Identification of Polymeric Materials P. F. K K U S E , JR.’, AND W. B. WALLACE iwaterials Laboratory, Sandia Corp., Albuquerque, .V. M .
A
METHOD for the identification of polymeric materials by infrared analysis of their pyrolysis products has been developed in this laboratory, apparently almost simultaneously with the work and method reported by Harms (1). One gram of the polymeric material, preferably shredded or cubed, is dry-distilled, and the products of decomposition are collected in 1 ml. of carbon tetrachloride. For the pyrolysis an aluminum cylinder 3.5 X 1.5 inches in diameter was drilled to a depth of 2.75 inches to accommodate a 15 X 100 mm. test tube, fitted with a 4-mm. glass inverted U delivery arm. A potentiometer, with the thermocouple in a S/srinch hole parallel and close to the test tube well, served to measure the temperature of the heating block. Using a hot, blue Bunsen burner flame, the block attains and holds temoeratures in the ranee of 830’ to 870’ F. The tube. containing the sample, is thenlinserted in the well. When del composition products begin to issue from the delivery arm, it is immersed in the carbon tetrachloride. I t is well to carry out the pyrolysis in a hood. For most rubbers and plastics, pyrolysis for 2 minutes produces a sufficiently concentrated solution for identification. I t is well to ensure that some liquid products are collected. The solution is dried by filtering through a column of anhydrous sodium sulfate protected with a calcium chloride drying tube, and then allowed to stand over anhydrous
sodium sulfate. The spectrum of the mixture of products is obtained by compensating for the solvent, This procedure has the advantages of (1)favoring close reproduction of spectra by pyrolyzing all samples a t the same controlled conditions and (2) minimizing fogging of the cell windows by moisture present in the pyrolyzates. As suggested by Harms (I), the solubility of pyrolysis products in carbon tetrachloride poses a problem. Fortunately, in practice, there have been only two experiments (pyrolyzates from urea and urea-melamine plastics) out of more than 100 pyrolyses into carbon tetrachloride where not enough solute was present to yield a characterizing spectrum. The usefulness of the infrared-pyrolysis technique is demonstrat,edin the int,eresting examples given by Harms. LITERATURE CITED
(1) Harms, D. L., A s . 4 ~ CHEM., . 25, 1140 (1953). RECEIVED for review February 14, 1953. Accepted June 12, 1953. Presented at the Eighth Southwest Regional Meeting, AMERICAN CHEMICAL SOCIETY, Little Rock, . i r k . , Decemher 5 , 1952
Two New Forms of Melting Point Calorimeters For Determining the Purity of Liquids of Condensed Gases JOHN T. CLARKE, IIERRICK L. JOHNSTON, AND W.4RREN D E SORBO* The Cryogenic Laboratory and Department of Chemistry, The Ohio State University, Columbus, Ohio
I
S CARRTISG out low temperature calorimetric a ork on gas-
eous hydrides of boron in this laboratory, it was necessary to make relatively accurate purity determinations in advance of introducing the compounds into the condensed gas calorimeter, to avoid the hazard that would be associated with formation of plugs in the narrow inlet tube due to condensable impurity. The two forms of “melting point calorimeter” described in the paper were developed to make these purity determinations. They have proved successful for the purpose, they can readily be utilized for purity determination on other liquids or condensable gases, and are relatively simple to construct and operate. Freezing point determinations have long been used to determine the degree of purity of liquid samples. The method can be made quantitative, without requiring knonledge of the exact nature of the impurity, or impurities, if it can safely be assumed that the impurity is soluble in the liquid but not in the solid phase of the solvent component. Most forms of the method depend on adding or withdrawing heat a t an approximately constant rate and determining the portion melted from a time temperature graph ( 2 , 5 ) . Cooling or warmingrates possess the disadvantage, compared with calorimetry, that the energy measurements are less accurately determined and that constant temperature gradients may exist in the system. Aston, Fink, Tooke, and Cines ( 1 ) 1
Present address, The Samuel Roberts Koble Foundation, Inc , Ard-
more, Okla.
