ANALYTICAL CHEMISTRY
844 ture waa raised by small temperature increments from below the melting point of the wax t o temperatures considerably above the melting point of the wax; the other set of values (falling) was obtained by lowering the temperature by increments from above the melting point of the wax to the point at which the wax would no longer flow. DISCUSSION
Results of the viscosity tests are shown in Figures 1 and 2. In the case of the crystalline paraffins and the plastic microcrystalline waxes (Figure 1) there is no pronounced rise in viscosity as the solidification point of the wax is approached from either the rising temperature or the falling temperature side, nor is there any indication of hysteresis for the crystalline and plastic waxes.
40
petrolatum melting point is the value of the temperature at which the viscosity of the molten wax is approximately 50 cs. rather than the true melting point of the wax. Values of the Continental solid point are uniformly low, probably because of the rapid rate of cooling, the time lag in reading the thermometer, and the lack of stem correction. A method that merits some discussion is that of the ring and plunger (Crown Cork and Seal softening point). This method gives values which are in fair correlation with the Fisher melting point block. Readings of the melting block method in this work were taken a t the first indication (visual) of liquefaction of any of the crystals. It was also noted in the course of the viscosity tests that there seemed to be a range of temperature through which the waxes melt. In some cases actual flow a t very high viscosity was possible while unmelted wax crystals could be seen suspended in the partially liquefied wax. This WBS not observed on the falling temperature curves, however; the wax appeared t o solidify instantly. It is possible that a shell formed on the o u b side of the viscosity tube and that in the center of the solid mass some liquid did exist. The ring and plunger method thus seems to indicate the “thaw point” or eutectic temperature (6) of the wax (for all these waxes are in reality multicomponent systems), and this value is approximately t h e true value of the “softening point” of the wax.
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SUMMARY
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There are three critical points near the transition point from solid to liquid wax. The first is the thaw point or the eutectic temperature of the wax, and this is approximated by the ring and plunger method. The second is the melt point or the point a t which all wax is actually liquid, best evaluated by the A.S.T.M. paraffin melting point (1) for all waxes. The third is the flow point or the point a t which the viscosity is reduced to about 50 cs. best evaluated by the A.S.T.M. petrolatum melting point (8) method. High melting, hard microcrystalline waxes are subject t o viscosity hysteresis a t the melting point, but other petroleum waxes are not.
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90 100 I10 TEMPERATURE DEGREES CEff TIGRADE
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Figure 2. Viscosity-Temperature Relationship near Melting Point for Microcrystalline Waxes of High Melting Point I n the case of the hard microcrystalline waxes there is a pronounced hysteresis in viscosity on the “rising temperature” curve aa may be seen in Figure 2, which is a typical member of this class. The viscosities immediately above the melting point are very high in comparison t o the “falling temperature” value of the viscosity in this temperature range. These waxes, near their melting point, seem to depart from the usual viscosity characteristics of petroleum liquid. This effect definitely increaaes the spread between the actual melting point of the wax and the melting point as determined by the A.S.T.M. petrolatum melting point method. The three most popular melting point methods (A.S.T.M. petrolatum, A.S.T.M. paraffi, and Continental solid point) are indicated on each of the viscosity curves. These do not show wide divergence in the caae of the paraffis and plastic microcrystalline waxes. In the light of the viscosity data shown i t is indicated that the A.S.T.M. paraffin melting point most nearly represents the actual melting point of the microcrystallines and is also about average for the crystalline and plastic waxes as well. This is supported by the agreement of this method with the other methods mentioned above. By extrapolation of the viscosity curves it may be seen that the actual value of the A.S.T.M.
The writer wishes t o acknowledge the assistance of J. A. Graves, M. E. Bolton, and J. D. Anderson in accumulating the data presented in this paper. LITERATURE CITED
Am. SOC.Testing Materials, “Standards for Petroleum Products,” 1947. Ibid.,D 127-30. Ibid., D 445-46T. Ibid., E 28-421’. Glaeatone, 6.. “Textbook of Physical Chemistry,” New York. D. Vsn Nostrand Co., 1940. Holde and Mueller, “Examination of Hydrocarbon Oils and SaDonifiable Fats and Waxes.” 2nd ed.. New York. John Wilev &&ne, 1922. (7) U. S. Army Ordnance, Method AXS 1015, 6-24-43. ( 8 ) Warth, A. H., “Chemistry and Technology of Waxes,” New York, Reinhold Publishing Corp, 1947. RECEIVED August 9, 1949.
Correction In the article on “Determination of Liquid-Vapor Equilibria” [Feller, Morris, and McDonald, H. J., ANAL. CHEM.,22, 338 (1950)], the last line on page 338 should read: “the sample is -0.1 mm. of mercury per millimeter.” I n Table 11, page 340, the headings of the dew point and bubble point columns should be mm. Hg instead of a C.
