Phenolic resin is a type of resin produced by the addition condensation reaction of phenolic compounds (such as phenol, cresol, xylenol, resorcinol, tert butyl phenol, bisphenol A, etc.) and aldehyde compounds (such as formaldehyde, acetaldehyde, polyformaldehyde, furfural, etc.) under alkaline or acidic catalysts. The reaction between phenol and aldehyde is quite complex, and due to the molar ratio of phenol to formaldehyde, the different catalysts used, the rate and products of addition and condensation reactions also vary.

1、 Reaction of alkaline catalyst

Many inorganic and organic bases can be used as alkaline catalysts, such as sodium hydroxide, barium hydroxide, ammonium hydroxide, calcium hydroxide, and ethylamine in laminated adhesives. Under alkaline catalyst conditions, the addition reaction of 1mol (sometimes up to 2.5mol) formaldehyde is dominant, while the condensation reaction proceeds slowly. The initial resin generated is formaldehyde formaldehyde resin, and the main reaction process is as follows:

1. Addition reaction (hydroxymethylation)

Phenol and formaldehyde undergo an addition reaction first, producing 1~3 hydroxymethylphenol

2. Condensation reaction (methylation)

Hydroxymethylphenol further condenses to form initial resins, also known as thermosetting phenolic resins (resols), A-stage resins, or one-step resins.

(1) Phenol reacts with hydroxymethylphenol to produce di (hydroxyphenylmethane)

(2) Reaction between hydroxymethylphenols

(3) Phenol or hydroxymethyl reacts with dimers or multi panel adhesive polymers, and polymers react with each other.

2、 Reaction of acidic catalyst

Acidic catalysts are strong acids, including inorganic acids and organic acids. Commonly used ones include hydrochloric acid, sulfuric acid, oxalic acid, benzenesulfonic acid, petroleum sulfonic acid, chloroacetic acid, etc. In acidic catalytic reactions, the molar ratio of phenol to formaldehyde is generally greater than 1:0.9, and the condensation rate of the generated hydroxymethyl group with the phenol nucleus far exceeds the addition rate of formaldehyde and phenol. The resulting resin has a linear structure and is soluble. Therefore, it is called thermoplastic phenolic resin (novolak) or linear phenolic resin. The reaction process is as follows:

(1) Formaldehyde can combine with water to form methylene glycol (HOCH2OH), which in acidic media generates hydroxymethyl cations; The (+CH2OH) hydroxymethyl cation undergoes electrophilic substitution reactions on the ortho and para positions of phenol, producing ortho hydroxymethylphenol and para hydroxymethylphenol

(2) Hydroxymethylphenol undergoes phenol condensation to produce dihydroxydiphenyl methane, which has three isomers: 2,2 '- di (hydroxyphenyl) methane; 2,4 '- di (hydroxyphenyl) methane and 4,4' - di (hydroxyphenyl) methane. The proportion of the three isomers is related to the pH of the reaction medium. In an acidic reaction medium, the two isomers are the main products after laminating.

The above three isomers undergo addition reaction with formaldehyde, and the resulting hydroxymethyl group then undergoes condensation reaction with phenol. Repeat this process to form phenolic resin oligomers. When synthesizing thermoplastic phenolic resin, the excess of phenol limits the increase in relative molecular weight of the resin. On average, each molecule contains about 5-6 benzene rings, and there are no unreacted hydroxymethyl groups. Even if heated for a long time, it can only melt and will not solidify, hence it is called thermoplastic phenolic resin. Because not all three functional groups of phenol react, it can be converted into thermosetting phenolic resin when heated with hexamethylenetetramine and polyformaldehyde.

Products and transformations of phenol formaldehyde reaction

The reaction rate and products of phenol and formaldehyde vary with different molar ratios, medium pH values, and catalyst types, as shown in the table

Reaction rate and products of phenol and formaldehyde