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



trie cover rings and a side steam-vent is especially suitable, but any 

 substantial dish or pan may be used for heating the water which controls 

 the temperature of the monomer during this operation. A large pan or 

 pail filled with ice-water or with running tap-water should also be pro- 

 vided for chilling the monomer in the flask at the end of the operation 

 or in case polymerization tends to proceed too rapidly. 



4. Adjust the temperature of the electric hot plate so that the water 

 bath is kept boiling gently, and let the flask remain in the bath for 

 about 5 minutes. At the end of this time— or earlier if any indications 

 of boiling are noted in the monomer — the flask should be removed from 

 the water bath and shaken or swirled in a manner to equalize the dis- 

 tribution of heat in the liquid. The flask should then be replaced in 

 the bath and heated under close watching and with frequent shakings 

 to maintain uniform heat distribution and guard against too rapid 

 progress of the polymerization reaction. In any instance where the 

 boiling does not subside readily or tends to become more violent after 

 shaking, the flask should be immersed promptly in cold water (pref- 

 erably ice water or running cold water in a large container) and moved 

 about in a manner to chill the sirup as quickly as possible in order to 

 prevent the reaction from getting out of hand. 



The partial polymerization should be continued in this manner until 

 the desired viscosity is attained; then the reaction should be completely 

 arrested by chilling the flask for 10 to 20 minutes, or preferably longer, 

 in the cold water. If, after being thoroughly cooled, the sirup shows a 

 tendency to warm up again upon standing at room temperature, further 

 chilling is necessary because this is an indication that the reaction is 

 still going on and, if not checked, will continue at an increasingly rapid 

 rate until polymerization is completed and the plastic becomes a solid 

 mass, which would be useless for embedding purposes. 



In judging the viscosity attained before discontinuing the heating, it 

 should be borne in mind that the sirup will become slightly thicker when 

 cooled. If too high a viscosity has developed, it is usually possible to 

 thin the sirup after cooling by adding a small amount of monomer and 

 then shaking or stirring the mixture until it becomes uniform. In any 

 case where the plastic has become solid through complete polymeriza- 

 tion, it may be reclaimed slowly by adding a small amount of catalyzed 

 monomer and allowing it to stand for a day or two under refrigeration 

 until the monomer has dissolved enough of the hard polymer to form a 

 sirupy mixture that is usable for casting. As soon as this sirup reaches 

 the desired viscosity, it should be transferred to a separate storage 

 vessel and more catalyzed monomer poured over the hard polymer in 

 the original container to dissolve an additional quantity. This may be 

 repeated until all of the polymer has finally been converted into usable 

 casting sirup. 



5. When the polymerizing reaction has been arrested beyond any 

 doubt, the sirup should be transferred to a storage jar having a metal- 

 lined screw cap or to a flask fitted with a cellophane-wrapped stopper. 



Tinfoil is not suitable for wrapping stoppers to be used beyond this 

 stage because it tends to become torn as a result of the increased viscos- 

 ity of the sirup, and shreds of it get into the sirup. Partial polymer 

 should be stored under refrigeration at a temperature of 4° C. (40° F.), 

 or lower, at all times except when it is being used, because it has a tend- 



