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The online version of this article can be found at:

DOI: 10.1177/0731684409345413September 2009

2010 29: 2124 originally published online 8Journal of Reinforced Plastics and CompositesP. Noorunnisa Khanam, G. Ramachandra Reddy, K. Raghu and S. Venkata Naidu

CompositesTensile, Flexural, and Compressive Properties of Coir/Silk Fiber-reinforced Hybrid

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Tensile, Flexural, and Compressive

Properties of Coir/Silk Fiber-reinforcedHybrid Composites

P. NOORUNNISAKHANAM,* G. RAMACHANDRA REDDY, K. RAGHU AND

S. VENKATANAIDU

Department of Polymer Science & Technology, Sri Krishna

Devaraya University, Anantapur-515003, Andhra Pradesh, India

ABSTRACT: The hybrid composites of coir/silk unsaturated polyester-based hybrid compositeswith different fiber lengths were prepared. Coirsilk fibers are taken in the ratio of 1 : 1, andthese fibers are incorporated with unsaturated polyester resin with different fiber lengths like 1, 2,and 3 cm. The variation of mechanical properties such as tensile strength, flexural strength, andcompressive strength of these composites with different fiber lengths has been studied. In the presentwork hand lay-up method was used for making the composites. Coir fibers are treated with NaOHand the effect of alkali treatment on the tensile, flexural, and compressive properties of the coir/silkhybrid composites has also been studied. Significant improvement in tensile, flexural, and compres-sive strengths of the coir/silk hybrid composites has been observed by these treatments.

KEY WORDS: coir fiber, silk fiber, unsaturated polyester resin, hybrid composites, tensilestrength, flexural strength, compressive strength.

INTRODUCTION

NATURAL FIBERS ARE a major renewable resource material throughout the worldspecifically in the tropics. According to the food and agricultural organization survey,natural fibers like jute, sisal, coir, banana, etc. are abundantly available in developing

countries. Recent reports indicate that plant fibers can be used as reinforcement in poly-

mer composites replacing to some extent more expensive and non-renewable synthetic

fibers such as glass, carbon, etc. Among all reinforcing fibers, natural fibers have gainedsubstantial importance as reinforcements in polymer matrix composites. A lot of work has

been done by many researchers on the composites based on these fibers [15].

Natural fibers are an attractive research area because they are eco-friendly, inexpensive,

abundant and renewable, lightweight, low density, high toughness, high specific properties,

biodegradability and non-abrasive to processing characteristics, and lack of residues upon

incineration. Therefore, natural fibers can serve as reinforcements by improving the strength

and stiffness and also by reducing the weight of the resulting biocomposite materials

although the properties of natural fibers vary with their source and treatments.

*Author to whom correspondence should be addressed. E-mail: noor_phd@yahoo.com

Journal ofREINFORCED PLASTICS AND COMPOSITES, Vol. 29, No. 14/2010 2124

0731-6844/10/14 21244 $10.00/0 DOI: 10.1177/0731684409345413 The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

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Natural fiber composites or biocomposites are defined as composite materials composed

of biodegradable natural fibers as reinforcement and biodegradable or non-biodegradable

polymers as matrix. Natural fibers are largely divided into two categories depending on

their origin: plant based and animal based. In general plant-based fibers are lignocellulose

in nature composed of cellulose, hemicellulose, and lignin, whereas animal-based fibers are

of proteins e.g., silk and wool.

Hybrid composites are materials made by combining two or more different types

of fibers in a common matrix. They offer a range of properties that cannot be obtained

with a single kind of reinforcement [68]. Indicula et al. [9] studied the mechanical proper-

ties of banana/sisal hybrid composites. John and Venkata Naidu [1012] studied the

tensile, flexural, impact, and compressive properties of sisal/glass hybrid composites.

Noorunnisa Khanam et al. [13] studied the tensile, flexural, and compressive properties

of sisal/silk polyester-based hybrid composites. Pothan et al. [14] studied the static and

dynamic mechanical properties of banana and glass fiber woven fabric-reinforced polyes-

ter composites. In the present work the author studied the tensile, flexural, and compres-

sive properties of untreated and treated coir/silk fiber-reinforced hybrid composites.The coir fiber is abundantly available in nature and the silk wastes are abundantly avail-

able in silk reeling and twisting units, which are unutilized. Further coir and silk are

natural biodegradable materials and they can be combined in the same matrix to produce

hybrid composites that offer a range of properties.

