# Selective Depolymerization and Effects of Homolysis of Poly(L-lactic acid) in a Blend with Polypropylene

Authors: Haruo Nishida; Yoshiro Arazoe; Takayuki Tsukegi; Wang Yan; Yoshihito Shirai
Journal: International Journal of Polymer Science
 (2009)

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## Abstract

Blends of poly(L-lactic acid) (PLLA) and polypropylene (PP), which are candidates for the practical use of PLLA, were investigated for selective degradation of PLLA, resulting in quantitative conversion of PLLA components into cyclic monomers, lactides, using magnesium oxide (MgO) as a depolymerization catalyst. Obviously, the catalyst MgO selectively accelerated only the PLLA depolymerization in the blends, dominantly generating L,L-lactide as a volatile product and separating the PP component. Expected effects of homolysis in the blend system were also determined as slight changes in activation energy of degradation for both the components and through the suppression of degradation by an antioxidant.


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## Body

## 1. Introduction

The move towards a future more
sustainable society requires renewable resources to be used more effectively.
One material derived from such resources, poly(L-lactic acid) (PLLA) has
been attracting the attention of many researchers because of its biomass-origin [1] and recyclability based on its
biodegradation [2],
hydrolysis [3],
and thermal depolymerization [4] functions. On the basis of its superior thermal degradation and hydrolysis, PLLA
has potential as a feedstock recycling material ideally suited to a future
sustainable society.Although PLLA is synthesized from renewable resources such as corn and sugarcane, the
production of the polymeric form requires much processing, energy, and fuel.
Typically, PLLA is being produced via starch saccharification, lactic acid
fermentation, oligomerization, cyclic monomer formation, and ring-opening
polymerization of the cyclic monomer. If the feedstock recycling of PLLA to the
cyclic monomer, L,L-lactide, can
be achieved more effectively, it will then become possible to renew PLLA by the
shortest, most energy-efficient route. Once this is achieved, PLLA may become widely used as a basic
material in a sustainable society.Although
PLLA has many useful physical properties, there are some shortcomings, such as
brittleness and poor water barrier properties, which should be improved for actual uses. Use of polymer
alloy is a common way to improve the properties of PLLA [5],
but from the viewpoint of feedstock recycling it only brings attendant contamination problems.
Therefore, for the selective feedstock recycling of PLLA
from such polymer alloys and blends,
it is necessary to clarify the influence of such blended polymers on the feedstock recycling of PLLA.Polypropylene
(PP) is a commonly used polyolefin in electric and electronic appliances due to
its mechanical properties and processability by injection molding, thus PP has
been alloyed with many kinds of materials by copolymerization, blending,
lamination, and composite formation to improve its properties so as to suit
particular applications. A blend of PP with PLLA is also being developed to
apply to various household appliances in recognition of the environmental
appeal of the carbon neutral feature of PLLA [6]. Improvements in the mechanical properties of
the PLLA/PP blend have been achieved by the use of effective
compatibilizers suited for the
requirements of particular applications. Since PP has a wide range of applications, the blend of PLLA and PP is a candidate for the practical use of PLLA.Recently, the depolymerization behavior
of PLLA has been investigated in detail. In some reports, depolymerization kinetics [7–9], catalysts [10, 11], and the effects of chemical structures [12] have
been discussed. Depolymerization catalysts for the feedstock recycling of PLLA
have been investigated by our group,
which have paid particular attention to alkali
earth metals, such as magnesium oxide (MgO) [10], which acted as an
effective catalyst for the depolymerization of PLLA in a temperature range of
250–300°C with slight
racemization to meso-lactide. However, selective depolymerization and
thereby separation of PLLA component
from polymer alloys has not been reported. There are few reports
regarding other blends with commodity resins. Tsukegi et al., and Omura et al., already reported the
selective depolymerization of PLLA component in PLLA/LLDPE [13, 14], PLLA/PSt [15], and PLLA/PBS blends [16]. There is no information for the blend with PP in spite of the importance for practical uses of PLLA. However, influences of the PLLA thermal degradation to coexisting PP component are quite uncertain
and must be important for the mechanical recycling of PP. The demand to the
solution of this subject may gradually rise with increase in practical uses.It is well
known that PP decomposes randomly by homolytic chain scission via various
mechanisms such as hydrogen abstraction, back biting, andβ-scission [17–20]. Although the
degradation of PLLA proceeds mainly in nonradical mechanisms [7, 9, 21], previous
studies by McNeill and Leiper [22, 23], and Tsuji et al. [24] have suggested
that the homolysis of PLLA occurs in a closed system. Södergård et al. [25, 26]
have also suggested the stabilization of PLLA with various types of peroxides.
However, the contribution of homolysis to the
feedstock recycling of PLLA in a blend system has not been discussed in
previous studies. If the radical mechanism significantly contributes to the
depolymerization of PLLA, the generated radical species may affect PP
degradation. Conversely, radical species originating from PP may also influence
the degradation behavior of PLLA. In this paper, selective depolymerization of
PLLA components in PLLA/PP blends with MgO was investigated. The effects of
homolysis in PLLA/PP blends were discussed.


## 2. Experimental Section

### 2.1. Materials

PLLA (LACEA
H-100J,Mn 88,000, Mw 170,000, Sn content:
52.7 ppm) and isotactic polypropylene
(PP) (NOVATEC FY7HA, MFR = 1.7) were obtained from Mitsui Chemicals and Japan
Polypropylene Corporation, respectively. Catalyst:
magnesium oxide (MgO
columnar crystalline, size 10×10×70μm) was purchased from Wako
Pure Chemical Industries, Ltd. and used as received. Antioxidant:
2,2-bis(hydroxymethyl)-1,3-propanediol tetra[3-(4-hdroxy-3,5-di-t-butylphenyl)propanoate
(BHPP) was purchased from Ciba Specialty Chemicals and characterized by H1
NMR (purity ~100%).


