Pyrolysis-GC/MS/IR Analysis of Nylon 6/6

Application Note 228-106

Authors Matthew S. Klee and Imogene L. Chang Hewlett-Packard Co. Avondale Division

Abstract The pyrolysis gas chromato­graphic characterization of Nylon 6/6 is described. De­tection of capillary-column effluent and identification of pyrolysates was accom­plished with an MSD and IRD connected in parallel. Cryo­genic cooling of the inlet and column was used to focus broad pyrolysate peaks for high-resolution analysis. Programmable column pres­sure control allowed fast analysis while maintaining chromatographic efficiency.

Introduction Pyrolysis gas chromatogTaphy is a useful tool for characterizing polymers. Polymers which are not volatile enough for standard gas chromatogTaphic analysis can be thermally cleaved into small, volatile fragments. The resulting chromatograms (pyrograms) yield characteristic fingerprints relat­ing to the composition of the

polymer and its molecular struc­ture (branching and defects), as well as additives.

High-resolution chromatography is very important for efficient separation of the many fragments which are produced during the pyrolysis process. Capillary column chromatographs with low dead volume connections and cryogenic cooling focus the wide pyrolysis injection bands into narrow bands and ensure maxi­mum resolution of pyrolysates.

Two new features of the HP 5890 SERIES II Gas Chromatograph inlet were used to ensure sharp injection profiles of solute bands and narrow chromatographic peaks. These inlet features are programmable pressure control and independent inlet tempera­ture control. Since the flow rate through a capillary column decreases with increasing oven temperature (the gas is more viscous, causing decreased flow with fixed head pressure), stan­dard pressure-controlled chro­matographs can suffer both losses in efficiency and increases in analysis time, especially when large temperature changes are necessary for the analysis. The

Fhfl'l HEWLETT ~~PACKARD

Gas Chromatography

August 1989

HP 5890 SERIES II GC used in this study automatically adjusted the column head pressure to maintain column flow rate during the run (CONSTANT FLOW mode). By rapidly increasing the inlet temperature (inlet tempera­ture control is controlled inde­pendently from the oven tempera­ture), pyrolysates which had been cryogenically trapped in the inlet during the pyrolysis were then transferred into the column oven zone in sharp bands.

Selective detection of pyrolysates often simplifies the task of char­acterizing the starting polymer, the mechanisms of its thermal degradation, and in identifying plasticizers and residual solvents in the polymer. Mass spectral and infrared detectors (MSD and IRD) are selective detectors which provide complimentary informa­tion on molecular structure and molecular orientation. Corn­pound identification, which is difficult to do by mass spectrome­try, can often be done by infrared spectroscopy and vice versa. Combining these two instruments for simultaneous chromatographic detection is, therefore, extremely effective for polymer pyrolysis studies.

Experimental The instrumentation and condi­tions used for this study are listed in table 1. A block diagram of the chromatographic system is given in figure 1.

Pyrolyses were carried out in a heated interface assembly (CDS, Oxford, PA) which is supplied for use with HP 5890 GCs. The standard interface needle assem­bly was too large to fit into the on-column injector and was replaced with a 28-gauge needle via a 1/8 in.-1/16 in. Swagelok union and graphiteN espel ferrule. The pyrolyzer-GC interface was used without the standard quartz liner and was fed with an auxiliary supply of helium carrier gas. This auxiliary helium was connected to an event-controlled solenoid (in the Purge B position) such that it could be turned off and on within a time program. As long as the auxiliary pressure supplied to the pyrolyzer-GC interface exceeds that of the programmable pres­sure controller (PPC), it overrides the PPC and supplies most of chromatographic system's carrier gas flow through the interface. As soon as the solenoid turns off the gas flow through the interface, the PPC resumes control of the system head pressure and no more helium flows throug-h the pyrolysis-GC interface.

Nylon 6/6 (a copolymer of 1,6-hexanedioic acid and 1,6-hexanediamine) was acquired from Chemical Data Systems, Oxford, PA. Pyrolyses were carried out at 700°C and 900°C in quartz tubes using a platinum coil pyrolysis probe. Sample amounts are stated in the caption of figure 2 and were less than 1 mg.

