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Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 1674–1683 www.elsevier.com/locate/compscitech
Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites H.N. Dhakal *, Z.Y. Zhang, M.O.W. Richardson Advanced Polymer and Composites (APC) Research Group, Department of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth, Hampshire PO1 3DJ, UK Received 12 April 2006; received in revised form 22 June 2006; accepted 29 June 2006 Available online 14 September 2006
Abstract Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to water immersion tests in order to study the effects of water absorption on the mechanical properties. HFRUPE composites specimens containing 0, 0.10, 0.15, 0.21 and 0.26 fibre volume fraction were prepared. Water absorption tests were conducted by immersing specimens in a de-ionised water bath at 25 C and 100 C for different time durations. The tensile and flexural properties of water immersed specimens subjected to both aging conditions were evaluated and compared alongside dry composite specimens. The percentage of moisture uptake increased as the fibre volume fraction increased due to the high cellulose content. The tensile and flexural properties of HFRUPE specimens were found to decrease with increase in percentage moisture uptake. Moisture induced degradation of composite samples was significant at elevated temperature. The water absorption pattern of these composites at room temperature was found to follow Fickian behaviour, whereas at elevated temperatures it exhibited non-Fickian. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Polymer–matrix composites; Natural fibre; B. Mechanical properties; D. Mechanical testing
1. Introduction The use of natural plant fibres as reinforcement in polymer composites for making low cost engineering materials has generated much interest in recent years. New environmental legislation as well as consumer pressure has forced manufacturing industries (particularly automotive, construction and packaging) to search for new materials that can substitute for conventional non-renewable reinforcing materials such as glass fibre [1]. The advantages of natural plant fibres over traditional glass fibres are acceptable as good specific strengths and modulus, economical viability, low density, reduced tool wear, enhanced energy recovery, reduced dermal and respiratory irritation and good biodegradability [2]. Natural plant fibre reinforced polymeric composites, also have some disadvantages such as the *
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0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.06.019
incompatibility between the hydrophilic natural fibres and hydrophobic thermoplastic and thermoset matrices requiring appropriate use of physical and chemical treatments to enhance the adhesion between fibre and the matrix [3]. Hemp is also called cannabis sativa. It is an annual herbaceous plant native to Asia and widely cultivated in Europe [4]. Hemp and flax are the only commercial sources of long natural fibres grown in the UK Plant stems are processed by various mechanical methods to extract the fibre [5]. Fibres from hemp stems have been widely used in the production of cords and clothing, and have potential for reinforcement in polymer–matrix composites (PMCs). Recently, car manufacturers have started manufacturing non-structural components using hemp and flax fibres due to their higher specific strength and lower price compared to conventional reinforcements [6]. All polymer composites absorb moisture in humid atmosphere and when immersed in water. The effect of
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absorption of moisture leads to the degradation of fibre– matrix interface region creating poor stress transfer efficiencies resulting in a reduction of mechanical and dimensional properties [7]. One of the main concerns for the use of natural fibre reinforced composite materials is their susceptibility to moisture absorption and the effect on physical, mechanical and thermal properties [8]. It is important therefore that this problem is addressed in order that natural fibre may be considered as a viable reinforcement in composite materials. Several studies in the use of natural fibre reinforced polymeric composites have shown that the sensitivity of certain mechanical and thermal properties to moisture uptake can be reduced by the use of coupling agents and fibre surface treatments [9,10]. Moisture diffusion in polymeric composites has shown to be governed by three different mechanisms [11,12]. The first involves of diffusion of water molecules inside the micro gaps between polymer chains. The second involves capillary transport into the gaps and flaws at the interfaces between fibre and the matrix. This is a result of poor wetting and impregnation during the initial manufacturing stage. The third involves transport of microcracks in the matrix arising from the swelling of fibres (particularly in the case of natural fibre composites). Generally, based on these mechanisms, diffusion behaviour of polymeric composites can further be classified according to the relative mobility of the penetrant and of the polymer segments, which is related to either Fickian, non-Fickian or anomalous, and an intermediate behaviour between Fickian and non-Fickian [13,14]. In general moisture diffusion in a composite depends on factors such as volume fraction of fibre, voids, viscosity of matrix, humidity and temperature [15]. The objective of this work was to compare the influence of both fibre reinforcement and water uptake on mechanical properties of hemp fibre reinforcement unsaturated polyester composites and the related kinetics and characteristics of the water absorption.
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and the structure of hemp fibre are presented in Table 1 [16]. The mechanical and physical properties of the polyester, hemp, and glass fibre used in this study are presented in Table 2 [17]. 2.2. Processing A combination of hand lay-up and compression moulding method was used to prepare the HFRUPE composite samples. Non-woven hemp fibre mat was first dried at 100 C to remove storage moisture in a fan-assisted oven. The storage moisture was recorded for hemp mat approximately 9%. A measured quantity of unsaturated polyester resin mixed with a catalyst (MEKP) for rapid curing was poured on a pre-weighed amount of non-woven hemp fibre mat, which was placed in a mould. The mould was coated with a semi-permanent, polymer mould release agent, Frekote FRP90-NC. After pouring the resin, each layer was left for a few minutes to allow the resin to soak into the fibre mat. Trapped air was gently squeezed out using a roller. The hemp fibre and polyester resin were then left for about 3 min to allow air bubbles to escape from the surface of the resin. The mould was closed and the composite panel was left to cure in a hydraulic press at a temperature of 22 C and at a compaction pressure of 10 bar for 1.5 h. The fabrication route of the HFRUPE composites is depicted in Fig. 1. The schematic of a hydraulic press used to consolidate composite panels is shown in Fig. 2. After being taken out from the hydraulic press, the panel was left to cure at a temperature of 22 C for 24 h before being removed from the mould. Subsequently, post curing was carried out at a temperature of 80 C for 3 h. In addition to this non-woven hemp fibre, a randomly oriented chopped strand mat (E-glass fibre 40 w/w%) was used to prepare reference glass fibre composite sample fabricated using similar procedure for comparison purpose. 2.3. Water absorption tests The effect of water absorption on hemp fibre reinforced unsaturated composites were investigated in accordance with BS EN ISO 62:1999 [18]. The samples for tensile and
2. Experimental procedure 2.1. Materials The matrix material used in this study was based on a commercially available unsaturated polyester, Trade Name ‘‘NORPOL 444-M888’’ supplied by Reichhold UK Ltd. The matrix was mixed with curing catalyst, methyl ethyl ketone peroxide (MEKP) at a concentration of 0.01 w/w of the matrix for curing. Needle punched randomly oriented non-woven hemp fibre, fabric weight 330 g/m2, was used as the reinforcement and was provided by JB Plant Fibres Enterprises Ltd. The typical chemical composition
Table 2 Comparative values of physical and mechanical properties of hemp with E-glass fibre Fibre
Density (g/cm3)
Elongation to break (%)
Tensile strength (MPa)
Young’s modulus (GPa)
Hemp E-glassa
1.14 2.50
1.6 2.5
690 2000–3500
30–60 70
a
For comparison purpose.
