Author: Yan Guohui, Wang Dezhou
Fāngyuán Anti-Corrosion Materials Co., Ltd. (associated with Shengli Oilfield)
This article introduces the production principle of pipe powder. It elaborates on the key indicators of pipe powder, including its cohesion time, particle size distribution, magnetic material content, and heat release, and combines the company's years of production experience to discuss how to control powder quality during the production process. It also analyzes the various factors that affect these indicators.
0. Introduction
Powder coatings are environmentally friendly coatings that have experienced rapid development in China in recent decades. In particular, fused epoxy powder coatings, with their excellent mechanical properties, superior corrosion resistance, and good resistance to chemical corrosion from acids, alkalis, and salts, are widely used for protecting the inner and outer walls of pipelines. With the accelerated pace of national construction, the demand for pipeline powder coatings is also increasing rapidly. Users are also demanding higher quality powder coatings. Therefore, controlling product quality during the production process is particularly important.
The production principle of powder coatings involves mixing solid materials such as resin, hardener, pigment, filler, and additives in a high-speed mixer according to a specific ratio for preliminary mixing. The materials are then extruded, melted, and mixed in an extruder, followed by pressing, cooling, and crushing. The resulting flakes are ground into powder using a grinding machine, and then passed through a cyclone separator to separate the powder. The separated powder is then screened to obtain the finished powder coating. Coarse particles are re-pulverized through a recycling system, and fine particles are collected in a dust collection box through a filter bag.
1. Factors affecting the powder consolidation time
The definition of setting time is: the time required for a specific volume of powder coating to become a non-deformable material under specified conditions after melting. Longer setting time results in better film smoothness, while shorter setting time results in poorer film smoothness. This effect is more pronounced in rapidly solidifying powder coatings.
The primary factors determining the curing time are the formulation design, but the most critical processes affecting the variation in curing time in the production of powder coatings are the heat mixing and extrusion processes. Heat mixing and extrusion processes are also among the most important processes in powder coating production, and the extruder is a key piece of equipment in powder coating production, directly determining the production efficiency and quality of the powder coating. Extruders primarily used in powder coating production include single-screw reciprocating damping extruders and twin-screw extruders.
Main technical features of single-screw reciprocating extruder: low shear strength on materials; long material residence time within the barrel; capable of completing mixing and extrusion at lower temperatures; high adaptability to different formulations; complex machine structure and high manufacturing costs; high selling price. Main technical features of twin-screw extruder: high shear strength on materials; short material residence time within the barrel; high production efficiency; lower selling price compared to twin-screw reciprocating extruders with screw diameters equal to those of the twin-screw extruder. We use twin-screw extruders, which are widely selected by most powder production manufacturers in China.
The transmission structure and screw structure of the twin-screw extruder are complex, and the rotation directions of the two screws are the same, resulting in increased shearing. The amount of material extruded by the extruder is determined by the diameter, length, and rotational speed of the screws. The arrangement, length, and gaps of the screw section in the twin-screw extruder determine the mixing and kneading effect. The screw ratio D/L, which is the ratio of the effective length of the screw to its diameter, is an important parameter that affects the kneading time of powder coatings. A larger D/L ratio provides the material with sufficient melting time in the screw, resulting in more complete mixing and better kneading. However, for rapidly solidifying pipe powders, prolonged residence time in the screw can lead to increased impurities and reduced leveling and shortening of the kneading time. For the production of heavy-duty anti-corrosion powders, it is recommended to use screws with a D/L ratio of 12:1 to 14:1, ensuring that the mixing time within the machine is less than 20 seconds. This ensures that the kneading time of the powder remains relatively constant, resulting in good film gloss and smoothness. If a larger D/L ratio is selected, particularly if the material remains in the screw for more than 30 seconds, the kneading time of the powder will be significantly shortened, resulting in poorer film smoothness and reduced gloss.
Figure 1: Screw with a length-to-diameter ratio of 18:1 (top) and 12:1 (bottom)
In our tests, SLE01 single-layer fused epoxy powder coating, manufactured using two different screw extruders (as shown in Figure 1) and then screened, was applied to substrates with different aspect ratios (18:1 and 12:1) at the same formulation and extrusion temperature. The curing time for the 18:1 aspect ratio sample was 8 seconds (at 230°C), while the 12:1 aspect ratio sample required 10 seconds (at 230°C). This demonstrates that the aspect ratio of the screw significantly affects the curing time of fast-setting powders.
2. Factors affecting particle size distribution
The particle size distribution of powder coatings directly affects the charge and movement properties of the powder particles in an electrostatic field, thereby influencing the powder coating's throwing power and quality. The ideal powder coating particle size should be controlled within the range of 10～60μm. Powder coatings with a wide particle size distribution, especially those containing a large amount of particles greater than 80μm, have a significant impact on the smoothness of the coating. Additionally, due to the gravitational force exceeding its electrostatic force, these large particles can also reduce the powder coating's throwing power and decrease the amount of powder applied per unit area. When the content of ultra-fine particles (less than 10μm) is high, the throwing power is significantly reduced due to the low charge on the ultra-fine particles. Furthermore, these particles are prone to agglomeration, leading to fluctuations in the flow of the powder coating, uneven feeding, and ultimately affecting the spraying quality. For pipe coatings, due to their relatively thick spraying thickness, the average particle size is generally controlled between 45～55μm. If the pipe coating is a corrosion-resistant powder coating, the average particle size can be adjusted to a lower value, around 40μm, to improve leveling performance.
