Effect of the β-crystal formation in isotactic polypropylene used for eco-friendly insulating material

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Effect of the β-crystal formation in isotactic
polypropylene used for eco-friendly insulating material
Wei Zhang, Man Xu, Ming-Xuan He, George Chen
State Key Laboratory of Electrical Insulation and Power Equipment
Xi’an Jiaotong University
Xi’an, 710049, China
Shuai Hou, Ming-li Fu
Electric Power Research Institute
China Southern Grid
Guangzhou, 510080, China
Hao Zheng, Qing-Tao Chen
Anhui Electric Power Company
State Grid Corporation of China
Hefei, 230061, China
Abstract- In this paper, the melt blending method was used to
prepare isotactic polypropylene (iPP) samples. The nucleating
agents N, N’-dicyclohexyl-2, 6-naphthalenedicarboxamide (TMB-
5) was chosen to induce the β-crystal. The quenching, 130 °C
isothermal crystallization, natural cooling molding processes were
used to control α- and / or β-crystals in the iPP samples. The
crystalline modifications and crystallinity were analyzed by X-ray
diffraction (XRD) and differential scanning calorimeter (DSC)
analysis. The mechanical and thermal degradation properties of
these samples were investigated by tensile test and
thermogravimetric analysis (TGA). The breakdown tests and the
volume resistivity tests were conducted to characterize the
electrical property of the samples. The experimental results show
that the three molding processes does affect the content and size of
β-crystals in the iPP sample, and the natural cooling and
quenching molding will produce more β-crystals. Compared to the
microcrystalline of α-crystal, the growth of β-crystals effectively
improved the thermal and electrical properties of iPP, such as the
thermal stability and breakdown strength. On another hand, the
thinner β-crystal can effectively improve the mechanical and
provide better stability in volume resistivity as temperature
increases. Based on the above results, the mechanism of
crystalline modifications influence on the electrical property of
iPP discussed.
Keyword ：polypropylene, β-crystal, volume resistivity,
breakdown strength
I. INTRODUCTION
Cross-linked polyethylene (XLPE) has been employed as
the insulating material in most modern extruded high voltage
cables. XLPE has excellent thermo-mechanical properties, is
relatively cheap and has a low dielectric loss [1]. Unfortunately,
XLPE is not easily recycled at the end of its lifetime leading to
questions concerning its long-term sustainability [2].
Compared to XLPE, polypropylene (PP) has higher operation
temperature range and good recycling properties making it a
potential candidate as eco-friendly advanced insulating
material [3, 4]. Traditional isotactic polypropylene (iPP) have
always had the problem of being too brittle for inclusion into
practical cable designs.
Recently, more efforts have been devoted to the
development iPP used as high performance extruded high
voltage cables. The rubbers like EPR, POE or EPDM, which
are called thermoplastic polyolefin (TPOs), are often used to
improve the mechanical properties of iPP, but this method can
obviously reduce the rigidity and thermal stability of iPP [5-9].
For this reason, nano-inorganic fillers are used to improve the
modulus of the iPP. However, due to huge surface energy, their
agglomeration in polymer matrix seriously restrict the
development of high-performance extruded high voltage cables
[10, 11]. Compared to the way of blending and doping, it is
possible to improve the electrical properties of polyolefin by
designing crystalline modifications, which can bring great
advantages to prepare the extruded high voltage cables. Several
crystalline morphologies, such as α, β, and γ, may exist in iPP,
the α and β-crystals have better stability [12]. In addition, it has
been reported that β-crystal can effectively suppress the space
charge accumulation [13]. So iPP contain β-crystal electrical
properties is important that should be deeply explored to
prepare high performance insulating materials.
In this paper, we aim at designing the arrangement of β-
crystal in iPP by doping N, N’-dicyclohexyl-2, 6-
naphthalenedicarboxamide (TMB-5) which acts as nucleating
agent. The iPP with TMB-5 was prepared by melt blending
method. This paper provides different crystal morphology by
controlling crystallization process, to study the relation
between the crystal structure and the mechanical and thermal
properties. Besides, the stability of electrical with the increase
of the temperature and electrical breakdown properties were
also discussed.
II. EXPERIMENT
A. Sample
Isotactic polypropylene (T30-S) with a density of 0.91 kg/L
was provided by Beijing YanShan Petrochemical Co., P. R.
China. TMB-5 was obtained from Shanxi Provincial Institute
of Chemical Industry Fine Chemicals, P. R. China.