Table 1 Typical chemical composition and structure parameters of hemp fibre Cellulose
Hemicellulose
Lignin
Pectins
Wax
Cell length (mm)
Spiral angle (Deg)
Moisture content (%)
74.4
17.9
3.7
0.9
0.8
23.0
6.2
10.8
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Unsaturated polyester
Resin/catalyst/hemp fibre
Resin/catalyst mixing/hemp fibre drying
Mixture of Resin/catalyst/hemp fibre
Catalyst (MEKP)
Hemp fibre
Drying Time: 1 hr Temp: 100 dc
Mixing
Mixture of resin/catalyst
Mould frame
Hemp fibre
Hand lay up
Hemp fibre/mixture of resin and catalyst
Processing
Hydraulic press consolidation Time: 1.5 hrs, Pressure: 10 Bars Temp: 25 dc
Post curing
Post curing Time: 3 hrs, temp:80 dc
Composite laminate
Composites
Fig. 1. Process flow chart showing the applied fabrication route of HFRUPE composites.
Heat and pressure
Hot press platen
conduction plate
Mixture of UPE/catalyst/hemp in a mould frame
Top/bottom mould plates
conduction plate
Hot press platen
Heat and pressure Fig. 2. Schematic of the composite consolidation.
flexural tests containing different fibre volume fractions of reinforcement were machined to a size of 150 · 20 · 3 mm3 and 60 · 15 · 3 mm3, respectively. First all the specimens were dried in an oven at 50 C and then were allowed them to cool to room temperature in a desiccator before weighing them to the nearest 0.1 mg. This process was repeated until the mass of the specimens were reached constant. Water absorption tests were conducted by immersing the HFRUPE specimens in a de-ionised water bath at 25 C for different time durations. After immersion for 24 h, the specimens were taken out from the water and all surface water was removed with a clean dry cloth. The specimens were reweighed to the nearest 0.1 mg within 1 min of removing them from the water. The specimens were
weighed regularly at 24, 48, 98, 196, 392 up to 888 h exposure. Similarly, the specimens were immersed in water at 100 C to determine water absorption at a higher temperature. For this test, the specimens were placed in a container of boiling de-ionised water. After 30 min of immersion, the specimens were removed from the boiling water, cooled in de-ionised water for 15 min at room temperature then removed and weighed to the nearest 0.1 mg. The weight of the samples was measured at different time intervals up to 31 h of exposure until the water content reached saturation. The moisture absorption was calculated by the weight difference. The percentage weight gain of the samples was measured at different time intervals and the moisture content versus square root of time was plotted.
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2.4. Mechanical testing 2.4.1. Tensile testing The tensile strength and modulus of the hemp fibre reinforced composites before and after water immersion were with a crosshead speed of 10 mm/min in accordance with BS EN 2747:1998 [19]. Test specimens were individually cut using a diamond wheel into rectangular beams from the laminate slabs fabricated by a hand lay-up process. The cut edges were then smoothed using 240 Grade SiC paper. 2.4.2. Flexural testing The flexural strength and modulus of the composite before and after water immersion were determined using three-point bending test method following BS EN 2746:1998 test method [20]. A span of 48 mm, maintaining a span to depth ratio of 16:1, was used in a 30 kN load cell. The load was placed midway between the supports. The crosshead speed applied was 2 mm/min. Each sample was loaded until the core broke and their average is reported.
where mi is the initial weight of the moisture in the material and ms is the weight moisture in the material when the material is fully saturated, in equilibrium with its environment. D is the mass diffusivity in the composite. This is an effective diffusivity since all the heterogeneities of the composites have been neglected. h is thickness of specimen and t is the time and j is the summation index. The diffusion coefficient is an important parameter in Fick’s law. Solving the diffusion equation for the weight of moisture, and rearranging in terms of the percent moisture content, the following relationship is obtained: 4M m t 0:5 0:5 M¼ Dx ð3Þ h p where Mm is the equilibrium moisture content of the sample. Using the weight gain data of the material with respect to time, a graph of weight gain versus time is plotted. The diffusion coefficient can be calculated using the following formula: D¼
2.4.3. Scanning electron microscopy In order to understand the effect of water absorption on the microstructure of composites the surfaces of the waterimmersed specimens were examined using a scanning electron microscope (SEM) JSM 6100. 3. Results and discussion The results obtained from this experimental study can be divided into two parts. The first part considers the nature of the diffusion into the hemp reinforced composites and the second evaluates the effects of water absorption at room temperature and at 100 C exposure on the mechanical properties. 3.1. Sorption behaviour The percentage of water absorption in the composites was calculated by weight difference between the samples immersed in water and the dry samples using the following equation: mt mo 100 mo
d2 p2 t70
ð4Þ
Where d is sample thickness in mm and t70 is time taken to reach 70% saturation in seconds. The diffusion properties of composites described by Fick’s laws was evaluated by weight gain measurements of pre-dried specimen immersed in water by considering the slope of the first part of the weight gain curve versus square root of time by using the following equation [25]. The coefficient of diffusion (D) defined as the slope of the p normalised mass uptake against t and has the form: 2 kh D¼p ð5Þ 4M m where, k is the initial slope of a plot of M(t) versus t1/2, Mm is the maximum weight gain and h is the thickness of the composites. Fig. 3 shows percentage of weight gain as a function of square root of time for UPE and various loading levels of UPE only
UPE/2 Layer hemp
UPE/ 3 Layer hemp
UPE/4 Layer hemp
UPE/5 Layer hmep
UPE/CSM
12
ð1Þ
10
where DM(t) is moisture uptake, Mo and Mt are the mass of the specimen before and during aging, respectively. Different models have been developed in order to describe the moisture absorption behaviour of the materials [21,22]. For one-dimensional moisture absorption each sample is exposed, on both sides, to the same environment, the total moisture content G can be expressed as follows [23,24]: " # 2 1 m mi 8 X 1 ð2j þ 1Þ p2 Dx t G ¼1 2 exp p j¼0 ð2j þ 1Þ2 ms mi h2 ð2Þ
Weight gain (%)
DMðtÞ ¼
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8 6 4 2 0 0
10
20
Time (Hours)
30
40
1/2
Fig. 3. Water absorption curves at RT for different specimens.
