When formaldehyde / phenol (molar ratio) is less than 1, thermoplastic products can be obtained, which is called thermoplastic phenolic resin, namely linear phenolic resin. It does not contain further condensation groups, and can be cured by adding curing agent and heating. Such as hexamethylene tetramine as curing agent, curing temperature 150 ℃, mixed with fillers made of molding powder, commonly known as bakelite powder. When the molar ratio of formaldehyde to phenol is greater than 1, the first step resin, namely thermosetting phenolic resin, is obtained under the catalysis of alkali. It can be dissolved in organic solvents. The first stage resin contains hydroxymethyl which can be further polycondensated, so it can be cured without adding curing agent. The second stage resin, also known as semi soluble phenolic resin, is insoluble and insoluble, but can be swelling and softening. Then, the insoluble phase C resin, also known as insoluble phenolic resin, was obtained by further reaction. Phase a resin can also self cure after long-term storage.

The curing forms of thermosetting phenolic resin can be divided into room temperature curing and thermal curing. Non toxic curing agent NL, benzenesulfonyl chloride or petroleum sulfonic acid can be used for room temperature curing, but the latter two materials are toxic and irritant.

The polycondensation products of phenols and aldehydes are generally known as phenolic resins, which are usually synthesized by condensation of phenol and formaldehyde. They are the earliest thermosetting resins.

Although phenolic resin is the oldest thermosetting resin, it is still widely used in the manufacture of glass fiber reinforced plastics and carbon fiber reinforced plastics because of its easy raw materials, convenient synthesis, good mechanical strength and heat resistance, especially outstanding instantaneous high temperature ablation resistance, and the resin itself has room for extensive modification Composite materials. Phenolic resin composites, especially in the aerospace industry (space vehicles, rockets, missiles, etc.) as instantaneous high temperature and ablation resistant structural materials have very important applications.

The synthesis and curing process of phenolic resin completely follow the law of bulk polycondensation. Two kinds of phenolic resins can be obtained by controlling different synthesis conditions (such as the ratio of phenol to aldehyde, the type of catalyst used, etc.): one is called thermosetting phenolic resin, which contains hydroxyl methyl active group which can be further reacted. If the synthesis is not controlled, the shape polycondensation reaction will continue until the formation of insoluble and insoluble three-dimensional network This kind of resin is also called first-order resin; the other is called thermoplastic phenolic resin, which is linear resin. It will not form three-way network structure in the synthesis process. In the further curing process, curing agent must be added. This kind of resin is also called second-order resin. These two kinds of bakelite powder are thermosetting materials, which are different from general thermoplastic materials. They have excellent differences and thermoplastic properties.