TM 1-1500-204-23-11
4-8. PRE-PREGS. Pre-preg offers several advan-
adhesive is unsuitable as a laminating resin or for
tages over wet layup. These advantages often lead
injection repair due to its high viscosity.
to a stronger repair and consist of: exact resin mix
ratio, exact resin to iber content ratio, and ease of
c. Foaming Adhesive. Foaming adhesives (core
handling/cutting the material. The main disadvantage
splice adhesives) consist of a thin, unsupported epoxy
to pre-preg materials is their refrigerated/frozen stor-
ilm containing a blowing agent. During the rise to
age requirement and short shelf life. Since frozen
cure temperature, an inert gas is liberated causing an
storage of pre-pregs is required, they will typically be
expansion or foaming action in the ilm. The expansion
used only for depot repairs and manufacturing. Refer
must be performed under positive pressure to prevent
to the system speciic DMWR for more information
over expansion and reduced strength. Following
on pre-pregs.
expansion, the adhesive is cured into a strong, highly
structured foam. It is used as a lightweight splice
4-9. ADHESIVES. Film and foaming adhesives have
material for splicing honeycomb core repair sections.
required a high temperature cure cycle. They also
require frozen storage and are typically limited to
4-10. CORE. Core materials are used in conjunction
depot repairs. Refer to the system speciic DMWR
with metal or composite skins to create sandwich
for more information on ilm and foaming adhesives
panels. In these constructions, the high modulus
skins carry tensile and compressive loads while the
a. Film Adhesives. Film adhesives are used in
low density core separates the skins and carries
the construction of parts manufactured from advanced
shear loads. Thanks in part to their core materials;
composite materials to bond honeycomb core and/or
sandwich panels offer the highest strength-to-weight
substructure members to laminate skins. These adhe-
and rigidity-to-weight ratios available. Core materials
sives may be cocured during the laminate curing
are typically thick, compared to the skins, and come
process or they may be secondarily cured after the
in a variety of materials and geometries. Honeycomb
laminate curing process is complete.
core is the most common core material used for
aircraft parts.
b. Paste Adhesives. Compared to ilm adhesives
a. Honeycomb. Honeycomb core is commonly
, paste adhesives have the advantage of lower tem-
perature cure cycles and are easier to store and ship.
produced from aluminum, Nomex paper, plastic, and
However, they have lower strength, poorer bondline
composite materials and is available with several types
thickness control, and higher overall repair weight
of cell geometry. This type of core is generally lighter
than ilm adhesives. The base resin of many paste
than solid cores, offers lexibility for forming to part
adhesives requires refrigerated storage to ensure its
contours; and, depending on material, offers a wide
shelf life is not compromised; however, there are
range of temperature, electrical transmissibility/shield-
some paste adhesives that can routinely be stored at
ing, and heat transfer capabilities. See Figures 4-13
room temperature for 6 months to a year. Selection
and 4-14 for core nomenclature and details on nam-
of an adhesive type is dependent upon the structural
ing/numbering core products.
application and repair type. For example, a paste
4-16
