Ordnance Department Plastic Developments1 E. T. MCBRIDE Office of Chief of Ordnance, War Department, Washington, D. C.
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HEN the President declared a limited national emergency two years ago, it was apparent that aluminum, brass, and zinc would become strategic. As the emergency developed and the United States became the arsenal of democracy it was also apparent that substitutes would have to be found for these metals in many applications. The Ordnance Department immediately investigated the possibility of substituting plastic materials for critical metal in some of its materiel. The substitution program is an effort of the Ordnance Department to conserve highly strategic materials and utilize the less strategic. Plastics, although critical, are more available than the metals formerly used, but plastics are not metals and they cannot be expected to reproduce the physical properties of metals. They are expected to provide moldings having adequate strength characteristics, stability, and temperature resistance. When a metal is declared critical by the War Production Board, immediate action is taken to effect a substitute for that metal. This substitute must be satisfactory to the using services from the standpoint of its ability to function efficiently wherever the theater of operations may be. It must be readily available in quantity at a nonprohibitive price and capable of withstanding long periods of storage in various climates under the most adverse conditions. It must provide permanence of form and resistance to wear, abrasion, and impact, whether in the subzero weather of the Arctic outposts or the torrid humidity of the tropics. At this time only the thermosetting compounds show definite promise. They alone have satisfactorily filled ordnance requirements. For these reasons thermoplastics have never generally been approved for materiel manufactured for the combat forces. The Small Arms Division, for example, has been particularly active in plastic substitution. For almost a year this organization considered plastics for the M-1 rifle stock and hand guard, the Browning automatic rifle stock, and machine gun belt links. Although satisfactory results have been obtained in utilizing special phenolic molding compounds for the Browning automatic rifle stock, it has been shown that plastics have not reached the state where they can take the place of black walnut generally. The automatic rifle stocks are slightly heavier than the 1 Presented before the Division of Paint, Varnish, and Plastics Chemistry at the 103rd Meeting of the AMERICAN CHEMICAL SOCIETY, Memphis,
Tenn., April 20 to 2 3 , 1942.
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wood, but the mechanical properties of the material compensate for this excess in weight by their ability to withstand the cracking that the wood was subjected to when certain parts of the firing mechanisms traveled to the rear. This stock has been standardized as a molded fiber-filled phenolic. Plastic substitution for the M-l rifle furniture has been abandoned because of the cost, weight factor, tendency of plastics to absorb and hold heat, and •^relatively poor flexibility. The belt link, believed an ideal thermoplastic application, cost too much, stretched under the weight of the ammunition, warped, and fouled before reaching the guns. The rifle-bore reflecting device, formerly a brass component, has been shown to be an ideal plastic application. This device is inserted in the breech of the rifles and machine guns for the purpose of examining the rifling to locate fouling, powder contamination, and defects in the bore. This group has found that molded grips and handles, being exact replicas for the dies in which they were molded, ensure uniform and accurate assembly. They have found that grips and handles for pistols, machine guns, and bayonets which are far superior to walnut furniture may be obtained at no extra cost. They have, therefore, termed as "alternate standard material" any medium impact phenolic having an impact strength of 0.23 foot pound energy to break, as determined by the Izod notched bar impact test, which can meet requirements of Springfield Armory's tentative specification for molded phenolic shapes. Thermoplastics, although they do not generally meet Ordnance Department specifications, have contributed to the development of this program. The bayonet scabbard on which Rock Island Arsenal prepared a tentative specification is a cotton mill duck impregnated cellulose acetate butyrate composition. Although a large number of these scabbards have been procured, a variety of thermosetting samples are being investigated and a comparative destructive test is being made with the hope of standardizing a scabbard best capable of withstanding storage, weathering, and combat usage. The Ammunition Division has also been actively investigating utilization of plastics. The well-known M-52 fuse is an excellent example of cooperation between the Army and industry. This item is now a standard plastic ammunition component and has shown itself to he highly satisfactory when fired from the 60-mm. and 81-mm. trench mortars. This fuse is
