Studies on mechanical, thermal and dynamic mechanical properties of

Materials and Design 59 (2014) 63–69

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Studies on mechanical, thermal and dynamic mechanical properties of untreated (raw) and treated coconut sheath fiber reinforced epoxy composites S.M. Suresh Kumar, D. Duraibabu, K. Subramanian ⇑ Department of Chemistry, Anna University, Chennai 600025, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 31 July 2013 Accepted 8 February 2014 Available online 21 February 2014

a b s t r a c t The untreated (raw) coconut sheath fiber reinforced epoxy (UTCSE) composite and treated coconut sheath fiber reinforced epoxy (TCSE) composite have been fabricated using hand layup followed by compression molding technique. The prepared specimens were characterized by Fourier transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM) techniques. The prepared specimens are cut as per ASTM Standards to measure tensile, flexural and impact strengths by using universal testing machine and izod impact tester respectively. The treated coconut sheath fiber reinforced epoxy composite (TCSE) posses higher mechanical strength and thermal stability compared to untreated (raw) coconut sheath fiber reinforced epoxy composite (UTCSE). In the SEM fracture analysis, TCSE composite showed better fiber–matrix bonding and absence of voids compared to UTCSE composite. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Natural fibers are considered as an alternative to glass fibers for use in composites as reinforcing materials. The advantages of natural fibers over glass fibers are low cost, low density, easy availability, low energy content and recyclability [1]. The industrial use of natural fibers as reinforcing agent in composite materials started at the beginning of the 20th century with the manufacturing of sheets, tubes and pipes for electronic purposes. Some limitations of natural fiber composites are poor wettability, incompatibility of fibers with some polymeric materials and high moisture absorption of fibers, make them undesirable for certain applications. The main problem often encountered is the fiber and matrix adhesion problem, due to the incompatibility between hydrophilic nature of natural fibers and hydrophobic nature of polymeric matrices. Chemical modification of fibers is a common method for modifying the fiber surface which enhances the bonding between a natural fiber and a polymeric matrix [2]. Several articles have been reported on composites based on natural fiber and their surface modification such as bagasse [3], Moroccan hemp [4], Sansevieria cylindrica [5], banana [6,7], sugar palm [8] and coir [9] as reinforcing materials in polymeric matrices. The preliminary ⇑ Corresponding author. Tel.: +91 44 22358660; fax: +91 44 22200660. E-mail addresses: [email protected], [email protected] (K. Subramanian). http://dx.doi.org/10.1016/j.matdes.2014.02.013 0261-3069/Ó 2014 Elsevier Ltd. All rights reserved.

kathsubramanianan

studies of coconut sheath fibers have been reported in literatures [10,11]. From the literature review, lack of information was obtained for untreated (raw) and treated coconut sheath fiber as reinforcing material in epoxy matrix. The coconut sheath fibers are huge waste material and it was obtained from coconut tree. Hence, we planned to study the coconut sheath fiber as reinforcement in epoxy matrix. In the present research work, we are reporting the usage of untreated (raw) and treated coconut sheath fiber as reinforcing material in epoxy matrix and also we have carried out the mechanical, thermal and dynamic mechanical behavior of untreated (raw) coconut sheath fiber and treated coconut sheath fiber reinforced epoxy composites (UTCSE and TCSE).

2. Materials and methods 2.1. Materials Epoxy resin (LY 556) and triethylenetetramine (HY 951) were purchased from Huntsman, USA. The density of epoxy resin and triethylenetetramine were 1.15–1.20 g/cm3 and 0.97–0.99 g/cm3 respectively. (This was obtained from the manufacturer’s test report). Sodium hydroxide and acetic acid were purchased from SRL Pvt Ltd., India. The coconut sheath fibers were collected from local agricultural resources (Tanjore region, Tamilnadu, India).

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S.M. Suresh Kumar et al. / Materials and Design 59 (2014) 63–69

2.2. Treatment of coconut sheath fibers

2.4. Characterization

The raw coconut sheath fibers were washed several times with water for the complete removal of plant debris and dried at room temperature for 24 h. After this, the coconut sheath fibers were immersed in 5% sodium hydroxide solution for an hour and then neutralized with 5% acetic acid solution [11]. Finally, the treated coconut sheath fiber was dried at 110 °C for 1 h and used as a reinforcing material for polymer composites.