2

Table 1. Instrumentation and Conditions

Gas Chromatograph

Cooling

Mass Selective Detector Mass Range

EMV Transfer Line Data System

Infrared Detector Resolution Transfer A,B Cell Data System

Column He Flow Oven Program Injector Program

Pyrolyzer Interface Probe Ramp Interval Pyrolysis Temperatures

Auxiliary He Supply Pressure OFF Time

Hewlett-Packard 5890 SERIES II Cold On-Column Injector w/Programmable Pressure and Temperature Control Cryo Blast, C02 Cryogenic (Oven and Injector)

Hewlett-Packard 5970 19-250 amu (0 to 8 min) 37-400 amu (8 min to END) + 0 V relative 325°C Hewlett-Packard 9836C (Rev. 3.1.1)

Hewlett-Packard 5965A 8 em -1 (4000 to 700 em -1 ) 300°C 300°C Hewlett-Packard 59970C (Rev. 1.0)

30m Series 530 Jl., 2.65~-tm HP-5 (Part No.19095J-123) 5 psi@ 0°C, Constant Flow Mode ooc (1 min)-> 275°C (2 min) @ 15°C/min -10°C (0.5 min)-> 300°C (1 min)@ 120°C/min

Chemical Data Systems (Oxford, PA) Model122 275 Coil Off (Ballistic) 20 sec 700°C, 900°C

10-12psi O.Smin

Figure 1. Block diagram of the pyrolysis-GC system. The external pressure source (P) can be turned ON/OFF by solenoid (S). A 28-gauge needle protrudes from the bottom of the pyrolysis interface (I) and is inserted into the cool on-column inlet. Liquid C02 cools both the inlet and the column. The mass selective d etector (MSD) and the infrared detector (IRD) are used for parallel detection of column effiuent.

Pyrolysates were separated by an HP-5 (5% phenyl methyl-silicone) capillary colwnn and detected by both MSD and IRD connected in parallel. The inlet end of the colwnn was connected to the cool on-colwnn injector. The outlet of the colwnn was connected to one side of a 1/16 in. zero dead-volwne Swagelok union. To the other end of the union, two transfer lines were connected via a two-holed graphite/Vespel ferrule: a short piece of 0.32 mm HP-5 coated capillary colwnn was used for con­nection to the IRD, and a 2m x 0.10 mm id blank column was used for connection to the MSD.

Results And Discussion Pyrograms corresponding to the 700°C and 900°C pyrolyses of Nylon 6/6 are shown in figures 2 and 3. Figure 2 represents the total ion chromatograms (mass spectral). Figure 3 represents the total reconstructed chromato­grams (infrared). The identities of the major pyrolysis fragments and several of the minor fragments are given in table 2. The 900°C pyrolysis yielded a higher propor­tion of smaller fragments (early­eluting compounds) as well as larger variety of compounds formed by molecular rearrange­ments; there were many more minor peaks throughout the whole chromatogram compared with the 700 oc pyrolysis. This is expected because the higher temperature pyrolysis is more energetic, caus­ing more secondary pyrolyses and molecular recombinations.

Differences in the relative heights of the peaks in the TIC compared to the TRC are a function of the differences in the abundance of ion fragments produced by a given

compound in the MSD relative to

Figure 2. Comparison of total ion chromatograms for the pyrolysis of Nylon 6/6 . A) 0.45 mg at 900°C for 20 sec. B) 0.77 mg at 700°C for 20 sec.

Figure 3. Total reconstructed chromatograms (infrared) corresponding to the same samples as in figure 1.

Table 2. Major Pyrolysis Fragments of Nylon 6/6

Peak No. Compound Name

1 1·propene 2 butene 3 acetonttrile, bp 82 4 acrylonrtrile, bp 77 5 propanenrtrile, bp 97 6 1,5-hexadiene, bp 60 7 1-hexene, bp 68 8 butylamine, bp 78 9 cyclopentanone, bp 130

1 0 cyclohexenone, bp 168 11 hexylamine, bp 131 12 caprolactam, 7 -member cyclic amide,

bp = 137

3

the strength of its infrared-ab­sorption in the IRD. It is appar­ent that the MSD was more sensitive than the IRD for most compounds.

Peaks 1 and 2 (propene and butene) in figures 2 and 3 are relatively broad. This is because ooc was not cold enough to focus them on the column, so the half-width of these peaks is approximately the width of the pyrolysis itself (20 sec). 0 oc column temperature was low enough, however, to chromato­graphically separate them so that they could be spectrally identified.

In order to demonstrate the power of the combined detector system, several pyrolysate peaks for which mass spectral library results were inconclusive were selected for further interpretation. Figure 4 shows the top three hits from a library search of mass spectra for peak 7 around 6 min. Due to the similarity of the spectra for hexene and chlorohexane, un­equivocal identification could not be made based solely on the mass spectrum; the search correlations are given in table 3. Rationally, of course, it is improbable that a chlorinated compound would be present in Nylon 6/6 except as a solvent or additive, but in many applications, additional spectral evidence is necessary for confir­mation. Even though the infrared spectrum for peak 7 is noisy, it confirms that the compound is unsaturated and it matches well with an IR library spectrum for hexene (figure 5).

