PRESERVATION OF SPECIMENS IN PLASTICS 



35 



progress too rapidly with consequent formation of vapor bubbles which 

 remain permanently in the final product. 



In their solid polymeric form these methacrylate esters are hard, 

 durable plastics that are free from toxic action and have a slow burning 

 rate comparable to that of hard wood or compressed paper. They are 

 resistant to water and to dilute solutions of alkalies and inorganic acids 

 and to their salts. They are, however, attacked by concentrated inor- 

 ganic reagents and by certain organic acids. They are more or less 

 readily soluble in acetone, ethyl acetate, and various other ketones and 

 esters as well as in toluene (toluol), benzene (benzol), glacial acetic 

 acid, dioxane, chloroform and similar chlorinated solvents, and also in 

 their own monomers. In addition to the active solvents enumerated, 

 various other organic liquids tend to dissolve, soften, or swell them. 

 These plastics are not dissolved or softened by petroleum hydrocarbon 

 solvents such as mineral spirits and kerosene or by mineral oils, nor are 

 they affected by glycerol and other polyhydric alcohols. 



Polymethyl and polyethyl methacrylates are entirely stable toward 

 moderate heat and may be raised to temperatures of 200° C. without 

 discoloration or other change. They begin to soften below 100° C. 

 (212° F.) but do not melt. At temperatures much in excess of 300° C. 

 they depolymerize to monomeric vapor which may be recovered in the 

 liquid form by condensation. 



These plastic polymers are clear and colorless and have exceptionally 

 high light transmission — approximately 92 percent in the visual range. 

 They are, furthermore, very stable to light under ordinary conditions, 

 and can withstand prolonged exposure to direct sunlight with almost 

 no change. 



Table 2. — Principal physical characteristics of methyl and 

 ethyl methacrylates 1 





Methyl methacrylate 



Ethyl methacrylate 



Property 



Monomer 



Polymer 



Monomer Polymer 



Melting point (°C.) 



-48 



100.3 

 61 

 46 



.950 



Does not melt 





Does not melt. 



Boiling point (°C.)__ 



760 mm 



200 mm 



[Does not boil;] 

 1 depolymerizes to 1 

 | monomer vapor [ 

 [above 300° C. j 



1.18 



220. 





fDoes not boil; 

 1 depolymerizes to 



100 mm 



Specific gravity (at 

 60°/60° F.) 



Hardness (Pfund) 

 at25°C. (grams) 



0.913 



| monomer vapor 

 [above 300° C. 



I.ll(at25 /4°C) 

 141. 



Tensile strength 



(p.s.i.) 





9,000. 





5,000. 



Toughness (arbi- 

 trary units) 





98 





174. 



Impact strength 

 (Kilogram-centi- 

 meters per square 

 centimeter) 





10.5 





7.1 



Refractive index 



1.417 



1.490 



1.414 



1.485. 



1 Based mainlv on data from Strain, Kennelly, and Dittmar (18) and Du Pont de 

 Nemours & Co. (2). 