The iPP with 0.3 wt % TMB-5 was prepared by melting
route and followed by compressive molding technology using
flat vulcanizing machine. Firstly, iPP pellets, 0.3wt % TMB-5
were dry blended in the stirred tank. Then the mixtures were
extruded using twin-screw extruder (Φ=21.7 mm, L/D=42)
with the screw speed of 30 rpm at 200 °C .
The iPP with TMB-5 was placed in a stainless steel mold
and compressively molded using flat vulcanizing machine with
a pressure of 15 MPa at 200 °C for 15 minutes. Subsequently,
different cooling molding processes were used to prepared iPP
with TMB-5 samples. A subset of samples were cooled in
room temperature for 45 minutes, and another subset of
samples were quenched directly in 0 °C cold water for 5
minutes, and the rest were immediately transferred to a
temperature-controlled fat vulcanizing machine of 130 °C
isothermal crystallization for 30 minutes then cooled in room
temperature 45 minutes. The three kinds of samples named
iPP-TMB-N, iPP-TMB-Q and iPP-TMB-I as shown in TABLE
I. The iPP sample was prepared as iPP-TMB-N, extruded using
twin-screw extruder and then cooled at room temperature.
TABLE I Cooling molding process of iPP and iPP with TMB.
samples
Cooling molding process
iPP
cooled at room temperature for 45 minutes
iPP-TMB-N
cooled at room temperature for 45 minutes
iPP-TMB-Q
quenched directly at 0 °C cold water until to room
temperature for 5 minutes
iPP-TMB-I
130 °C isothermal crystallization for 30 minutes then
cooled at room temperature for 45 minutes
B. Experiments
Crystal structures of iPP, iPP-TMB-N, iPP-TMB-Q and iPP-
TMB-I samples were characterized by an X-ray diffractometer
(XRD) using Cu-Ka radiation (Rigaku, D/MAX-RB).A
differential scanning calorimetry (DSC) (TA-Q200) was
carried out under nitrogen environments. Temperature accuracy
was set at ±1 °C. Temperature was increased from 30 to 200 °C
with 10 °C/min. Tensile testing was undertaken at room
temperature using a CMT-4503 mechanical tensile machine
(Meitesi Industry System, China) at a fixed speed of 100
mm/min. Thermal weight loss measurement of the material
was conducted by using a METTLER TOLEDO
TGA/SDTA851º thermogravimetric analyzer under nitrogen
atmosphere. Temperature was increased from 30 to 600 °C
with 10 °C/min. The dependence of electrical resistivity on
different temperature was tested in an oven using a Keithley
6517A, range from 20 to 90 °C . Electrical breakdown strength
measurements were performed by using an AHDZ-10/100 AC
dielectric strength tester (Shanghai Lanpotronics Corp., China)
with copper ball electrodes of fixed diameter (25 mm for the
bottom and top ones). All the measurements were performed at
room temperature and fifteen breakdown tests were performed
on each kind of sample.
III. RESULTS AND DISCUSSION
A. Structure analysis
The effect of the TMB-5 on the formation of β-crystal
morphology in iPP was investigated using XRD and DSC.
From Figure. 1, it can be seen that the characteristic diffraction
peaks of iPP are at 2θ=14.2°, 17.1°, 18.6°, 21.2°, and 21.9°,
which are, respectively, corresponded to the α (110), α (040), α
(130), α (111), and α (131), indicating that there only exists α-
crystal morphology in iPP [14]. However, for the iPP with
TMB-5, the diffraction peak at 2θ=16.2° is found and its peak
is corresponded to β (300), indicating the appearance of β-
crystal in iPP. Thus, it can be concluded that the doping of
TMB-5 into iPP could induce the formation of β-crystal, and
almost all of α-crystal turned into the β one when the cooling
molding were quenched and natural cooled.
The relative content of β-crystal (Kβ) was calculated
according to the well-known Turner-Jones equation defined as
follows [15]:
(300)
(110) (040) (130) (300)
H
KH H H H



  
  
