Tensile strength of elephant grass fiber reinforced polypropylene composites.
Kumara, N. Ravi ; Ramji, K. ; Prasad, A.V. Ratna 等
Introduction
Natural fibers are lignocellulosic in nature and the most abundant
renewable biomaterial of photosynthesis on earth. Underutilized natural
fiber residues are readily available rich resources of lignocellulosic
materials. Since last decade, there is considerable worldwide interest
in the potential of substituting natural fibers (agro fibers) for either
wood or man made fiber (eg., fiber glass ) in composite materials.
Composites consisting lignocellulosic fibers and synthetic
thermoplastics have received substantial attention in scientific
literature as well as in industry, primarily due to improvements in
process technology and economic factor. Natural fibers such as jute,
flux, hemp, etc. can be alternately used to reduce the cost of the
composites (Mohanty et al., 2002).
The prominent advantages of natural fibers include acceptable
specific strength properties, low cost, low density and high toughness
(Biagiotti et al., 2004). The mechanical properties of some natural
fibers such as jute, sisal, and flaw fibers were compared to glass
fibers and it was observed that specific moduli of these fibers are
comparable to or better than those of glass fibers (Nabi Saheb et al.,
1999 ) . The physical and mechanical properties of wood, water hyacinth,
Kenaf, banana and empty fruit bunch of oil palm fibers filled
polypropylene composites has been reported (Myrtha Karina et al., 2007
). Different composites based on polypropylene and reinforced with flax
and glass have been made and their mechanical properties are measured
together with the distribution of the fiber size and the fiber diameter(
Amirhossein Esfandiari, 2007). Composites of polypropylene and four
different types of natural fibers including wood flour, rice hulls,
kenaf fibers, and newsprint were prepared at 25 and 50% fiber contents
and their dynamic mechanical properties were studied and compared with
the pure plastic ( Mehdi Tajvidi et al., 2006). The mechanical
properties of bamboo fiber- reinforced polypropylene composites are
compared with commercially available wood pulp board and it is reported
that bamboo fiber composites are lighter, water-resistant, cheaper and
has more tensile strength than wood pulp composites (Xiaoya Chen et al.,
1998). A systematic study of the mechanical properties of the composites
as a function of fiber loading, and fiber treatment time has been made
for sisal polypropylene composites (Smita Mohanty et al., 2004,
P.V.Joseph et al., 1999).
The main objective of this paper was an Elephant grass
fibre(Scientific name: Pennisetum purpureum), is identified as potential
reinforcement for making composites. Elephant grass fiber reinforced
polypropylene matrix composites have been developed by injection molding
technique with varying percentages of weight (0%, 10%, 15%, 20%, and
25%). The developed composites were then tested for their tensile
Properties.
Experimental
A. Materials
The composites were produced using Elephant grass fibre and
polypropylene pellets. The Elephant grass fibers were chopped into a
length of 3 mm. Then the composites were developed with 0, 5, 10, 15, 20
and 25% (by weight) of Elephant grass Fiber.
B. Extraction of Fibre
In this method, the culms of elephant grass were cut at their base
and the leaves at the nodes and end of the culms were trimmed. After
trimming, the culms were dried in shade for a period of one week. The
node portions were removed by cutting, and the culms were separated into
pieces. The short culms separated are composed of exodermis (bark),
vascular bundle sheaths, soft tissue cells and endodermis (inner surface
layers). The hollow cylindrical portion of culms was taken for
extracting fibre and made into four strips peeling them in longitudinal
direction. These strips of elephant grass were soaked in water for about
3weeks. After this process the strips were subjected to a mechanical
process, by beating them gently with a plastic mallet in order to loosen
and separate the fibre. The resulting fibre bundle was scrapped with
sharp knife and combed until individual fibers were obtained.
C. Composite Fabrication
An oven of size 450 X 450 X 450 mm (model CIC-12) is used to dry
the extracted fibers. The oven has an automatic temperature control unit
with an operating range 0-350[degrees]C. Proper proportions of fibers
(0, 5, 10, 15, 20 and 25%) by weight and polypropylene pellets were
properly mixed to get a homogeneous mixture. The mixture was then placed
in a 2.5 tonne hydraulic Injection Molding Machine, Model JIM-1 HDB,
supplied by Texair Plastics Limited, Coimbatore. At a temperature of
210[degrees]C and pressure of 1100 kgf/[cm.sup.2], composites of
different weight fractions were developed. Five specimens were made for
each weight fraction of Elephant grass fiber composites.
D. Testing
A 2 ton capacity-Electronic tensometer, METM 2000 ER-I model
supplied by M/S Mikrotech, Pune was used to find the tensile strength of
the specimens. Tensile test specimens (Fig.1) were made in accordance
with ASTM-D 638M to measure the tensile properties. The samples were
tested at a crosshead speed of 0.5 mm/min and the strain was measured
with an extensometer of the machine. The density of the composite was
measured using picnometric procedure.
