228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



familiar to all as the wrapping material Cellophane and the common 

 or viscose type of rayon. 



The basic raw material, cellulose, is obtainable in fairly pure, 

 fibrous condition as either ordinary cotton or pulped wood. Treat- 

 ment with chemicals converts cellulose into compounds which are 

 characterized by the ease with which they can be formed into desir- 

 able shapes. Cellulose plastics excel in toughness and are especially 

 useful in thm sheets which have remarkable flexibility. These plas- 

 tics conduct heat slowly and can be made substantially tasteless, 

 odorless, and transparent. Their principal applications, in addition 

 to those mentioned above, include photographic film, safety glass, 

 flexible window material, artificial leather, airplane dopes, and 

 lacquers. 



Protein plastics. — These are perhaps best known according to the 

 source of the raw material — for example, casein from skimmed milk 

 and soybean meal from soybeans. These protein substances are thor- 

 oughly kneaded into a colloidal mass, which is then formed into 

 sheets, rods, or tubes by suitable presses or extrusion devices. The 

 formed pieces are hardened by treatment with formaldehyde. The 

 finished products, such as buttons, buckles, beads, and game counters, 

 may be machined from blanks cut from the hardened material or may 

 be shaped from the colloidal casein mass and then hardened. This 

 latter process is now common practice because of the shorter curing 

 time required for the thin pieces. 



Classification on basis of heat effect. — The plastics used in the 

 molding industry may be divided into two groups, based on their 

 behavior toward heat, as (1) thermoplastic and (2) thermosetting. 

 The thermoplastic materials are permanentl}'' fusible, that is to say, 

 they alternately melt or soften when heated and harden when cooled. 

 If they are subjected to very high temperatures, vaporization or 

 decomposition takes place. The cellulose derivatives, some synthetic 

 resins, and most of the natural resins are examples of this type. The 

 thermosetting plastics, on the other hand, may be made permanently 

 infusible. This group is usually further subdivided into three stages 

 on the basis of changes in physical and chemical properties. The 

 product of stage (A) is called the initial condensation product and 

 may be liquid or solid; it is both fusible and soluble. The inter- 

 mediate or stage (B) product is insoluble and difficultly fusible, but 

 it can be molded by the proper application of heat and pressure. 

 This is the usual condition of the resin in the molding composition 

 when it is received from the manufacturer. Further heating of 

 this material, as in the process of molding, converts it to the final 

 or stage (C) product, which has a permanent set and maximum 



