Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.8, pp.671-681, 2010 Printed in the USA. All rights reserved
Studies on Mechanical Characterization of Polyprop yl en e/ Na+-MMT
V. Selvakumara,*, K. Palanikumarb, K. Palaniveluc
aSathyabama University, Chennai-119.
bSri Sai Ram Institute of Technology, Chennai.
cCentral Institute of Plastics Engineering & Technology, Chennai.
*Corresponding author:
This article addresses the effect of montmorillonite (MMT) on the morphology, and mechanical
properties of polypropylene (PP). PP/MMT nanocomposites have been prepared by melt mixing
using maleic anhydride grafted polypropylene (MAH-g-PP) as compatibilizing agents. Melt
mixing was achieved using twin screw extruder. The MAH-g-PP used as compatibilizer helped
the dispersion of the MMT in PP matrix. The influence of MMT on the impact fracture
morphology of the nanocomposites was studied by scanning electron microscopy (SEM). The
polymer composites were characterized by using different techniques such as X-ray diffraction
(XRD), tranmission electron microscopy (TEM) and mechanical characterization as per ASTM
standards. The mechanical properties of strength and modulus of the nanocomposites increases
with addition of 5 wt% of nanoclay and impact strength and hardness of the nanocomposites
increses with addition of 3 wt% of nanoclay.
Keywords: Polypropylene; Montmorillonite; X-ray diffraction (XRD); scanning electron
microscopy (SEM);Tranmission electron microscopy (TEM).
Polypropylene is one of the fastest growing classes of thermoplastics. This growth is attributed to
its attractive combination of low cost, low density, and high heat distortion temperature (HDT).
Currently, automotive and appliance applications employ glass or mineral-filled systems with
loading levels ranging from 15 to 50 wt%. This approach improves most mechanical properties,
672 V. Selvakumar, K. Palanikumar, K. Palanivelu Vol.9, No.8
but polypropylene’s ease of processing is somewhat compromised. Furthermore, the need for
higher filler loading leads to greater molded part weight [1].
Nanoclay is the most commonly used tool for the preparation of nanocomposites. MMT,
hectorite, and saponite are the most commonly used layered silicates. Layered silicates have two
types of structure: tetrahedral-substituted and octahedral substituted. In the case of tetrahedrally
substituted layered silicates the negative charge is located on the surface of silicate layers, and
hence, the polymer matrices can react interact more readily with these than with octahedrally-
substituted material [2]. The main attraction of polypropylene (PP) is its high performance-to-
cost ratio. PP can also be easily modified to achieve greatly enhanced properties. With regard to
reinforcement effects, considerable research can be found in recent literature [3-6] on improving
mechanical properties of PP using various kinds of inorganic fillers. It is now well recognized
that the use of inorganic fillers is a useful tool for improving stiffness, toughness, hardness,
chemical resistance, dimension stability, and gas barrier properties of PP [7-9]. PNCs are now
prepared by different methods, namely in situ polymerization, solvent process and melt
compounding. PNCs are also made using a large variety of thermosetting and thermoplastic
polymers [10-12].
In present investigation melt-blending technique has been adopted to synthesize PP-layered
silicate nanocomposites using PP-g-MAH as compatibiliszr. The aim of this work is to report
and explain the effect of Na+ -MMT on mechanical characterization and morphological
properties of the nanocomposites.
2.1 Materials
The polypropylene (H110MA) with density of 0.910 g/cc and MFI of 11 g/10 min (measured at
230oC under 2.16 kg load), obtained from Reliance LtD., India was used as the base matrix for
the present study. PP-g-MA was used in this study is supplied by Exxon mobile India Pvt
Ltd.india, under the trade name Exxlor PO 1020 and has melt flow index 125gm/10 min with
percentage of grafting of MA is 0.75%. Clay i.e. Na+-MMT (unmodified having CEC 92.6
meq/100 g clay), were obtained from Southern Clay Products Inc., USA.
2.2 Preparation of Nanocomposites
Melt blending of PP, PP-g-MAH (10 wt %) and the nanoclays (Na+-MMT) of 1, 3, 5 and 7wt%
was carried out in an intermeshing counter rotating twin screw extruder (ctw-100, Haake-
Germany) having barrel length of 300mm and angle of entry 90o. Prior to extrusion, the matrix
polymer and the nanoclay were dehumidified in a vacuum oven at 60oC for a period of 6 hours.
