This document summarizes a study on the effect of crystallite size of zinc oxide (ZnO) filler on the properties of polypropylene (PP)/ZnO nanocomposites. ZnO nanoparticles were prepared using two different methods, resulting in nanoparticles with crystallite sizes of 13.4 nm (NZO) and 29.2 nm (CZO). PP/ZnO composites containing 0-5% ZnO were produced by melt mixing. Composites with NZO exhibited higher mechanical properties, dynamic properties, and thermal stability than those with CZO, due to the smaller crystallite size and more uniform dispersion of NZO in PP. Transparency of the composites improved with decreasing ZnO crystallite

1. Composites: Part B 69 (2015) 145–153
Contents lists available at ScienceDirect
Composites:Part B
journal homepage: w ww . elsevier . com/locate/compositesb
Effect of crystallite size of zinc oxide on the mechanical, thermal and
flow properties of polypropylene/zinc oxide nanocomposites
Saisy K. Esthappan
a,b
, Ajalesh B. Nair
a
, Rani Joseph
a,⇑
a
Department of Polymer Science and Rubber Technology, Cochin University of Science & Technology, Cochin 682 022,
India
b
Department of Chemistry, Mangalam College of Engineering, Ettumanoor 686 631, India
a r t i c l e i n f o
Article history:
Receiv ed 24 September 2012
Receiv ed in revised form 9 June 2013
Accepted 12 August 2013
Av ailable online 20 August 2013
Key words:
A. Nano-structures
B. Mechanical properties
B. Thermal properties
E. Compression moulding
a b s t r a c t
ZnO nanoparticles were prepared using zinc chloride and sodium hy droxide in chitosan medium. Pre-pared ZnO
(NZO) and commercial ZnO (CZO) was characterized by scanning electron microscopic and X-ray diff raction
studies. PP/ZnO nanocomposites were prepared using 0–5 wt% of zinc oxide by melt mixing. It was then
compression moulded into f ilms. Transparency of the composite films were improv ed by reducing the crystallite
size of ZnO. Melt f low index studies rev ealed that NZO increased the f low char-acteristics of PP while CZO
decreased. X-ray diff raction studies indicated a-f orm of isotactic poly propy l-ene. An increase in mechanical
properties, dy namic mechanical properties and thermal stability of the composites were observ ed by the addition of
ZnO. Unif orm dispersion of the ZnO was observ ed in the scanning electron micrographs of the tensile f ractured
surf ace of composites.
2013 Published by Elsev ier Ltd.
1. Introduction
Polymer nanocomposites have attracted great attention due to their
enhanced mechanical strength and thermal properties at low filler
loadings [1–3]. Now a days, nanostructured versions of conventional
inorganic fillers are used in plastic composites. There are many reports
on the enhancement of properties of polymer by adding inorganic
nanofillers [4–7]. Recently, nanocomposites based on PP matrix
constitute a major challenge for industry as they represent the route to
substantially improve the mechanical and physical properties of PP.
PP is one of the most w idely used thermoplastic polymers due to its
good physical and mechanical properties as well as the ease of
processing at a relatively low cost. There are a number of
investigations on the PP nanocomposites filled w ith different types of
fillers such as carbon nanotubes [8–10], nanoclay [11,12], talc, mica
and fibrous fillers.
The enhanced properties are due to the synergistic effects of
nanoscale structure and interaction of fillers w ith polymers. The size
and structure of the dispersed phase significantly influence the
properties of polymer nanocomposites [13]. Recently, there are some
studies on the influence of micro- and nano-sized ZnO on the
properties of PP [14,15]. Morphology of the filler also plays a major
role on the properties of a polymer.
⇑ Corresponding author. Tel.: +91 484 2575723; fax: +91 484 2577747. E-
mail address: rani@cusat.ac.in (R. Joseph).
http://dx.doi.org/10.1016/j.compositesb.2013.08.010
1359-8368/ 2013 Published by Elsevier Ltd.
