High-Tg Thermosetting Epoxy Powder Coating for External Application on Oil Pipelines

Linglong Hao, Fang Jun

(Aishi Hua Jia Coatings Co., Ltd., Huangshan, Anhui, 245061)

Summary:This document combines the characteristics of the operating environment of pipelines, selects raw materials, adjusts the process, and selects the optimal coating formulation to develop a high-Tg pipeline anti-corrosion epoxy powder coating. The coating exhibits excellent properties and fully meets the requirements of CSA Z245.20 System 1B.

Keywords:Epoxy powder coatings, pipe corrosion protection, glass transition temperature (Tg)

0. Introduction

Fusion Bonded Epoxy (FBE) powder coatings are a type of coating that is applied in powder form and then melted to form a film. They are VOC-free, easy to apply, have a fast curing speed, and offer excellent performance. FBE powder coatings are widely used in pipeline corrosion protection due to their excellent mechanical properties, chemical resistance, and ease of application.

Fusion-bonded epoxy (FBE) has been used as a standalone coating, as well as a primer for single-layer or multi-layer systems, for over 40 years due to its excellent long-term performance. As a result, it has become the standard anti-corrosion coating material for pipelines worldwide. Over the past decade, the oil and gas exploration and transportation industry has undergone significant development, and the industry has continuously strived to slow down the corrosion of strategic and valuable assets (such as pipelines). This has stimulated the development of new fusion-bonded epoxy (FBE) anti-corrosion coatings.The FBE formulation utilizes solid-state epoxy resin. When formulated correctly, solid-state epoxy resin can achieve higher performance requirements, enabling the FBE coating to meet the more challenging demands of on-site use.

As deepwater oil and gas exploration and development continue to advance, the temperature of the produced liquid typically ranges from 80°C to 150°C. Ordinary fused epoxy powders are no longer suitable for providing safe and reliable corrosion protection for pipelines. To ensure the long-term service performance of FBE coatings, it is generally required that the Tg (glass transition temperature) of the epoxy powder should be at least 5～20°C higher than the operating temperature.

The Tg value of FBE coatings refers to the temperature at which the coating transitions from a hard, brittle solid state to a soft, resilient elastic state. When the coating reaches or exceeds this Tg value, the rate at which oxygen, water, and other ionic substances penetrate the coating and permeate into the substrate increases significantly, leading to the breakdown of the polymer structure and ultimately affecting the adhesion between the FBE coating and the substrate.

1. Experimental Part

1.1 Experimental Materials

- Domestic high-Tg epoxy resin A- Domestic bisphenol A type epoxy resin B- Domestic amine curing agent- Domestic reaction promoter- Estron-based flow agent- Yangzhou Tianli silica- Homemade additives

1.2 Preparation and Spraying Process of Coatings

Accurately weigh out each component raw material according to the formula, and mix them at high speed in a mixer. Then, extrude and cool the resulting material using a twin-screw extruder, and sieve the cooled material through a 120-mesh screen after grinding with coffee grounds to obtain the finished powder coating.

After sandblasting, the test samples are placed in an oven for preheating. The powder coating is then sprayed onto the preheated samples to a specified thickness (＞300μm) using a static sprayer. After curing and cooling, the samples are ready for testing.

2. Results and Discussion

2.1 Performance Testing

Perform performance testing according to CSA Z245.20 System 1B.

CSA Z 245.20 standard defines a threshold for Tg (glass transition temperature) of 115°C as the demarcation criterion between high-temperature and standard epoxy powder. Powders with Tg > 115°C are subjected to performance testing according to CSA Z245.20 System 1B, which is more stringent than System 1A. The specific differences are outlined in the following table:

Table 1: Differences between Systems 1A and 1B as defined in CSA Z 245.20

2.2 Selection of Epoxy Resin

The structure and properties of epoxy resins are the primary factors determining the performance of coatings and coating films. When selecting epoxy resins, factors such as melting temperature, decomposition temperature, viscosity, electrical properties, resin stability, and adhesion should be considered. From an industrial production perspective, factors such as the synthesis process, raw material sources, cost, and toxicity of epoxy resins should also be considered.

