172 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 5 



Tarle 4. — Moduli of elasticity in tension and specific gravities of plastics 



and metals 



Material 



Specific 

 gravity 



Modulus of 

 elasticity 



Modulus/S.G. 



Ratio 



(stainless 



= 100) 



Stainless steel 



Chrome-moly steel 



Aluminum alloy 



Magnesium alloy 



Pregwood 



Paper laminate 



Glass fabric laminate 



Impact phenolic, molded 



Canvas laminate..- 



Asbestos laminate 



Wood-flour-phenolic, molded 



7.85 

 7.85 

 2.80 

 1.81 

 1.30 

 1.33 

 1.50 

 1.38 

 1.33 

 1.80 

 1.36 



30xl0« 3.8xl0« 



29... 

 10.4. 

 6.5.. 

 3.7.. 

 3.0.. 

 2.... 

 1.8.. 

 1.5.. 

 1.5.. 

 0.96. 



3.7xl0«_. 

 3.7xl0«- 

 3.6xl0«... 

 2.84 x 10 «., 

 2.25 x 10 ». 

 1.34 x 10 ». 

 1.30 x 10 «. 

 1.12xl0«. 

 0.84 x 10 «. 

 0.70 x 10 «- 



100 

 98 

 98 

 94 

 75 

 59 

 35 

 34 

 29 

 22 

 18 



SPECIFIC MATERIALS 



A recitation of many branches of the plastics family tree may help 

 to clarify the possibilities of future applications and their relations 

 to metals. The two basic classifications of thermosetting and thermo- 

 plastic are merely behavior classifications. First let us remember that 

 plastics are the children of organic chemistry — permutations and 

 combinations of the five elements carbon, hydrogen, oxygen, nitrogen, 

 and chlorine, with minor assistance from many others. 



The top division among plastics would group them into rigid 

 and nonrigid, rigid plastics being those which require substantial 

 force to deform, and which show only a moderate deformation under 

 the stress of usage. This would probably mean an elongation in ten- 

 sion of less than 100 percent and possession of appreciable flexural 

 strength. This is the general class of materials meant when we 

 talk about plastics. The materials that lie outside the rigid group are 

 the elastomers such as the Bunas, Butyl, Thiokol, Neoprene, and the 

 like. 



The rigid plastics may then be divided into three general groups, 

 the thermosetting resins, the thermoplastic resins, and the cellulosic. 

 Even after that break-down we should perhaps recognize a fourth 

 and straddling group of "natural" plastics to accommodate such 

 materials as lignin, casein, cashew nuts, redwood, coffee, and such. 



Among the thermosetting resins there are phenolformaldehydes 

 (commonly called phenolics), phenolfurfural, urea formaldehydes, 

 and melamine formaldehydes. They are all furnished to the molder 

 or laminator in a preliminary state of chemical change and are com- 

 pleted or cured by the heat applied in the forming operations. A 

 chemical reaction takes place in the mold so that when they come 

 out of the mold or laminating press they are different chemically 

 from the materials that went into the presses. Each will maintain 

 its new identity indefinitely if properly treated. 