[FIGURE 1 OMITTED]
Results and Discussion
The density of the elephant grass fiber is 817.53 kg/[m.sup.3]
which is very less compared to established fibers like sisal, jute, coir
and banana. So, elephant grass fiber can be used for designing light
weight materials. The diameter of Elephant grass fibre under
consideration varied between 190 [micro]m and 400 [micro]m. The
percentage yield in quantity of fibers extracted by process of retting
was 56.
Table 1 shows the variation of tensile properties of polypropylene
composites with fiber weight percentages.
Fig.2 shows the average tensile stress Vs % weight fraction of the
fibre. It can be observed that upto 10 % weight fraction of the fibre,
the tensile strength has increased than pure polypropylene composite.
However at higher weight percentages, the strength gets reduced. The
tensile strength of the pure polypropylene composite is 16.746 MPa. The
maximum tensile strength of the Elephant grass composite is 20.98 MPa
and it occurs in 10 percent wt. fibre composite. After 10 percent wt.
fiber as reinforcement in the composites, tensile strength was decreased
with higher percentages of fiber. The incorporation of fibers into
thermoplastics leads to poor dispersion of fibers due to strong inter
fiber hydrogen bonding which holds the fibers together. Improper
adhesion hinders the considerable increment of tensile strength. Thus,
as fibre percentage increases, gathering of fibers takes place instead
of dispersion and melted polypropylene cannot wet them properly due to
non entrance of melt through the adjacent two fibers. Since no adhesion
is present between the fibers and fibers are also not bonded with
matrix, failure occurs before attaining the theoretical strength of
composite. Thus high fiber content was limited by the incompatibility
issue unless coupling agent is used.
Fig.3 shows the average tensile modulus Vs % weight fraction of the
fibre. The tensile modulus of the pure polypropylene composite is
105.255 MPa. The maximum tensile modulus value of Elephant grass
composite is 618.554Mpa and it occurs in 25 percent wt. fibre composite.
The tensile modulus changes in an irregular manner. This is mainly due
to the fiber to fiber interactions occurring at high fiber loading. At
low fiber loading the matrix is not restrained by enough fibers and
highly localised strain occurs in the matrix at low stresses, causing
the bond between the matrix and fiber to break leaving the matrix
diluted by non-reinforcing debonded fibers.
Fig.4 shows the specific tensile strength Vs % weight fraction of
the fibre. The specific tensile strength of the pure polypropylene
composite is 0.017163 MPa/ Kg/[m.sup.3]. The maximum specific tensile
strength value of Elephant grass composite is 0.021002 MPa/Kg/[m.sup.3]
and it occurs in 10 percent wt. fibre composite.
Fig.5 shows the specific tensile modulus Vs % weight fraction of
the fibre. The specific tensile modulus of the pure polypropylene
composite is 0.107879 MPa/ Kg/[m.sup.3]. The maximum specific tensile
modulus value of Elephant grass composite is 0.62285 MPa/Kg/[m.sup.3]
and it occurs in 25 percent wt. fibre composite.
The curves drawn between percentage weight fraction of fiber and
specific values of strength and modulus as shown in Figs 4 and 5 exhibit
the similar trend observed for tensile strength and modulus and the same
cause is attributed as stated above.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Conclusions
Vegetation associated with agriculture and forestry is a large
source for extracting fibers, which has been largely under utilized.
Fibers that can be extracted from the vegetation with water retting
process are inexpensive.
The process of extraction of Elephant grass fiber is simple and
results in an excellent quality of fiber. 10 percent fiber weight
composites has better tensile strength compared 5, 15, 20&25 weight
percent fiber composites. 25 percent fiber weight composites have better
tensile modulus compared to other fiber weight percentage composites.
The density of the Elephant grass fiber is less than that of well
established natural and synthetic fibers. So, elephant grass fiber can
be used as natural reinforcement in the composites for the design of
light weight materials.
Acknowledgements
The authors gratefully acknowledge the financial support extended
by the All India Council for Technical Education, New Delhi, India, (F.
No. 8023/RID/BOR/MOD12) to carryout the research project.
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N. Ravi Kumar (a) *, K. Ramji (b), A. V. Ratna Prasad (c) and K.
Murali Mohan Rao (d)
(a) Mechanical Engineering Department, V.R. Siddhartha Engineering
College, Vijayawada-520 007, India
* Corresponding Author: Email:
[email protected]
(b) Mech. Engg. Dept., College of Engineering, Andhra University,
Visakhapatnam, India
(c) Mech. Engg. Dept., V.R. Siddhartha Engineering College,
Vijayawada-520 007, India
(d) Principal, Vivek Institute of Technology, Vijayawada, A.P.,
India
Table 1: Variation of tensile properties of polypropylene
composite with fiber weight percentages.
Fiber content Tensile Strength Tensile Modulus Elongation at
(MPa) (MPa) break (%)
0 16.746 105.255 6.086
5 18.211 200.155 4.696
10 20.978 171.397 6.87
15 9.9106 147.676 2.957
20 8.2 282.701 1.67
25 7.55 618.554 0.591