For corrosion protection of high-temperature pipelines, the glass transition temperature (Tg) of the FBE coating must be at least 5°C higher than the highest design temperature of the pipeline. Additionally, the FBE coating must have sufficient flexibility to prevent cracking in the coating during pipeline bending during on-site use. The combination of a high glass transition temperature (Tg) and good flexibility is often a challenge in conventional epoxy resin systems, as a higher Tg typically results in reduced flexibility. Higher glass transition temperatures (Tg) are typically achieved by increasing the crosslinking density of the polymer; however, increasing crosslinking density typically reduces the flexibility of the coating. The glass transition temperature (Tg) of a polymer is influenced by three variables: crosslinking density, chain stiffness, and intermolecular forces. These variables can be adjusted to achieve the desired balance of properties.

To balance crosslinking density and flexibility, the experiment selected high-Tg epoxy resin A and bisphenol A-type epoxy resin B, and the bending test results are shown in Table 2:

Table 2: Epoxy Resin Experimental Results

As shown in Table 2, using epoxy resin A and epoxy resin B together can achieve both the cross-linking density and flexibility of the coating, and the ratio of the two can be selected according to customer requirements and cost considerations.

Steel bar bending performance demonstration:

Figure 1: Bending performance of steel bars

Specific parameters for epoxy resin A and epoxy resin B used in the experiment are shown in Table 3:

Table 3: Epoxy Resin Specifications

2.3 Selection of Hardener

Epoxy resin itself is a thermoplastic material that requires a curing agent to undergo a cross-linking reaction under specific conditions, forming a three-dimensional networked thermoset product. This allows it to exhibit various excellent properties and become a valuable epoxy material. Therefore, the curing agent is essential and even plays a decisive role in the application of epoxy resins.

Generally, the hardener should be in a solid form at room temperature, such as powder, granular, or flake-like; in the manufacturing of powder coatings, it is used in the molten state.

During the mixing process, the material should not react chemically with resins or other components. After being prepared as a powder coating, the powder coating should have good storage stability and should not react chemically with resins or other components, nor should it clump or aggregate.

Common hardening agents used in powder coatings include, but are not limited to, dicyanamide and its derivatives, anhydrides, imidazoles, imidazolines, cyclic imidazoles, phenolic resins, phenolic resins, polyester resins, and acrylic resins. This experiment considers mechanical properties, crosslinking density, and cost, and selects amine-based curing agents.

In amine-based curing agents, dicyandiamide is a typical representative that has been widely used. Dicyandiamide is a compound with the following chemical structure:

Due to the high melting point ([207-209 °C]) and the presence of CN functional groups, the reaction between dicyandiamide and epoxy resins requires a temperature of at least 150 °C. Therefore, to improve its low reactivity, catalysts or promoters such as imidazole or addition imidazole should be added to accelerate the reaction. The mixture obtained by directly adding such promoters to dicyandiamide is called accelerated dicyandiamide, while the dicyandiamide obtained by introducing substituents through derivatization is called substituted dicyandiamide.

Dicyandiamide-cured coatings exhibit high cross-linking density, resulting in improved density, heat resistance, solvent resistance, and corrosion resistance, making them suitable for use in pipeline environments.

2.4 Selection of Fillers

Fillers can enhance the mechanical strength and other protective properties of the coating. The fillers used in powder coatings for pipes should have the following properties:

(1) Excellent dispersibility, good flowability, free from foreign matter, and available in colorless or white.

(2) Exhibits excellent resistance to water and organic solvents, and good chemical resistance.

(3) It does not react with other components in the formulation, nor does it reduce the physical and mechanical properties of the coating film.

Silica can also be used as a filler in pipe coatings due to its low shrinkage, low oil absorption, good electrical properties, and non-toxicity. Its needle-like structure also provides excellent protection against corrosive media.

2.5 Selection of Additives

The additives for pipe powder coating include: leveling agents, accelerators, anti-scratch agents, hardeners, toughening agents, powder de-agglomerants, degassing agents, and adhesion promoters, among others. These are selected based on the specific product requirements.

