the results have purposely been equated t o a 4.0% wt. level. Between the quadruplicate samples analyzed, a total variance less than the relative error was obtained. I n additional studies a total of eight polyols, including ethylene glycol, propylene glycol, diethylene glycol, 1,3butylene glycol, dipropylene glycol, trimethylene glycol (1,3 - propylene glycol), glycerine, and 1,2,&hexanetriol, were easily separated by the described instrumental technique. However, only propylene glycol, diethylene
glycol, and glycerine were found to be present in the commercial cigarettes treated, and inasmuch as our primary coiicerii was with a technique for the analysis of cigarettes, detailed studies of the estimation of these other compounds were not made. IThile we do not feel that sugars present in the casing mixture used on coiiiniercial cigarette tobaccos would interfere with the described analytical technique, further nork by someone in the tobacco industry having an intimate knowledge of various casing
solutions should be conducted t o confirm or refute our viewpoint. ACKNOWLEDGMENT
The authors express their appreciation t o W.D. Hanus and A. C. Cocuzaa for their assistance in the above work. RECEIVED for revien- December 7, 1961. Resubmitted August 2, 1962. Accepted
October li, 1962. Presented in part a t the 15th Tobacco Chemists' Research Conference, Philadelphia, Pa., October 4-6, 1961.
Pyrolysis-Gas Chromatographic Technique Effect of Temperature on Thermal Degradation of Polymers KITTY ETTRE and P. F. VARADl The Machlett laboratories, Inc., Springdale, Conn.
p A new pyrolysis apparatus was developed, which allows exact thermal degradation studies in a wide temperature range. Three polymersnitrocellulose, poly(vitiyl alcohol), and poly(n-butyl methacrylate)-were pyrolyzed between 300" and 950" C. The composition of the breakdown products at the different temperatures i s tabulated. The results are compared with those obtained using flash pyrolysis technique.
1
previous studies ( 2 ) , a flash filament pyrolysis unit was used to investigate polymers at a measured temperature of 650" C., and a complete qualitative and quantitative analysis of the pyrolysis products was given for each polymer at this temperature. Subsequent work proved, however, that if the breakdown has t o be studied a t different temperatures, the flash pyrolysis technique is inadequate. The pyrolysis apparatus described in the literature can be divided into two groups, those using flash filament pyrolysis and t h e others using reactor chamber techniques. The instrumentation for flash filamcnt pyrolysis consists mainly of a filament heated electrically t o the desired temperature. The sample is either dissolved in a solvent and coated on t h e filament in form of a thin layer or measured in solid form into a small cup or boat which is placed in the heating coil. Both procedures have definite disadvantages. T17ith the coating technique (1. 4, 6. 7 . 10 13. I d ) , the sample cannot be N OUR
analyzed in solid form b u t has to be dissolved and then applied to the filament. The dissolved form of the material, however, does not necessarily decompose identically to the undissolved (solid) substance; and the breakdowi products of the solvent may also appear in the chromatogram. Quantitative measurement is not possible, since the original amount cannot be weighed, nor can the amount and characteristics of the residue be determined. Finally, t h e glowing wire may act catalytically on t h e decomposition products to change their nature by secondary reactions. The second mode of operation (use of a small boat) (9, 11) eliminates most of these errors, but introduces a new major problem: The heat-up time of the sample is no longer instantaneous. The boat or cup placed in the heating coil reduces the heating rate of the filament and slows down its heat-up time. The boat itself takes over the final temperature of the filament only after a certain time, depending on its material and wall thickness; thus, the total heat-up time of the sample may vary between 20 and 40 seconds b u t can hardly be reduced belovi 20 seconds. This means t h a t the sample itself goes through t h e n-hole temperature range before reaching the desired temperature, and therefore the composition of the breakdown products reflects t h e pgrolysis not at a certain temperature but up t o a certain temperature. The products of the lower temperature pyrolysis may also react further; thus, a combination of primary and secondary
pyrolysis products may appear in the chromatogram. The fact that most flash pyrolysis setups do not allow higher carrier gas flow rates also contributes t o the possibility of secondary reactions, because the primary breakdown products stay too long in contact with the heated part- of the pyroly sis zone. A separate problem of all flash pyrolysis units ib that it is practically impossible t o measure the exact pyrolysis temperature. Therefore. in many cases the temperature of the glov-ing n-ire is evaluated only visually, by observing its color (5, 1 1 ) . Thus, although studies Kith flash pyrolysis result in reproducible data, they may be misleading in many cases. Recently, some re3earchers described the uae of reaction chambers for pyrolysis studies (3, S, 1 2 ) . These units, USUally made of a stainless steel tube, have overcome some of the difficulties of flash pyrolysis. The error of sloyer heat-up is eliminated because thi. sample is introduced (injected) directly a t the temperature of pa rolysis: the temperature is also more controllable and its nieasuremeiit is more accurate. Finally, higher flow rates can be applied, and thus the possibility of secondary reactions can be minimized There remain. however, some problems n hich are yet unsolved, such as the iiitroduction of solid samples and t h e possibility of measurements over a wide tempernture range. Further, the hot stainless steel surface of the chamber may have some catalytic effects, resulting again in secondary reactions. '
VOL. 35, NO. 1, JANUARY 1963
69
Our aim was to develop a pyrolysis system which eliminates all these errors and assures conditions that allow controlled, accurately measured pyrolysis processes. This system is capable of taking samples in solid or liquid form, and accurately measures the sample and its residue. The sample is heated instantaneously to the desired temperature, and the products formed are swept out with a high flow of carrier gas. The temperature can be measured very exactly and pyrolysis can be performed a t any selected temperature in a wide temperature range (150' to 950' C.). Finally, series of analyses (five to 10 samples) may be carried out without disconnecting the chamber.
