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Title

Degradation behavior of polymer blend of
isotactic polypropylenes with and without
unsaturated chain end group

Author(s)Nakatani, Hisayuki; Kurniawan, Dodik; Taniike,
Toshiaki; Terano, Minoru

Citation

Science and Technology of Advanced Materials, 9:024401‑1‑024401‑5

Issue Date

2008‑07‑07

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/8821

RightsCopyright (C) Institute of Physics and IOP
Publishing Limited 2008. This is the author‑
created version of Institute of Physics and IOP
Publishing Limited, Hisayuki Nakatani, Dodik
Kurniawan, Toshiaki Taniike and Minoru Terano,
Science and Technology of Advanced Materials, 9,2008, 024401‑1‑024401‑5.
http://dx.doi.org/10.1088/1468‑6996/9/2/024401

Description

Japan Advanced Institute of Science and Technology

IOP PUBLISHINGSCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS

Sci. Technol. Adv. Mater. 9 (2008) 024401 (5pp)

doi:10.1088/1468-6996/9/2/024401

Degradation behavior of polymer blend of
isotactic polypropylenes with and without
unsaturated chain end group
Hisayuki Nakatani1 , Dodik Kurniawan2 , Toshiaki Taniike2
and Minoru Terano2
1

Department of Applied and Environmental Chemistry, Kitami Institute of Technology, 165 Koen-cho,
Kitami, Hokkaido, 090-8507, Japan
2
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi,
Ishikawa, 923-1292, Japan
E-mail: nakatani@chem.kitami-it.ac.jp

Received 26 November 2007
Accepted for publication 1 February 2008
Published 7 July 2008
Online at stacks.iop.org/STAM/9/024401
Abstract
In this work, the relationship between the unsaturated chain end group content and the thermaloxidative degradation rate was systematically studied with binary polymer blends of isotactic
polypropylene (iPP) with and without the unsaturated chain end group. The iPPs with andwithout the unsaturated chain end group were synthesized by a metallocene catalyst in theabsence of hydrogen and by a Ziegler catalyst in the presence of one, respectively. The thermaloxidative degradation rate of the binary iPP blends was estimated from the molecular weight
and the apparent activation energy (1E), which were obtained through size exclusion
chromatography (SEC) and thermogravimetric analysis (TGA) measurements, respectively.
These values exhibited a negative correlation against the mole content of the unsaturated chain
end group. The thermal oxidative degradation rate apparently depends on the content of theunsaturated chain end group. This tendency suggests that the unsaturated chain end acts as aradical initiator of the iPP degradation reaction.
Keywords: polypropylene, unsaturated chain end groups, activation energy, radical initiator

becomes a radical initiator by itself under general degradation
conditions. The production of a radical species and its spreadare an important factor in oxidative degradation.
Based on the detailed kinetic studies of iPP degradation
using chemiluiminescence (CL), Celina and co-workers
proposed ‘a spreading model’ for the progress of the thermaloxidative reaction [6–8]. This model was constructedassuming that the unoxidized polymer fraction was
infected by the spatial expansion from the initial oxidizedzone. The validity of the model has been confirmed
experimentally [9–12]. Low-molecular-weight materials,
such as dialkyl peroxides, hydroperoxides and macroradical
species, have been considered as initiators [6, 9, 11, 12].
We have reported that unsaturated chain end groups
influence the rate of iPP degradation [13]. Considering

1. Introduction
Isotactic polypropylene (iPP) has been widely used asmaterials in the household appliance, medical ware,automotive and other industrial sections. However, iPP
easily undergoes oxidative degradation under the influenceof elevated temperature and sunlight, and the degradation
leads to a deterioration of the mechanical properties andyellowing [1–5].
The reaction mechanism of the degradation has beenrecognized as a free-radical chain reaction, leading to polymer
chain scission [1–5]. This radical reaction is a cyclic reaction
and is called ‘autoxidation’. Here, it is noted that the presenceof a radical species is indispensable for the initiation of
autoxidation. Since iPP is a saturated hydrocarbon, it rarely
1468-6996/08/024401+05$30.00

