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



Both of these plastics have been found to be suitable for use as em- 

 bedding mediums for biological specimens. The choice between them 

 depends on only minor differences in their characteristics. Methyl 

 methacrylate polymer is considerably harder and has a higher thermal 

 yield point, but the monomer has a lower boiling point and is more likely 

 to develop vapor bubbles during polymerization. Ethyl methacrylate 

 with its higher boiling point can be polymerized somewhat more satis- 

 factorily, and the polymer is less likely to chip or crack, if dropped, 

 because of its greater toughness; but its impact strength is less, and it 

 has a slightly lower abrasion resistance. A polymer with characteristics 

 intermediate between those of methyl and of ethyl methacrylate may 

 be obtained by mixing the two monomers together in suitable propor- 

 tions and then polymerizing the mixture. 



The characteristics of methyl and ethyl methacrylates that are of 

 particular interest in connection with their use as specimen-embedding 

 mediums are summarized in table 2. Additional data may be found by 

 consulting the publications listed under Selected Bibliography, page 73. 



PREPARATION OF PLASTIC 

 Precautions for Handling Methacrylate Monomer 



Because of the inflammability and toxicity of the monomeric meth- 

 acrylate liquids, special precautions are necessary in handling them. 

 All work with them should be performed in a well ventilated room and 

 as far as possible under a fume hood or similar enclosure provided with 

 an exhaust fan or other effective means for removing the vapor. (See 

 cautions under "Equipment and Supplies," page 32 et seq.) All 

 monomer and partial polymer sirup should be kept in properly closed 

 containers, and spilling should be avoided as much as possible. 



Open flames, exposed electrical heating elements, sparking contacts, 

 or other possible means of igniting the liquid or the vapor must never 

 be permitted to come near the work. Ovens used for heating the liquid 

 during polymerization should preferably be steam heated. If electric 

 ovens are used they must have external thermostat contacts, and their 

 doors should be equipped with spring catches instead of positive latches 

 as a safeguard against explosive rupture in case of accidental ignition of 

 vapor inside. 



Care must be exercised not to inhale dangerous concentrations of 

 vapor, and it should be borne in mind that both liquid and vapor can be 

 taken into the system by absorption through the skin as well as by 

 inhalation or ingestion. No food or open beverage containers that 

 might absorb the vapor should be put into refrigerators where monomeric 

 or incompletely polymerized methacrylate is kept. 



Because of the tendency of the methacrylate monomers to polymerize 

 more or less rapidly at room temperature, they are customarily supplied 

 by the manufacturer in a temporarily stabilized, or inhibited, state. 

 Hydroquinone is the inhibitor generally employed, and before the 

 monomer can be used for embedment, or similar casting purposes where 

 a colorless polymer is required, this inhibitor must be removed as 

 directed in the following section of this publication. It is best, how- 

 ever, to store the monomer in the inhibited state in which it is received 

 until just prior to its use, and as an extra precaution against premature 



