Лако-красочные материалы — производство

5.4.1

Chemistry of Phenolic Resins

The starting monomers of phenolic resins are phenol and formaldehyde or parafor­maldehyde. These are allowed to transform into low-molecular-weight resins called ‘novolaks’ (Figure 5.8) by a ‘careful’ polyaddition reaction under acid conditions, followed by transformation into resol resins under alkaline conditions (Figure 5.9).

C-0

Formation of novolak from p-hydroxymethylphenol

Figure 5.8 The chemistry of novolak.

Novolak and resol-type resins are still meltable or soluble, and serve as the starting products for phenolic resins and phenolic resin adhesives, respectively, that are generated by polycondensation reactions and the liberation of water. Novolaks condense (cure) under mild conditions to become hard and brittle thermosets; this occurs with the addition of either formaldehyde or substances that split off

Resonance structures of phenolate ion

CH2OH

Methylol phenol becomes di or trimethylol phenol, with ortho and para positions Figure 5.9 The chemistry of resol.

formaldehyde, such as hexamethylenetetramine. This further enhances the conden­sation reaction owing to the generation of alkalinity (ammonia). Novolaks may be provided with epoxy groups (via the OH-groups of the phenol), thus increasing the heat resistance of epoxy resin adhesives. Novolaks may also be employed as low — temperature base resins for the production ofwood adhesives (see Section 8.4) [21].

Initially, resol-type resins were used in the production of adhesives since, after precondensation under alkaline conditions and neutralization, they cure in a self­condensation reaction when heated to about 150-170 °C by reaction of the hydro­xymethyl groups with orthohydrogen or parahydrogen atoms of other phenol nuclei and the liberation of water.

In common phenolic resin adhesives there are no dimethylene ether bridges between the two hydroxymethyl groups of two phenol rings which were postulated earlier; rather, there are methylene bridges [22]. Without any modifications, a hard and brittle thermoset is also generated which has a high durability and has been used for joining wood since the 1930s.

5.4.2