Development of Biowaste Encapsulated Polypropylene

Composites: Thermal, Optical, Dielectric, Flame

Retardant, Mechanical, and Morphological Properties

Atta ur Rehman Shah,1 M.N. Prabhakar,1 Mohsin Saleem,2 Jung-Il Song1

1
Department of Mechanical Engineering, Changwon National University, Changwon, Korea

2
Department of Electrical Functionality Material Engineering, University of Science and
Technology, Daejeon, Korea

Polymer matrix composites have recently gained attrac- effects of petroleum-based products, modern research is
tion due to their light weight, low cost, and easy avail- being focused on the development of biocomposites [1–3].
ability, but they are also known for their higher burning
rate and low thermal stability. Polypropylene (PP) is one
Consumption of seafood is causing a large amount of
of the most used thermoplastic polymers, its thermal waste in the form of shells. This material causes landfills
stability and flammability have been modified in this and pollution in coastal areas, which is a hazard for public
study by incorporation of oyster shell powder (OSP) into health and environment [4]. Governments throughout the
PP as a filler. Besides flammability and thermal proper- world are currently focusing more on ecofriendly policies.
ties, OSP incorporation has some effects on dielectric
and mechanical properties of PP. Effect of concentration
Recycling shell waste can therefore be an effective way to
of OSP (0–50 wt%) on the properties of OSP/PP compo- utilize it in making useful products. Currently waste shells
sites was studied. The burning rate was observed to are being used in agriculture as soil conditioners and low-
decrease while thermal stability was increased as a cost absorbent [5]. The use of these natural fillers in indus-
result of OSP addition into PP. Similarly, relative permit- trial applications may allow the underdeveloped countries
tivity (dielectric constant) of OSP/PP composites was
found higher than that of pure PP. Flame retardancy,
to utilize their local resources [6].
thermal stability, and dielectric properties are functions The Korean peninsula is surrounded by sea, which
of OSP concentration. Optical tests including fluores- gives it an opportunity to explore in aquaculture industry.
cence spectroscopy and FT-IR were conducted to study According to a report of FAO fisheries and aquaculture
any possible interactions between OSP and PP. A slight department, in 2012 the aquaculture production in Korea
decrease in the tensile strength was observed. FESEM
was more than 1,506,730 tons [7]. Pacific oyster is one of
images revealed that the decrease in tensile strength is
a result of interfacial flaws between filler particles and the abundantly found aqua-specie in this region, its shells
matrix. Tensile modulus was found to increase which is are found as waste material on the coasts of Korea. This
an evidence of stiffness, this is also correlated to the waste material can be utilized as filler in polymer matri-
improvement in thermal stability. POLYM. COMPOS., ces to enhance various properties.
C 2015 Society of Plastics Engineers
00:000–000, 2015. V
Polymers are generally light weight with acceptable
mechanical properties but they degrade frequently under
INTRODUCTION high temperature and fire propagation [8]. Thermal stabil-
In the last decades, utilizing biowaste and other bio- ity and flame retardancy of the polymers may be
based fillers in making useful products has become a key improved by various techniques, particulate reinforcement
area of interest. Due to high cost and sever environmental is one of the famous methods [9]. Various types of fillers
have been studied by the researchers to improve thermal
stability and fire retardancy of polymer composites
Correspondence to: Jung Il Song; e-mail: jisong@changwon.ac.kr
Contract grant sponsor: This work is supported by the National Research
[10–21]. These fillers may also improve the dielectric
Foundation of Korea (NRF) grant funded by the Government of the properties of polymers [22, 23]. Dielectric properties of
Republic of Korea (Ministry of Science, ICT and Future Planning polymers have not been studied much in literature, cur-
(MISP)) (No. 2013R1A2A2A0107108 and No. 2011-0030058). And also rently it is more famous about ceramics-based materials
supported financially by Changwon National University (CWNU) during [24, 25]. In this study, polypropylene (PP) has been rein-
2013-2014.
DOI 10.1002/pc.23580
forced with oyster shell powder to study its effects on
Published online in Wiley Online Library (wileyonlinelibrary.com). thermal stability, fire retardancy, dielectric properties,
C 2015 Society of Plastics Engineers
V optical parameters, and mechanical properties of

