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Abstract. This paper deals with the evaluation of water absorption properties of natural fibre composites consisting of bamboo fibre as reinforcement, epoxy as ...

Original Article

Studies on water absorption behaviour of bamboo–epoxy composite filled with cenosphere

Journal of Reinforced Plastics and Composites 2014, Vol. 33(11) 1059–1068 ! The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684414523325 jrp.sagepub.com

Hemalata Jena, Arun Kumar Pradhan and Mihir Kumar Pandit

Abstract This paper deals with the evaluation of water absorption properties of natural fibre composites consisting of bamboo fibre as reinforcement, epoxy as matrix and cenosphere as particulate filler at different environmental conditions. Hand lay-up technique is used to fabricate the composites with varying number of layers of bamboo fibre and cenosphere filler content. Water absorption kinetics of the composites is presented in this paper. It is observed that the rate of water absorption depends on the fibre content as well as filler content. Addition of filler in the layered bamboo–epoxy composite decreases the moisture absorption capacity and maximum reduction is observed to be 21% and 32% for distilled and sea water conditions, respectively, in seven-layered composite with 3.0 wt% filler.

Keywords Cenosphere, bamboo fibre, epoxy, water absorption behaviour

Introduction Environmental awareness motivates researchers towards the study of natural fibre-reinforced composite as a replacement of conventional materials. The natural fibre as reinforcement in composites is a cost-effective option with advantages like high specific strength, ample availability, biodegradability as compared to synthetic fibre.1 Amongst commonly used natural fibre in polymer composites, bamboo fibre is very popular. This agricultural crop has a harvesting percentage of 65%, 28% and 7% in Asia, America and Africa, respectively.2 Bamboo composite possesses high specific strength which is superior to that of ordinary glass fibre-reinforced composites, with a specific strength three to four times that of mild steel.3 But being hydrophilic in nature, the bamboo fibre leads to weak interfacial bonding between fibres and matrix which in turn deteriorates its mechanical properties4–7 and causes dimensional instability.8 This restricts its long-term use for outdoor applications. The rate of water absorption of composites depends on resistance of the fibres to water absorption, reaction between water with the matrix, chemical composition and microstructure of polymer matrix. Moisture penetration into composite materials occurs by diffusion of water molecules inside

micro gaps between polymer chains, capillary transport into gaps and flaws at interfaces between fibres and polymer. Generally, based on these mechanisms, diffusion behaviour of composites can be classified as Fickian, non-Fickian, anomalous, or an intermediate behaviour between Fickian and non-Fickian.9,10 A number of studies on water absorption behaviour of natural fibre composites are reported.11–14 Various efforts have been made to decrease water absorption of natural fibre composites. Coupling agents, compatibilizers or other chemical modifications are used to improve the moisture resistance of composites.15–17 The moisture absorption of wood plastics prepared from sawdust and virgin and/or recycled plastics is studied by Najafi et al.18 Hybrid composites consist of two or more reinforcements which are complementary to each other in terms

School of Mechanical Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar, India Corresponding author: Hemalata Jena, School of Mechanical Sciences, Indian Institute of Technology Bhubaneswar, Samantapuri, Bhuabaneswar 751013, India. Email: [email protected]

1060 of properties, so that the demerit of one is compensated by the merit of the other. Hybridization of natural fibre with different synthetic fibres like carbon, aramid, glass, etc. can improve its moisture resistance.19–21 Incorporation of fibre, micro or nano size particulate filler or whisker is also an option to use as a second phase reinforcement in the polymer matrix composites. For that, a proper material design is required for making composite with good performance and cost effective. The third phase in the composite plays an important role in determining the properties of the composite.22,23 It is well known that cenosphere (aluminosilicate micro hollowsphere) is an industrial waste produced during burning of coal in thermal power plants24 and could be a unique class of particulate filler in polymer matrix composite due to its fine dispersion, homogeneity, inertness, low water absorption and chemical stability. There are several instances of investigation of mechanical properties of the cenosphere-filled polymer composites.25–27 It is reported that the mechanical characteristics of bamboo composites can be modified by alkali treatment of fibre28 and addition of cenosphere filler.29 It is also reported that the mechanical properties of bamboo–epoxy composites under different loading conditions can be modified by changing the number of laminae and configuration of laminae in resin.3,30 However, the effect of cenosphere as a filler material on the water absorption properties of bamboo fibrereinforced composites has not yet been reported. Therefore, the present study investigates the effect of filler on the water absorption properties of bamboo fibre-reinforced epoxy composite with different number of layers of bamboo fibre mats and cenosphere filler loading. In addition, the effect of filler and fibre concentration on the diffusion kinetics of the composites is also investigated.