EXPERIMENTAL

Materials

Coir fibers taken from local sources and the twisted waste silk fiber (un-degummed)

taken from twisting units of Dharmavaram were used for the present study. Unsaturated

polyester resin obtained from Allied Marketing Co., Secunderabad was used as matrix.

Methyl ethyl ketone peroxide and cobalt napthenate of commercial grade supplied by M/s.

Bakelite Hylam, Hyderabad were used as a catalyst and accelerator, respectively.

Fiber Treatment

The coir fibers were taken in a glass tray. A 2% NaOH solution was added in to the tray

and the fibers were allowed to soak in the solution for 1 h. The fibers were then washed

thoroughly with water to remove the excess of NaOH sticking to the fibers. Final washing

was carried out with distilled water and the fibers were then dried in hot air oven at 70 C

for 3 h. The fibers were chopped into short fiber lengths of 1, 2, and 3 cm for molding the

composites.

Preparation of Composites

Unsaturated polyester resin and styrene were mixed in the ratio of 100: 25 parts by

weight respectively. Later, 1 wt% methyl ethyl ketone peroxide and 1 wt% cobalt naphtha-

nate were added and mixed thoroughly. Hand lay-up technique for making test specimens

was used. To make the test specimens, the matrix system was poured into a mold wherefibers were reinforced and air bubbles were removed carefully with a roller. The closed

Properties of Coir/Silk Fiber-reinforced Hybrid Composites 2125

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mold was kept under pressure for 24 h at room temperature. To ensure complete curing the

composite samples were post-cured at 80C for 1 h and test specimens of the required size

were cut out from sheets. Composites with different fiber lengths like 1, 2 and 3 cm, treated

and untreated were prepared by keeping the weight ratio of coir/silk at 1 : 1.

Mechanical Tests

The tensile strength, flexural strength, and compressive strength of 1, 2, and 3 cm lengths

of treated and untreated coir/silk polyester-based hybrid composites were carried out in

UTM. Five samples were tested in each case and the average value was reported.

RESULTS AND DISCUSSIONS

Tensile Strength

The tensile strength, modulus, and percent elongation of the composites were measured

by using an INSTRON model 3369 UTM instrument. Tensile properties of randomly

oriented coir-silk fiber hybrid composites with different fiber lengths are presented in

Table 1. From the table, it is seen that 2 cm fiber length composites have higher tensile

strength than 3 and 1 cm fiber length composites. Further it is observed that treated

composites possess higher tensile strength than untreated. This is due to the fact that

alkali treatment improves the adhesive characteristics of coir fiber surface by removing

hemicellulose and lignin. This surface offers better fibermatrix interface adhesion and an

increase in mechanical properties.

Flexural Strength

Flexural strength was determined by using an INSTRON model 3369 Instrument. The

variation of flexural strength with different fiber lengths are presented in Table 1. It is

observed from the table that the 2 cm fiber length composites have higher flexural strength.

It is also observed that the alkali-treated composites showed superior flexural properties

than untreated composites, because alkali treatment improves the adhesive characteristics

of coir fiber surface by removing hemicellulose and lignin. This surface offers better

fiber

matrix interface adhesion and an increase in mechanical properties.

Table 1. Tensile strength, flexural strength, and compressive strength of untreated andtreated polysted-based coir/silk hybrid composites with different fiber lengths.

Tensile strength

(MPa)

Flexural strength

(MPa)

Compressive strength

(MPa)

S. no.

Fiber

length (cm)

Untreated

composites

Treated

composites

Untreated

composites

Treated

composites

Untreated

composites

Treated

composites

1 1 11.419 15.014 37.419 39.533 134.895 154.034

2 2 15.624 17.24 43.744 45.067 142.087 162.975

3 3 12.924 16.144 39.692 42.018 138.401 159.822

2126 P.N. KHANAM ET AL.

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Compressive Strength

Compressive strength was determined by using an INSTRON model 3369 Instrument.

Results are tabulated in Table 1. From this table it is observed that 2 cm fiber length

composites have higher strength than 1 and 2 cm. Further these values are found to be

slightly higher to treated fiber composites, when compared to untreated composites.