POLYMER COMPOSITES—2015
polypropylene. PP is economically, ecologically, and
technically suitable matrix for biofiller composites [26]. It
is among the most commonly used polymeric materials
because of its low cost and easy availability [27]. Gener-
ally, it is filled with different types of fillers including
fibers, nanoparticles, and powders, etc. to tailor the
desired properties. Enough literature is available on PP
based composites [28–33].
Oyster shell is a food waste and its powder has been
used as filler in PP for this study. A chemical analysis of
this biowaste material revealed that it is composed of 96%
of calcium carbonate (CaCO3) [34], which makes it a fire
retardant agent. A flame retardant is a molecule which
obstructs flame growth either chemically or physically.
Reactive type flame retardants act chemically and modify FIG. 1. Particle size distribution of OSP. [Color figure can be viewed
the combustion mechanism while the additive type flame in the online issue, which is available at wileyonlinelibrary.com.]
retardants act physically and make a char layer on the sur-
face against flame propagation and release nonflammable Preparation of OSP/PP Composites
gases (e.g., H2O, CO2) to slow down the burning rate [35].
In this study, oyster shell powder reinforced in PP acts as All ingredients including PP, OSP, and MAPP were
an additive type flame retardant filler. On heating (or burn- mixed together manually, blends with different concentra-
ing), the CaCO3 (present in the oyster shell) decomposes tions of OSP (0, 10, 20, 30, 40, and 50% by weight) were
and releases CO2 gas. CO2 is a fire extinguishing gas and prepared. After manual mixing each blend was melt
it resists flame propagation during burning process. Oyster mixed using a twin-screw extruder (Brabender Technolo-
shell powder was added to PP resin in different concentra- gie, Korea) and then cut into small pellets of 2 mm size.
tions varying from 0 to 50% by weight. Maleic anhydride The small pellets were re-extruded in the twin-screw
grafted polypropylene (MAPP) was added as a coupling extruder to obtain better mixing of all the components.
agent in an amount of 2.5% by weight [36]. All materials Extrusion temperature was kept between 210 and 2208C
were mixed, extruded, and then injected into the required [37]. The extruded material was pelletized again and
specimen shapes. The shell powder was characterized by dried in oven at 608C for 24 h to remove any possible
means of X-RD to study its crystalline nature. TGA, DSC, moisture content. The dried pellets were then injected
FT-IR, fluorescence spectroscopy, impedance test, univer- into the required test specimens using a horizontal injec-
sal tensile test, and FESEM studies of the OSP/PP compo- tion molding machine (Woojin Selex, Korea).
sites were carried out. It was observed that thermal
degradation and fire retardancy of PP were enhanced after Characterization and Tests
OSP incorporation. Dielectric properties were also
improved. Optical tests (FT-IR and florescence spectros- X-RD Analysis. X-RD analysis was conducted to evalu-
copy) revealed no chemical reaction between OSP and PP, ate crystalline nature of the OSP using Bruker D8 Dis-
but a physical interaction exists. Tensile modulus was also cover equipment (Germany). Analysis was performed
observed to increase as a function of OSP concentration, between 108 and 808 at a rate of 18/min and 30 mA
which is a sign of stiffness. A slight decrease in the tensile current.
strength was observed. Scanning electron microscopy dis-
covered poor interfacial bonding, which caused decrease in Thermal Analyses and Flame Test. Thermal analyses
the mechanical strength. including TGA and DSC were performed using SDT
Q600 measuring equipment. Each specimens was 4 to
9 mg in weight. Tests were conducted between room tem-
MATERIALS AND EXPERIMENTAL METHODS
perature and 8008C in air flow and nitrogen atmosphere
separately at a heating rate of 158C/min. TGA measured
Materials
weight loss (degradation) as a function of temperature,
PP resin was used in the form of pellets of size 1 to while DSC test gave results of change in Tm as a function
1.5 mm, which was purchased from GS Caltex Corpora- of OSP concentration in PP.
tion Korea. The oyster shell powder was purchased from ASTM D635 horizontal flame test was conducted to
Haesung Korea. MAPP was obtained from Sigma Aldrich calculate burning time and burning rate of pure PP and
(USA). Particle size of OSP was measured using Malvern OSP/PP composites as a function of OSP concentration.
Mastersizer instrument. Average particle size of the OSP
was found to be approximately 148 lm. The particle size Impedance Test. Silver electrodes were deposited on
distribution is shown in Fig. 1. both sides of the specimens, and dielectric properties

2 POLYMER COMPOSITES—2015 DOI 10.1002/pc

 spectra were recorded from 4,000 to 400 cm21 region
with 32 scans in each case at a resolution of 4 cm21.
Fluorescence spectroscopy was conducted between 200
and 665 nm wave length ranges using Shimadzu RF-5301
PC spectrofluorophotometer (USA). Absorption behavior
of the specimens was studied.

