Abstract: The building materials industry applies film protection to the surface of aluminum profiles, effectively avoiding dust pollution, mechanical scratches, and abrasions, preserving the original high smoothness of the product surface and improving the secondary processing yield.

However, in the process of use, it is common to encounter situations where the protective film peels off and residual glue remains, which can easily leave dark spots, flower spots, and even fading effects on the surface of the powder coating, affecting the decorative effect of the coating.

Through technical analysis, the main factors affecting the detachment of protective film adhesive on the surface of profiles include protective film, powder coating, profile storage environment, and the residence time of protective film on the surface of profiles.

This article focuses on the analysis and research of the film printing problem that occurs after powder coating and protective film are applied to building materials.

1. Overview

The Chinese aluminum industry started in the 1950s and has undergone nearly 70 years of development, becoming a global producer and consumer of aluminum.

The protective film is a layer of plastic protective film attached to aluminum materials, which aims to protect the produced aluminum materials from damage during a series of processes such as handling, inventory, transportation, processing, and installation. After the installation of the aluminum materials is completed, the installation team will tear them off to make the surface of the aluminum materials smooth and new, and achieve the desired decorative effect.

At present, there are various types of aluminum products on the market, and the surface treatment technology of aluminum is constantly improving.

Various surface treatment techniques for aluminum materials include mechanical polishing, chemical polishing, anodizing coloring, electrophoretic coating, chemical coloring, fluorocarbon spraying, electrostatic powder spraying, surface pattern treatment, etc. Different adhesive protective films need to be selected for different aluminum surfaces.

This article focuses on the analysis and research of the film printing problem that occurs after powder coating and protective film are applied to building materials.

2. Introduction to protective film

Protective film is a shortened term for pressure-sensitive protective film in the building materials industry, and it is also one of the most important products in pressure-sensitive adhesives (PSA).

Protective film is usually made of polyolefin (PE/PVB) plastic film as the substrate, acrylic polymer as the matrix resin of pressure-sensitive adhesive, and several specific adhesive additives coated and processed.

The protective film is generally of polyacrylate type, which is divided into solvent type and water-soluble polyacrylate adhesive.

2.1 Solvent based polyacrylate adhesive

Solvent based polyacrylate adhesive uses organic solvents as a medium to dissolve acrylic monomers. The colloid is very transparent, has low initial viscosity, is very resistant to aging, and can last up to 10 years.

2.2 Water soluble polyacrylate adhesive

Water soluble polyacrylate adhesive is a type of adhesive that uses water as a medium to dissolve acrylic monomers, and basically has the characteristics of solvent based polyacrylate adhesive. However, the adhesive should avoid and reduce contact with water vapor to avoid residue.

Due to the environmentally friendly nature of water-soluble polyacrylate adhesive, which does not require a solvent recovery device, it is currently widely used to produce protective films.

The basic structure of polyacrylate protective film (Figure 1): from top to bottom: isolation layer; Printing layer; film; Adhesive layer.

3. Analysis of Film Printing Phenomenon in Powder Coating Coating after Applying Protective Film

3.1 Description of Film Printing Phenomenon

The protective film only serves as a temporary protective measure, and the profile still needs to remove the protective film after transportation, cutting, installation, and construction are completed.

However, in the process of use, it is common to encounter situations where the protective film peels off and residual glue remains, which can easily leave dark spots, flower spots, and even fading effects on the surface of the powder coating, affecting the decorative effect of the coating.

3.2 Analysis of Reasons for Film Printing Formation

Through technical analysis, the main influencing factors of the film imprint phenomenon where the protective film adhesive falls off on the surface of the profile include the protective film, powder coating, profile storage environment, and the residence time of the protective film on the surface of the profile.

3.2.1 Factors of protective film

There are five main reasons why protective film causes adhesive to peel off and form film marks: a) fluctuations in raw material quality; b) Insufficient curing of adhesive layer; c) The adhesive layer has not been dried; d) The adhesion of the protective film is relatively high; e) The corona treatment is not in place.

3.2.2 Powder coating factors

Powder coating melts at a certain temperature and undergoes cross-linking reaction to form a dense coating, thus providing good protection for building profiles.

However, there are some small molecule additives in the coating that do not participate in the formation of the network structure. They are only covered by film-forming substances and are easily detached or bonded to other floating and sinking substances under certain external adhesive forces.

Or it may interact with the pressure-sensitive adhesive acrylic ester substance in the protective film, causing the adhesive to peel off and form a film imprint from the appearance.

3.2.3 Factors affecting the residence time of protective film on profiles

If the protective film stays on the surface of the profile for too long, it will be affected by the following factors:

a) The surface of the board has oxidizing properties;

b) The surface of the board has a catalytic degradation effect on the adhesive layer;

c) The small molecular substances contained in the board penetrate into the adhesive layer, resulting in weak cohesion in the adhesive. The probability of adhesive detachment from the protective film increases with the prolonged residence time of the protective film on the surface of the profile.

3.2.4 Environmental factors for storage of protective film and profiles

The main factors causing film printing in the storage environment are that the storage environment does not meet the requirements, resulting in the following problems:

a) There is moisture on the surface of the profile;

b) Oil stains on the surface of the profile;

c) Storage temperature is too high;

d) Excessive negative pressure.

The existence of these problems will significantly increase the probability of adhesive detachment from the protective film.

4. Paint formula testing and result discussion

4.1 Influence of Coating Crosslinking Density

Select 96/4 polyester and 93/7 polyester respectively, adjust the reaction rate and promote sufficient reaction by adding catalyst. From Table 1, it can be seen that the higher the acid value of the resin, the higher the cross-linking density of the coating, which helps to improve the film printing.