* Research .4ssociate, The General Electric Research Laboratory, Sehenectady, ti.Y.
developed a melting point calorimeter to which heat is added in measured increments and time is allowed b e t w e n heating periods for attainment of thermal equilihrium under adiabatic conditions. This is essentially a simplified form of the more accurate vacuum calorimeters used in the measurement of specific and latent heats at low temperatures, and employs, for the impurity determination, the “premelting heat capacity” treatment employed originally by Johnston and Giauque (3,4 ) . The two melting point calorimeters described in this paper are less suitable than that of Aston and roworkers for determination of premelting heat capacities, arid hence do not employ the accurate premelting heat capacity treatment. They are, however, well adapted for measurement of melting point as a function of fraction melted, and calorimeter I1 is well adapted for heat of fusion measurements. Both calorimeters are simple to construct and to operate and an impurity determination may be made in less than 2 hours, IT-ith calorimeter I, provided there is previous knowledge of the heat of fusion, and in about 3 hours with calorimeter 11, including a fusion heat determination. Calorimetera I and I1 require only 4 and 3 ml. of sample, respectively. CONSTRUCTIOY AND OPER4TIOY OF CALORIMETER I
Calorimeter I is the simpler of the two, both in construction and operation. Its design is shown in Figure 1. The calorimeter proper is a borosilicate glass tube 35 cm. long by 12 mm. in outside diameter, sealed a t the bottom. It contains a platinum resistance thermometer element made of No. 40 B and S gage wire, coiled in a helix, and mounted on a notched mica cross 31 1156
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1157
mm. long. The thermoelement was 90% platinum and 10% rhodium-as this material was readily available-and its dR/dT was accurately known. The resistance of the thermometer a t room temperature is approximately 100 ohms. Two current and two potential leads (Xo. 40 B and S gage platinum for 5 cm. above the terminals. then KO.32 B and S gage platinum the rest of the way) are carried out through a de Khotinsky seal a t the top of the calorimeter tube. These leads may go either to a potentiometer or a bridge for rei.istance measurements. I n operation the resistance thermometer also serves as a heater, with electrical energy supplied by batteries that may be connected through an appropriate resistance to the pair of current leads. The 12-mm. calorimeter tube is surrounded by a 40-mm. outside diameter vacuum-tight, borosilicate glass tube to which it is attached by a ring seal. This t,ube can be evacuated to 10-6 mm. of mercury, or better, and serves as a vacuum jacket to attain nearly adiabatic conditions during a purity determination. T w o concentric cylinders of 0.125-mm. aluminum are suspended between the calorimeter and the vacuum jacket to serve as a double radiation shield. ?;arrow windows are cut in this double shield to permit observation of the liquid level in the calorimeter. -1thin copper foil surrounds the resistance thermometer element. Its purpose is to distribute heat vertically through the mass of liquid contained in the calorimeter. The sample is introduced through the inlet shown a t the top of the diagram, in sufficient amount to cover the thermometer junction to a depth of about 12 mm. This amounts to about 4 ml. of sample. -4suitable refrigerant bath is then placed around the calorimeter assembly, the vacuum is broken by introducing hydrogen or helium into the vacuum space within the 40-mm. tube, and the sample is allowed to cool to a temperature 15" or 20" C. below the melting point'. If t)he sample under investigation is gaseous a t room temperature the refrigerant is, of course, introduced in advance of admission of the sample and the sample is condensed in the calorimeter. After cooling has been accomplished the vacuum jacket is evacuated to 1 0 - 6 mm. of mercury, and the run is started. The purpose of cooling 15' to 20" below the melting point is to permit a preliminary measurement of heat capacities and of drift rates to establish the rate of heat leak to or from the calorimeter. Heat is added to the calorimeter by passing current through the combination resistance thermomet'er-heater for 1-minute heating periods and the calorimeter is allowed to equilibrate for 10 or 15 minutes between heating periods. Drift rates are established by making resistance thermometer readings a t 1- or 2-minute intervals during the equilibrat,ion periods. To avoid possible influence of thermoelectric effects a t potential lead junctions. especially where leads of dissimilar compositions are joined, it is advisable to read the resistance of the resistance thermometer with cirw i t s reversed and to average the normal and reverse readings. These measurements determine the rate (calories per minute) a t which heat is received by conduction down the glass wall of the calorimeter tube. down the electrical leads, and across The vacuum space. Finally, the calorimeter is heated to the melting point of the substances and heating is continued until the sample is completely liquid. Small known fractions are melted using alternation of 1- or 2-minute heating periods with 10-minute equilibrium periods. This provides adequate information (provided the heat of fusion is already known) to determine the degree of purity. CALCULATIOX OF RESULTS
The following equat'ion for melting point lowering x a s used:
XT?
=
AH",r(T"
-
Tm)/R(T')'
(1)
Two new forms of melting point calorimeters were dew eloped for estimating the degree of purity of gaseous hydrides of boron. These calorimeters may be found useful for comparatively rapid purity determinations of low boiling liquids. The first calorimeter uses a platinum thermometer immersed in the sample and has given reliable results when the heat of fusion is known or when the cryoscopic constant is determined by adding an impurity to the sample. This apparatus uses a platinum resistance thermometer embedded in the sample, and is easily
in which X,= mole fraction of impurity in the melt AHj" = molal heat of fusion of pure material R = the molar gas constant, equal t o 1.986 calories T o = the melting point of pure substance Yfl, = the melting point of sample This is the limiting equation for the freezing point lowering in a solution that contains no solid soluble impurity and is applicable with little error for the melting points of samples of comparatively high puritj- provided the fraction melted is not too low.
