Requirements for Binders for Al2O3-SiC-C Bricks (Alumina Silicon Carbide Carbon Bricks)
Binders play a pivotal role in the production of Al₂O₃-SiC-C bricks (alumina-silicon carbide-carbon bricks), significantly influencing the mixing and forming properties of the green mix, as well as the microstructure of the final product. The primary requirements for binders used in Al₂O₃-SiC-C bricks are as follows:
- (1) They must exhibit excellent wettability with the Al₂O₃-based refractory aggregates and matrix materials.
- (2) They should contain no—or minimal—components that are harmful to human health.
- (3) The properties of the mixed batch should remain relatively stable over time, and the extent of chemical reaction with the aggregates should be minimal.
- (4) During the heating process of the product, the binder should maintain a high residual carbon yield; furthermore, the polymer structure formed after carbonization must possess excellent high-temperature strength.
Good wettability between the binder and the refractory aggregates and graphite—combined with appropriate viscosity—can significantly enhance the bulk density and mechanical strength of the final product. Moreover, excellent wettability with graphite facilitates the uniform dispersion of graphite particles throughout the product, ideally forming a continuous network structure. Upon carbonization, this network evolves into a continuous carbon-bonded skeleton, thereby substantially improving both the mechanical strength and high-temperature slag resistance of the product. Consequently, the selection of an appropriate binder is of paramount importance. In the manufacture of Al₂O₃-SiC-C bricks, phenolic resins and aluminum dihydrogen phosphate are frequently selected as the binders.
Phenolic Resins
Phenolic resins are synthesized through a polycondensation reaction involving phenolic compounds (such as cresol, phenol, xylenol, and resorcinol) and aldehyde compounds (such as formaldehyde and furfural), catalyzed by acids or bases. In the refractory industry, phenolic resins are widely utilized in the production of carbon-containing refractories due to their high carbon yield, excellent bonding strength—which enhances product mechanical strength—low emission of harmful volatile organic compounds, and high thermal stability. Based on their thermal behavior and structural morphology, they are primarily classified into two categories: thermosetting phenolic resins and thermoplastic phenolic resins.
In the fabrication of Al2O3-SiC-C bricks, thermosetting phenolic resins are typically selected as the binder; they are generally added at a concentration of approximately 5%, resulting in refractory products with relatively low porosity after molding. When subjected to heat in a neutral or reducing atmosphere, phenolic resins undergo thermal decomposition, generating gaseous products such as CO2, CO, CH4, H2, and H2O.
Within the temperature range of 200°C to 1000°C, phenolic resins continuously decompose to generate gases. As these gases volatilize, they create open pores, thereby increasing the apparent porosity of the refractory material and negatively impacting the mechanical strength, oxidation resistance, and slag resistance of carbon-containing refractory products. Consequently, to ensure the optimal performance of refractory products, two key measures must be taken: firstly, selecting a resin binder characterized by low gas evolution and a high carbon yield; and secondly, determining and utilizing an appropriate addition level for the resin.
Aluminum Dihydrogen Phosphate
Aluminum phosphate is widely utilized in the preparation of refractory materials. One of its primary advantages as a binder for refractories is that the resulting bonded structure exhibits excellent properties at intermediate temperatures. Aluminum phosphate is typically synthesized through the reaction of aluminum hydroxide with phosphoric acid. Depending on the degree of neutralization during this reaction, three distinct products can be formed: aluminum dihydrogen phosphate [Al(H₂PO₄)₃], aluminum hydrogen phosphate [Al₂(HPO₄)₃], and aluminum orthophosphate [AlPO₄]. For refractory applications, aluminum dihydrogen phosphate is the preferred choice of binder; its primary mechanism involves the polymerization of reaction products upon heating to a specific temperature, thereby enhancing the intermediate-temperature performance of the refractory product. At room temperature, aluminum dihydrogen phosphate is water-soluble; however, when heated to a certain temperature, it decomposes into aluminum pyrophosphate and aluminum metaphosphate, simultaneously undergoing polymerization reactions.
The formation and polymerization of aluminum metaphosphate [Al(PO₃)₃]n generate strong adhesive forces, thereby imparting strength to the refractory material at intermediate temperatures. As the temperature continues to rise, the aluminum metaphosphate decomposes to yield AlPO₄ and P₂O₅. The P₂O₅ can further react with the Al₂O₃ present in the refractory material to form additional AlPO₄, thereby further enhancing the intermediate-temperature strength of the refractory product.
Below 500°C, the aluminum dihydrogen phosphate primarily undergoes a dehydration process as the temperature increases. As the free water within the refractory product is expelled, the product undergoes shrinkage. During this dehydration phase—prior to reaching 500°C—the overall volume of the product remains relatively stable; however, the porosity of the binder phase increases, while its bulk density decreases. At this stage—due to the gradual precipitation of AlPO₄ and the subsequent formation and polymerization of aluminum pyrophosphate and aluminum metaphosphate—the density of the bonded structure may decrease slightly, yet its mechanical strength increases significantly.
When the temperature exceeds 500°C, the dehydration process within the bonded structure diminishes, and the product’s weight loss becomes negligible. Prior to the onset of high-temperature ceramic bonding within the material, there are no significant changes observed in the product’s porosity or density.
For refractory specimens utilizing aluminum dihydrogen phosphate as a binder, the cold-state strength begins to decline once the temperature surpasses a turning point of approximately 500°C; the strength does not begin to rise again until high-temperature ceramic bonding is established within the interior of the specimen. In contrast to its cold strength, the hot strength of the specimen increased continuously, reaching a maximum value at a temperature of 900°C. This increase in hot strength with rising temperature is likely attributable to the formation of compounds such as AlPO4 and Al(PO3)3 during the heating process; furthermore, the thermal expansion of the material fills the pores, resulting in a denser structure.
Between 900°C and 1000°C, the material’s hot strength declined significantly. This decline may be attributed to two factors: on one hand, the P2O5—generated from the decomposition of aluminum phosphate and aluminum pyrophosphate within the product—begins to volatilize; on the other hand, it may be related to the crystallographic phase transformation of AlPO4. As the temperature rises, AlPO4 undergoes a series of phase transformations: the low-temperature quartz form (low-temperature berlinite) transforms into the high-temperature quartz form (high-temperature berlinite) at 586°C. The high-temperature berlinite form can further transform into the tridymite form at 815°C, and the tridymite form transforms into the cristobalite form at 1025°C. During these phase transformations, the various crystalline forms of AlPO4 undergo volume expansion or contraction; this disrupts the product’s structural integrity, induces cracking, and consequently reduces the product’s bonding strength. When the temperature exceeds 1000°C, aluminum dihydrogen phosphate completely decomposes to yield AlPO4 and P2O5 (note that AlPO4 itself does not undergo decomposition to form M2O3 and P2O5 until reaching 1760°C); once the P2O5 has volatilized, only AlPO4 remains. Calculations based on solid-solution formulas indicate that, at high temperatures, AlPO4 can react vigorously with SiO2 to produce mullite and P2O5.