4

~(496

100001 2/ 29 5000 I /

0 I I I -)1,1:.'1lrl;;~f :·:,:· :,1'1"

I I : .-:·.

of. HRTT:NYL_R3.0

69

/ I

t52s:::::kHex-e ne-:· <BClSCIJ .fr-om . ORTR: NBS_ REVE:. L

'41' (t529)

: ·:::1 ~ <· •.

I.:,J.I ,.,, . 69

""­I

84 ·:.·\ ···

84 /

I

:·m,· '"·"""" if"' '":\~'"· ""'"""' ·r 5 0 . ~ <+295 1) +2951

] 100001 f7 _3..9 a: $000 I

0

1000JltS40 ) +540 Cycl~rop.n~ . propy1-it9Cil

5000 2/ ;

9 41 j, I ~

0- l 11 I._ .I . . __ . 30 35 4·0 · . "·45 .·:: -:-50 . 55 · .. -··:·60

Ha.s s /Ch ~-~-1;1-~ .·:·

from ORTR:NBS REVE.L

6 .5

69

" I . 70 ' ' 75 .aa ,:,:-::-. es·.

Figure 4. Graphic results of the mass-spectral library search of peak 7 from the 700°C pyrolysis of nylon.

Table 3. MS Correlation for Peak No. 7

1: 1-Hexene (8CI9CI) 2: Hexane, 2-chloro- (8CI9CI) 3: Cyclopropane, propyl- (9CI) 4: Hexane, 1-fluoro- (8CI9CI) 5: 1-Pentene, 2-methyl- (8CI9CI) 6: 1-Hexene, 1-chloro-, (E)- (9CI) 7: Cyclopropane, butyl- (9CI) 8: 1-Hexanol (9CI) 9: Cyclopentane, methyl- (8CI9CI)

10: 2(3H)-Furanone, dihydro-3-methyl- (8CI9CI)

CAS#

000592-41-6 000638-28-8 002415-72-7 000373-14-8 000763-29-1 050586-19-1 000930-5 7-4 000111-27-3 000096-37-7 001679-47-6

Library Index#

529 2951

540 1614 538

2792 1135 1476 524

1254

ASP 5.904 - 5.925 mln MKIRD3: NYL_A3 . D

:J . --:-:-:-: 2. 0 a: E

:J a: E

Match Quality

9699 9694 9495 9169 9155 8876 8862 8840 8831 8827

Figure 5. Comparison of the infrared spectrum of peak 7 from the 700°C pyrolysis and the library spectrum of 1-hexene.

The main pyrolysis fragment, peak 9, elutes at around 10 min. The library search of its mass spectrum, shown in figure 6, suggests compounds which do not seem reasonable based on the retention time and the molecular structure of Nylon 6/6. The infrared spectrum of peak 9, on the other hand, matches well with the spectrum for cyclopentanone (figure 7), which has previously been identified as a Nylon 6/6 pyrolysis fragment (1).

The mass spectrum of peak 10, at around 11.1 min, is also incon­clusive with library searching (figure 8). The spectrum indicates a molecular weight of 96. The IR library search matched the spec­trum of peak 10 with that of cyclopentenone (figure 9), which has a molecular weight of 82. The analyst might combine the mass spectral and infrared information to conclude that there is a high probability that peak 10 is cyclo­hexenone. In this case, the complementary information provided by the two detectors greatly simplified the task of preliminary peak identification.

·:·.:.:-·-:::;:;.::-·

]00001 :';000

0

Figure 6. Graphic results of the mass-spectral library search of the peak 9 from the 700°C pyrolysis of Nylon 6/6.

-·-:-:.:-;_:::;::· ::::::;:- _._::::;:;:: RSP =l"li!.2 4,9- 10.320 MODIFIED

-:.·-:::-:·._.:.::·:-: ... :::=:<150

0~=r==~~~~~~~==~~~~~~~~~~~ <1 0 00

' · f:1sf:> ··=..-e=tT; ·-:-:-:-:.;- :-:--:-:

-~::::=::: r::::?: \rt~-- ·-:

~ ., "' · a+-=---~~~~~~~~~~~~~~~~~~~99~

<1000

Figure 7. Comparison of the infrared spectrum of peak 9 from the 700°C pyrolysis a n d the library spectrum of cyclopen t anone.

