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.
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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
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
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
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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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
Journal of Reinforced Plastics and Composites 33(11)
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
A0 B0 C0 D0
0.589 0.504 0.580 0.550
0.046 0.149 0.058 0.0192
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
C0 C2 C4 C6 C8
0.580 0.561 0.495 0.436 0.477
0.058 0.059 0.155 0.311 0.299
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
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
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)
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
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
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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)
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
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
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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