The production and processing range for internally and externally epoxy-coated elbows spans φDN80 to ΦDN1800.

Epoxy-coated elbows—both internally and externally coated—are widely used in fire-fighting water supply systems. In the field of building fire protection systems, these elbows play a crucial role. Therefore, the quality of epoxy-coated elbows—both internally and externally coated—is of utmost importance. Epoxy-coated elbow samples (4 pieces) serve as a key component for ensuring the reliability of fire protection systems. Next, we will introduce the production process of epoxy-coated elbows, the properties of the powder (epoxy resin) used, and the performance characteristics of these elbows.

Production process

For internally and externally epoxy-coated elbows, the first step is to pre-treat the base pipe—grinding the elbow (Advantage: Automatic spraying ensures uniform coating on both the inner and outer walls of the substrate, with excellent leveling properties, meeting the standards of CJ/T120-2016 and GB/T5135.20-2010). Next, the ground elbows are placed in an electric heating oven for preheating; once the temperature reaches 230°–250°C (typically taking about one and a half hours), the preheated pipes are ready for coating. After coating, the pipes are transferred to a curing oven for curing (maintaining the temperature at 200°C for 15 minutes). Finally, the finished product is obtained. Following spraying, the heating equipment is preheated to 180°C to cure the coated substrate. Advantage: At the time of spraying, the epoxy resin powder has not yet fully fused to the inner and outer walls of the substrate. After 30 minutes of preheating and curing, the curing agent within the epoxy resin fully hardens, significantly enhancing the adhesion of the coating on both the inner and outer walls.

In China, earlier internal and external epoxy-coated pipe fittings were all made from galvanized elbows. Later, it was discovered that the zinc coating on the inner wall did not actually enhance the adhesion of the inner coating; rather, it had an adverse effect and also increased the cost of the galvanizing process. To improve the adhesion of the inner coating, it became necessary to remove the zinc layer from the inside of the galvanized elbows—a step that further raised costs and led to energy waste. Consequently, some manufacturers have started using welded elbows directly coated without prior galvanization. Before applying the plastic coating, both internal and external epoxy-coated pipe fittings should undergo a sandblasting process to remove rust, oil stains, and any existing zinc coating from the metal surface; this process is commonly referred to as surface pretreatment. While the construction industry does not specify quantitative requirements for surface pretreatment, the petroleum industry has relatively more detailed guidelines in this regard.

Epoxy Resin Performance

1. High mechanical performance: Epoxy resin has strong cohesive forces and a dense molecular structure, which gives it superior mechanical properties compared to general-purpose thermosetting resins such as phenolic resin and unsaturated polyester.
2. Strong adhesion. The epoxy resin curing system contains highly reactive epoxy groups, hydroxyl groups, as well as polar functional groups such as ether bonds, amine bonds, and ester bonds, which impart excellent adhesion of the cured epoxy to polar substrates including metals, ceramics, glass, concrete, and wood.
3. It exhibits low curing shrinkage—typically ranging from 1% to 2%. It is one of the thermosetting resins with relatively low curing shrinkage (phenolic resins have a shrinkage rate of 8% to 10%; unsaturated polyester resins, 4% to 6%; and silicone resins, 4% to 8%). It also has a very low coefficient of linear thermal expansion, generally around 6 × 10⁻⁵/°C. Consequently, its volume change after curing is minimal.
4. Excellent processability: During curing, epoxy resins virtually do not release low-molecular-weight volatiles, allowing them to be molded under low pressure or by contact molding. They can be formulated with various curing agents to produce environmentally friendly coatings such as solvent-free, high-solids, powder coatings, and waterborne coatings.
5. Excellent electrical insulation properties. Epoxy resin is one of the thermosetting resins with relatively good dielectric performance.
6. Good stability and excellent resistance to chemicals.
7. The heat resistance of epoxy cured products generally ranges from 80 to 100°C. Epoxy resins with enhanced heat resistance can withstand temperatures up to 200°C or even higher.

The internally and externally epoxy-coated bends, made from a heavy-duty, modified epoxy resin that is a thermosetting plastic, boast excellent temperature resistance, allowing them to be used continuously over a wide temperature range—from -30°C to 200°C—and are non-combustible. Additionally, these bends exhibit outstanding resistance to chemical corrosion, fire-extinguishing agent corrosion, high temperatures, and woodpile fires, as well as superior corrosion and flame-retardant performance in both wet and dry conditions. They are particularly well-suited for firefighting applications involving water and gas delivery. The internally and externally epoxy-coated bends offer exceptional resistance to chemical corrosion, water, and solvents, as well as to proteins and high-expansion foam fire extinguishing agents. By addressing the corrosive effects of water, underground installation, and exposure to acids, alkalis, and salts on metal pipelines, these bends significantly extend the service life of firefighting and water (gas) supply pipelines—up to over 50 years under optimal conditions. Moreover, their overall maintenance costs are only one-sixth of those associated with conventional galvanized pipe bends.