CS Cil T r o m

126

"' I

Figure 8. Graphic results of the mass-spectral library sea rch of the peak 10 from the 700°C pyrolysis of Nylon 6/6.

5

The same sort of approach could be used for peak 11 at 11.5 min. The mass spectrum of peak 11 indicates a molecular weight of 101 (figure 10) and the library search was inconclusive. The infrared spectrum of peak 11 matches well with octylamine (figure 11), indicating a primary amine with a saturated hydrocar­bon chain. The combination of this information with that of the mass spectrum would make hexylamine a reasonable choice for the identity of peak 11.

6

Figure 9. Comparison of the infrared spectrum of peak 10 from the 700°C pyrolysis and the library spectrum of 2-cyclopentenone.

Figure 10. Graphic results of the mass-spectral library search of peak 11 min from the 700°C pyrolysis of Nylon 6/6.

Figure 11. Comparison of the infrared spectrum of peak 11 from the 700°C pyrolysis and the library spectrum of octylamine.

Figure 12 is a comparison of the selected-wavelength chromat­ograms for the 700°C pyrolysis. The lower chromatogram (1735-1745 cm-1) indicates compounds with ketone carbonyls such as peak 9, cyclopentanone. The top chromatogram (1705-1720 cm-1) shows peaks relating to com­pounds with amide carbonyl groups. Figure 13B is an ex­panded TIC around the amide­containing peaks at 19 min. The mass spectrum of the peak at 18.8 min (figure 13A) indicates a molecular weight of 113 but gave no reasonable library match. The infrared spectrum of the same peak (figure 14) matched well with that of hexahydro-2H-azepin-2-one (caprolactam), the structure of which is shown below.

The selected-wavelength chrom­atogram, therefore, helped to flag peaks with functional groups of interest for confirmation of structure, or so that they could be further interpreted. This is yet another attractive attribute of selective detection which is extremely useful in interpreting complex pyrograms.

1705-l l ?@ ¢~;c·i, AMIIli'; •• CARilONYL 12

A

20 25

CARBONYL

B

Figure 12. Comparison of the selected-wavelength chromatograms of the 700°C pyrogram, indicating compounds with amide carbonyls (A) and keto carbonyls (8).

( 1361 -

/'1 ~3

8

Figure 13. Expanded TIC for the 900°C pyrogram, showing the multitude of minor components in the 17-21 min time frame (8). Mass spec­trum for the peak 12 (A) indicating a molecular weight of 113.

Figure 14. Comparison of infrared chromatograms for peak 12 (A) and caprolactam (8).

7

Conclusion The combination of the HP 5890 SERIES II GC with parallel detection by an MSD and IRD has been shown to be very effective for Nylon 6/6 pyrolysis studies. Cryogenic cooling and the use of an on-column inlet 'Nith program­mable pressure and temperature control ensured high-resolution separation of all except the most volatile pyrolysates. The HP 5890 SERIES II GC has many useful and flexible features which sim­plify complex chromatography experiments such as pyrolysis GC/MS/IR. The complimentary information from MSD and IRD greatly simplified the interpreta­tion of pyrograms, and in identifi­cation of pyrolysates.

References 1. Hajime Ohtani, Tamio Nagaya, Yoshiihiro Sugimura, Shin Tsuge, "Studies on Thermal Degradation of Aliphatic Polyamides by Pyrolysis­Glass Capillary Gas Chromatogra­phy," Journal of Analytical and Applied Pyrolysis , 4 (1982) 117-131.

rf/o- HEWLETT .:~PACKARD

For more information, call your local Hewlett-Packard Sales Office or nearest Regional Office:

United States:

Eastern Region (301) 670-4300

Midwestern Region (312) 255-9800

Southern Region (404) 955-1500

Western Region (818) 505-5600

Canada (416) 678-9430

Austria (02 22) 25 00 0

Belgium (02) 761 3111

Denmark (02) 8166 40

Finland (0358) 0887 21

France (1) 6077 5391

Germany (061 72) 400 0

Italy (02) 92 36 91

Netherlands (20) 471825

Norway (2) 24 60 90

Spain (91) 637 00 11

Sweden (8) 750 20 00

Switzerland (057) 31 21 11

United Kingdom (0734) 784774 Middle East and Africa Operations (022) 780 81 11

Far East 852-5-848-7777 Japan 81-422-55-95 71

Australia 61-2-888-4444

Latin America 55-11-421-1311

Mexico 52-5-596-8077

Venezuela 58-2-239-4133

Elsewhere in the World (415) 857-5027

Copyright © 1989

Hewlett-Packard Company

Printed in USA 8 /89 43-5952-0294