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Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites
Composites Science and Technology 69 (2009) 1942–1948
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites Hazizan Md Akil *, Leong Wei Cheng, Z.A. Mohd Ishak, A. Abu Bakar, M.A. Abd Rahman School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 21 August 2008 Received in revised form 17 April 2009 Accepted 19 April 2009 Available online 3 May 2009 Keywords: A. Fibres A. Polymer–matrix composites (PMCs) B. Environmental degradation B. Mechanical property E. Pultrusion
a b s t r a c t Water absorption of natural fibre plastic composites is a serious concern especially for their potential outdoor applications. In this research, jute fibre reinforced with unsaturated polyester composites are subjected to water immersion tests in order to study the effects of water absorption in its mechanical properties. Water absorption tests were conducted by immersing composite specimens into three different environmental conditions included distilled water, sea water and acidic solutions at room temperature for a period up to 3 weeks. Water absorption curves obtained and characteristic parameter D (diffusion coefficient) and Mm (maximum moisture content) were determined. The water absorption of jute fibre reinforced unsaturated polyester composites were found to follow a so-called pseudo-Fickian behaviour. The effects of the immersion treatment on the flexural and compression characteristics were investigated. The flexural and compression properties were found to decrease with the increase in percentage water uptake. These flexural and compression behaviours were explained by the plasticization of the matrix–fibre interface and swelling of the jute fibres. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the course of nature life, composite materials are subjected to both mechanical loading and exposed to severe environmental conditions. Degradation behaviour of polymeric composites upon exposure to environmental conditions such as temperature and humidity is one of the most important issues. The natural fibre reinforced composites are reasonably strong, lightweight, and free from health hazards, biodegradable and hence they have the potential to be used as building materials. Despite the advantages listed above, they suffer from some limitations such as lower modulus, poor moisture resistance especially absorption and low strength compared with synthetic fibres such as glass and carbon [1]. Among all the natural fibre reinforcing materials, jute appears to be a promising material because it is relatively inexpensive and commercially available in the required form. However, its mechanical and physical properties are highly inconsistent and depend on geographic origin, climatic growth conditions and processing techniques [2]. Jute is one of the most important natural vegetal fibres, and is produced in India, Bangladesh, Thailand, Vietnam and other countries. It contains 56–64% cellulose, 29–25% hemicelluloses, 11–14% lignin and a small proportion of fats, pectin, ash and waxes [3]. Ben Daly et al. [4] have studied the effects of water absorption in pultruded composites containing fillers and low profile additives. Dash et al. [5] studied the processing, mechanical properties * Corresponding author. Tel.: +60 4 5996161; fax: +60 4 5941011. E-mail address: [email protected] (H.M. Akil). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.04.014
and SEM analysis on low cost jute-polyester composites. Munikenche Gowda et al. [2] studied some mechanical properties of unsaturated jute reinforced polyester composites. Alam and Khan [6] studied the comparative study of water absorption behaviour jute reinforced composites using neutron radiography technique. Saha et al. [7] studied of jute fibre reinforced polyester composites by Dynamic Mechanical Analysis. From previous studies [2,4–7], moisture diffusion in polymeric composites have shown to be governed by three different mechanisms. The first mechanism involves diffusion of water molecules inside the micro gaps between polymer chains. The second mechanism involves capillary transport into the gaps and flaws which interfaces between fibre and the matrix. The third mechanism involves transport of microcracks in the matrix arising from the swelling of fibres (particularly in the case of natural fibre composites) [8]. Based on these mechanisms, absorption behaviour can be categorized into several types. These include: (1) linear Fickian behaviour, where the moisture weight gains gradually attains equilibrium after a rapid initial take off; (2) pseudo-Fickian behaviour where the moisture weight gains never reached equilibrium after initial take off; (3) two stage diffusion process with an abrupt jump in the moisture weight gains after initial take off; (4) rapid moisture gain results from fibre/matrix debonding and matrix cracking; and (5) moisture weight gains follows a decrease trend after the initial take off, an irreversible process as a result of the leaching out of the material from the bulk following chemical or physical breakdown [4]. This paper is focused on the water absorption study and its effects on pultruded jute fibre reinforced unsaturated polyester composite. Residual
1943
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
Fig. 1. Weight gain as a function of time for (a) flexural specimen and (b) compression specimen of pultruded jute reinforced unsaturated polyester composite after exposure to distilled water, sea water and acidic solution at room temperature.
mechanical properties under flexure and compression after immersion process at specified time were determined using rod composite specimen.
