Materials Sciences and Applications, 2010, 1, 301-309
doi:10.4236/msa.2010.15044 Published Online November 2010 (http://www.SciRP.org/journal/msa)
301
Fracture Toughness Studies of Polypropylene
- Clay Nanocomposites and Glass Fibre
Reinfoerced Polypropylene Composites
A. Ramsaroop, K. Kanny, T. P. Mohan
Composites Research Group, Department of Mechanical Engineering, Durban University of Technology, Durban, South Africa.
Received July 7th, 2010; revised November 4th, 2010; accepted November 5th, 2010.
ABSTRACT
In this paper, a comparative study on the fracture toughness of woven glass fibre reinforced polypropylene, chopped
glass fibre reinforced polypropylene and nanoclay filled polypropylene composites is presented. Nanoclays (Cloisite
15A) of 1 wt. % to 5 wt. % were filled in polypropylene (PP) matrix and they were subjected to fracture toughness stud-
ies. The specimen with 5 wt. % nanoclay showed 1.75 times and 3 times improvement in critical stress intensity factor
(KIC) and strain energy release rate (GIC), respectively, over virgin PP. On the other hand, 3 wt. % nanoclay PP
composites showed superior crack containment properties. These structural changes of composite specimens were ex-
amined using Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD) methods. It showed that exfoliated
nanocomposite structures were formed up to 3 wt. % nanoclay, whereas, intercalated nanocomposite structures formed
above 3 wt. % nanoclay in the PP matrix. Furthermore, the woven fibre reinforced PP composites demonstrated supe-
rior crack resistant properties than that of clay filled nanocomposites and chopped fibre PP composites. However, KIC
and GIC values for woven fibre composites were lesser than that of chopped fibre composites. Moreover, KIC and GIC
values for both nanoclay filled PP composites and woven fibre composites are comparable even though the clay filled
PP demonstrated catastrophic failure. Also, the crack propagation rate of PP-nanoclay composites is comparable to
that of chopped fibre composites.
Keywords: Fibre Reinforced Composites, Polymer Clay Nanocomposites, Stress Intensity Factor, Crack Growth, Crack
Initiation, Fracture Toughness
1. Introduction
Fibre reinforced plastics (FRP) are commonly used in
aerospace, automotive and other engineering applica-
tions mainly because of their high strength-to-weight
ratio, high stiffness, good resistance to fatigue, and
corrosion resistance. Reinforced fibres are usually
added in the form of continuous or chopped fibres in a
polymer matrix. Each type of these reinforcement fi-
bres has their benefits and limitations in applications.
A short fibre reinforced composites can have better
processing properties and can be mouldable into com-
plex shaped components. Long fibre reinforced com-
posites, on the other hand, provide enhanced strength
and stiffness properties as per the desired directions.
Various types of synthetic and natural fibres reinforced
plastics are presently studied in literature, namely,
glass fibre, carbon fibre, boron, alumina, oxide/carbide
and sisal/jute based fibres in a polymer matrix [1-5].
In addition to these fibre reinforced composites, parti-
cle reinforced polymer composites are also widely inves-
tigated. In these particle filled composites, various types
of nano- and micro-scale particles are used. Nanoparticle
reinforced polymer composites gained special attention
due to their superior and improved properties when
compared to their corresponding micro-scale particles.
Nanoclays, carbon nanotubes and alumina/oxide based
particles are widely used as nanoparticle reinforcements
in polymer matrices [6-8]. Polymer-Clay Nanocompo-
sites (PCN) is a relatively new area of research in particle
filled composites, and consists of nanoclay as the rein-
forcement and a polymer serves as the matrix material.
