Wang Kai Tu, Liu Weihua, Zheng Hang, and Miao Bao Lu
Yanmei Chemical Co., Ltd., Langfang City
Summary:This chapter evaluates the impact of different powder coating systems on the performance of valve and pipe components through three sets of experiments. The pure epoxy system provides the best functional protection for valve and pipe components, while the mixed system performs the worst. In terms of decorative aspects, the pure polyester system provides the best performance, while the pure epoxy system performs the worst.
1. Introduction
In recent years, with the development of the piping industry, domestic valve and fitting products are also evolving towards high-tech content, high standards, strong corrosion resistance, and long lifespan. This has increased the demands on valve coating and finishing technologies. Cast iron valves are increasingly used in water systems. Valve damage and replacement are primarily due to corrosion of the main body, and severe contact between the valve interior and the medium leads to significant corrosion. After corrosion, the valve diameter decreases, increasing flow resistance, and severely affecting the transmission of the medium. Valves are often installed above or below ground, and their surfaces are exposed to humid air, making them highly susceptible to corrosion. Therefore, the main material of the valve must be protected against corrosion to extend its lifespan. Additionally, direct contact between cast iron valves and water can cause secondary pollution of the water. Therefore, the performance requirements for valve coatings are primarily functional protection, with decorative aspects as a secondary consideration. With increasing emphasis on environmental protection by the government, powder coatings have become the main coating for valves. However, there are different opinions on which powder coating system is most suitable for valves and fittings.
The commonly used testing standards for valve powder are mainly as follows: 1) GB/T 34202-2017 "Epoxy Coating (Heavy Corrosion Protection) for Cast Iron Pipes, Fittings, and Accessories"; 2) CJ/T 120-2016 "Plastic-Coated Composite Steel Pipes for Water Supply"; and 3) EN14901:2006 "Cast Iron Pipes, Fittings and Accessories - Epoxy Coating (Heavy Duty) for Cast Iron Pipes, Fittings and Accessories - Requirements and Test Methods." While these three standards explicitly mention epoxy coating systems in their titles or content, it is undeniable that non-pure epoxy systems exist in the market, such as epoxy-polyester mixed systems and pure polyester systems. There are both internationally renowned large brands and outstanding domestic brands that produce these non-epoxy systems. When describing their products, some falsely claim to be epoxy systems, some use vague names (e.g., powder coatings), and some honestly state that they meet or exceed the standards. So, how should we understand this situation in the market? Instead of arguing, let's focus on the facts. As an example, we will design three formulations with similar resin content and curing time, and compare them one by one, using the most common blue color.
2. Formulation Design
The materials used in this experiment are as follows: Bisphenol A epoxy resin 903, Bisphenol A epoxy resin 904H, polyester resin 6055, polyester resin SJ4867, curing agent 969A-2, curing agent TGIC, leveling agent 988A, titanium white R-902+, silica powder, BaSO4, and other functional additives.
2.1 Design of pure epoxy system formulations
2.2 Design of epoxy-polyester type system formulations
2.3 Design of pure polyester-based system formulations
3. Main instruments and equipment for preparing and testing powder coatings
4. Preparation of Powder Coatings for Valves
Weigh the resin, hardener, filler, and additives separately, then mix them thoroughly in a high-speed mixer. The resulting mixture is then fed into a twin-screw extruder for melting and extrusion. The extruded strips are cooled, and then pulverized using a high-speed grinder. Finally, the pulverized material is sieved using a 100-mesh screen.
