Properties of Paraffin Waxes COMPOSITION BY MASS SPECTROMETER ANALYSIS WILLIAM R. TURNER, DONALD S. BROWN, AND DONALD V. HARRISON Research and Development Department, Atlantic Rejining Co., Philadelphia, Pa.
T
HE uses of paraffin waxes extend into many industries-
packaging, candle, polish, rubber, and others. Ordinarily each particular use can best be filled by a wax having certain specific properties. A wax for use in decorative candles should burn well and impart rigidity to the candle; a wax for coating a paper milk container should have good low temperature flexibility to minimize leakage in service. The properties of waxes can be controlled to a large extent during processing by proper choice of stocks and operating variables. I n doing this the producer is in effect controlling the composition of the hydrocarbon mixture making up the wax, and is changing the distribution of the components to accentuate a desired property. How the chemical composition of paraffin waxes is changed through processing could only be estimated until recently. The development of the high temperature mass spectrometer has thrown new light on the subject of wax composition. This paper reports mass spectrometer analyses of a number of laboratory-prepared and commercial paraffin waxes, and shows how variations in chemical composition, thus determined, may affect physical properties. The utility of mass spectrometer as an analytical device for determining the composition of hydrocarbon mixtures in the paraffin wax range has been brought out in recent publications (4, 1 6 , 1 7 ) . Because of its unique insight into the distribution of wax components by molecular weight and type, it is a powerful tool for anticipating or adjusting the performance of a commercial paraffin in a particular application. During the past two decades petroleum technologists have become increasingly aware of the fact that petroleum waxes comprise a complex mixture of hydrocarbons which are not entirely of the n-paraffin type but also contain branched-chain paraffins and cycloparaffins, sometimes in large percentages (6, 9-12, 1 5 ) . They have also recognized that as the boiling point of the wax increases, the mixture tends to become more complex in differing chemical structures. Until fairly recent years, determination of the composition of hydrocarbon mixtures present in paraffin waxes required complex separations b y distillation, crystallization, selective adsorption, and chemical reaction. Frequently, even then identification of a “pure” component was based largely upon a comparison of its physical properties with those of standards reported in the literature, which themselves were not always in good agreement. With the work of BPI Project 42 in preparing purified hydrocarbons in the paraffin wax range, and n-ith the development of the high temperature mass spectrometer, a much better picture of the composition of paraffin waxes has become available. O’Neal and Weir ( 1 7 ) describe the technique of mass spectrometry of heavy hydrocarbons and report the analysis of a paraffin wax with a molecular weight of about 350. Brown, Melpolder, and Young ( 4 ) also discuss techniques and report analysis of a paraffin wax of 280 molecular weight, and of its fractions obtained by molecular distillation. O’Neal ( 1 6 ) mentions a series of n-alkane fractions prepared from petroleum wax b y distillation and crystallization. On the
basis of ordinary analytical techniques these fractions were believed to be very pure. However, when run in the mass spectrometer, the preparations were found to be complex mixtures. This laboratory has confirmed the fact that distillation and crystallization may be ineffective as andytical tools for such hydrocarbon mixtures. Several years ago n-paraffin concentrates were prepared from refined paraffin waxes by distillation and recrystallization techniques. One of these concentrates was believed to be a very pure sample of n-heptacosane (G7Hjg). This belief was based upon a comparison of its physical properties with those reported in the literature for pure compounds. After development of the technique for using the high temperature mass spectrometer in wax analysis, this sample of “C2,Hj6” mas run in the instrument. It was found to consist of at least eleven compounds (Table I, wax A), and although it was 98% n-paraffins, it contained only 70% of n-heptacosane. The mass spectrometer makes available for the first time an analytical tool particularly well suited for the study of paraffin waxes. The authors have attempted, therefore, to relate composition as determined b y this instrument to the physical properties of waxes. At the start of the work there was hope that this new method, by giving a better picture of wax composition than ever before possible, would go a long way toward eliminating the many tests customarily made to evaluate paraffin waxes. As the work proceeded, however, i t became apparent that this Tvas expecting too much of the method in its present state of development. It was found that an attempt to predict some property of a was from its mass spectrum n-as frequently successful if a “bulk” property such as hardness was concerned-that is, one on which minor percentages of a component had no strong effect. If, on the other hand, the property in question was strongly influenced b y traces of certain components, as is often the case with tensile strength, for example, prediction from a study of the mass spectrum was less successful. This may simply reflect the inability of the instrument to measure accurately in a complex mixture those components mhich are present in very small percentages, especially the isoparaffin and cycloparaffin types. It is to be expected that as additional pure hydrocarbons become available for calibration purposes, and improved methods are developed for preparing samples for analysis, still better precision of the data will be attained. The value of the data in correlations with physical properties then will be correspondingly enhanced. The effects of various components of the wax on its properties in the solid state can be very complex, and may be responsible in part for those cases where composition and physical properties do not correlate exactly. Such actions are a t present incompletely understood, and further study will be needed before it can be said just how composition influences certain physical properties. EXPERIMENTAL METHODS
Mass Spectrometer Analysis.
The instrument used t o determine the compositions of the wayes reported in this paper was a
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