Laminated transparent plastics are well suited to
Transparent plastics, reinforced plastics, and
pressurized applications in aircraft because of their
composite materials are common materials used in
shatter resistance, which is much higher than that of the
aircraft construction. Sandwich construction is used for
stretched solid plastics.
radomes as well as for structural areas where strength
and rigidity are important. Laminate construction is
Stretched acrylic is a thermoplastic conforming to
also used in aircraft construction.
Military
Specification
MIL-P-25690.
This
specification covers transparent, solid, modified acrylic
TRANSPARENT PLASTICS
sheet material having superior crack propagation
resistance (shatter resistance, craze resistance, fatigue
Transparent plastic materials used in aircraft
resistance) as a result of proper hot stretching.
canopies, windshields, and other transparent enclosures
may be divided into two major classes, or groups,
Stretched acrylic is prepared from modified acrylic
depending on their reaction to heat. They are the
sheets, using a processing technique in which the sheet
thermoplastic materials and the thermosetting
is heated to its forming temperature and then
materials.
mechanically stretched so as to increase its area
approximately three or four times with a resultant
Thermoplastic materials will soften when heated
decrease in its thickness. Most of the Navy's high-speed
and harden when cooled. These materials can be heated
aircraft are equipped with canopies made from
until soft, formed into the desired shape, and when
stretched acrylic plastic.
cooled, will retain this shape. The same piece of plastic
can be reheated and reshaped any number of times
Identification
without changing the chemical composition of the
material.
Most transparent plastic sheet used in naval aircraft
is manufactured in accordance with various military
Thermosetting plastics harden upon heating, and
specifications, some of which are listed in table 2-6.
reheating has no softening effect. They cannot be
Individual sheets are covered with a heavy masking
reshaped after once being fully cured by the application
paper on which the specification number appears. In
of heat. These materials are rapidly being phased out in
addition to serving as a means of identification, the
favor of stretched acrylic, a thermoplastic material.
masking paper helps to prevent accidental scratching of
Transparent plastics are manufactured in two forms
the plastic during storage and handling.
of material--solid (monolithic) and laminated.
Identification of unmasked sheets of plastic is often
Laminated plastic consists of two sheets of solid plastic
difficult; however, the following information may serve
bonded to a rubbery inner layer of material similar to
as an aid. MIL-P-8184, a modified acrylic plastic, has a
the sandwich materials used in plate glass.
Table 2-6.--Transparent Plastics
Type
Specification No.
Solid Thermoplastic
Thermoplastic
Heat-resistant acrylic
MIL-P-5425
Modified acrylic
MIL-P-8184
Stretched modified
MIL-P-25690
acrylic (8184)
Thermosetting
Polyester craze
MIL-P-8257
resistant
Laminated
Laminated modified
MIL-P-25374
acrylic (8184)
2-40