Experimental procedure Materials In the present work, the composite consists of bamboo fibre as reinforcement and cenosphere as filler material in the epoxy matrix. Bamboo fibre is a woven type mat (Figure 1). Each fibre has an average thickness of 1.5 mm and the width of the fibre used in the mats is 4.5 mm. The density of the fibre is 0.95 gm/cc. Ash cenosphere is used as a filler material in the composite having a particle density of 0.45–0.80 gm/ml and a size of 60–94 mm. It is grey in colour. Its melting temperature is 1300–1500 C. Diglycidyl ether of bisphenol-A (DGEBA), a medium viscosity epoxy resin is used as matrix. Triethylene tetra-amine (TETA), a room temperature-curing agent is used as hardener.

Journal of Reinforced Plastics and Composites 33(11)

Figure 1. Cross-sectional image of bamboo fibre mat.

Fabrication The conventional hand lay-up technique is used to fabricate composites having three, five, seven and nine layers of woven bamboo mats. The epoxy is kept at 110 C in an oven in order to remove air bubbles. Then, it is mixed with hardener in a ratio of 10:1 by weight. Then, the cenosphere is mixed with epoxy resin at 25 C temperature with varying weight percentages (0%, 1.5%, 3.0%, 4.5% and 6.0%). In the present experiment, this cenosphere–epoxy mixture is used to impregnate the bamboo fibre mats kept in a mould of dimension 200 mm  10 mm under a uniform load. The composites are cured for 24 h at room temperature and post cured in air for another 24 h after removal from the mould. For easy removal of the composite, teflon sheet and silicon spray are used which prevent adhesion between the surface of mould and composite sample. The composites are then cut into desired dimensions for its water absorption test at different environmental conditions. The composite specimens with varying weight % of fibre and cenosphere filler are designated as described in Table 1.

Water absorption test and thickness swelling Water absorption and thickness swelling of the composites are performed as per ASTM D 570-98. Three test specimens are prepared from each type of composite having dimensions of length 64 mm and width 12.4 mm. The weights of the composite specimens are taken before immersing in two types of aqueous environments, which are distilled water (pH ¼ 7) and sea water (pH ¼ 8) at room temperature. After immersion in water, the samples are removed at 24 h and wiped

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Table 1. Designation of composite specimens. Composite with varying weight % of cenosphere

Designation

Composite with varying laminae

Composite with varying weight % of fibre

A0 B0 C0 D0

Three-layered bamboo–epoxy composite Five-layered bamboo–epoxy composite Seven-layered bamboo–epoxy composite Nine-layered bamboo–epoxy composite

18 28 33 43

0 0 0 0

C2 C4 C6 C8

Seven-layered bamboo–epoxy composite

33

1.5 3.0 4.5 6.0

with tissue paper to remove surface water. These samples are reweighed with an analytical weighing balance having a resolution of 0.001 mg. During this process, water absorption takes place through the surface and the edges of the specimen resulting in weight gain. The above process is repeated in regular intervals of 24 h until an equilibrium value is reached. The percentage weight gain of the samples is measured by using the following relation Mt ¼

ðwt  w0 Þ  100 w0

where Mt : Moisture gain percentage. w0 : Mass of the specimen before aging. wt : Mass of the specimen at different time intervals during aging. The percentage weight gain of these samples is measured at different time intervals and the moisture content versus time is plotted. Equilibrium moisture content (EMC) of the sample is the moisture content in which the weight change for two successive readings of the sample is less than 0.1%. The thickness swelling is determined by the same procedure using the following relation T¼

Tt  T0 100 T0

where Tt and T0 are the composite thickness after and before immersion in water, respectively.

