Author: Li Dongyang1, PENT1, Zhao Yihua2, Ren Xianghui3, Du Juan1, Chen Lin3
(1. National Pipe Network Group North China Pipeline Technology Research Center; 2. First Gas Production Plant, Fifth Purification Plant, Changqing Oilfield; 3. National Pipe Network Group North China Pipeline Company)
Summary:This document focuses on the critical performance characteristics of epoxy powder coatings for pipe fusion welding, including properties such as thermal characteristics, density, particle size distribution, volatility, curing time, adhesion, impact resistance, and flexural strength. It compares and analyzes relevant standards from ISO, Canada, and China. The document clarifies the specific meaning of each performance indicator and analyzes the factors that influence these characteristics from the perspective of epoxy powder coating formulations and production processes. It proposes corresponding quality control measures, recommending that epoxy powder coating manufacturers should control the quality of pipe fusion welding epoxy powder coatings through rational formulation design and improved processing techniques, thereby enhancing the overall quality of the pipe coating.
Keywords:Pipes; Epoxy powder for jointing; Standard; Quality control
Study on Quality Control of Fusion Bonded Epoxy Powder Coating for Pipelines
Li Dongyang, Pan Teng, Zhao Yihua, Ren Xianghui, Du Juan, Chen Lin
(1. Pipeline R&D Center, Pipe China North Pipeline Company; 2. The Fifth Purification Plant of the First Gas Production Plant of Chang-qing Oilfield, Yinchuan; 3. Pipe China North Pipeline Company)
Abstract: In view of the key technical specifications for bonded epoxy powder coatings for pipelines, such as thermal characteristics, density, particle size distribution, volatile content, gelation time, curing time, adhesion, impact resistance, and bending resistance, the relevant standards of ISO, Canada, and China were compared and analyzed. This paper presents the specific significance of each technical specification, discusses the factors influencing these specifications in terms of powder coating formulation and production process, and proposes corresponding quality control measures. It is suggested that powder coating manufacturers should control the quality of fusion bonded epoxy powder coatings for pipelines through reasonable formulation design and improvement of processing technology to upgrade the overall quality of pipeline coatings.
Key words: pipeline; fusion bonded epoxy powder; standard; quality control
0. Introduction
Since the early 1970s, fused epoxy powder coatings have developed into a recognized 4E-type coating product that offers high production efficiency, excellent coating performance, environmental friendliness, and cost-effectiveness.[1]。Epoxy powder coatings exhibit excellent mechanical properties, superior corrosion resistance, and good chemical resistance, making them widely used both domestically and internationally for coating the inner and outer walls of various diameter pipelines for transporting oil, gas, and water.[2].
This article compares key performance indicators of epoxy powder coatings for pipelines based on relevant domestic and international standards. It analyzes the various factors that affect these indicators and proposes improvement measures. This research is of great significance for further improving the performance of fused epoxy powder coatings, optimizing the quality control of the production process, and enhancing the anti-corrosion quality of fused epoxy powder coatings for pipelines.
1 Compliance with domestic and international standards
1.1 International Standards
CASZ245.20—2018《Standards for Protective Coating on Epoxy Powder Applied to Pipe for Fusion Welding》specifies requirements for the identification, application, inspection, testing, handling, and storage of six coating systems for applying fusion-welded epoxy (FBE) coating to the outer walls of steel pipes. ISO21809-2:2014《Oil and Gas Industry [1] Protective Coatings for Buried or Submerged Pipelines in Transport Systems – Part 2: Single-Layer Fusion-Welded Epoxy Coating》specifies requirements for the identification, application, testing, and handling of single-layer fusion-welded epoxy (FBE) coatings on welded and seamless steel pipes used in oil and gas pipelines. ISO21809-1:2018《Oil and Gas Industry – Protective Coatings for Buried or Submerged Pipelines in Transport Systems – Part 1: Polyene Coating (Three Layers of Polyethylene and Three Layers of Polypropylene)》specifies technical requirements for the application of three-layer polyethylene (3PE) or three-layer polypropylene (3PP) protective coatings on the outer surfaces of welded and seamless steel pipes used in oil and gas pipelines, including requirements for FBE coatings.
