Pyrolysis gas chromatography and radioactive tracer techniques have
been used independently to study the thermal degradation of polymers. In
these laboratories the two techniques have been combined to elucidate
some of the mechanisms of the thermal degradation of epoxy resins and
polyimides. This paper describes the apparatus developed for this work

Bishop, D. P.

Moffitt, B. R.

Smith, D. A.

English

Cranfield CERES

ji,111 1111 NI! CoA Memo . No . 168 Novemberl 1968  THE COLLEGE OF AERONAUTICS DEPARTMENT OF MATERIALS Combined pyrolysis and radiochemical gas chromatography for studying the thermal degradation of polymers - by - D.P. Bishop, B.R. Moffitt and D.A. Smith Synopsis ------ Pyrolysis gas chromatography and radioactive tracer techniques have been used independently to study the thermal degradation of polymers. 	 In these laboratories the two techniques have been combined to elucidate some of the mechanisms of the thermal degradation of epoxy resins and polyimides. This paper describes the apparatus developed for this work. Introduction Pyrolysis gas chromatography has become one of the most extensively used technioues for investigating the thermal breakdown of organic compounds. It has proved particularly useful for studying the mechanisms of polymer degradation, especially in those cases where the pyrolysis products are simply related to the original structure of the polymer. When complex polymers, such as cured phenolic or epoxy resins, are pyrolysed large numbers of degradation products are formed which are not simply related to the original structure (1)(2)(3). 	 Chromatographic separation of these products gives a characteristic pyrogram for each polymer. These 'finger print' pyrograms may be used for polymer identification, but they are of limited value in determining the mechanisms of thermal breakdown because although the components of each pyrogram can be identified, it is not often possible to say how they were formed. 'Radioactive tracer techniques have a unique advantage in that specific atoms or groups in complex molecules can be labelled, and hence their fate through any number of operations can be followed. This facility is obviously useful for following degradation processes and has been exploited in a number of polymer degradation (4)-(7) and stabilisation (8)(9) studies. In these laboratories the thermal degradation of cured epoxy resins (2)(10)(11)(12) and polyimides has been .tudied. 	 The use - 2 - of pyrolysis gas chromatography alone made it possible to postulate degradation mechanisms for these polymers, and it was considered that isotopic techniques would provide conclusive evidence for some of the mechanisms. Consequently an attempt has been made to combine the two techniques to form a continuous and quantitative pyrolysis gas chromatography and radiochemical analysis system. This system is described below; details of the polymer degradation work will appear in future publications. The pyrolysis gas chromatography unit Pyrolysis gas chromatography techniques have been compre-hensively reviewed by Levy (13)(14) and need no further discussion here. The pyrolysis unit used for the present work consisted of an electrically heated nichrome filament housed in a pyrex glass tube (2)(10). This tube was built into a Perkin Elmer 452 gas chromatograph so teat degradation products that were volatile at the chromatograph oven temperature were swept into the column and separated. Pyrolysis temperatures were obtained using a chromelialumel thermocouple attached to the mid point of the filament. The filament temperature was correlated with the supply voltage for various carrier gas flow rates and oven teuaeratures. The gas chromatograph was fitted with a thermal conductivity detector, and since nitrogen was used as the carrier gas throughout this work positive and negative peaks appear on the pyrograms. (Figs. 4 and 5). 