It is particularly important to note that, to meet the requirements of CSA Z245.20 System 1B, specifically the water immersion and electrochemical detachment test at 95°C @ 28d, an adhesion promoter must be added to the formulation.

The experiment selected an epoxy resin A:epoxy resin B ratio of 1:2. The experiment compared the long-term cathodic delamination and boiling performance under 95°C @ 28d conditions with and without the addition of an adhesion promoter. Specific performance testing results are shown in Table 4:

Table 4: Impact of Adhesion Promoters on Coating Performance

As shown in Table 4, the adhesion promoter can significantly improve the adhesion of the coating, enabling the coating to meet the strict requirements for long-term cathodic delamination and boiling water resistance tests.

Demonstration of long-term cathodic corrosion protection performance:

Figure 2: Long-term cathodic delamination performance

2.6 Summary of Formulation Performance

based on the selection of raw materials, the designed formulation, and the production of sample putty powder, the material was applied to the workpiece and tested according to CSA Z245.20 System 1B. The test results are shown in Table 5.

Table 5: Summary of Performance Testing Results for Formulations

Product DSC Performance Testing:

Figure 3: Product DSC Performance

3. Construction Process

(1) Pipe blast cleaning to NACE near white (Sa2.5), with a surface roughness of 50-112 μm;

(2) If the sandblasted surface contains water-soluble salts, clean it with phosphoric acid and distilled water.

(3) Heat the pipes to 437-473°F (225-245°C);

(4) Apply high-Tg powder coating products to the pipes, achieving the required thickness as per customer specifications.

(5) Perform post-curing according to the curing guidelines recommended in Table 6, and determine the coating's curing completion using DSC or other methods.

(6) Check for frost point.

Table 6: Product Curing Guide

Table 6's curing guidelines specify the shortest curing time required for the coating to achieve specific performance characteristics. Due to differences in pipe wall thickness, the cooling rate of the pipes varies, which cannot be prevented, but can be determined through online monitoring. High Tg pipes have a higher crosslinking density, requiring a higher curing temperature and longer curing time. The table lists the recommended construction temperature range, which typically does not require post-curing, but must adhere to the minimum construction temperature (actual surface temperature of the pipe) and shortest quenching time as determined in the table.

Under specific temperatures, the time it takes for the coating to undergo cross-linking and hardening to meet performance requirements is referred to as the curing time. A key difference between traditional heat-curing epoxy powders and high-Tg heat-curing epoxy powders is the curing time. High-Tg heat-curing epoxy powders require a higher degree of cross-linking to achieve the required coating performance. A more rigid and densely cross-linked molecular structure can improve the coating's performance at higher temperatures with minimal impact on mechanical and chemical resistance. This is why high-Tg coatings have a longer curing time.

4. Conclusion

(1) Hot-melt epoxy (FBE) powder coatings exhibit excellent adhesion, impact resistance, and corrosion resistance, meeting the requirements for pipeline usage. For environments with higher temperature requirements, it is necessary to design pipeline powder coating products with higher membrane Tg (glass transition temperature) to ensure the service life of the pipelines.

(2) Considering the service environment of the pipes and potential bending requirements during on-site construction, when selecting epoxy resins, it is necessary to consider both the membrane's Tg (glass transition temperature) and its flexibility requirements.

(3) To meet the requirements of the coating's 95°C long-term water resistance and cathodic protection performance after 28 days, an adhesion promoter must be added to the formulation.

(4) High-Tg pipe powder has a large cross-linking density, requiring a higher curing temperature and longer curing time. Experiments and construction must be performed strictly according to the curing guidelines.

References

[1] "Preparation of Epoxy Powder Coatings for Oil Pipeline Use," Lanzhou Institute of Technology, College of Oil Chemistry and Chemical Engineering, Lanzhou, Gansu, 2005

[2] "Research on the Application of High-Temperature Epoxy Coatings in the Anti-Corrosion of Submarine Pipelines," PetroChina Energy Development Co., Ltd., Pipeline Engineering Division, 2015

[3] Zhang Junzhi, Zhou Shiyue, "Powder Coatings and Coating Technology," Chemical Industry Press, 2008