1070-4272/03/7602-0307 $25.00 C 2003 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 76, No. 2, 2003, pp. 307!309. Translated from Zhurnal Prikladnoi Khimii, Vol. 76, No. 2, 2003,
Original Russian Text Copyright + 2003 by Gankov.
AND POLYMERIC MATERIALS
Thermal Oxidative Degradation of Caprolactam
in Melting and Storage in Molten State
in the Presence of Some Trace Impurities
N. P. Gankov
Ekomedbio EAD Business Innovation Center, Sofia, Bulgaria
Received January 22, 2002
Abstract-The influence of trace amounts of cyclohexanol, cyclohexanone, and cyclohexanone oxime on
the quality and stability of caprolactam melt was considered.
Caprolactam is the main raw material for produc-
tion of polycaproamide fibers. It is well known that
caprolactam is stable to heat treatment in an inert
medium; however, even trace amounts of impurities
make it unstable . The presence of concomitant im-
purities, such as cyclohexanol, cyclohexanone, and
cyclohexanone oxime, is inevitable and is caused by
the process conditions.
Under conditions of caprolactam melting, these
impurities are oxidized under the action of minor
amounts of oxygen present in the inert gas or adsorbed
on crystalline caprolactam, which can affect its char-
The influence of impurities on the particular char-
acteristics of caprolactam was considered previously
; however, more specific information on the
maximum permissible concentrations of each impurity
was not given.
Previously , we found that trace amounts of cy-
clohexanol, cyclohexanone, and cyclohexanone oxime
inhibit polymerization and cause formation of a poly-
mer with cross-linked structure and high polydisper-
sity. Up to now, these processes have not been studied
We found no published data on thermal oxidation
processes in melting of impure caprolactam and its
storage in the liquid state.
To study the effect of trace impurities, cyclohex-
anol, cyclohexanone, and cyclohexanone oxime in
the amount of 10
wt % (the amounts of these
impurities in commercial caprolactam are usually
smaller) were added to crystalline caprolactam con-
taining 99.9553% main substance. Caprolactam sam-
ples with impurities were preliminarily homogenized.
The samples were melted at 85oC in hermetically
sealed glass ampules. Before sealing, the ampules
filled with caprolactam and additives were blown with
nitrogen for a long time. The main characteristics
of caprolactam were determined by the procedures
described previously .
The dependences of the main quality characteristics
of caprolactam and the degree of oxidation on the type
and amount of impurities (wt % relative to caprolac-
tam) are shown in Fig. 1.
Determination of permanganate extinction number
(PEN) involves determination of the optical density
of a caprolactam solution in sulfuric acid 10 min after
addition of a definite amount of a KMnO
Figure 1a shows that the presence of cyclohexanone
oxime in caprolactam affects PEN insignificantly.
Cyclohexanol and cyclohexanone appeared to be more
active in this case, with their effect strongly depend-
ing on their concentrations. The PEN value noticeably
decreases at the content of cyclohexanol and cyclo-
hexanone exceeding 10
wt % and becomes lower
than that prescribed by the standard. Previously 
it has been established that aniline has the strongest
effect on PEN.
The optical density A
of a 50% aqueous solution
of caprolactam was determined on a spectrophotome-
ter at a wavelength l = 390 nm. The color expressed
in Hasen degrees X
was determined from the optical
density of 50% aqueous solution of caprolactam at
wavelength l = 390 nm.