Application of powder coating in architectural aluminum formwork

Construction projects require a large number of formwork for frame construction when pouring concrete. For cost control, builders usually use aluminum formwork that is durable and can be used repeatedly. In order to facilitate the rapid disassembly of the aluminum formwork after the concrete structure is solidified, it is common practice to apply a layer of release agent on the surface of the aluminum formwork to prevent it from adhering to the concrete. Common aluminum template release agents are water-based or oil-based liquid coating agents, but these coating agents need to be applied multiple times and it takes 5-8 hours to form an isolation film. In addition, the conditions for applying release agent to aluminum film panels are relatively high. Special attention must be paid to the thickness and uniformity during the application process. The aluminum template being painted must always be in a horizontal state, otherwise the release agent will easily flow down. The space and cross-operation restrictions in the construction environment seriously affect the film-forming effect of the film remover. Therefore, construction companies urgently need fast, efficient and low-cost solutions to replace water-based or oil-based release agents.

Powder coatings have become an ideal substitute for aluminum formwork release agents for construction due to their high alkali resistance, water resistance, high hardness and high wear resistance. Not only does it have no volatile gases but it can also be used repeatedly, which greatly improves the construction progress of the construction project and ensures the physical and mental health of the construction workers. In this article, we will learn with you the details of powder coating in the field of aluminum membrane panels for construction.

1. Selection of types of powder coatings

From the application fields and usage environment of aluminum formwork, we can understand that the coating used in its coating must have a certain degree of weather resistance to prevent the coating from powdering when exposed to the sun outdoors; it also needs to have high alkali resistance and high water resistance to prevent pouring After concrete corrodes the aluminum formwork, thereby reducing the flatness of the concrete surface and the service life of the aluminum formwork; there is a large amount of sand and stones in the concrete, which can easily scratch and impact the coating surface during the pouring process, causing damage to the coating and affecting its protective effect. and demoulding effect, so the coating must have high hardness and high wear resistance; in addition, in order to improve the demoulding effect, the coating surface needs to have low surface tension and strong hydrophobicity to ensure that the concrete is difficult to adhere to during demoulding. On the coated surface, it can be reused after simple cleaning. The performance characterization of powder coatings for architectural aluminum formwork is shown in Table 1.

Performance	Reference standards	Assessment indicators
Coating appearance	Visual inspection	Flat and smooth
Alkali resistance (saturated sodium hydroxide)	GB/T 9265-2009	No blistering or peeling of the coating after 500 hours
Alkali resistance (30% sodium hydroxide)	GB/T 9265-2009	72 hours without blistering or peeling of the coating
Concrete demolding test (40-50 times)		No mucosa, coating does not detach
Impact performance (50kg. em)	GB/T 1732-1993	Positive passage
Adhesion	GB/T 9286-1998	0
Flexibility / mm	GB/T 1731-1993	≤2
Hardness	GB/T 6739-2006	4H-5H
Xenon lamp aging (1000h)	GB/T 1865-2009	Light retention rate above 50%, adhesion level 0

Table 1 ~ Performance characterization of powder coatings for building aluminum formwork

Considering the environment in which powder coatings for aluminum templates are used, they should have certain weather resistance and extremely high alkali resistance. Currently, the weather resistance systems mainly include acrylic systems, polyester systems, and polyurethane systems. Compared with acrylic systems, polyester systems and polyurethane systems contain a large number of ester groups after curing, and ester groups are easy to decompose when exposed to alkali. Therefore, the alkali resistance of polyester systems and polyurethane systems is relatively poor, so acrylic systems are given priority. Acrylic powder coating not only has excellent weather resistance, but also has excellent alkali resistance, and the surface tension of the coating is small, so the concrete mortar demoulding effect is also good.

2. Effects of different acrylic film-forming systems on powder coating properties

There are many varieties of acrylic powder coatings, but from the perspective of storage stability and comprehensive performance of the coating film, industrial weather-resistant acrylic powder coatings are mainly polycarboxylic acid curing systems for glycidyl acrylic resins and blocked polyisocyanates for hydroxy acrylic resins. Curing system. This article examines several weather-resistant acrylic film-forming systems, and the results are shown in Table 2.

Acrylic film-forming system
Project	Glycidyl based acrylic resin / polyacrylic acid	Glycidyl based acrylic resin / carboxyl based polyester resin	Hydroxyacrylic resin / blocked polyisocyanate
Alkali resistance (saturated calcium hydroxide)	500h coating without bubbles or peeling
Alkali resistance (30% sodium hydroxide)	72h coating foaming and peeling	72h coating without bubbles or peeling
Concrete demolding test	31 demolding coatings peeling off	30 demolding coatings peeling off	28 demolding coatings peeling off
Impact performance (50kg. cm)	30kg.cm front pass	40kg.cm front pass
Adhesion level	0
Flexibility	2
Hardness	HB	1H	1H
Xenon lamp aging (1000h)	92% light retention rate, adhesion level 0	90% light retention rate, adhesion level 0	89% light retention rate, adhesion level 0

Table 2 ~ Coating performance of acrylic film-forming systems with different weather resistance

As can be seen from the results in Table 2, the alkali resistance and weather resistance of different acrylic systems are excellent. Compared with the other two curing systems, the polybasic acid curing system for glycidyl acrylic resin has poorer impact properties and hardness. The properties of the carboxyl polyester resin curing system for glycidyl acrylic resin and the blocked polyisocyanate curing system for hydroxyl acrylic resin are relatively close. Taking into account the high cost of blocked polyisocyanate and the environmental pollution caused by the release of blocking agent during the curing process, the carboxyl polyester resin curing system for glycidyl acrylic resin was finally selected. However, in order to appropriately improve the cross-functionality of the film-forming product The coupling density is increased, and a small amount of dodecanoic dicarboxylic acid is further used to obtain a uniform cured coating film.