Smectic clays, particularly montmorillonite (MMT)
minerals, serve as good nanoclay fillers owing to their
ease of dispersion in the organic matrix. MMT consists
of nanosized layers of alumina/silicate sheets where the

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
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302
alumina sheets are sandwiched between two silicate
sheets thus causing a net negative charge. This charge is
counterbalanced by exchangeable metal ions present in
the surface of the layers. MMT clays are generally hy-
drophilic in nature and can be converted into organo-
philic by replacing the exchangeable metal ions with
cations such as alkylamonium cations. The addition of
small amounts of clay (~3 to 5 wt.%) in an organic
polymer matrix leads to improved mechanical, thermal
and barrier properties.
The addition of nanoclays in a polymer matrix may
result in the formation of two types of nanocomposite
structures, namely, intercalated and exfoliated nanos-
tructures. In an intercalated structure, the host polymer
matrix enters into the interlayer spacing of the nanoclay
and increases the interlayer spacing but maintains the
parallel arrangement of the nanolayers of clay in the ma-
trix. If the nanolayers of clays are randomly dispersed in
the matrix, then the structure is called an exfoliated struc-
ture. In practice, exfoliated structures provide enhanced
and improved properties due to their excellent disper-
sions and improved aspect ratio. If MMT clay is not sub-
jected to any surface treatments, then when it is added
into a polymer matrix, no nanolayer dispersions of the
clay takes place and it acts as a micro-scale filler particle
with meagre improvements in properties [9-12].
Polypropylene-clay based nanocomposites are of tre-
mendous research interest due to the improved properties
with low clay content as well as the clay serving as a
valuable cost effective additive. In general, 0 to 5 wt.%
of organically treated clay is added in PP polymer matrix.
The addition of the nanoclay in the PP matrix increases
the thermal stability in air medium, increases physical
properties (dimensional stability), improves flame retar-
dant properties (increased thermal-oxidative stability and
reduced Heat Release Rate), improves mechanical prop-
erties, fracture properties and gas barrier properties. Sev-
eral studies were conducted with various types of organo
clays, clay concentrations and compatabilizers [13-18].
Research into fracture toughness of thermoplastic na-
nocomposites is not well known and relatively very few
studies are available. Siddiqui et al. [19] reported an in-
crease of 26% in flexural modulus and 60% increase in
fracture toughness with the addition of 3 wt.% nanoclays.
Cotterell et al. [20] have carried out fracture and fracture
toughness studies on spherical and platelet like nano
structure filled semi-crystalline polymers. They observed
that fracture toughness is a complex study in platelet
structure which depends on the aspect ratio of clay plate-
lets, orientation and distribution as well as processing
effect. Lingyu Sun et al. [21] have completed a review
on energy absorption mechanisms of various nanoparti-
cle filled polymer composite systems. The influence of
some key control parameters such as shape and size of
nano-fillers, mechanical properties of nano-fillers and
matrix materials, interfacial adhesion, interphase charac-
teristics, as well as the volume fraction and dispersion of
fillers in the matrix were studied. They observed that for
a weak bonding, the nano-filler will dissipate more en-
ergy by pull-out or debonding. For strong bonding, the
nano-filler will dissipate energy by tension up to fracture
of the filler. Weiping Liu et al. [22] have carried out
fracture toughness studies in epoxy-clay based nano-
composites. Fracture toughness of this epoxy system has
been greatly enhanced with the addition of nanoclays.
With the addition of only 4.5 phr of clay, the strain en-
ergy release rate of the epoxy was increased 5.8 times
from the original value. Martin N. Bureau et al. [23] have
studied fracture analysis for PP based polymer at 2 wt.%
nanoclay content with various types of coupling agents.
They reported that the toughness improvements were
attributed to higher voiding stresses and improved matrix
resistance attributed to finer, more oriented clay nanopar-
ticles in the matrix polymer. S.C. Tjong and S.P. Bao [24]
showed a two-fold increase of impact fracture toughness
of the HDPE/2%Org-MMT and HDPE/4%Org-MMT
nanocomposites by adding 10%SEBS-g-MA. Yuan Xu
and Suong Van Hoa [25] observed an 85% increase in
interlaminar fracture toughness of Carbon Fibre Rein-
forced Epoxy Nanoclay composites with the introduction
of 4 phr nanoclay in epoxy.
