Chemistry Reference
In-Depth Information
O
O
H
(R)
H
N
+
+
O
H
O
O
N
H
In amine or ammonia-catalyzed reactions [
145
], the additions and the condensations occur almost
simultaneously with each other. Methylol groups are still present in the finished resins to the extent of
15-30 groups per 100 phenol residues. The structures are branched and the degree of branching
depends upon the amine used.
7.12.4 Typical Commercial Preparations
The resols are usually prepared in typical reaction kettles, using 1.5-2.0 moles of formaldehyde per
mole of the phenol. The reactions are rapid and the condensations to resoles might be accomplished in
1 h. Formaldehyde is often added in the form of formalin. The quantity of the added caustic or
ammonia might comprise one percent of the phenol in the reaction mixture. These reactions are
carried out at water reflux for a specified time. The pH is then lowered to neutral and the water is
distilled off, usually at reduced pressure. The progress of the condensation is followed by measuring
the melting point, the gel time (time required for the material to become thermoset at a specified
temperature), solubility, or free phenol content.
Better quality novolacs and resoles are prepared in stainless steel resin kettles. For novolacs, a
typical recipe might call for mole of phenol to 0.8 moles of formaldehyde (usually added as formalin,
a 37% solution in water). Acid catalysts, like oxalic, hydrochloric, or others, are added in amounts of
1-2% by weight of the phenol. Oxalic acid is favored over hydrochloric, sulfuric, or phosphoric due
to corrosion problems. In addition, vapors of hydrochloric acid tend to react with vapors of formal-
dehyde and form a carcinogenic compound, 1,1
0
-dichlorodimethyl ether. The reactions are conducted
at the reflux temperature of water for 2-4 h. Maleic acid is sometimes used to form high melting
novolacs. In a typical preparations of novolacs, molten phenol, usually kept at 65
C, is introduced
into the reaction kettle and heated to 95
C. The catalyst is then added. This is followed by addition of
the formaldehyde solution to the kettle with stirring, at a rate that allows a gentle reflux. After
addition, heating and stirring are continued until almost all the formaldehyde is used up. At that point,
the resins separate from the aqueous phase. Water is distilled off and the temperature is raised in the
process to about 160
C. The unreacted phenol is removed by vacuum distillation. The end of the
reaction may be determined by the melting point of the product or by its melt viscosity.
Cresols are also often used in preparations of phenolic resins. These may be individual isomers or
mixtures of all three. Cresilic acids, mixtures of all three isomers, rich in
m
-cresol and low in
o
-cresol
are preferred.
Xylenols (all six isomers) are now also in common use to form alkali-resistant grades of phenolic
resins. High 3,5-xylenol mixtures are preferred. Also, resorcinol, which forms very reactive phenolic
resins, is used in preparations of cold-setting adhesives. Higher homologues of phenol, like Bisphenol
A, are used to prepare special phenol-formaldehyde condensates.
