Practical applications and functions of phenolic resin in different refractory materials

Practical applications and functions of phenolic resin in different refractory materials

Phenolic resin, as an important binder in refractory materials, offers advantages over bitumen in terms of superior wetting properties with refractories and graphite, higher residual carbon content, better adhesion, and the ability to be mixed and molded at room temperature. It produces high-strength green bodies with lower levels of harmful substances, improving the working environment. Currently, it is widely used as a binder for carbon-containing refractories. This paper analyzes and discusses the application of phenolic resin in carbon composite shaped refractories such as magnesia-carbon bricks and alumina-carbon bricks, as well as in unshaped refractories such as dry tundish refractories and anhydrous taphole clay in blast furnaces.

Research on phenolic resins for refractory materials mainly focuses on resin modification. Improving the structure and properties of resin residual carbon through catalytic reactions is a major future research trend. Binders are indispensable components in refractory materials, serving to bind granular and powdered refractory materials together. Synthesizing novel interpenetrating network high-performance phenolic resins is another future research trend. This approach mainly focuses on modifying the resin’s structure to create a more complex network structure after curing than ordinary phenolic resins. This improves the material’s low-temperature baking strength and medium-temperature strength, while simultaneously reducing the amount of phenol used and lowering the cost of the phenolic resin.

Phenolic Resins

Phenolic resins are a large class of synthetic resins produced by the condensation polymerization of phenolic and aldehyde compounds. The primary aldehyde compound used is formaldehyde, but other commonly used aldehydes include acetaldehyde, formaldehyde, furfural, or mixtures of several aldehydes. The unique chemical structure and macromolecular cross-linked network structure of phenolic resins endow them with excellent adhesion, superior heat resistance, unique ablation resistance, and good flame retardancy. Therefore, phenolic resins are widely used in fiberglass, molding compounds, adhesives, thermal and electrical insulation materials, coatings, etc.

Phenolic resins can be classified into two types according to their structural morphology and heating characteristics: thermosetting phenolic resins and thermoplastic phenolic resins. Thermosetting phenolic resins are produced by reacting a resin containing hydroxymethyl active groups that can be further reacted, using an alkali such as potassium hydroxide, sodium hydroxide, barium hydroxide, or calcium hydroxide as a catalyst, with a formaldehyde/phenol molar ratio >1, pH >7, and at a certain temperature for a certain period of time. Thermoplastic phenolic resin, commonly known as phenolic varnish or linear phenolic resin, is a phenolic resin produced by reacting formaldehyde (F) and excess phenol (P) under the action of acidic catalysts such as oxalic acid, hydrochloric acid, sulfuric acid, and formic acid (where the molar ratio (F/P) is 0.6-0.9). Regardless of whether it is a thermosetting or thermoplastic resin, the final cured structure is the same: a three-dimensional network structure. The residual carbon obtained from the high-temperature carbonization of the resin is an important structural component supporting the refractory material and ensuring its lifespan.

Application of Phenolic Resin in Carbon Composite Shaped Refractory Materials

Currently, due to the development of the metal smelting industry, especially the steel industry, the demand for refractory materials has greatly increased, leading to a year-on-year increase in the application of phenolic resin in the refractory materials industry. Refractory materials are widely used in the metal smelting industry, cement industry, and ceramics industry. The binder is an indispensable component in the production of refractory materials, playing a role in binding refractory particles and powder together. Moreover, in the production practice of refractory materials, it has been recognized that the quality and stability of phenolic resin are one of the main factors affecting the performance of carbon-containing refractory materials, highlighting the increasing importance of phenolic resin.

