1. Introduction
　　
　　Types and Applications
　　
　　In 1872, German chemist Baeyer first synthesized phenolic resin. In 1907, Belgian-American Baekeland proposed a heat-curing method that enabled its industrial production. In 1910, the world's first synthetic phenolic resin factory was established in Berlin, Germany, ushering in the era of synthetic polymers. A wide variety of phenolic resins can be produced by varying the phenols, aldehydes, catalysts and phenol-to-aldehyde molar ratios used. These include linear, thermosetting, oil-soluble and water-soluble phenolic resins. Their main uses include producing molding powders and laminated plastics; making varnishes and insulating or corrosion-resistant coatings; manufacturing household articles and decorative products; and producing sound-insulating and heat-insulating materials, wood-based panels, foundry materials and refractory materials.
　　
　　Phenolic resin, abbreviated PF, is a yellow, transparent, amorphous solid in block form. Free phenol gives it a slightly reddish color. It has a specific gravity of 1.25–1.30, is readily soluble in alcohol but insoluble in water, and is stable in water and weak acid or weak alkali solutions. It is made by catalytic polycondensation of phenol and formaldehyde, followed by neutralization and washing with water. Depending on the catalyst selected, it can be divided into thermosetting and thermoplastic types. Phenolic resin has good acid resistance, mechanical properties and heat resistance, and is widely used in corrosion-protection engineering, adhesives, flame-retardant materials and grinding-wheel manufacture.

Liquid phenolic resins are yellow or dark-brown liquids. For example, alkaline phenolic resin is mainly used as a foundry binder.
　　
　　2. Principles of Phenolic Resin Synthesis
　　
　　Solid phenolic resin is a colorless or yellow-brown transparent substance. Commercial products often contain colorants and are sold in red, yellow, black, green, brown, blue and other colors, in granular or powdered form. It resists weak acids and weak alkalis, decomposes in strong acids, and is attacked by strong alkalis. It is insoluble in water but soluble in organic solvents such as acetone and alcohol. It is obtained by the polycondensation of phenol and formaldehyde. Types include linear, thermosetting and oil-soluble phenolic resins. Its main uses include producing molding powders and laminated plastics; making varnishes and insulating or corrosion-resistant coatings; manufacturing household articles and decorative products; and producing sound-insulating and heat-insulating materials. Common examples include low-voltage electrical sockets and plastic furniture handles.
　　
　　Phenolic Resin Laminates
　　
　　Phenolic resin, abbreviated PF, is a yellow, transparent, amorphous solid in lumps. It may have a slight reddish tint due to free phenol. Its specific gravity is 1.25–1.30. It dissolves readily in alcohol, is insoluble in water and remains stable in water and weak acid or alkaline solutions. It is made by the catalytic polycondensation of phenol and formaldehyde, followed by neutralization and washing with water. Depending on the phenol-to-formaldehyde molar ratio and the catalyst selected, it is divided into thermosetting and thermoplastic types. When the formaldehyde-to-phenol molar ratio is greater than one and an alkaline catalyst is used, thermosetting phenolic resin is formed. When the ratio is less than one and an acidic catalyst is used, thermoplastic phenolic resin is formed. Phenolic resin has good acid resistance, mechanical properties and heat resistance. It is widely used in corrosion-protection engineering, adhesives, flame-retardant materials, grinding-wheel manufacture and other industries.
　　
　　Phenolic resin was the first synthetic resin to be industrialized.
　　
　　3. Important Properties of Phenolic Resin
　　
　　High-Temperature Performance
　　
　　The most important characteristic of phenolic resin is its high-temperature resistance. It maintains structural integrity and dimensional stability even at very high temperatures. This is why phenolic resin is used in high-temperature applications such as refractory materials, friction materials, binders and the foundry industry.
　　
　　Phenolic Resin Refractories
　　
　　Bond Strength
　　
　　One important application of phenolic resin is as a binder. Phenolic resin is a versatile material compatible with a wide variety of organic and inorganic fillers. Properly formulated phenolic resins wet surfaces particularly quickly. After crosslinking, they provide the mechanical strength, heat resistance and electrical properties required for abrasives, refractory materials, friction materials and Bakelite molding powder.

Water-soluble or alcohol-soluble phenolic resins are used to impregnate paper, cotton cloth, glass, asbestos and similar materials to give them mechanical strength, electrical properties and other characteristics. Typical examples include electrical-insulation and mechanical laminates, clutch plates and filter paper for automotive filters.
　　
　　High Char Yield
　　
　　In an inert atmosphere at approximately 1000°C, phenolic resin produces a high char yield, helping maintain its structural stability. This property is another important reason for its use in refractory materials.
　　
　　Low Smoke and Low Toxicity
　　
　　Compared with other resin systems, phenolic resin systems offer the advantages of low smoke and low toxicity. During combustion, scientifically formulated phenolic resin systems decompose slowly, producing hydrogen, hydrocarbons, water vapor and carbon oxides. The decomposition process generates relatively little smoke with relatively low toxicity. These characteristics make phenolic resin suitable for public transport and applications with very strict safety requirements, such as mining, protective barriers and construction.
　　
　　Chemical Resistance
　　
　　Crosslinked phenolic resin resists decomposition by chemicals such as gasoline, petroleum, alcohol, ethylene glycol and various hydrocarbons.
　　
　　Heat Treatment
　　
　　Heat treatment raises the glass-transition temperature of cured resin and can further improve its properties. The glass transition is similar to the melting behavior of crystalline solids such as polypropylene. The initial glass-transition temperature of phenolic resin is related to the curing temperature used during the initial curing stage. Heat treatment can increase the mobility of the crosslinked resin and promote further reaction. It can also remove residual volatile phenol, reduce shrinkage, and improve dimensional stability, hardness and high-temperature strength. At the same time, the resin tends to shrink and become more brittle. The heating curve for resin post-treatment depends on the initial curing conditions and the resin system.