As the name suggests, refractory plastic is a material that can withstand high temperatures and has a certain degree of plasticity. Plasticity mainly refers to the ability to take any shape under the action of external force, and to maintain the same shape when the external force contacts it. So what materials are refractory plastic made of, and what are its characteristics?
Refractory plastic is made of 70%~80% granular and powdery materials as the main materials, 10~25% plastic clay and other binders and an appropriate amount of plasticizer. It is a hard mud paste and an amorphous refractory material that maintains high plasticity for a long time.
Refractory plastic is mainly used in various heating furnaces that are not directly in contact with the melt. The materials are mostly clay and high aluminum, and there are also silicon, magnesium, chromium, zircon and silicon carbide. If classified by binder, there are clay-bonded, water glass-bonded, phosphate-bonded, sulfate-bonded plastics, etc.
What are the characteristics of refractory plastic? In order to ensure smooth construction and normal use at high temperatures, refractory plastics should have the following basic properties: ① have a certain plasticity to facilitate construction; ② have a certain shelf life to ensure that the molding performance remains unchanged during the specified storage period; ③ have a certain strength after room temperature curing to facilitate the removal of the frame or transportation after construction; ④ have a certain high-temperature volume stability to prevent the furnace lining structure from being damaged due to excessive deformation.
Therefore, the difference between refractory plastics and other refractory castables is that they have strong plasticity, so how to enhance plasticity?
The plasticity of plastics has a direct impact on the characteristics and amount of clay added. It is also related to the amount of water added. Plasticity increases with the increase of water added. But it cannot be too high, generally 5~10%. To improve plasticity, it is necessary to control the amount of clay and water added in plastics, and plasticizers can be added.
The role of plasticizers in refractory plastics: ① Increase the hygroscopicity of clay particles, disperse clay particles and be coated with water film; ② Solify clay particles; ③ Increase the electrostatic repulsion between clay particles and stabilize the sol; ④ Exclude ions that hinder the solubilization as insoluble salts from the system; ⑤) Increase the viscosity of water in clay to form a solid water film, etc. Commonly used plasticizers include pulp waste liquid, cyclohexane acid, lignin sulfonate, lignin phosphate, lignin chromate, etc.
The choice of binder also has a certain influence on the plasticity of plastics. Instead of using a hydraulic binder, a thermosetting or air-hardening binder can be selected. If an air-hardening binder is used, sealing measures should be taken during storage. For long-term storage, slow setting measures should also be taken.
What are the hardening characteristics and heat treatment characteristics of refractory plastics?
Plastics with soft clay as a binder harden slowly after construction and have very low strength at room temperature. For this reason, an appropriate amount of air-hardening and thermosetting binders should be added. The strength of plastics with sodium silicate changes rapidly after construction. As long as the drying speed is properly controlled, shrinkage cracks can be prevented. The mold can be removed quickly after construction. However, during the drying process, sodium silicate may migrate to the surface of the combination, hindering the smooth removal of moisture and causing deformation of the surface. Therefore, it is used in large kilns and furnace roofs with long construction periods.
Aluminum phosphate is a thermosetting binder and is widely used. After construction, it can obtain high strength after drying and baking. During the heat treatment process, because plastics contain more clay and moisture, they will shrink greatly. Therefore, expansion agents should be added. For example, 5~15% kyanite fine powder is added to the fine powder to offset the shrinkage under high temperature; an appropriate amount of electrofused corundum, raw zircon, quartz and other micron-level barren fine powder is added to replace part of the clay to reduce drying shrinkage. Compared with sintered refractory products and other amorphous refractory materials of the same material, plastics have better thermal shock stability.
From the above, we can see that refractory plastics are classified into many categories according to the different materials and binders, but they all have strong plasticity, structural strength, thermal shock resistance, and convenient construction. They can be used to directly build new linings or to repair linings with severe local wear, with significant results.
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