*Author for correspondence E-mail: karthirajme@gmail.com
Effect of alkali treatment and fiber length on impact behavior of coir fiber reinforced epoxy composites
A Karthikeyan1* and K Balamurugan2
1Department of Mechanical Engineering, KSR College of Engineering, Tiruchengode 637 215, India
2Department of Mechanical Engineering, Institute of Road and Transport Technology, Erode 638 316, India
Received 02 April 2012; revised 01 August 2012; accepted 02 August 2012
This study presents effect of fiber length and sodium hydroxide (NaOH) treatment on impact behavior of coir fiber composites. Coir fibers were treated with NaOH (conc. 2, 4, 6, 8 and 10%) for 10 days. For each group of coir, fiber length was 10, 20 and 30 mm. Coir fiber was used as a reinforcement and epoxy as a matrix to fabricate composites by hand lay-up technique.
Alkali treated specimens showed an improvement in impact strength of 15% when compared with untreated fiber.
Keywords: Alkali treatment, Coir fiber, Fiber length, Impact strength, Scanning electron microscope
Introduction
Natural fibers are being used as reinforcing components for thermoplastic and thermoset matrices, because of renewability, biodegradability, availability and environmental friendliness offered by natural fibers1,2. Natural fibers (sisal, coir, jute, ramie, pineapple leaf, and kenaf) have potential to be used as a replacement for glass or other traditional reinforcement materials in composites, which are increasingly adopted to replace synthetic polymers in industrial applications3,4. Coir fibers can substitute wood and other materials in fabrication of composites5,6. Mechanical and electrical properties of coir fiber reinforced polypropylene composite have been reported7. Coir fiber reinforced rubber materials have found wider application8,9. It is estimated that 55 billion coconuts are produced annually in the world, but only 15% of husk fibers are actually recovered for use, leaving most husks abandoned. Coir fibers are usually treated to improve resin-fiber interfacial bonding9,10. Coir fiber- polyester composites were tested as helmets, as roofing and post-boxes11. Treated sugar palm fiber reinforced epoxy composite with 0.5M NaOH solution showed 12.85% increase in impact strength compared with untreated composite12. Short pine apple leaf fiber with 4% NaOH showed improved mechanical properties13. Treated screw pine fiber polyester composites exhibited
improvement in mechanical properties14. Composites with coir loading (9-15 wt%) have a flexural strength of 38 MPa. Coir polyester composites with coir fibers (fiber loading, 17 wt%) were tested in tension, flexure and notched Izod impact15. Other studies16,17 were found on fiber treatments on performance of coir-polyester composites.
This study presents potential use of coir fiber as a reinforcing material in polymer composites and investigates its effect on impact behavior of resulting composites with alkali treatment.
Experimental Section
Coconut husks were soaked in water for 5 months for retting, and then husks were beaten gently with a hummer. Coir fibers were removed from shell and separated with a comb. After drying in room temperature (RT), coir fibers were combed in a carding frame to further separate fibers into individual state. Coir fibers were treated in NaOH solutions (conc. 2, 4, 6, 8 and 10%) at RT (27-29°C) for 10 days. After alkali treatment, fibers were immersed in distilled water for 1 h to remove residual NaOH.
Specimen Preparation
Treated and untreated coconut coir fibers were chopped (length 10, 20 and 30 mm). Various composite materials were fabricated by hand lay-up technique. Short coconut coir fibers were used to reinforce Epoxy LY
556 resin (Bisphenol A Diglycidyl Ether), which was used as matrix material. Low temperature curing epoxy resin (Araldite LY 556) and corresponding hardener (HY951), supplied by Covai Seenu & company, were mixed in ratio of 10:1 by wt%. Coir fiber was collected from rural area of Tamil Nadu, India. Three different types of composites18 [C1 {epoxy (70 wt%) + coir fiber (length 10 mm)}, C2 {epoxy (70 wt%) + coir fiber (length 20 mm)} and C3 {epoxy (70 wt%) + coir fiber (length 30 mm)}] were fabricated with and without chemical treatment of NaOH.
Impact Strength
Low velocity instrumented impact tests were carried out on composites specimens. Specimens were cut from fabricated plates in accordance with ASTM D256.
Specimen was fixed in slot and impact load was applied, by releasing pendulum. Load required to break specimen was noted down and procedure was repeated for different
trials. A minimum of 10 specimens were tested in each group.
