PLASTICS AND METALS — SCRIBNER 171 



they can shape within certain limits laminated structures that have 

 been considered completely cured. With those reservations our defini- 

 tion of thermosetting is still good.) 



The thermoplastic materials, the other general class of plastics, are 

 those which have the quality of softening under heat and hardening 

 when cool ; a simple analogy in this case is paraffin. 



The thermosetting group are usuable on an average up to around 

 300° F. ; some will go to 400° F. The moldable materials that go above 

 those figures are based on cement binders as a rule and are not classed 

 as plastics. Thermoplastic materials, as a rule, are not rated much 

 above 130° F. although the laboratories promise some that can be 

 boiled, for production in the near future. 



There is one outstanding characteristic about metals that we who 

 handle plastics regard with wistful envy. The word "metals" covers a 

 number of different materials and everybody accepts that fact without 

 question. So does the word "plastics," but few people seem to recog- 

 nize that fact. Perhaps an illustration will make significance of this 

 statement clearer : 



Suppose the hardware stores stocked ordinary house gutters of two 

 or three different kinds of plastic materials, and that John Doe decided 

 to buy one and bought the cheapest he could find — to be specific, one 

 made of cellulose acetate. On an August day in Philadelphia it would 

 probably sag from the heat and become useless. The odds are that he 

 thought of the gutter as "plastics" rather than as an acetate plastic, 

 and when it proved unsatisfactory he would simply condemn all plas- 

 tics and decide never to buy another plastics gutter. 



On the other hand, suppose he bought a cheap black iron gutter and 

 in 3 or 4 months that gutter rusted out. Again the odds are that he 

 would merely scratch his head and admit that he should have known 

 better and bought a copper gutter in the first place. He definitely 

 would not have condemned metal gutters. 



The moral is that we to whom plastics are bread and butter have 

 been lax in educating the public to the fact that there are a whole series 

 of plastic materials, each of which has a different set of qualities, so 

 that if a wrong application is made of one of the series (and there are 

 bound to be mistakes made) the whole class of plastics will not be con- 

 demned. We should begin that education with engineers and then 

 try to make it filter down to the general public. Unfortunately, even 

 some engineers disparage plastics generally because they made a mis- 

 take in the choice of one of the line. 



