RESULTS AND DISCUSSION
Since the results of this gas counting is directly dependent on the average length of time the sample spends in the counter as the mixed gas stream mrpes through it, a total integrated count per sample was recorded rather than a count rate. The residence time in the counter is difficult to determine accurately except by comparisons using standardized samples; therefore no attempt was made to express the results in the form of the specific activity of the sample. Also, because of the difficulty in precisely duplicating flow rates from d a y to
day, only those results obtained on a given day were used t o calculate the precision of the measurements. The results are listed in Table 11. The difference in precision of the results obtained by counting radioactive samples in a liquid scintillation counter and those obtained with a flow-through proportional counter probably results from small changes in gas flow rates, and from short counting times in the gas counter with consequently greater statistical variation. RECEIVED for review January 19, 1967. Accepted March 14, 1967.
A Pyrolysis Oven Which Utilizes a Preheated Helium Stream R . A. Prosser, J. T. Stapler, and W. E. C. Yelland Materials Research Brunch, U. S . Army Nutick Laborutories, Nutick, Muss.
As PART OF A STUDY of the thermal degradation of polymers, attempts were made to determine the effects of additives such as metals and metallic oxides on pyrolysis reactions and products. This requires that polymer samples be pyrolyzed under known, reproducible, and reasonably uniform conditions so that the results can be subsequently compared with those obtained from mixtures of polymer and additive. The pyrolysis should be done in such a way that the additive will not seriously affect the temperature distribution throughout the sample. Although these conditions can be achieved by placing a sealed vessel containing the charge in a hot oven a t temperature equilibrium, the approach suffers from a serious disadvantage. The first products of degradation may undergo further reactions. .4s a result, the establishment of a mechanism of decoinposition becomes more difficult, and the effects of the additives may be obscured. Secondary degradation can be reduced by sweeping the sample with an inert gas as is the case when the pyrolysis is carried out in the carrier stream of a gas-liquid chromatograph. The decomposition products are thereby quickly removed !tom the oven and can be either swept directly onto the column ofthe gas-liquid chromatograph or cooled quickly and stored .it a low temperature. Sweeping as normally carried out can create a problem, because i f the temperature of the inert gas is different from that of the oven, a considerable variation in the temperature throughout the sample may exist. The initial experiments showed that the temperature difference between the helium and the wall at the sample position was as high as 100" C , depending on the flow rate. Serious temperature differences have been reported by others ( I ) . -!'he presence of an additive can also affect the temperature distribution in the sample. For a powdered polymer sample containing a finely dispersed metal, conduction and radiation from the oven wall are important sources of heat. Also, the presence of the metal in the polymer mixture tends to equalize the temperature throughout the sampie. I n a sample containing a metallic oxide, the time required for the entire charge to reach pyrolysis temperature will generally be increased. The pure polymer, when molten, is in many cases transparent and will absorb little radiant energy. Therefore, the surface temperature of the sample will be close to that of the inert gas; the bottom layer, which is in contact with the container, will be a t (1) I3. A. Vassalln, ANAL. CHEM., 33, 1823 (1961).
694
ANALYTICAL CHEMISTRY
In
Figure 1.
Pyrolysis oven
( A ) Hot Watt heater ( B ) External heater ( C ) Stainless-steel test tube (D) Sample holder ( E ) 34/45 ground joints ( F ) Concave region ( G ) Controlling thermocouple (H) Power leads (He) Helium (Q) Quartz wool insulation
the same temperature as the container wall (especially when bulk charges of 3 grams or more of the polymer are pyrolyzed). When the temperature of the inert gas is well below that of the wall of the oven, the time to reach temperature equilibrium and the temperature distribution in the various samples could be sufficiently different to change the relative amounts of pyrolysis products. This could obscure or even mask the chemical and/or catalytic effects of the additive on the pyrolysis, especially when attempting to ascertain the initial degradation products by carrying out the pyrolysis just above the threshold of decomposition. Madorsky ( 2 ) found wide differences in the temperatures obtained by several workers in similar uegradation studies of polystyrene. It is difficult to assess the degree of nonuniformity of temperature in a charge. One way to minimize this problem is t o place the charge in an oven so hot that pyrolysis takes place immediately. However, Radell and Strutz ( 3 ) found that the pyrciyzate of a pure polymer is not reproducible above a n upper temperature limit. Another way is to heat the sample
(2) S. L. Madorsky, J. Res., Natl. Bur. Std.. 62, 219 (i95Y). (3) E. A. RadeIl and H. C. Strutz, ANAL. CHEM., 31, 1890 (1959).
by means of an inert gas. This should bring all of the exposed surfaces of the polymer and its container to a uniform and known temperature We have used the latter approach in our work and found that chromatograms of the pyrolyzate obtained with metals were significantly different statistically from chromatograms of pyrolyzates obtained from multiple and reproducible pyrolyses of the pure polymer. I t is felt that the differences in the A-romatograms can now be reasonably attributed to chemical or catalytic effects of the metals. This article describes a pyrolysis oven which minimizes secondary degradakon by sweeping the sample with an inert gas, and the effect of additives on the temperature distribution throughout the sample by preheating the inert gas t o the pyrolysis temperature. Other ovens (4-13) were considered, but none could be readily adapted to this problem.