2. Experimental 2.1. Materials Jute fibres were supplied by Alam Fiber Impex Ltd., Bangladesh in twisted roving form. Unsaturated polyester resin (Crystic P9901) was purchased from Revertex Company, Malaysia.
The continuous jute fibres were first impregnated with unsaturated polyester resin in a resin impregnation tank. There was a pulling device to pull the resin impregnated jute fibre though steel die to obtain the desired shape. Curing process was carried out in a curing die which is a precision machined to impart the final shape. A pulling device draws the stock through a dies and also determines the production speed [10]. Finally, the profile was cut into desired length. The average diameter of all composites rod is 12.7 mm. Table 1 Effect of immersion environments on the diffusion coefficient, D and the maximum moisture content, Mm of jute fibre reinforced unsaturated polyester composite.
2.2. Preparation of pultruded composites The composites were prepared using a thermoset Pultrusion machine at MMFG Composites Sdn. Bhd, Subang Jaya, Selangor, Malaysia. During manufacturing of composites, jute fibres (roving) were placed on a creel merely of bookcase-type shelves and equipped with roving guide to lead the strands to the resin bath. Roving guider was used to ensure the strands do not scrape across one another as this will generate considerable static and cause ‘‘fuzz balls” to build up in the resin bath, raising its viscosity [9].
Flexural specimen
Compression specimen
Distilled water D (m2/s) Mm (%)
6.54 1012 10.71
2.67 1011 14.32
Sea water D (m2/s) Mm (%)
4.84 1012 10.25
2.02 1011 12.96
Acidic solution D (m2/s) Mm (%)
5.93 1012 10.34
2.20 1011 13.37
1944
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
2.3. Material characterization 2.3.1. Water absorption investigation Specimens were immersed into three different aqueous environments, which were distilled water, sea water and acidic solution. For the water absorption measurements, the specimens were withdrawn from the waters, wiped dry to remove the surface moisture, and then weighted using an electronic balance accurate
to 104 g to monitor the mass during the aging process. The moisture content, M(t) absorbed by each specimen is calculated from its weight before, w0 and after, wt absorption as follows:
wt w0 MðtÞ ¼ 100 w0
ð1Þ
Diffusion coefficient, D is calculated from the slope of moisture content versus the square root of time by:
Fig. 2. Variations of (a) flexural strength, (b) maximum flexural strain, and (c) flexural modulus for pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
2 2 h M2 M1 pffiffiffiffi pffiffiffiffi D¼p 4M m t2 t1
ð2Þ
Assuming the absorption process is linear at an early stage of immersion; times are taken at the beginning of absorption process, so that the weight change is expected to vary linearly with the square root of time.
1945
2.3.2. Flexural testing Flexural test was carried out using Instron 8802 according to the standard ASTM D4476–03. Specimens (pultruded rod with diameter of 12.7 mm) were cut into two parts with the cross section of each part is smaller than a half-round section. The total specimen length is 125 mm with overhang length of 12.5 mm at both supports. The crosshead speed for flexural test was set at
Fig. 3. Flexural stress–strain curves of pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
1946
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
5 mm/min. Three specimens for each condition are used to obtain a satisfactory result.
3. Results and discussions 3.1. Water absorption investigation
2.3.3. Compression testing Compression test was carried out using Instron 3367 according to the standard ASTM D 695-02a. The diameter and length of the specimen was 12.7 mm and 25.4 mm, respectively. The crosshead speed for flexural test was set at 5 mm/min. Three specimens for each condition are used to minimise the error.
