42 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



rate of polymerization is usually slower because the available oxygen 

 is released less readily than is the case with benzojd peroxide. This 

 catalyst is generally applied in the same proportions as the solid cata- 

 lysts, i.e., between 0.2 and 0.05 percent on weight of the monomer. 

 Except in cases when the monomer contains particles of desiccant or 

 other substances that impair its clarity, no nitration of the catalyzed 

 monomer is needed when a completely soluble liquid catalyst of this 

 type is used. 



Preparing Partial Polymer Casting Sirup 



For most casting purposes it is desirable, after catalyzing the 

 monomer, to convert it to a partially polymerized sirupy form that 

 is somewhat less volatile than the original free-flowing liquid. This 

 sirup has the further advantage that it requires less time for final 

 polymerization and shrinks less when converted to the solid form than 

 the original liquid. It is best to prepare two grades of this partial 

 polymer casting sirup, one about the consistency of moderately viscous 

 honey, and the other just slightly more viscous than glycerine. The 

 former is to be used principally for casting the supporting base and the 

 layers above the specimen, and the latter for impregnating the specimen 

 and for casting the layers in which it is embedded. An ample supply of 

 unthickened monomer should also be reserved for use in rinsing the 

 specimen and for impregnation in certain instances, and it may further- 

 more be needed for thinning the partial polymer sirup in cases where 

 it has become too thick for use. 



Water-bath method. — Preparation of the partial polymer sirup is 

 preferably carried out by applying moderate heat to the catalyzed 

 monomer by means of a water bath until the desired degree of viscosity 

 has been obtained, and then chilling it thoroughly to arrest further 

 polymerization. The steps required for this operation are as follows: 



1 . Clean and dry a Pyrex-type glass Erlenmeyer flask (preferably of 

 1 liter capacity) that is equipped with a vented stopper to permit equali- 

 zation of pressure without loss of vapor. For this purpose a distilling 

 column may be used or, if preferred, a suitable stopper can be impro- 

 vised by cutting off the upper tube portion of a 50-ml. transfer pipette 

 and inserting it in inverted position in the neck of the flask (fig. 12). 

 The enlarged part of the pipette should be wrapped with cellophane so 

 that it will fit snugly enough to retain the vapor but at the same tirm 

 be free to release itself in case of sudden building up of internal pressuee. 

 Cork or rubber stoppers should not be used in this connection because 

 of the likelihood of substances being extracted from them that will 

 discolor the monomer or inhibit its polymerization. 



2. Pour into the flask to approximately one-third of its capacity 

 catalyzed monomer that has the same temperature as the room. If 

 the monomer is to be taken from a container removed from the refrigera- 

 tor, it should be allowed to warm up before pouring, in order to avoid 

 the possibility of introducing water into the monomer as a result of 

 condensation of atmospheric moisture. After pouring the monomer 

 into the flask, put the vented stopper in position. As a safeguard 

 against possible entrance of water during heating in the water bath, 

 it is advisable to wrap the joint with a piece of cellophane about 4 by 8 