### 2.2. Preparation of PLLA/PP, PLLA/PP/MgO, and PLLA/BHPP Blends

PLLA and PP were dried at 40°C for 2 hours at ca. 0.1 kPa in
a vacuum oven just before melt processing. PLLA/PP, PLLA/MgO, PP/MgO, PLLA/PP/MgO,
and PLLA/BHPP blends (see Tables 1 and 2) were prepared by melt blending with
an Imoto Machinery Co., Ltd. twin-screw extruder model 160B (diameter 20 mm,
L/D 25) equipped with an air vent under conditions of 180°C temperature and a
screw rotational speed of 25 rpm.Table 1
Preparation of pellet samples with an extruder.
EntryMelt-processed sample (wt ratio)Molecular weight of PLLA component after preparationMnMw1–1PLLA62 000207 0001–2PP——1–3PLLA/PP (20:80)66 000181 0001–4PLLA/MgO  (100:5)37 00085 0001–5PP/MgO (100:5)——1–6PLLA/PP/MgO (20:80:5)8 00015 000Table 2
Preparation of PLLA/BHPP blends with an extruder.
EntryPLLA/BHPP (wt/wt)MnMw2–1100/079 000150 0002–2100/0.280 000152 0002–3100/180 000152 0002–4100/584 000176 0002–5100/2085 000171 000


### 2.3. Characterization

Molecular weights of the samples were measured by size exclusion chromatography (SEC) on a TOSOH HLC-8120 GPC system with a refractive index (RI) detector
at 40°C using a TOSOH TSKgel Super HM-M column and chloroform eluent (0.6 mL min−1). The
calibration curves for GPC analysis were obtained using polystyrene standards
with a low polydispersity (5.0×102, 1.05×103,
2.5×103, 5.87×103, 9.49×103, 1.71×104, 3.72×104, 9.89×104, 1.89×105, 3.97×105, 7.07×105, 1.11×106, TOSOH
Corporation). The sample (12 mg) was dissolved in chloroform (2 mL) and the solution
was filtered through a membrane filter with a 0.45 μm pore size.Proton (H1) NMR spectra
were recorded on a 500-MHz JEOL JNM-ECP500 FT NMR system. Chloroform-d was
used as solvent. Chemical shifts were reported as δ values (ppm) relative to
internal tetramethylsilane (TMS) in CDCl3 unless otherwise noted.Fourier transfer infrared (FTIR) spectra were recorded on a Perkin
Elmer Spectrum GX2000R. Reflection spectra of as-pyrolyzed samples were measured on a Golden Gate Diamond ATR(10500)
module with a germanium crystal, by the single-reflection ATR method.Optical and scanning electron microscopy for PLLA/PP and PLLA/PP/MgO were
performed with a KEYENCE digital microscope VH-5000 and a HITACHI S-3000N WET-SEM at an
accelerating voltage of 15 kV.


### 2.4. Dynamic Pyrolysis in TG and Py-GC/MS

Thermal degradation behavior of PLLA/PP blends was analyzed by a thermogravimeter (TG) and the
pyrolysis-gas chromatograph/mass spectrometer (Py-GC/MS). TG was conducted on a
Seiko Instrumental Inc. EXSTAR 6200 TG/DTA system in an aluminum pan (5 mm in diameter) under a constant
nitrogen flow (100 mLmin−1) using about 5-6 mg of film sample. Dynamic thermal degradation
of the sample was conducted at
prescribed heating rates, φ, of 1, 3, 5, 7, and 9°C min−1 in a temperature range of 60 to 500°C. The thermal degradation data were
collected at regular intervals (about 20 times per degree) by an EXSTAR 6000
data platform and recorded into an analytical computer system.Py-GC/MS
was conducted on a Frontier Lab PY-2020D double-shot pyrolyzer connected to a
Shimadzu GC/MS-QP5050 chromatograph/mass spectrometer, which was equipped with
an Ultra Alloy+-5 capillary column (30 m × 0.25 mm i.d.; film
thickness, 0.25 μm). High-purity helium was used as a carrier gas under a
constant flow of 100 mL min−1.
In the dynamic heating process with Py-GC/MS, about 0.5 mg of sample was added
to the pyrolyzer and heated from 60°C to a prescribed temperature at a heating
rate of 9°C min−1.
Volatile pyrolysis products were introduced into the GC through a Frontier Lab
SS-1010E selective sampler. The temperature of the column oven was first set at
40°C. After the pyrolysis process had finished, the column was heated according
to the following program: 40°C for 1 minute, 40–120°C at 5°C min−1, 120–320°C at 20°C min−1, and 320°C for
13 minutes. Then, the separated
pyrolyzates were analyzed directly with the quadrupole mass spectrometer (70 eV, electron impact
ionization: EI). Mass
spectrum measurements were recorded 2 times s−1 during this period.Thermal degradation behaviors of related blends as references were also
analyzed in the same manner as the PLLA/PP blend.


### 2.5. Isothermal Pyrolysis in Glass Tube Oven

About a 65 mg sample of PLLA/BHPP was put in a sample bottle and set at the waiting position in
a Shibata GTO-350D glass tube oven. The air in the oven chamber was completely exchanged
withN2. The oven was heated rapidly to 300°C under a N2 atmosphere.
Then, the sample was moved to a
heated position thermostated at 300°C and kept there for 30 minutes. After heating, the residual sample was quickly taken
out from the reactor and quenched in an ice bath at 0°C. The recovered residual sample was dried, weighed,
and its molecular weight was measured with SEC. Vaporized pyrolyzates were condensed in a chilled trap,
collected by dissolving in acetone, and dried in a vacuum rotary evaporator to produce white
solids. The obtained solid product was analyzed by GC for characterization.The experiment was repeated 3 times in the same manner
and the obtained results were averaged and listed in Table3 with their
associated standard deviations.Table 3
Changes in weight and molecular weight of PLLA in PLLA/BHPP blends after heating at 300°C for 30 minutes.
EntryPLLA/BHPP (wt/wt)Weight loss (%) (SD(a))Mn (SD)Mw (SD)3–1100/014.6 (0.6)5 500 (355)18 800 (1095)3–2100/0.211.7 (0.4)6 800 (338)20 800 (645)3–3100/16.7 (0.9)8 100 (929)23 100 (1694)3–4100/57.9 (0.3)8 100 (698)20 800 (1319)3–5100/207.8 (0.3)7 100 (118)18 500 (1452)(a)Standard deviation.