The FT-IR spectra of UTCSE composite and TCSE composite were recorded in the range of 400–4000 cm1 using Perkin Elmer instrument. The samples were taken in powder form and prepared into pellets by using KBr. The tensile and flexural tests were carried out on the UTM machine (Model H10K-S Tinius Olsen) with cross head speed of 5 mm/min according to the ASTM: D3039/D3039 M-08 and ASTM: D790-10 standards. For tensile tests, the dimensions of 200 mm 25 mm 4 mm and 200 mm 25 mm 4.1 mm were cut from the UTCSE composite and TCSE composite plates manufactured. The tensile geometry of UTCSE composite and TCSE composite are shown in Fig. 1a and b. For flexural tests, the dimensions of 127 mm 12.7 mm 4 mm and 127 mm 12.7 mm 4.1 mm were cut from the UTCSE composite and TCSE composite plates manufactured. Five samples were taken for each test and the average of the results was taken. The flexural geometry of UTCSE composite and TCSE composite are shown in Fig. 2a and b. The impact test was carried out on the Izod impact tester according to the ASTM: D256-10 standard. Five samples were taken for each test and the average of the results was taken. The surface morphology of untreated (raw) and treated coconut sheath fibers and their reinforcing epoxy composites fractured were studied by using FE SEM Hitachi

2.3. Fabrication of composites The mould plate used for preparing untreated coconut sheath fiber reinforced epoxy composite (UTCSE) was made from two rectangular mild steel plates with a dimension of 220 mm 220 mm. Four beadings were used to maintain a 4 mm thickness all around the mould plates. The moulds are cleaned, dried and silicon spray was used as releasing agent, before applying the uncured epoxy resin mixture. The raw coconut sheath fibers were cut into an appropriate size of 200 mm 200 mm. Five layers of raw coconut sheath fiber were preweighed and corresponding amount of epoxy resin was taken. The epoxy resin and hardener were taken in the ratio of 10:1. The stirrer was used to homogenous the mixture. Then, the resin mixture was used to fabricate five layers of coconut sheath fiber by hand layup followed by compression molding technique. The epoxy resin homogeneous mixture was poured into mould plate, then first layer of coconut sheath fiber was placed in the mould plate (hand layup) and then epoxy resin mixture was applied. Then, second layer of coconut sheath fiber was placed in the mould plate (hand layup) and then epoxy resin mixture was applied. This type of procedure was repeated for the subsequent layers, and then the mould plate was closed. It was kept on compression molding machine. Then, uniform pressure of 3.92 MPa was applied over the mould plates (purpose of this is to maintain uniform thickness and to avoid void formation during curing) for 24 h at room temperature curing. After curing, moulds were opened to remove the UTCSE composite. Finally, UTCSE composite was weighed to find out epoxy content. Similar procedure was adopted to fabricate treated coconut sheath fiber reinforced epoxy composite (TCSE). The resulting thickness of UTCSE and TCSE composites were 4 mm and 4.1 mm respectively. After compression we consider, Resin weight (g) = Laminate weight (g) Fiber weight (g) The fiber volume fraction of UTCSE and TCSE composites was calculated by using the formula (1) [12].

Vf ¼

ðW f =qf Þ ðW f =qf Þ þ ðW m =qm Þ

ð1Þ Fig. 1. Tensile geometry of (a) UTCSE composite and (b) TCSE composite.

Vf is fiber volume fraction, Wf and Wm are the weight (g) of fiber and matrix respectively, qf and qm are the density (g/cm3) of fiber and matrix, respectively. The density measurement of the untreated and treated coconut sheath fibers was measured according to the ASTM: D3800M-11 standard test method for density of high modulus fibers. Benzene was used as immersion liquid. Prior to the measurements, the samples were dried overnight in an oven at 80 °C. The density of the fibers was calculating using the formula (2) [13].

qf ¼ q1 W fa =ðW fa W fs Þ

ð2Þ

where ql is the density of benzene (0.8765 g/cm3), Wfa is the weight of fiber in air and Wfs is the weight of fiber in liquid. The density of untreated and treated coconut sheath fiber was 1.20 g/cm3 and 1.25 g/cm3. Fiber volume fractions of UTCSE and TCSE composites were 0.47–0.48 and 0.43–0.44 respectively.