(1)
Where Hβ(300) is the diffraction intensity of the (300) plane
of trigonal β-crystal, while Hα(110), Hα(040), and Hα(130) are
the intensities of the three strongest a-crystal peaks attributed
to the (110), (040), and (130) planes of monoclinic cell,
respectively.
The total crystallization degree (Xc) was determined as
follows:
c= 100%
crystalline
crystalline amorphous
H
XHH




(2)
(300) (110) (040) (130)
crystalline
H H H H H
  
   

(3)
Where Hamorphous are the fitted areas of amorphous.
The average crystal size (L) was determined using the
Scherrer equations [16]:
=cos
K
LB


(4)
The X-ray wavelength λ=0.154 nm, K is the Scherrer
constant, a somewhat arbitrary value that falls in the range
0.87-1.0, taken as 0.94, B is defined as the width at half
maximum of the peak and θ the position (angle) of the peak.
For these calculations, the (110), (040) and (130) peak of the
typical α-crystal and the (300) peak of the typical β-crystal
have been used. According to Equation. (1), (2), (3) and (4),
the Kβ, L, Xc and Xβc value, obtained by calculation from the
XRD profiles is shown in TABLE II.
Based on the calculate, it is clearly seen that the Kβ value
increases with the quenching and natural cooling, reaches
maximum values of 82.0 % and 87.2 %. The Kβ value in the
sample for 130 °C isothermal crystallization is 30.4 %. The
result shows that the iPP with TMB-5 samples conclude α-and
β-crystal and quenching and natural cooling can produce more
β-crystal.
It is clearly seen that, the crystal size of iPP-TMB has been
influenced by molding process. From the crystal size of α-
crystal, L(110), L(040) and L(130) of iPP, is 18.0 nm, 19.6 nm
and 16.8 nm, and the size of iPP-TMB-I is 12.8 nm, 14.5 nm
and 14.7 nm. On the other hand, the β-crystal of iPP-TMB-N is
the biggest for L(300), is 29.7 nm, and the size of β-crystal in
iPP-TMB-I, L(300) is 17.9 nm.