According to Figure 2, the following parameters are of particular interest in the powder coating industry: D(4,3) and D50. D(4,3) represents the weighted average of particle size with respect to volume or weight, also known as the average particle size by volume or weight. In Figure 2, the D(4,3) particle size is 44.20 μm. D50 represents the median particle size, which is the size at which 50% of the measured powder particles are larger or smaller. In Figure 2, the D50 value is 41.64. Particles smaller than 12.02 μm make up 10%, and particles larger than 80.10 μm make up 10%.
The primary factor affecting the particle size distribution of powder coatings is the ACM grinder. The ACM grinder operates as follows: The feeder delivers material flakes from the hopper into the ACM grinder. The impact of the rotating grinding disc, combined with the impact of the material against the gear and the material's mutual impact, causes the material to be pulverized. Under the influence of airflow and the secondary grinding action, the fine particles are separated and pass through the pipeline into the cyclone separator for coarse and fine particle separation. The ultra-fine particles enter the collection box with a filter bag and are separated. The remaining powder settles at the bottom of the cyclone separator and is discharged by the discharge device into the rotating screen feeder, which then enters the rotating screen for filtration and separation. Coarse particles are discharged from the other end for recovery.
Several factors can influence the particle size distribution throughout the entire process: the thickness of the flakes, the feed rate, the rotational speed of the main and auxiliary grinders, the airflow volume, and the wear of the impactor and gear ring. For a single ACM grinder, the midpoint diameter D50, the lower limit diameter D10, and the upper limit diameter D90 can be adjusted primarily by adjusting the airflow volume and the parameters of the main and auxiliary grinders. The size of larger particles can also be controlled by adjusting the mesh size of the screening net, which is typically 100 mesh for pipeline powders. In most production processes, the particle size distribution is adjusted primarily by adjusting the parameters of the main and auxiliary grinders. If adjusting the parameters of the main and auxiliary grinders does not achieve the desired particle size distribution, the airflow volume should be adjusted. Generally, as the parameters of the main and auxiliary grinders increase, the average particle size of the powder decreases; as the airflow volume increases, the average particle size of the powder increases. In addition, during the production process, the following two points should be noted: one is to ensure that the material thickness is consistent; and two is to pay attention to the wear of the impactor and gear ring. As the wear of the impactor and gear ring increases, the particle size of the powder will become finer. In the production process, the parameters of the main and auxiliary grinders should be adjusted in a timely manner, or new impactors and gear rings should be replaced.
3. Factors affecting the content of magnetic materials
The content of magnetic particles is an important indicator for powder coatings. Exceeding the specified magnetic particle content can lead to more pinholes in the powder coating, significantly affecting the anti-corrosion performance of the pipes. In the petroleum and natural gas industry standard SY/T 0315-2013 "Technical Specifications for Epoxy Powder Coating on Steel Pipes," the requirement for magnetic particle content is ≤0.002%.
Factors that influence the content of magnetic materials include: the inherent magnetic material content in the raw materials (especially magnetic fillers) and the magnetic materials generated from equipment wear. When establishing a formula and selecting raw materials, it is crucial to control and match the materials, and to choose raw materials with low magnetic material content. Wearable components in the production process include: screws and screw sleeves; impactors and gears. The parts of the screw that are prone to wear are the conveying block and pre-mixing block, while the mixing and kneading block is less prone to wear.
Methods to reduce the content of magnetic materials: firstly, improve the formulation by reducing filler content and lowering powder hardness; secondly, appropriately increase the extrusion temperature to accelerate the melting of materials and reduce wear on screws and barrels; thirdly, replace more wear-resistant materials to reduce wear on screws, barrels, and cams; fourthly, add a magnetic separation system to adsorb magnetic materials from the material, thereby reducing the content of magnetic materials.
Considering factors such as overall manufacturing cost and performance, it is recommended to use materials that are more resistant to abrasion, thereby reducing wear and tear on screws, screw sleeves, and impact pins and gear rings. The following are the results of experiments conducted on replacing different materials in our equipment:
The experimental results indicate that, although the cost of ceramic materials is higher, they exhibit minimal wear and tear, allowing for long-term use without the need for frequent replacement, which ultimately reduces the cost of powder production.
4. Factors affecting the heat release (ΔH)
In powder coatings, the heat released during the chemical reaction under heat conditions is referred to as the reaction heat, also known as the heat of reaction. This is represented by ΔH. According to the standard SY/T 0315-2013 "Technical Specifications for Epoxy Powder Coating on Steel Pipe Linings in the Petroleum and Natural Gas Industry," the requirement for ΔH is ≥45J/g, and some engineering projects require ΔH ≥55J/g.
The amount of heat generated is closely related to the design of the curing system. Different curing systems have significantly different heat generation rates. In the production of the same powder formulation, several factors can also influence the heat generation:1. **Extrusion process:** The extrusion process has a significant impact on heat generation. The longer the material mixes in the screw, the lower the heat generation. Therefore, when producing powders for pipelines, it is important to select a screw with the appropriate length-to-diameter ratio. In addition, the mixing time should be minimized without affecting the mixing effect.2. **Cooling and crushing process:** During production, it is important to ensure that the cooling water in the pressing roller is unobstructed. The temperature of the crushed material should be kept as low as possible, ideally below 30°C.3. **Grinding process:** During grinding, it is crucial to control the grinding chamber temperature. When the air temperature is high, a cooling fan should be used to maintain a low grinding chamber temperature.4. **Storage temperature:** The powder product should be stored in a warehouse with a temperature below 35°C.
Closing remarks
The quality of powder coatings is influenced by many factors. The control of powder coating manufacturing equipment and processes, along with the quality of raw materials, are equally important factors in determining the quality of powder coatings. Therefore, to improve the quality of powder coatings, a comprehensive quality management system must be established throughout the production process.