Fig. 2. Flexural geometry of (a) UTCSE composite and (b) TCSE composite.

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possibly be due to the cleavage of the a ether linkages between lignin and hemicelluloses [18]. The cellulose is the major portion of natural fiber which provides strength and influences the mechanical properties of natural fibers [19]. After the chemical treatment of coconut sheath fiber the cellulose percentage was found to be increased. 3.2. FT-IR spectroscopy

Fig. 3. DMA geometry of (a) UTCSE composite and (b) TCSE composite.

S-4800 instrument at 10 kV. The dynamic mechanical analysis were carried out on SII Nanotechnology (Model DMS-6000 Japan) in a three point bending configuration, at a temperature range of 25–150 °C using a specimen with the dimension of 50 mm 10 mm 4 mm and 50 mm 10 mm 4.1 mm for UTCSE composite and TCSE composite. The DMA geometry of UTCSE composite and TCSE composite are shown in Fig. 3a and b. The frequency used with 1.0 Hz at the heating rate of 2 °C/ min for UTCSE composite and TCSE composite. Thermo gravimetric analysis (TGA) was carried out using TGA Q50 TA instrument with a heating rate of 10 °C/min.

The FT-IR spectra of UTCSE composite and TCSE composite are shown in Fig. 4a and b. The UTCSE composite showed the intense absorption band at 1738 cm1 which corresponds to C@O stretching of hemicelluloses [20]. But in the case of TCSE composite there was no such intense absorption band. It clearly indicates that, the hemicelluloses content was decreased by chemical treatment. The UTCSE composite and TCSE composite showed the broad band’s at 3400 cm1 region, due to OH absorption band. 3.3. SEM images of untreated (raw) and treated coconut sheath fiber surfaces The Fig. 5a and b shows the SEM images of untreated (raw) and treated coconut sheath fiber surfaces respectively. The raw coconut sheath fiber surface showed cloud like pattern, due to agglomeration of impurities on their surface (Fig. 5a). But treated coconut sheath fiber surface showed, clear fiber surface and there was no impurities found on the surface (Fig. 5b). 3.4. Mechanical properties of UTCSE and TCSE composites

3. Results and discussion 3.1. Chemical composition of untreated (raw) and treated coconut sheath fibers The chemical composition of the untreated (raw) and treated coconut sheath fibers were analyzed as per the reported procedure [14] and results are shown in Table 1. It can be seen that untreated coconut sheath fiber contains higher percentage of hemicelluloses, lignin and lowest percentage of cellulose. After the alkali (NaOH) treatment we observed that there is decrease in percentage of hemicelluloses, lignin and increase in percentage of cellulose when compared to untreated coconut sheath fibers. The reaction between native cellulose – I (coconut sheath fiber) and alkali treatment is shown in Scheme 1. During the alkali treatment, native cellulose – I is converted into alkali cellulose (cell-ONa+) and also removal of other materials. In this reaction sodium ion expands space into the lattice plane of the native cellulose – I. This leads to the formation of alkali cellulose (intermediate) and it has large space between cellulose molecules, when compared to native cellulose – I. Due to large space, the water molecules can easily enter into the lattice space thus resulting in swelling [15,16]. Then, the alkali cellulose (cell-ONa+) was found to react with acetic acid to form cellulose – II (short crystalline) as shown in Scheme 1. The hemicellulose is partially hydrolyzed and the lignin is depolymerized, giving rise to sugars and phenolic compounds that are soluble in water thus decreasing their content after the alkali treatment [17]. The high solubility of lignin and hemicelluloses might