Water absorption curves for flexural and compression specimens of jute fibre reinforced unsaturated polyester composite are shown in Fig. 1a and b. Specimens were immersed into three types of different aqueous environments, which are distilled water, sea water (pH 8) and acidic solution (pH 3). Each curve represents an
Fig. 4. Variations of (a) Compression strength, (b) maximum compression strain, and (c) compression modulus for pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
average data of three specimens. From the water absorption curves, it shown that absorbed water contents increased with increasing immersion time. According to a comprehensive review on moisture absorption behaviour which presented by Weitsman [11], jute fibre reinforced unsaturated polyester composite exhibited a so-called pseudo-Fickian behaviour, where the moisture weight gains never reaches equilibrium after initial take off. Diffusion coefficient, D
1947
and maximum of the moisture content, Mm for flexural specimens and compression specimens of jute fibre reinforced unsaturated polyester composite were represented in Table 1. Jute fibre reinforced unsaturated polyester composite gives higher diffusion coefficient, D and maximum of moisture content, Mm if immersed into distilled water, follows by acidic solution and lastly sea water. Table 1 indicates that both of the diffusion coefficient, D and maximum of
Fig. 5. Stress–strain curves for compression specimen of pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
1948
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
the moisture content, Mm decrease if the specimen is immersed in sea water and acidic solution compared to distilled water. It is due to the presence of large salt molecules in sea water (notably sodium chloride) slows the diffusion process into the matrix of the composite materials, resulting in a lower absorption kinetic parameters [4]. Weakly acidic solutions, such as acetic acid solutions buffered at pH values of 3–5, do not cause significantly more extensive corrosion than distilled water and this finding is in agreement with previously reported work [12]. Both of the diffusion coefficient, D and maximum of moisture content, Mm for compression specimens are higher if compared to the flexural specimens. This may due to the influence of geometry and dimension of specimen, in which the cross sectional area of compression specimen was circular and flexural specimen was semicircular.
the composite materials rather than the intrinsic compressive failure [13]. The decrease in mechanical properties with increasing in moisture content could be attributed to the formation of hydrogen bonding between the water molecules and cellulose fibre [8]. The elementary unit of cellulose in jute fibre contains three hydroxyl (–OH) groups. These hydroxyl groups form an intramolecular hydrogen bonds inside the molecule and intermolecular hydrogen bonds with other cellulose molecules as well as hydroxyl groups of moist air [14]. With the presence of high hydroxyl group, natural fibre such as jute tends to show low moisture resistance. This may lead to dimensional variation and poor interfacial bonding between the fibre and matrix. The rigidity of the cellulose structure is destroyed by the water molecules in the cellulose network structure in which water acts as a plasticiser and it permits cellulose molecules to move freely [8].
3.2. Flexural testing 4. Conclusion Variations of flexural strength, maximum flexural strain and flexural modulus for jute fibre reinforced unsaturated polyester composite after exposed to various environmental conditions are summarised in Fig. 2a–c, respectively. Each value represents an average data of three specimens. Consequently, flexural stress– strain curves corresponding to the samples immersed in distilled water, sea water and acidic solution are depicted in Fig. 3a–c, respectively. From Fig. 2, it is clear that for all environmental conditions, as the immersion time increases, both flexural strength and flexural modulus decreases. This could be attributed to jute fibre which tends to absorb high amount of moisture after exposed to aqueous environmental and leads to higher degradation rate. This tendency can be also related to the weak fibre–matrix interface due to water absorption after exposed to aqueous environments [8]. On the other hand, the maximum flexural strain increases dramatically as the immersion time increases. It is believed that the cellulose content of jute fibre in the composites will be reduced as a result of water absorption process and making jute fibre more flexible as a result of plasticization effect. Absorbed water molecules fill into cavities and cracks within the composite and act as a plasticiser to the composite and render the structure more flexible [8]. Comparing the flexural properties with the aqueous environments, the highest degradation rate for jute fibre reinforced unsaturated polyester composite is recorded for specimen after exposed to distilled water, follows by acidic solution and sea water. In accordance with the diffusion coefficient, D and maximum of moisture content, Mm that shown in Table 1, higher diffusion coefficient and maximum of moisture content tend to impart higher degradation in flexural strength and flexural modulus with an improvement in flexural strain. 