Crack initiation and propagation in composites occurs
as a result of the three basic fracture modes; which in-
clude the opening mode (Mode I), the sliding shear mode
(Mode II), and the scissoring shear mode (Mode III). In
order to predict crack initiation and propagation, a mate-
rial's fracture toughness needs to be determined. This is
usually expressed in terms of the critical stress intensity
factor, KIC, KIIC and KIIIC. In this study, Mode I type
testing was carried out for PP-nanoclay composite series.
From the literature it was found that structural changes
taking place in polymer clay nanocomposites due to va-
rying nanoclay content and their resultant characteristic
features of fracture toughness and the analysis of their
merits and demerits are relatively scarce. Moreover this
new PCN material class will be encountered in various
structural loading in practical applications, and hence
studying their fracture toughness properties is essential.
The objective of this paper is to study the Mode I fracture
type of nanoclay filled PP composites with various con-
centrations (0 to 5 wt.%). The effect of nanoclay on
Mode I type fracture toughness is studied and the char-
acteristics of failure is examined by studying the struc-
tural changes taking place in the nanocomposites by
TEM and XRD techniques. Furthermore, the fracture
toughness behaviour of these nanocomposites is com-

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
Composites
303
pared with woven glass fibre and chopped glass fibre
reinforced PP composites. Their merits and demerits of
KIC characteristics are discussed in this work.
2. Experimental Studies
2.1. Materials
Cloisite 15A clay was obtained from Southern Clay
Products Inc, USA. This is natural montmorillonite
clay that was organically treated with a quaternary
ammonium salt. The organic modifiers used were di-
methyl, dihydrogenatedtallow, quaternary ammonium
salt. Polypropylene pellets (melting point 168°C) were
procured from Chempro, South Africa. Woven rovings
of S2 glass fibre and chopped S2 glass fibre (length 15
mm and diameter of 1 m) were purchased from Vetro-
tex, S.A.
2.2. Composite Preparation
The nanocomposite panel was manufactured using a melt
blending technique. In this technique, the polypropylene
pellets and the nanoclay of desired weight concentration
(1, 2, 3, 4 and 5 wt. %) were combined in a REIFFEN-
HAEUSER screw extruder. The extruder has a 40 mm
diameter single rotating screw with a length / diameter
ratio (L/D) of 24 and driven by a 7.5 kW motor. Three
heating zones along the length of the screw were set up
to gradually heat the pellet/clay mixture. The tempera-
tures in these zones were as follows: Zone 1 (pellet load-
ing end) was set at 170°C, Zone 2 (centre region of screw)
was at 190°C, and Zone 3 (extrusion end) was main-
tained at 210°C. This temperature gradient setup was
created to avoid thermal shock.
For the preparation of PP-glass fibre (chopped and
woven) composites panel, the fibre weight fraction for
both these composite structures was 0.3. In the woven
fibre polypropylene composites, 4 layers of woven S2
glass fibre were used as the reinforcement. The panels
were manufactured using a compression moulding tech-
nique. The mould temperature was set at 170ºC and
mould pressure was maintained at 20 kN. The chopped
fibre composites were also produced using the compres-
sion moulding technique at 170°C and 20 kN mould
pressure.
2.3. Characterization
The structure of the nanocomposites was studied by
using XRD and TEM studies. A Philips PW1050 dif-
fractometer was used to obtain the X–Ray diffraction
patterns using CuKα lines (λ = 1.5406 Å). The diffrac-
trograms were scanned from 2.5˚ to 12° (2θ) in steps of
0.02˚ using a scanning rate of 0.5˚/min. X-ray diffrac-
trograms were taken on Cloisite 15A clay particles and
on polypropylene clay nanocomposites. Microscopic
investigation of selected nanocomposite specimens at
the various weight compositions were conducted using
a Philips CM120 BioTWIN transmission electron mi-
croscope with an operation voltage of 20 to 120 kV.