Results and discussion Effect of fibre loading on water absorption It is known that bamboo fibre is hydrophilic in nature because it contains hydroxyl groups. Effect of this fibre

Figure 2. Variation of water absorption of bamboo–epoxy composite with immersion time for distilled water.

with matrix during aging is essential. Figures 2 and 3 show the variation of the water absorption of bamboo– epoxy composites as a function of time for different fibre loadings in distilled water and sea water conditions. Water absorption increases with immersion time, reaching a saturation point beyond which it remains constant. The time to reach the saturation point is different for two different environmental conditions. It is observed that the saturation time is 216 h for distilled water and 168 h for sea water conditions. The figures indicate that the initial rate of water absorption and equilibrium absorption of water increases with increase in fibre content and it is maximum at maximum fibre content, i.e. D0 composite for both environmental conditions. The reason for increased water absorption may be due to higher hydrophilic nature of cellulosic fibre and its greater capillary effects as compared to the epoxy resin matrix. Moreover, the observed absorption of water in this case could have also occurred through micro cracks present inside the

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Journal of Reinforced Plastics and Composites 33(11) of the lignocellulosic fibre of composite in water, the thickness swelling of composite occurs. The hydrophilic nature of fibre and the capillary action through micro pores are the main causes to absorb water during immersion and thus increase the thickness of the composite.32,33 From the figure, it is observed that for different environmental conditions the thickness swelling increases with an increase in fibre content and immersion time. The thickness swelling rate for D0 composite is the highest, i.e. 5.9% and 4.7% for distilled and sea water conditions, respectively.

Effect of filler loading on water absorption Figure 3. Variation of water absorption of bamboo–epoxy composite with immersion time for sea water.

Figure 4. Plot of EMC vs. fibre loading for bamboo–epoxy composites at different environmental conditions. EMC: Equilibrium moisture content

composite. When the layers of fibre mat increases, it is obvious that the cellulose content increases, which in turn results in more water absorption. The behaviour is almost linear as previously reported by taking different weight % of natural fibre.11–14 Figure 4 shows the EMC of composites of different fibre loadings at different environmental conditions. It is observed that the EMC value increases with increasing fibre content. The absorption rate in case of sea water is found to be less than that of distilled water. This is due to the presence of large amount of NaCl present in sea water, which slows down the diffusion process into the matrix of the composite material.31 For A0 composite, the maximum absorption is 3.26% for distilled water and 4.92% for sea water conditions. Figure 5(a) and (b) shows the thickness swelling rate of the composite samples as a function of time for different environmental conditions. Due to the exposure

Figures 6 and 7 show the moisture absorption of composite type C with different weight % of cenosphere filler. Addition of cenosphere to bamboo fibre composite reduces the maximum moisture absorption for both distilled and sea water conditions. The maximum absorption decreases to 21% and 32% for distilled and sea water conditions, respectively. The EMC of the composites for filler loading is shown in Figure 8. It is observed that the EMC value reduces with the addition of cenosphere to the composites. The maximum water absorption rate in the bamboo–epoxy composite with cenosphere filler is in the increasing order as C4 < C2 < C6 < C8 < C0. From the figure, it is observed that the moisture absorption increases and reaches an equilibrium state after the initial take-off. So, this behaviour can be considered as Fickian process. A comparison of thickness swelling of the bamboo– epoxy composites with different weight % of cenosphere is shown in Figure 9(a) and (b) for different environmental conditions. It shows that the long-term immersion in water causes the dimensional instability of the composites. The thickness swelling for C0 composite shows a value of 4.8% and 3.5% for distilled water and sea water conditions, respectively. The increase in cenosphere content decreases the thickness swelling up to C4 composite type having 3% filler weight fraction after which the thickness swelling is increased on further addition of filler. The thickness swelling for C4 composite shows a value of 3.03% and 2.03% for distilled water and sea water conditions, respectively. It indicates that the thickness swelling of bamboo–epoxy composite with cenosphere filler is less than that of bamboo–epoxy composite without cenosphere filler. The pattern for thickness swelling is similar to the weight gain in composite for both environmental conditions shown in Figures 6 and 7. A comparison graph for both environmental conditions is shown in Figure 10, which indicates that the thickness swelling is more in distilled water than sea water condition.