3.3. Compression testing Fig. 4a–c shows the variations of compression strength, maximum compression strain and compression modulus for jute fibre reinforced unsaturated polyester composite after exposed to distilled water, sea water and acidic solutions, respectively. On the other hand, Fig. 5a–c shows the corresponding stress–strain curves for each environmental condition (i.e. distilled water, sea water and acidic solution) investigated. Each value represents an average data of three specimens. Fig. 4 indicates a trend of decrease in compression strength and compression modulus with increasing immersion time; however, there is a trend of increasing maximum compression strain as immersion time increases after exposed to aqueous environments. These compression results show a similar trend with the flexural testing as discussed in Section 3.2. There are fluctuations as indicated by the error bars causing variations in the result which could be attributed to the uneven surfaces of the compression specimen. As a result, a slightly eccentric applied load will cause premature buckling in
Comparative study of water absorption on the mechanical properties of jute fibre reinforced unsaturated polyester composite has been studied following different aqueous environments: distilled water, sea water and acidic solutions at room temperature. The water absorption pattern is found to follow pseudo-Fickian behaviour and therefore is predictable over a period of time. Exposure of the natural fibre composite materials to aqueous environments resulted in significantly drops in strength and modulus due to the weakening of interface between fibre and matrix. However, a significant increase in the maximum strain is observed due to the increase in ductility of natural fibre as a result of breakdown of cellulose structure after immersion process. Acknowledgements The authors are grateful to Universiti Sains Malaysia (USM) and Construction Industry Development Board of Malaysia (CIDB) for their assistance and contribution that has resulted in this article. References [1] Velmurugan R, Manikandan V. Mechanical properties of palmyra/glass fiber hybrid composites. Compos: Part A 2007;38:2216–26. [2] Munikenche Gowda T, Naidu ACB, Chhaya Rajput. Some mechanical properties of untreated jute fabric-reinforced polyester composites. Compos: Part A 1999;30:277–84. [3] Murphy John. Reinforced plastics handbook. 2nd ed. Elsevier Advanced Technology; 1998. [4] Ben Daly H, Ben Brahim H, Hfaied N, Harchay M, Boukhili R. Investigation of water absorption in pultruded composites containing fillers and low profile additives. Polym Compos 2007;28:355–64. [5] Dash B, Rana AK, Mishra HK, Nayak SK, Mishra SC, Tripathy SS. Novel, low cost jute-polyester composites: Part 1: processing, mechanical properties, and SEM analysis. Polym Compos 1999;20:1. [6] Alam MK, Khan MA. Comparative study of water absorption behavior in Biopol and jute-reinforced Biopol composite using neutron radiography technique. Reinf Plast Compos 2006;25:11. [7] Saha AK, Das S, Bhatta D, Mitra BC. Study of jute reinforced polyester composites by dynamic mechanical analysis. Appl Polym Sci 1999;71:1505–13. [8] Dhakal HN, Zhang ZY, Richardson MOW. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos Sci Technol 2007;67:1674–83. [9] Meyer Raymond W. Handbook of pultrusion technology. New York: Chapman and Hall; 1985. [10] Callister Jr William D. Materials science, engineering: an introduction. 6th ed. John Wiley & Sons, Inc.; 2003. [11] Weitsman YJ. Effects of fluids on polymeric composites – a review. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 2. Elsevier; 2000. p. 369–401. [12] Jones Russell H. Environmental effects on engineered materials. Marcel Dekker, Inc.; 2001. [13] Davis Harmer E, Troxell George Earl, Hauck George FW. The testing of engineering materials. 4th ed. McGraw-Hill Book Company; 1982. [14] Alam MK, Khan MA. Comparative study of water absorption behavior in Biopol and jute-reinforced Biopol composite using neutron radiography technique. Reinf Plast Compos 2006;25:11.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Water absorption study on pultruded jute fibre reinforced unsaturated polyester composites Hazizan Md Akil *, Leong Wei Cheng, Z.A. Mohd Ishak, A. Abu Bakar, M.A. Abd Rahman School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 21 August 2008 Received in revised form 17 April 2009 Accepted 19 April 2009 Available online 3 May 2009 Keywords: A. Fibres A. Polymer–matrix composites (PMCs) B. Environmental degradation B. Mechanical property E. Pultrusion