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UPE/hemp reinforced samples immersed in de-ionised water at room temperature (23 C). The maximum percentage weight gain for UPE, 3, 4 and 5 layers of hemp fibre reinforced specimens, corresponding to 0, 0.15, 0.21 and 0.26 fibre volume fractions, respectively, immersed at room temperature for 888 h is 0.879, 5.63, 8.16 and 10.97%, respectively. The water uptake process for all specimens except CSM, which hardly absorbs any water, is linear in the beginning, then slows and approaches saturation after prolonged time, following a Fickian diffusion process. Both the initial rate of water absorption and the maximum water uptake increases for all HFRUPE composites samples as the fibre volume fraction increases. This phenomenon can be explained by considering the water uptake characteristics of hemp fibre. When the composite is exposed to moisture, the hydrophilic hemp fibre swells. As a result of fibre swelling, micro cracking of the brittle thermosetting resin (like unsaturated polyester) occurs. The high cellulose content in hemp fibre (approximately 74%) further contributes to more water penetrating into the interface through the micro cracks induced by swelling of fibres creating swelling stresses leading to composite failure [26]. As the composite cracks and gets damaged, capillarity and transport via micro cracks become active. The capillarity mechanism involves the flow of water molecules along fibre–matrix interfaces and a process of diffusion through the bulk matrix. The water molecules are actively attack the interface, resulting in debonding of the fibre and the matrix [27]. The SEM evidence in Fig. 4 supports this explanation. Fig. 5 shows the percentage of weight gain for UPE, 3, 4 and 5 layers of hemp specimens immersed in water at 100 C. For UPE, 3, 4 and 5 layer hemp reinforced specimens the percentage of moisture absorption is 1.947, 7.366, 9.12 and 13.53%, respectively. The effect of fibre volume fraction and temperature on water absorption can be clearly seen. The rate of approach to equilibrium is clearly more rapid for the 100 C specimens than the samples immersed at RT. Higher temperatures seem to accelerate the moisture uptake behaviour. When the temperature of immersion is increased, the moisture saturation time (MST) is greatly shortened. For 5 layer hemp samples at room temperature, it takes 888 h to reach MST whereas for 100 C samples, the MST is 31 h. The MST in this case was shortened by 857 h. This shows that sorption at room
16
UPE only
UPE/3 Layer hemp
UPE/4 Layer hemp
UPE/5 Layer hemp
14 12
Weight gain (%)
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10 8 6 4 2 0 0
2
4
Time (Hours )
6
8
1/2
Fig. 5. Water absorption curves at boiling temperature for different specimens.
temperature takes far longer period to reach equilibrium than sorption at elevated temperatures. In addition to the increase in weight gain percentage, it also shows the weight gain is higher for samples immersed in boiling water than at room temperature. For 5 layer hemp samples, the weight gain percentage at moisture saturation point at boiling temperature is approximately 23% higher than at room temperature. It is evident that there is a different sorption behaviour for immersion at room temperature than for elevated temperature indicating different aging mechanisms. The higher and faster weight gain upon exposure to boiling water may be attributed to the different diffusivity of water into the material leading to moisture induced interfacial cracks at an accelerated rate as a result of degradation in the fibre–matrix interface region as well as the state of water molecules existing in the HFRUPE composites. Other studies also have reported a similar trend for ageing of polymer composites at elevated temperatures [28]. Table 3 presents the diffusion coefficients for both room temperature and 100 C water-immersed specimens. It can be seen that the maximum moisture content and the diffusion coefficient values increases steadily with an increase in fibre volume fraction. The increase is more pronounced for the specimens immersed at 100 C than those of immersed
Fig. 4. Failure showing (a) matrix cracking, (b) fracture running along the interface and (c) fibre–matrix debonding due to attack by water molecules.
H.N. Dhakal et al. / Composites Science and Technology 67 (2007) 1674–1683 Table 3 Moisture uptake of hemp fibre composites immersed in water at RT and 100 C Saturation moisture uptake Mm (%) RT 0 (UPE only) 10 (2L hemp) 15 (3L hemp) 21 (4L hemp) 26 (5L hemp)
0.879 3.441 5.639 8.161 10.972
100 C 1.947 – 7.366 9.125 13.533
Initial slope of plot (k) M(t) versus t1/2
Diffusion coefficient, D, ·103 (m2/s)
RT
100 C
RT
100 C
0.102 0.102 0.247 0.346 0.496
0.437 – 1.178 1.562 2.375
5.714 1.551 3.618 3.841 4.367
88 – 48 62 67
Stress for samples without moisture absorption
70
Data in table are means with a sample size of 3 for each specimen group.
3.2. Effect of moisture absorption on mechanical properties 3.2.1. Tensile properties The tensile stresses and strain versus fibre volume fraction results for these samples are shown in Figs. 7 and 8. For both dry and water aged samples (exposure time 888 h at RT), the stress–strain curves are linear up to the point of failure. There is no affect of water absorption on tensile stress for UPE samples. The tensile stress was rather increased after water immersion of 888 h. Similarly, for 2 layer hemp reinforced samples, the tensile stress is increased by 22% after immersion in water. This increase in tensile
Fig. 6. Degradation of composite showing (a) crack development (b) lost of resin particles due to high accelerated ageing at 100 C.
60 50 40 30 20 10 0 0.1
0.15
0.21
0.26
UPE
Fibre volume fraction Fig. 7. Tensile stress versus fibre volume fraction.
Strain for samples without moisture absorption
14
Strain for samples with moisture absorption
12 10
Strain (%)
at RT. Higher fibre loaded samples, as would be expected, contain a greater diffusivity due to higher cellulose content. The moisture uptake at elevated temperatures compared to RT seems to obey non-Fickian behaviour showing a 23% higher moisture uptake for 5 layer hemp fibre reinforced composites. The moisture uptake results in this study show Fickian behaviour at room temperature and non-Fickian at boiling temperature. This is attributed due to the moist, high temperature environment, and microcracks developed on the surface and inside the materials [29]. As the cracks develops material is actually lost, most likely in the form of resin particles [30,31] as can be seen in Fig. 6a and b. After the occurrence of damage in the composites water transport mechanisms become more active [32]. The deviation from Fickian water uptake behaviour at 100 C is attributed to the development of micro cracks in the composites [33].
