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Tensile strength of elephant grass fiber reinforced polypropylene composites.


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.


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.


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.






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.


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.


[1] Mohanty, A. K., Misra, M. and Drzal, L. T. (2002): Sustainable Bio-Composites from Renewable Resources:Opportunities and Challenges in the Green Materials World, Journal of Polymers and the Environment, Vol. 10, No. 1-2, pp. 19-26.

[2] Biagiotti.J., S.Fiori, L.Torre, M.A. Lopez-Manchado and J.M.Kenny, 2004, Mechanical properties of polypropylene matrix composites reinforced with natural fibers: A statistical approach. Polymer Composites, 25(1): 26-36.

[3] Nabi Saheb, D. and Jog, J. P. (1999). Natural Fiber Polymer Composites: A Review, Adv. Polym. Tech.,18(4): 351-363.

[4] Myrtha Karina, Holia Onggo and Anung Syampurwadi. 2007, Physical and Mechanical Properties of Natural fibers filled polypropylene composites and its recycle, Journal of Biological Sciences,7 (2): 393-396.

[5] Amirhossein Esfandiari, 2007, Mechanical properties of PP/Jute and Glass Fibers Composites: The statistical Investigation, Journal of Applied Sciences 7 (24), 3943-3950.

[6] Mehdi Tajvidi, Robert H. Falk, John C. Hermanson.2006, Effect of Natural Fibers on Thermal and Mechanical Properties of Natural Fiber Polypropylene Composites Studied by Dynamic Mechanical Analysis, Journal of Applied Polymer Science, Vol. 101, 4341-4349 .

[7] Xiaoya Chen, Qipeng Guo, Yongli Mi, 1998, Bamboo fiber reinforced polypropylene composites: A study of the Mechanical Properties, Journal of applied polymer science, Vol.69, 1891-1899.

[8] Smita mohanty, Sushil K.verma, Sanjay K. Nayak, Sudhansu S. Tripathy, Influence of fiber treatment on the performance of Sisal polypropylene composites, Journal of applied polymer science, Vol.94, 1336-1345.

[9] P.v. Joseph, Kuruvilla Joseph, Sabu Thomas,1999, Effect of processing variables on the mechanical properties of sisal fiber reinforced polypropylene composites, Composites Science and Technology, 59, 1625-1640.

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:

(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
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Article Details
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Author:Kumara, N. Ravi; Ramji, K.; Prasad, A.V. Ratna; Rao, K. Murali Mohan
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Nov 1, 2009
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