The basic principle of phenolic resin synthesis

Phenolic resins are phenolic compounds (such as phenol, cresol, xylenol, resorcinol, tert-butylphenol, bisphenol A, etc.) and aldehyde compounds (such as formaldehyde, acetaldehyde, paraformaldehyde, furfural, etc.) Under the action of alkaline or acidic catalysts, the resins obtained by addition polycondensation are collectively referred to as phenolic resins. The reaction of phenols with aldehydes is relatively complicated. Due to the molar ratio of phenol to formaldehyde and the different catalysts used, there are also differences in the rate of addition and condensation polymerization reaction and products.

First, the reaction of alkaline catalyst

Many inorganic bases and organic bases can be used as a basic catalyst. Commonly used are sodium hydroxide, cesium hydroxide, ammonium hydroxide, calcium hydroxide, ethylamine, and the like. 1 mol (sometimes as high as 2.5 mol) formaldehyde under the alkaline catalyst conditions, the addition reaction predominates, while the condensation reaction proceeds slowly, the initial resin generated is resole phenolic resin, the main reaction process is as follows:

1, Addition reaction (methylation)

Phenol and formaldehyde are first subjected to an addition reaction to generate 1-3 methylolphenols

2, condensation reaction (methylene)

Hydroxymethylphenol is further condensed to form initial resins, or thermosols, A-stage resins, and one-step resins.

(1) Phenol reacts with hydroxymethyl phenol to form bis(hydroxyphenyl methane)

(2) Reaction between methylol phenols

(3) Phenol or hydroxymethyl groups react with dimers or multimers and react between polymers.

Second, the reaction of acidic catalyst

Acidic catalysts are strong acids, including inorganic and organic acids. Commonly used are hydrochloric acid, sulfuric acid, oxalic acid, benzenesulfonic acid, petroleum sulfonic acid, chloroacetic acid, and the like. In the acidic catalytic reaction, the molar ratio of phenol to formaldehyde is generally greater than 1:0.9. The condensation rate of methylol and phenolic nucleus is much faster than the addition rate of formaldehyde and phenol, and the obtained resin has a linear structure. Is meltable. It is therefore called a novolak or novolak resin. The reaction process is as follows:

(1) Formaldehyde combines with water to form methylene glycol (HOCH2OH). In acid medium, methylene glycol forms hydroxymethyl cation; (+CH2OH) hydroxymethyl cation is in the ortho position of phenol. Electrophilic substitution reaction at the paraposition to generate o-methylolphenol and p-hydroxymethylphenol

(2), hydroxymethyl phenol condensation of phenol to produce bis (hydroxyphenyl) methane (dihydroxydiphenyl methane) bis (hydroxyphenyl) methane There are 3 isomers: 2,2 '- bis (hydroxyphenyl) methane; 2,4'-bis(hydroxyphenyl)methane and 4,4'-bis(hydroxyphenyl)methane. The ratio of the three isomers is related to the pH of the reaction medium. In the acidic reaction medium, the latter two isomers are the main products.

The above three isomers are subjected to an addition reaction with formaldehyde, and the resulting hydroxymethyl groups are further subjected to a condensation reaction with phenol, so that the repeated production of phenolic resin oligomers is repeated. In the synthesis of thermoplastic phenolic resins, the excess of phenol restricts the increase of the relative molecular mass of the resin. On average, each molecule contains about 5 to 6 benzene rings. There is no unreacted hydroxymethyl group, and even if it is heated for a long time, it can only melt. It does not cure so it is called a thermoplastic phenolic resin. Because the three functional groups of phenol are not completely reacted, hexamethylenetetramine and paraformaldehyde can be converted into thermosetting phenolic resins when heated.

Phenol and formaldehyde reaction products and transformation

The reaction of phenol with formaldehyde varies with the molar ratio, the pH of the medium and the type of catalyst, and the reaction speed and product are also different from each other.

Reaction speed and product of phenol and formaldehyde