Figure. 1 X-ray diffraction patterns for iPP and iPP with TMB-5 under
different cooling process samples.
TABLE II Summary of XRD data of iPP and iPP with TMB-5 under different
cooling process samples.
samples
L (nm)
Kβ
(%)
Xc
(%)
Xβc
(%)
α
β
110
040
130
300
iPP
18.0
19.6
16.8
/
0
46.2
0
iPP-TMB-N
/
/
/
29.7
87.2
54.1
49.9
iPP-TMB-Q
/
/
/
18.9
82.0
55.2
45.8
iPP-TMB-I
12.8
14.5
14.7
17.9
30.4
60.9
16.5
The crystallinity in iPP is about 46.2 %, for the iPP-TMB-N,
iPP-TMB-Q and iPP-TMB-I the crystallinity (Xc) are 54.1 %,
55.2 % and 60.9 %. Furthermore, the β-crystal crystallization
degree (Xβc) is 49.9 %, 45.8 % and 16.5 %, respectively. The
result shows that the different crystallization process not only
affect the content of β-crystal but also affect the size of crystal.
To further determine, the crystal morphologies in iPP and
iPP-TMB with different cooling processes, the DSC testing
was performed. The melting curves were shown in Figure 2,
and the DSC data is listed in Table III.
For pure iPP, it can be seen that there is only one melting
peak at 164.2 °C and the related melting enthalpies (ΔHm) is
73.1 J/g. However, for the iPP-TMB-N and iPP-TMB-Q,
another new melting peak occurs at about 155 °C, which is
attributed to the formation of β-crystal induced by TMB-5. For
the iPP-TMB-I, there is only one broad melting peak, which
contains two peaks, it means that more molecular chains
rearrange into crystal nucleus to form more α-crystal at 130 °C.
The result agrees well with XRD. For iPP-TMB-N and iPP-
TMB-Q, the ΔHm is 83.6 J/g and 80.2 J/g. For the iPP-TMB-I,
the ΔHm increases to 94.3 J/g, which is due to the increased
fraction of α-crystal resulted from the 130 °C isothermal
crystallization.
The TMB-5 can serve as one crystal nucleus, which will
induce the growth of β-crystal along one or two dimension on
the TMB-5 surface. That is, the whole surface of TMB-5 is the
core of nucleation and the microcrystallites begin to grow
around its surface. In this case, it is not necessary to form
crystal nucleus depending on the rearrangement of molecular
chains, which is different from the pure iPP with α-crystal.
Figure. 2 DSC melting curves for iPP and iPP with TMB-5 under different
cooling process samples.
TABLE III Summary of DSC data of iPP and iPP with TMB-5 under different
cooling process samples.
samples
Tm1 (°C)
Tm2 (°C)
ΔHm (J/g)
iPP
\
164.2
73.1
iPP-TMB-N
154.2
166.2
83.6
iPP-TMB-Q
155.0
165.8
80.2
iPP-TMB-I
156.9
163.9
94.3
B. Tensile testing
Mechanical property is an important consideration when
selecting cable insulation material. Figure. 3 shows the typical
stress–strain curves of iPP and iPP with TMB-5. Furthermore,
the breaking elongation, elastic modulus, tensile strength and
fracture energy of the samples are listed in Table IV.
The stress was proportional to the strain up to 2 %, which
allowed the modulus to be calculated. All of the samples
showed high elastic modulus above 1000 MPa, especially iPP
and iPP-TMB-I as hard and brittle of α-crystal. Following
130 °C isothermal crystallization, iPP-TMB-I sample failure at
21.9 % strain, whereas iPP exhibits a similar failure at 17.8 %
strain. Under the tensile stress, the small crystal melted and
recrystallized along tensile direction, so iPP-TMB-Q shows a
characteristic necking behavior above 340 % strain. The
remaining materials, iPP-TMB-N was failure at 74.5 %. The
breaking elongation and the fracture energy increases as the β-
crystal size increases. Whilst the brittle nature of iPP is widely
appreciated, but for an insulating material application, a more
flexible, less brittle system is preferred, which clearly rules out
both iPP and iPP-TMB-I.
The stress was proportional to the strain up to 2 %, which
allowed the modulus to be calculated. All of the samples
showed high elastic modulus above 1000 MPa, especially iPP
and iPP-TMB-I as hard and brittle of α-crystal. Following
130 °C isothermal crystallization, iPP-TMB-I sample failure at
21.9 % strain, whereas iPP exhibits a similar failure at 17.8 %
strain. Under the tensile stress, the small crystal melted and
recrystallized along tensile direction, so iPP-TMB-Q shows a
characteristic necking behavior above 340 % strain. The
remaining materials, iPP-TMB-N was failure at 74.5 %. The
breaking elongation and the fracture energy increases as the β-
crystal size increases. Whilst the brittle nature of iPP is widely
appreciated, but for an insulating material application, a more

