An infrared Absorption Method for Measuring Phase Transitions of Waxes J. M. MARTIN, Jr.,
R. W. B. JOHNSTON, H.J. CANNON, and M. J. O'NEAL
Houston Research Laboratory, Shell
Oil Co., Houston, Tex.
A method based on infrared absorption in the 13- to 14-micron region has been developed for determining transition temperatures of commercial waxes. The melting temperature and the temperatures a t which solid phase transitions occur can b e obtained in 30 to 45 minutes on as little as 1 gram of sample.
C
properties of commercial waxes, such as tensile strength, ductility, seal strength, and blocking characteristics, change markedly with changes in crystal structure. It is important, therefore, to develop rapid and convenient methods for detecting both these crystalline changes and the temperatures a t which they occur. Such methods can be applied in studying the relationship between the performance properties and conipositional or structural changes occurring in conimercial paraffin waxes. The phase transitions of normal paraffins have been determined in a variety of ways ( 1 , 6-9). Templin ( I S ) , who measured the thermal expansion of paraffins and petroleum waxes, found significant volume changes for paraffin ERTAIN
waxes a t the melting point and a t about 10' C. below the melting point. He also refers to a number of important investigations. A refractometric method for determination of phase transformations in commercial paraffin waxes is reported by Johnson (4). Infrared studies of polyethylenes, glycerides, etc., revealed that abrupt changes in the infrared spectra sometimes occur as the result of polymorphic crystalline phase transitions (6, 3, 5, 11). The changes in absorption characteristics were particularly marked in the 13.5- to 14.0-micron region. The authors observed such changes in a large number of commercial paraffin waxes. The change occurs as a splitting of the 13.88-micron methylene rocking vibration (IO,16) into a 13.72- to 13.92-micron doublet a t the transition temperature. This phenomenon has been utilized in developing a rapid method for measuring the transition temperatures of commercial waxes.
sodium chloride optics. The sample cell (Figure 1) is similar in size and basic design to a cell manufactured by the Beckman Instrument Co. Nichrome heating elements were wound in each of the Transite cell-assembly plates for reheating or slowing down the cooling rate of the sample. Temperature measurements were made with an iron-constantan thermocouple recessed in a 1'-shaped groove just below the inner surface of one of the salt windows. This arrangement permitted intimate contact between sample and thermocouple, thus minimizing temperature lag. Cooling of the sample to the transition temperature was controlled by introducing dry nitrogen through a piece of perforated copper tubing attached to one side of the cell holder. The dry nitrogen vapor was blown through the perforations and onto the cell. If the transition temperature was below ambient, the nitrogen was precooled by passage through a coil placed in a temperature control bath. The sample cell was loaded by introducing a liquid sample into the preheated cell from a heated hypodermic syringe.
APPARATUS
The infrared measurements were obtained with a single-beam, Beckman IR-2 spectrophotometer equipped with
EXPLO R A T 0 RY
A sample of 138-40' F. (ASThl) melting point, fully refined commercial paraffin (FRP) wax was chosen for the initial study of phase transition. Infrared spectra of the wax (Figure 2) exemplify the effect of temperature on the infrared absorption a t the 13.73micron position. The spectrum of the wax, scanned in the solid phase but above the transition temperature, shows a single absorption band a t 13.88 microns. The spectrum from a scan obtained below the transition range clearly exhibits the doublet a t 13.73 and 13.92 microns. The infrared measurements of transition range were checked by a differential thermal analysis that established the occurrence of a phase change a t the temperature of the infrared band splitting. X-ray measurements made above and below the transition temperature also confirmed this phase change. METHOD
Figure 1.
Cell holder and heated cell for melting and transition point determination
Transition temperatures were determined a t the fixed wave length of 13.73 microns. The instrument slit width was adjusted to give a n initial transmittance level of a t least 50y0T with the molten VOL. 30, NO. 5, MAY 1958
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I I38
I34
I42
WAVE LENGTH,Microns
TEMPERATURE
Figure 3. Cooling curve from infrared absorption measurements a t 13.73 microns
Figure 2. Infrared absorption of a fully refined paraffin w a x
>p., I.138
Table
I.
F.
Melting and Transition Temperature Data
Wax Sample 138/140" F. FRP wax A
- 40'
Melting3 Temp., ASTM Infrared method 87-42 rnethod 139.5 124.0 120.5
123' F. FRP wax 118" F. FRP wax 138/140' F. FRP wax B 139.0 124/126' F. FRP wax 125.0 Heavy wax 156.5 HMP@Fax 182.5 a High melting point wax.
for Refinery Waxes
A
Transitkon Temp., F., Infrared Method
138.8 123.2
118.8
0.7 0.8 1.7
114.5 102.0 101.o
139.1 125 2 156.8 184.8
0.1 0.2 0.3 2.3
112.0 88.5 138.0 ...
OF.
show reasonable agreement. The infrared method will also determine melting points with a reproducibility of 10.5' F. which correlate well with measurements by ASTM Method 87-42. LITERATURE CITED
(1) Andrew, E. R.,
nax sample. Recording of the transmittance was begun immediately as the molten sample was placed in the light path, The cooling was adjusted between the melting point and transition point to provide a rate of fall of 2' to 3' F. per minute. The transmittance was recorded continuously until after the phase transition had been completed. Temperature of the sample was recorded a t intervals of approximately 1" F. The melting point may be obtained from the cooling curve as the initial temperature of deflection from the transmittance plateau established above the solidification point. The temperature of this point is interpolated to the nearest 0.5' F. The transition temperature is picked in a similar manner,
ANALYTICAL CHEMISTRY
using the plateau established between the melting and transition points. Figure 3 presents an example of the determinations from a curve traced from the instrument record for a representritive commercial wax. ANALYTICAL RESULTS
Table I tabulates transition measurements for sereral commercial waxes. The reproducibility of the infrared method was ~ t 0 . 5 ' F. Transition temperatures by other methods were not available; however, the temperatures determined on several n-paraffins by the infrared method when compared with the data of Templin ( I S ) , obtained by volume expansion measurements,
J. Chem. Phys.
18,
607 (1950). (2) Chapman, D., Nature 176, 216 (1955). (3) Elliott, A., Ambrose, E. J., Temple, R. B., J. Chem. Phys. 16, 877 (1948). (4) Johnson, J. F., Ind. Eng. Chem. 46, 1046 (1954). 15'1 Kendall. D. N.. AKAL. CHEX. 25. 382 (1953). ' Koolvdort, 'J., J . Inst. Petrol. 24, 338 (1938). Mazee, W. hl., Rec. trav. chim. 67, 197 (1948). Piper, S. H., Chibnall, A. C., Hopkins, s. J., Pollard, A , , Smith, J. A. B., Williams, E. F., Biochem. J . 25, 2072 (1931). Seper, W. F., Patterson, R. F., Keays, J. L., J . Am. Chem. SOC.66, 179 (1944). Sheppard, hl., Sutherland, G. B. B. AI., Nature 159, 739 (1947). Stein, R. S., Sutherland, G. B. B. M, J . Chem. Phys. 22, 1993 (1954). (12) Sutherland, G. B. B. M., Jones, A. V., Sature 160, 567 (1947). (13) Templin, P. R., Ind. Eng. Chem. 48, 155 (1956). RECEIVEDfor review August 26, 1957. Accepted February 5, 1958. \
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