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molded from a medium impact phenolic by the transfer process. The windshield for 37-mm. armor-piercing ammunition will require high-impact rag-filled phenolic which is not only capable of withstanding "setback" stress, a force generated on firing, but also the rotational forces of flight without breaking or separating from the projectile before reaching the target. Setback is the inertia set up in the gun at the moment of firing. It results in a compressive stress and is measured as pounds of pressure per pound weight of projectile. Experimental development of plastic 75mm. windshield has been suspended by the Ordnance Department because of constant setback failures encountered in firing. The setback in the 75-mm. gun results in a force of 17,000 pounds per pound weight of projectile. All plastic windshields of this caliber fired to date have exhibited longitudinal cracks which start at the threaded section. This ehows that the differences in thermal expansion of metal and plastic are too great or the plastic materials do not have sufficient elasticity to give with the projectile. The M-50 dummy fuse which is used for 37-mm. gun target practice has been classified as "plastic alternate standard". This fuse is not being procured as a plastic because the powdered iron component, which is also acceptable, has provided a more inexpensive source. The substitute for this aluminum compound must be capable of meeting with weight, weathering, and temperature requirements. The fuses must be concentric and perform satisfactorily in the gun, in flight, and at the target which may be 4,000 yards from the gun. "Volume applications" or the use of plastics in ammunition components are definitely limited because of the inadequacy of modern plastics to meet ammunition requirements. In many cases plastic components have cracked badly in loading and feed mechanisms. In other cases, the temperatures of explosion exceed by far the upper limit of plastic temperature range. What is needed is a plastic material capable of withstanding high temperatures and high setback pressures simultaneously for short periods. The scarcity of aluminum has made necessary a great deal of substitution by the rapidly growing Aviation Ordnance Section. Satisfactory materials are needed for booster cups and bomb detonators. The M-103 bomb fuse, once thought of as a practical application, will probably never become a plastic component because of the "drop safe" requirements which no 849
a n d knobs molded with inserts for gun €Îîrectorsr aiming circles, and battery commanders' telescopes. These components aJso require medium-impact phenolics. eCTTOpflp,, am aggjttrodle of S,060 fee*. The» bomb so dropped as eaqpeilfced to Saocd with.— Toe Tank and Combat Vehicle Division o u i «fflfqt^F^iri'ffi'grn so tL&ait alBaed aviators m a y Etas been investigating t h e possibility of release boanab Bo&ds oroer fensnidlhr territory osing high-impact phenolics for elevating if a forced frammtrng bfcocmfis necessary. sbrnd traversing mechanisms on tank guns. The isSbeût-fàiïti&â pifosmoiôe· booster cttp These hand wheels will contain steel infrftg also faiîBfwfl THuâs coenrnpofiiieiiht. wiien serts a n d , if they are satisfactory with dropped 5 0 Ê ^ a â e r ^âiMwanrs a i —40 e F . znespect t o durability, temperature resistance, a n d physical properties, will be considered for adoption. There is a definite military requirement f o r "windshield blisters" on tanks. The wibâcfe is cwipmlhiftp of be&Bg stored for fomgg required blisters are similar to the transparent domes used on bombardment aircraft smd are fit over the tank comTlaBSSBCttnOBB wag ippspfwmguBJga firwr ftUn» « k i « n t rnander's turret when o n reconnaissance missions with the hatch open. The purpose of this windshield or blister is t o proj e c t the tank commander from dust, dirt, a n d gas fumes which m a y be present in Ifotgh enough concentration to impair his TSae Airtflteiy Divrisiam as «fievefiopôig a vision- T o b e satisfactory for this commuTTtiBw-ff· o f fise-eoaafapol cojnagwMTrefiite f r o m ponent, a material must have superior wwy^CTiMfr iiitiwrihPinM*attg Kyifi^*awn>p^)ir 0^Π&~' SE&Q abrasion resistance and ability to 'withendpoaoes for ffrpM glasses amil telescopes stand tropical and arctic temperatures, S S e DS890C mMpwpp^i fftniMnnt ngwdniimmnft—JpiMBKt|pBf