a b s t r a c t Water absorption of natural fibre plastic composites is a serious concern especially for their potential outdoor applications. In this research, jute fibre reinforced with unsaturated polyester composites are subjected to water immersion tests in order to study the effects of water absorption in its mechanical properties. Water absorption tests were conducted by immersing composite specimens into three different environmental conditions included distilled water, sea water and acidic solutions at room temperature for a period up to 3 weeks. Water absorption curves obtained and characteristic parameter D (diffusion coefficient) and Mm (maximum moisture content) were determined. The water absorption of jute fibre reinforced unsaturated polyester composites were found to follow a so-called pseudo-Fickian behaviour. The effects of the immersion treatment on the flexural and compression characteristics were investigated. The flexural and compression properties were found to decrease with the increase in percentage water uptake. These flexural and compression behaviours were explained by the plasticization of the matrix–fibre interface and swelling of the jute fibres. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In the course of nature life, composite materials are subjected to both mechanical loading and exposed to severe environmental conditions. Degradation behaviour of polymeric composites upon exposure to environmental conditions such as temperature and humidity is one of the most important issues. The natural fibre reinforced composites are reasonably strong, lightweight, and free from health hazards, biodegradable and hence they have the potential to be used as building materials. Despite the advantages listed above, they suffer from some limitations such as lower modulus, poor moisture resistance especially absorption and low strength compared with synthetic fibres such as glass and carbon [1]. Among all the natural fibre reinforcing materials, jute appears to be a promising material because it is relatively inexpensive and commercially available in the required form. However, its mechanical and physical properties are highly inconsistent and depend on geographic origin, climatic growth conditions and processing techniques [2]. Jute is one of the most important natural vegetal fibres, and is produced in India, Bangladesh, Thailand, Vietnam and other countries. It contains 56–64% cellulose, 29–25% hemicelluloses, 11–14% lignin and a small proportion of fats, pectin, ash and waxes [3]. Ben Daly et al. [4] have studied the effects of water absorption in pultruded composites containing fillers and low profile additives. Dash et al. [5] studied the processing, mechanical properties * Corresponding author. Tel.: +60 4 5996161; fax: +60 4 5941011. E-mail address: [email protected] (H.M. Akil). 0266-3538/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2009.04.014
and SEM analysis on low cost jute-polyester composites. Munikenche Gowda et al. [2] studied some mechanical properties of unsaturated jute reinforced polyester composites. Alam and Khan [6] studied the comparative study of water absorption behaviour jute reinforced composites using neutron radiography technique. Saha et al. [7] studied of jute fibre reinforced polyester composites by Dynamic Mechanical Analysis. From previous studies [2,4–7], moisture diffusion in polymeric composites have shown to be governed by three different mechanisms. The first mechanism involves diffusion of water molecules inside the micro gaps between polymer chains. The second mechanism involves capillary transport into the gaps and flaws which interfaces between fibre and the matrix. The third mechanism involves transport of microcracks in the matrix arising from the swelling of fibres (particularly in the case of natural fibre composites) [8]. Based on these mechanisms, absorption behaviour can be categorized into several types. These include: (1) linear Fickian behaviour, where the moisture weight gains gradually attains equilibrium after a rapid initial take off; (2) pseudo-Fickian behaviour where the moisture weight gains never reached equilibrium after initial take off; (3) two stage diffusion process with an abrupt jump in the moisture weight gains after initial take off; (4) rapid moisture gain results from fibre/matrix debonding and matrix cracking; and (5) moisture weight gains follows a decrease trend after the initial take off, an irreversible process as a result of the leaching out of the material from the bulk following chemical or physical breakdown [4]. This paper is focused on the water absorption study and its effects on pultruded jute fibre reinforced unsaturated polyester composite. Residual
1943
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
Fig. 1. Weight gain as a function of time for (a) flexural specimen and (b) compression specimen of pultruded jute reinforced unsaturated polyester composite after exposure to distilled water, sea water and acidic solution at room temperature.
mechanical properties under flexure and compression after immersion process at specified time were determined using rod composite specimen.