Figure 1.
3Ielting Point Calorimeter I
7'' may be obtained by plotting T observd x r s u s l / a where is the fraction melted a t the observed equilibrium temperature T observed, and emxpoiating the curve so obtained to ( l / a ) = 0. This treatment give- a. straight line plot which can be easily extrapolated. Kith T o arid TnLboth evaluated (the latter hy reading off T observedj at ( l l a = 11, Equation 1 is used to compute the mole per cent of liquid-soluble, solid-insoluble impurity. 01
EXPERIMENT4L R E S L L T S N ITH CALORIIIETER I
Figures 2 and 3 show data obtained with a sample of diborane obtained from the Naval Research Laboratory, on which were obtained four equilibrium periods within the melting range. The a's for Figure 3 were computed from the energies in excess of the C,dT integrals and from the allon.ances for heat leak during the equilibrium periods.
operated. I determination may be made in about 2 hours, on a 3-cc. sample. The second calorimeter is a crude form of vacuum calorimeter made from a copper-to-glass Housekeeper seal. It uses a copper "block" w-hose temperature must be carefully regulated, but good temperature control is obtained. It gives heats of fusion sufficiently accurate to determine the cryoscopic constant. It gives more accurate results than calorimeter I. A determination may be made, including a heat of fusion, in about 3 hours, with a 4-cc. sample.
ANALYTICAL CHEMISTRY
1158
6,RECIPRGCAL OF FRACTION MELTED Table I. Sample
Purity Determination on Diborane Tm, Determination 1 2 3 4
A A B C
Ohms 63.9568 63.9572 63.9564 63.9265
639680
Purity, % 99.950 99.938 99.968 99.30
3620
Results obtained in four determinations on three samples of diborane are given in Table I. Sample C (Table I ) was prepared from sample B by the addition of 0.59 mole % of ethane as an impurity. This determination was carried out to determine the reliability of the freezing point method with this particular impurity since there was some possibility that ethane might form mixed crystals with diborane owing to the similarity in formulas. The data gave a straight line in the l/a plot.
-
-
,9320
.9263 -
-
-
-
,9290
,9250
1 y
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I'
62.8
(I MIN) H E A T l M PERIOD
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IL
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50-gram cylinder from Naval Research Laboratoq
P d
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i
6396
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Figure 2.
X,
3.00 PM
10 TIME
20
3:30PM
40
50
block was provided with a heater, for regulation of its temperature, and with a thermocouple. The whole assembly is placed in a vacuum-tight glass jacket, to constitute a crude design of vacuum calorimeter. The glass jacket is immersed in a refrigerant bath. The calorimeter is cooled to the bath temperature by introduction of helium gas into the vacuum space, which is pumped out prior to a run. r M L D FOIL
Melting Curve for Diborane
REX WEE. O.D. 9/16
50-gram cylinder
It is not necessary to have a very accurate calibration of the resistance thermometer in this method, since the temperature can be expressed in resistance-thermometer ohms for the purpose of obtaining 01, T,, and T o . Conversion to degrees Kelvin need be made only for the final substitution into Equation 1. This equation is, moreover, not sensitive to a very exact value for To,and the percentage error in the amount of impurity will correspond to that in (dR/dT). Calorimeter I is not suitable for a determination of the heat of fusion. This results from the fact that a considerable portion of the solid is above the heater and fails to melt completely.
COPPER BLOCK HEATER
ORWTER BULB
OCK ASSEMBLY
~
OYE.IRX.012
v
TER WIRE GROOVE
CONSTRUCTION AND OPERATION OF CALORIMETER I1
NT HOLES.WI2
A new calorimeter m-as designed in which both heats of fusion and degree of purity may be determined. This calorimeter gives higher accuracy, in purity determination, than does calorimeter I. The design is illustrated in Figures 4 and 5. The calorimeter bulb consists of the copper portion of a Housekeeper seal ( 3/ginch outside diameter by 2-inch-long copper to 14-mm. borosilicate glass), closed a t the end, Four copper fins were soldered inside of the copper tube to aid in heat distribution. Fifty ohms of No. 40 double nylon-covered gold (copper would be a satisfactory substitute) wire wound on the outer surface of the copper tube, and cemented to the tube with General Electric adhesive No. 7031 constitutes a combination resistance thermometerheater. Current and potential leads are taken off from the thermometer terminals and brought into intimate thermal contact with the upper portion of the copper tube, and with a copper "block" attached to the glass tube a t a point 0.5 inch above the glass-to-copper seal, by means of Cerrobend alloy. ThiE
TOP VIEW
0-
GROOVE TO ACCOMMODATE LEAD WIRES TO OOLD THERMOMETER
Figure 4.