Stress for samples with moisture absorption
80
Tensile stress (MPa)
Composite fibre (vol%)
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8 6 4 2 0 0.1
0.15
0.21
0.26
UPE
Fibre volume fraction Fig. 8. Tensile strain versus fibre volume fraction.
stress for unreinforced and 2 layer hemp reinforced sample implies that further crosslinking or other mechanisms are taking place enhancing the material strength. The tensile stress however, drops by 38 and 15%, respectively, for 3 and 4 layer hemp reinforced specimens. Generally, for higher fibre volume composites samples immersed in water, it is expected that the relative extent of decrease in tensile properties is greater compared to dry samples. However, it is interesting to note that for 5 layer hemp reinforced samples, the ultimate tensile stress of wet samples is higher than that for dry samples. This could be due to the fact that high amounts of water causes swelling of the fibres, which could fill the gaps between the fibre and the polymer–matrix and eventually could lead to an increase in the mechanical properties of the composites [34]. Similar observations have been reported for jute fibre reinforced polymer composites where after 24 h of soaking in water the flexural strength increased
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by 28% and after 72 h of water immersion, and flexural strength was increased by 45% [35]. The failure tensile strain value for all water-immersed specimens was found to increase compared to dry specimens. The increase in failure strain upon exposure of the samples to a wet environment can be attributed to the plasticisation of hemp samples caused by moisture absorption. Fig. 9a shows a SEM picture of hemp fibre. At regular intervals along the fibre surface, kinks or nodes can be clearly seen. Fig. 9b shows a HFRUPE composite where the effect of kinks or nodes on the surface of the composite laminate reflect the misalignment of fibres. When these irregularly shaped fibres are placed in composites they do not seem aligned properly leading to fibre entanglement. Fibre alignment factors play a crucial role in the overall properties of composites. There is always a chance of fibre entanglement with randomly oriented fibre reinforced composites. The random orientation of fibres produces lower mechanical properties compared to long unidirectionally orientated fibres. This fibre entanglement can create resin rich areas, which can contribute to the formation of voids and porosity (Fig. 10). Voids and porosity can act as stress concentrators leading to failure of composite samples. Hence, the void content for 2, 3, 4 and 5 layers of hemp fibre composites specimen was found to be 12.56, 14.46, 16.60 and 18.64%, respectively. The void content of
Fig. 9. SEM micrograph of hemp fibre (a) showing kinks or nodes (b) showing fibre misalignment and entanglement.
composite was calculated using the following standard formula: wf wm V v ¼ 1 qc ð6Þ þ qf qm where Vv is the volume fraction of voids, qc the density of composite, wf the weight percent of fibre (%), wm the weight percent of matrix (%), qf the density of fibre g/cm3 and qm is the density of matrix g/cm3. As far as voids content in natural fibre composites is concern, the fabrication techniques are not yet fully developed and the natural origin of the fibre component necessarily induces an element of variation in to the composites; both factors contribute in creation of voids which affects to the overall composite properties. It is evident in this study that as the fibre volume fraction of hemp reinforced composite sample increases the void content also increases. 3.2.2. Flexural properties The flexural stress–strain versus fibre volume fraction results for dry and water immersed (exposure time 888 h at RT) HFRUPE composites are shown in Figs. 11 and 12. The observations made earlier for the effect of water absorption on tensile stress/strain properties are also relevant here. The flexural stress drops incrementally as the fibre volume fraction increases hence increased moisture uptake percentage. The decrease in flexural properties after water immersion can be related to the weak fibre–matrix interface due to water absorption. Flexural strain for water-immersed samples has increases dramatically compared to dry samples. Flexural strain for 5 layer hemp reinforced dry samples is 8% whereas after 888 h of water immersion the strain is almost doubled. HFRUPE composites become more rigid due to the lower flexibility of the unsaturated polyester chain. After water aging for 888 h, strain is almost doubled compared to dry specimens since natural fibre reinforced composites tend to be ductile once the loss of cellulose and integrity has taken place [36]. It has been reported that water molecules act as a plasticiser agent in the composite material, which normally leads to an
Fig. 10. Micrograph of water immersed hemp/UPE samples showing effects of voids (a) voids, (b) voids acting as reservoirs and (c) matrix cracking and delamination after 888 h of immersion.
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Dry sample
Water immersed sample
Flexural stress (MPa)
120 100 80 60 40 20 0 UPE
0.1
0.15
0.21
0.26
Fibre volume fraction Fig. 11. Flexural stress versus fibre volume fraction.
30
Dry sample
water immersed sample
Flexural strain (%)
25 20 15 10 5 0 UPE
0.1
0.15
0.21
0.26
Fibre volume fraction Fig. 12. Flexural strain versus fibre volume fraction.
increase of the maximum strain for the composites after water absorption [37]. The decrease in mechanical properties with increase in moisture content is may be caused by the formation of hydrogen bonding between the water molecules and cellulose fibre. Natural fibres are hydrophilic with many hydroxyl groups (–OH) in the fibre structure forming a large number of hydrogen bonds between the macromolecules of the cellulose and polymer [38]. With the presence of a high –OH group percentage, natural fibres such as hemp tend to show low moisture resistance. This leads to dimensional variation of composites products and poor interfacial bonding between the fibre and matrix, causes a decrease in the mechanical properties [39]. Water absorbed in polymers is generally divided into free water and bound water. Water molecules (which are contained in the free volume of polymer and are relatively free to travel through the micro voids and holes) are identified as free water. Water molecules that are dispersed in the polymer–matrix and attached to the polar groups of the polymer are designated as bound water [40]. The char-
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acteristics of water immersed specimens are influenced not only by the nature of the fibre and matrix materials but also by the relative humidity and manufacturing technique, which determines factors such as porosity and volume fraction of fibres. Water uptake can be advantageous for some natural fibres (such as Duralin fibre) at 66% relative humidity as can fibre plasticising effect as a result of from the presence of free water [41]. Excessive water absorption, however, leads to an increase in the absorbed bound water and a decrease in free water. In this situation, water can penetrate into the cellulose network of the fibre and into the capillaries and spaces between the fibrils and less bound areas of the fibrils. Water may attach itself by chemical links to groups in the cellulose molecules. 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. Consequently the mass of the cellulose is softened and can change the dimensions of the fibre easily with the application of forces. Observation of the fracture surface from the flexural test sample further emphasises the importance of fibre–matrix adhesion on flexural strength. 3.2.3. Influence of moisture on the modulus Table 4 represents the results of tensile modulus and flexural modulus for both dry and water-immersed specimens at RT. It can be seen that moisture absorption causes change in the modulus as determined by tensile and flexural tests. The tensile modulus decreases for all hemp reinforced samples. The reduction in tensile modulus for 3, 4 and 5 layer hemp reinforced specimens compared to dry specimens is 61, 97 and 87%, respectively. A plausible explanation for this would be that, the elastic modulus is a fibresensitive property in composites and is affected as a result of moisture absorption. This effect is particularly greater for the composites with higher fibre content, in which stress transfer capability between fibre and matrix interface gets sharply reduced due to moisture content. The flexural modulus, however, is not adversely affected by moisture absorption. The increase in flexural modulus is more pronounced with higher fibre content specimens, hence higher moisture content. It would be intuitive to assume that the effect of fibre reinforcement to be less critical for the flexural failure stress than in tensile failure Table 4 Tensile and flexural modulus for dry and wet samples Specimens
Fibre volume (%)
Tensile modulus (GPa)
Flexural modulus (GPa)
Dry
Wet
Dry
Wet
UPE only 2 Layer hemp 3 Layer hemp 4 Layer hemp 5 Layer hemp
0 10 15 21 26
0.56 0.72 1.0 1.22 1.27
0.60 0.64 0.62 0.62 0.68
5.51 4.20 5.34 7.30 6.49
5.81 5.76 6.08 6.06 8.05
Data in table are means with a sample size of 5 for dry and 3 for wet for each specimen group.