## 2.1. Materials

PLLA (LACEA
H-100J,Mn 88,000, Mw 170,000, Sn content:
52.7 ppm) and isotactic polypropylene
(PP) (NOVATEC FY7HA, MFR = 1.7) were obtained from Mitsui Chemicals and Japan
Polypropylene Corporation, respectively. Catalyst:
magnesium oxide (MgO
columnar crystalline, size 10×10×70μm) was purchased from Wako
Pure Chemical Industries, Ltd. and used as received. Antioxidant:
2,2-bis(hydroxymethyl)-1,3-propanediol tetra[3-(4-hdroxy-3,5-di-t-butylphenyl)propanoate
(BHPP) was purchased from Ciba Specialty Chemicals and characterized by H1
NMR (purity ~100%).


## 2.2. Preparation of PLLA/PP, PLLA/PP/MgO, and PLLA/BHPP Blends

PLLA and PP were dried at 40°C for 2 hours at ca. 0.1 kPa in
a vacuum oven just before melt processing. PLLA/PP, PLLA/MgO, PP/MgO, PLLA/PP/MgO,
and PLLA/BHPP blends (see Tables 1 and 2) were prepared by melt blending with
an Imoto Machinery Co., Ltd. twin-screw extruder model 160B (diameter 20 mm,
L/D 25) equipped with an air vent under conditions of 180°C temperature and a
screw rotational speed of 25 rpm.Table 1
Preparation of pellet samples with an extruder.
EntryMelt-processed sample (wt ratio)Molecular weight of PLLA component after preparationMnMw1–1PLLA62 000207 0001–2PP——1–3PLLA/PP (20:80)66 000181 0001–4PLLA/MgO  (100:5)37 00085 0001–5PP/MgO (100:5)——1–6PLLA/PP/MgO (20:80:5)8 00015 000Table 2
Preparation of PLLA/BHPP blends with an extruder.
EntryPLLA/BHPP (wt/wt)MnMw2–1100/079 000150 0002–2100/0.280 000152 0002–3100/180 000152 0002–4100/584 000176 0002–5100/2085 000171 000


## 2.3. Characterization

Molecular weights of the samples were measured by size exclusion chromatography (SEC) on a TOSOH HLC-8120 GPC system with a refractive index (RI) detector
at 40°C using a TOSOH TSKgel Super HM-M column and chloroform eluent (0.6 mL min−1). The
calibration curves for GPC analysis were obtained using polystyrene standards
with a low polydispersity (5.0×102, 1.05×103,
2.5×103, 5.87×103, 9.49×103, 1.71×104, 3.72×104, 9.89×104, 1.89×105, 3.97×105, 7.07×105, 1.11×106, TOSOH
Corporation). The sample (12 mg) was dissolved in chloroform (2 mL) and the solution
was filtered through a membrane filter with a 0.45 μm pore size.Proton (H1) NMR spectra
were recorded on a 500-MHz JEOL JNM-ECP500 FT NMR system. Chloroform-d was
used as solvent. Chemical shifts were reported as δ values (ppm) relative to
internal tetramethylsilane (TMS) in CDCl3 unless otherwise noted.Fourier transfer infrared (FTIR) spectra were recorded on a Perkin
Elmer Spectrum GX2000R. Reflection spectra of as-pyrolyzed samples were measured on a Golden Gate Diamond ATR(10500)
module with a germanium crystal, by the single-reflection ATR method.Optical and scanning electron microscopy for PLLA/PP and PLLA/PP/MgO were
performed with a KEYENCE digital microscope VH-5000 and a HITACHI S-3000N WET-SEM at an
accelerating voltage of 15 kV.


## 2.4. Dynamic Pyrolysis in TG and Py-GC/MS

Thermal degradation behavior of PLLA/PP blends was analyzed by a thermogravimeter (TG) and the
pyrolysis-gas chromatograph/mass spectrometer (Py-GC/MS). TG was conducted on a
Seiko Instrumental Inc. EXSTAR 6200 TG/DTA system in an aluminum pan (5 mm in diameter) under a constant
nitrogen flow (100 mLmin−1) using about 5-6 mg of film sample. Dynamic thermal degradation
of the sample was conducted at
prescribed heating rates, φ, of 1, 3, 5, 7, and 9°C min−1 in a temperature range of 60 to 500°C. The thermal degradation data were
collected at regular intervals (about 20 times per degree) by an EXSTAR 6000
data platform and recorded into an analytical computer system.Py-GC/MS
was conducted on a Frontier Lab PY-2020D double-shot pyrolyzer connected to a
Shimadzu GC/MS-QP5050 chromatograph/mass spectrometer, which was equipped with
an Ultra Alloy+-5 capillary column (30 m × 0.25 mm i.d.; film
thickness, 0.25 μm). High-purity helium was used as a carrier gas under a
constant flow of 100 mL min−1.
In the dynamic heating process with Py-GC/MS, about 0.5 mg of sample was added
to the pyrolyzer and heated from 60°C to a prescribed temperature at a heating
rate of 9°C min−1.
Volatile pyrolysis products were introduced into the GC through a Frontier Lab
SS-1010E selective sampler. The temperature of the column oven was first set at
40°C. After the pyrolysis process had finished, the column was heated according
to the following program: 40°C for 1 minute, 40–120°C at 5°C min−1, 120–320°C at 20°C min−1, and 320°C for
13 minutes. Then, the separated
pyrolyzates were analyzed directly with the quadrupole mass spectrometer (70 eV, electron impact
ionization: EI). Mass
spectrum measurements were recorded 2 times s−1 during this period.Thermal degradation behaviors of related blends as references were also
analyzed in the same manner as the PLLA/PP blend.