Canadian standard CASZ245.20 was first published in 1992, and many other standards refer to and cite this standard to varying degrees. The ISO standard was first published in 2011, but due to its influence, it has gradually become the most widely adopted standard in the pipeline corrosion protection industry.
1.2 Domestic Standards
SY/T0315—2013 "Technical Specifications for the Design, Construction, and Inspection of Single-Layer and Double-Layer Epoxy Powder Coating on Steel Pipe" specifies the design, construction, and inspection requirements for single-layer and double-layer epoxy powder coating on steel pipes. GB/T23257—2017 "Polyethylene Corrosion-Resistant Coating for Buried Steel Pipes" specifies the performance requirements for epoxy powder coatings and their coatings on steel pipes.
SY/T0442—2018 "Technical Standard for the Application of Epoxy Powder Coating on Steel Pipes" specifies the design, production, and inspection requirements for epoxy powder coating on the inner surface of steel pipes. SY/T0315—2013, as a technical standard for the application of epoxy powder coating on the outer surface of pipes, is widely used in China and is currently being upgraded to a national standard. GB/T23257—2017 is widely used in the domestic oil and gas pipeline industry, but this standard is primarily for 3PE anti-corrosion layers.
2. Key Performance Indicators Analysis and Quality Control
2.1 Thermal Properties
The thermal properties requirements for epoxy powder coatings, as shown in Table 1, are consistent both domestically and internationally.
Regarding thermal properties, international standards such as CSA Z245.20-2018, ISO 21809-1:2018, and ISO 21809-2:2014 define epoxy powders with a Tg2 value around 100°C as standard-grade epoxy powders. The industry typically uses whether Tg2 reaches 110°C or 120°C as the benchmark for distinguishing between standard-grade and high-temperature-resistant epoxy powders.[3]Currently, GB/T 23257-2017, SY/T 0315-2013, and SY/T 0442-2018 provide detailed specifications for thermal properties, while CSAZ245.20-2018 and ISO 21809-2:2014 do not impose such requirements.
It is worth noting that CSAZ245.20—2018 and SY/T0315—2013 have consistent requirements regarding the heating rate. CSAZ245.20—2018 specifies more detailed requirements, with different points for determining the glass transition temperature (Tg) for powders and coatings. SY/T0315—2013 uses the initial Tg point, while CSAZ245.20—2018 uses the intermediate Tg point. Different versions of CSAZ245.20 have different regulations for Differential Scanning Calorimetry (DSC), but the requirements are becoming increasingly reasonable and closer to practical production.
When the operating temperature exceeds Tg, the adhesion and impermeability of the coating will decrease, leading to the failure of the FBE coating, resulting in issues such as bonding failure, coating blistering, and coating detachment, thereby increasing the risk of corrosion.[4].
The thermal properties of epoxy powder also depend on the glass transition temperature (Tg) of the epoxy resin. As the Tg of the epoxy resin increases, the Tg of the powder also increases. Certain pigments and blowing agents can also improve the thermal properties of the powder coating. Therefore, manufacturers can improve the thermal properties of powder coatings by selecting epoxy resins with a higher Tg, appropriately increasing the quality percentage of pigments and fillers, and adding blowing agents.[5-6].
2.2 Density
Density requirements for domestically and internationally produced fused epoxy powder coating are shown in Table 2.