CONCLUSIONS

The natural fiber composites of coir/silk unsaturated polyester-based hybrid composites

were prepared with varying fiber lengths. The variation of tensile strength, flexural

strength, and compressive strength of unsaturated polyester-based coir/silk hybrid com-

posites has been studied as a function of fiber length. It is observed that 2 cm fiber length

hybrid composites are having higher tensile, flexural, and compressive strength than 1 and

3 cm. The effects of alkali on the tensile, flexural, and compressive properties have alsobeen studied. It is found that treated hybrid composites showed higher strength than

untreated composites.

REFERENCES

1. Nishino, T., Hirao, K., Kotera, M., Nakamae, K. and Inagaki, H. (2003). Kenaf Reinforced BiodegradableComposite,Composites Science and Technology, 63: 12811286.

2. Li, Y., Mai, Y.-W. and Ye, L. (2000). Sisal Fibre and its Composites: A Review of Recent Developments,Composites Science & Technology, 60: 20372055.

3. Mwaikambo, L. Y. and Ansell, M. P. (2002). Chemical Modification of Hemp, Sisal, Jute and Kepok fibersby Alkalization, Journal of Applied Polymer Science, 84: 22222234.

4. Han, S. O., Lee, S. M., Park, W. H. and Cho, D. (2006). Mechanical and Thermal Properties of WasteSilk Fiber Reinforced Poly(butylenes succinate) Bio Composites, Journal of Applied Polymer Science, 100:49724980.

5. Hill, C. A. S. and Abdul Khalil, H. P. S. (2006). Effect of Fibre Treatment on Mechanical Properties of Coiror Oil Palm Fiber Reinforced Polyester Composites, Journal of Applied Polymer Science, 78: 16851697.

6. Kalaprasad, G., Thomas, S., Parvithran, C., Nilakantan, N. R. and Balakrishna, S. (1996). Hybrid Effect inthe Mechanical Properties of Short Sisal/Glass Hybrid Fiber Reinforced Low Density PolyethyleneComposites,Journal of Reinforced Plastics & Composites, 15: 4873.

7. Mwaikambo, L. Y. and Bisanda, E. T. N. (1999). The Performance of Cotton-kapok Fabric PolyesterComposites, Polymer Testing, 18: 181198.

8. Hariharan, A. B. A. and Abdul Khalil, H. P. S. (2005). Lignocellulose-based Hybrid Bilayer LaminateComposite: Part 1 Studies on Tensile and Impact Behavior of Oil Palm Fiber-glass Fiber-reinforcedEpoxy Resin, Journal of Composite Materials, 39(8): 663684.

9. Indicula, M., Malhotra, S. K., Joseph, K. and Thomas, S. (2005). Dynamic Mechanical Analysis of

Randomly Oriented Intimately Mixed Short Banana/Sisal Hybrid Fibre Reinforced Polyester Composites,Composites Science & Technology, 65: 10771087.

10. John, K. and Venkata Naidu, S. (2004). Tensile Properties of Unsaturated Polyester based Sisal Fibre-glassFibre Hybrid Composites, Journal of Reinforced Plastics and Composites, 23(17): 18151819.

11. John, K. and Venkata Naidu, S. (2004). Effect of Fibre Content and Fibre Treatment on Flexural Propertiesof Sisal Fibre/Glass Fibre Hybrid Composites, Journal of Reinforced Plastics and Composites, 23(15):16011605.

12. John, K. and Venkata Naidu, S. (2004). Sisal Fiber/Glass Fibre Hybrid Composites: The Impact andCompressive Properties, Journal of Reinforced Plastics and Composites, 23(12): 12531258.

13. Noorunnisa Khanam, P., Mohan Reddy, M., Raghu, K., John, K. and Venkata Naidu, S. (2007). Tensile,Flexural and Compressive Properties of Sisal/Silk Hybrid Composites, Journal of Reinforced Plastics andComposites, 26(10): 10651070.

14. Pothan, L. A., Ptschke, P., Habler, R. and Thomas, S. (2005). The Static and Dynamic MechanicalProperties of Banana and Glass Fibre Woven Fabric-reinforced Polyester Composite, Journal ofComposite Materials, 39(11): 10771087.

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