## 2.5. Isothermal Pyrolysis in Glass Tube Oven

About a 65 mg sample of PLLA/BHPP was put in a sample bottle and set at the waiting position in
a Shibata GTO-350D glass tube oven. The air in the oven chamber was completely exchanged
withN2. The oven was heated rapidly to 300°C under a N2 atmosphere.
Then, the sample was moved to a
heated position thermostated at 300°C and kept there for 30 minutes. After heating, the residual sample was quickly taken
out from the reactor and quenched in an ice bath at 0°C. The recovered residual sample was dried, weighed,
and its molecular weight was measured with SEC. Vaporized pyrolyzates were condensed in a chilled trap,
collected by dissolving in acetone, and dried in a vacuum rotary evaporator to produce white
solids. The obtained solid product was analyzed by GC for characterization.The experiment was repeated 3 times in the same manner
and the obtained results were averaged and listed in Table3 with their
associated standard deviations.Table 3
Changes in weight and molecular weight of PLLA in PLLA/BHPP blends after heating at 300°C for 30 minutes.
EntryPLLA/BHPP (wt/wt)Weight loss (%) (SD(a))Mn (SD)Mw (SD)3–1100/014.6 (0.6)5 500 (355)18 800 (1095)3–2100/0.211.7 (0.4)6 800 (338)20 800 (645)3–3100/16.7 (0.9)8 100 (929)23 100 (1694)3–4100/57.9 (0.3)8 100 (698)20 800 (1319)3–5100/207.8 (0.3)7 100 (118)18 500 (1452)(a)Standard deviation.