- For density, international standards generally specify that it must comply with the manufacturer's specifications.- Domestic standards generally specify a range of 1.3~1.5 g/cm.3。The density of epoxy powder significantly affects the thickness of epoxy coatings produced. If the density of the epoxy powder is too low, the amount of powder sprayed by the gun per unit time (measured in volume) increases, resulting in a thicker epoxy coating. Due to the electrostatic repulsion phenomenon during electrostatic spraying, the epoxy coating surface becomes honeycomb-like, and after baking and curing, many pits or irregularities appear on the coating. Generally, the thickness of epoxy coatings produced by electrostatic powder spraying should be less than 150μm. If the density of the epoxy powder is too high, it will reduce the amount of powder sprayed by the gun per unit time, resulting in a thinner epoxy coating, which reduces the anti-corrosion effect and can also cause leaks. The thickness of the coating with electrostatic repulsion during powder spraying is also affected by factors such as the type of powder coating resin, the composition of the powder coating, the spraying voltage, and the material of the substrate. It needs to be determined based on actual experiments and production operations. Domestic standards specify the density of powder coatings, mainly to prevent some manufacturers from producing substandard products and affecting the quality of the powder coatings. The factors that have a significant impact on density are the content of resin in the epoxy powder.[7]Therefore, manufacturers should improve the density of epoxy powder by adjusting the resin content.
2.3 Particle Size Distribution
The particle size distribution index requirements for epoxy powder coatings, as shown in Table 3, are applicable both domestically and internationally.
The particle size of epoxy powder coating refers to the average particle size of irregular-shaped particles. The powder particles obtained from air classification grinding (ACM grinding) have varying sizes and exhibit a distribution, i.e., particle size distribution. According to both domestic and international standards, the particle size distribution for epoxy powder typically ranges from no more than 3% of the powder passing through a 150μm sieve and no more than 0.2% of the powder passing through a 250μm sieve. However, these standards may be overly broad for current epoxy powder production technology. The particle size distribution significantly affects the electrical properties, dusting rate, stability, leveling, thickness, and texture of epoxy powder coatings, and is a crucial technical indicator affecting the coating application performance.[8]。Controlling the particle size distribution of powder coatings, aiming to minimize the maximum particle size and control the content of ultrafine particles, is of great importance in the production process. The ability of the powder coating to meet the requirements is primarily influenced by fine grinding, classification, and screening equipment. These three elements form a coordinated system. The main equipment in this system includes air classifiers, spiral feeders, cyclone separators, pulverizers, bag filters, rotary valves, fans, and pulse vibrators. By analyzing and comparing the motor speed of the pulverizer's main mill and the speed of the classifying rotor (secondary mill) with the particle size distribution of the powder coating, selecting appropriate main mill and classifying rotor speeds can result in a more reasonable particle size distribution.
2.4 Volatile Organic Compounds (VOCs)
The requirements for volatile content of epoxy powder coatings from both domestic and international sources are shown in Table 4.
Regarding volatile content, CSAZ245.20-2018 specifies two methods for determining volatile content: (1) titration method, with a maximum volatile content of 0.5%; and (2) mass loss method, with a maximum volatile content of 0.6%. The requirement of volatile content ≤ 0.6% is consistent with domestic standards, although the terminology differs. Generally, powder coatings are solid, particulate materials that do not contain volatile organic solvents and water, and are considered environmentally friendly coatings. However, they may still contain small amounts of relatively low molecular weight, easily volatile components, which can lead to the occurrence of hazy conditions during the extrusion and baking processes of powder coatings, causing negative impacts on personnel and the environment.[9]The fumes released during the baking process of powder coatings, which have a pungent odor, are almost entirely derived from the raw materials, including resin, hardener, additives, and fillers. The solution can be approached from two aspects: "blocking" and "venting." In the "blocking" aspect, special materials with adsorption or decomposition properties can be selected and added to the powder formulation to adsorb or decompose volatile substances. These materials are characterized by their porous, fine particle size, and high heat resistance, while also not interfering with coating performance. In the "venting" aspect, ventilation ports and collection systems can be installed on the powder coating extruder to reduce the impact of volatile components on operators and the environment. Coating workshops should strengthen ventilation, enhance the collection and processing of volatile gases, and reduce fume concentration.
2.5 Curing Time
The requirements for the curing time of epoxy powder coating adhesive in both domestic and international standards are shown in Table 5.
