Polycondensation Methylol derivatives reaction

Polycondensation of Simple Methylol Derivatives. This is illustrated belijw jjy the reaction of equimolar quantities of formaldehyde and uiea ... 

Polycondensation, This t -pe of reaction is identical nit.h polycondensa-tion of simple methylol derivatives (p. 112). 

The most commonly known phenolic composite group is phenol formaldehyde polymers (phenoplasts). They are produced by polycondensation of a phenol and a mixture of phenols (phenol and phenol derivatives like cresol-resorcinol or para tertiary butyl phenol) with an aldehyde, usually formaldehyde and hexamethylene tetramine. Reaction of formaldehyde with phenol (up to 3 moles of formaldehyde can react with one mole of phenol - phenol acts as a three functional monomer) yields methylol groups in the ortho and para positions of the phenol molecule. In a further reaction, the methylol groups condenses with another molecule of phenol to form a methylene bridge. In practice, a prepolymer (usually a powder) is prepared first which is then cured later to the shape of the article in the mould. 

Other amino comonomers can be used, but their proportions in the materials are low compared to those of the two preceding molecules. The reaction mechanisms corresponding to the formation of the precursors and their polymerization were presented in Chapter 7. The precursors are generally prepared in batches from the comonomers in aqueous solution. The structure and the properties of the final material are determined by the composition of the reaction medium, its pH, and the time and temperature of reaction. The first step of the process is the formation of methy-lol derivatives, and the second step involves their condensation with elimination of formaldehyde or water. An acidic medium does not permit an easy control of the formation of methylol compounds, and polycondensations are generally carried out in slightly basic medium (pH 7.5 to 9.0). 

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