## 3. Results and Discussion

### 3.1. Thermal Degradation of Melt-Blended PLLA/PP and PLLA/PP/MgO

Melt-blended samples of PLLA/PP and PLLA/PP/MgO were prepared as pellets by hot extrusion
molding at 180°C using an extruder with a pelletizer (Table 1). Melt-extruded
samples of PLLA, PP, PLLA/MgO, and PP/MgO were also prepared as references in
the same manner. From microscopic and SEM observations, PLLA and PP components
showed a phase separation of the order of 10–20 μm and MgO
powder dispersed homogeneously in the PLLA/PP matrix. The Mn values of PLLA and PLLA component in the PLLA/PP
blend (entries 1–1 and 1–3 in Table 1)
decreased, whereas Mw increased slightly compared with the values of original PLLA. These findings
may result from a heterogeneous thermal degradation and condensation between carboxyl
and hydroxyl groups during the extrusion [27]. In the case of samples including
MgO, average values Mn and Mw of PLLA components
decreased significantly. In particular, the PLLA component in PLLA/PP/MgO
(entry 1–6) dramatically decreased, because of its higher concentration of the depolymerization catalyst
MgO to PLLA component than that in PLLA/MgO.Thermogravimetric
analyses of the blends and homopolymers as references were carried out with a
TG/DTA. In Figure1, TG curves corresponding to polymer components in PLLA, PP, PLLA/MgO, and PP/MgO
measured at φ=9°C min−1 are illustrated. The TG curve of PLLA/MgO was shifted to a lower-temperature
range by 50°C compared with that of PLLA homopolymer. Since, in a previous
report [28], PLLA (Mn 98,000, Mw 170,000)/MgO (100:5 (wt/wt)) prepared by mixing in a chloroform solution showed a similar TG
curve in a temperature range of 250–310°C, the
observed temperature shift in TG curve of PLLA/MgO must reflect a catalytic
effect of MgO. On the other hand, when compared with that of PP homopolymer, no
temperature shift of the TG curve of PP/MgO occurred except for a slight shift at
the onset of the thermal degradation. These results indicate that MgO is an
effective catalyst for PLLA-degradation, in which it has no appreciable effect
on the thermal degradation of PP.Figure 1
Thermogravimetric analysis of
PLLA, PP, PLLA/MgO, and PP/MgO.φ=9°C min−1.TG curves corresponding to polymer components in the
blends, PLLA/PP = 20:80 (wt/wt) and PLLA/PP/MgO = 20:80:5 (wt/wt/wt), are plotted
in Figure2. Typical two-step weight loss behaviors were observed for both
blends, reflecting the blend ratio of 20:80 (wt/wt). As shown in Figure 2, only
the PLLA component in PLLA/PP/MgO shifted to the lower-temperature range by the
catalytic effect of MgO in contrast to the PLLA component in PLLA/PP. At the
same time, the PP component remained within the same range with nonblended PP shown
in Figure 1. Obviously, the catalyst MgO selectively accelerated the
degradation of only the PLLA component in the blend.Figure 2
Thermogravimetric analysis (φ=9°C min−1) of PLLA/PP and
PLLA/PP/MgO blends. w: residual
weight ratio of PLLA/PP components.To investigate the interaction between both components during
thermal degradation, volatile degradation products from PLLA, PP, PLLA/PP, and
PLLA/PP/MgO blends were measured with Py-GC/MS. In Figure3, the changes in the
total ion count (TIC) spectra of pyrolyzates from PLLA/PP and PLLA/PP/MgO
blends are illustrated within the temperature ranges of PLLA component
degradation. From the PLLA/PP blend, many kinds of pyrolyzates including L,L-lactide, meso-lactide, and oligomers were detected at
retention time ranges of 11–12, 9–11, and 19–28 minutes,
respectively. In contrast, L,L-lactide
was predominantly detected as a volatile product from the PLLA/PP/MgO blend.
These results show the high selectivity of MgO catalyst for the PLLA
depolymerization.Pyrolysis-GC/MS analysis (φ=9°C min−1) of PLLA/PP
and PLLA/PP/MgO. Weight loss values of PLLA/PP components were estimated
from TG analysis (φ=9°C min−1).In Figure3,
it was observed that the detected pyrolyzates from PLLA/PP blend included other
series of volatile products in a similar range to the well-defined cyclic
oligomers composed of a lactate unit [29]. A typical TIC spectrum of
pyrolyzates from PLLA/PP blend at 370°C is shown in Figure 4 in comparison with
the TIC spectra of pyrolysis products from PLLA and PP homopolymers. The
signals from the other series of products detected in Figure 3 agreed with those
of the pyrolyzates from PP homopolymer at 400°C. In Figure 5, based on the TIC intensity-%, the changes in composition of pyrolyzates from
PLLA/PP and PLLA/PP/MgO blends are shown with the corresponding degradation
temperature. The pyrolyzates originating from PP component in the PLLA/PP blend
were detected at 350°C and higher temperatures, despite the weight loss of PP
homopolymer starting at 380°C under the same conditions. This result suggests
that the degradation of PP component was accelerated by the degradation of PLLA
component. However, in the degradation of PLLA/LLDPE blends [14], no
interaction between the components was observed.Figure 4
Pyrolysis-GC/MS analysis of PLLA,
PP, and PLLA/PP.φ=9°C min−1.Composition ratios of pyrolyzates from PLLA/PP (80:20 wt/wt) and PLLA/PP/MgO (80:20:5 wt/wt/wt).Considering the whole analytical results obtained, the highly
selective depolymerization of PLLA component to L,L-lactide
is possible from PLLA/PP blends, even when there is an interaction between both
components during degradation.To clarify the interaction between PLLA and PP components
during the thermal degradation
of the blend, the kinetic parameters for the degradation were evaluated. In
Figure6, changes in the activation energy (Ea) values of the thermal degradation of PLLA, PP, and PLLA/PP blend are plotted.
PLLA and PP homopolymers showed characteristic Ea values of 115–165 [30] and 200–230 kJ mol−1 [31] in separated temperature ranges
of 320–360 and 410–460°C,
respectively. The Ea plot for the PLLA/PP blend appeared over a wide temperature range of 310–460°C encompassing
both the temperature ranges corresponding to PLLA and PP homopolymers. Although
the Ea values of PLLA and PP components in the blend were in the same ranges as those for PLLA and PP
homopolymers, it is noteworthy that the Ea plot for PLLA component in the blend shifted to a lower-temperature range by
~10°C in spite of having nearly the same molecular weight with that of PLLA
homopolymer. Especially, during the initial period of the degradation, the departure
from the plot of PLLA homopolymer was significant. This departure in Ea value between PLLA homopolymer and PLLA component in the blend may be induced by the melt-blending
process and/or an interaction with the blended PP component.Figure 6
Activation energy,Ea, plots against temperature
for thermal degradation of PLLA, PP, and PLLA/PP blend. φ=5°C min−1.When theEa values were plotted against the residual weight ratio, w, of PLLA, PP, and both components in the blend, slight decreases of
5–10 kJ mol−1 were observed for the PLLA and PP components, suggesting that both components
influenced each together and induced minor reactions mutually accelerating the
degradation.In Figure7, the Ea changes with temperature during the thermal degradation of PLLA/MgO, PP/MgO,
and PLLA/PP/MgO blends are illustrated. Shifts to a lower-temperature range for
PLLA component and into lower-Ea values for the PP component in the PLLA/PP/MgO blend were clearly observed in
comparison with those of PLLA/MgO and PP/MgO, respectively. Plots against the
residual weight ratio, w, of both components
indicated a definite decrease in the Ea values of the PP component by 10–50 kJ mol−1 and a slight decrease for the PLLA component by ~3 kJ mol−1. This
considerable decrease in the Ea value
at the onset of the PP component degradation clearly indicates the influence of
PLLA degradation, for example, H-radical abstraction, on an initial degradation
mechanism of the PP component.Figure 7
Activation energy,Ea, plots against temperature
for thermal degradation of PLLA/MgO, PP/MgO, and PLLA/PP/MgO blend. φ=5°C min−1.


