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