Curing time refers to the total time required for powder coatings, from the molten state to the fully cross-linked state, during which the coating cannot be drawn into a thread.[10]The curing time is related to the epoxy powder formulation and directly affects the curing speed of the epoxy powder. Both domestic and international standards generally require the curing time to fall within the range specified by the powder manufacturer, or to meet the manufacturer's specified value ±20%. SY/T0315—2013 specifically requires the curing time to be ≤30s and to fall within the range specified by the powder manufacturer, which is mainly due to practical production considerations. For epoxy powders, if the curing time is too short, the epoxy powder will not have enough time to wet the steel pipe substrate, leading to poor leveling, reduced gloss, and even defects such as pinholes. If the curing time is too long, the coating will not fully cure within the specified time, resulting in reduced adhesion, impact resistance, and other performance characteristics. Therefore, curing time is a very important indicator in the production process. When applying 3PE and 3PP anti-corrosion coatings, it is necessary to apply an acrylic ester adhesive to the epoxy powder coating before the coating is fully cured, so that the adhesive and epoxy powder coating can be well bonded together, ensuring good bonding between the two coatings.
Temperature is a key factor influencing the curing time of the powder.[11]。Generally, higher temperatures lead to faster curing reaction of powder coatings, resulting in shorter gel times. In the production of powder coatings, the processes that most significantly affect the gel time are the heat mixing and extrusion processes. Furthermore, the type of curing agent also directly influences the gel time. Therefore, the gel time of powder coatings can be ensured by controlling temperature and time, and adding curing promoters if necessary.
2. 6 Curing Time
The curing time requirements for epoxy powder coatings, as shown in Table 6, are consistent both domestically and internationally.
The curing time of powder coatings significantly impacts the production efficiency of manufacturers applying coatings to pipelines. Both domestic and international standards generally require that the curing time meets the range specified by the powder coating manufacturer or the manufacturer's specified value ±20%. GB/T23257—2017 requires the curing time of epoxy powder to be ≤3min, and SY/T0315—2013 requires the curing time of epoxy powder to be ≤2min, while also meeting the range specified by the powder coating manufacturer. This is primarily due to the speed of steel pipe production. Shorter curing times result in faster production speeds. Achieving appropriate curing conditions is crucial for ensuring that the epoxy powder coating meets all performance requirements. Generally, epoxy powder coating manufacturers provide the curing temperature and time for their products, and under these specified temperature and time requirements, the curing rate of the epoxy powder reaches 95% or more. This demonstrates that the curing rate of epoxy powder is related to the curing temperature and time.
In a powder coating application line, the optimal curing zone is the distance between the powder coating spraying room and the water cooling room.[12]。During the anti-corrosion production of steel pipes using powder coatings, the solidification time is equal to the ratio of the solidification duration to the speed. The solidification duration is determined by the production line and remains constant. Therefore, the solidification time of the powder coating is inversely proportional to the linear travel speed of the steel pipe along the production line. The faster the speed, the shorter the solidification time. Typically, faster production speeds require higher heating temperatures for the steel pipes to achieve optimal solidification. Additionally, achieving the desired solidification rate can be achieved by increasing the solidification duration of the epoxy powder coating, which may require adjustments to the operating line.
2.7 Adhesion
The requirements for adhesion strength of epoxy powder coatings, as shown in Table 7, are applicable both domestically and internationally.