### 3.2. Thermal Degradation of Melt-Blended PLLA/BHPP

To determine active species taking part in the interaction between PLLA and PP
components during the thermal degradation of the blend, effects of an oxidation
inhibitor: BHPP on the thermal degradation of PLLA were investigated.
Generally, oxidation inhibitors are added to PP to suppress its oxidative
degradation by an amount less than 1 wt.-%. In this study, BHPP was melt blended
into PLLA at prescribed weight ratios of 0, 0.2, 1, 5, and 20 wt.-% to PLLA.
After melt processing,Mw value of PLLA decreased by 20 000,
but the decrease was suppressed by the BHPP (Table 2). Thermogravimetric
analysis of the PLLA/BHPP blends was carried out with TG/DTA, resulting in TG
curves illustrated in Figure 8. As shown by interpolation in Figure 8, the TG
curve of PLLA shifted to higher-temperature ranges with increase in the dosage
of BHPP, suggesting the inhibitive action of BHPP to PLLA degradation.Figure 8
Thermogravimetric analysis of
PLLA, PLLA/BHPP (0.2, 1.0, 5, and 20 wt%).φ=9°C min−1.The changes inEa values of PLLA/BHPP blends are plotted in Figure 9
against residual weight w. The Ea values of PLLA shifted from a range of 120 to 170 kJ mol−1 into higher ranges with
increase in the addition of BHPP, especially, in the first half of the weight
loss, resulting in an Ea range of 155 to 180 kJ mol−1 of PLLA/BHPP (1 wt.-%) blend. These
results suggest that BHPP captured some of the active species responsible for accelerating
the PLLA degradation, such as radical species.Figure 9
Activation energy,Ea, plots against residual
weight, w, of PLLA and PLLA component for thermal degradation of PLLA,
PLLA/BHPP-0.2 wt%, 1 wt%, and 5 wt% blends.To elucidate the inhibitive action of BHPP on the feedstock recycling of PLLA, changes in
weight loss and molecular weight of the PLLA/BHPP blends were measured after
heating at 300°C for 30 minutes under an N2 atmosphere. Obtained
results of the weight loss and the average molecular weights are shown with
standard deviations in Figure 10. The largest decreases in weight and average
molecular weights were observed for PLLA homopolymer without BHPP. The decreases became shorter with increase in the
addition of BHPP up to 1 wt.-%. McNeill and Leiper [23] also found an inhibiting
effect for diaminoanthraquinone (1 wt.-%) as a radical inhibitor on the
volatilization of PLLA at 230°C. A BHPP addition of more than 1 wt.-% showed an
increase in the weight loss and decreases in the average molecular weights.
These decreases resulting from the addition of large amounts of BHPP may be due
to the action of BHPP as a phenolic reactant. From the obtained results, it was
assumed that BHPP acts not only as an oxidation inhibitor by reacting with
radical species, but also as a phenolic reactant in breaking down the PLLA
chains.Weight loss values and average molecular weights of
PLLA in PLLA/BHPP blends after heating at 300°C for 30 minutes. Bar: standard
deviation.Volatile products from the samples were collected and characterized byH1 NMR
analysis as diastereoisomers of lactide (Figure 11). Interestingly, the
addition of BHPP tended to suppress the racemization of products up to 1 wt.-%
addition, resulting in the highest composition ratio of L,L-lactide occurring in the product from PLLA/BHPP. Based
on mechanisms of PLLA homolysis previously submitted by Kopinke et al. [21]
and Tsuji et al. [24], it is assumed that BHPP preferentially reacts with
radical species to stop expected radical chain reactions (Scheme 1). According
to this scheme, although the homolysis of PLLA chains induces the racemization
of the resulting lactides as volatile products, BHPP suppresses not only the
chain reaction of degradation by radical species, but also the racemization of the
products.Scheme 1
Expected inhibitive mechanism on PLLA homolysis by BHPP.Figure 11
Composition ratios of pyrolyzates evaporated from
PLLA/BHPP during heating at 300°C for 30 minutes.From the above
results, it was observed that the antioxidant BHPP affected the PLLA thermal
degradation, suppressing some degradation mechanisms, which may induce a
homolysis of PLLA chain bonds.When considering the feedstock recycling of PLLA/PP blends,
the antioxidant reagent in PP components may affect the selective depolymerization
of PLLA by MgO catalyst. When TG curves of PLLA/BHPP/MgO = 100:1:1 (wt/wt/wt)
and PLLA/MgO = 100:1 (wt/wt) were compared, a slight increase in the
degradation temperature range by~5°C was found to occur on the addition of BHPP. Moreover,
when the composition ratios of pyrolyzates from PLLA/MgO and PLLA/BHPP/MgO were
compared, similar composition and its changes with temperature were observed
for both samples. These results indicate that the function of MgO in the
selective depolymerization of PLLA component was preserved even in the presence
of BHPP.