3 Detection of radioactive compounds in  the gas chromatograph effluent 	  --------- (a) Discussion of methods In order to determine the fate of labelled atoms when a sample of labelled polymer is pyrolysed it is necessary to moritor the various breakdown products for radioactivity. The 'char' and tar like products which remain in the pyrolysis tube may be analysed by conventional liquid scintillation counting techniques, but the volatile products which emerge from the gas chromatograph are more difficult to analyse. The methods available for detecting and measuring radio-activity in the effluent of a gas chromatograph have been reviewed by Karmen (15). They may be divided into two main types; continuous methods in which radioactivity is detected and measured in the gas stream, and discontinuous or two stage methods in which the labelled compounds are condensed in a scintillator solution (or on scintillator crystals) and counted by conventional counting techniques. The continuous methods have the advantages of speed and convenience, but the time taken for an active compound to pass through the detector must be limited in order to preserve the resolution of the various compounds. It is therefore impossible to count low levels of activity with accuracy when using continuous techniques. The tendency for high boiling compounds to condense in the - 4 - detector may be overcome by either using a heated detector cell, or, if only 14C and 3H are to be detected, by burning all organic compounds to CO2 and H2 (and 14CO2  and 3H2). Heated proportional counters have been designed to operate at temperatures up to 200°C but the problem of adsorption of polar compounds onto the detector walls has not been satisfactorily overcome. Crystal scintillation detectors may be used at reasonably high temperatures. Anthracene for example melts at 216°C and presumably may be used at temperatures up to about 200°C, but the heated cell must be separated from the photomultiplier tube with a cooled light pipe in order to keep the thermionic noise from the tube at a .low level. Combustion of organic compounds to CO2  and H2 is easily achieved by passing them first over copper oxide heated to 700°C and then over iron filings or steel wool also at 700°C. The reaction with copper oxide gives CO2  and H20, and the reaction with iron converts H2O to H2. The ferric oxide formed in the latter reaction strongly absorbs water and it is necessary to keep the iron reduced by introducing hydrogen into the gas stream, (16). Combustion is not suitable when isotopes other than 14C and 3H are to be detected; halogen acids for example al'e retained by copper oxide and labelled halogens would therefore be undetected. In polymer degradation work it is desirable to collect the volatile breakdown products for characterisation. If radioactivity is measured using a two stage method the breakdown products are retained in solution or adsorbed on crystals ani can not be characterised easily. 	 If a continuous technique with combustion is used the products are destroyed. This difficulty may be overcome by using a stream splitter, but when a stream splitter is used only a small percentage of the effluent may be taken for fractionation because otherwise the level of activity reaching the detector becomes too low for accurate measurement. This may mean that insufficient material is available for identification of minor degradation products and the alternatives of increasing sample size or increasing the specific activity of the sample must be considered. This problem is less acute if a two stage method is used because here the counting time is unlimited and low activity samples can be counted with accuracy. A larger proportion of the gas stream may therefore be split off for characterisation. The main disadvantage of a two stage method is that every product must be collected and counted separately; a tedious and time consuming operation unless an automatic fraction collector and a scintillation counter with an automatic sample changer is available. (b) Description of the scintillation detector --------------------------- The apparatus developed for the present work was intended to be used as a flow through scintillation counter with or without a combustion train. It was constructed so that the flow cell could be removed easily and the unit used as a conventional scintillation counter. The following description refers to FIG. 1. The detector unit was a modified Nuclear Erterprises NE5504 6 - shielded scintillation head unit. A water cooled quartz light pipe (A) was fitted between the flow cell (B) and the face of the photomultiplier tube (C) so that the photomultiplier remained at constant room temperature while the flow cell was heated to pravent the condensation of organic compounds. The flow cell consisted of a pyrex glass spiral (30 cm. of 