Tensile Test. Tensile test was conducted according to

ASTM D638 using MTS 97 kN load cell with a cross
head speed of 3 mm/min. MTS extensometer model
634.11F-24 was used to measure strain readings precisely.
Tensile strength and modulus were calculated.

FIG. 2. X-RD pattern of oyster shell powder. [Color figure can be Surface Morphology. FESEM images of the fractured
viewed in the online issue, which is available at wileyonlinelibrary. tensile test specimens were taken using CZ/MIRA I LMH
com.] 1.5 nm resolution equipment. Images at different resolu-
tions were studied to know the interfacial conditions
were measured using an impedance analyzer (Agilent between filler and matrix.
tech 4294 A, Malaysia). The dielectric constant and tan-
gent loss were calculated at 1 kHz frequency and room RESULTS AND DISCUSSION
temperature. Capacitance (CP) value for each specimen
was obtained through experiments by the impedance ana-
lyzer, relative permittivity (also known as dielectric con- X-RD Analysis
stant) was calculated by the following expression. Generally, crystallinity also plays a role to decide the
mechanical properties of the polymer composites. In order
Cp 3 d
er ¼ (1) to probe this aspect in the present investigation, the crys-
eo 3A tal structure and impurities present in the oyster shell
where A is the cross-sectional area and d is thickness of powder were studied using X-RD. The X-RD pattern of
the specimen. eo is electric constant known as vacuum plain oyster shell powder is presented in Fig. 2. In the
permittivity, which is equal to 8.85 3 10215 F/mm. pattern, all the peaks matched very well with that of cal-
cium carbonate CaCO3 in the form of calcite and arago-
nite. These results clearly indicate the high purity of
Optical Tests. FT-IR spectroscopy was conducted by inorganic calcium carbonate in the bioinspired micropar-
potassium bromide (KBr) disk method using JASCO FT- ticles and no impurities were observed. In the present
IR-6300 spectrometer (UK) in dry air at ambient tempera- study, the calcite and aragonite phases of oyster shell
ture. Pellets of oven dried samples (at 608C) were mixed powder physically exist in the matrix which has no chem-
with KBr powder by vacuum pressing. % of transmittance ical reaction with the polymer matrix.

FIG. 3. TGA thermograms of PP and OSP/PP in (a) air (b) N2. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2015 3

 TABLE 1. Thermal and tensile properties of OSP/PP composites. improvement in the thermal stability of OSP/PP compo-
sites. Besides this, it was observed that pure PP was fully
Onset of
consumed during the analysis while some amount of OSP/
degradation (8C) Tensile properties
PP remained unconsumed till the ending temperature.
Strength Modulus
S. No Specimen In air In N2 (MPa) (GPa)
Differential Scanning Calorimetry
1 0% OSP 265 420 21.44 1.009
The thermal assets of PP and OSP/PP blend compo-
2 10% OSP 297 449 18.56 1.284
3 20% OSP 302 451 18.42 1.567 sites have been studied both in air and nitrogen atmos-
4 30% OSP 310 453 18.21 1.673 phere from ambient temperature to 2008C on the first
5 40% OSP 330 460 17.73 2.177 heating run. The DSC thermograms of samples containing
6 50% OSP 340 463 18.12 2.457 different concentrations of OSP are shown in the Fig. 4.
In both atmospheres, any pointed difference is not
observed except some thermal stability, the composites
Thermogravimetric Analysis were found thermally more stable in inert atmosphere
TGA was conducted to evaluate % weight loss of pure (nitrogen) as compared with air because of oxygen. The
PP and OSP/PP composites as a function of temperature. melting point was taken as the main peak of the endother-
PP is a low melting point (1708C) polymer which cannot mic curve. The results show no significant changes in Tm
withstand at higher temperatures. On the other hand OSP for PP and OSP/PP composites. These results imply that
has about five times higher melting point (8258C) than the presence of the oyster shell powder does not affect
pure PP. Therefore blending OSP with PP controls its Tm of PP. It was found from these results that Tm of the
thermal degradation, and improves its performance at polypropylene composites were influenced by the poly-
higher temperatures. Figure 3 shows TGA curves of pure propylene rather than filler loading. Tm did not change
PP and OSP/PP blends in air and N2 (inert) environment. significantly due to the same matrix. Furthermore, Tm of
The first onset of degradation is an important step of this the polypropylene composites plays an important role in
analysis which is summarized in Table 1. Results indicate their manufacturing.
that pure PP is weakest against thermal loadings among
all compositions. In air, the onset of degradation for PP
Flame Test
started at 2658C while for OSP/PP (10, 20, 30, 40, 50
wt%) composites it started at 2978C, 3028C, 3108C, Burning time and burning rate of pure PP and OSP/PP
3308C, and 3408C, respectively. Similarly in N2 environ- composites were calculated using horizontal flame test.
ment, the onset point is increasing with the increasing OSP reinforcement caused a significant increase in the
concentration of OSP in PP. In N2, the onset of degrada- burning time of specimens consequently reducing burning
tion starts at 4208C, 4498C, 4518C, 4538C, 4608C, and rate. This effect goes on increasing with the increasing
4638C for pure PP and OSP/PP (10, 20, 30, 40, 50 wt%) concentration of OSP reinforcement in OSP/PP compo-
composites. Due to the absence of oxygen, values sites. Results of the flame test are shown in Fig. 5. The
are higher in N2 as compared with air, which shows 50 wt% OSP reinforced OSP/PP composite displayed