In this work, we investigated the effect of crystallite size and
morphology of ZnO on mechanical, dynamic mechanical, transpar-
ency, morphology, thermal and flow properties of the PP/ZnO com-
posites prepared by melt mixing.
2. Experimental
2.1. Materials
Isotactic PP homopolymer (REPOL H200MA) with melt flow index
of 25 g/min w as supplied by M/s. Reliance Industries limited.
2.2. Methods
2.2.1. Preparation of zinc oxide
Zinc oxide nanoparticles were prepared by reacting zinc chlo-ride
and sodium hydroxide in chitosan medium. In this method zinc chloride
(5 g in 500 ml 1% acetic acid in water) was added to chitosan (5 g in
500 ml 1% acetic acid in w ater) with vigorous stir-ring using
mechanical stirrer. This was allowed to react for 24 h. During this
period, stabilization of the complex take place. Then sodium hydroxide
(25 g in 500 ml 1% acetic acid in water) was added drop w ise from
burette to the above solution w ith stirring using mechanical stirrer. The
whole mixture was allowed to digest for 12 h at room temperature.
This w as to obtain homogeneous dif-fusion of OH
_
and Cl
_
to the
matrix. The precipitate formed w as washed several times w ith distiled
w ater untilcomplete removal

2. 146 S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153
of sodium chloride and dried at 100 LC. Then it w as calcined at 550
LC for 4 h.
2.2.2. Preparation of PP/ZnO composites: compression moulding
The melt compounding w as performed using a Thermo Haake
Polylab system operating with counter rotating screws at 40 rpm for 8
min at 170 LC w ith a capacity of 60 cm3. Composites of differ-ent
concentrations (0–5 wt%) of ZnO were prepared. The hot mix
immediately pressed after mixing using a hydraulic press. The
samples w ere then made into films using compression moulding at
180 LC for 6 min in an electrically heated hydraulic press.
2.2.3. Mechanical properties of PP/ZnO composites
Mechanical properties of the compression moulded samples of PP
and PP/ZnO composites were studied using a Universal testing
machine (UTM, Shimadzu, model-AG1) w ith a load cell of 10 kN
capacity. The specimens used were rectangular strips of dimen-sions
10 _ 1 cm. The gauge length betw een the jaws at the start of each test
was adjusted to 40 mm and the measurements were carried out at a
cross-head speed of 50 mm/min (ASTM D 882).
2.2.4. Dynamic mechanical analysis (DMA)
DMA studies were carried out on rectangular shaped specimens of
dimensions 3 _ 1 cm by temperature sweep (temperature ramp from
30 LC to 150 LC at 3 LC/min) method at a constant frequency of 1 Hz.
The dynamic storage modulus, loss modulus and tan d were
measured.
2.2.5. Scanning electron microscopy
The morphology of the tensile fractured surface of PP and com-
posites was studied using scanning electron microscope (JOEL model
JSM 6390 LV).
2.2.6. Thermogravimetric analysis
Thermogravimetric analyzer (TGA Q-50, TA instruments) was used
to study the effect of ZnO on the thermal stability of PP. Approximately
10 mg of the samples w ere heated at the rate of 20 LC/min from
ambient to 800 LC in nitrogen atmosphere.
2.2.7. Melt flow index (MFI) measurements
MFI of the composites were studied using CEAST melt flow
modular line indexer (ITALY) at 190 LC and 2.16 and 5 kg wt. A pre-
heating time of 6 min is given before each experiment. The w eight of
the substance extruded in 10 min in grams is then measured.
2.2.8. X-ray diffraction studies
X-ray diffraction studies were carried out using Rigaku Geiger-flex
at wavelength Cu Ka = 1.54 Å. Crystallite size of ZnOwas calcu-lated
using Debye Sherrer equation:
CS ¼ 0:9k=b cos h
where CS is the crystallite size, b is the full width at half-maximum of
an hkl peak at h value, h is the half of the scattering angle.