The specimens were microtomed using a LKB/Wallac
Type 8801 Ultramicrotome with Ultratome III 8802A
Control Unit. Ultra thin transverse sections, approxi-
mately 80-100 nm in thickness were sliced at room
temperature using a diamond coated blade. The sec-
tions were supported by a 100 copper mesh grid sput-
ter-coated with a 3 nm thick carbon layer.
2.4. Testing
Single edge notch beam (SENB) tests were performed
using Lloyds tensile testing machine fitted with a 20kN
load cell and at a cross-head speed of 1 mm/min. The
ASTM D 5045 testing standard was used for fracture
toughness studies. The dimension of the test specimen
for plane strain conditions is shown in Figure 1. The
length (S) of the test specimen is 150 mm with an ini-
tial crack length of 5 mm. Five specimens of each
composite type were tested and the average values
were reported. Also, the standard deviation of test val-
ues within 3% is only considered for average value
measurements. Mode I stress intensity factor, KI, of the
composite specimens was determined as per equation
(1), where f(a/W) is a correction factor in accordance
standard. The fracture surface was examined using
JEOL JSM 840A scanning electron microscope (SEM).
I
Pa
Kf
W
BW



(1)
5
25
S
Figure 1. Dimensions of SENB specimen that satisfy
plane strain conditions.

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
Composites
304
3. Results and Discussions
3.1. Structure and Morphology of PP-Clay
Nanocomposites
The structure and morphology of nanoclay filled PP
composites was examined using XRD and TEM methods.
Figure 2 shows the XRD patterns of PP-clay series. The
Cloisite 15A nanoclay shows a diffraction peak of 2θ at
3.3° and corresponds to an interlayer spacing of nanoclay
(d-spacing) of 26.75Å (calculated from Bragg’s diffrac-
tion law of 2dSinθ = nλ). From 1 wt.% to 3 wt.% of
nanoclay in the PP polymer, there is an absence of a dif-
fraction peak, and this suggests that the matrix polymer
has entered into the interlayer spacing of the nanoclay
and increased the original nanoclay spacing above the
level of 7.5 nm (in which Bragg’s law cannot satisfy), or
the nanolayers of clays could have randomly dispersed in
the PP polymer. Hence, it can be concluded that the
nanoclays in the PP polymer from 1 to 3 wt.% formed an
ordered exfoliated structure, or a randomly dispersed
clay exfoliated structure. Above 3 wt.% nanoclay on-
wards, there exists a diffraction peak in the composite
specimens. In PP with 4 wt.% nanoclay, the nanoclay
diffraction peak shifted to a lower 2θ value of 2.68° and
this corresponds to an interlayer of clay of 33.5Å. At 5
wt.% nanoclay, the diffraction peak occurs at a 2θ value
of 2.9° and corresponds to the interlayer spacing of nano-
clays of 30.5Å. XRD results for 4 wt.% and 5 wt.%
nanoclay shows that the composite formed intercalated
nanocomposite structures.
To further study the dispersion of nanoclays in the PP
polymer, TEM morphology of the samples was examined.
Figure 3 shows the bright field TEM images of PP-nano-
0.00 3.00 6.00 9.0012.00
2 theta
Intensity (arbitrary unit)
(e)
(b)
(c)
(d)
(f)
(a)
Figure 2. XRD patterns of (a) Cloisite 15A, PP with (b) 5
wt.% clay, (c) 4 wt.% clay, (d) 3 wt.% clay, (e) 2 wt.% clay
and (f) 1 wt.% clay.