FIG. 4. DSC curves of pure PP and OSP/PP composites in (a) air (b) N2. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]

4 POLYMER COMPOSITES—2015 DOI 10.1002/pc

FIG. 5. Effect of OSP concentration on burning time and burning rate.

FIG. 7. FT-IR spectra of PP, OSP, and OSP/PP. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
32% increase in the burning time and 24% decrease in com.]
the burning rate as compared with pure PP. Reduction in
the flame propagation rate practically verifies the composite. Results show that the relative permittivity
improvement in flame retardancy. As discussed in Intro- increases after addition of OSP in PP, and it keeps on
duction section, this resistance to flame is due to the pro- increasing with the increasing concentration of OSP. Pure
duction of CO2 during the burning process of CaCO3 PP has lowest value of er while 50 wt% OSP/PP has the
present in the oyster shell. OSP reinforcement also helps highest value. These high values of relative permittivity and
in increasing stiffness and glass transition temperature of thermal stability (as discussed in Thermogravimetric Analy-
the composite which reduces dripping rate during burning sis section) make OSP/PP composite a suitable choice for
process. During the flame test, it was observed that pure insulations applications in electronics industry. The results
PP was soft against the flame and dripping rate was rapid, also expressed an increase in the tangent loss of OSP/PP
whereas the response of the OSP/PP composites against composites. This shows a dissipation of energy due to the
flame increased harder and harder with the increasing internal flaws and holes in the OSP/PP composite speci-
concentration of OSP. mens, which can be observed in FESEM images. The
increase in tangent loss is very small, it can be controlled
by reducing the possible internal flaws in the material.
Impedance Test
In Fig. 6, relative permittivity (er) and tangent loss (tan FT-IR Spectroscopy
d) are plotted against the concentration of OSP in OSP/PP
The infrared spectra of virgin OSP, virgin PP, and
OSP loaded PP is presented in Fig. 7. Results of only one

FIG. 8. Fluorescence spectra of pure PP and OSP/PP composites.

FIG. 6. Relative permittivity and tangent loss of PP and OSP/PP at [Color figure can be viewed in the online issue, which is available at
1 kHz. wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—2015 5

 bands at 1,365 cm21 are the symmetric bending vibration
of CH3 functional group, and the absorption bands at
1,461 cm21 are the asymmetric bending vibration of
ACH3 groups and the scissor bending vibration of
ACH2A groups. The absorption bands at 2,873 cm21 and
2,915 cm21 are believed to be the symmetric stretching
vibration of ACH3 and the asymmetric stretching
vibrations of CH2, respectively. On the other hand, the
intensities of the CAO bands of oyster shell between
1400 cm21 and 500 cm21 were the strongest. We
observed CAO stretching vibrations at 1,440 to
1,450 cm21 and out-of-plane CAO bending vibrations at
870 to 880 cm21. At the same time in the results, no new
characteristic absorption bands of OSP loaded PP compo-
FIG. 9. Effect of OSP concentration on tensile properties. [Color figure
sites were observed, this clearly explains there was no
can be viewed in the online issue, which is available at wileyonlineli- chemical reaction between PP and OSP.
brary.com.]