3. Results and discussion
3.1. X-ray diffraction studies of zinc oxide
XDD pattern of NZO and CZO is show n in Figs. 1a and 1b respec-
tively. The figures show the characteristics peaks of hexagonal crystal
structure. The peaks obtained correspond to (100), (002), (101), (102),
(110), (103), (112), (201), (004), (202), (104) planes. The (101) plane
is most prominent. The crystallite size of
Fig. 1a. XRD pattern of ZnO prepared in chitosan medium.
Fig. 1b. XRD pattern of commercial ZnO.
ZnO are calculated using Debye Sherrer equation is 13.4 nm for NZO
and 29.2 nm for CZO.
3.2. Scanning electron micrographs of zinc oxide
Scanning electron micrographs of two ZnO which is taken for the
study are shown in Fig. 2. Similar morphologies and different particle
sizes are seen in SEM. ZnO show n in Fig. 2a depicted as NZO and
Fig. 2b is denoted as CZO. Both show sphere like morphol-ogy and
small particles are observed in the scanning electron micrographs of
NZO w hen compared to CZO.
3.3. Mechanical properties of PP/ZnO composites
Figs. 3 and 4 show variation in tensile strength and modulus of PP
with ZnO content. The incorporation of ZnO in the PP matrix result in
an increase in the tensile strength and modulus. It reaches maximum
at 1.5 w t% concentration of ZnO and then decreases. In the case of
PP w ith NZO the tensile strength gets increased from 31.75 to 44.37
N/mm2 and tensile modulus from 1105.35 to 1897.02 N/mm2 at 1.5
wt% NZO. CZO filled PP shows an increase in tensile strength from
31.8 to 41.2 N/mm2 at 1.5 w t% CZO and tensile modulus from
1105.75 to 1422.9 N/mm2 at 0.5 wt% of CZO. The increase in
properties may be due to the interface inter-action between
nanoparticles and a polymer matrix that can trans-fer stress, which is
beneficial for the improvement of the tensile strength of composite
films. How ever, w ith increasing content of nanoparticles, aggregation
occurs, which leads to a decrease in the contact area between the
nanoparticles and polymer matrix and results in the formation of
defects in the composites.

3. S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153 147
Fig. 2. Scanning electron micrographs of (a) NZOand (b) CZO.
Fig. 3. Effect of particle size of ZnOon tensile strength of PP/ZnO composites. Fig. 5. Effect of particle size of ZnOon elongation at break of PP/ZnO composites.
Fig. 4. Effect of particle size of ZnOon modulus of PP/ZnO composites.
Therefore, the effective interfacial interaction is reduced, and the
tensile strength of the films gets decreased [6]. The mechanical
properties also depend on the dispersion of nanoparticles in the
matrix. The improvement of tensile modulus and strength of PP/ ZnO
nanocomposites is related to the inherent stiffness and quality of the
dispersion of ZnO [16,17]. NZO filled PP shows higher mechanical
properties than CZO filled PP. This is due to the differ-ence in
morphology and crystallite size of filler. ZnO w ith smaller crystallite
size shows better properties than ZnO with higher crys-tallite size.
Sphere like morphology of NZO results in the uniform distribution of
ZnO particles in the PP and thus improve the mechanical properties of
PP. Elongation at break (Fig. 5) of PP is decreased by the addition of
low concentration of ZnO, at a higher concentration it is increased.
3.4. Dynamical mechanicalanalysis of PP/ZnO composites
Dynamic storage modulus is the most important property to assess
the load-bearing capability of a material. The storage mod-ulus of neat
PP and PP/ZnO nanocomposites as a function of tem-perature at 1 Hz
are shown in Fig. 6. Storage modulus of the PP is increased with the
addition of ZnO. The increase in storage mod-ulus is significant at low
temperature like 40 LC. The increase may be due to the stiffening
effects of ZnO and efficient stress transfer between the polymer matrix
and nano ZnO.