(a)
(b)
(c)
(d)

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
Composites
305
(e)
Figure 3. TEM pictures of PP with (a) 1 wt.%, (b) 2 wt.%, (c)
3 wt.%, (d) 4 wt.% and (e) 5 wt.% nanoclay.
clay composite series. The bright region of the picture is
the matrix phase and the dark region is the particle phase.
From 1 wt.% to 3 wt. % nanoclay in the PP polymer ma-
trix, the nanolayers of clays were well separated and dis-
persed randomly in the polymer matrix. This shows the
formation of an exfoliated structure. In 4 wt.% and 5 wt.
% nanoclay in the PP matrix, the nanolayers of clays
were arranged parallel to each other in the PP matrix and
this shows the formation of intercalated nanocomposite
structures. The TEM pictures support the XRD results for
the nanocomposite series.
3.2. Fracture Toughness
3.2.1. Fracture Toughness of Nanocomposite
Structures
SENB tests were performed on virgin and nano-rein-
forced polypropylene structures infused with 1, 2, 3, 4
and 5 wt.% of Cloisite 15A. All tests were conducted
under Mode I test conditions and the critical stress in-
tensity factor (KIC), strain energy release rate (GIC)
and other related properties for each nanoclay infused
structure was determined and is shown in Table 1. The
table shows that the addition of nanoclays in the PP
polymer continuously increased the failure load, KIC
and GIC values with increasing clay content. Maxi-
mum improvement of KIC and GIC of 1.75 times and
2.66 times, respectively, was observed in the 5 wt.%
nanoclay filled PP composite when compared with the
virgin PP polymer. Even though the addition of nano-
clay continuously increased the KIC and GIC values,
the rate of increase was slower at higher clay concen-
trations (3 wt.% and above). This slower rate increase
was possibly due to the formation of intercalated ag-
gregates of nanoclay particles in the PP matrix. The
exfoliated structures contributed more toughness prop-
erties towards the loading directions due to the increase
of the net aspect ratio; whereas in the intercalated
Table 1. Average Failure Load, Critical Stress Intensity
Factor and Strain Energy Release Rate values for clay
weight loadings of 0, 1, 2, 3, 4 and 5%.
Material Failure
Load (N)
Critical Stress
Intensity Factor KIC
(MPa √m)
Strain energy
release rate GIC
(kJ/m2)
PP 275 2.00 16.0
PP + 1 wt.% clay373 2.75 24.3
PP + 2 wt.% clay480 3.17 29.7
PP + 3 wt.% clay560 4.09 36.3
PP + 4 wt.% clay708 4.56 42.1
PP + 5 wt.% clay768 5.49 48.3
structure, the net and effective aspect ratio was reduced
and hence contributed less fracture toughness proper-
ties towards the loading direction.
Figure 4 shows the crack propagation versus time of
the nanocomposite series. PP shows a propagation of
cracks at a relatively shorter time period than nanoclay
filled composite specimens. Time for crack propagation
increased as nanoclay content increases in the PP poly-
mer up to 3 wt.%, and above 3 wt.% nanoclay content
the crack propagation time reduced. Even though KIC of
the composite series continuously increased as clay con-
tent increased in the matrix, the nanoclay concentrations
of above 3 wt.% contributed less towards crack confine-
ment due to their structural features. The crack propaga-
tion rate for 3%, 4% and 5% nanoclay composites were
3.53 mm/min, 3.83 mm/min and 3.58 mm/min, respec-
tively. Crack propagation rate for virgin PP was 8.62
mm/min. To understand this behaviour, the fractured
surface of these composite specimens was investigated
using the SEM process.
0. 0
5. 0
10.0
15.0
20.0
25.0
0100 200300 400500
time, s
crack length, mm
PP
PP + 1% clay
PP + 2% clay
PP + 3% clay
PP + 4% clay
PP + 5% clay
Figure 4. Crack Length versus Time responses for PP and
PP-clay composite series.

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
Composites
306
Figure 5 shows the SEM images of the fractured sur-
face of composite specimens. Pure PP and PP with 5 wt.
