formaldehyde - Sciencemadness Dot Org

USES OF FORMALDEHYDE 305
fusibility and insolubility are probably inherent characteristics of compounds
having a cross-linked structure. The formation of thea? resins
is illustrated by the following equation in which (A)H, indicates a simple
organic compound containing three reactive hydrogen atom? and .r equals a
number greater than 1 (usually about 1.5 to 2.0):
I !
. - CH—(A)— CH—(A)-CH—(A)—CH«—(A* ..
! I
I 1 "
K(A)H3 -f xn CHaO -* xn H90 + .. CH3—(A)— CH— (A)—CH— (A)—CH— (A) ..
I " I
I I
• - -CH2—(Aj^-CHs—(A)—CHs—(A)—CH2~(A). ,.
Usually the reactions are so controlled that cross-linking is held in check
and is not completed until the primary products are subjected to a final
heating or curing operation. These primary products are the essential
ingredients of molding powders and liquid casting resins. In many cases
the thermosetting process involves reaction with additional formaldehyde
or hexamethylenetetramine.
Although resins of this type can be synthesized from a wide variety of
diversified chemical compounds, those derived from phenols and urea are
produced in quantities far in excess of any other synthetic formaldehyde
resins. IT. S. production figures for the principal varieties of synthetic
resins are compared with those for phenol-formaldehyde and urea-formaldehyde
resins in Table 28 which covers 1939, 1940 and 194L These
figures are based on those reported by the U. S. tariff commission 4 .
Although phenolic resins include those prepared from cresol, xylenol, and
miscellaneous tar acid products, approximately 70 per cent of the total
production volume is due to resins prepared from unsubstituted phenol.
Phenol is particularly satisfactory because it can be readily obtained in a
high state of purity and is accordingly uniform and dependable in its behavior.
A large portion of the remainder is derived from cresols and a
small amount from xylenols.
Other important raw materials from which thermosetting synthetic
resins are produced include aniline, dicyandiamide, and melamine.
Numerous applications of these thermosetting resins are discussed in
detail in books dealing with plastics and synthetic resins. In addition to
their well known applications in the production of molded parts for radios,
airplanes, telephones, electrical instruments, etc., they are also used as
binders in grinding wheels, plywood adhesives, thermosetting baking finishes
and laminating varnishes. Molded phenolic resins are strong, resili-
CH3
CH2
CH*
CH,