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REACTIONS OF FORMALDEHYDE WITH PHENOLS 187
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O
H
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CHs CHa CH3
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•CH2 CK,- •GH,—
According to his work, the average molecular weights of these resins are
low, ranging in the neighborhood of 300 to 700.
The presence of phenol hydroxy groups in Novolaks is indicated by
Eoeb&er's finding that these products are soluble in alcoholic sodium hydroxide.
The dielectric properties of thermo-plastic phenolics is reported
to be due to the rotation of these polar hydroxy groups 29 . Meyer's studies 40
on phenol-resin structures led him to the conclusion that the Xovolaks
obtained by acid condensation of phenol and formaldehyde have terminal
p-HOCaH*- groups,
As previously pointed out, these theories do not account for the resin
from 2,4-dimethylphenol reported by Granger 24 .
Infusible Resins. The cross-linked resins which are infusible and insoluble
in organic solvents may be regarded as the products of polyrnethylolphenols
-which can condense with other phenolic nuclei in three dimensions
to form cross-linked macro-molecules. They are most readily prepared
from phenols hi which the three active, 2, 4 and 6, ring positions are unsubstituted.
Alkali catalysts favor their formation and a formaldehydeto-phenol
molar ratio of greater than one is desirable. Granger 25 points
out that, when acid catalysts are employed, considerable excess formaldehyde
is required and much of it does not react and is wasted. Satisfactory
results are obtained by acid condensation of polymethylolphenols produced
under alkaline conditions.
The formation of infusible resins usually takes place in stages. Baekelaud
3 recognized three fairly distinct stages producing bodies which he
described as "A", "B" and ''C 1 ' resins. This formulation has been generally
accepted by resin chemists. "A" resins are liquid or solid initial
condensation products which are soluble and can be melted. "B" resins
are solvent-sensitive and can be melted or softened a limited number of
times. "G" resins are the final, infusible, insoluble products. The names
resols, resitols and resites, respectively, have also been used for these resins,
The transition to the final condensation product is known as ft eure."
Xovolaks are incurable, whereas "A" resins are cured to "C", passing
through the intermediate £i B" in the process.
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