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(a)

(b)

Figure 5. Thickness swelling (%) of bamboo–epoxy composite for (a) distilled and (b) sea water condition.

Figure 6. Variation of water absorption of bamboo–epoxy– cenosphere composite with immersion time for distilled water.

Figure 8. Plot of EMC vs. filler loading for bamboo–epoxy composites at different environmental conditions. EMC: Equlibrium moisture content

Moisture absorption kinetics In order to investigate the type of diffusion mechanism, the absorption data have been fitted to the following relations13  log

 Mt ¼ logðkÞ þ n logðtÞ Mm

where Mt: Water absorption at time t. Mm: Water absorption at the saturation point. k and n: Constants.

Figure 7. Variation of water absorption of bamboo–epoxy– cenosphere composite with immersion time for sea water.

The value of n indicates the type of transport mechanism and k indicates the interaction between the sample and water in addition to its structural characteristics of polymer network. Figure 11 shows the example of fitting the curve of log(Mt/Mm) as a

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(a)

(b)

Figure 9. Thickness swelling (%) of bamboo–epoxy–cenosphere for (a) distilled and (b) sea water condition.

Figure 10. Comparison graph for thickness swelling of the composite samples under two environmental conditions.

function of log(t) for bamboo–epoxy composites to determine the value of n and k. They are determined by linear regression analysis, and their values are given in Tables 2 and 3 for bamboo–epoxy composite of different bamboo fibre and cenosphere filler loading, respectively. It is observed from the tables that the values of n are close to 0.5 for all composite types. Therefore, it can be concluded that the water absorption of the composites follows the Fickian behaviour. For a Fickian diffusion mechanism, n has a value of 0.5. When n ¼ 1.0, the mechanism is non-Fickian and when it lies between 0.5 and 1.0, the diffusion is anomalous.20 The diffusion coefficient (D) is one of the important parameters of Fick’s model and shows the ability of water molecules to penetrate inside the composite structures. The values of D can be obtained from pffiffi the initial slope of the plot of Mt/Mm against time t using the following equation34    h 2 M2  M1 2 pffiffiffiffi pffiffiffiffi D¼ 4Mm t2  t1 where Mm : Equilibrium moisture content. h: Thickness of the sample. t1 and t2 : Selected time points in the initial linear portion of curve. M1 and M2 : Moisture content at time t1 and t2.

Figure 11. Diffusion curve fitting for bamboo–epoxy composites.

Figure 12 shows the diffusion coefficient curve-fitting plot for composites. It is observed that the diffusivity (D) values increased with fibre loading for both environmental conditions as shown in Figure 13. The maximum diffusivity value is found for D0 composite type, which is about 56% and 121% higher as compared to composite of type A0 for both environmental

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Table 2. The dependence of moisture absorption constants n and k on fibre loading for bamboo–epoxy composites without cenosphere filler. Composite type

Environment

n

k  102

Environment

n

k  102

A0 B0 C0 D0

Distilled water

0.589 0.504 0.580 0.550

0.046 0.149 0.058 0.0192

Sea water

0.551 0.573 0.492 0.471

0.088 0.069 0.194 0.262

Table 3. The dependence of moisture absorption constants n and k on cenosphere filler loading for bamboo–epoxy composites. Composite type

Environment

n

k  102

Environment

n

k  102

C0 C2 C4 C6 C8

Distilled water

0.580 0.561 0.495 0.436 0.477

0.058 0.059 0.155 0.311 0.299

Sea water

0.492 0.438 0.460 0.477 0.456

0.194 0.345 0.257 0.200 0.287

Figure 12. Diffusion curve-fitting plots for the diffusion coefficient for bamboo–epoxy composites under water condition.

