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National Advisory Committee for Aeronautics, Technical Notes - Curing of Resin Wood Combinations by High Frequency Heating

Within recent Years combinations of wood and synthetic
resins have played an. increasingly important role in air—
craft manufacture. The earliest development was the use of
resins, such as phenol and urea formaldehydes, as adhesives
possessing superior resistance to deteriorating influences
of moisture and decay. A series of highly moisture—
resistant plywoods and laminated wood parts of many kinds
followed. Later it was discovered that the resins could be
used as impregnants and that impregnation increased so_m_e_
of the mechanical properties and introduced a large measure
of dimensional stability. Finally, it was found that
resin—impregnated wbod'could be compressed at elevated
temperatures and pressures to a hard, dense mass of spe—
cific gravity as high as 1.4 or mops.

Certain of the resins employed as adhesives can be
made to "set" at' ordinary temperatures, but the best of
them require heat, and even the "cold—setting" adhesives
cure more rapidly as their temperatures are increased.
The impregnants-are almost universally thermosetting.

_ Heat is customarily introduced into the wood—resin
combination by pressing between _hot plates. Heat flows
into the mass from the heated surface, but, on account of
the poor heat conductivity of both wood and resin, hours
may be required to raise the centers of _heavy masses to
the curing temperature, Furthermore, the surface may have
to _ba_maintaine.d at a higher temperature than is desirable
in order that the— center may be brought to the minimum
necessary temperature in a reasonable time. Simultaneous,
uniform, and rapid heating throughoutthetdmhamass is the
ideal process for resin—impregnated parts, such as propel—
lers. Selective heating of the bonding layers in glued—up
stock, irrespective of its thickness, is desirable in
laminated or cross—banded construction.

Such heating is p_ossible if the part to be glued or
cured is inserted in a high—freque_ncy electrical field.
The heat instead of being conducted from the outside, can
be generated inside by making use of the dielectric loss
of the material. In this way the curing may be accelerated,
no temperature—gradient is- involved, and the heat input
may be controlled with great flexibility. In addition,
selective heating of single layers becomes possible if they
can be made to absorb more field_energy than the rest of
the mass.