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mode. This is because the flexural samples fail in combination of compression, shear and tension mode. 4. Conclusions The effect of water absorption on the mechanical properties of non-woven hemp fibre reinforced unsaturated polyester composites has been studied following immersion at room temperature and boiling temperature. It shows that moisture uptake increase with fibre volume fraction increases due to increased voids and cellulose content. The water absorption pattern of these composites at room temperature is found to follow Fickian behaviour, whereas at the elevated temperature the absorption behaviour is non-Fickian. Water uptake behaviour is radically altered at elevated temperatures due to significant moisture induced degradation. Exposure to moisture results in significant drops in tensile and flexural properties due to the degradation of the fibre–matrix interface. References [1] Bledzki AK, Gassan J. Composites reinforced with cellulose base fibres. Progr Polym Sci 1999;24:221–74. [2] Bolton J. The potential of plant fibres as crops for industrial use. Outlook Agric 1995;24:85–9. [3] Gassan J, Cutowski VS. Effect of corona discharge and UV treatment on the properties of jute–fiber epoxy composites. Compos Sci Technol 2000;60:2857–63. [4] Richardson MOW, Santana MTJ, Hague J. Natural fibre composites – the potential for the Asian markets. Progr Rubber Plast Technol 1998;14:174–88. [5] Hepworth DG, Hobson RN, Bruce DM, Farrent JW. The use of unretted hemp fibre in composite manufacture. Compos Part A 2000;31:1279–83. [6] Karus M, Kaup M. Natural fibres in the European automotive industry. J Ind Hemp 2002;7:117–29. [7] Yang GC, Zeng HM, Li JJ, Jian NB, Zhang WB. Relation of modification and tensile properties of sisal fiber. Acta Sci Nat Uni Sunyatseni 1996;35:53–7. [8] Thwe MM, Liao K. Effects of environmental ageing on the mechanical properties of bamboo-glass fiber reinforced polymer matrix hybrid composites. Compos Part A 2002;33:43–52. [9] Joseph K, Thomas S. Effect of chemical treatment on the tensile properties of short sisal fibre-reinforced polyethylene composites. Polymer 1996;37:5139–49. [10] Mwaikambo LY, Ansel MP. Chemical modification of hemp, sisal, jute and kapok fibres by alkalization. J Appl Polym Sci 2002;84:2222–34. [11] Espert A, Vilaplana F, Karlsson S. Comparison of water absorption in natural cellulosic fibres from wood and one-year crops in polypropylene composites and its influence on their mechanical properties. Compos Part A 2002;35:1267–76. [12] Karmaker AC. Effect of water absorption on dimensional stability and impact energy of jute fibre reinforced polypropylene. J Mater Sci Lett 1997;16:462. [13] Collings TA. Moisture absorption–Fickian diffusion kinetics and moisture profiles. In: Jones FR, editor. Handbook of polymer fibre composites. UK: Longman Scientific and Technical; 1994. p. 366–71. [14] Shen CH, Springer G. Moisture absorption and desorption of composite materials. J Compos Mater 1999;10:2–20. [15] Thomasan JL. The interface region in glass fibre-reinforced epoxy resin composites: 2. Water absorption, voids and the interface. Composites 1995;26:477–85.
[16] Bledzki AK, Reihmane S, Gassan J. Properties and modification methods for vegetable fibres for natural fibre composites. J Appl Polym Sci 1996;59:1329–36. [17] Bledzki AK, Zhang W, Chate A. Natural fibre reinforced polyurethane microfoams. Compos Sci Technol 2001;61:2405–11. [18] Plastics – determination of water absorption, BS EN ISO 1999;62:1– 8. [19] Glass fibre reinforced plastics-tensile test. BS EN 1998;2747:1–11. [20] Glass fibre reinforced plastics-flexural test – three point bend method. BS EN1998;2746:1–6. [21] Wang W, Sain M, Cooper PA. Study of moisture absorption in natural fibre composites. Compos Sci Technol 2006;66:379–86. [22] Sreekala MS, Thomas S. Effect of fibre surface treatment on watersorption characteristics of oil palm fibres. Compos Sci Technol 2003;63:861–9. [23] Geroge J, Carter HG, Kibler KG. Langmuir-type model for anomalous moisture diffusion in composite resins. J Compos Mater 1978;12:118–31. [24] Camino G, Luda MP, Polishchuk A, Revellino Ya, Blancon M, Merle RG, et al. Kinetic aspects of water sorption in polyester resin/ glass–fibre composites. J Compos Sci Technol 1997;57:1469–82. [25] William MB, Dumolin F, Hayward D, Pethrick A, Li ZC. Nondestructive examination of composite joint structures: a correlation of water absorption and high-frequency dielectric propagation. J Phys D: Appl Phys 1996;29:233–9. [26] Bismarck A, Askargorta IA, Springer J, Lampke T, Wielage B, Stamboulis A, et al. Surface characterization of flax, hemp and cellulose fibres; surface properties and the water uptake behaviour. Polym Compos 2002;23:872–94. [27] Marom G. The role of water transport in composite materials. In: Comyn J, editor. Polymer permeability. Elsevier Applied Science; 1985 [chapter 9]. [28] Liao K, Schulttheisz CR, Hunston DL. Effects of environmental ageing on the properties of pultruded GFRP. Compos Part B 1999;30:485–93. [29] Zhou J, Lucas JP. The effects of water environment on anomalous absorption behaviour in graphite epoxy composites. Compos Sci Technol 1995;53:57–64. [30] Loos AC, Springer GS. Moisture absorption of polyester-E glass composites. In: Vinson JR, editor. Environmental effects on composites materials. ASTM STP, 658. p. 43–60. [31] Loos AC, Springer GS. Effect of thermal spiking on graphite/epoxy composites. In: Springer GS, editor. Environmental effects on composites materials. CT, USA: Technomic Publishing Co. Inc.; 1981. p. 109–25. [32] Comyn J. Polymer permeability. Chapman and Hull, Printed in Great Britain by Ipswich Book Co., Ipswich, Suffolk, 1985 (reprinted 1994). [33] Verghese NE, Hayes M, Garcia K, Carrier C, Wood J, Lesko JJ. Effects of temperature sequencing during hydrothermal ageing of polymers and polymer matrix composites: the reverse thermal effect, fibre composites in infrastructure. In: Proceedings of the second international conference on fibre composites in infrastructure ICCI; 1998. p. 720–39. [34] Karmakar AC, Hoffmann A, Hinrichsen G. Influence of water uptake on the mechanical properties of jute fibre reinforced polypropylene. J Appl Polym Sci 1994;54:1803–7. [35] Ayensu A. Interfacial debonding of natural fibre reinforced composites. Quart Sci Vision 2000;6:25–34. [36] Joseph PV, Rabello MS, Maattoso LHC, Joseph K, Thomas S. Environmental effects on the degradation behavior of sisal fibre reinforced polypropylene composites. Compos Sci Technol 2002;62:1357–72. [37] Stamboulis A, Baillie CA, Peijs T. Effects of environmental conditions on mechanical and physical properties of flax fibers. Compos Part A Appl Sci Manuf 2001;32:1105–15. [38] Sombastsompop N, Chaochanchaikul K. Effect of moisture content on mechanical properties, thermal and structural stability and