The loss modulus of the PP and composites are given in Fig. 7.
The loss modulus also increase substantially w ith the ZnO concen-
tration. Maximum improvement is shown by PP w ith 1.5 wt% NZO.
Composite of PP w ith NZO shows significant improvement com-pared
to CZO. Reinforcing effect of ZnO increases with decrease in
crystallite size of ZnO.
Fig. 6. Effect of ZnO on storage modulus of PP/ZnO composites.

4. 148 S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153
Fig. 7. Effect of ZnO on loss modulus of PP/ZnO composites.
Fig. 8. Effect of ZnO on tan d values of PP/ZnO composites.
The tan d curves of PP and composites are shown in Fig. 8. It is
evident from the figure that there is an increase in tan d value on
addition of ZnO. This indicates an increase in damping characteris-tics
of the composites. It is obtained in many cases that the improvement
of stiffness markedly decreases the ductility. But PP/ZnO composite
show ed increased stiffnesswithout reduction in ductility.
3.5. Torque studies
Fig. 9 indicates the variation of torque w ith mixing time for neat PP
and PP/ZnO composites. Torque is increased rapidly during
initial mixing and then dropped to stabilize a line w ith higher mixing
time.
This indicates a good level of mixing at the specified conditions.
Also the torque value of the PP/NZO composites are higher than that
of neat PP and PP/CZO composites. This is mainly due to the increase
in interfacialinteraction betw een the nanoparticles and polymer [18].
3.6. X-ray diffraction analysis of composites
XRD plots of neat PP and PP/ZnO composites are given in Fig. 10.
The peaks obtained are corresponding to the planes (110), (040),
(130) represents a form of isotactic PP. Gopinath et al. observed the
similar peaks in the XRD pattern of isotactic PP [19]. X-ray dif-fraction
pattern of nanocomposites show sharp and highly intense peaks
whereas neat PP shows less intense peaks. This may due to the
development of crystallinity in the polymer. In both neat PP and PP
w ith ZnO, the crystalplane of PP is monoclinic.
3.7. Scanning electron micrographs (SEM) of the nanocomposites
Fig. 11a–c represents the SEM photographs of fractured surfaces
of neat PP, composites of PP w ith NZO and CZO at 1.5 wt% respec-
tively. The SEM of PP w ith 1.5 wt% NZO shows formation of fibre like
structure which results increase in mechanical properties. Uni-form
distribution of ZnO is observed in the SEM photographs. Fig. 11d and
e shows SEM photographs of PP w ith NZO and PP w ith CZO at 5 wt%
respectively. At higher percentage large particles are observed due to
the agglomeration of the zinc oxide particles. This may be the reason
for the decrease in mechanical properties at higher concentration.
3.8. EDAX of the composites
EDAX is used for identifying the chemical composition of a
specimen. Fig. 12 represents the EDAX of neat PP, 1.5 w t% NZO filled
PP and 1.5 w t% CZOfilled PP.
EDAX shows the presence of ZnO in the PP matrix. EDAX of the
composites shows the peak compared to zinc and oxygen indicat-ing
the presence of ZnO in the composites. Lighter elements such as
hydrogen cannot be observed in EDAX due to the beryllium w in-dow
that isolates the cooled detector from the vacuum system. So the peak
of hydrogen is not seen in the spectrum.
3.9. Thermogravimetric analysis (TGA)
Fig. 13 represents the thermogram of neat PP and composites. The
values are tabulated in Table 1. Thermal stability of PP/ZnO
Fig. 9. Variation of torquewith time duringmixing. Fig. 10. X-ray diffraction patternof neat PP and ZnOfilled composites.

