PRESERVATION OF SPECIMENS IN PLASTICS 41 



11. Upon completion of the alkali-extraction operation, rinse the 

 separatory funnel and other glassware with water, and allow them to 

 stand in the open air until they are dry and all traces of monomer have 

 evaporated. 



Catalyzing the Monomer 



In order to facilitate 'polymerization within the moderate temperature 

 range necessary for satisfactory embedment of most biological speci- 

 mens, a small amount of an oxidizing catalyst is usually added to the 

 monomer after the inhibitor has been removed. 



Among the most suitable catalysts that have been found for this 

 purpose is benzoyl peroxide. Since this is rated as a dangerous fire 

 promoting substance, it should preferably be used in the form of a mix- 

 ture containing a preponderance of nonoxidizing substances. Such a 

 mixture, which contains 75 percent of inert material, is available under 

 the trade name "Luperco-A". However, since the inert portion of this 

 mixture is more or less insoluble in the monomer, filtration is necessary 

 after the soluble portion has gone into solution. 



The amount of benzoyl peroxide that can be employed ranges from 

 0.05 percent to 0.2 percent on the weight of the monomer, according 

 to the rate of polymerization desired; but in most instances it is advisable 

 to use 0.1 percent or less, to avoid unduly rapid polymerization. When 

 Luperco-A is used, four times the net amount is required, because this 

 mixture contains only 25 percent of active benzoyl peroxide. 



In the catalyzing operation, and all the steps that follow, it is im- 

 portant that all glassware be entirely clean and dry. The steps for this 

 operation are as follows: 



1 . Weigh the amount of catalyst needed for the quantity of monomer 

 to-be catalyzed, in accordance with the proportions given above, and 

 place it in an Erlenmeyer flask or other suitable glass container provided 

 with a tinfoil- wrapped stopper. 



2. Pour the monomer into the flask and allow it to stand for about 

 30 minutes. Stir or shake frequently during this time to obtain more 

 rapid solution of the soluble portion of the catalyst mixture. 



3. When the active catalyst is all in solution, filter the monomer 

 through fine-mesh filter paper to remove all trace of cloudiness. A 

 paper at least as fine as Whatman No . 40 or its equivalent will be needed 

 to insure complete clarity of the filtrate. Keep both funnel and receiv- 

 ing flask suitably covered to prevent escape of vapor. Several sets of 

 funnels and flasks may be used if desired to cut down the time required 

 for filtering through this fine paper, or vacuum filtration may be 

 employed. 



4. Accumulate the clear filtered monomer in bottles provided with 

 tight fitting, metal-lined screw-caps, and store under refrigeration at 

 4° C. (approximately 40° F.) or below. It should be borne in mind 

 that catalyzed monomer has an active tendency to polymerize at room 

 temperature and should therefore be kept cold at all times except just 

 prior to use. 



In place of the crystalline benzoyl peroxide type of catalyst, it is also 

 possible to use a liquid type, such as tertiary butyl hydroperoxide, 

 which is somewhat safer to handle and is completely soluble in the 

 monomer. If tertiary butyl hydroperoxide is employed, however, the 