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Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites H.N. Dhakal *, Z.Y. Zhang, M.O.W. Richardson Advanced Polymer and Composites (APC) Research Group, Department of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth, Hampshire PO1 3DJ, UK Received 12 April 2006; received in revised form 22 June 2006; accepted 29 June 2006 Available online 14 September 2006
Abstract Hemp fibre reinforced unsaturated polyester composites (HFRUPE) were subjected to water immersion tests in order to study the effects of water absorption on the mechanical properties. HFRUPE composites specimens containing 0, 0.10, 0.15, 0.21 and 0.26 fibre volume fraction were prepared. Water absorption tests were conducted by immersing specimens in a de-ionised water bath at 25 C and 100 C for different time durations. The tensile and flexural properties of water immersed specimens subjected to both aging conditions were evaluated and compared alongside dry composite specimens. The percentage of moisture uptake increased as the fibre volume fraction increased due to the high cellulose content. The tensile and flexural properties of HFRUPE specimens were found to decrease with increase in percentage moisture uptake. Moisture induced degradation of composite samples was significant at elevated temperature. The water absorption pattern of these composites at room temperature was found to follow Fickian behaviour, whereas at elevated temperatures it exhibited non-Fickian. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Polymer–matrix composites; Natural fibre; B. Mechanical properties; D. Mechanical testing
1. Introduction The use of natural plant fibres as reinforcement in polymer composites for making low cost engineering materials has generated much interest in recent years. New environmental legislation as well as consumer pressure has forced manufacturing industries (particularly automotive, construction and packaging) to search for new materials that can substitute for conventional non-renewable reinforcing materials such as glass fibre [1]. The advantages of natural plant fibres over traditional glass fibres are acceptable as good specific strengths and modulus, economical viability, low density, reduced tool wear, enhanced energy recovery, reduced dermal and respiratory irritation and good biodegradability [2]. Natural plant fibre reinforced polymeric composites, also have some disadvantages such as the *
Corresponding author. Tel.: +44 23 9284 2396; fax: +44 23 9284 2351. E-mail address: [email protected] (H.N. Dhakal).
0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.06.019
incompatibility between the hydrophilic natural fibres and hydrophobic thermoplastic and thermoset matrices requiring appropriate use of physical and chemical treatments to enhance the adhesion between fibre and the matrix [3]. Hemp is also called cannabis sativa. It is an annual herbaceous plant native to Asia and widely cultivated in Europe [4]. Hemp and flax are the only commercial sources of long natural fibres grown in the UK Plant stems are processed by various mechanical methods to extract the fibre [5]. Fibres from hemp stems have been widely used in the production of cords and clothing, and have potential for reinforcement in polymer–matrix composites (PMCs). Recently, car manufacturers have started manufacturing non-structural components using hemp and flax fibres due to their higher specific strength and lower price compared to conventional reinforcements [6]. All polymer composites absorb moisture in humid atmosphere and when immersed in water. The effect of
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absorption of moisture leads to the degradation of fibre– matrix interface region creating poor stress transfer efficiencies resulting in a reduction of mechanical and dimensional properties [7]. One of the main concerns for the use of natural fibre reinforced composite materials is their susceptibility to moisture absorption and the effect on physical, mechanical and thermal properties [8]. It is important therefore that this problem is addressed in order that natural fibre may be considered as a viable reinforcement in composite materials. Several studies in the use of natural fibre reinforced polymeric composites have shown that the sensitivity of certain mechanical and thermal properties to moisture uptake can be reduced by the use of coupling agents and fibre surface treatments [9,10]. Moisture diffusion in polymeric composites has shown to be governed by three different mechanisms [11,12]. The first involves of diffusion of water molecules inside the micro gaps between polymer chains. The second involves capillary transport into the gaps and flaws at the interfaces between fibre and the matrix. This is a result of poor wetting and impregnation during the initial manufacturing stage. The third involves transport of microcracks in the matrix arising from the swelling of fibres (particularly in the case of natural fibre composites). Generally, based on these mechanisms, diffusion behaviour of polymeric composites can further be classified according to the relative mobility of the penetrant and of the polymer segments, which is related to either Fickian, non-Fickian or anomalous, and an intermediate behaviour between Fickian and non-Fickian [13,14]. In general moisture diffusion in a composite depends on factors such as volume fraction of fibre, voids, viscosity of matrix, humidity and temperature [15]. The objective of this work was to compare the influence of both fibre reinforcement and water uptake on mechanical properties of hemp fibre reinforcement unsaturated polyester composites and the related kinetics and characteristics of the water absorption.
