GAS A N D FUEL CHEMISTRY-HIGH POLYMER CHE MISTRY D I V I S I O N O F G A S A N D FUEL CHEMISTRY
Spark Ignition Study Results in Rules to Predict Fuel Properties • 13 rules set up to allow prediction of changes in magnitude and direction of spark ignition energy brought on by changes in structure T^LAMMABILITY limits of paraffinic hydro·** carbons in air and the effects of flammability limit, molecular structure, and initial temperatures on "the ignition spark energy were the topics considered in a brief session of the division. Main work of the division was presented in a joint symposium with the Division of Petroleum Chemistry described elsewhere on these pages. One of the fundamental properties of a fuel is its minimum spark ignition energy, according to H. F. Calcote of Experiment Inc. Ignition energy is the energy of the spark just necessary to ignite a combust ible fuel mixture, and a plot of the igni tion energy vs. the electrode distance gives a parabolic-type curve, the minimum of which is defined as the minimum spark ignition energy. Although the minimum spark ignition energy is a function of electrode shape (that is pointed or flat disk), the relative values for any one shape are functions of fuel type alone. Dr. Calcote's apparatus consisted of a small, thermostated stainless steel cham ber equipped with movable rod electrodes. The combination of the stainless steel chamber and Teflon packings and Teflon inlet and outlet tubes minimized the pos sibility of introducing variables through fuel contamination. Both electrodes were capable of microm eter adjustment. The fuel-air mixture,
generally of stoichiometric composition, was fed into the chamber, the spark passed in a short—10"8 seconds—interval of time, and whether or not combustion occurred at the particular electrode distance and spark energy was determined either by visually noting the flash of combustion or, in those instances where there was no flash, by the slight pressure rise re sulting from combustion. Generalizations on the results from ap proximately 60 compounds parallels those which have been known for some time to be factors in fuel properties. Branched chain hydrocarbons require higher ener gies for ignition than do straight chain hydrocarbons. Electronegative substitutents raise the ignition energy as d o sixmembered rings, whereas olefinic linkages and three-membered rings lower t h e ig nition energy. Five-membered rings and aromatics have little effect on the igoition energy. By correlating the data o n the compounds tested, a set of 13 rules has been set up for predicting both the direc tion and the magnitude a change in stnicture will have on the spark ignition en ergy. Turning to the effect of initial tempera ture, Dr. Calcote found that the logarithm of the ignition spark energy increases linearly with decreasing initial tempera ture, except at low temperatures where the increase is rapid.
D I V I S I O N O F H I G H POLYMER CHEMISTRY
Association of Polystyrene In Toluene Indicated • Radioactive tracer techniques evaluate initia tor efficiencies in vinyl polymerization of a slight amount of INDICATIONS sociation of polystyrene in toluene,
as re lated inversely to solution temperature, have been observed by H. P. Frank and Ε. Η. Immergut of the Polytechnic Insti tute of Brooklyn, as reported by Herman Mark in the Division of High Polymer Chemistry. Osmotic pressure measure
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ments revealed an increase in apparent molecular weight from 200,000 to 240,000 as temperature was decreased, Dr. Mark stated. Addition of a small amount of polar solvent eliminated the association tendency, as did the use of a pure poly styrene, polymerized in the absence oF oxy gen. It has been concluded, said. Dr.
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Maurice Morton, University of Akron, dis cusses propagation rate determination in the emulsion polymerization of butadiene Mark, that osmotic* pressure measurements can be utilized to determine true molecu lar weights, and also to reveal evidence of incipient association. Intrinsic viscosity measurements, he said, do not show up the small degrees of association which might be involved. Radioactive tracer techniques have been utilized in vinyl polymerizations, accord ing to L. M. Arnett of Du Pont, to deter mine initiator efficiency—defined as the ratio of radicals which initiate chains to the total radicals generated by initiator decomposition. Measurements were made by polymerizing representative vinyl monomers with an aliphatic azodinitrile containing carbon 14 .id determining the radioactivity of ' \ e resulting polymers. Measured efficiencies in polymerizations of methyl methacrylate, styrene, vinyl ace tate, vinyl chloride, and acrylonitrile have been found to range from 50% for methyl methacrylate to 100% for acrylonitrile. Comparison of the osmotic molecular weight and the amount of combined initiator fragments in polymethyl methac rylate, said Dr. Arnett, indicates that termination is by combination, since an initiator fragment is attached to each end of the polymer molecule. A study of the emulsion polymerization of butadiene, with the object of evaluating the absolute propagation rate from the rate of polymerization per particle of latex, was reported by Maurice Morton of the University of Akron. A persulfate-mereaptan system was found unsuitable for the purpose, as the rate of polymerization per particle showed a dependence on the soap concentration, but not on the temperature of the system. A hydroperoxide-polyamine system, however, did behave in the man ner of a true emulsion polymerization, giving a rate per particle independent of activator concentration, and dependent only upon temperature.
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