Find out what layers of materials are made of non-metallic expansion joints first, and stop asking layman questions such as "which material is best"
Two days ago, I met a customer and asked, "Which material is the best for your non-metallic expansion joint?" I was stunned for a moment and asked him, "How many degrees does your pipe run? How many MPa is the pressure? Is there any corrosive medium?" He couldn't answer. This question itself is wrong-non-metallic expansion joints never depend on a single material to dominate the world, but multi-layer composite structures work together. You take apart a typical non-metallic expansion joint (also called a fabric fiber expansion joint). From the inside out, it is roughly: a fluoroplastic/rubber sealing layer, a ceramic fiber or glass fiber insulation layer, a wire mesh or fabric reinforcement layer, and a weather-resistant rubber/fluororubber protective layer on the outermost layer. Each layer does each layer of work, and as soon as the temperature changes, the scenes of each layer of materials have to be rearranged. So don't ask "which is the best", ask "what temperature range is your working condition in" first.
Normal temperature to 200℃: Performance boundary and typical application scenarios of rubber and fluoroplastic materials
This interval is the comfort zone for rubber compensators and PTFE compensators. For ordinary EPDM and neoprene, it is not a big problem to carry 120℃ for a long time, and it can be tolerated to rush to 150℃ in an instant. But if you expect the rubber to remain elastic above 150℃, you are rogue-the rubber will harden, crack, and even carbonize. On the fluoroplastics side, PTFE (polytetrafluoroethylene) has a temperature resistance of over 200℃, which is excellent in corrosion resistance. However, it has large creep and poor resilience, so it is usually made into a lining or composite layer, which does not bear force alone.
In actual projects, the most common ones from room temperature to 200℃ are flue gas pipes, ventilation pipes and low-temperature water vapor pipes. Choose rubber PTFE compensator or non-metallic expansion joint. The key is to see whether the medium is corroded. There are acid and base gases? The inner layer must be covered with PTFE or fluoroplastic. No corrosion? A regular rubber compensator is enough, cheap and durable. There is no metaphysics in this area, just don't overheat.
200℃ to 400℃: the true performance of silica gel, fluororubber and PTFE composite layers, which parameters will suddenly deteriorate?
Above 200℃, ordinary rubber is basically out. The only materials that can be applied are silica gel, fluororubber (FKM), and PTFE composite layers. But you think they can easily carry it to 400℃? Naive.
The continuous use temperature of silica gel generally reaches 250℃, and it can rush to 300℃ in a short time. However, after exceeding 250℃, the mechanical properties of silica gel drop from a cliff-the tensile strength and tear strength drop so much that you doubt your life. The temperature resistance of fluororubber is slightly better than that of silica gel, and it can reach 250℃-280℃ for a long time. However, when it exceeds 300℃, it begins to defluorinate, releasing corrosive gas, and when it is superimposed with acidic medium, the aging speed takes off directly. PTFE, in theory, can be used continuously at 260℃, but it creeps greatly at high temperature and high pressure, and PTFE will release a very small amount of fluorine above 260℃, which is a disaster for some precision equipment.
Therefore, in this interval, real engineers will make a "composite layer": ceramic fiber or glass fiber is used as the thermal insulation skeleton inside, fluoroplastic film is attached to the medium side, and then silicone or fluororubber is used as the outer seal. Many of the non-metallic expansion joints (fabric fiber expansion joints) you see are of this structure. The core of this interval material selection is not which material to choose, but how to combine it. One parameter is most susceptible to sudden deterioration: interlaminar adhesion. When the temperature is high, the adhesive fails first, the layers break off, the expansion joint bulges, and then the whole is scrapped. When selecting, be sure to ask what bonding process the manufacturer uses, and don't just look at the temperature-resistant paper data.
Above 400℃: Temperature Resistance Limit and Failure Mode of Ceramic Fiber, Glass Fiber and Wire Mesh Reinforcement Layer
Above 400℃, polymer materials are basically destroyed, leaving inorganic fibers and metals. Ceramic fiber is the protagonist, with long-term temperature resistance above 1000℃, while glass fiber is slightly lower, but it is stable at 500℃-600℃. The real bottleneck lies in the sealing layer: it is impossible to use rubber or fluoroplastics at this temperature, and it can only be laminated into a "metal-non-metal composite" by wire mesh + ceramic fiber, which is called metal mesh-reinforced non-metal expansion joint in China.