1© 2008 National Institute for Materials Science

Printed in the UKSci. Technol. Adv. Mater. 9 (2008) 024401

H Nakatani et al

the higher resonance stability of the allyl radical, theunsaturated chain end group is expected to function as an
initiator of degradation. In addition, iPP certainly contains
the unsaturated chain end group, which is produced by achain transfer reaction during polymerization [14, 15] and byphysical iPP chain scission during molding [16]. Its role in
the initial stage of degradation should be clarified in detailsince the unsaturated end group may act as not only theradical initiator but the capture agent. The double bond caneasily react with radical species, including the allyl radical
in another chain. In this case, the unsaturated chain end
group would be the degradation inhibitor such as antioxidant.
It is important to examine which characteristics of theunsaturated chain end group are dominant during the initialstage of degradation. The characteristics naturally depend on
various conditions such as the content and the molecularmobility of the unsaturated chain end group. The elucidation
requires a systematic iPP series with various contents of
the unsaturated chain end group. It is difficult, however, todirectly synthesize such an iPP series. The unsaturated chain
end group is produced by chain transfer reactions such as
monomer transfer and hydrogen elimination [14]. The change
in content requires various polymerization conditions, leading
to changes in the molecular characteristics such as molecularweight and tacticity. A better method is to blend iPPs without
and with the unsaturated chain end group to generate suchan iPP series. Recently, many studies of PPs synthesized
by metallocene catalyst have been reported subsequent to
the discovery made by Kaminsky et al [17]. The feature of
the metallocene catalyst is that it enables the production of
tailor-made PP through a ligand design at the metal center[14, 17, 18], and this catalyst is naturally able to yield iPP
with unsaturated chain end groups.
In this study, the iPP series with varying unsaturated end
group content was prepared using the binary iPP blend of
Ziegler-catalyzed iPP (ZiPP) without unsaturated chain end
groups and metallocene-catalyzed iPP (MiPP) with 0.7 mol.%of unsaturated chain end groups, and the effect of the content
of the unsaturated group on thermal oxidative degradation
was studied by thermogravimetric analysis (TGA) and size
exclusion chromatography (SEC).

chain end group was a saturated hydrocarbon. In order toremove unfavorable catalyst residues, ZiPP was stirred with
distilled water containing a small amount of ethanol at room
temperature overnight, dried, and then was reprecipitatedfrom a boiling xylene solution into methanol under nitrogen
atmosphere.

2. Experimental

2.6. SEC analysis

2.1. Materials

The sample in the small vial was dissolved in 5 ml ofo-dichlorobenzene containing 2,6-di-t-butyl- p-cresol as anantioxidant, and the obtained sample solution was directly
measured by SEC. The molecular weight was determined by
SEC (Senshu, SSC-7100) with styrene-divinylbenzene
gel columns (SHODEX, HT-806M) at 140 ◦ C usingo-dichlorobenzene as a solvent.

2.3. Metallocene-catalyzed iPP (MiPP)
The meso-ethylenebis(1-indenyl)zirconium dichloride andMAO catalyst system simultaneously yielded a slightly
low isotactic iPP and a considerably low one. The lower
iPP (MiPP) contained a number of unsaturated chain end
groups [13]. In this work, the polymerization was performed
under a propylene pressure of the 5 kgf cm−2 with an Al/Zr
molar ratio of 3000 at 50 ◦ C for 3 h in toluene as solvent.
In order to remove catalyst residues, the whole iPP was
reprecipitated from a boiling xylene solution into methanol
under nitrogen atmosphere and then dried in vacuo. This
whole iPP was fractionated by boiling n-hexane in a soxhlet
apparatus for 6 h, and the obtained soluble part was used as
MiPP after drying it in vacuo.
2.4. Polymer blend preparation
The ZiPP and the MiPP were blended in a boiling xylene
solution in pure nitrogen at atmospheric pressure. The
compositions of the components in blends of ZiPP/MiPP
were 99/1, 95/5, and 90/10 (w/w). All of polymer blends
were precipitated in methyl alcohol and then dried in a
vacuum oven at 60 ◦ C for 6 h.
2.5. 1 C- and 13 C-NMR measurements
The contents of the unsaturated chain end group and meso
pentad fraction (mmmm) were determined by 1 H- and
13
C-NMR spectroscopy using a Varian Gemini-300
spectrometer at 120 ◦ C on 20% (w/v) solution in hexachloro1,3-butadiene. 1,1,2,2-Tetrachloroethane-d2 was added as aninternal lock and used as an internal chemical shift reference.

Propylene of research grade (donated by Chisso Co.),
nitrogen (purchased from Uno Sanso Co.), meso-ethylenebis(1-indenyl)zirconium dichloride (purchased from Wako
Pure Chemical Industry), and methylaluminoxane (MAO;
purchased from Tosoh Finechem Co. in the toluene solution)
were used without further purification. Toluene and xylene
solvent was purified by passing them through a molecularsieve 13 X column.

2.7. Thermogravimetric analysis (TGA)
The decrease in the weight of the sample was investigatedby thermogravimetric analysis (TGA, METTER TG50) at
several constant temperatures (130, 135, 140 and 145 ◦ C)
in air.