5. S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153 149
Fig. 11. SEM images of fractured surface of (a) neat PP, (b) PP + 1.5 wt% NZO, (c) PP + 1.5 wt% CZO, (d) PP + 5 wt% NZO, and (e) PP + 5 wt% CZO filled composites.
composites are greater than neat PP. The properties studied by TGA
indicate an improvement in thermal stability of PP by the addition of
ZnO. Onset of degradation (temperature at which degradation starts)
is increased by the addition of ZnO. The increase is signifi-cant when
the particle size of ZnO decreases. Onset of degradation of neat PP is
391 LC w here as 1 wt% NZO added PP is 422.7 LC. The temperature
at w hich maximum degradation takes place is increased by the
addition of ZnO. Rate of degradation is decreased with filler loading.
The increase in thermal stability of the compos-ites may be due to the
strong interaction between the ZnO and PP. TGA studies show that
inorganic fillers, which are w idely used industrially to improve the
mechanical properties of polymer materials, have various effects on
the thermal oxidation of PP. Gilman [20] suggested that the thermal
stability of polymers in presence of fillers is due to the hindered
thermal motion of poly-mer chains.
3.10. Kinetic analysis of thermal decomposition
Coats–Redfern method was used to study the kinetics of thermal
degradation of PP and PP/ZnO composites [21]. This
method is an integral method and thermal degradation functions for
the Coats–Redfern method are given in Table 2.
Thermogravimetric function g(a) and activation energy (E) is
obtained fromthe equation:
ln½gðaÞ=T2& ¼ lnðAR=UEÞð1 _ 2RT=EÞ _ E=RT ð1Þ
where a is the decomposed fraction at any temperature and is given
as:
a ¼ Ci _ C=Ci _ Cf
where C is the w eight at the chosen temperature, Ci is the w eight at
initial temperature, Cf is the w eight at final temperature, a is the
heating rate, and E is the activation energy for decomposition. Acti-
vation energy (E) can be calculated from the slope of the curve and
pre-exponential factor (A) using the intercept value of the plot of ln
[g(a)/T2] against the reciprocal of absolute temperature (1/T). The
order of decomposition reaction was measured using the best linear fit
of the kinetic curve and that gives the maximum correlation coefficient.
The form of g(a), which best represents the experimen-tal data gives
the proper mechanism. From these calculations it is observed that the
Mampel equation (_ln(1 _ a)) fits in w ell. The

6. 150 S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153
Fig. 12. EDAXof (a) neat PP, (b) PP + 5 wt% NZO, and (c) PP + 5 wt% CZOcomposites.
linear correlation coefficients suggest that the F1 model is the most
appropriate to describe the experimental results (Table 2).
From Table 3 it is clear that the activation energy of PP increased
with the addition of ZnO nanoparticles. Activation energy (E) obtained
for neat PP is 126.52 kJ/mol, 1.5 w t% NZO added PP is 136.84
kJ/mol. Significant increase in activation energy indicates high thermal
stability. Representative plot of Coats–Red-fern equation for neat PP,
PP/1.5 w t% NZOand PP/3 w t% NZOis show n in Fig. 14.
3.11. Melt flow index (MFI)
MFI gives an idea about the flow characteristics of the ther-
moplastics. It depends on the molecular properties such as molecular
weight and structure of polymers [22]. Fig. 15a and b shows the effect
of ZnO on the MFI of PP at 5 kg and 2.16 kg respectively. MFI of PP is
decreased by the addition of
CZO indicate a decrease in flow characteristics of the polymers. In
NZO filled PP, MFI is increased by the addition of low concen-tration of
NZO indicates an increase in the flow properties of the polymers.
Adding the low concentration of nanoparticles may provide a flow
favouring orientation due to the small size of NZO as depicted in Fig.
16. An increase in MFI is reported when adding multi w alled carbon
nanotube to PP [23]. After adding 1 w t% NZO to the PP the MFI value
decreases gradually. It indi-cates the structure of nanoparticles was
interconnected to hinder the molecular motion of polymer chains [24].