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and the structure of hemp fibre are presented in Table 1 [16]. The mechanical and physical properties of the polyester, hemp, and glass fibre used in this study are presented in Table 2 [17]. 2.2. Processing A combination of hand lay-up and compression moulding method was used to prepare the HFRUPE composite samples. Non-woven hemp fibre mat was first dried at 100 C to remove storage moisture in a fan-assisted oven. The storage moisture was recorded for hemp mat approximately 9%. A measured quantity of unsaturated polyester resin mixed with a catalyst (MEKP) for rapid curing was poured on a pre-weighed amount of non-woven hemp fibre mat, which was placed in a mould. The mould was coated with a semi-permanent, polymer mould release agent, Frekote FRP90-NC. After pouring the resin, each layer was left for a few minutes to allow the resin to soak into the fibre mat. Trapped air was gently squeezed out using a roller. The hemp fibre and polyester resin were then left for about 3 min to allow air bubbles to escape from the surface of the resin. The mould was closed and the composite panel was left to cure in a hydraulic press at a temperature of 22 C and at a compaction pressure of 10 bar for 1.5 h. The fabrication route of the HFRUPE composites is depicted in Fig. 1. The schematic of a hydraulic press used to consolidate composite panels is shown in Fig. 2. After being taken out from the hydraulic press, the panel was left to cure at a temperature of 22 C for 24 h before being removed from the mould. Subsequently, post curing was carried out at a temperature of 80 C for 3 h. In addition to this non-woven hemp fibre, a randomly oriented chopped strand mat (E-glass fibre 40 w/w%) was used to prepare reference glass fibre composite sample fabricated using similar procedure for comparison purpose. 2.3. Water absorption tests The effect of water absorption on hemp fibre reinforced unsaturated composites were investigated in accordance with BS EN ISO 62:1999 [18]. The samples for tensile and
2. Experimental procedure 2.1. Materials The matrix material used in this study was based on a commercially available unsaturated polyester, Trade Name ‘‘NORPOL 444-M888’’ supplied by Reichhold UK Ltd. The matrix was mixed with curing catalyst, methyl ethyl ketone peroxide (MEKP) at a concentration of 0.01 w/w of the matrix for curing. Needle punched randomly oriented non-woven hemp fibre, fabric weight 330 g/m2, was used as the reinforcement and was provided by JB Plant Fibres Enterprises Ltd. The typical chemical composition
Table 2 Comparative values of physical and mechanical properties of hemp with E-glass fibre Fibre
Density (g/cm3)
Elongation to break (%)
Tensile strength (MPa)
Young’s modulus (GPa)
Hemp E-glassa
1.14 2.50
1.6 2.5
690 2000–3500
30–60 70
a
For comparison purpose.
Table 1 Typical chemical composition and structure parameters of hemp fibre Cellulose
Hemicellulose
Lignin
Pectins
Wax
Cell length (mm)
Spiral angle (Deg)
Moisture content (%)
74.4
17.9
3.7
0.9
0.8
23.0
6.2
10.8
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Unsaturated polyester
Resin/catalyst/hemp fibre
Resin/catalyst mixing/hemp fibre drying
Mixture of Resin/catalyst/hemp fibre
Catalyst (MEKP)
Hemp fibre
Drying Time: 1 hr Temp: 100 dc
Mixing
Mixture of resin/catalyst
Mould frame
Hemp fibre
Hand lay up
Hemp fibre/mixture of resin and catalyst
Processing
Hydraulic press consolidation Time: 1.5 hrs, Pressure: 10 Bars Temp: 25 dc
Post curing
Post curing Time: 3 hrs, temp:80 dc
Composite laminate
Composites
Fig. 1. Process flow chart showing the applied fabrication route of HFRUPE composites.
Heat and pressure
Hot press platen
conduction plate
Mixture of UPE/catalyst/hemp in a mould frame
Top/bottom mould plates
conduction plate
Hot press platen
Heat and pressure Fig. 2. Schematic of the composite consolidation.
flexural tests containing different fibre volume fractions of reinforcement were machined to a size of 150 · 20 · 3 mm3 and 60 · 15 · 3 mm3, respectively. First all the specimens were dried in an oven at 50 C and then were allowed them to cool to room temperature in a desiccator before weighing them to the nearest 0.1 mg. This process was repeated until the mass of the specimens were reached constant. Water absorption tests were conducted by immersing the HFRUPE specimens in a de-ionised water bath at 25 C for different time durations. After immersion for 24 h, the specimens were taken out from the water and all surface water was removed with a clean dry cloth. The specimens were reweighed to the nearest 0.1 mg within 1 min of removing them from the water. The specimens were
weighed regularly at 24, 48, 98, 196, 392 up to 888 h exposure. Similarly, the specimens were immersed in water at 100 C to determine water absorption at a higher temperature. For this test, the specimens were placed in a container of boiling de-ionised water. After 30 min of immersion, the specimens were removed from the boiling water, cooled in de-ionised water for 15 min at room temperature then removed and weighed to the nearest 0.1 mg. The weight of the samples was measured at different time intervals up to 31 h of exposure until the water content reached saturation. The moisture absorption was calculated by the weight difference. The percentage weight gain of the samples was measured at different time intervals and the moisture content versus square root of time was plotted.
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2.4. Mechanical testing 2.4.1. Tensile testing The tensile strength and modulus of the hemp fibre reinforced composites before and after water immersion were with a crosshead speed of 10 mm/min in accordance with BS EN 2747:1998 [19]. Test specimens were individually cut using a diamond wheel into rectangular beams from the laminate slabs fabricated by a hand lay-up process. The cut edges were then smoothed using 240 Grade SiC paper. 2.4.2. Flexural testing The flexural strength and modulus of the composite before and after water immersion were determined using three-point bending test method following BS EN 2746:1998 test method [20]. A span of 48 mm, maintaining a span to depth ratio of 16:1, was used in a 30 kN load cell. The load was placed midway between the supports. The crosshead speed applied was 2 mm/min. Each sample was loaded until the core broke and their average is reported.
