Table 1-6.Transparent Plastics
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shatter resistance, which is much higher than that of the
stretched solid plastics.
Stretched acrylic is a thermoplastic conforming to
Military Specification MIL-P-25690. This specification
covers transparent, solid, modified acrylic sheet
material having superior crack propagation resistance
(shatter resistance, craze resistance, fatigue resistance)
as a result of proper hot stretching.
Stretched acrylic is prepared from modified acrylic
sheets, using a processing technique in which the sheet
is heated to its forming temperature and then
mechanically stretched so as to increase its area
approximately three or four times with a resultant
decrease in its thickness. Most of the Navys high-speed
aircraft are equipped with canopies made from stretched
Most transparent plastic sheet used in naval aircraft
is manufactured in accordance with various military
specifications, some of which are listed in table 1-6.
Individual sheets are covered with a heavy masking
paper on which the specification number appears. In
addition to serving as a means of identification, the
masking paper helps to prevent accidental scratching of
the plastic during storage and handling.
Identification of unmasked sheets of plastic is often
difficult; however, the following information may serve
as an aid. MIL-P-8184, a modified acrylic plastic, has a
slight yellowish tint when viewed from the edge;
MIL-P-8257, a thermosetting polyester plastic, has a
bluish or blue-green tint; and MIL-P-5425, a heat-
resistant acrylic, is practically clear. In addition,
stretched acrylic sheets and fabricated assemblies are
permanently marked to ensure positive identification.
Plastic enclosures on aircraft maybe distinguished
from plate glass enclosures by tapping lightly with a
blunt instrument. Plastic will resound with a dull thud
or soft sound, whereas plate glass will resound with a
metallic sound or ring.
Storage and Handling
Transparent plastic sheets are available in a number
of thicknesses and sizes that can be cut and formed to
required sizes and shapes. These plastics will soften
and/or deform when heated sufficiently; therefore,
storage areas having high temperatures must be avoided.
Plastic sheets should be kept away from heating coils,
radiators, hot water, and steam lines. Storage should be
in a cool, dry location away from solvent fumes, such
as may exist near paint spray and paint storage areas.
Paper masked transparent plastic sheets should be kept
indoors as direct rays of the sun will accelerate
deterioration of the masking paper adhesive, causing it
to cling to the plastic so that removal is difficult.
Plastic sheets should be stored, with the masking
paper in place, in bins that are tilted at approximately 10
degrees from the vertical to prevent buckling. If it is
necessary to store sheets horizontally, you should take
care to avoid chips and dirt getting between the sheets.
Stacks should not be over 18 inches high, and small