## 3.1. Thermal Degradation of Melt-Blended PLLA/PP and PLLA/PP/MgO

Melt-blended samples of PLLA/PP and PLLA/PP/MgO were prepared as pellets by hot extrusion
molding at 180°C using an extruder with a pelletizer (Table 1). Melt-extruded
samples of PLLA, PP, PLLA/MgO, and PP/MgO were also prepared as references in
the same manner. From microscopic and SEM observations, PLLA and PP components
showed a phase separation of the order of 10–20 μm and MgO
powder dispersed homogeneously in the PLLA/PP matrix. The Mn values of PLLA and PLLA component in the PLLA/PP
blend (entries 1–1 and 1–3 in Table 1)
decreased, whereas Mw increased slightly compared with the values of original PLLA. These findings
may result from a heterogeneous thermal degradation and condensation between carboxyl
and hydroxyl groups during the extrusion [27]. In the case of samples including
MgO, average values Mn and Mw of PLLA components
decreased significantly. In particular, the PLLA component in PLLA/PP/MgO
(entry 1–6) dramatically decreased, because of its higher concentration of the depolymerization catalyst
MgO to PLLA component than that in PLLA/MgO.Thermogravimetric
analyses of the blends and homopolymers as references were carried out with a
TG/DTA. In Figure1, TG curves corresponding to polymer components in PLLA, PP, PLLA/MgO, and PP/MgO
measured at φ=9°C min−1 are illustrated. The TG curve of PLLA/MgO was shifted to a lower-temperature
range by 50°C compared with that of PLLA homopolymer. Since, in a previous
report [28], PLLA (Mn 98,000, Mw 170,000)/MgO (100:5 (wt/wt)) prepared by mixing in a chloroform solution showed a similar TG
curve in a temperature range of 250–310°C, the
observed temperature shift in TG curve of PLLA/MgO must reflect a catalytic
effect of MgO. On the other hand, when compared with that of PP homopolymer, no
temperature shift of the TG curve of PP/MgO occurred except for a slight shift at
the onset of the thermal degradation. These results indicate that MgO is an
effective catalyst for PLLA-degradation, in which it has no appreciable effect
on the thermal degradation of PP.Figure 1
Thermogravimetric analysis of
PLLA, PP, PLLA/MgO, and PP/MgO.φ=9°C min−1.TG curves corresponding to polymer components in the
blends, PLLA/PP = 20:80 (wt/wt) and PLLA/PP/MgO = 20:80:5 (wt/wt/wt), are plotted
in Figure2. Typical two-step weight loss behaviors were observed for both
blends, reflecting the blend ratio of 20:80 (wt/wt). As shown in Figure 2, only
the PLLA component in PLLA/PP/MgO shifted to the lower-temperature range by the
catalytic effect of MgO in contrast to the PLLA component in PLLA/PP. At the
same time, the PP component remained within the same range with nonblended PP shown
in Figure 1. Obviously, the catalyst MgO selectively accelerated the
degradation of only the PLLA component in the blend.Figure 2
Thermogravimetric analysis (φ=9°C min−1) of PLLA/PP and
PLLA/PP/MgO blends. w: residual
weight ratio of PLLA/PP components.To investigate the interaction between both components during
thermal degradation, volatile degradation products from PLLA, PP, PLLA/PP, and
PLLA/PP/MgO blends were measured with Py-GC/MS. In Figure3, the changes in the
total ion count (TIC) spectra of pyrolyzates from PLLA/PP and PLLA/PP/MgO
blends are illustrated within the temperature ranges of PLLA component
degradation. From the PLLA/PP blend, many kinds of pyrolyzates including L,L-lactide, meso-lactide, and oligomers were detected at
retention time ranges of 11–12, 9–11, and 19–28 minutes,
respectively. In contrast, L,L-lactide
was predominantly detected as a volatile product from the PLLA/PP/MgO blend.
These results show the high selectivity of MgO catalyst for the PLLA
depolymerization.Pyrolysis-GC/MS analysis (φ=9°C min−1) of PLLA/PP
and PLLA/PP/MgO. Weight loss values of PLLA/PP components were estimated
from TG analysis (φ=9°C min−1).In Figure3,
it was observed that the detected pyrolyzates from PLLA/PP blend included other
series of volatile products in a similar range to the well-defined cyclic
oligomers composed of a lactate unit [29]. A typical TIC spectrum of
pyrolyzates from PLLA/PP blend at 370°C is shown in Figure 4 in comparison with
the TIC spectra of pyrolysis products from PLLA and PP homopolymers. The
signals from the other series of products detected in Figure 3 agreed with those
of the pyrolyzates from PP homopolymer at 400°C. In Figure 5, based on the TIC intensity-%, the changes in composition of pyrolyzates from
PLLA/PP and PLLA/PP/MgO blends are shown with the corresponding degradation
temperature. The pyrolyzates originating from PP component in the PLLA/PP blend
were detected at 350°C and higher temperatures, despite the weight loss of PP
homopolymer starting at 380°C under the same conditions. This result suggests
that the degradation of PP component was accelerated by the degradation of PLLA
component. However, in the degradation of PLLA/LLDPE blends [14], no
interaction between the components was observed.Figure 4
Pyrolysis-GC/MS analysis of PLLA,
PP, and PLLA/PP.φ=9°C min−1.Composition ratios of pyrolyzates from PLLA/PP (80:20 wt/wt) and PLLA/PP/MgO (80:20:5 wt/wt/wt).Considering the whole analytical results obtained, the highly
selective depolymerization of PLLA component to L,L-lactide
is possible from PLLA/PP blends, even when there is an interaction between both
components during degradation.To clarify the interaction between PLLA and PP components
during the thermal degradation
of the blend, the kinetic parameters for the degradation were evaluated. In
Figure6, changes in the activation energy (Ea) values of the thermal degradation of PLLA, PP, and PLLA/PP blend are plotted.
PLLA and PP homopolymers showed characteristic Ea values of 115–165 [30] and 200–230 kJ mol−1 [31] in separated temperature ranges
of 320–360 and 410–460°C,
respectively. The Ea plot for the PLLA/PP blend appeared over a wide temperature range of 310–460°C encompassing
both the temperature ranges corresponding to PLLA and PP homopolymers. Although
the Ea values of PLLA and PP components in the blend were in the same ranges as those for PLLA and PP
homopolymers, it is noteworthy that the Ea plot for PLLA component in the blend shifted to a lower-temperature range by
~10°C in spite of having nearly the same molecular weight with that of PLLA
homopolymer. Especially, during the initial period of the degradation, the departure
from the plot of PLLA homopolymer was significant. This departure in Ea value between PLLA homopolymer and PLLA component in the blend may be induced by the melt-blending
process and/or an interaction with the blended PP component.Figure 6
Activation energy,Ea, plots against temperature
for thermal degradation of PLLA, PP, and PLLA/PP blend. φ=5°C min−1.When theEa values were plotted against the residual weight ratio, w, of PLLA, PP, and both components in the blend, slight decreases of
5–10 kJ mol−1 were observed for the PLLA and PP components, suggesting that both components
influenced each together and induced minor reactions mutually accelerating the
degradation.In Figure7, the Ea changes with temperature during the thermal degradation of PLLA/MgO, PP/MgO,
and PLLA/PP/MgO blends are illustrated. Shifts to a lower-temperature range for
PLLA component and into lower-Ea values for the PP component in the PLLA/PP/MgO blend were clearly observed in
comparison with those of PLLA/MgO and PP/MgO, respectively. Plots against the
residual weight ratio, w, of both components
indicated a definite decrease in the Ea values of the PP component by 10–50 kJ mol−1 and a slight decrease for the PLLA component by ~3 kJ mol−1. This
considerable decrease in the Ea value
at the onset of the PP component degradation clearly indicates the influence of
PLLA degradation, for example, H-radical abstraction, on an initial degradation
mechanism of the PP component.Figure 7
Activation energy,Ea, plots against temperature
for thermal degradation of PLLA/MgO, PP/MgO, and PLLA/PP/MgO blend. φ=5°C min−1.