- Adhesion testing periods are generally specified as 24h, 48h, and 28d in national and international standards, with performance grades typically ranging from 1 to 3. SY/T0442—2018 also specifies the use of the "peeling" method to measure coating adhesion. Short-term adhesion testing is used to quickly assess the adhesion of coatings and epoxy powder coatings, while 28d testing serves as a type test to control and differentiate the quality of epoxy powder coatings and epoxy powder-based coatings. The temperature of the water bath used in adhesion testing is determined based on the operating temperature of the pipeline. Higher temperatures and longer durations require higher quality epoxy powder coatings.[13]。
The adhesion of coatings is affected by various factors, including surface preparation of the substrate, coating application methods, and coating formulations.[14]。When preparing the substrate, first remove any grease, dirt, or other contaminants from the steel surface. Preheat the steel and then perform surface treatment. The surface preparation should meet Sa2.5 level, with a surface roughness of 50-75 μm. Increasing the spraying temperature improves the adhesion of the coating. In powder coating formulation design, epoxy powder coatings have the best adhesion, followed by epoxy-polyester powder coatings, then polyester powder coatings, and finally polyurethane and acrylic powder coatings. The degree of complete curing of the coating also significantly affects its adhesion. If the coating is not fully cured, its adhesion, impact resistance, and bending resistance will be poor. To ensure complete curing of the coating, adjust the powder coating's curing time or modify the curing process.
2.8 Flexural Strength Performance
The requirements for flexural strength of epoxy powder coatings from both domestic and international standards are shown in Table 8.
based on the environmental conditions and temperature requirements for pipeline installation, both domestic and international standards specify bending resistance tests at 0 °C, -20 °C, and -30 °C. The specific temperature differences are primarily due to inconsistencies in the on-site construction environments and operating conditions between domestic and international standards. Users should select the appropriate standard based on actual conditions.
The flexural performance of powder coatings is primarily determined by the formulation. The following aspects can be considered to improve the coating's flexural performance:First, the choice of resin: Selecting resins with a flexible structure, high molecular weight, and low epoxy value, hydroxyl value, and acid value is beneficial for improving the coating's flexural performance.Second, the choice of curing agent: Selecting curing agents with a flexible and long-chain structure can also improve the coating's flexural performance.Furthermore, when the resin and curing agent are fully reacted, the coating's flexural performance is optimal. Therefore, selecting powder coating systems with high reactivity facilitates complete curing of the coating, which in turn improves its flexural performance. Additionally, in the design of powder coating formulations, shortening the powder coating's gelation time, appropriately adding curing reaction promoters, or adjusting the curing process can ensure complete curing of the coating, thereby improving its flexural performance.[15]。
2.9 Impact Resistance
The requirements for impact resistance of epoxy powder coatings, as shown in Table 9, are consistent both domestically and internationally.
Pipe coatings are inevitably subjected to impacts during transportation, processing, and installation. Their resistance to mechanical damage depends on their impact resistance. This requires epoxy powder coatings to exhibit good impact resistance under external forces. The impact resistance of the coating is related to its adhesion and hardness. Furthermore, environmental temperature significantly affects the impact resistance of the coating. Lower temperatures result in poorer impact resistance. Therefore, both domestic and international standards generally specify that impact resistance tests should be conducted at ambient temperature or -30°C.
The impact resistance of the coating is largely related to the formulation. To improve the impact resistance of the coating, it is necessary to select resins with high reactivity and a high epoxide value, acid value, and hydroxyl value. These resins react more completely with the hardener, resulting in a higher cross-linking density in the coating, which in turn increases the impact resistance. Furthermore, when the Tg (glass transition temperature) of the coating is high, the coating hardness also increases, but this can reduce the impact resistance. Conversely, a lower Tg can improve the impact resistance, but it can also reduce the storage performance of the powder coating. Therefore, when selecting resins, it is necessary to consider the Tg comprehensively. In addition, adding plasticizers, toughening agents, or thermoplastic resins can also improve the impact resistance of the coating.[15]。
3. Conclusion
The key performance indicators for epoxy powder coatings for pipe fusion include thermal properties, density, particle size distribution, adhesion, impact resistance, and flexural resistance, among others. Requirements for these indicators vary among domestic and international standards. The formulation design and processing methods of epoxy powder coatings for pipe fusion significantly impact these indicators. Powder coating manufacturers should control product quality through appropriate formulation design and improved processing methods to enhance the overall quality of the pipe coating.
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Article published in "Coatings Industry" Magazine, Issue 7, 2021.