2.1 Application in Magnesia-Carbon Bricks

Magnesia-carbon bricks are high-temperature composite materials of carbon complexes, co-fired from fused magnesia, graphite, and a binder. They have a long service life, with a furnace life exceeding 1600 heats, and have therefore seen significant development in recent years. Magnesia-carbon bricks are widely used in the iron and steel metallurgical industry, mainly due to their excellent performance. However, the performance of the binder, phenolic resin, can affect the performance of magnesia-carbon bricks. Phenolic resin for magnesia-carbon bricks should have characteristics such as high solid content, low relative molecular mass, high residual carbon rate, and low water content.

Through research on the relationship between various resin properties and their raw material ratios and synthesis processes, the optimal raw material ratio for synthesizing phenolic resin for magnesia-carbon bricks is determined to be: m(F)/m(P) = 1.0~1.3; the catalyst is a 50% NaOH solution with w(NaOH) added at 1%~1.5% of the phenol mass; the optimal synthesis process is: first react at 60~70℃ for 1 hour, then raise the temperature to 80~90℃ and react for 1~2 hours. Phenolic resin synthesized under optimal raw material ratios and process conditions has a relative molecular weight of 300-400, a viscosity of 15-20 Pa·s, a solid content of 77.0%-82.0% (w), and a residual carbon content of 43.0%-49.0% (w), meeting the performance requirements for phenolic resin in magnesia-carbon bricks.

Since the performance of magnesia-carbon bricks largely depends on the carbon structure formed by the bonding system, and phenolic resin pyrolysis forms a disordered, brittle, homogeneous glassy carbon structure, which is highly sensitive to oxidation and stress, magnesia-carbon bricks often use resin combined with other binders such as bitumen to improve oxidation resistance and mechanical properties.

2.2 Application in Alumina-Carbon Bricks

Alumina-carbon bricks are made from graphite and corundum, mixed with a certain amount of binder and additives, and then pressed. Since Al₂O₃ and C do not undergo a solid-phase reaction at the firing temperature of the product, the bonding strength of alumina-carbon bricks mainly comes from the additives and binder. Phenolic resin is widely used in alumina-carbon bricks due to its excellent properties. Currently, phenolic resin is widely used as a binder in aluminum-carbon refractories. The bonding principle is mainly through the cross-linking reaction of phenolic resin molecular chains during the curing process to form a network structure, thereby improving the bonding force between the binder and the material.

Applications of Phenolic Resin in Unshaped Refractories

Phenolic resin is mainly used as a binder in carbon-containing shaped refractories. In recent years, its application areas have been continuously expanding, and it is now used as a binder in unshaped refractories.

3.1 Application in Anhydrous Blast Furnace Sticking

Compared to other unshaped refractories, phenolic resin is most widely used in sticking. Early sticking, using tar and pitch as binders, primarily employed tar and pitch. However, during use, tar and pitch produced carcinogens and black smoke, causing significant environmental harm, and were also difficult to sinter and had slow hardening speeds. As a binder for anhydrous sticking, phenolic resin not only avoids the pollution problems caused by using tar as a binder, but also improves the corrosion resistance, sintering properties, and wear resistance of the sticking.

3.2 Application in Tundish Dry-Type Materials

The application of tundish dry-type materials is increasing. Phenolic resin is the most widely used binder in tundish dry-type processes both domestically and internationally, due to its excellent workability and performance. However, due to the disadvantages of phenolic resin, such as increasing carbon and hydrogen in molten steel, high cost, and low strength at medium and high temperatures, the application of phenolic resin in dry tundish materials has been greatly hindered to a certain extent.

Conclusion In summary

This paper analyzes the application of phenolic resin in various types of refractory materials, including magnesia-carbon bricks, alumina-carbon bricks, dry tundish refractories, and anhydrous taphole clay for blast furnaces. As one of the earliest synthetic resins, phenolic resin is widely used due to its advantages such as high heat hardening properties, high drying strength, and high-temperature strength. In the field of refractory materials, it is used as a good binder in the preparation of carbon composite refractory materials. However, phenolic resin has two major drawbacks: high cost and the fact that its pyrolytic carbon is glassy carbon, which to some extent affects its application in refractory materials.