2
v, 4 0
TOP VIEW
G
EXPERIMENTAL
Pyrolysis Unit and I t s Operation. Figure 1 shows the schematic of the new pyrolysis device. It is composed of two parts: the pyrolysis chamber and the furnace with the electrical control unit. The chamber is connected through tubulations 1 and 12 directly into the gas-sampling valve of the gas chromatograph. The main part of the chamber is a quartz tube 2 inches long and inch wide, which is heated in the furnace to the desired temperature. The entering tube, 1, is continued in a flexible metal tubulation, 2, and is connected by a vacuum-tight flat metal joint, 3, to the pyrolysis chamber. This joint is tightened by screws and a neoprene or Teflon gasket is used for sealing. The joint is connected by a metal-to-glass seal, 4, to a glass tube, 5, which is then continued through a graded seal, 6, to the quartz tubc, 7. At a certain place, a "bump," 8, is formed in the quartz tube, in order t o stop the sample boat always a t the same point during each analysis. The quartz tube is continued in a second graded seal, 9, a short glass tube, 10, and a second glass-to-metal joint, 11, which connects the exit port, 12, to the pyrolysis chamber. Parts 10 to 12 are heated to avoid condensation. Two other glass tubes are attached to the main glass-to-quartz tube. The first, 13, is attached at a 30" angle at the side of the chamber and is placed in the same horizontal plane; it is terminated through a glass-tometal seal, 14, to a flat metal flange seal, 15. This side glass tube has a connecting tubulation, 16, to the main chamber, to accelerate the purge-out of this tube by the carrier gas. The second attachment to the horizontal tube is a vertically placed glass tube, 17, terminated through a glass-to-metal seal, 18, in a flat metal flange, 19. The horizontal and vertical tubes serve as storage places of the sample boats before and after pyrolysis, respectively. The quartz tube is heated in a combustion type furnace, 20, equipped with a heat shield and a thermocouple, 21, for exact temperature measurement. The electric current to the furnace can be regulated by a variable
70
ANALYTICAL CHEMISTRY
VAL VE
Figure 1.
Schematic diagram of pyrolysis new device
transformer, 23. The thermocouple is connected to a millivoltmeter, 24, calibrated directly in degrees centigrade. Pyrolysis. The samples are weighed in small ceramic boats, 25, which are introduced through port, 15 of the side tube into the chamber. Between two boats, small nickel rods, 26, are placed; then the side tube is closed by the flat metal disk. Up to this point the tube was bypassed by the carrier gas by means of the gas-sampling valve; iiow the valve is turned to the second position and the whole chamber is purged with the carrier gas. At the same time, the furnace is heated to the desired temperature. Meanwhile, the first boat is pushed with the small nickel rod placed behind it (moved with a n outside magnet) into the cold zone of the main tube. After the base line of the chromatographic recorder is stabilized, this boat is pushed very quickly (3 to 5 seconds) with the help of the metal pusher, 22, activated again with an outside magnet, into the hot zone. The oven and quartz tube represent a high hot mass; thus the material to be pyrolyzed is heated instantaneously to the desired temperature, the pyrolysis takes place immediately, and the volatile decomposition products are swept with the carrier gas into the gas chromatograph for separation and analysis. After the analysis is accomplished, the boat is removed from the hot zone and pulled back with the metal pusher into the vertical storage tube, 17. The whole procedure is repeated with the next sample. I n this way, six t o eight measurements can be performed under identical conditions without disconnecting the pyrolysis chamber. Gas Chromatograph. A standard Perkin-Elmer Model 154-D Vapor Fractometer equipped with a thermistor detector and connected to a Leeds & Northrup 5 m v . potentiometer
recorder was used for the investigations. The columns were the same as previously (2j, but the lengths of the adsorption columns and the partition column with diisodecyl phthalate liquid phase were doubled (4 meters). The column temperatures were identical to those in the previous work. A second principal change was in the inlet pressure: 15 p.s.i.g. instead of 7 and 10 p.s.i.g. This relatively high inlet pressure was possible because the grid seals of the new system permitted a much higher flow rate without leaking. Actually, carrier gas inlet pressures up to 30 p.s.i.g. could be used, while with the previous flash pyrolysis unit 8 to 10 p.s.i.g. was the maximum inlet pressure that could be applied continuously without leakage. Procedure. The same three polymers were investigated as in the first work (2): nitrocellulose, poly(%-butyl methacrylate), and poly(viny1 alcohol). The characteristics and sources of these materials have been described ( 2 ) . The amount of sample used for the present pyrolysis studies was on the order of 12 to 13 mg. The pyrolysis measurements were performed in the temperature range of 300" tJo 950" C. Each polymer was pyrolyzed subsequently in this range with steps of 100" C. At least two parallel pyrolyses were made at each temperature; the results agreed within &5%. The pyrolysis products were analyzed on different separation columns: The light inorganic gases and hydrocarbons were separated on a Molecular Sieve and a silica gel column and the other organic substances on a phthalate and a Carbowax column (2). Comparison with pure standards permitted evaluation of the chromatograms. Using these columns, a complete quantitative and qualitative analysis was performed for each polymer a t each measured temperature.
~~
Table 1.
~
~~
Composition of Breakdown Products of Nitrocellulose
Temperature, "C. 400
300
500
600
700
800
900
950
Weight per cent of original polymer
+
Carbon dioxide nitrogen oxides Carbonmonoxide Nitrogen Methane Ethylene Acetaldehyde Methanol + ethanol Methyl acetate Unidentified Water Residue
T
I
;
~
~
1
I
"
5
IO MINUTES
0
Figure 2. Typical chromatogram obtained from pyrolysisof nitrocellulose at
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