2.2. Ziegler-catalyzed isotactic polypropylene (ZiPP)
ZiPP was supplied by Toho Catalyst Co., Ltd. Since thepolymer chain transfer was carried out with hydrogen, its
2

Sci. Technol. Adv. Mater. 9 (2008) 024401

H Nakatani et al
HC

HG
HC

Polymer

HF

Polymer
PolymerorHD

HD

PolymerHE

HA

Vinylene end group

Terminal vinyl end group

HB

Vinylidene end group
HD
HE HF

HC
HG

HA
HB

5.9

5.8

5.7

5.6

5.1

5.5

5.0

4.9

4.8

4.7

ppm

Figure 1. Region of unsaturated chain end groups in 1 H-MR spectrum of MiPP.
Table 1. Molecular characteristics of ZiPP, MiPP and ZiPP/MiPP polymer blends.
Sample
ZiPP
MiPP
ZiPP/MiPP (99/1)∗
ZiPP/MiPP (95/5)∗
ZiPP/MiPP (90/10)∗∗

Meso pentad fraction
mmmm (mol.%)Mn(g mol.−1 )MW /MnChain end groupcontent∗∗ (mol.%)

97
72
–
–
–

9.4 104
3.0 10 3
–
–
–

4.9
2.6
–
–
–

≈0
0.7
0.007
0.035
0.07

Blend ratio: (wt.%/wt.%). ∗ ∗ Calculation from dilution factor.

3. Results and discussion
1.0

Figure 1 shows the 1 H-NMR spectrum of MiPP. The existenceof vinylidene, vinyl and vinylene unsaturated chain end
groups can be confirmed. The total content of unsaturated
chain end groups was estimated to be 0.7 mol.% from the areaof each corresponding peak. ZiPP contained no unsaturated
chain end group because of the polymer chain transfer byhydrogen. The polymer blend (ZiPP/MiPP) provided an iPP
series with various contents of the unsaturated chain end
group. The blend ratios (wt.%/wt.%) of ZiPP/MiPP were
99/1, 95/5 and 90/10. The molecular characteristics of ZiPP,
MiPP and ZiPP/MiPP are summarized in table 1.
The content dependence of thermal oxidative degradation
was investigated by SEC measurement. Figure 2 shows
the changes in the ratio of molecular weights after andbefore thermal oxidative degradation. The molecular weightdecreases for all amounts of additional unsaturated chain end
groups. The rate of change apparently depends on the content
of unsaturated chain end groups, indicating that the existenceof unsaturated groups accelerates the degradation reaction.
In our previous works [13, 19], we have reported that
TGA measurement is a useful method of studying the rateof thermal oxidative degradation. The weight change curves
obtained by TGA are shown in figure 3. The curves exhibit
multiple stages composing the three periods of constant,Mn,t / Mn,0

0.8
0.6

0.4

ZiPP

0.2

ZiPP/MiPP (99/1)
ZiPP/MiPP (95/5)
ZiPP/MiPP (90/10)

0
0

10

20

30

40

50

60

Degradation time (min)Figure 2. Changes in ratio of molecular weights (Mn,t /Mn,0 ) of
ZiPP and ZiPP/MiPP during thermal oxidative degradation at
130 ◦ C; Mn,t and Mn,0 are number average molecular weights afterand before thermal oxidative degradation, respectively.

increasing and decreasing weight. Here the increase anddecrease of weight are attributed to the incorporation ofoxygen and the volatilization of highly degraded parts,
respectively. The start of the increase in weight means the endof the induction period of thermal oxidative degradation. In
3

Sci. Technol. Adv. Mater. 9 (2008) 024401

H Nakatani et al

Table 2. Value of tdi at various temperatures and apparent activation energies (1E).
Sample130 ◦ C
ZiPP
ZiPP/MiPP (99/1)
ZiPP/MiPP (95/5)
ZiPP/MiPP (90/10)

tdi (×103 s)
135 ◦ C 140 ◦ C

8.4
7.6
5.8
4.7

6.1
5.1
3.8
2.9

4.0
3.6
2.7
2.1

1E(kJ mol.−1 )145 ◦ C
2.3
2.2
1.8
1.6

102

125

101

120

100

∆ E (kJ mol–1)

Weight ratio (wt.%)(wt %)

119
113
108
101

99
98

ZiPP
ZiPP/MiPP (99/1)
ZiPP/MiPP (95/5)
ZiPP/MiPP (90/10)

97
96
95
94

115
110
105
100

0

5000
10000 15000
Degradation time (s)

20000

95

ZiPP/MiPP (99/1)–ln tdi

–8.0

–8.5

–9.0

2.44

1/T (1000 K–1)

.08
0.08

RH → R·
R· + O2 → ROO·
ROO· + RH → R· + ROOH
Chain branching (including chain scission):
ROOH → carbonyls + R·
ROOH + ROOH → carbonyls + R·
Initiation:
Propagation:

2.40

.0
0.06

summarized in table 2. It is noted here that the 1E value
depends on the content of the unsaturated chain end group. As
shown in figure 5, 1E exhibits a negative correlation with themole content of unsaturated chain end groups. The fall of 1E
suggests that the existence of unsaturated chain end groups
affects the reaction mechanism of iPP degradation.
There is much literature on the mechanisms of the
iPP autoxidation process [1–5, 20]. Many researchers agree
fundamentally with the following mechanistic scheme.