## 3.2. Thermal Degradation of Melt-Blended PLLA/BHPP

To determine active species taking part in the interaction between PLLA and PP
components during the thermal degradation of the blend, effects of an oxidation
inhibitor: BHPP on the thermal degradation of PLLA were investigated.
Generally, oxidation inhibitors are added to PP to suppress its oxidative
degradation by an amount less than 1 wt.-%. In this study, BHPP was melt blended
into PLLA at prescribed weight ratios of 0, 0.2, 1, 5, and 20 wt.-% to PLLA.
After melt processing,Mw value of PLLA decreased by 20 000,
but the decrease was suppressed by the BHPP (Table 2). Thermogravimetric
analysis of the PLLA/BHPP blends was carried out with TG/DTA, resulting in TG
curves illustrated in Figure 8. As shown by interpolation in Figure 8, the TG
curve of PLLA shifted to higher-temperature ranges with increase in the dosage
of BHPP, suggesting the inhibitive action of BHPP to PLLA degradation.Figure 8
Thermogravimetric analysis of
PLLA, PLLA/BHPP (0.2, 1.0, 5, and 20 wt%).φ=9°C min−1.The changes inEa values of PLLA/BHPP blends are plotted in Figure 9
against residual weight w. The Ea values of PLLA shifted from a range of 120 to 170 kJ mol−1 into higher ranges with
increase in the addition of BHPP, especially, in the first half of the weight
loss, resulting in an Ea range of 155 to 180 kJ mol−1 of PLLA/BHPP (1 wt.-%) blend. These
results suggest that BHPP captured some of the active species responsible for accelerating
the PLLA degradation, such as radical species.Figure 9
Activation energy,Ea, plots against residual
weight, w, of PLLA and PLLA component for thermal degradation of PLLA,
PLLA/BHPP-0.2 wt%, 1 wt%, and 5 wt% blends.To elucidate the inhibitive action of BHPP on the feedstock recycling of PLLA, changes in
weight loss and molecular weight of the PLLA/BHPP blends were measured after
heating at 300°C for 30 minutes under an N2 atmosphere. Obtained
results of the weight loss and the average molecular weights are shown with
standard deviations in Figure 10. The largest decreases in weight and average
molecular weights were observed for PLLA homopolymer without BHPP. The decreases became shorter with increase in the
addition of BHPP up to 1 wt.-%. McNeill and Leiper [23] also found an inhibiting
effect for diaminoanthraquinone (1 wt.-%) as a radical inhibitor on the
volatilization of PLLA at 230°C. A BHPP addition of more than 1 wt.-% showed an
increase in the weight loss and decreases in the average molecular weights.
These decreases resulting from the addition of large amounts of BHPP may be due
to the action of BHPP as a phenolic reactant. From the obtained results, it was
assumed that BHPP acts not only as an oxidation inhibitor by reacting with
radical species, but also as a phenolic reactant in breaking down the PLLA
chains.Weight loss values and average molecular weights of
PLLA in PLLA/BHPP blends after heating at 300°C for 30 minutes. Bar: standard
deviation.Volatile products from the samples were collected and characterized byH1 NMR
analysis as diastereoisomers of lactide (Figure 11). Interestingly, the
addition of BHPP tended to suppress the racemization of products up to 1 wt.-%
addition, resulting in the highest composition ratio of L,L-lactide occurring in the product from PLLA/BHPP. Based
on mechanisms of PLLA homolysis previously submitted by Kopinke et al. [21]
and Tsuji et al. [24], it is assumed that BHPP preferentially reacts with
radical species to stop expected radical chain reactions (Scheme 1). According
to this scheme, although the homolysis of PLLA chains induces the racemization
of the resulting lactides as volatile products, BHPP suppresses not only the
chain reaction of degradation by radical species, but also the racemization of the
products.Scheme 1
Expected inhibitive mechanism on PLLA homolysis by BHPP.Figure 11
Composition ratios of pyrolyzates evaporated from
PLLA/BHPP during heating at 300°C for 30 minutes.From the above
results, it was observed that the antioxidant BHPP affected the PLLA thermal
degradation, suppressing some degradation mechanisms, which may induce a
homolysis of PLLA chain bonds.When considering the feedstock recycling of PLLA/PP blends,
the antioxidant reagent in PP components may affect the selective depolymerization
of PLLA by MgO catalyst. When TG curves of PLLA/BHPP/MgO = 100:1:1 (wt/wt/wt)
and PLLA/MgO = 100:1 (wt/wt) were compared, a slight increase in the
degradation temperature range by~5°C was found to occur on the addition of BHPP. Moreover,
when the composition ratios of pyrolyzates from PLLA/MgO and PLLA/BHPP/MgO were
compared, similar composition and its changes with temperature were observed
for both samples. These results indicate that the function of MgO in the
selective depolymerization of PLLA component was preserved even in the presence
of BHPP.


## 4. Conclusions

The selective depolymerization of PLLA component in a PLLA/PP
blend was investigated to determine influences of coexisting PP component. The
TG curve of PLLA component shifted into a lower-temperature range upon the
addition of MgO, while the TG curve for PP component retained its original position
without any shift in the range. The catalyst MgO selectively accelerated the
degradation of PLLA component in the blend. From Py-GC/MS analysis, L,L-lactide was dominantly detected as a
volatile product from PLLA/PP/MgO, demonstrating the selective depolymerization
of PLLA component by the catalytic action of MgO.TheEa plot of PLLA component degradation in the blend shifted to lower temperatures
and the Ea value of PP
component also decreased by 10–50 kJ mol−1.
Py-GC/MS analysis of the blend showed the accelerated degradation of PP
component. These results suggest that there are interactions between both
components during the thermal degradation of the blend. From the suppression effects
of antioxidant BHPP on PLLA degradation, the homolysis of PLLA chains by the
contribution of radical species was expected.



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*Source: 287547-2009-02-11.xml*