5-7 OCTOBERISFR2011

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TOLEDO (SPAIN)AN UPGRADE CHEMICAL RECYCLING OF POLYAMIDES
Akio Kamimura1, Kouji Kaiso,1,2 Shuzo Suzuki,1 Yusuke Oishi,1 Yuki Ohara,1,2 Tsunemi Sugimoto,2Kohichi Kashiwagi,2 and Makoto Yoshimoto1
1Department Applied Molecular Bioscience, Graduate School of Medicine, Yamaguchi University, Ube 755-8611, Japan
2 Ube Laboratory, Ube Industries Ltd., Ube 755-8633, Japane-mail: ak10@yamaguchi-u.ac.jp

AbstractPolyamides such as nylon 6 and nylon 12 were effectively converted to Z-hydroxyalkanoic acid by treatment withsupercritical MeOH. The reaction produced corresponding monomeric Z-aminoaminoalkanoic acid derivatives including
lactams as intermediate products, which underwent nucleophilic substitution to give methyl Z-hydroxyalkanoate or
elimination reaction to give methyl Z-hydroxyalkenoate. The reaction was sensitive to acid/base catalyst and solvent.
Kinetic analysis was performed by simulating products distribution and kinetic parameters were estimated.

Keywords: upgrade recycling, supercritical MeOH, depolymerization, polyamides, -hydroxyalkanoic acid.

1. Introduction
Development of effective methods for recycling plastics isdesired as a key technology for sustainable developmentof the world.[1] Chemical recycling is regarded as a key
technology for reserving carbon resources because it
provides rebuilt plastics from waste plastics. However,chemical recycling contains economical problems
because practical chemical recycling process usuallyrequires huge costs. If the process provides morevaluable products than simple rebuilt plastic, this strategy
may overcome such problems.
Polyamides are important polymers to support our daily
lives and tons of products are produced in every year.
We already found that use of supercritical secondary
alcohol effectively converted nylon-6 into monomericcaprolactams in more than 90% yields. [2] Ionic liquids
such as [PP13][TFSA] also achieved an interesting
depolymerization.[3] During the course of our investigation
to develop a new chemical recycling reaction for plastics,
we found that treatment of polyamides with supercritical
MeOH resulted in an effective conversion to Zhydroxyalkanoic acids, which were usually more valuable
chemicals than corresponding monomeric compounds. [4]Thus, the present method will open a new avenue for
upgrade chemical recycling technology. In this paper weshow details of the conversion and some kinetic analysis.

2. Materials and Methods
Procedures for depolymerization of polyamides in
MeOH: Polyamide chips (0.3 g, Mw = 22000) and MeOH
(4 g) were added to a 10 mL reaction vessel under argon
atmosphere. After sealing, the reaction vessel wasplaced in a hot oven (300 to 370 °C) for an appropriate
time. The reaction vessel was immediately cooled by dryice/MeOH bath, and an internal standard was added. Theinteriores libro.indd 113

products were analyzed by GC (Shimadzu GC-2014,
Intercap 5 (0.25 mm id x 30 m) column) and quantified by
the curve fitting method.

3. Results and Discussion
Nylon 6 and MeOH were heated to 330 ºC in a sealed
tube for several hours. All of solid nylon chip disappeared
within 1 h and most of polyamide linkages were cleaved
to give monomeric compounds. The products wereidentified and quantified by GC analyses using the curve
fitting method. GC analyses revealed that the following
six compounds were formed as major products:
caprolactam 1, N-methylcaprolactam 2, methyl 6-(N,Ndimethyl)amino capronate 3, methyl 6-hydroxycapronate
4, methyl 5-hexenoate 5, and methyl 6methoxycapronate 6 (Scheme 1).

