Chemistry Reference
In-Depth Information
condensates form instead. These urea-formaldehyde condensates separate as colloidal dispersions
that are stabilized by association with excess formaldehyde [
153
]. The cross-linking reaction consists
of agglomeration of colloidal particles with an accompanying release of formaldehyde. This opinion
is supported by several observations: (1) when one plots the logarithm of solution viscosity against
time during the polymerization, the plot exhibits a sharp break. Also, the plot differs from similar
ones for phenol-formaldehyde condensation reactions that show continuous increases in viscosity.
(2) Scanning electron micrographs of the fully cured resins show surface characteristics that resemble
more the surfaces of coagulated and coalesced colloidal particles than those of high molecular weight
polymers. (3) X-ray diffraction patterns and laser Raman spectra of the cross-linked resins show that
there are crystalline areas in the material and absence of water. Similar patterns are obtained from
hydrogen-bonded proteins with close chain packing. On the other hand, FT-IR studies [
154
] show that
methylene and ether cross-links are present in the cured resin. There are also indications of the
presence of cyclic ether units. The above information also suggests that the final structure of the urea-
formaldehyde resin may be a function of the feed ratio and the pH at which it was formed.
Urea-formaldehyde resins for surface coatings are commonly modified for solubility in organic
solvents by reacting them with alcohols to form ether groups. Usually,
n
-butyl alcohol is used.
The reaction is carried out under basic conditions, before acidification:
O
O
C
4
H
9
+
C
4
H
9
OH
N
2
N
NOH
+
H
2
O
H
2
N
NO
H
H
After etherification, the reaction mixture is acidified and the resin is further reacted to acquire the
desired degree of condensation. A typical butylated urea-formaldehyde resin contains 0.5-1.0 moles
of butyl ether groups per mole of urea.
7.13.2 Melamine-Formaldehyde Resins
These resins are quite similar to urea-formaldehyde condensates and, probably, for that reason, find
similar applications. Melamine reacts with formaldehyde under slightly alkaline conditions to form
mixtures of various methylolmelamines [
155
]:
NH
2
H
N
OH
N
N
+
OH
N
N
O
H
2
N
N
NH
2
H
2
N
N
NH
2
H
H
N
OH
N
OH
+
N
N
+
N
N
H
2
N
N
N
OH
HO
N
N
N H
H
H
H