The failure mode has also changed. Low temperature is aging cracking, high temperature is fiber pulverization and wire mesh oxidation. What are ceramic fibers most afraid of? Thermal shock-the fiber breaks and pulverizes and falls off when the temperature changes suddenly. Wire mesh is obviously oxidized when it exceeds 500℃, especially in flue gas with sulfur and chloride ions. Corrosion + oxidation are combined, and the life is directly cut in half. Therefore, the surface of non-metallic expansion joints above 400℃ is usually coated with high-temperature resistant coating (such as aluminosilicate coating) to separate oxygen from corrosive media. In addition, do not neglect the thickness of the insulation layer in this temperature range. Some manufacturers make the insulation layer thin in order to save costs. As soon as the thermal bridge effect comes out, the outer surface is hot, and the metal flange may be deformed.
Comparison table of type selection in different temperature intervals: from flue gas duct to high temperature air duct, direct copy of material selection logic
If it is not completed, go directly to the table:
- ≤150℃: Ordinary EPDM, neoprene, used in flue gas pipeline, dust removal pipeline. Typical Product: Rubber Compensator.
- 150℃—200℃: PTFE lining + rubber outer layer, or fluororubber integral molding to deal with weak corrosive media. Typical products: PTFE compensator, rubber PTFE compensator.
- 200℃—350℃: Silicone/fluororubber + glass fiber composite layer, suitable for hot air duct, boiler flue. Typical products: Non-metallic expansion joints (fabric fiber expansion joints).
- 350℃—600℃: Ceramic fiber + glass fiber + wire mesh reinforcement, outer layer with high temperature resistant coating. Suitable for high temperature air duct and power station flue gas system.
- ＞600℃: All-ceramic fiber layer + metal mesh interlayer, and the inner layer is even superimposed with refractory castable. This kind of working condition generally has to be customized, and conventional products can't do it.
The higher the temperature, the thinner the sealing layer, the thicker the insulation layer, and the denser the reinforcement layer. You see those rectangular non-metallic expansion joints, with large cross-sections and many layers. They are not showing off, and each layer is forced out by working conditions.
In addition to material selection, it also depends on the working conditions: how do pressure, corrosive medium and displacement affect the upper limit of temperature?
Temperature is just the ticket, and what really determines lifespan is the superposition of three things.
Number one, stress. The higher the pressure, the easier the material is to be "topped" out of the bulge. In the same 250℃ working condition, the material selection of 1kPa flue gas pipeline and 50kPa compression air duct is completely different. Under high-pressure working conditions, even if the temperature is not high, part of the temperature resistance must be sacrificed to change the strength-for example, the number of wire mesh layers must be increased, because the fiber layer is compacted as soon as it is compressed, the heat insulation effect decreases, the "internal conduction" of temperature intensifies, and the surface material ages faster.
Second, the corrosive medium. Sulfur oxides and chloride ions in flue gas can corrode and crack stainless steel mesh at 200℃, let alone fluororubber. When encountering acidic medium, inner PTFE is almost necessary, but the temperature resistance of PTFE is limited. At this time, you have to struggle: Do you keep temperature resistance or anti-corrosion? The realistic approach is to adopt the gradient structure of "multi-layer PTFE + ceramic fiber", so that PTFE only faces the medium side, and the temperature is blocked by the heat insulation layer. Doesn't it sound simple? However, many small factories can't do a good job in interlayer compounding, and they are delaminated after a period of time.
Third, the amount of displacement. The advantage of non-metallic expansion joint is large displacement compensation, but the displacement amount is too large, the fiber layer bends repeatedly, and the fatigue fracture is faster than over-temperature. Especially at high temperature, the ceramic fiber itself is brittle, and then superimposed with frequent displacement, the fiber pulverization speed doubles. Therefore, in the case of large displacement, it is necessary to increase the thickness of fiber layer or adopt corrugated fabric structure instead of simply stacking materials.
Have you noticed? The performance comparison of non-metallic expansion joint materials in different temperature ranges finally requires "working condition selection". Simply asking what degree of temperature resistance is as amateur as asking "which material is the best". Next time, choose the non-metallic expansion joint. First, throw the four cards of temperature, pressure, medium and displacement to the manufacturer, and let the manufacturer arrange the composite layer structure for you. If the manufacturer only reports a temperature resistance number, this product can't be asked for.