3.12. Transparency of the films
The percentage transmittance of neat PP and composites is given
in Fig. 17. By the addition of ZnO the transmittance of the film is
decreased. NZOfilled PP films show higher

7. S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153 151
Fig. 13 (continued)
Fig. 13. Thermogram of PP andPP/ZnO composites.
Table 1
Ef f ect of ZnO particlesize on the thermal stability of PP.
Concentration Temperature Onset of End set of Residue Rate
of ZnO (%) at which degradation degradation (%)
maximum (LC) (LC)
degradation
take place
(LC)
NZO CZO NZO CZO NZO CZO NZO CZO NZO CZO
0 471.6 471.6 391.0 391.0 500.7 500.7 1.4 1.4 56.3 56.3
0.5 472.9 471.8 409.8 390.1 500.1 492.9 0.8 0.9 54.5 43.8
1 474.4 472.4 422.7 392.9 499.9 498.9 1.6 1.8 53.4 47.6
1.5 475.4 472.9 416.3 396.6 499.8 498.5 2.6 2.2 53.9 51.1
2 475.3 475.4 412.7 401.7 497.2 498.6 3.5 3.3 53.8 52.6
3 473.9 471.6 406.3 402.6 499.1 494.6 3.9 3.6 52.4 51.7
Table 2
The mechanisms of solid-state thermal degradation reaction and corresponding
thermal degradation functions g(a).
g(a) = kt Sy mbol Rate controlling process
Deceleratory at curves
(a) Based on dif fusionmechanism
a
2 D
1 One-dimensional diffusion
a + (1 _ a)
ln(1 ) D Two-dimensional diffusion
1/3
]
2_ a 2
Three-dimensional diffusion[1 _ (1 _ a) D3
1 _ (2/
_ a)
2/3 D4 Three-dimensional diffusion
3)a _ (1 (Gistling–Brounshtein equation)
(b) Based on geometrical models
1 _ (1 _ a)
1/n
Rn Phase-boundary reaction; n = 1, 2 and 3
(one, two and three dimensional,
respectively
(c) Based on ‘order’ of reaction
_ln(1 _ a)
F1 First order (Mampel equation)
Table 3
Apparent activation energy (E) andcorrelation coefficients (R) for neat PP and PP/ZnO
composites by Coats–Redfern method.
Sample name R E
Neat PP 0.999 126.52
PP + 0.5% NZO 0.999 134.73
PP + 1.5% NZO 0.999 136.84
PP + 3% NZO 0.999 133.61
transparency when compared to CZO filled PP films. As the crys-tallite
size of ZnO decreases the transparency of the composite film
increases.

8. 152 S.K. Esthappan et al. / Composites: Part B 69 (2015) 145–153
Fig. 14. Representative plot of Coats–Redfernequation for neat PP and PP/ZnO
nanocomposites. Fig. 17. Visible-IR transmittance of neat PP and PP/ZnO composites.
Fig. 15. Effect of ZnOparticle size on the melt flow index of PPusing (a) 2.16 kgand
(b) 5 kg weight.
Fig. 16. Schematic representation of flow behaviour of (a) PP/NZO and (b) PP/CZO
composites.
4. Conclusion
NZO shows smaller crystallite size compared to CZO. PP/ZnO
composites are prepared by melt mixing method. Mechanical and
dynamic mechanical properties of PP are improved by the addition of
ZnO. PP shows better thermal stability in presence of ZnO. NZO filled
PP shows higher mechanical and thermal properties than CZO filled
PP and neat polymer. X-ray diffraction studies of neat PP and
composites indicate the presence of a phase of monoclinic PP. Melt
flow index increases by adding low concentration of NZO whereas
CZO added PP shows a decrease in MFI. Transparency of the PP
films is decreased by the addition of ZnO. PP w ith NZO filled film
show shigher transparency when compared to CZO filled PP films.
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