PARTⅣ BIOMASS DEGRADATIONOLIGOMERIZATION OF POLY(L-LACTIC ACID) BY
HIGH-PRESSURE STEAM TREATMENT FOR FEEDSTOCKRECYCLING
Yan Wang, Takayuki Tsukegi, Yoshihito Shirai, Haruo Nishida*1

Eco-Town Collaborative R&D Center for the Environment and Recycling, Kyushu
Institute of Technology, Hibikino 2-4, Wakamatsu-ku, Kitakyushu,Fukuoka 808-0196, Japan.
E-mail: nishida@lsse.kyutech.ac.jp

Abstract: High-pressure steam hydrolysis of a poly(L-lactic acid) (PLLA) / poly(bisphenol Acarbonate) (PC) (50:50 wt/wt) immiscible blend was investigated as a pre-treatment process
before feedstock recycling of the PLLA component, resulting in the selective oligomerizationof the PLLA component. After the pre-treatment by steam, the PLLA component waspreferentially hydrolyzed in a manner of autocatalytic random degradation, whereas the PC
component showed no change in molecular weight. This characteristic hydrolysis behavior of
the blend will facilitate the crushing of moldings and subsequent feedstock recycling ofPLLA.
1. Introduction
In recent years, great interest has been shown in polymers derived from renewable
resources. Poly(L-lactic acid) (PLLA), in particular, because of its high melting point, goodtransparency and superior biocompatibility has been considered as the most interesting of thebiomass-based materials. However, its brittleness and low glass transition temperature have
limited its practical application. Therefore, various approaches including copolymerization
and blending with other polymers, such as polyethylene[1], have been employed to try toimprove these limitations in PLLA’s mechanical and thermal properties. Especially, a blend
with poly(bisphenol A carbonate) (PC)[2], which has good transparency and high impact
resistance, was able to correct the defects of PLLA and applied to a notebook computer in2005.
Feedstock recycling is an attractive strategy for plastic waste management. Monomer
recovery, especially, is an ideal approach, because it enables plastics to completely regenerate.
The monomer recovery from PLLA to L,L-lactide[3] and L-lactic acid[4]has been developedby pyrolysis and hydrolysis approaches, respectively, giving a reasonable yield. Whenconsidering the total process of PLLA feedstock recycling, the oligomerization of PLLA
moldings is an interesting pre-treatment process. This is because, before monomer recovery,- 205 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
the oligomerization facilitates the crushing of molded products into flakes [5], which are in aconvenient form for transporting and separating from other kinds of plastics.
Recently, it has been reported that PLLA oligomerization can be effectively achieved
using high pressure steam up to the critical point [6], at which point the homogeneous random
degradation process shifts into a heterogeneous decomposition process inducing easy crushing.
In the present work, to confirm the pre-treatment effect of using high pressure steam, thehydrolysis behavior of PLLA/PC blend was investigated to achieve the selective
oligomerization of the PLLA component.
2. Experimental
2.1 Materials and Preparation of PLLA/PC melt blend
PLLA (LACEA H-100J; Mn 76,000 and Mw 142,000) and PC (Iupilon S-3000; Mn
18,500 and Mw 36,100) were obtained from Mitsui Chemicals, Inc. and Mitsubishi
Engineering-Plastics Corp., respectively, and used as received. PLLA/PC (50/50 wt/wt) blend
film was prepared by a convenient solvent casting method from a chloroform solution (10w/v-%) and subsequent compression molding at 200 °C for 5 min under a pressure of 8.0MPa
to produce a blend film (10×10 mm2) with a thickness of 100-120 µm.
2.2 Hydrolysis of PLLA/PC blend film under high-pressure steam
Hydrolysis of blend films under high-pressure steam was carried out in an autoclave(Tomy autoclave model SS-325). The films were treated with steam at 100 (0.102), 110(0.145), 120 (0.202), and 130 °C (0.276 MPa) for prescribed periods. The internal
temperature of the autoclave was thermostated to within ± 0.5 °C. Temperatures of both thehydrolysis start and stop were controlled at 70 °C.
2.3 Measurements
Differential scanning calorimetry (DSC) thermograms were measured on a Seiko
Instruments Inc. EXSTAR 6200 DSC system at a constant heating rate of 10 °C·min-1.
Scanning electron microscopic (SEM) images were observed on a Hitachi S-3000N
WET-SEM at an accelerating voltage of 5kV. Before observation, the sample films werefractured in liquid N2, and spattered with Pt. Size exclusion chromatograms (SEC) were
measured on a TOSOH HLC-8120 GPC system equipped with refractive index (RI) and ultra
violet (UV) dual detectors at 40 °C using a TOSOH TSKGel Super HM-M column and achloroform eluent (0.6 mL·min-1).
3. Results and Discussion
3.1 Morphology of PLLA/PC blend
Thermal properties of PLLA/PC (50/50 wt/wt) blend were determined using DSC andcompared with PC and PLLA homopolymers. Transition temperatures of the blend were- 206 -PARTⅣ BIOMASS DEGRADATION
observed at 59.4, 105.0, 169.3, and 231.8 °C as Tg,PLLA, Tc,PLLA, Tm,PLLA, Tm,PC, respectively.
These transition temperatures almost agreed with the transition temperatures of the PLLA andPC homopolymers. The Tg,PC around 150 °C was not determined due to overlapping withTm,PLLA. From the results, it is clear that PLLA and PC are immiscible and thus both phases
exist independently in the blend film.
Morphological structure of the blend was a key factor influencing its hydrolytic behavior.
Therefore, the morphology of the blend film was observed with SEM before and after thehigh pressure steam treatment at 130 °C for 2 h. Fig. 1 shows the fractured surface images ofblend film. In good agreement with the DSC result, the PC/PLLA blend was segregated in
two phases with particle domains of 5-20 µm in diameter being dispersed in a matrix phase.
The rough surface morphology of the original particles was changed to a smooth surface after
the hydrolysis. Taking into account the melting at 200 °C, PLLA would form a continuous
matrix with PC being dispersed as particle domains in the matrix.

