is below 50mm, especially for white products, which cannot meet the requirements in actual painting.

For this reason, we have appropriately increased the content of pigments to provide high hiding power like liquid paint. Among them, white products are more special, and we must choose the strongest hiding power of rutile titanium dioxide powder and increase the dosage at the same time, otherwise we cannot meet the requirements.

As the film thickness decreases, the sensitivity of the coating to covering power increases exponentially. During the development process, we found that it is necessary to consider taking a series of measures to prevent uneven melting of the product as the pigment content increases.

One is to use resin with good melting performance; The second is to use titanium dioxide with good melting performance or titanium dioxide treated with encapsulation. In addition, it is necessary to enhance the kneading effect during extrusion. Therefore, in order to achieve very good covering power, corresponding improvements should be made in the powder formula.

2. Leveling property

The leveling performance and sagging phenomenon of ordinary powder coatings are a contradiction, and when the leveling performance is good, it is easy to sag. Ultra fine powder coatings are less prone to sagging due to thin coating, so the amount of leveling agent can be increased to achieve better leveling performance. Ultra fine powder has a very flat coating surface due to its uniform particle size and uniform spraying.

3. Powder charge

Ultrafine powder has a small mass and is not easy to powder. In theory, some electric agents should be added to improve the powder loading rate. However, in practical applications, it has been found that a lower one-time powder loading rate is actually an advantage.

Due to the low powder loading rate, the selectivity of spraying is enhanced, making it easy to obtain a uniform coating thickness during spraying. Because ultrafine powder solves the fundamental problem of powder fluidization, there is no problem with the recycling and use of ultrafine powder.

4. Cost

Due to the significant reduction in film thickness, the cost of ultrafine powder coating will be greatly reduced. But the percentage of cost reduction is not directly related to the percentage of powder savings. Due to the increased production costs caused by the use of many high-end raw materials, ultrafine powder is usually much more expensive than ordinary powder.

And the cost increase of white ultrafine powder products is more than that of other color products due to the large use of high-end titanium dioxide powder. The cost increase of dark ultrafine powder is minimal. Overall, ultrafine powder coating still has significant advantages in overall cost, and the more high-end products, the more significant the cost reduction.

The market experience in the past six months is that due to insufficient understanding of new products, customers who first use ultrafine powder are not interested in the cost reduction factor of coating, and their driving force is to improve the product grade in order to achieve a flat surface effect. Of course, even though the cost savings now are significantly higher than the increase in costs.

Coating of ultrafine powder

1. Spraying equipment and processes

The starting point for tackling the issue of ultrafine powder coatings is to completely solve the fluidization problem of ultrafine powders, so that theoretically no coating process will be changed.

In practical applications, ultrafine powder can indeed undergo complete fluidization like ordinary coarse powder without the problem of poor fluidization such as gun blockage. However, the spraying of ultrafine powder still has its unique characteristics. Ultrafine powder has a smaller mass and larger surface area. Although individual particles have less charge, the overall charge increases significantly.

After more than a year of testing and application, we have found that the spraying equipment basically does not need to be changed. Whether it is a manual spray gun in a simple spray room or the most advanced automated spray gun in a modern spray room, it can be sprayed freely.

However, the spraying process requires minor changes based on the specific situation, such as the distance being slightly closer and the voltage being slightly lower. The spraying process parameters of ultrafine powder are the same as those of ordinary coarse powder, and each production line has its own optimal coating conditions that meet its own requirements. Technical personnel need to conduct certain exploratory tests on site.

2. Powder loading rate and selectivity

There are many fine powders in the recycled powder of ordinary coarse powder, which often cause poor fluidization problems such as clumping and spitting during repeated use, causing trouble for the recycling and utilization of powder coatings. Therefore, it is necessary to mix recycled powder and coarse powder in a certain proportion before recycling.

Due to its small mass, ultrafine powder is not as easy to reach the surface of the workpiece as coarse powder. The one-time powder application rate is lower than that of coarse powder, but this is not a bad thing. The poor powder application rate makes it easy to obtain a uniform and thin film coating when choosing a better ultrafine powder spraying, which is difficult for coarse powder to achieve.

On the other hand, as ultrafine powder solves the problem of fine powder fluidization, there is no problem of poor flowability in the recovered powder. As long as the equipment has a recycling device, there is no problem with painting.

3. Recycling performance and recoating performance

The commonly used recycling systems now include cyclone recycling and filter bag recycling. Both methods can effectively recover all unused powders. Both coarse and ultrafine powders contain particles of varying sizes, but with varying proportions of content.

Although the average particle size of ultrafine powder is much smaller than that of coarse powder, its particle size is within the range designed for recycling by existing recycling equipment. It should be said that normal recycling is not a problem. Recycling powder usually has a smaller particle size than the powder used for the first time.

The key technology of ultrafine powder coatings is to thoroughly solve the fluidization problem of ultrafine powder, so the recovered powder of ultrafine powder has the characteristics and applications of ultrafine powder coatings.

4. Painting performance

The coating of ultrafine powder is actually a successful spraying process achieved by solving the fluidization of fine powder. Due to the following reasons, ultrafine powder coatings have different coating properties from ordinary powder.