The mating ground joint for the left end is conventional. On the right end the joint is so constructed that, when installed, the tapered portion (F) remains exposed to the air for cooling. It has a Teflon sleeve of reduced length. All metal-to-glass joints have Swagelok unions (not shown) with Viton-A or silicone O-rings. The helium enters the left end of the oven and flows over the Nichrome wire coils of the Hot Watt heater and between the oven wall and the stainless-steel test tube to the far right end, at which point it sweeps out the residual air. Then the helium enters between the stainless-steel test tube and the sample holder and flows into the mouth of the sample holder, over the controlling thermocouple and sample, and through the outlet tube to the collection traps.
DESCRIPTION OF OVEN
With an empty sample tube in place, the current supplied to the external heater is adjusted until the controlling thermocouple equilibrates a t between 10 and 15" C below the desired pyrolysis temperature. Then the Thermac Controller is set so that the helium is preheated sufficiently to raise the temperature of the controlling thermocouple and polymer the additional 10" to 15" C . Thus the Thermac Controller is allowed a temperature span in which to function, and the helium is maintained at the desired pyrolysis temperature. A thermocouple probe placed at the sample position showed a decrease of 4" C per inch along the sample holder when the controlling thermocouple registered 450" C and the helium flow rate was 106 cc/minute. The temperature gradient depends on: the inert gas used and its flow rate; the length, insulation, and winding of the external heater; and the length of the stainless-steel test tube and sample holder. By suitable adjustment of these variables the gradient can be reduced further. After the oven has reached equilibrium, the powdered sample is inserted into a sampler holder, which then replaces the empty sample tube in the oven. During this step the ground joint is momentarily left open to flush out any residual air. The time required for the system (with an empty sample tube) to recover to 450" C is 7 minutes. Should the ground joint on the right end jam, it can be loosened by placing a little dry ice on its concave surface. A limitation of this oven is that the temperature gradient inside the sample tube becomes severe at slow flow rates of the inert gas. A loss of heat through the walls of the tube is indicated. This loss might be minimized by winding the external heater wires progressively closer together along the oven wall toward the oven exit.
The oven (Figure 1) consists of a borosilicate glass or quartz tube 30 inches long and 1-inch 0.d. with 34/45 ground joints ( E ) . The oven has an internal heater (A), consisting of a cylindrical ceramic rod 14 inches long and 0.875 inch in diameter with 12 axial holes through which finely coiled Nichrome wire has beehthreaded (left portion of the oven). This heater, custom built by Hot Watt, Inc., 128 Maple St., Danvers, Mass., is capable of delivering 1600 W and is automatically controlled by a Thermac Controller (Research Inc., Box 6164, Minneapolis 24, Minn.), utilizing a thermocouple (G) located just to the left of the sample position. An external heater ( B ) consisting of layers of asbestos, Nichrome wire, and asbestos, covers the external right portion of the tube. The current is controlled by a Variac. A stainless-steel test tube ( C ) 8 inches long and 0.75 inch in diameter is inserted into the right end of the oven, with the closed end between I and 2 inches from the Hot Watt heater. A stainless-steel sample holder (D),consisting of a test tube 4 inches long and 0.625 inch in diameter, which is welded coaxially to a stainless-steel outlet tube 12 inches long and 0.25 inch in diameter, fits into the stainless-steel test tube (C) with the outlet tube protruding from the right end of the oven. (4) D. Ettre and P. F. Varadi, Zbid., 34,752 (1962). ( 5 ) C. E. R, Jones and A. F. Moyles, Nafure, 189,222 (1961). (6) C. E. R. Jones and A. F. Moyles, Ibid., 191, 663 (1961). 33, 673 (7) F. A. Lehrnann and G. M. Brauer, ANAL. CHEM., (1961). (8) M. M. Markowitz itnd D. A. Boryta, Zbid., 32, 1588 (1960). (9) W. H. Parriss and P. D. Holland, Brifish Plastics, 33, 372 (1960). (IO) R. S. Porter, A. S. Hoffman, and J. F. Johnson, ANAL.CHEM., 34, 1179 (1962). (1 1) J. Strassburger, G . M. Brauer, M. Tryon, and A. F. Forziati, Ibid., 32, 454 (1960). (12) W. B. Swann and]'. P. Dux, Zbid., 33,654(1961). (13) H. Szyrnanski, C. Salinas, and P. Kwitowski, Nuture, 188, 403 (1960).
OPERATION
RECEIVED for review August 1; 1966. Accepted January 10, 1967.
V O i 39, NO. 6 . M A Y 1967
695