3. The influence of resin on the performance of powder coating

Because powder coatings for building aluminum formwork coatings need to have high alkali resistance, high water resistance, high hardness, high impact properties, etc., the selected glycidyl acrylic resin and carboxyl polyester resin need to have good performance. We have examined several glycidyl acrylic resin and carboxyl polyester resin. The results are shown in Tables 3 and 4.

Glycidyl based acrylic resin
Project	A	B	C
Glass transition stability / ℃	56	68	60
Epoxy equivalent	610	750	980
Alkali resistance (saturated calcium hydroxide)	500h coating without bubbles or peeling
Alkali resistance (30% sodium hydroxide)	72h coating foaming and peeling	72h coating without bubbles or peeling
Impact performance (50kg. cm)	40kg.cm front pass		50kg.cm front pass
Adhesion level	0	1	0
Flexibility	2
Hardness	HB	2H	2H

Table 3 ~ Coating properties prepared by different glycidyl based acrylic resins

Carboxyl polyester resin
Project	A	B	C
Glass transition stability / ℃	71	76	66
Acid value	26	28	24
Alkali resistance (saturated calcium hydroxide)	500h coating without bubbles or peeling
Alkali resistance (30% sodium hydroxide)	72h coating foaming and peeling	72h coating without bubbles or peeling
Impact performance (50kg. cm)	50kg.cm front pass	40kg.cm front pass	50kg.cm front pass
Adhesion level	0	0	0
Flexibility	2
Hardness	2H	2H	1H

Table 4 ~ Coating performance of different hydroxyl polyester resins prepared

It can be seen from the results in Table 3 and Table 4 that choosing glycidyl acrylic resin and carboxyl polyester resin with high glass transition temperature and high functionality can significantly improve the hardness of the coating film, but the high glass transition temperature of the resin will cause The brittleness of the coating will increase, and the adhesion and impact performance will decrease to a certain extent, so in the end we chose glycidyl acrylic resin B and carboxyl polyester resin a.

4. The influence of fillers on the performance of powder coatings

In powder coatings, adding appropriate fillers can improve the hardness, scratch resistance, wear resistance and other physical properties of the coating film. We examined several fillers and the results are shown in Table 5.

Filler
Project	/	Barium sulfate	Silicon carbide micro powder	Porous quartz powder
Impact performance(50 kg.cm)	Positive passage	Frontal fissure	Frontal fissure	Positive passage
Hardness	2H	2H	4H	4H

Table 5~Performance of Powder Coatings with Different Fillers

As can be seen from the table above, adding barium sulfate does not significantly improve the hardness of the coating. Adding silicon carbide powder and porous quartz can significantly increase the hardness. However, silicon carbide powder has a certain impact on the impact performance, while porous quartz has a certain impact on the impact properties. It does not reduce the impact performance of the coating. The main reason is that the surface of porous quartz contains a certain number of hydroxyl groups. As polar functional groups, it is used in powder coatings to help improve the adhesion and cohesion of the coating film. At the same time, porous quartz contains Zirconium dioxide has stable properties and high hardness. Coupled with the porous structure of porous quartz, the coating has good hardness and impact resistance.

5. Effect of additives on powder coating properties

In order to improve the demoulding effect of powder coating coated concrete, it is necessary to reduce the surface tension of the coating film surface. The smaller the surface tension, the harder it is for the concrete mortar to adhere to the surface. It is easy to clean after pouring and molding, and it is not easy to cause the coating to fall off or crack. Commonly used leveling agents have the function of reducing surface tension, but the function of silicone-modified acrylate polymer leveling agents in reducing surface tension is more significant than that of unmodified acrylic polymer leveling agents. After passing the experimental test, adding silicone Modified acrylic polymer leveling agent can better improve the demoulding effect of the coating film. The number of times is increased from 30 times to more than 40 times before the demoulding coating starts to fall off.

At the same time, adding polytetrafluoroethylene wax can significantly improve the hardness and scratch resistance of the coating film, and can also improve the demoulding effect of the coating film. The most appropriate addition amount is 0.4% of the total formula.

Conclusion

Powder coating offers a promising alternative to traditional release agents for architectural aluminum formwork. Its durability, demolding efficiency, and worker safety make it a compelling choice. Careful selection of acrylic systems, resins, fillers, and additives is crucial for optimal performance. By embracing powder coating, construction companies can streamline project timelines and enhance worker well-being.