blend of OSP/PP are plotted to keep the figure clean and Fluorescence Spectroscopy
readable, the general trend of all OSP/PP blends is same. Fluorescence spectroscopy is a useful tool to identify
The absorption bands at 721 cm21 represent the rocking the interactions among polymers and other molecules in
vibration of the group [ACH2ACHA]n. The absorption blend polymers. The fluorescence spectra of pure PP and

FIG. 10. FESEM images of fractured surfaces of tensile test specimens. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]

6 POLYMER COMPOSITES—2015 DOI 10.1002/pc

10 wt% OSP/PP are shown in Fig. 8. This one blend of at the filler-matrix interface act as stress concentration
OSP/PP represents all compositions of OSP/PP as their points and the increase in external tensile forces cause
general trend is the same. In the figure, there is a signifi- propagation of internal cracks which results in early fail-
cant quenching/intensity of fluorescence in OSP/PP com- ure of such particulate filled composites.
posite as compared with that in the virgin PP. The
fluorescence quenching in OSP/PP blend can be attributed CONCLUSION
to (1) photo-induced charge transfer from excited PP mol-
Abundantly available oyster shell waste is found to be
ecules to OSP and (2) resonance energy transfer between
a useful biofiller for polymers to improve some of their
species in the excited state, respectively, due to an over-
properties. It acts as a nucleating agent to enhance the
lap of emission and absorption spectra. The resonance
thermal and fire retardant properties of polypropylene.
energy transfer and photo-induced transfer are closely
The flame test and thermal analyses proved that OSP is
related to the charge of the participating donor and
helpful in increasing thermal stability and to control the
acceptor molecules. The fluorescence quenching may be
burning rate of PP. Thermal stability was improved due
due to the electron transported from excited PP to OSP
to the high melting point of OSP as compared with PP
indicated by reduced emission intensity. The quenching
while burning rate was decreased due to CO2 production
effects of fluorescence intensity are more significant as
during combustion process. Relative permittivity was also
OSP microparticles were incorporated into the PP matrix.
observed to increase as a result of OSP addition in PP
When OSP content was added, physical contact between
because OSP/PP composites have high values of capaci-
PP and OSP or between OSP molecules was built and
tance. The FT-IR and fluorescence spectroscopies have
therefore improved the charge transfer, contributing to
revealed that OSP and PP particles have some physical
increased fluorescence quenching effect.
interaction but no evidence of any chemical reaction was
found. Both the filler and matrix are inert to each other.
Tensile Test Meanwhile tensile strength was the only property
observed to decrease after increasing OSP concentration
The trend of tensile properties of PP as a function
in PP. Poor interfacial bonding between PP and OSP par-
of OSP concentration was observed, which is shown in
ticles was discovered in the FESEM micrographs which
Fig. 9. Tensile test revealed that tensile strength decreases
is the main reason of reduction in the tensile strength. On
slightly while tensile modulus increases as a result of
the other hand, tensile modulus was increased which is a
OSP reinforcement in PP. This is a common occurrence
sign of greater stiffness in OSP/PP composites. Higher
in case of particulate filled composites [38]. The decrease
stiffness is also one of the reasons of high thermal stabil-
in tensile strength is mainly because of the poor interfa-
ity of OSP/PP composites. Overall OSP/PP was found
cial bonding between filler and the matrix. The increase
thermally stable and fire retardant material which can be
in modulus is an indication of stiffness, high stiffness of
used in interior parts of automobiles, insulations, and
the OSP/PP composites makes them thermally more sta-
packaging industry.
ble. Decrease in the tensile strength is undesired but this
decrease is small, powder fillers are usually week against
tension unless they have some chemical compatibility NOTATIONS
with the matrix. Oyster shell powder has no chemical
compatibility with the polymer matrices hence it does not OSP Oyster shell powder
perform well under tensile loads. After 50 wt% OSP rein- PP Polypropylene
forcement in PP, the fall observed in the tensile strength N2 Nitrogen
is about 15% while increase in the tensile modulus is Tm Melting temperature
143%. Tensile properties of all compositions are given in TGA Thermogravimetric analysis
Table 1. DSC Differential scanning calorimetry
FT-IR Fourier transform-infrared spectroscopy.
X-RD X-rays diffraction
Surface Morphology FESEM Field emission scanning electron microscopy.
FESEM micrographic images of fractured surfaces of
tensile test specimens are shown in Fig. 10. Images of
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8 POLYMER COMPOSITES—2015 DOI 10.1002/pc