The tensile, flexural and impact strengths of UTCSE composite and TCSE composite are shown in Table 2. TCSE composite showed higher tensile, flexural and impact strengths compared to UTCSE composite, the treated coconut sheath fiber have better interaction with epoxy resin due to decrease of hemicelluloses and lignin, also removal of some other materials present on the fiber surface by chemical treatment and it also formed inter molecular hydrogen bonding between fiber and epoxy resin. When the untreated (raw) coconut sheath fiber was incorporated into epoxy matrix, it showed less tensile, flexural and impact strengths of UTCSE composite, due to presence of impurities agglomerate on the coconut sheath fiber surface (Fig. 5a). So, the fiber–matrix interaction was less compared to TCSE composite. The tensile, flexural and impact strengths of TCSE composite was increased to 21.19%, 18.88% and 24.39% respectively when compared to UTCSE composite. 3.4.1. Tensile, flexural and impact fractured surfaces of UTCSE and TCSE composites The Fig. 6a and b shows tensile fractured surfaces of UTCSE composite and TCSE composite. In UTCSE composite fractured surface have some voids and de-bonding. In TCSE composite fractured surface, there was absence of voids and better bonding between coconut sheath fiber and epoxy resin compared to UTCSE composite. So the tensile strength of TCSE composite was higher compared to UTCSE composite. The Fig. 7a and b shows the flexural fractured surfaces of UTCSE composite and TCSE composite. In UTCSE composite fractured surface showed poor bonding and bending of some

Table 1 Chemical composition of untreated (raw) and treated coconut sheath fibers. Fibers

Cellulose (%)

Hemicelluloses (%)

Lignin (%)

Other materials (%)

Untreated (raw) coconut sheath fiber Treated coconut sheath fiber

40(±3) 58(±5)

26(±2) 17(±3)

27(±1) 23(±2)

±7 ±2

% – Percentage.

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Cellulose

OH

Cellulose

NaOH

Native cellulose -I

ONa

H2 O

Other materials

Alkali cellulose -I dil. CH3COOH

Cellulose

OH

Cellulose -II Scheme 1. Schematic representation of native cellulose – I, alkali cellulose – I and cellulose – II.

100

a b

UTCSE TCSE

60

1738

Transmittance (%)

80

a

40 b

20

0 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1) Fig. 4. FT-IR spectra of (a) UTCSE composite and (b) TCSE composite.

fibers. But in the TCSE composite fractured surface some of the fibers are bend and rest of the fibers are broken, but no de-bonding. So the flexural strength of TCSE composite was higher than UTCSE composite. The Fig. 8a and b shows impact fractured surfaces of UTCSE composite and TCSE composite. In UTCSE composite fractured surface have some broken fibers and also presence of voids. In the TCSE composite fractured surface showed some broken fibers and there are no voids on the fiber surface. So the impact strength of TCSE composite was higher than UTCSE composite.

3.5. Dynamic Mechanical Analysis (DMA) The mechanical behavior of coconut sheath fiber reinforced epoxy composites was studied using dynamic mechanical analysis (DMA) to investigate the properties of stiffness and damping, these are reported as storage modulus and tan d. 3.5.1. Storage modulus (E0 ) The variation of storage modulus with temperature for UTCSE composite and TCSE composite at the frequency range of 1 Hz are shown in Fig. 9a and b. The higher value of storage modulus for TCSE composite (3.2 GPa) is due to increase in inter molecular hydrogen bonding resulting from the hydroxy groups, when compared to UTCSE composite (2.1 GPa). This leads to the closed packing of the cellulose chain in the treated coconut sheath fiber than untreated coconut sheath fiber, this increases the stiffness of the fibers which is reflected in the increasing in storage modulus of TCSE composite than UTCSE composite [6]. The close packing of the cellulose in the treated fiber restricts the movement of the epoxy resin in the TCSE composite, which increases the storage modulus of TCSE composite than UTCSE composite. The reinforcement imparted by the treated fibers induced better stress transfer and better fiber matrix adhesion [21]. 3.5.2. Damping parameter (tan d) The glass transition temperature (Tg) was determined from the peak position of damping parameter (tan d), due to the transformation of rigid to elastic state of the material. The variation of tan d with temperature for UTCSE composite and TCSE composite at

Fig. 5. SEM images of (a) untreated (raw) coconut sheath fiber surface and (b) treated coconut sheath fiber surface.