ZiPP/MiPP (95/5)
ZiPP/MiPP (90/10)

–9.5

0.0
0.04

Figure 5. Apparent activation energy (1E) versus content of
unsaturated chain end group.

ZiPP

–7.5

.02
0.02

Content off unsaturated chain
end group (mol.%)

Figure 3. Weight change curves of ZiPP and ZiPP/MiPP at 130 ◦ C
in air.
–7.0

0

2.48

Figure 4. Arrhenius plots of duration of induction period (tdi ) for
thermal oxidative degradation of ZiPP and ZiPP/MiPP.

(I)
(II)
(III)
(IV)
(V)

Among these reaction steps, the hydroperoxide
decompositions (IV) and/or (V) are the rate-determining
steps [4, 20, 21]. The 1Es obtained here corresponds tothose of the hydroperoxide decomposition [20]. In addition,
the 1E of bimolecular hydroperoxide decomposition (V) is
considerably lower than that of the unimolecular one (IV)[4, 20]. The existence of adjacent hydroperoxides leads tothe bimolecular decomposition reaction (V) path [4, 20] and,thus, apparently lowers the 1E observed in the degradation
reaction. If the formation rate of adjacent hydroperoxides
increases with the increase in the content of the unsaturated
chain end group, the observed 1E changes can be explainedby the competition between the bimolecular decomposition
with a lower 1E and unimolecular one with a higher 1E.

order to evaluate the degradation rate, in several samples, thedurations of the induction period (tdi ) were determined using
the weight change curves obtained from the measurements atseveral constant temperatures (130, 135, 140 and 145 ◦ C). The
tdi has been defined as the time (second) at the intersectionof the tangent at the inflection point with the plateau line of
the induction period [13, 19]. The apparent activation energy(1E) was calculated from the Arrhenius plot of ln tdi versus
the reciprocal of temperature.
Figure 4 shows the Arrhenius plots of tdi of ZiPP and
ZiPP/MiPP. tdi apparently obeys the Arrhenius law within
the chosen temperature limit. The values of tdi and 1E are4

Sci. Technol. Adv. Mater. 9 (2008) 024401

H Nakatani et al

The behavior of the lower 1E is thought to be closely related
to the increasing population of adjacent hydroperoxides.
As materials that accelerate the thermal oxidative
degradation of iPP, benzoyl peroxide (BPO) [11],formaldehyde [11, 22] and organic acids [11, 23] have beenreported. The acceleration effect of BPO is due to its
radical-initiating ability [11], whereas the formaldehyde
and organic acid additives are considered to catalyze thehydroperoxide decomposition as prodegradants [11, 22, 23].
Considering the chemical structure, the acceleration effectof unsaturated chain end groups is likely due to not only thecatalysis of the hydroperoxide decomposition, but also theiraction as degradation initiators. The unsaturated chain end
group should be the initiator because the allyl radical to be
produced has higher resonance stability. In fact, in the caseof polyethylene, Allen and colleagues have already pointed
out that the unsaturated group is the initiator [24, 25]. The
added MiPP would be expected to function as the degradation
initiator. The amount of hydroperoxide increases with theaddition of MiPP, and the ratio of adjacent hydroperoxides
becomes higher. The decrease of 1E originates from thismechanism.
These results demonstrate that the unsaturated chain end
group becomes the radical initiator, not the capture agent.
It is natural that this behavior is also associated with thechemical structure and molecular weight of the added polymer
containing unsaturated chain end groups. The MiPP has the
iPP structure and a considerably low molecular weight (Mn =3000 g mol.−1 ). Thus, the added MiPP is uniformly dispersedin the ZiPP and have such a slow molecular chain motion thatthe possibility of encountering other radicals is considerably
low. The degradation appears to proceed uniformly andefficiently upon the addition.

Acknowledgment
This work was supported by a Grant-in-Aid for Scientific
Research, No. 17550194 from the Japan Society for the
Promotion of Science.

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4. Conclusion
In order to clarify the function mechanism of the unsaturated
chain end group in iPP thermal oxidative degradation, thecorrelation between the unsaturated chain end group content
and degradation rate was studied in a ZiPP/MiPP polymer
blend. This blend allowed the unsaturated chain end groupcontent to be varied in order to clarify a role of the group asa degradation initiator. The changes in the molecular weightand 1E showed that the increase of the content led to theacceleration of the iPP degradation reaction. These results
suggested that the unsaturated chain end group acted as aradical initiator.

5