Methyl Methacrylate as Imbedding Agent H. F. HALENZ and L. W. BOTIMER Emmanuel Missionary College, Berrien Springs, Michigan
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ETHYL methacrylate monomer gives, upon polymenzatlon, . . a colorless, highly transparent solid which in recent years has found many industrial applications. Because of its optical properties and relative ease of handling, it may be used as a medium for the imbedding of many types of objects for purposes of preservation or display, etc. So far as the writer is aware, work of this nature, as yet unpublished, was first done some years ago by C. E. Sando of the Bureau of Chemistry and Soils. Hibhen (1)a t about that time reported on imbedding of some biological specimens. Inquiry among teachers of chemistry reveals a definite interest in this use of methyl methacrylate, though none of those asked had attempted to carry out the process. The starting material is the monomer, obtainable from the E. I. du Pont de Nemours and Company ( 2 ) . Pertinent information on this and other methacrylate resins will be found in the recent literature (3, 4, 5). The monomer is a clear, colorless liquid of characteristic odor. Polymerization of the commercial product
is prevented by the add~tionof a trace of inhibitor. The latter may be removed by vacuum distillation through a short packed column (boiling point 100°C. a t 760 mm., 61°C. a t 200 mm.). A simpler method is to wash out the inhibitor by shaking equal quantities of a two per cent aqueous sodium hydroxide solution and monomer in a separatory funnel. The heavier alkaline solution is discarded. The washing process should be repeated until the alkali layer remains colorless; the monomer is then washed twice with distilled water. Any water remaining is removed by the use of calcium chloride 01 anhydrous sodium sulfate. The uninhibited monomer may be kept in a refrigerator for several days. A small amount of the monomer is now carefully refluxed until it becomes thick enough to retain bubbles. This will take but a few minutes. Only sufficient material should be prepared to use within a day. The monomer nearest the container walls thickens more quickly; upon shaking i t redissolves in the less viscous portion of the liquid. When i t has cooled there is added about 0.1 per cent by weight of benzoyl peroxide. The latter speeds up the polymerization very appreciably. The amount of benzoyl peroxide to he used is somewhat critical; solidification takes place rather slowly if less than 0.05 per cent by weight is used; a larger amount tends to increase bubble formation; even frothing may occur. The partially polymerized material is now transferred to a mold. An ordinary wide-mouthed bottle is quite satisfactory. It is placed in an oven a t 45" to 50°C. and will become quite hard in from one to two days. A higher temperature increases the reaction rate, but also the extent of bubble formation. During polymerization the container must be tightly stoppered, since the monomer is highly volatile. A foil-covered stopper is advised. When a layer of the material, of suitable thickness, has solidified, the specimen to be imbedded is placed on it and partly polymerized monomer is poured around it. The heating process is then continued. When the polymer has become solid the temperature
may be raised to 80' or 90°C.in order to make certain that polymerization is complete. The material will become perfectly hard at the lower temperature also if sufficient time is allowed. The temperature to be used will depend on the nature of the specimen which is imbedded. In order to prevent the formation of bubbles and the distortion of any specimens that may not be perfectly rigid, it is advisable to add the prepared monomer in two or three installments. The various layers will blend smoothly: no line of demarcation is ordinarily . amarent. Casts prepared in this fashion show no evidence of strain under polarized light. The m a t e r a may be colored by use of certain dyes. Many of the more common dyes will not dissolve in methyl methactylate, but amongthose which were found L .
useful are Chrysarobin, Sudan 111, and Indianthrene Brilliant Violet, which give light brown, red, and purple colors, respectively. An idea of the appearance of objects imbedded by the above method may be obtained from the photographs. LITERATURE CITED
(1) HIBEEN,"The preservation of biological specimens by means of transparent plastics," Science, 86, 247 (1937). ' (2) "Polymerization of methyl methawvlate monomer." A
mimeographed sheet of~informatiohfrom the E. I. du Pont de Nemours and Company. (3), nu Porn DE NEMOURSAND "Methacrvlate r&ns." - ~ -- COMPANY. ~ ~-~ - - - - ---~ ,.-... . ....., Isd. Eng. Chem.,28,1160 (1938). (4) NEHER. "Anylic resins," ibid., 28,267 (1936). ( 5 ) STRAIN, KENNELLY, AND DITTMAR, "Methacrylate resins," ibid., 31,382 (1939).
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