formaldehyde - Sciencemadness Dot Org

154
FORMALDEHYDE
ghieinol itseli are obtained. On heating with formaldehyde in the presence
01 relatively high concentrations of hydrochloric acid, phloroglucinol k
converted to a dark reddish brown, water-insoluble resin whose composition
is approximately that represented by the empirical formula CTHS^,
This end-product of the condensation reaction is apparently a polymethylene
derivative in which the ratio of methylene radicals to aromatic nuclei
is approximately 1:L
Methylene-bis-pyrogallol (m.p. 241°C with deeompn.) was prepared
hy Caro 16 by treating 3 parts of pyrogallol in 20 parts of 10 per cent hydrochloric
acid with one part commercial formaldehyde, and allowing the reaction
to proceed for 12 hours at room temperature.
PHEX'OL-FOEMALDEHYDE RESINS
Phenol-formaldehyde resins are the final roducts of most reactions
involving formaldehyde and phenol. Although formaldehyde solution is
the principal form of formaldehyde employed in their preparation, paraformaldehyde
and hexamethylenetetramine (page 303) are also utilized
to a considerable extent. Anhydrous gaseous formaldehyde may likewise
be employed, as illustrated by a process devised by Aylsworth 2 in which
formaldehyde gas is introduced into an autoclave containing hot phenol
or cresol.
In general, these resins ma}" be defined as mixtures of polymethylene
compounds in which phenolic radicals are linked by methylene groups.
They are broadly classified as belonging to two distinct types:
!l'i Fusible resins in which the component molecules have a linear or
chain-like structure.
(2) Infusible resins in which the component molecules are made up of a
three-dimensional network consisting of cross-linked molecules.
Mixed types, is which there is a minimum of cross-linkage or in which
both linear and cross-linked molecules are present, form an intermediate
group.
The type of resin obtained from a given condensation reaction is governed
by the same factors that detennine the nature of the primary reaction
products, viz.? the structure of the phenol and the nature of the reaction
catalyst. The molar ratio of formaldehyde to phenol is also of particular
importance in resin reactions.
The manner in which methylene groups are attached to the phenolic
radicals in resins has been a subject of considerable controversy. However,
the main body of evidence indicates that the majority of these linkages
connect nuclear carbon atoms. Absorption spectra made on thin (0.03mm)
sections of hardened phenolics give definite indication that this is the