flexible, less brittle system is preferred, which clearly rules out
both iPP and iPP-TMB-I.
Figure.3 Tensile test results from iPP and iPP with TMB-5 under different
cooling process samples.
TABLE IV Summary of tensile test data of iPP and iPP with TMB-5 under
different cooling process samples.
samples
Breaking
Elongation
(%)
Elastic
Modulus
(MPa)
Tensile
strength
(MPa)
Fracture
Energy
(kJ/m2)
iPP
17.8
1482.6
38.0
0.5
iPP-TMB-N
74.5
1075.4
24.5
1.2
iPP-TMB-Q
343.6
1167.9
28.9
4.2
iPP-TMB-I
21.9
1581.2
34.3
0.4
C. TGA analysis
The thermal degradation properties of the iPP and iPP with
TMB-5 samples were investigated via TGA. The 5 % weight
loss temperature, T5%, and the maximum decomposition
temperature, Tp, represent the thermal degradation resistance.
Figure. 4 TG curves of iPP and iPP with TMB-5 under different cooling
process samples.
TABLE V Summary of TG data of iPP and iPP with TMB-5 under different
cooling process samples.
samples
T5% (°C )
Tp (°C )
Residual (%)
iPP
413.1
451.9
0.5
iPP-TMB-N
420.3
471.6
1.8
iPP-TMB-Q
422.3
470.2
2.1
iPP-TMB-I
419.1
470.2
3.6
Higher values of T5% and Tp correspond to excellent thermal
stability of the iPP and iPP with TMB-5 material. Table V
shows the statistical data of the TGA parameters of samples.
Results show that, for iPP with TMB-5, the T5%, Tp and residual
increase. For iPP samples, T5% is 413.1 °C and Tp is 451.9 °C .
However, the T5% and Tp of iPP with TMB-5 samples is about
420 °C and 470 °C ，respectively. The residual of iPP with
TMB-5 are more than iPP. These experimental phenomena
reflect the thermal degradation of β-crystal is more stability
than α-crystal of iPP.
D. Volume resistivity
The volume resistivity and breakdown strength were
investigated to evaluate insulating properties. Figure 5 shows
the volume resistivity of samples decreased with the increased
temperature.
Figure 5. Temperature dependence of volume resistivity of iPP and iPP with
TMB-5 under different cooling process.
The volume resistivity is deeply influenced by temperature.
It means that the conductivity would increase with temperature
increase. However, the descent rate resistivity of the samples
with different cooling molding was remarkable. To be specific,
the iPP is violently affected by temperature; the β-crystal of
iPP can improve the property of heat-resisting. Evidently, at
room temperature, there is no nucleating agent, no more
conductive factor, so volume resistivity of iPP is maximum.
When the temperature is above 70 °C , the increase of
crystallinity and β-crystal limit electronic hopping, the volume
resistivity of iPP with TMB-5 decreases less. In these samples,
the iPP-TMB-I ΔHm is maximum, so volume resistivity
decreases least at high temperature. The variation of volume
resistivity under different temperature of iPP with β-crystal is
better than polyethylene material.
E. Breakdown strength
As shown in Figure. 6, the breakdown strength of iPP and
iPP with TMB-5 were evaluated by the Weibull analysis
according to the following formula:
( ) 1 exp[1 ( / ) ]
ib
P E E E

  
(5)
Where the P(Ei) is the failure probability, E is the measured
electrical breakdown strength, Eb is the characteristic electrical
breakdown strength, and β is the shape parameter.

The results show that the characteristic breakdown strength
increases from 65.9 kV/mm of iPP to 83.0 and 86.2 kV/mm
and 77.6 kV/mm by introducing β-crystal. In addition, more β-
crystal higher breakdown strength. This phenomenon may
attribute to the TMB-5 induces the β-crystal that can
effectively protect the formation of electrons tunnels and
improve the insulating property. Besides, the crystallites grown
on the TMB-5 surface have wide bend gap to restrict the
transportation of charge carriers, improving the breakdown
strength.
Figure.6 Weibull distribution of the breakdown field.
TABLE VI Weibull parameters of iPP and iPP with TMB-5 under different
cooling process samples.
samples
β
Eb/kV
iPP
19.1
65.9
iPP-TMB-N
11.7
83.0
iPP-TMB-Q
8.9
86.2
iPP-TMB-I
12.2
77.6
Quenched sample displays increased performance compared
to isothermally crystallized sample and nature cooled sample.
Table VI shows that compared with the iPP with TMB-5
samples the iPP sample have bigger β values. Particularly, iPP-
TMB-Q sample manifests one-half β value, indicating bigger
data dispersion property. This phenomenon may attribute to the
crystal boundary between α-crystal and β-crystal.
IV. CONCLUSION
The effect of β-crystal content on the properties of the iPP
samples can be summarized as below:
Natural cooling process is helpful to β-crystal formation.
The α-crystal and β-crystal fully grown in iPP at 130 °C
isothermal crystallization process. The crystallization time of
quenched iPP with TMB-5 is short, so the crystal size is small,
that is beneficial to mechanical properties. During the tensile
process, the smaller crystal rearrangement increases the
toughness. Besides, the β-crystal in iPP improving the
breakdown strength, while the breakdown strength of iPP-
TMB-I increases less, due to the presence of α-crystal. The
volume resistivity of iPP with β-crystal decreases less above
70 °C .
Introduced of β-crystal in iPP can result in enhanced
mechanical flexibility, good enough thermal properties and
enhanced electrical properties (i.e., stability volume resistivity
and high breakdown strength). Therefore, iPP with β-crystal is
a promising candidate for eco-friendly advanced material for
extruded high voltage cables.
ACKNOWLENDGMENT
The authors gratefully acknowledge experimental support of
the Science & Technology Projects of China Southern Grid (K-
KY2014-31) and the National Key Research and Development
Program of China (2016YFB0900702).
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