TM 1-1500-204-23-11
Generally speaking, the ibers in most composite
nonlammable characteristics. PMR polyimide materi-
laminates are capable of much higher temperatures
als have seen very limited use as matrix materials and
than matrix resins. For example, the very best epoxy
have been used exclusively on engine components.
resins are only capable of temperatures around 400
F, while the carbon ibers used to make a laminate
d. Cyanate Esters. Cyanate esters are a family
are capable of many times that. The service temper-
of high temperature thermosetting resins that bridge
ature of any given resin system is determined in a lab
the gap in thermal performance between epoxies and
and has a relationship to the resin s glass transition
polyimides. In the uncured state, cyanate ester resins
temperature, or Tg. The Tg is the temperature at
are prone to moisture absorption which adversely
which the resin goes from being a "glassy solid" to a
affects the physical properties of the cured resin.
softer, rubbery solid. This is the point that the resin
Many of the beneicial characteristics of cyanate ester
begins to degrade and, in most cases, is permanently
resins contrast with those of epoxies. Properties
compromised.
of cyanate esters include lower moisture absorption,
higher diffusibility, low cure shrinkage, low coeficient
4-5. RESIN TYPES. The following paragraphs pro-
of thermal expansion, and low dielectric constant and
vide a general description of the types of resins and
dielectric loss when compared to epoxy and BMI.
adhesives; always refer to the manufacturer s material
These attributes are particularly useful for structures
data sheet for speciic information on the product you
with stable moisture and thermal properties, radomes,
are using.
and low dielectric applications. Cynate ester repair
resins may also reach very low viscosities during cure
which require special bagging precautions to prevent
a. Epoxy. Epoxy resins are probably the most
common two part system available. The majority of
advanced composite parts in aircraft applications use
e. Vinyl Esters. Vinyl ester resins are thermoset-
epoxy resins as the matrix material. Epoxies generally
exhibit excellent physical properties, good adhesion,
ting resins that are a combination of polyester and
epoxy structures. Vinyl esters have the appearance,
chemical and moisture resistance. Depending on the
handling properties, and curing characteristics of con-
between 70 and 350 F with a corresponding range
mechanical properties are improved over polyester
of Tg from 200 to 400 F. Table 4-2 provides shelf
due to the epoxy structure in the resin.
life data for several common two part epoxies.
f. Polyesters. Polyester resins are formed by the
b. Bismaleimides. Bismaleimide resins are inter-
reaction of dysfunctional acids and alcohols. While
mediate service temperature polyimide resins. They
common in the sporting goods and marine industries,
typically cure at 375 to 450 F and have a service
polyesters are rarely found in Army Aviation since they
temperature of 350 F. Generally, they are used on
lack the necessary strength and service temperature
areas of the aircraft that experience operating tem-
properties. Polyesters are not acceptable substitutions
peratures in excess of 250 F but less than 350 F.
for epoxies. Polyesters also give off hydrogen cyanide
BMI resins combine a number of unique features,
gas when burned and are therefore never used for
including excellent physical property retention at ele-
interior aircraft parts.
vated temperatures and in wet environments; almost
constant electrical properties over a wide range of
g. Thermoplastics. Thermoplastics
are
long
temperatures, and nonlammable properties. BMIs
chained carbon molecules that do not crosslink like
have epoxy like processing characteristics yet have
thermoset resins. Most industrial plastics, such as
a higher temperature use limit. One challenge when
polyethylene, polycabonate, or acrylic, are thermo-
repairing a BMI part is the very low viscosity the resin
plastics and usually soften at lower temperatures
reaches during cure. Special bagging precautions are
than thermosetting resins. Polycarbonate and acrylic
required to prevent over bleeding a BMI repair and
are commonly used for aircraft windows. Engineered
the high cure temperatures.
thermoplastics, such as polyetheretherketone (PEEK)
have not yet been used on Army aircraft for structural
c. Polyimides. Polyimide resins used as matrix
applications. Temporary thermoplastic repair patches
materials are high service temperature resins. They
which soften when mild heat is applied can be used
cure at 650 F and have a service temperature of
for secondary structure repair to provide an environ-
550 F. Polyimides are dificult to process due to
mental enclosure until a permanent repair can be
their complex cure chemistry, but they provide high
made.
thermal stability, excellent mechanical properties, and
4-13