Firstly, the particles are fine, the coating is dense, and the surface is flat, thus enhancing the scratch resistance and smoothness of the surface;

Secondly, the thin coating avoids drawbacks such as coating detachment caused by thick coating.

In addition, it is difficult to obtain thin coatings with coarse powder coating, while excessively thick coatings actually bring great waste. To save costs, conventional powder coatings must add a large amount of cheap fillers.

Although these fillers do not affect the covering power of the coating, they to some extent affect the chemical properties and corrosion resistance of the coating. Due to the thin coating, ultrafine powder requires high covering power and requires the use of the best raw materials. Moreover, due to the thin coating, it can also afford to use the best raw materials.

Therefore, the performance of ultrafine powder coatings is significantly superior to that of ordinary powder coatings in many aspects, such as corrosion resistance, weather resistance, softness, adhesion and hardness. Of course, if strong mechanical friction resistance is required, then a thin coating is not as good as a thick coating.

Application examples of ultrafine powder coatings

1. Black components

Most internal parts of automobiles can be coated with powder coating, with the main function being to protect them. However, due to the inability of ordinary powder to achieve thin coating, the coating film thickness is usually between 60-100pm, so it is not widely used.

However, black ultrafine powder has no issues with covering power and surface smoothness. In practical applications, the film thickness has been reduced to an average of 20pm, saving a lot of costs. The corrosion resistance of the coating is comparable to that of ordinary powder, and the hardness and adhesion have also increased.

2. Indoor products such as furniture and containers

Some customers choose ultrafine powder for producing export products with high surface requirements. In order to achieve the best apparent quality, the reduction in film thickness was minimal, resulting in no cost reduction. This indicates that in the low-priced indoor product market, the application of ultrafine powder currently does not have a cost advantage.

3. Outdoor weather resistant products

The spraying of aluminum profiles and other materials first requires excellent weather resistance, followed by good apparent performance. Ultra fine powder uses high-end weather resistant polyester and inorganic weather resistant pigments, which can obtain coating products with excellent performance in both aspects. More importantly, cost savings are significant.

4. Automotive sector

For many years, automotive gloss varnish has been considered a difficult field for powder coatings to enter. However, due to the advantages of powder coatings in terms of economy and environmental friendliness.

Recently, since the successful application of bottom integrated powder coatings in the field of automotive painting, automobile manufacturers and coating manufacturers have conducted extensive research in this area.

BMW is the first car manufacturer in the world to use powder gloss varnish in its standard products. By the end of 2000, powder gloss varnish had been commercially produced at BMW's German factory, producing a total of 500000 cars.

5. Other application markets

Other markets for powder coatings include anti-corrosion powder coatings used in pipelines and for reinforcing threaded steel bars. This type of powder coating is mainly based on pure epoxy systems (fusion bonded).

In Europe, the rebar market is almost ignored, but from a technical perspective, this field is considered to have great growth potential. Due to the different statistical classification methods in different regions, it will be very difficult to evaluate the consumption of powder coatings in different fields.

When comparing powder coatings economically with some VOC (volatile organic compound) coatings that comply with selection principles, it is necessary to pay attention to the total cost, including coating costs.

Compared to other environmentally friendly coatings, the advantage of using powder coatings in the actual coating process is that the utilization rate of raw materials has increased from 95% to 99%; Energy loss is reduced by 30% (compared to traditional low solid carbon paint); Labor costs will be reduced by 40% to 50%; The material discarded due to surface defects is reduced by approximately 4-6 times, resulting in an almost 90% reduction in waste.

Progress of ultrafine powder coatings in various fields

The latest development in coating equipment is the manufacturing of coating production lines that are easier to clean and change colors faster; The trend of developing low-temperature curing and high reactivity powder coatings can improve production line speed, save energy, and make the economic advantages of powder coatings more attractive.

MDF adopts powder electrostatic spraying, which has advantages such as excellent coating performance, high construction efficiency, and low energy consumption. After sealing wood products with 100% solid powder coating, it can prevent the volatilization of harmful substances inside the wood, making it a truly green product.

Of course, this still needs improvement, such as the chemical storage stability of the powder: latent catalysts or additives that can prevent the reaction of the base material at storage temperature without affecting the curing conditions; Physical storage stability: Increase the glass temperature of the system, but do not affect the reaction rate and viscosity of the system.

Coil steel coating is a pre coating process that differs from traditional "post coating" processes. Due to its advantages in simplifying production processes, efficient construction, saving investment and operating costs, complying with environmental regulations, and superior film performance to traditional methods, coil steel coating has become one of the development directions of the current coating industry.

Color plate production technology was first introduced in the United States in 1927, and China began to introduce coil coating and coating technology in the 1980s.

In the late 1990s, the consumption and production of color plates began to rise in China, with an exceptionally rapid growth momentum. By the end of 2003, 124 enterprises had completed 169 coating units with a production capacity of 8.74 million tons.

The powder coating device is located in a strong electrostatic field, and the powder rotating brush generates a coating powder cloud. The solid coating particles of the powder cloud carry high charges, which fly towards the high-speed running substrate, generating sufficient boundary penetration force, and the powder particles are evenly deposited on the surface of the strip steel.

With the strict environmental protection requirements and cost-effectiveness considerations of the country, the powder coating of coil materials will gradually be recognized by people and become a development trend of coil coating.