Vol.9, No.8 Studies on Mechanical Characterization 673
PP was fed at the rate of 5 kg/hour and the nanoclay was subsequently introduced at the melting
zone. The process was carried out at a screw speed of 150 rpm and a temperature difference of
160, 170 and 180oC between feed zones to die zone, followed by granulation in a pelletizer
(Fission, Germany) and drying. These granules were further injection molded using injection
moulding machine (SP 130 Windsor Clocknar Ltd) having clamping force 800kN fitted with a
dehumidifier at a temperature range of 195–220oC and mold temperature of 80oC, for preparation
of test specimens of tensile, flexural and impact strength as per ASTMD.
2.3 Characterization
2.3.1 Mechanical characterization
Specimens of virgin PP and PP/nanocomposites of dimensions 165X13X3 mm were subjected to
tensile test as per ASTM D-638 using universal testing machine (UTM) LR-100K (Lloyd
Instrument Ltd U.K). A cross head speed of 50mm/min and gauge length of 50mm was used for
carrying out the test. Specimens of virgin PP and PP/Nanocomposites of dimensions 80X12.7X3
mm were taken for flexural test under three point bending using the same universal testing
machine accordance with ASTM-D 790 at a cross head speed of 1.3 mm/min and a span length
of 50mm.Similarly,Izod impact strength was determined from the specimens having dimensions
63.5X12.7X3mm with a “V” notch depth of 2.54 mm and notch angle of 45o as per ASTM-D
256 using impact meter 6545(Ceast.Italy).For analyzing the mechanical properties test specimens
were initially conditioned at 23+1o C and 55+2% RH. Five replicate specimens were used for
each test and the data reported are the average of five tests. Corresponding standard deviations
along with measurement uncertainty values for the experimental data showing the maximum
standard deviation is also included.
2.3.2 Scanning electron microscopy
The morphology of the impact fractured surfaces of neat PP and its nanocomposites, the fracture
surfaces were coated with thin layers of gold of about 10 A. All specimens were examined with
JEOL, JSM 840A scanning electron microscope with an accelerating of 10kv.
2.3.3 Wide angle X-Ray diffraction (WAXD)
Wide angle X-ray scans (WAXS) were made using a Philips X’Pert MPD (Japan) X-ray
diffractometer in the reflection mode which had a graphite monochromator and a Cu Ka
radiation source (k ¼ 1.54A˚) at a scan rate of 0.5o/min over the range of 2h ¼ 1–10o . X-ray
analyses were performed at room temperature on as molded specimens. Both for the clays and
the nanocomposites, XRD was recorded using, operated at 40Kv and 30mA.The basal spacing
or d001reflection of the samples was calculated from Bragg’s equation by monitoring the
674 V. Selvakumar, K. Palanikumar, K. Palanivelu Vol.9, No.8
diffraction angle 2h from 2–10o at a scanning rate of 0.5o/min.
2.3.4 Transmission Electron Microscopy (TEM)
Thin sections for transmission electron microscopy (TEM) analysis were microtomed from the
central and skin regions of an Izod bar; the cuts were made parallel and perpendicular to
the flow direction 3–4 cm away from the far end of a 13 cm Izod bar and halfway between the
top and bottom surfaces of the bar.Ultra thin sections ranging from 70 to 100 nm in thickness
were cryogenically cut with a diamond knife using Reichert-Jung FC4E ultra cryomicrotome
cutter (Mager Scientific,Inc.,Dexter,MI) at temperature of 100oC.Sections were collected on 300
mesh copper TEM grids and subsequently dried with filter paper. The sections were examined
by TEM using a JEOL-EM-2000 FX Electron Microscope with 200 kV accelerating voltage.
3.1. Mechanical Properties
The mechanical properties of PP/Na+-MMT nanocomposites are summarized in Table 1.
Table 1.Mechanical properties of PP and its nanocomposites
Sample Tensile
PP 31.95 3158.54 35.58 1137.60 1696.67 82.6
PP/1%MMT 32.56 3354.39 36.76 1269.96 1750.93 87.26
PP/3%MMT 34.68 3774.66 38.12 1364.96 1988.93 89
PP/5%MMT 36.34 4240.64 40.25 1653.98 1971.00 87.36
PP/7%MMT 30.85 3033.34 36.54 1344.19 1944.47 87.28
Both the strength and modulus of the nanocomposites increase in tandem up to 5 wt. % clay
concentrations, thereafter significant degradation occurs.The average tensile strength and
flexural strength for 5 wt. % structures is approximately 14% and 13% respectively greater than
neat polypropylene. These values peak at 36.34 MPa tensile strength and 40.25 MPa for flexural
strength as per Table 1. The average tensile modulus and flexural modulus for 5 wt. % structures
is approximately 34% and 45% respectively greater than neat polypropylene. These values peak
at 4240.64MPa tensile modulus and 1653.98 MPa for flexural modulus as per Table 1. A sharp
decrease subsequently occurs with 7 wt. % specimens dropping to 30.85 MPa and 36.54 MPa for
Vol.9, No.8 Studies on Mechanical Characterization 675
the strength respectively shown is Figure 1. A similar trend for the modulus of the
nanocomposites is shown in Figure 2.
