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 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 sheets should be stacked on the larger ones to avoid unsupported overhead. Storage of transparent plastic sheets presents no special fire hazard, as they are slow burning.

Masking paper should be left on the plastic sheet as long as possible. You should take care to avoid scratches and gouges, which may be caused by sliding sheets against one another or across rough or dirty tables. Formed sections should be stored so that they are amply supported and there is no tendency for them to lose their shape. Vertical nesting should be avoided. Protect formed parts from temperatures higher than 120F. Protection from scratches may be provided by applying a protective coating of masking paper or other approved materials.

If masking paper adhesive deteriorates through long or improper storage, making removal of paper difficult, moisten the paper with aliphatic naphtha, which will loosen the adhesive. Sheets so treated should be washed immediately with clear water.

CAUTION

Aliphatic naphtha is highly volatile and flammable. You should exercise extreme care when using this solvent.

Do not use gasoline, alcohol, kerosene, xylene, ketones, lacquer thinners, aromatic hydrocarbons, ethers, glass cleaning compounds, or other unapproved solvents on transparent acrylic plastics to remove masking paper or other foreign material, as these will soften and/or craze the plastic surface.

NOTE: Just as woods split and metals crack in areas of high, localized stress, plastic materials develop, under similar conditions, small surfaces fissures called "crazing." These tiny cracks are approximately perpendicular to the surface, very narrow in width, and usually not over 0.01 inch in depth. These tiny fissures are not only an optical defect, but also a mechanical defect, inasmuch as there is a separation or parting of the material. Once a part has been crazed, neither the optical nor mechanical defect can be removed permanently; therefore, prevention of crazing is a necessity.

When it is necessary to remove masking paper from the plastic sheet during fabrication, the surface should be remasked as soon as possible. Either replace the original paper on relatively flat parts or apply a protective coating on curved parts.