INDUSTRIAL A N D ENGINEERING CHEMISTRY PUBLISHED BY THE A M E R I C A N C H E M I C A L S O C I E T Y WALTER J. MURPHY, EDITOR
Emulsion Polymerization of Synthetic Rubber in I O - G r a m Systems A n Experimental Technique CHARLES F. FRYLING, The B. F. Goodrich Company, Akron, O h i o The experimental procedure consists of sealing the ingredients of a polymerization recipe into a test tube and rotating the tube at a constant temperature, following the course of the reaction b y noting the decrease in volume of the system. The latex is removed for coagulation when the polymerization has proceeded as far as desired, and the yield is determined b y weighing the dried stabilized coagulum.
Control testing of raw matvials. The particular properties essential for polymerization of all shipments of materials intended for large-scale production can be tested quickly. A good correlation can be obtained between the experimental results of this method and behavior on a manufacturing scale.
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N DEVELOPING practical recipes for the manufacture of synthetic rubber, it was desirable to investigate the effects on the polymerization process of a large variety of highly purified substances. Only small quantities of many materials were available. Furthermore, in order to obtain valid comparisons between experiments conducted over a period of time, it was necessary to keep standardized samples of the major components and to use them as econonomically as possible. These considerations led to the development of a small-scale polymerization technique whereby from 10 to 20 grams of monomers could be employed. By this method an extraordinary amount of valuable information was obtainable. This included:
The most serious limitation to this technique is that materials cannot be added to or subtracted from the system once polymerization has started. [Balandina et al. described a similar technique, the details of which are not readily available to English-speaking investigators ( I ) . ] RECIPES FOR POLYMERIZING SYNTHETIC RUBBER
The development of satisfactory polymerization recipes is one of the important objectives of synthetic rubber research. Patent literature contains many examples of such. The following, from a U.S. patent issued to Wollthan and Becker ( 5 ) ,and recalculated to the scale of this technique, is perhaps typical:
1. Yield 2. A polymerization reaction curve from which to estimate length of induction period, if any; rate of polymerization a t any desired time; and over-all conversion at any desired time 3. Kind of emulsion-Le., whether fluid, viscous, gelatinous, or heterogeneous, at any stage in the process 4. pH of emulsion at end of process 5. Qualitative observations on coagulation 6. Production of a sample of synthetic rubber sufficiently large to determine solubility, milling characteristics, and cured properties by procedures described by Garvey (9)
Butadiene Styrene Isohexyl mercaptan Water Sodium oleate Ammonium persulfate Temperature Time Yield
This small-scale technique has been employed for investigating polymerization of a large number of monomers and comonomer mixtures, for evaluating emulsifying agents, for determining the effect of impurities in the reagents employed, for varying the comonomer ratio and the ratio of hydrocarbons to aqueous phase, and for investigating behavior of various initiators, inhibitors, and other ingredients of the polymerization recipe. It proved to be especially advantageous in providing information on the effect of some one ingredient over a range of concentrations. The influence of reaction temperature was readily determined. I n general, the advantages of the technique were particularly apparent in:
7.5 gramn 2.6 gram 0 . 0 5 gram 18.0 grams 2 . 0 grams 0 . 0 3 gram 30’ C. “Several days” “Excellent”
An understanding of the function of each ingredient is essential I n the above example, the butadiene and styrene are the monomers, which, by copolymerizing, form synthetic rubber. The isohexyl mercaptan is described as a substance exerting a “regulating effect”-i.e., it possibly decreases the branching chargcteristics of the resulting polymer. Sodium oleate is the emulsifying agent, and the ammonium persulfate acts as the polymerization initiator, also called the “polymerization catalyst”. The possibilities of research on such a system are great and are increased by the fact that a variation introduced in any one ingredient may require a concomitant change in some other ingredient. For example, substituting another substance for the initiator might require a change in the type of emulsifying agent employed in order to get satisfactory results.
Preliminary surveys, where wide areas of investi ation had to be covered in the shortest possible time. The mekod is amenable to simple labor-saving tricks, such as filling a large number of reaction tubes a t the same time with solutions of a given emulsifying agent. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 16, No. 1
GENERAL CONSIDERATIONS ON TECHNIQUE
Polymerimtion, while a science, is also an art. The way in which things are done-thst is, the niceties of experimental technique employed-is of equal importance to the scientific aspects of the subject. Unless this viewpoint is kept clearly in mind, the investigator is frequently confronted by baffling failures. Emulsion polymerization is particularly susceptible to the influence of traces of cont~mimnts. The equipment of a research laboratory may be covered with dust which contains inhibitors or accelerators of polymerization, indeed, some substances, which under certain conditions inhibit polymerization, may under sightly different conditions act as catalysts. Neverthefess, the difficulties confronting the investigator can be avoided wit.h a little ewe and forethought.
There is no substitute for constant care and ckanliness on the part of the investigator. A careful experimenter can easily adjust the weight of small portions of certain monomers using a clean medicine dropper, while a crtreless experimenter (performing the same operation) can ruin a large number of experiments by allowing the monomer to come into contact with the rubber bulb of tho medicine dropper. EXPERIMENTAL PROCEDURE
The insrredientsof a aolvmerieation recioe an3 sealed into a ter3t e. The course