By adding a catalyst, the reaction is more complete and the coating density is better. Small molecules in the protective film are not easily able to penetrate into the coating.

4.2 Effects of Wax Powder

Add 0, 0.5%, 1%, 1.5%, 3% polyethylene wax powder and pure polytetrafluoroethylene wax to the formula respectively, and test their effects on the film printing effect.

The following conclusion can be drawn from the experiment in Table 2: under the condition of ensuring complete reaction, 0-1.5% wax powder has little effect on film printing, but exceeding 1.5% will increase the probability of forming clear film printing.

High melting point polytetrafluoroethylene wax can reduce the surface tension of the coating and decrease the penetration of unreacted acrylic monomers and small molecules in pressure-sensitive adhesives. At the same time, PTFE has poor compatibility with acrylic substances, so adding PTFE wax powder can help eliminate membrane printing.

4.3 Effects of Different Fillers

The effects of using barium sulfate, garnet powder, and aluminum hydroxide as fillers on film printing

From Table 3, it can be concluded that under the premise of complete curing, different fillers have no effect on film printing.

4.4 Effects of different leveling agents

Four sets of formula tests were conducted using ether leveling agents, acrylic leveling agents, or a mixture of them to investigate their effects on film printing.

From Table 4, it can be concluded that non acrylic ether leveling agents are not prone to producing film marks, but they affect leveling and require the addition of a higher proportion of leveling agents, which increases the formulation cost.

Acrylic ester leveling agent is a small molecule homopolymer of butyl acrylate, which has a similar solubility principle with the acrylic material of pressure-sensitive adhesives. Therefore, pressure-sensitive adhesives are more likely to penetrate the surface of the coating and leave clear film marks. In addition, it is difficult to avoid the complete absence of film marks when applying high-temperature film.

5. Summary and Analysis

Incomplete coating reaction can easily cause adhesive to peel off and form film marks. The unreacted small molecule substances in the coating film bond with the pressure-sensitive adhesive on the protective film, forming a strong adhesive force. Once the adhesive force exceeds the peeling force between the pressure-sensitive adhesive and the PE film itself, the adhesive will peel off and fall onto the surface of the coating film, forming a film imprint.

5.2 If the wax powder content is less than 1.5%, it has almost no effect on the film printing. The reason for this is that the wax powder in the coating with low content can be wrapped by the dense film-forming material, and a small amount of wax powder floating on the surface has little effect due to its low surface tension and smoothness.

Only when the package cannot be held, does it begin to affect the film printing. Pure polytetrafluoroethylene wax powder, due to its high melting point, acts as a filler wrapped in resin in the coating film, while also possessing the smoothness of ordinary polyethylene wax powder, which helps to eliminate film printing.

5.3 Different fillers may have slight structural differences, but due to the generally fine particle size of powder fillers, they are tightly coated with barium resin during the melting, extrusion, and solidification process, making it difficult for them to affect the surface material. Therefore, fillers have no effect on the production of film marks.

5.4 Ether leveling agents can effectively avoid film marks, but they affect leveling. Mixing ether leveling agents and acrylic ester leveling agents can reduce leveling losses and decrease the formation of film marks, with almost no obvious film marks.

Acrylic ester leveling agent is a small molecule homopolymer of butyl acrylate, which has a similar solubility principle with the acrylic material of pressure-sensitive adhesives. Therefore, pressure-sensitive adhesives are more likely to penetrate the surface of the coating and leave clear film marks.

5.5 Under special storage conditions such as excessive negative pressure, high temperature, and moisture, it is impossible to completely solve the film printing problem from the powder formula. As the temperature increases, the load compression strengthens the adhesion between the coating and the adhesive.

6. Conclusion

The factors that affect the phenomenon of film printing include the quality of protective film, the quality of powder coating, and the storage environment of profiles. The value proportion of protective film and powder coating in the aluminum industry is very small, and they are only auxiliary materials in the building materials industry.

However, if the two are incompatible, the harm is not only that they cannot provide protection, but may also damage the surface quality of the product, causing tangible material losses and intangible brand image losses to users, ultimately affecting the price position and sales of the product manufacturer.

Therefore, when designing the formula for powder coatings used in building materials, we need to consider the film printing phenomenon induced by adverse environments during the production and storage of aluminum profiles, and try to avoid and reduce the occurrence of film printing problems as much as possible. Yongji Industrial Co., Ltd. is located in; Hometown of Chinese Coatings& mdash;— Shunde, Guangdong, covering an area of nearly 10000 square meters, is a high-tech outward oriented powder coating production enterprise that integrates research and development and operation.

The company was founded in 1997, and we have always adhered to the principle of 'riding'; User first, quality first; Our business philosophy is that in 2003, the company fully passed the ISO9001-2000 international quality management system certification. In 2004, it was evaluated by the National Quality Evaluation Center as; Chinese Quality Credit Enterprise;, The Yongji brand, built with high-quality products and excellent services, has won a good reputation among customers. In July 2018, the company underwent a comprehensive upgrade and passed the ISO9001:2015&ISO14001:2015 Quality&Environmental Management System certification.

Since its establishment, the company has maintained healthy, stable, and rapid growth. From two small production lines and a small factory with more than ten people in the early stages of construction, it has developed into a modern enterprise with an experienced and skilled technical and management team, with an annual output of over 15000 tons and annual sales exceeding 200 million yuan.