0.3 cm. I.D. tubing) loosely packed with scintillation grade anthracene crystals (m.p. 216°C). A plane mirror (D) was mounted above the spiral to reflect scintillations occurring near the top surface of the spiral into the light pipe and thence to the photo-multiplier tube. A highly polished brass collar (E) surrounded the spiral and mirror, and the enclosure thus formed was filled with an optically clear grade of silicone oil (F). The function of the oil was to make optical contact between the glass spiral and the light pipe, and also to make thermal contact between the brass collar and the spiral. The brass collar was heated by the heating coil (G). The temperature of the flow cell was recorded using a chromel/alumel thermocouple (H) dipping into the silicone oil. This system detected '4C labelled compounds with about 20% efficiency and there vas no tendency for active material to be retained in the flow cell. When the cell was operated at temperatures above about 30°C however the cooling system was inadequate and the photomultiplier gave excessive noise as it became warm. Consequently when high boiling compounds were being separated a combustion train was placed between the gas chromato-graph and the scinitillation detector. 7 In order to use the detector as a conventional liquid scintillation counter the flow cell, mirror, and thermocouple were lifted out and replaced by a 12 ml. glass counting bottle containing the sample to be counted. Counting was found to be about 65% efficient using 0.540 butyl P.B.D. in redistilled A.R. toluene as the liquid scintillator. Efficiencies approaching 10C% may normally be expected with this scintillator and the low efficiency of the present system may be attributed to light loss in the quartz light pipe. The integrator  unit The integrator unit was designed to drive a chart recorder with the events per second data obtained from the photomultiplier tube, and to integrate each radioactive peak and display the total count digitally. The following description of the system refers to FIG. 2. Pulses from the photomultiplier tube (caused by scintillations in the flow cell) are amplified and fed via the 'UP' gate into the REVERSIBLE BINARY COUNTER. At the end of each 1 sec. period the count is transferred to STORE, I which drives the CHART 'CORDER via the DIGITAL TO ANALOGUE CONVERTER, producing a deflection proportional to the number of events in that 1 sec. period. This reading is held until STORE I is set to the count for the next 1 sec. period, thus a histogram type record is produced on the chart. At the same time as STORE I is loaded (i.e. at the end of a 1 sec. period) the UP/DOWN STORE is set to I dowr I . 	 This isolates 3 the counter from the data source and connects it to CLOCK I, which causes the counter to count down towards zero at 106  pulses per second. When zero is detected the UP/DOWN STORE is set to 'ups and the next counting period begins. The capacity of the counter is 255 so the maximum time for reset is 0.25 millisec., giving a negligible error in the 1 sec. gating periods. Since the number of pulses required to bring the counter back to zero equals the number reached when counting data pulses, there is at the output of the DOWN GATE, during the resetting phase, a train of that number of pulses. 	 This is used to drive 'Llhe DECIMAL COUNTER via the INTEGRATE GATE which is controlled by the THRESHOLD DETECTOR. If the preset threshold (background count) is reached or passed by the data count the reset pulse train is fed into the DECIMAL COUNTER and added to the number already there, but if the threshold is not reached the reset pulse train does not enter the DECIMAL COUNTER. During a radioactive chromatogram peak the threshold is reached during every data count and the count total accumulates in the decimal counter. When at the end of the peak the threshold is not reached the DISPLAY CONTROL is armed and the INTEGRATE GATE is closed. The DISPLAY CONTROL causes STORE 2 to hold the number reached by the decimal counter, and the counter is reset to zero ready for the next peak. The DIGITAL DISPLAY shows the number held in STORE 2. 	 In this way the total count for each peak is displayed until the next total has been accumulated, and may be noted on the recorder chart or otherwise. 