Fig 1: Strength of PP/MMT nanocomposites
Fig 2: Modulus of PP/MMT nanocomposites
Te nsile strength
Flexural strength
Strength (MPa)
% of Nanoclay
Flexural m odulus
Tensile modulus
Modulus (MPa)
% of Nanoclay
676 V. Selvakumar, K. Palanikumar, K. Palanivelu Vol.9, No.8
As a result, the strength and the modulus of the PP nanocomposites increase with MMT
loadings. A fraction of intercalated structure decreases with increasing nanoclay content. At
higher clay concentrations, aggregation of the nanoclays may occur decreasing the strength. The
increased mechanical properties at low concentration of nanoclays may be due to the uniformly
dispersed MMT tactoid and intercalated structures of the nanocomposites shown in TEM and X-
RD analysis. The increase in both hardness and impact strength for a nanoclay loading up to 3
wt. % is by the formation of intercalated and exfoliated nanoclay structures shown in the TEM
analysis. Figure 3 shows an increase in both the impact strength as well as the Vickers hardness
number up to 3 wt. % loading.
Fig 3: Impact strength and Rockwell hardness number of PP/MMT nanocomposites
3.2 X-Ray Diffraction
Figure 4 shows the X-ray diffraction patterns of MMT silicate. As shown in this figure, the peak
maximum shifts from 2θ = 6.98°, corresponding to basal spacing d001 of 12.6 Ao.The XRD
pattern of polypropylene infused with 1, 3, 5, and 7 wt. % MMT is shown in Figure 5. During
mixing, the polymer infuses and intercalates between the intergallery spacing of layered silicates
and makes the clay layers to move apart. The disappearance of peak indicates the separation of
clay layers and the formation of nanocomposites at 1 wt. % to 5 wt. % clay loadings. This is
confirmed by TEM studies indicating that predominantly exfoliated nanoclay structures have
formed in 1 wt. % and 3 wt. % nanoclay polypropylene specimens. Polypropylene infused with 5
wt. % nanoclays has more intercalated clay structures.
Im pact strength
Hardness num ber
% of N anoclay
Impact strength (J/m)
Rockwell hardness numb er (RHN)
Vol.9, No.8 Studies on Mechanical Characterization 677
Fig 4: X-RD pattern of Na-MMT clay
Fig 5: TEM image of PP/MMT nanocomposites with different MMT contents
(a)1%,(b)3%,(c)5% and (d)7%
3.3 Transmission Electron Microscopy (TEM) Analysis
Figure 6 (a) shows an image of a polypropylene structure infused with 1 wt. % of nanoclays. In
this image various structures can be seen. Darker stacked arrangement of lines and random lines
are visible against the lighter background. This suggests that mixed morphologies of exfoliated
and intercalated nanostructures have been synthesised. Even at this low weight loading of clay,
the modulus and strength of polypropylene have increased.Figure 6(b) shows an image of a
section of polypropylene infused with 1 wt. % of nanoclays.
678 V. Selvakumar, K. Palanikumar, K. Palanivelu Vol.9, No.8
Fig 6: TEM image of PP/MMT nanocomposites with different MMT contents
(a) 1%,(b) 3%,(c) 5% and (d) 7%
Three types of structures may again be seen. Stacked clay tactoids representative of intercalated
morphology, random clay platelets representative of exfoliated morphology and large dark
structures occurring amongst some of the clay tactoids are visible. Another finding is that the
mixed morphologies of intercalated and exfoliated structures are in close proximity of each
other. The closeness of these structures suggests that the polymer matrix is becoming
increasingly reinforced. This phenomenon explains the increasing hardness and impact strength
properties. Figure 6(c) shows an image generated by exploring a section of polypropylene
infused with 2 wt. % of nanoclays. The image displays darker lines against the lighter polymer
background section. The lines show an ordered stacked arrangement with clearly visible
interspaces. This suggests the formation of a tactoid nanoclay structure. The structure consists of
1 nm thick platelets arranged in a stacked formation where the interlayer space between each
platelet is occupied by chains of polymer molecules. Their nanocomposite structures showed
increases in strength and modulus at 5 wt % loadings of nanoclay. This arrangement of clay
platelets can be associated with that of an intercalated nanocomposite structure. At this clay
loading, maximum enhancement in properties occurred. Figure 6(d) shows an image of
polypropylene infused with 7 wt. % of nanoclays. This image represents sites that frequently
occurred across this section. Three distinct morphologies are clearly visible. Region A shows
areas where arrangements of stacked lines are prevalent. This indicates presence of intercalated
morphology. Region B shows areas where darker lines or patches are noticeable. These sites
show little penetration by the polymer into the interlayer resulting in agglomerated clay sites.