2. Experimental 2.1. Materials Jute fibres were supplied by Alam Fiber Impex Ltd., Bangladesh in twisted roving form. Unsaturated polyester resin (Crystic P9901) was purchased from Revertex Company, Malaysia.
The continuous jute fibres were first impregnated with unsaturated polyester resin in a resin impregnation tank. There was a pulling device to pull the resin impregnated jute fibre though steel die to obtain the desired shape. Curing process was carried out in a curing die which is a precision machined to impart the final shape. A pulling device draws the stock through a dies and also determines the production speed [10]. Finally, the profile was cut into desired length. The average diameter of all composites rod is 12.7 mm. Table 1 Effect of immersion environments on the diffusion coefficient, D and the maximum moisture content, Mm of jute fibre reinforced unsaturated polyester composite.
2.2. Preparation of pultruded composites The composites were prepared using a thermoset Pultrusion machine at MMFG Composites Sdn. Bhd, Subang Jaya, Selangor, Malaysia. During manufacturing of composites, jute fibres (roving) were placed on a creel merely of bookcase-type shelves and equipped with roving guide to lead the strands to the resin bath. Roving guider was used to ensure the strands do not scrape across one another as this will generate considerable static and cause ‘‘fuzz balls” to build up in the resin bath, raising its viscosity [9].
Flexural specimen
Compression specimen
Distilled water D (m2/s) Mm (%)
6.54 1012 10.71
2.67 1011 14.32
Sea water D (m2/s) Mm (%)
4.84 1012 10.25
2.02 1011 12.96
Acidic solution D (m2/s) Mm (%)
5.93 1012 10.34
2.20 1011 13.37
1944
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
2.3. Material characterization 2.3.1. Water absorption investigation Specimens were immersed into three different aqueous environments, which were distilled water, sea water and acidic solution. For the water absorption measurements, the specimens were withdrawn from the waters, wiped dry to remove the surface moisture, and then weighted using an electronic balance accurate
to 104 g to monitor the mass during the aging process. The moisture content, M(t) absorbed by each specimen is calculated from its weight before, w0 and after, wt absorption as follows:
wt w0 MðtÞ ¼ 100 w0
ð1Þ
Diffusion coefficient, D is calculated from the slope of moisture content versus the square root of time by:
Fig. 2. Variations of (a) flexural strength, (b) maximum flexural strain, and (c) flexural modulus for pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
2 2 h M2 M1 pffiffiffiffi pffiffiffiffi D¼p 4M m t2 t1
ð2Þ
Assuming the absorption process is linear at an early stage of immersion; times are taken at the beginning of absorption process, so that the weight change is expected to vary linearly with the square root of time.
1945
2.3.2. Flexural testing Flexural test was carried out using Instron 8802 according to the standard ASTM D4476–03. Specimens (pultruded rod with diameter of 12.7 mm) were cut into two parts with the cross section of each part is smaller than a half-round section. The total specimen length is 125 mm with overhang length of 12.5 mm at both supports. The crosshead speed for flexural test was set at
Fig. 3. Flexural stress–strain curves of pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
1946
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
5 mm/min. Three specimens for each condition are used to obtain a satisfactory result.
3. Results and discussions 3.1. Water absorption investigation
2.3.3. Compression testing Compression test was carried out using Instron 3367 according to the standard ASTM D 695-02a. The diameter and length of the specimen was 12.7 mm and 25.4 mm, respectively. The crosshead speed for flexural test was set at 5 mm/min. Three specimens for each condition are used to minimise the error.