5 Powder and Coating Performance Testing
Through communication with experts from PetroChina and Sinopec, national-level testing institutions, the group reached a general consensus. For the detection of buried pipelines in the petrochemical industry, the detection standards for valves and fittings using coatings are too simple, and revisions and strengthening are inevitable. In reality, many valve and fitting coating manufacturers have also incorporated their own standards. This detection introduces the mature standard SY/T 0315-2013, "Technical Specifications for External Epoxy Powder Coating of Steel Pipes," which is widely referenced in pipeline detection standards. The following key items are introduced:
(1) Thermal property testing
Thermal characteristics include the heat release of epoxy powder, Tg1, Tg2 of epoxy powder coatings, and Tg3, Tg4 of the coating. The degree of heat release indicates the cross-linking density of the coating, Tg1 indicates the storage stability of the coating, and Tg2, Tg3, and Tg4 reflect the heat resistance of the powder coating. The cross-linking density, which determines the chemical resistance and durability of the coating, is a crucial factor that directly affects its service life. SY/T 0315-2013 clearly states that the glass transition temperature (Tg2) of the powder must be above the working temperature and ambient temperature of the coated pipes (above 40°C). GB/T 34202-2017 specifies that this standard applies to water pipes, fittings, and accessories coated with spherical cast iron, with a maximum operating temperature of 60°C, which can be buried in corrosive soil or water with an ambient temperature not exceeding 50°C. based on these regulations, the glass transition temperature of powder coatings suitable for valves and fittings should ideally be at least 90°C. These regulations are the result of years of practical experience and testing.
(2) Cathodic Disbondment Detection
We believe that incorporating this item is necessary, as buried valves and pipelines, as well as buried petroleum and petrochemical pipelines, also require cathodic protection. This is a crucial inspection that directly affects the service life of the components.
The routine inspection items for valve powder coatings include curing time, magnetic particle content, particle size distribution, acetone resistance, high-temperature yellowing resistance, impact resistance, adhesion, and corrosion resistance. Specific coating testing items and inspection results can be found in Table 5, Table 6, and Table 7.
5.1 Performance Testing of Pure Epoxy Powder Coating Systems
5.2 Performance Testing of Epoxy-Polyester Type Powder Coating System
5.3 Performance Testing of Pure Polyester System Powder Coating
6. Data Analysis and Summary
based on the experimental comparison of the three different valve powder systems described above, the following conclusions can be drawn:
(1) There is a significant difference in heat generation. The heat generation of the epoxy system is approximately twice that of the other two systems, which demonstrates that the epoxy system has higher reactivity and can achieve sufficient curing of the coating in the shortest time at the same temperature, making it suitable for the curing process of many pipe fittings and valves with residual heat. The glass transition temperature of the powder coating is only above 90°C for the pure epoxy system, while the other two systems do not meet the requirements.
(2) Regarding resistance to hot water immersion and cathodic delamination, there are significant differences among the three systems. The experiment specified a 48-hour, 75°C immersion condition, and while all three systems met the requirements, the pure epoxy system performed the best. based on previous long-term hot water immersion tests, only the pure epoxy system maintained adhesion above a Level 2 after 28 days. The cathodic delamination differences were even more pronounced. Although the above experiment included adhesion promoters in the epoxy-polyester and pure polyester systems to improve cathodic delamination resistance, they had little noticeable effect on the epoxy-polyester and pure polyester powder coatings. only the pure epoxy system met the requirements. After 48 hours of hot water immersion, the surfaces of all three systems became opaque, but the pure epoxy system exhibited the least degree of opacity, followed by the epoxy-polyester system, and then the pure polyester system. This indicates that water molecules rapidly interact with the epoxy-polyester and pure polyester systems, which can significantly shorten the service life of the coatings.
(3) Regarding frozen bending, epoxy-polyester powder coatings exhibit the poorest performance. Under extreme conditions, deformation of valve fittings can cause the coating to crack, which in turn can affect its service life.
(4) Regarding color, the pure epoxy system exhibits the poorest brightness and color saturation, while the polyester system performs the best. Some epoxy systems find it difficult to achieve vibrant colors, and the epoxy system is also the most prone to color change under different baking temperatures. This is also a significant reason why polyester and epoxy-polyester powder formulations are often marketed as epoxy formulations. Furthermore, most valve and pipe fittings are buried underground, but some are exposed to the elements, which requires consideration of weather resistance. In this regard, the epoxy system is the poorest among the three systems.
Currently, from a cost perspective, epoxy systems are the most expensive among the three systems. Regarding the market situation for valve and pipe fittings, all three systems' powder coatings are available. There is no consensus on which system is superior. Each powder coating manufacturer has its own perspective, and the data obtained through experiments using three different systems may be biased. We hope for corrections and feedback.