Region C shows the formation of a micro void due to aeration or where micron sized clay
agglomerate fell away from the polymer structure during microtome sectioning. Formation of
Vol.9, No.8 Studies on Mechanical Characterization 679
micro voids and clay agglomerates may be the cause for degraded mechanical properties at this
clay loading.
3.4 Scanning Electron Microscopy (SEM) Analysis
Figure 7 shows the impact fractured surface of PP and its nanocomposites.From Figure 7(a) the
surface shows a fairly homogenous polymer with minimal high stress zones. The high stress
zones depicted by the impact fracured hole size is large and number of holes are less.In Figure
7(b, c and d) arrows indicate there is a distinct change in the fractured surface when compared to
PP sample.The impact fractured hole size is small and number of holes are more. These
structures show several high stress zones which indicate the increased reinforcement of the
polymer matrix and good dispersion of the nanoclay in nanocomposites. It is finer and more
particulate in nature.
Fig 7: SEM image of PP and PP/MMT nanocomposites with different MMT contents
(a) PP,(b) 1%,(c) 3%,(d) 5% and (e) 7%
680 V. Selvakumar, K. Palanikumar, K. Palanivelu Vol.9, No.8
This high density grain boundary shows a strengthened matrix. These structural phenomena may
be linked to the increase in the strength; modulus, impact strength and hardness of the
nanocomposites as shown in Table1. In Figure 7(e) two distinct structures are seen. One is
particulate in nature and the other is a large agglomerated structure. The large agglomeration
may be a micron sized clay tactoid caused by poor nanoclay dispersion. The fractured surface
shows areas where no dispersion occurred and agglomerated clay sites. These structures impact
on the interfacial interactions of the polymer molecules causing poor interfacial adhesion leading
to a reduction in mechanical properties and embrittlement in the PP nanocomposite structure.
The mechanical properties of strength and modulus of the nanocomposites increases with
addition of 5 wt% of nanoclay and impact strength and hardness of the nanocomposites increses
with addition of 3 wt% of nanoclay. In the XRD analysis of the pristine PP and PP
nanocomposites, the disappearance of the peak in the PP nanocomposites indicates the separation
of clay layers and the formation of intercalated or exfoliated structures. This is confirmed in the
TEM studies. TEM images at 1wt. %, 3wt. % and 5%clay loadings show evidence of intercalated
and exfoliated structures. These structures may be responsible for the increase in properties at
low weight loadings. SEM studies of the tensile fractured surfaces at low clay loadings shows
that the morphology changes to homogenous and fibrillated. This indicates there is good
interfacial adhesion between the nanoclays and polymer which may be responsible for the
improvement in mechanical properties.
The author would like to thank the Central Instiute of Plastics Engineering and Technology,
Chennai, India for manufacturing the nanocomposites.The author also would like to thank
Sathyabama University and department of mechanical and production engineering, chennai-119.
[1]. Handbook of Polypropylene and Polypropylene Composites, second
Edition, Marcel dekker.Inc.(2003).
[2]. J.I.Weon, H.-J.Sue: Journal of Material Science 41 (2006) 2291-2300.
[3]. Gu-Su Jang, Won-Jei Cho, Chang-Sik Ha: Journal of Polymer Science: part B: Polymer
Physics 39 (2001) 1001-1016.
[4]. Roya Khalil, Andrew George chryss: Journal of Material Science 42 (2007) 10219-10227.
[5]. K.Kanny.P.Jawahar, V.K.Moodley: Journal of Material Science 43(2008) 7230-7238.
[6]. Jeong Hyun Park et al: Journal of Physics and Chemistry of Solids 69 (2008), 1375-1378.
[7]. M.Garcia et al: Rev.Adv.Mater.Sci. 6 (2004) 169-175.
Vol.9, No.8 Studies on Mechanical Characterization 681
[8]. Xiaohui Liu, Qiuju Wu: Polymer 42 (2001) 10013-10019
[9]. Ludovic cauvin et al: C.R.Mecanique 335 (2007) 702-707.
[10]. D.Garcia-lopez et al: Polymer Bulletin, 59 (2007) 667-676.
[11]. T.C.Chung: Journal of Organ Metallic Chemistry 690 (2005) 6292-6199.
[12]. Laszlo Szazdi et al: Polymer, 46(2005) 8001-8010.