conditions. The diffusivity for both fibre and filler loading is more in distilled water than the sea water condition. Addition of cenosphere filler into the composites results in lowering of diffusivity (D) values of the composites as shown in Figure 14. The minimum diffusivity is observed for C4 composite type. As mentioned, one of the common mechanisms of water molecules penetration into composite materials is capillary transport into the gaps and flaws at the interfaces between fibres and polymer because of incomplete wettability and impregnation.35 Addition of Cenosphere filler into the

composite gives better adhesion and wettability between matrix and fibres.29 Hence, the velocities of the diffusion processes decrease as there may be fewer gaps in the interfacial region. But filler content more than 3.0 wt% in the bamboo–epoxy composite shows the adverse effect to the diffusion processes. Absorption coefficient is another important factor to determine the kinetics of water absorption behaviour. The permeability of water molecules through the composite depends on the absorption of water through the fibres. Therefore, the absorption

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Journal of Reinforced Plastics and Composites 33(11) coefficient is related to the equilibrium absorption of the penetrant and it is calculated by using the following relation36 S¼

wm wp

where wm : Mass of the solvent taken up at equilibrium swelling. wp : Mass of the sample.

Figure 13. The dependence of diffusivity, D, on bamboo–epoxy composites with different fibre loadings.

The values of diffusion coefficient (D) and absorption coefficient (S) of bamboo–epoxy composites at different fibre loadings and filler loading under different environmental conditions are shown in Tables 4 and 5, respectively. The values of S are observed to increase with increase in fibre weight % for both distilled and sea water environments. On the other hand, filler loading in composites has decreased the value of S. And maximum decrement is 24.51% and 31.99% for distilled and sea water environmental conditions, respectively, at C4 composite type. The permeability coefficient (P) which represents the net effect of absorption and diffusion is given by the relation36 P¼DS The values of P of the composite samples having different fibre and filler loadings are presented in the same table. From the table, it is clear that the permeability coefficient follows the same trend as that of S and D.

Conclusion Figure 14. The dependence of diffusivity, D, on bamboo–epoxy composites with different filler loadings.

The water absorption property of woven bamboo fibrereinforced epoxy composites with and without cenosphere has been studied. Two different environmental

Table 4. Values of D, S and P for bamboo–epoxy composite with different fibre loadings. Composite type

Absorption coefficient (S) (g/g1)

Diffusivity (D)  108 (cm2/s1)

Permeability (P)  109 (cm2/s1)

Distilled water

A0 B0 C0 D0

0.049 0.063 0.070 0.076

9.62 12.01 19.54 22.95

7.74 7.58 13.70 17.64

Sea water

A0 B0 C0 D0

0.017 0.023 0.026 0.033

7.86 11.78 15.37 18.40

1.34 2.73 4.08 6.07

Environment

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Table 5. Values of D, S and P for bamboo–epoxy composite with different filler loadings. Composite type

Absorption coefficient (S) (g/g1)

Diffusivity (D)  108 (cm2/s1)

Permeability (P)  109 (cm2/s1)

Distilled water

C0 C2 C4 C6 C8

0.070 0.060 0.052 0.063 0.065

19.54 11.57 10.41 12.13 16.99

13.7 6.99 5.50 7.68 11.12

Sea water

C0 C2 C4 C6 C8

0.054 0.040 0.036 0.047 0.051

15.37 10.66 8.93 12.26 14.57

8.36 4.33 3.30 5.81 7.43

Environment

conditions, i.e. distilled water and sea water, are considered for this purpose. Following conclusions are drawn from the present study. 1. Moisture absorption and thickness swelling increase with increase in number of layers for both environmental conditions. The maximum absorption is observed in D0 composite having 43 wt% of fibre. 2. The water absorption pattern of bamboo–epoxy composites with fibre and filler loading at both environmental conditions is found to follow Fickian behaviour. 3. The addition of cenosphere improves the water absorption resistance of the composites, but it depends upon the amount of cenosphere which is limited to 3.0 wt%. 4. Addition of cenosphere filler into bamboo–epoxy composite reduces the EMC to 21% and 32% for distilled and sea water conditions. 5. The maximum weight gain and thickness swelling per cent is higher in the case of distilled water as compared to that of sea water for both fibre and filler loading.

Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of interest None declared.

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