Table 1-6.—Transparent Plastics Type Specification No. Solid Thermoplastic Thermoplastic Heat-resistant MIL-P-5425 acrylic Modified acrylic MIL-P-8184 Stretched modified MIL-P-25690 acrylic (8184) Thermosetting Polyester craze MIL-P-8257 resist ant - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- --------------------------------------- Laminated Laminated modified MIL-P-25374 acrylic (8184) 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 Navy’s high-speed aircraft are equipped with canopies made from stretched acrylic plastic. Identification 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 1-42