% nanoclay shows a smooth fracture surface. This failure
mechanism is characteristic of a brittle material suggest-
ing that the material had undergone a reduced deforma-
tion under failure. However, the SEM image of PP with 3
wt.% nanoclay does not show any brittle type failure
mechanism, but rather show extended and elongated de-
formations before failure. This deformation mechanism
possibly induced more time for crack to failure. Fur-
thermore, the exfoliated nanocomposite structure (up to 3
(
a
)
(
b
)
(
c
)
Figure 5. SEM of fractured surface of (a) PP, (b) PP with 5
wt.% nanoclay and (c) PP with 3 wt. % nanoclay.
wt. % clay) forced cracks to undertake torturous paths
whereas the intercalated structure provided brittle-like
failure. This result further suggests that there existed a
structural change in the nanocomposite as clay content
increased in the PP polymer matrix. Moreover, Ho et al.
[26] and Moodley et al. [27] found that the nanoclay ad-
dition improved hardness values, and hence, in our re-
sults, the improved hardness values also could have re-
sulted in the improved fracture toughness properties.
3.2.2. Fracture Toughness – Fibre-Reinforced
Polypropylene Composites
The two fibre-reinforced composite types (Polypro-
pylene/Chopped S2 Glass and Polypropylene/Woven
S2 Glass) were tested under Mode I conditions via the
SENB test to establish the crack propagation rates, KIC
and GIC. Figure 6 illustrates the specimen loading
with regards to laminate orientation in the woven fibre
composites. The loading was perpendicular to the plane
of the fibre layers.
The load versus stress intensity factor (KI) of the
composite series is shown in Figure 7. It shows that the
chopped fibre specimen exhibited a superior load versus
KIC response than the other composite materials. The
chopped fibre coupon achieved a critical stress intensity
factor (KIC) (initiation) of 7.92 MPa√m at a load of 960
N, while a KIC (initiation) value of 5.78 MPa√m at a
load of 700 N was obtained in the woven fibre case. Also,
the strain energy release rate is superior for chopped fibre
reinforced PP composites as shown in Table 2.
The poor failure loading performance of the woven fi-
bre composite was due to a “kinking” phenomenon as
shown in Figure 8. The “kinking” observed on the top-
edge of the test coupon in Figure 8(a) occurred directly
beneath the loading point and directly above the crack
front, as shown in Figure 8(b). This occurrence was un-
noticeable in the chopped fibre specimens and also in the
nanoclay filled PP composites. This caused significant
Figure 6. Illustration showing specimen loading with re-
gards to laminate orientation.

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
Composites
307
Table 2. Fracture toughness properties of fibre reinforced
composites.
Material Failure
Load (N)
Critical Stress
Intensity Factor
KIC (MPa √m)
Strain energy
release rate GIC
(kJ/m2)
PP +
chopped GF 960 7.92 181
PP + woven
GF 700 5.78 51
0
100
200
300
400
500
600
700
800
900
1000
0.0 2.0 4.0 6.0 8.010.012.
0
Stress Intensity Factor (MPa √m)
Load (N)
PP
PP + 5% clay
PP + woven GF
PP + chopped GF
Figure 7. Load versus Stress Intensity Factor responses for
Polypropylene / Chopped S2 Glass and Polypropylene /
Woven S2 Glass.
(a)
(b)
Figure 8. Photographs of Polypropylene/Woven S2 Glass
specimen after SENB test showing “kinking” phenomenon
from (a) top edge and (b) front surface.
local stresses to be induced at the loading point. Al-
though these stresses were not greater than the breaking
stress of the fibres, it was greater than the matrix thresh-
old. Thus the matrix succumbed to the induced localised
stresses and was compressed by the test fixture, with no
evidence of fibre damage. Upon further loading the stress
concentration at the crack front increased to the point
where it was large enough to initiate crack growth. Due
to the test fixture compression of the coupon, the applied
load was not directly above the crack front anymore but
applied to adjacent areas as depicted by the arrows in
Figure 8(b).
