When the molar ratio of formaldehyde/phenol is less than 1, a thermoplastic product can be obtained, called thermoplastic phenolic resin, or linear phenolic resin. It does not contain further condensed groups and can only be cured by adding a curing agent and heating. If hexamethylenetetramine is used as a curing agent, the curing temperature is 150 ℃, and the molding powder made by mixing fillers is commonly known as electric wood powder. When the molar ratio of formaldehyde/phenol is greater than 1, a thermosetting phenolic resin is obtained under alkaline catalysis, which is soluble in organic solvents. The thermosetting phenolic resin contains hydroxymethyl groups that can be further condensed, so it can be cured without the need for a curing agent. Under heating, an ethylene stage resin, also known as a semi soluble phenolic resin, is obtained, which is insoluble and non melting but can swell and soften. Further reaction results in an insoluble and non melting type structure of C-stage resin, also known as insoluble phenolic resin. Resin in Stage A can also self solidify after long-term storage.

The curing forms of thermosetting phenolic resin are divided into two types: room temperature curing and heat curing. Room temperature curing can use non-toxic room temperature curing agent NL, as well as benzenesulfonyl chloride or petroleum sulfonic acid, but the latter two materials are more toxic and irritating.

The condensation products of phenols and aldehydes are commonly known as phenolic resins, which generally refer to synthetic resins obtained by the condensation reaction of phenol and formaldehyde. It is the earliest type of thermosetting resin synthesized.

Although phenolic resin is the oldest type of thermosetting resin, it is still widely used in the manufacture of composite materials such as glass fiber reinforced plastics and carbon fiber reinforced plastics due to its easy availability of raw materials, convenient synthesis, good mechanical strength and heat resistance, especially outstanding instantaneous high-temperature ablation resistance, and extensive room for modification of the resin itself. Phenolic resin composite materials, especially in the aerospace industry (spacecraft, rockets, missiles, etc.), have very important applications as structural materials that can withstand instantaneous high temperatures and erosion.

The synthesis and curing process of phenolic resin completely follows the law of bulk polymerization reaction. By controlling different synthesis conditions (such as the ratio of phenols and aldehydes, the type of catalyst used, etc.), two different types of phenolic resins can be obtained: one type is called thermosetting phenolic resin, which is a resin containing hydroxymethyl active groups that can be further reacted. If the synthesis is not controlled, the condensation reaction will continue until a solidified resin with a three-dimensional network structure that is non melting and insoluble is formed. Therefore, this type of resin is also called first-order resin; Another type is called thermoplastic phenolic resin, which is a linear resin that does not form a three-dimensional network structure during synthesis. In the further curing process, a curing agent must be added. This type of resin is also known as second-order resin. These two types of electric wood powder are different from thermosetting materials and general thermoplastic materials, as they have excellent properties that differ from thermoplastics.