Scheme 1. Depolymerization of nylon 6 in supercritical
MeOH at 330 ºC.
Caprolactam 1 was formed as an initial product and theyield reached about 30% in 80 min. Then the yield of 1
decreased and the compounds 2 and 3 were produced;
they reached their peak yields at 120 min. The final
reaction products 4 and 5 gradually increased their yields
in proportion to the reaction time and reached themaximum yields at 240 min. The yields of 4 and 5 were

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6th International Symposium on Feedstock Recycling of Polymeric Materials

unchanged thereafter so that these two products werestable under the reaction conditions. It should be notedthat almost no formation of compound 6 was observed.
This clearly suggested that no direct substitution of theamino group in compound 3 by MeOH occurred. The
sums of the yields of compounds 1 to 6 exceeded 80%
so that we supposed that most of products of the reaction
were successfully traced. The reaction time course wasdepicted in Fig. 1.

Scheme 2. Kinetic simulation of the depolymerization ofnylon 6 in supercritical MeOH at 330 ºC.
Table 1. Kinetic parameters of the depolymerization at330 ºC.
First order ratek (10-3 min-1)

Fig. 1. The time course of the reaction of nylon 6 insupercritical MeOH at 330 ºC.
The reaction performed under much higher temperature
conditions progressed faster than the reaction at 330 ºC,
while decrease of the final products 4 and 5 wasobserved. This suggests that compounds 4 and 5
decomposed under the reaction conditions at 350 ºC orhigher. Products ratio between 4 and 5 was usually 1.5:1and the ratio did not change during the reaction. We
supposed that the chemoselectivity between 4 and 5 wasdetermined kinetically. To improve the chemoselectivityof compound 4, we examined various additives for thereaction. To our surprise, the ratio was enhanced to 6.7:1
when excess amounts of glycolic acid were added in thereaction. Finally compound 4 was successfully obtainedin 69% yield in a highly chemoselective manner. The
present methodology was useful for the treatment ofother polyamide. For example, use of nylon 12 and nylon
66 gave methyl Z-hydroxydodecanoate and 1,6hexanediol in good yields, respectively.
To analyze kinetics of the reaction, we performed asimulation of products distribution. Numerical integrations
and optimizations using the Runge-Kutta method andSimplex algorithm were performed to estimate the kinetic
parameters and to simulate the reactions, respectively.
We hypothesized that the reaction took place through thepathway shown in Scheme 2, and that each step
progressed as a first-order reaction. To simplify thesimulation, the sum of the yields of compounds 2 and 3
was treated as a single intermediate. To fit the simulationwith actual data, we introduced an induction period of thereaction. This is reasonable because 10 to 30 min wererequired until heating up the reaction vessel to thedesired temperature. The kinetic parameters obtained by
the simulation were summarized in Table 1.

interiores libro.indd 114

k1
14.7

k2
29.1

KOH
11.9

Kelim
8.2

The simulation was also successfully achieved for thereaction at 300 ºC, 350 ºC, and 370 ºC. Based on thesefirst order rate constants, the activation energies for eachstep were estimated using the Arrhenius plots. The
estimated activation energies were 94.2 kJ/mol for the
first step, nylon-6 to compound 1, 100.1 kJ/mol for thesecond step, N-alkylation from compound 1, 84.8 kJ/molfor the formation of compound 4, and 94.4 kJ/mol for theformation of compound 5.

4. Conclusions
We have successfully developed a new chemical
conversion of polyamide. Treatment of polyamides withsupercritical MeOH as a reaction solvent resulted in the
direct conversion of amino group into hydroxyl group to
give Z-hydroxycarboxylic acids or diols, which are morevaluable compounds than simple monomeric materials of
the polyamides. The hydroxy/alkene selectivity is greatly
improved by the presence of extra amounts of carboxylic
acids. This methodology will open a new concept forupgrade chemical recycle which will be expected as a
key technology to overcome the economical problems in
the plastic chemical recycling.

References[1] Technology for Feedstock Recycling of Plastic
Wastes, Research Association of Feedstock Recycling ofPlastics, Japan, Ed. CMC books, Tokyo, Japan, 2005.
[2] A. Kamimura, Y. Oishi, K. Kaiso, T. Sugimoto, K.
Kashiwagi, ChemSusChem 2008, 1, 82–84.
[3] A. Kamimura, S. Yamamoto, Org. Lett. 2007, 9,2533–2535; S. Yamamoto, A. Kamimura, Chem. Lett.
2009, 39, 1016 – 1017
[4] See homepage of JETRO; http://www.jetro.go.jp

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