9 The effects of overflow in the two counters were considered and measures were taken to prevent the generation of wrong data. A signal is produced if the REVERSIBLE COUNTER reaches full scale which isolates the counter input thus storing full scale in the counter. At the end of the 1 sec. period this count is transferred in the normal way to STORE I producing full scale deflection on the chart recorder. In addition a warning light shows that an overflow has occurred. An overflow in the DECIMAL COUNTER produces a carry signal which sets a store so connected that when the count next reaches 3000 the input is isolated locking the counter at that number. When the display is set a warning light is also set if an overflow has occurred. The display was therefore interpreted as follows: Display 	 Overflow 	 True Count 	< 9999 	No 	 As display 	< B000 	 Yes 	 As display 10,C00 	Moo 	 Yes 	 18,000 By interposing a suitable voltage to frequency converter between the input of this instrument and a chromatograph detector having a voltage output, the peak integration facility would be available for conventional gas chromatography. The complete pyrolysis radio gas chromatograph is shown in FIG. 3 Results The apparatus has been used in a study of the pyrolytic - 1_0 degradation of epoxy resins and polyimides. Details of this work will appear in separate publications but FIGS. 4 and 5 show some pyrograms and 'radiopyrograms' obtained on pyrolysis of the diglycidyl ether of bisphenol A (DGEBPA) cured with phthallic anhydride (PA). The resin was 14C labelled at the 1,3 atoms of the glycidyl groups (FIG. 6). References (1) Lee, ,H., J. Polymer Sci.„ A3, 859-882, (1965). Stuart, J.M., Smith, D.A., J. Appl. Polymer Sol., 9, 3195-3214, (1965). Madorsk, S.L., Straus, S., Modern Plastics, 39, No. 6, 134-210, (1961) Cameron, C.G., Grassie, N., Makromol. Chem., 51, 130-136, (1962). Richards, D.H.„ Salter, D.A., Polymer, 8, 153-159, (1967). Moiseev, V.D., Neiman, M.B.„ Kovarskayal B.M., Zenova, Guryanova, V.V., Soviet Plastics, 6, 12-15, June 1962 (7) Bowen, D.O., Modern Plastics, 44, 127-164, Aug. 1967. (8) Frye, A.H., Horst;  R.W., Paliobagis, 3. Polymer Sci., A2, 1765-1614 (X964). (9) n?ye, A.11,, Horst, R.W., Int. J. Appl. Rad. Isotopes, 15, 169-i74, (1964). (10) Keenan, M.A., Smith;  D.A., J.Appl.Polymer Sci., 11, 1009-1026, (1967) (il) Bishop, D.P., Smith, D.A., Ind.Eng.Chem., 59 No. 8, 32-39, Aug.(1967). (12) Patterson-Jones, J.C., Smith, D.A., J.Appl.Polymer Sol.;  12, 1601-1620, (1966). (3.3) Levy, R.L., Chromatographic Reviews, 8, 48-49, (1966). (14) Levy, R.L., .T.Gas Chromatography, 5, 107-113, (1967). (15) Karm,]n, A., Packard Technical Bulletin No. 14, June 1965. (16) Karmen, A., McCaffrey, I., Winkelman, J., Bowman, R.L., Anal. Chem., 35, 536-542, (1963). F 0 b A C D —  AMPLIFIER i 	 The scintillation detector unit CLOCK 1 pps CLOCK 2 1 1 PPS DOWN STORE DOWN GATE UPTECA  INTEGRATE GATE SCINTILLATION DETECTOR.  REVERSIBLE BINARY COUNTER   IN•tiPLIF E9 ZERO DETECT  THRED-401.0 DETECTOR STORE 1 DIGITAL TO ANALOGUE CONVITOR L  DISPLAY CONTROL CHART RECORDER FIG. 2 	 Block diagram of the integrator and Misplay unit DECIMAL COUNTER STORE 2 DIGITAL DISPLAY FIGURE 3 The combined pyrolysis gas chromatography and radiochemical analysis apparatus (with combustion train). HYDROGEN METHANE PYROGRAM 4 CARBON MONOXIDE CARBON DIOXIDE r--  CV L.464644406,1".044.46610464#46.40140.468.414msehtrr) 	 D I OPY ROGRAM #0,0"1411 0 	10 	 20 TIME - MINS FIG. 4 	 Separation of gaseous degradation products from DGEBPA (14C 1,3-glycidy1)/ PA (20 mg). Pyrolysis temp.: 700°C. Column: 30-60 mesh silica gel. Oven temp. : 50°C. Carrier gas: Nitrogen, 20 ml. I min. WATER ACETALDEHYDE 1 MIXED GASES R ADIOPYRO GRAM mindivuoprate- 10 	2p 	 30 TIME - MINS FIG. 5 Separation of low boiling degradation products from DGEBPA (14C 1„ 3-glycidyl)/ PA. (50 mg.) Pyrolysis temp. : 700oC. Column: 7, 8 benzoquinoline/60-80 mesh Chromosorb P, 15/85. Oven temp.: 50°C. Carrier gas: Nitrogen, 20 ml. /min. 0)( 	 1 	 11 0)( 	 0 -CH-CH2 _0 	 0_6H, _CH_cH, _o_c • -0-co X= H OR -0-CO *denotes 14C labelled  FIG. 6 	Structure a DGEBPA (14C I, 3-glycidy1)/ PA. 

Combined pyrolysis and radiochemical gas chromatography for studying the thermal degradation of polymers

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Combined pyrolysis and radiochemical gas chromatography for studying the thermal degradation of polymers

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