Fig.1 SEM images of fractured surfaces of the PLLA/PC blend film
after high pressure steam treatment at 130 °C for 90 min. bar: 20 µm.
3.2 Hydrolytic behavior of the PLLA/PC blend
For evaluating the changes in molecular weight during the hydrolysis, SEC profiles weremeasured with the RI and UV dual detectors. The peak separation was carried out based on
the contribution of each component. Each fraction of the SEC analysis was separated into two
compositional sub-fractions then individual SEC profiles of the components were constructedby integration of the compositional sub-fractions.
To calculate the molecular weigh, all the response coefficients of components were taken
into account for the composition calculation of each fraction. Expressions employed for
calculating the weight sub-fractions of PLLA and PC components at elution point “i”: wi,PLLA
and wi,PC, are as follows:
wi×PLLA =Ai×UV ×mix - RPC × Ai×RI ×mix
( AUV ×PLLA - AUV ×PC ) - RPC × ( ARI ×PLLA - ARI ×PC )Ai×UV ×mix - RPLLA × Ai×RI ×mix
wi×PC =RPLLA × ( ARI ×PLLA - ARI ×PC ) - ( AUV ×PLLA - AUV ×PC )(1)- 207 -The 5th ISFR (October 11-14, 2009, Chengdu, China)
where Ai,UVmix and Ai,RImix are the UV and RI responses of a fraction of the blend atelution point “i”, AUV,PLLA, ARI,PLLA, AUV,PC, and ARI,PC are the response coefficients for PLLAand PC with the UV and RI detectors, and RPLLA and RPC are response ratios: AUV,PLLA/ARI,PLLA
and AUV,PC/ARI,PC, respectively. All the coefficient values were determined by using the same
polymer concentration (6 mg·mL-1).

Fig. 2 Changes in SEC profiles of PLLA and PC components
during high pressure steam hydrolysis at 130 °C.
Fig. 2 illustrates changes in the SEC profiles of PLLA and PC components, which weretreated by steam at 130 °C. The SEC profile of the PLLA component gradually shifted into a
low molecular weight range with time (Fig. 2a), whereas no shift in the profile of the PC
component was observed (Fig. 2b). Similar results were obtained with time of steam
treatments at 100, 110, and 120 °C. These results indicate that a selective hydrolysis of thePLLA component in the blend proceeded without any deterioration in the PC component.
To evaluate parameters: rate constant (kh) and activation energy (Ea) values for thehigh-pressure steam hydrolysis of the PLLA component, the Mw value was employed.
Previously, Mohd-Adnan et al.[6] reported that estimation using the Mw value was morereliable than when using the Mn value in the analysis of the hydrolysis process. In Fig.3,relationships between lnMw and hydrolysis time are plotted. A linear relationship wasobserved for each plot.[6] This linear relationship means that the homogeneous autocatalytic
random degradation occurred during the treatment. The kh values were calculated from slopesof the linear plots. The Ea value was estimated as being 82.4 kJ·mol-1 using the Arrhenius plot
from these kh values. The steam hydrolysis of the PLLA homopolymer film was also
examined as a reference and its Ea value was calculated as being 82.8 kJ·mol-1 from its khvalues.
Nearly equal hydrolysis parameters: kh and Ea values were shown for the PLLA
component in the PLLA/PC blend and the PLLA homopolymer film. The PC domain had noeffect on the hydrolysis of the PLLA matrix. Thus, the steam can easily diffuse in the matrixof the blend and hydrolyze the PLLA chains preferentially without any effect on the PC
particle domain.
- 208 -PARTⅣ BIOMASS DEGRADATION

Fig. 3 Plots of ln Mw vs. hydrolysis time for PLLA component
under high pressure steam in a temperature range of 100-130 °C.
4. Conclusion
The high-pressure steam treatment of PLLA/PC (50:50 wt/wt) immiscible blend induced
the preferential hydrolysis of PLLA component as a matrix phase. The PLLA component washydrolyzed in a manner of the autocatalytic random degradation into oligomers, whereas thePC component showed no change in molecular weight during the treatment. Obtainedhydrolysis parameters: kh and Ea values for the PLLA component were nearly the same as
those of PLLA homopolymer film, suggesting no effect from the PC component.
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[2] Y. Hayata, Finechemical 2007, 36, 47-52.
[3] Y. Fan, H. Nishida, T. Mori, Y. Shirai, T. Endo, Polymer 2004, 45, 1197-1205.
[4] T. Saeki, T. Tsukegi, H. Tsuji, H. Daimon, K. Fujie, Kobunshi Ronbunshu 2004, 61, 561.
[5] N. A. Weir, F. J. Buchanan, J. F. Orr, D. F. Farrar, G. R. Dickson, Proc. Instn. Mech.
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