refineries were run under the experimental conditions described above. These samples varied in sulfur content from 0.3 to 3%. Some samples were liquid, whereas others were so highly paraffinic t h a t they were solid a t room temperature. The iron and vanadium contents varied with no relation to the nickel content. The results of running these samples are shown in Table 111. The conventional analysis shown for nickel \vas determined by wet-ashing the sample and analyzing the ash by the A.R.L. Quantorneter (6). The reproducibility of this method is about 0.1 p.p.m. of nickel (2 u ) . Figure 3 is a crosi plot of x-ray analysis and
conventional laboratory analysis for the 21 plant samples. Statistical analysis of these data in the 0- to 0.5-p.p.m. range shows a 2 value of 0.07 p.p.m. The x-ray method of direct nickel determination presented in this paper shows conclusively the ability to analyze samples to A0.07 p.p.m. of nickel with 95y0 confidence limits. This limit is equal to the error one would calculate from the statistics of counting under the experimental conditions. Because the method is rapid and independent of matrix changes, extension to fields other than petroleum should be successful.
LITERATURE CITED
(1) Dwiggins, C. W.,Dunning, H. X., A N A L . CHEM. 31, 1040 (1959). (2) Hansen, J., Hodgkins, C. R., I b i d . , 30,368 (19%). ( 3 ) Kang, C. C., Keel, E. W.,Solomon, E., I b i d . , 32, 221 (1960). (4) Liebhafsky, H. A., Pfeiffer, H. G., Zemany, P. D., I h i d . , 27, 1257 (1955). (5) Zemany, P. D., Pfeiffer, H. G., Liebhafsky, H. A,, Ibzd., 31, 1776 (1959).
RECEIVEDfor review J u l y 15, 1960. Accepted October 6, 1960. Division of Analytical Chemistry, 138th Meeting, ACS, New York, N. Y., September 13, 1960.
Estimating Thermal Stability of Experimental Polymers by Empirical Thermogravimetric Analysis C. D. DOYLE General Engineering laboratory, General Elecfric Co., Schenecfady,
b Thermogravimetric analysis in inert atmosphere is used as a method for empirically assessing the ihermal stabilities of experimental polymers. Methods of interpreting the data record are discussed,
I
THE development of thermally stable polymeric materials, it is generally impractical to use timeconsuming procedures in initially evaluating and comparing experimental products. As a rule, it is only in the subsequent evaluation of relatively few survivors that rigorously scientific methods or specialized functional-environmental procedures can be applied economically. For this reason, several rapid analytical techniques have been tried as methods for empirically assessing intrinsic thermal stability or, more accurately, apparent thermal stability in inert atmosphere. Methods which have been applied in this manner include : isoteniscopic analysis ( I ) , thermogravimetric analysis ( 2 , S), differential thermal analysis ( d ) , and analysis by colloidal particle counting techniques (6), whereby the tcmperature range of interest is scanned for evidence of decomposition as implied in terms of vapor pressure, residual weight, heat content, or concentration of gas-borne colloidal particles. Such circumstantial evidence of decomposition is fallible, of course, so that none Qf these scanning methods can be applied exclusively as a sufficient test of thermal stability. Rather, the most consistently applicable method is used
N
N. Y.
as a primary test, while the less versatile methods are retained for corroborative testing. For the preliminary comparison of diverse polymeric materials, the method which has so far proved most versatile is thermogravimetric analysis in dry nitrogen. This is true primarily because the record of residual weight fraction us. temperature, unlike the records of such transitory effects as vapor pressure, differrntial temperature, or smoke particle concentration, is cumulative. As such, it is consistently and unambiguously informative, whether it is ’devoid of recognizable features, as in some cases of slow decomposition occurring over broad temperature ranges, or whether it displays a confusing profusion of features, as in cases of stepwise decomposition. Furthermore, it is possible to summarize this information in a consistent manner in a single comprehensive index of apparent thermal stability under the procedural conditions employed. PROCEDURAL DECOMPOSITIONTEMPERATURES
It is natural and convenient to express indices of thermal stability as decomposition temperatures, but in doing so, it must always be borne in mind that when they have been determined empirically, such temperature data are usually highly trivial. Their measured values depend not only on how decomposition is sensed and how the data record is interpreted but also on a long list of powerfully influential procedural details, such as the size and
fineness of the sample, the size and shape of its container, the type of atmospheric gas and its rate of flow and, especially, the rate of heating (2-4). As a precaution against absently regarding such trivial data as definitive, it has been the custom in this investigation to refer to them as “procedural decomposition temperatures.” In this report, all procedural decomposition temperatures refer to the thermogravimetric analysis of 200-mg. pulverized samples in a Chevenard X-Y recording thermobalance. All samples were heated in a 000 Coors porcelain crucible to 900’ C. a t 180’ C. per hour in an atmosphere of dry nitrogen flowing a t 314 cc. per minute. Instrument drift due to heating was corrected and care \vas taken to prevent the accumulation of easily condensable decomposition products on moving parts of the balance. Two general types of procedural decomposition temperature have SO far been defined for thermogravimetric analysis in inert atmosphere. One of these, called the “differential procedural decomposition temperature” (dpdt), was devised as a means of defining the locations of knees in normalized data records. As such, the dpdt is neither consistently available nor unique and is not considered further here. The second type of procedural decomposition temperature, the “integral procedural decomposition temperature” (ipdt), n-as devised as a means of summing up the whole shape of the normalized data curve. As such, it is consistently available from the cumulative data VOL. 33, NO. 1, JANUARY 1961
77
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