JOURNAL OF POLYMER SCIENCE VOL. XIII, (1954)

LETTERS TO THE EDITORS

The Thermal Degradation of 66 Nylon

The thermal decomposition of 66 nylon (polyhexamethylene adipamide) and its copolymers has been the subject of several recent publication^.'-^ This topic has been under investigation in this laboratory for some time, and it is desired to describe briefly certain results which extend consider- ably and correlate the &dings of previous workers.

Carbon dioxide has been shown to be one of the principal volatile prod- ucts of nylon decomp~sition,’-~ but measurements of the quantity pro- ducedls2 have revealed Wiculties in the interpretation of its origin. Ach- hammer, Reinhart, and Klinel considered the following reaction occurring at adipic acid ends, and showed that it was insufficient to explain the quantities of carbon dioxide found:

-NHCO(CH&COOH ------+ H + COZ + cyclopentanone

They concluded that a large part of the carbon dioxide arises either from sorption on the original material, or from parts of the chains other than the ends. The latter reaction was suggested to result from preliminary fission of CO-NH bonds, giving CO(CH2),C0 units which might decompose to give: (a) cyclopentanone and carbon monoxide, or (b) by an unspecified reaction, hydrocarbons and carbon dioxide. The figures of Achhammer, Reinhart, and Kline reveal a CO : cyclopentanone ratio of 1 : 4 and a CO, : hy- drocarbon molar ratio of 10-15: 1, which do not constitute adequate evi- dence for schemes (a) and (b).

Hasselstrom and co-workers2 similarly recognized a dificulty in explain- ing the origin of carbon dioxide, and suggested that it arises together with ammonia, by decomposition at salt linkages which were assumed to occur a t points along the polyamide chains. However, 110 evidence was provided in support of this hypothesis.

In the present work. expeiiments have been made with thermally de- graded 66 nylon polymer, and with model amides. When heated in a nitrogen atmosphere at 350 O for up to 5 hours, monosubstituted acid amides RCONHR’ (R and R’ both alkyl) and diamides derived from hexamethyl- enediamine, RCONH(CH2)6NHCOR (R = n-CJl11 or C a b ) , decomposed only very slightly, and recoveries of 95% or upward of original material could be made. On the other hand, in the same conditions a series of N,N’-di-n-butylamides, BuNHCO(CH2),CONHBu (z = 3, 4, 5, S), all decomposed in varying but substantial amounts, reaching virtual com- pleteness in the case of N,N’-dibutyl adipamide (x = 4). Whereas the stable amides produced only traces of nitriles, the principal volatile product from the dibutyl amides was n-butylamine, accompanied by minor amounts of hydrocarbons and carbon monoxide, and in the case of dibutyl adipamide alone by substantial quantities of carbon dioxide. The composition of the

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176 LETTERS TO THE EDITORS

volatile products from dibutyl adipamide was as shown in Table I. Neither cyclopentanone nor its condensation products was produced in the pyroly- sis of dibutyl adipamide; likewise, cyclohexanone was not detected in the pyrolysis of dibutyl pimelamide (z = 5). This has an important bearing upon the hypotheses of Achhammer et al., for, if their scheme (a) were correct, cyclic ketones should hale been formed in both cases, while in respect to scheme (b) there is again the difficulty of the high COz:hydro- carbon ratio, and the more formidable objection that scheme (b) neither explains nor predicts the formation of carbon dioxide from the substituted adipamide alone.

TABLE I ANALYSIS OF VOLATILE PRODUCTS FROM N,N’-DIBUTYL ADIPAMIDE

SUbStancs Yield, mole/rnole amide pydyzed

n-Butylamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.961 coz.. . . . . . . . 0.466 co.. . . . . . . . . 0.008 C4Hio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C4Hs . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.003 NH3 . . . . . . . . . . 0.000005

The decomposition of di-n-hexyl adipamide occurs similarly a t 350 O to give n-hexylamine and carbon dioxide as principal volatile products, and these experiments establish a unique reaction of N-substituted adipamides, in which carbon dioxide is formed on heating in anhydrous conditions without the production of equivalent quantities of hydrocarbons, and in the absence of carboxylic acid groups. The occurrence of this reaction in 66 nylon would explain the disproportionate quantities of carbon dioxide found by previous workers, while avoiding the necessity of the postulate of Hasselstrom et al. and the difficulty of linking its formation with that of cyclopentanone or hydrocarbons according to the proposals of Achhammer et al.