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S.M. Suresh Kumar et al. / Materials and Design 59 (2014) 63–69 Table 2 Tensile, flexural and impact strengths of UTCSE composite and TCSE composite. Composites

Number of samples tested

Tensile strength (MPa)

Average tensile strength (MPa)

Flexural strength (MPa)

Average flexural strength (MPa)

Impact strength (J/mm)

Average impact strength (J/mm)

UTCSE

1 2 3 4 5 1 2 3 4 5

43.50 49.15 51.35 47.65 50.10 55.34 60.81 56.62 59.74 60.49

48.35

62.26 65.71 63.49 66.14 65.40 75.07 78.83 76.91 79.57 73.62

64.60

4.29 4.98 4.06 5.04 4.18 5.48 5.79 5.28 5.94 5.56

4.51

TCSE

58.60

76.80

5.61

Mega Pascal (MPa). Joules/millimeter (J/mm).

Fig. 6. Tensile fractured surfaces of (a) UTCSE composite and (b) TCSE composite.

Fig. 7. Flexural fractured surfaces of (a) UTCSE composite and (b) TCSE composite.

the frequency range of 1 Hz are shown in Fig. 10a and b. The tan d value of UTCSE composite (0.3248 at 84.1 °C) was higher than that of TCSE composite (0.2692 at 82.4 °C). The incorporation of treated fiber reduces the damping property of the TCSE composite when compared to UTCSE composite, due to restricted movement of polymeric molecules, raised the storage modulus value and reduce the viscoelastic lag between stress and strain [22]. As a result, the amount of matrix (volume) is inadequate to dissipate the vibrational energy properly and hence tan d decreases. The lower value

of tan d for TCSE, indicates that the TCSE composite has good load bearing capacity, high interfacial adhesion and improved stress transfer [21]. 3.6. Thermo gravimetric analysis (TGA) The TGA curves of UTCSE composite and TCSE composite are shown in Fig. 11a and b. The UTCSE composite and TCSE composite showed the initial weight loss at 100–105 °C indicates the removal

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Fig. 8. Impact fractured surfaces of (a) UTCSE composite and (b) TCSE composite.

a b

4x109

100

UTCSE TCSE

a b

UTCSE TCSE

80 9

Weight (%)

E' (Pa)

3x10

b 9

2x10

60

a

40

a 9

1x10

20 b

0

0 50

100

150

0

Fig. 9. The variation of storage modulus with temperature for (a) UTCSE composite and (b) TCSE composite at 1 Hz frequency.

0.4

a b

Tan δ

0.3

UTCSE TCSE

800

Fig. 11. TGA curves of (a) UTCSE composite and (b) TCSE composite.

higher than TCSE composite. It indicates the less amount of lignin present in the TCSE composite than UTCSE composite.

b

The natural fiber reinforced polymer composites, UTCSE and TCSE have been fabricated using hand layup followed by compression molding technique. The prepared samples were characterized and results are reported. After chemical treatment of coconut sheath fiber, the cellulose content was increased, but hemicelluloses, lignin and other materials content was decreased compared to untreated (raw) coconut sheath fiber. The decrease in hemicelluloses was confirmed by FT-IR spectra. The TCSE composite has shown higher tensile, flexural and impact strengths compared to UTCSE composite. The interfacial bond between fiber and matrix was improved in TCSE composite. The percentage increment in TCSE composite was found 21.19%, 18.88% and 24.39% for tensile, flexural and impact tests when compared to UTCSE composite and their fractured surfaces TCSE composite shown better fiber– matrix interaction and absence of voids compared to UTCSE composite. From dynamic mechanical analysis, the storage modulus (E0 ) value was increased and damping parameter (tan d) was decreased for TCSE composite. It shows higher adhesion between treated coconut sheath fiber and epoxy resin than UTCSE composite. The thermo gravimetric analysis, TCSE composite showed higher thermal stability and less char yield than UTCSE composite.

0.0 80

600

4. Conclusions

0.1

60

400

a

0.2

40

200

Temperature ( °C)

Temperature (°C)

100

120

140

Temperature ( °C) Fig. 10. The variation of tan d with temperature for (a) UTCSE composite and (b) TCSE composite at 1 Hz frequency.

of moisture. Both UTCSE composite and TCSE composite showed weight loss between 230 and 300 °C due to the decomposition of low molecular weight hemicelluloses and the weight loss occurred at 300–400 °C for thermal degradation of cellulose and epoxy. The higher char yield depends on composition or amount of lignin present on the fiber [23]. The char yield of UTCSE composite was


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