Binder Selection And Action Mechanism For Carbon Composite Refractories

Binders occupy a very important position in carbon-composite refractory materials. They significantly influence the mixing and forming properties of the batch, as well as the microstructure of the final products.

In terms of mixing and forming, binders are expected to exhibit good wettability toward refractory aggregates and graphite, along with appropriate viscosity, in order to improve the mixing quality of the batch and the bulk density of green bricks. In addition, good wettability enables the binder to distribute uniformly over the surfaces of particles and graphite, forming a continuous network as much as possible. After carbonization, a continuous bonded carbon skeleton can be formed, which helps enhance the strength and corrosion resistance of the products.

The type of binder and the carbonization conditions greatly affect the microstructure and properties of the bonded carbon. Different binders undergo different carbonization processes, resulting in substantial differences in the structure of the bonded carbon formed. Common binders include tar, pitch, and resin.

Asphalt

Asphalt can be broadly classified into two categories: coal tar pitch and petroleum asphalt. It is a mixture mainly composed of aromatic and aliphatic structures. Generally, coal tar pitch contains more aromatic hydrocarbons than petroleum asphalt.

In the refractory industry, asphalt is used as a non-aqueous binder mainly for carbon-containing refractories. It can be used alone or compounded with tar, phenolic resin, and other materials. Coal tar pitch is the primary asphalt binder used in refractories due to its low cost. Compared with resin-derived carbon, the carbon produced from coal tar pitch has a higher degree of crystallization and better oxidation resistance. Meanwhile, carbon-containing refractories bonded with coal tar pitch exhibit excellent service performance; therefore, coal tar pitch has been widely used in carbon-containing refractory products.

When coal tar pitch is used as the binder for carbon-containing refractories, the structure and properties of its carbonized products affect key performances such as strength and corrosion resistance. Therefore, coal tar pitch is required to have a high carbon yield, produce carbonized products with high compactness, and possess good oxidation resistance.

Phenolic Resin

Phenolic resin is a type of non-aqueous organic binder used in refractory materials. It is prepared through the polycondensation of phenol (or cresol, xylenol, or resorcinol) with formaldehyde (or furfural) in the presence of a catalyst.

With a high carbon yield comparable to that of pitch, phenolic resin can be mixed and shaped at room temperature and exhibits good wettability toward refractory aggregates and graphite. In addition, it causes relatively little environmental pollution. For these reasons, phenolic resin, serving as both a binder and a carbon source, is increasingly replacing traditional carbon-forming materials such as pitch in the production of shaped and unshaped refractory products.

Resin-bonded refractories exhibit the following main characteristics:

a. Their production process is environmentally friendly and energy-saving during mixing.

b. They can be prepared in either cured or uncured states. During curing and carbonization, they do not pass through a distinct plastic stage, which gives them excellent deformation resistance.

In addition, a larger amount of carbonaceous material (such as graphite or carbon black) can be incorporated, thereby providing the excellent wear resistance and slag resistance required for converter applications. Practical applications have shown that carbon-bonded refractories using phenolic resin as the carbon source exhibit superior service performance.

Phenolic resins are mainly classified into resole phenolic resin and novolac phenolic resin.

Resole phenolic resin undergoes a dehydration reaction at 100–150 °C, accompanied by a mass loss of about 10% due to water removal. It can be cured by heating or by using formic acid at room temperature and is conventionally regarded as a thermosetting resin.

Novolac phenolic resin itself possesses thermoplasticity. When compounded with hexamine or solid resole resin, it hardens upon heating with the release of ammonia and without water generation. Owing to its inherent thermoplasticity, novolac resin is less susceptible to aging caused by ambient temperature variations. Compared with resole resin, it exhibits much better storage stability, both for the resin itself and for prepared batches. However, its reaction with hexamine releases ammonia, resulting in an unpleasant odor.

Due to the presence of methylol groups in resole phenolic resin, the holding time between batching and forming can be shortened, and the shaped products are less prone to delamination. Nevertheless, its wettability toward refractory aggregates is inferior to that of novolac resin.

Resole resin generates water during curing, while ethylene glycol-commonly used as a solvent for novolac resin-is hygroscopic. Therefore, when additives that react with metals or water are used, strict attention must be paid to the drying conditions.

Composite Binders

The combined use of liquid resin with 10% solid resin can improve the strength of refractory materials. Thermogravimetric (TG) and differential thermal analysis (DTA) of typical thermosetting and thermoplastic phenolic resin binders (Figure 4) show that both exhibit a minor low-temperature endothermic valley (at 133 °C and 120 °C, respectively), a small exothermic peak (at 213 °C and 200 °C, respectively), and a major exothermic peak (at 467 °C and 524 °C, respectively).

The endothermic valley is attributed to the removal of adsorbed water from the carbonized products, while the major exothermic peak results from carbon oxidation. In addition to the removal of adsorbed water from the binder, solvent volatilization also occurs in the temperature range corresponding to the endothermic valley. The temperature of the small exothermic peak corresponds to the rapid curing temperature of the binder, whereas the temperature of the major exothermic peak corresponds to the temperature at which the carbon–oxygen reaction becomes intense.

Accordingly, the following three technical principles should be followed during the heat treatment of green bodies of carbon-containing refractory products prepared with phenolic resin binders:

The optimal drying and curing temperature should be set at the temperature corresponding to the small exothermic peak.

The preheating temperature for tar impregnation should not reach the temperature corresponding to the major exothermic peak.

Unmodified anhydrous resins are unsuitable for the production of products containing free CaO because of their dehydration reaction.

Preliminary applications in the production of certain carbon-containing refractory products have shown that composite resin has a remarkable effect on improving product quality. Composite resin combines two types of resins. It retains the advantages of resole resin (direct thermal curing) and novolac resin (good fluidity in ethanol), while avoiding the disadvantages of resole resin, such as short effective shelf life and limited adaptability to mixing equipment.

Long-term production practice indicates that the optimal dosage for product quality is 5.5%–6% when using a novolac phenolic resin binder, whereas the optimal dosage for resole resin and composite resin is 3.5%–4%.

Binders decompose upon heating, and part of the decomposition products is released in gaseous form. The discharge pathways of these gases eventually become open pores. Therefore, to control open porosity, gas generation should be minimized, which requires a high carbon yield. In addition, the carbonized material is expected to be as dense as possible and to possess good oxidation resistance.

Carbon derived from pitch undergoes graphitization at relatively low temperatures and exhibits high density; however, its microstructure contains more pores and is less compact than that of resin-derived carbon. Resin-derived carbon has the advantage of producing fewer pores after carbonization but suffers from poor oxidation resistance and insufficient densification.

When pitch and resin are blended in an appropriate proportion, the carbon yield of the mixture becomes higher than that of either pure pitch or pure resin alone. Since both pitch and resin possess unique characteristics, their combination can achieve optimal overall performance and microstructure.