Water absorption curves for flexural and compression specimens of jute fibre reinforced unsaturated polyester composite are shown in Fig. 1a and b. Specimens were immersed into three types of different aqueous environments, which are distilled water, sea water (pH 8) and acidic solution (pH 3). Each curve represents an
Fig. 4. Variations of (a) Compression strength, (b) maximum compression strain, and (c) compression modulus for pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
average data of three specimens. From the water absorption curves, it shown that absorbed water contents increased with increasing immersion time. According to a comprehensive review on moisture absorption behaviour which presented by Weitsman [11], jute fibre reinforced unsaturated polyester composite exhibited a so-called pseudo-Fickian behaviour, where the moisture weight gains never reaches equilibrium after initial take off. Diffusion coefficient, D
1947
and maximum of the moisture content, Mm for flexural specimens and compression specimens of jute fibre reinforced unsaturated polyester composite were represented in Table 1. Jute fibre reinforced unsaturated polyester composite gives higher diffusion coefficient, D and maximum of moisture content, Mm if immersed into distilled water, follows by acidic solution and lastly sea water. Table 1 indicates that both of the diffusion coefficient, D and maximum of
Fig. 5. Stress–strain curves for compression specimen of pultruded jute fibre reinforced unsaturated polyester composite after exposure to (a) distilled water, (b) sea water, and (c) acidic solution.
1948
H.M. Akil et al. / Composites Science and Technology 69 (2009) 1942–1948
the moisture content, Mm decrease if the specimen is immersed in sea water and acidic solution compared to distilled water. It is due to the presence of large salt molecules in sea water (notably sodium chloride) slows the diffusion process into the matrix of the composite materials, resulting in a lower absorption kinetic parameters [4]. Weakly acidic solutions, such as acetic acid solutions buffered at pH values of 3–5, do not cause significantly more extensive corrosion than distilled water and this finding is in agreement with previously reported work [12]. Both of the diffusion coefficient, D and maximum of moisture content, Mm for compression specimens are higher if compared to the flexural specimens. This may due to the influence of geometry and dimension of specimen, in which the cross sectional area of compression specimen was circular and flexural specimen was semicircular.
the composite materials rather than the intrinsic compressive failure [13]. The decrease in mechanical properties with increasing in moisture content could be attributed to the formation of hydrogen bonding between the water molecules and cellulose fibre [8]. The elementary unit of cellulose in jute fibre contains three hydroxyl (–OH) groups. These hydroxyl groups form an intramolecular hydrogen bonds inside the molecule and intermolecular hydrogen bonds with other cellulose molecules as well as hydroxyl groups of moist air [14]. With the presence of high hydroxyl group, natural fibre such as jute tends to show low moisture resistance. This may lead to dimensional variation and poor interfacial bonding between the fibre and matrix. The rigidity of the cellulose structure is destroyed by the water molecules in the cellulose network structure in which water acts as a plasticiser and it permits cellulose molecules to move freely [8].
3.2. Flexural testing 4. Conclusion Variations of flexural strength, maximum flexural strain and flexural modulus for jute fibre reinforced unsaturated polyester composite after exposed to various environmental conditions are summarised in Fig. 2a–c, respectively. Each value represents an average data of three specimens. Consequently, flexural stress– strain curves corresponding to the samples immersed in distilled water, sea water and acidic solution are depicted in Fig. 3a–c, respectively. From Fig. 2, it is clear that for all environmental conditions, as the immersion time increases, both flexural strength and flexural modulus decreases. This could be attributed to jute fibre which tends to absorb high amount of moisture after exposed to aqueous environmental and leads to higher degradation rate. This tendency can be also related to the weak fibre–matrix interface due to water absorption after exposed to aqueous environments [8]. On the other hand, the maximum flexural strain increases dramatically as the immersion time increases. It is believed that the cellulose content of jute fibre in the composites will be reduced as a result of water absorption process and making jute fibre more flexible as a result of plasticization effect. Absorbed water molecules fill into cavities and cracks within the composite and act as a plasticiser to the composite and render the structure more flexible [8]. Comparing the flexural properties with the aqueous environments, the highest degradation rate for jute fibre reinforced unsaturated polyester composite is recorded for specimen after exposed to distilled water, follows by acidic solution and sea water. In accordance with the diffusion coefficient, D and maximum of moisture content, Mm that shown in Table 1, higher diffusion coefficient and maximum of moisture content tend to impart higher degradation in flexural strength and flexural modulus with an improvement in flexural strain. 