Chemistry Reference
In-Depth Information
7.13 Amino Polymers
Currently, the bulk of the commercial polymers that would fit into this category are urea-formaldehyde
and melamine-formaldehyde resins [
151
]. Over the years, however, many other materials that might fit
into this group were prepared but not adopted for use for various reasons.
7.13.1 Urea-Formaldehyde Resins
These thermosetting resins find applications in coatings, adhesives, laminating, and molding
compositions. The materials are formed in water at a pH above 7 at the start of the reaction, because
the methylol derivatives that form condense rapidly at acidic conditions. The initial step, where urea
undergoes a nucleophilic addition of formaldehyde, can be shown as follows:
O
O
+
2
H
2
N
NOH
H
2
N
NH
2
3
O
H
In the past, it was believed by some that further condensations that take place at pH below 7
include formations of cyclic intermediates. This, however, was never demonstrated [
151
]. NMR
spectra of urea-formaldehyde resins show [
152
] that condensations under acidic conditions proceed
via formations of methylene linkages:
O
O
+
H
2
N
N
OH
H
2
N
N
OH
H
H
O
O
H
2
N
N
N
N
OH
H
H
H
Under alkaline conditions, on the other hand, dimethylene ether groups form instead [
152
]:
O
O
+
H
2
N
N
OH
H
2
N
N
OH
H
H
O
O
H
2
N
N
O
N
NH
2
H
H
In addition, the more highly condensed water-soluble resins contain hemiformal groups.
Further reactions may not result in formation of polymeric materials [
153
]. This is especially true
when the ratios of formaldehyde to urea are low. Some are of the opinion that linear oligomeric