In order to elucidate the nature of this reaction, the pyrolysis residues of dialkyl adipamides are being studied. Dibutyl adipamide yields a series of distillable substances with molecular weights in the range 200-300, and a residue involatile in high vacuum. A t least two of the distillable sub- stances contain a single nonbasic nitrogen atom per molecule; this is present neither in an amide or a nitrile group, nor in a hydrolyzable form. A t least one of the compounds is also oxygen free. To accommodate these findings the nitrogen atom was postulated to occur in a 5-membered hetero- cyclic ring, and this view was conGrmed by the strong Ehrlich reaction (reddish purple color with p-dimethylaminobenzaldehyde hydrochloride) given by all the distillate fractions, similar to the color produced by numer- ous substances containing a pyrrole nucleus.

Ultraviolet absorption measurements have confirmed that a common nucleus is present in the various compounds, and the occurrence of an intense absorption band in the region 270-280 mp (the position of the maximum differs somewhat in the various compounds) indicates that the

LETTERS TO THE EDITORS 177

absorbing structure cannot be a single pyrrole ring. The absorption char- acteristics of certain alkylated indoles recorded in the literature6-* and during the course of the present work, taken together with the elementary analyses of the compounds, are consistent with the view that the pyrolysis residues from dialkyl adipamides are composed largely of substituted indoles. The formation of fully aromatized structures from aliphatic amides is of considerable interest, and the hydrogen eliminated during the cyclization is inferred to become attached, a t least in part, to a nitrogen atom as volatile primary amine.

Since hexamethylenediamine and ammonia are also products of the pyrolysis of 66 nylon p ~ l y m e r , ~ , ~ a similar abstraction of hydrogen may occur, and in confirmation of this it has been observed by Mr. 0. B. Edgar of these laboratories that nylon polymer degraded a t 290" and subse- quently treated with the Ehrlich reagent acquires a purple-red color. increasing in intensity with time of heating (the undegraded polymer gives no reaction). Heating polymer at 290" also results in the appearance of an ultraviolet absorption band a t 280 mp, similar to that of the degradation products of dibutyl adipamide. Furthermore, continued thermal degrada- tion of nylon polymer leads to an increase in the ratio of C/H and a de- crease in N/C, consistent with the formation of aromatic heterocyclic nuclei.

The changes in ultraviolet absorption are reproduced in Figures 1-4. Measurements on nylon polymer were made on melt-pressed films prepared from normal polymer and from polymer degraded for 9 hours at 290" (Figs. 1 and 3) ; the spectra of dibutyl adipamide and one of its degradation products (Figs. 2 and 4) were measured in methanol solution. All the spectra were recorded on a Cary photoelectric spectrophotometer.

0

WAVELENGTH (ME)-- WAVELENGTH (h4kJ--

Fig. 1. Undegraded 66 nylon polyrnel Fig. 2. N,N'-Dibutyl adipamide (film thickness 0.002 cm.; measured (concn. 21.2 g./l.; measured against against air). methanol, 1 cm. cells).

178 LETTERS TO THE EDITORS

WAVELENGTH (My) + WAVELENGTH (M d - Fig. 3. 66 Nylon polymer degraded for Fig. 4. Dibutyl adipamide decom-

9 hours at 290" (film thickness 0.0008 position fraction of b.p. 115-130" at em.; measured against air). 0.5 mm. (concn. 0.196 g./l.; measured

against methanol, 1 cm. cells).

Liquori, Mele, and Carelli4 have recently observed that solutions of 66 nylon in 60% sulfuric acid show an absorption band at 290 mp, increasing in intensity with time of previous heating of the polymer at 275'. These results, which they were unable to interpret, are explicable in terms of the present postulate that the degradation of nylon polymer leads to aromatized nuclei related to indole, by a reaction which simultaneously liberates carbon dioxide and ammonia or amines.

Work is continuing here on the constitution of the pyrolysis products of nylon and model amides, with a view to elucidating more precisely the nature of the thermal degradation reaction and the irreversible crosslink- ing which occurs on continued heating.

References

(1) B. G. Achhammer, F. W. Reinhart, and G. M. mine, J. Applied Chem., 1, 301

(2) T. Hasselstrom, H. W. Coles, C. E. Balmer, M. Hannigan, M. M. Keeler, and

(3) H. Hopff, Kudsfoffe, 42,423 (1952). (4) A. M. Liquori, A. Mele, and V. Carelli, J. Polymer Sci., 10, 510 (1953). (5) R. Robinson and J. Saxton, J. Chem. Soc., 1950,3136. (6) W. A. Jacobs, L. C. Craig, and A. Rothen, Science, 83,166 (1936). (7) B. G. Edwards, Arch. Biochem., 21, 103 (1949). (8) M-M. Janot and R. Goutarel, Bull. soc. chim., 18,588 (1951). (9) G. B. Taylor, J. Am. Chem. Soc., 69, 635 (1947).

(19512.

R. J. Brown, Teztik Reseurch J., 22, 742 (1952).

ISAAC GOODMAN Research Department, Imperial Chemical Industries Limited Dyestuffs Division, Hexagon House Blackley, Manchester, 9, England

Received August 4,1953