3.3. Compression testing Fig. 4a–c shows the variations of compression strength, maximum compression strain and compression modulus for jute fibre reinforced unsaturated polyester composite after exposed to distilled water, sea water and acidic solutions, respectively. On the other hand, Fig. 5a–c shows the corresponding stress–strain curves for each environmental condition (i.e. distilled water, sea water and acidic solution) investigated. Each value represents an average data of three specimens. Fig. 4 indicates a trend of decrease in compression strength and compression modulus with increasing immersion time; however, there is a trend of increasing maximum compression strain as immersion time increases after exposed to aqueous environments. These compression results show a similar trend with the flexural testing as discussed in Section 3.2. There are fluctuations as indicated by the error bars causing variations in the result which could be attributed to the uneven surfaces of the compression specimen. As a result, a slightly eccentric applied load will cause premature buckling in
Comparative study of water absorption on the mechanical properties of jute fibre reinforced unsaturated polyester composite has been studied following different aqueous environments: distilled water, sea water and acidic solutions at room temperature. The water absorption pattern is found to follow pseudo-Fickian behaviour and therefore is predictable over a period of time. Exposure of the natural fibre composite materials to aqueous environments resulted in significantly drops in strength and modulus due to the weakening of interface between fibre and matrix. However, a significant increase in the maximum strain is observed due to the increase in ductility of natural fibre as a result of breakdown of cellulose structure after immersion process. Acknowledgements The authors are grateful to Universiti Sains Malaysia (USM) and Construction Industry Development Board of Malaysia (CIDB) for their assistance and contribution that has resulted in this article. References [1] Velmurugan R, Manikandan V. Mechanical properties of palmyra/glass fiber hybrid composites. Compos: Part A 2007;38:2216–26. [2] Munikenche Gowda T, Naidu ACB, Chhaya Rajput. Some mechanical properties of untreated jute fabric-reinforced polyester composites. Compos: Part A 1999;30:277–84. [3] Murphy John. Reinforced plastics handbook. 2nd ed. Elsevier Advanced Technology; 1998. [4] Ben Daly H, Ben Brahim H, Hfaied N, Harchay M, Boukhili R. Investigation of water absorption in pultruded composites containing fillers and low profile additives. Polym Compos 2007;28:355–64. [5] Dash B, Rana AK, Mishra HK, Nayak SK, Mishra SC, Tripathy SS. Novel, low cost jute-polyester composites: Part 1: processing, mechanical properties, and SEM analysis. Polym Compos 1999;20:1. [6] Alam MK, Khan MA. Comparative study of water absorption behavior in Biopol and jute-reinforced Biopol composite using neutron radiography technique. Reinf Plast Compos 2006;25:11. [7] Saha AK, Das S, Bhatta D, Mitra BC. Study of jute reinforced polyester composites by dynamic mechanical analysis. Appl Polym Sci 1999;71:1505–13. [8] Dhakal HN, Zhang ZY, Richardson MOW. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos Sci Technol 2007;67:1674–83. [9] Meyer Raymond W. Handbook of pultrusion technology. New York: Chapman and Hall; 1985. [10] Callister Jr William D. Materials science, engineering: an introduction. 6th ed. John Wiley & Sons, Inc.; 2003. [11] Weitsman YJ. Effects of fluids on polymeric composites – a review. In: Kelly A, Zweben C, editors. Comprehensive composite materials, vol. 2. Elsevier; 2000. p. 369–401. [12] Jones Russell H. Environmental effects on engineered materials. Marcel Dekker, Inc.; 2001. [13] Davis Harmer E, Troxell George Earl, Hauck George FW. The testing of engineering materials. 4th ed. McGraw-Hill Book Company; 1982. [14] Alam MK, Khan MA. Comparative study of water absorption behavior in Biopol and jute-reinforced Biopol composite using neutron radiography technique. Reinf Plast Compos 2006;25:11.