High Pressure Steam Hydrolysis for Chemical
Recycling of Poly( L -lactic acid)（ 九 工 大 院 ・ 生 命 体 ）（ 学 ） ○ Ahamad-Faris Mohd-Adnan（ 九 工 大 ・ ETC）（ 正 ） 西 田 治 男 ＊ 、（ 九 工 大 院 ・ 生 命 体 ） 白 井 義 人
AbstractPoly( L -lactic acid) (PLLA) was treated with high pressure steam as a pre-treatment forchemical recycling. PLLA degraded smoothly with decrease in molecular weight. Using
appropriate analytical models and mechanistic assumptions, the mode of hydrolysis was verified
to proceed via an auto-catalytic random degradation. Exact rate constant (k h ) and activationenergy (E a ) values were estimated to be 8.4×10 -5 (100°C) to 7.2×10 -4 s -1 (130°C) and 87.2 kJ
mol -1 , respectively.
Introduction
Recently, biomass plastics have gained popularity as an alternative to conventional plastics.
PLLA, which is produced chemically via the lactic acid fermentation from renewable resources 1 ,have been promoted as a ‘green’ alternative in the packaging industries. PLLA was originallyconceived as a biodegradable plastic, degrading in the natural environment to CO 2 and H 2 O. In
many efforts to utilize natural resources efficiently, several recycling processes have been
established to recover L -lactic acid and L,L-lactide by hydrolysis 2 and pyrolysis 3 , respectively. In
this study, an effective method to hydrolyze PLLA by using high pressure steam is proposed as a
pre-treatment process of the chemical recycling.
Materials and Methods
PLLA pellets (LACEA H-100J; Sn content: 40ppm; M n 23700, M w 56300; dimension:
w3.92×d3.48×h4.52mm) were obtained from Mitsui Chemicals, Inc. and used as received.
Hydrolysis of PLLA pellets under high pressure steam was done in an autoclave (Tomyautoclave model SS-325 unobstructed capacity 55L) for prescribed periods at 100°C (0.102MPa),
110°C (0.145MPa), 120°C (0.202MPa) and 130°C(0.276 MPa). Internal temperature of the autoclave
wasthermostatedtowithin± 0.5°C.

After

autoclaving, the PLLA samples were dried in vacuo
at room temperature for one night and refrigerated
at -30°C until following analysis.
Molecularweight(MW)ofPLLA

wasmeasured on a size exclusion chromatograph (SEC)
at 40°C using chloroform as an eluent. SEC traces
were analyzed by universal calibration method

Figure 1. Changes in SEC profile of
PLLA sample during the hydrolysis byhigh pressure steam at 110°C.

(UCM) 4 using Mark-Houwink-Sakurada parameters for PLLA and polystyrene.
Results and Discussion
The pre-treatment of PLLA pellets under thehighpressuresteam

resultedinthesmoothhydrolysis with decrease in MW. SEC profiles
gradually
multi-modal

shiftedfroma

uni-modaltoashapewith

anappearanceofashoulder at a certain point (Figure 1). Critical point
has been defined as a specific point, at which therandom degradation changes from a homogeneous
reaction to a heterogeneous one, exhibiting thechanges in the shape in SEC profile as shown inFigure 2. Plots of lnM n and lnM w of
PLLA sample hydrolyzed at 110°C andtheoretical approximations.

Figure 1 and the departure of actual plots from thetheoretical approximation lines as shown in Figure 2.
To determine exact rate constants (k h ) and activation energy (E a ) values for the high pressure
steam hydrolysis of PLLA, relative MW values based on polystyrene standards were re-calculated
into exact values MW UCM by using UCM. To avoid the uncertain evaluation by M n value alone,both values of M n and M w were used for the evaluation. Moreover, to determine the mode ofhydrolysis, the MW UCM data were applied into kinetic equations for several models.
As results, it was found that the mode of hydrolysis of PLLA under the high pressure steam
was a typical autocatalytic random degradation. According to the appropriate kinetic model
formulated by Nishida et al 5 , exact k h and E a values were estimated as being 8.4×10 -5 (100°C) to
7.2×10 -4 s -1 (130°C) and 87.2 kJ mol -1 , respectively.
Conclusion
In order to confirm the precise kinetics parameters: k and E a values, PLLA hydrolysis was
carried out under high-pressure steam and analyzed the MW changes by UCM without any
influence of weight loss. The changes in molecular weight were successfully fitted with thesimulation lines generated according to the auto-catalytic hydrolysis mechanism, resulting in theevaluation of exact k and E a values.
References1)

J. Lunt, Poly .Degrad. Stab., 59, 145-52 (1998).

2)

H. Tsuji et al., Biomacromolecules, 4, 835-840 (2003).

3)

H. Nishida et al., Ind. Eng. Chem. Res., 44, 1433-1437 (2005).

4)

Z. Grubisic et al., Polym. Lett., 5, 753-759 (1967).

5)

H. Nishida et al., Macromolecules, 33, 6595-6601 (2000).

【連絡先】〒808‑0196 福岡県北九州市若松区ひびきの 2‑4 九州工業大学エコタウン実証研究センター
西田治男 Tel：093‑695‑6233 FAX：093‑695‑6060 E‑mail：nishida@lsse.kyutech.ac.jp