A comparison of crack propagation rates was con-
ducted to determine which composite type had superior
crack containment properties. Figure 9 shows the rela-
tionship between Crack Length versus Time for the
chopped and woven fibre-reinforced, and the nanoclay
filled polypropylene specimens. For all specimens, the
crack initiated directly beneath the loading point, which
is typical of a Mode I crack. The chopped fibre compos-
ites and nanoclay filled composites experienced rapid
crack growth compared to the slow crack growth of the
woven fibre case. The crack propagation rate for the
chopped fibre and 5wt.% nanoclay filled samples were
3.37 mm/min and 3.58 mm/min, respectively, while the
woven fibre case had a crack propagation rate of 1.08
mm/min. The woven fibre specimen therefore demon-
strated superior crack containment properties as it had a
lower crack propagation rate. This containment may be-
attributed to the structured fibre layout of the Polypro-
pylene/Woven S2 Glass composite. Even though kinking
occurred in woven fibre composites, they demonstrated
superior crack arresting properties. Moreover, another
0.0
5.0
10. 0
15. 0
20. 0
25. 0
0200400 600 80010001200
time, s
crack length, mm
PP + chopped GF
PP + woven GF
PP + 3% nanoclay
PP + 5% nanoclay
Figure 9. Crack length versus Time responses for PP/
Chopped fibre, PP/Woven fibre and nanoclay (3% and 5
wt.%) filled PP composites.

Fracture Toughness Studies of Polypropylene - Clay Nanocomposites and Glass Fibre Reinfoerced Polypropylene
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308
interesting observation was that the crack propagation
rate was comparable in the nanoclay filled composites
and the chopped fibre composites.
Although the nano-reinforced polypropylene showed
poor crack containment, the 5 wt.% specimen was able to
bear a higher load than the woven fibre specimen. This
nanocomposite had a critical stress intensity factor of
5.49 MPa√m, and was 5% less than the woven fibre case
which had a critical stress intensity factor of 5.78
MPa√m. This suggests that the 5 wt.% nanocomposite
has a similar ability to resist crack initiation as a woven
fibre structure with 6 times the reinforcement weight.
This characteristic should be considered more important,
because a material with a high resistance to crack initia-
tion can sustain higher loads. Such a material may be
used in applications where high tensile/flexural loading
conditions are prominent.
4. Conclusions
PP polymer filled with nanoclay at various concentra-
tions (1, 2, 3, 4 and 5 wt.%) was prepared by using
melt blending extrusion method. It was observed that
up to 3 wt.% nanoclay in the PP matrix, an exfoliated
structure was formed and further addition of nanoclay
formed an intercalated structure. Even though the KIC
and GIC values continuously increased as clay content
increased in the nanocomposites, the rate of increase
was reduced at higher clay content due to intercalated
structures that affected the net aspect ratio of the clay
platelets.
In the second stage, KIC and GIC were measured for
chopped fibre and woven fibre reinforced PP composites.
The results showed that the chopped fibres demonstrated
an enhanced load bearing capacity and higher fracture
toughness values. However, in terms of crack arresting,
the woven fibre reinforced PP composites showed supe-
rior improvement over the chopped fibre composites. On
comparing all three types of composites, the nanocompo-
site with 5 wt.% clay loading was able to bear a higher
load than the woven fibre case, and also demonstrated a
comparable critical stress intensity factor with that of the
woven glass fibre-reinforced composites. Moreover, the
crack arresting properties can be comparable for nano-
composites (3 wt.% and 5 wt.% nanoclay PP composites)
and the chopped fibre composites. One of the important
points that were noticed was that the weight of fibre re-
inforcement was 6 times more than the nanoclay weight
in the nanocomposite.
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