Results and Discussion
Mechanical Characteristics of Composites
Fiber length and alkali treatment have significant effect on impact strength (Fig. 1). When stress level exceeds fiber strength, fiber fracture occurs. Fractured fiber may be pulled out of matrix, which involves energy dissipation18. Resistance to impact loading of coir fiber reinforced epoxy resin (CF-ER) composites improves with increase in fiber length (Fig. 1). Scanning electron micrograph (SEM) of an impact tested specimen (Fig. 2) of CF-ER composite shows pulled out fiber in the composite. Fiber has offered resistance and has absorbed energy in its own fracture (Fig. 2a). Fiber pull- out is longer and fiber surface is cleaner, indicating an even worse adhesion between coir fiber and epoxy resin (Fig. 2b).
Fig. 1—Impact characteristics of coir fiber composites
Fig. 2—SEM of untreated coir/epoxy specimen after impact testing: a) fiber resistance; and b) adhesion between coir fiber and epoxy resin coir fiber 2% NaOH 4% NaOH 6%NaOH 8% NaOH 10% NaOH
10 mm 20 mm 30 mm
Impact energy, kJ/m2
Effect of Chemical Concentration on Impact Strength
SEM images of coir fiber before alkali treatment (Fig. 3a) shows that surface of coir fiber is covered with a layer of substance, which may include pectin, lignin and other impurities. Surface is not smooth, spread with nodes and irregular strips. After alkali treatment (Fig. 3 b-f), important modification done is the disruption
of hydrogen bonding in network structure, thereby increasing surface roughness. Most of the lignin and pectin are removed resulting in a rough surface. Rows of pits noticed on the surface would increase mechanical bonding between matrix and coir fiber in composite fabrication. It is observed that alkali led to an increase in amorphous cellulose content (Fig. 4a) at the expense of
Fig. 3—SEM image of coir fiber: a) before alkali treatment; b) 2% NaOH treatment; c) 4% NaOH treatment; d) 6% NaOH treatment; e) 8%
NaOH treatment; and f) 10% NaOH treatment
Fig. 4—NaOH treatment versus: a) cellulose content in fiber; b) lignin content in fiber; and c) fiber diameter
a) b)
c)
NaOH treatment
Chemical conc., % Chemical conc., %
Chemical conc., %
NaOH treatment NaOH treatment
Cellulose, % Lignin, %
Fibre diam, mm
crystalline cellulose. Thus NaOH treatment increases surface roughness, resulting in better mechanical interlocking, and increases amount of cellulose exposed on fiber surface, increasing the number of possible reaction sites19. Denser NaOH solution provided more Na+ and OHions to react with substance on fiber, causing greater amount of lignin, pectin to leach out (Fig. 4b). A decreased trend is seen in fiber lignin content with increased NaOH density20. A decrease in diameter observed with increasing in chemical concentration shows that increase in chemical concentration decreases in fiber strength (Fig. 4c). After alkali treatment, resultant rough fiber surface might improve adhesive ability of fiber with matrix. SEM picture of an impact tested specimen made of NaOH treated CF-ER composite shows no pulled out fiber in this composite (Fig. 5). Higher impact strenght of NaOH treated coir/epoxy specimen was due to good interface bonding between fiber and matrix (Fig. 5a).
No fiber pull-out in composite surface indicates (Fig. 5b) improvement in adhesion between coir fiber and epoxy resin.
Conclusions
CF-ER composites with different lengths, treated with and without alkali were fabricated by hand layup techniques. Alkali treated CF-ER composite had better impact strength (27 kJ/m2) along with increased fiber length. Coir fiber (30 mm) showed better impact strength compared to other lengths. Untreated coir fiber resulted in low impact strength due to poor interfacial bonding.
However, adhesion was improved by surface modification of coir fiber. No significant difference in impact strength of composite was noticed for 6% and 8% alkali concentrations. When alkali concentration was increased
to 10%, decrease in fiber strength was observed. Among alkali treated CF-ER composites, 6% alkali treated composite showed better impact strength compared to untreated CF-ER composites.
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Fig. 5—SEM of NaOH treated coir/epoxy specimen after impact testing: a) interface bonding between fiber and matrix; and b) adhesion between coir fiber and epoxy resin
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