Melting Point Calorimeter I1
In operation the graduated side tube is cooled below the boiling point of the sample and 2 to 3 ml. of sample are allowed to condense a t a temperature preferably just above the melting point. The calorimeter is cooled to a convenient temperature and the measured amount of sample is distilled into it. The calorimeter is then cooled to a temperature several degrees below the melting point and the block is heated just to the meltpoint of the sample after a good vacuum mm.) has been obtained in the system. Electrical energy is then added to the calorimeter, for short heating periods, and drift rates are established after attainment of thermal equilibrium. This process is repeated until the sample
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
1159
is completely melted. From these data total heat capacities (calories per ohm) and the heat of fusion are determined. Values o f ' a , the fraction melted, are computed in the same manner as described for calorimeter I and the same plot of l / a versus temperature (ohms) is employed to obtain impurity mole fraction.
Figure 5.
fusion, in conjunction with the ( l / a )plot, yields 99.79% for the purity of t h e n -pentane. To this sample of n-pentane, 2.40 mole % impurity of isooctane was added. The results are given by crosses in Figure 6. There is noticeable curvature of T us. l/a plot a t high values of l / a , but this is to be expected as the concentration of impurity is appreciable (25 mole %) when l / a = 10 with this relatively impure material. However, from the slope of the line for l / a < 2.0 one obtains 97.58 mole % ' for the purity as compared with a calculated value of 97.53 mole %.
Detailed Assembly of Calorimeter I1
Because of the better temperature control, data obtained with Calorimeter I1 are superior to those obtained with Calorimeter I.
x 24.95
S A H E + 2 . 2 6 MOLE % ISO-OCTANE
RESULTS WITH CALORIMETER I1
Calorimeter I1 was first tested with pure n-pentane and npentane to which iso-octane had been added as an impurity. The over-all results in terms of resistance (temperature) us. time are shown graphically in Figure 6. The circles represent the data where 1 cm. = 0.62" C. and the heating periods are represented by Roman numerals. Heating period I is entirely in the solid, while during period XI the sample is completely melted. The readings taken during melting are represented also by crosses on a scale of 1 cm. = 0.062' C. For purity determination these data have been converted to fraction melted after each equilibration period by allowing for the heat necessary to raise the temperature of solid and liquid and for the amount of heat leak from radiation and conduction. The results are plotted as circles, in terms of resistance us. (1/a) in Figure 7. The heat of fusion determined from this run was 2090 calories per mole. This is 4y0 greater than the true heat of fusion but sufficiently accurate for the determination of the cryoscopic constant. The 2090-calorie heat of
0
2.0
6.0
4.0
1. a
80
10.0
I
FRACTION MELTED
Figure 7 . Melting Point Curve for n-Pentane
Calorimeter I1 was used by the authors to determine the purity of two different samples of pentaborane prior to their introduction, as a blend, into our precision calorimeter. One sample was analyzed as 99.99% and the other as 99.91% pure by calorimeter 11. These two samples were then blended in such proportions that the purity of the blend should be 99.94 mole %. A more accurate determination of the purity of the blend in the precision vacuum calorimeter yielded 99.949 mole %, which is within the precision of the Calorimeter I1 determination. Calorimeter I1 also gave a value for the heat of fusion of pentaborane that was within 5% of the accurate value determined in the precision vacuum calorimeter. LITERATURE CITED
(1) Aston, J. G., Fink, H. L., Tooke, J. W., and Cines, M. R., IND. ENG.CHEM., - 4 S A L . ED.,19,218 (1947). (2) Glasgow, A. R., Streiff, A. J., and Rossini, F. D., J . Reaearch Natl. Bur. Standards, 35, 355 (1945). ( 3 ) Johnston, H. L., and Giauque, W. F., J . Am. Chem. SOC.,51, 3194 (1929). >----,-
11:w
I
I
I
I
I
Il:O
11:zo
Il:W
11:40
ll:m
I
I
12:w 12:o
I
I
12:m
12:w
TIME
Figure 6.
Melting Curves for n-Pentane
I
1840
I
1*:m
(4) Ibid.r86,31 (1934). (5) Schwab, F. W., and Wichers, E., in "Temperature, Its Measurement and Control in Science and Industry," p. 256, New York, Reinhold Publishing Corp., 1941. RECEIVEDfor review December 1, 1950. Accepted May 27, 1953. Work supported in part by the Office of Naval Research under contract with The Ohio State University Research Foundation.