where mi is the initial weight of the moisture in the material and ms is the weight moisture in the material when the material is fully saturated, in equilibrium with its environment. D is the mass diffusivity in the composite. This is an effective diffusivity since all the heterogeneities of the composites have been neglected. h is thickness of specimen and t is the time and j is the summation index. The diffusion coefficient is an important parameter in Fick’s law. Solving the diffusion equation for the weight of moisture, and rearranging in terms of the percent moisture content, the following relationship is obtained: 4M m t 0:5 0:5 M¼ Dx ð3Þ h p where Mm is the equilibrium moisture content of the sample. Using the weight gain data of the material with respect to time, a graph of weight gain versus time is plotted. The diffusion coefficient can be calculated using the following formula: D¼
2.4.3. Scanning electron microscopy In order to understand the effect of water absorption on the microstructure of composites the surfaces of the waterimmersed specimens were examined using a scanning electron microscope (SEM) JSM 6100. 3. Results and discussion The results obtained from this experimental study can be divided into two parts. The first part considers the nature of the diffusion into the hemp reinforced composites and the second evaluates the effects of water absorption at room temperature and at 100 C exposure on the mechanical properties. 3.1. Sorption behaviour The percentage of water absorption in the composites was calculated by weight difference between the samples immersed in water and the dry samples using the following equation: mt mo 100 mo
d2 p2 t70
ð4Þ
Where d is sample thickness in mm and t70 is time taken to reach 70% saturation in seconds. The diffusion properties of composites described by Fick’s laws was evaluated by weight gain measurements of pre-dried specimen immersed in water by considering the slope of the first part of the weight gain curve versus square root of time by using the following equation [25]. The coefficient of diffusion (D) defined as the slope of the p normalised mass uptake against t and has the form: 2 kh D¼p ð5Þ 4M m where, k is the initial slope of a plot of M(t) versus t1/2, Mm is the maximum weight gain and h is the thickness of the composites. Fig. 3 shows percentage of weight gain as a function of square root of time for UPE and various loading levels of UPE only
UPE/2 Layer hemp
UPE/ 3 Layer hemp
UPE/4 Layer hemp
UPE/5 Layer hmep
UPE/CSM
12
ð1Þ
10
where DM(t) is moisture uptake, Mo and Mt are the mass of the specimen before and during aging, respectively. Different models have been developed in order to describe the moisture absorption behaviour of the materials [21,22]. For one-dimensional moisture absorption each sample is exposed, on both sides, to the same environment, the total moisture content G can be expressed as follows [23,24]: " # 2 1 m mi 8 X 1 ð2j þ 1Þ p2 Dx t G ¼1 2 exp p j¼0 ð2j þ 1Þ2 ms mi h2 ð2Þ
Weight gain (%)
DMðtÞ ¼
1677
8 6 4 2 0 0
10
20
Time (Hours)
30
40
1/2
Fig. 3. Water absorption curves at RT for different specimens.
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UPE/hemp reinforced samples immersed in de-ionised water at room temperature (23 C). The maximum percentage weight gain for UPE, 3, 4 and 5 layers of hemp fibre reinforced specimens, corresponding to 0, 0.15, 0.21 and 0.26 fibre volume fractions, respectively, immersed at room temperature for 888 h is 0.879, 5.63, 8.16 and 10.97%, respectively. The water uptake process for all specimens except CSM, which hardly absorbs any water, is linear in the beginning, then slows and approaches saturation after prolonged time, following a Fickian diffusion process. Both the initial rate of water absorption and the maximum water uptake increases for all HFRUPE composites samples as the fibre volume fraction increases. This phenomenon can be explained by considering the water uptake characteristics of hemp fibre. When the composite is exposed to moisture, the hydrophilic hemp fibre swells. As a result of fibre swelling, micro cracking of the brittle thermosetting resin (like unsaturated polyester) occurs. The high cellulose content in hemp fibre (approximately 74%) further contributes to more water penetrating into the interface through the micro cracks induced by swelling of fibres creating swelling stresses leading to composite failure [26]. As the composite cracks and gets damaged, capillarity and transport via micro cracks become active. The capillarity mechanism involves the flow of water molecules along fibre–matrix interfaces and a process of diffusion through the bulk matrix. The water molecules are actively attack the interface, resulting in debonding of the fibre and the matrix [27]. The SEM evidence in Fig. 4 supports this explanation. Fig. 5 shows the percentage of weight gain for UPE, 3, 4 and 5 layers of hemp specimens immersed in water at 100 C. For UPE, 3, 4 and 5 layer hemp reinforced specimens the percentage of moisture absorption is 1.947, 7.366, 9.12 and 13.53%, respectively. The effect of fibre volume fraction and temperature on water absorption can be clearly seen. The rate of approach to equilibrium is clearly more rapid for the 100 C specimens than the samples immersed at RT. Higher temperatures seem to accelerate the moisture uptake behaviour. When the temperature of immersion is increased, the moisture saturation time (MST) is greatly shortened. For 5 layer hemp samples at room temperature, it takes 888 h to reach MST whereas for 100 C samples, the MST is 31 h. The MST in this case was shortened by 857 h. This shows that sorption at room
16
UPE only
UPE/3 Layer hemp
UPE/4 Layer hemp
UPE/5 Layer hemp
14 12
Weight gain (%)
1678
10 8 6 4 2 0 0
2
4
Time (Hours )
6
8
1/2
Fig. 5. Water absorption curves at boiling temperature for different specimens.
temperature takes far longer period to reach equilibrium than sorption at elevated temperatures. In addition to the increase in weight gain percentage, it also shows the weight gain is higher for samples immersed in boiling water than at room temperature. For 5 layer hemp samples, the weight gain percentage at moisture saturation point at boiling temperature is approximately 23% higher than at room temperature. It is evident that there is a different sorption behaviour for immersion at room temperature than for elevated temperature indicating different aging mechanisms. The higher and faster weight gain upon exposure to boiling water may be attributed to the different diffusivity of water into the material leading to moisture induced interfacial cracks at an accelerated rate as a result of degradation in the fibre–matrix interface region as well as the state of water molecules existing in the HFRUPE composites. Other studies also have reported a similar trend for ageing of polymer composites at elevated temperatures [28]. Table 3 presents the diffusion coefficients for both room temperature and 100 C water-immersed specimens. It can be seen that the maximum moisture content and the diffusion coefficient values increases steadily with an increase in fibre volume fraction. The increase is more pronounced for the specimens immersed at 100 C than those of immersed
Fig. 4. Failure showing (a) matrix cracking, (b) fracture running along the interface and (c) fibre–matrix debonding due to attack by water molecules.
H.N. Dhakal et al. / Composites Science and Technology 67 (2007) 1674–1683 Table 3 Moisture uptake of hemp fibre composites immersed in water at RT and 100 C Saturation moisture uptake Mm (%) RT 0 (UPE only) 10 (2L hemp) 15 (3L hemp) 21 (4L hemp) 26 (5L hemp)
0.879 3.441 5.639 8.161 10.972
100 C 1.947 – 7.366 9.125 13.533
Initial slope of plot (k) M(t) versus t1/2
Diffusion coefficient, D, ·103 (m2/s)
RT
100 C
RT
100 C
0.102 0.102 0.247 0.346 0.496
0.437 – 1.178 1.562 2.375
5.714 1.551 3.618 3.841 4.367
88 – 48 62 67
Stress for samples without moisture absorption
70
Data in table are means with a sample size of 3 for each specimen group.
3.2. Effect of moisture absorption on mechanical properties 3.2.1. Tensile properties The tensile stresses and strain versus fibre volume fraction results for these samples are shown in Figs. 7 and 8. For both dry and water aged samples (exposure time 888 h at RT), the stress–strain curves are linear up to the point of failure. There is no affect of water absorption on tensile stress for UPE samples. The tensile stress was rather increased after water immersion of 888 h. Similarly, for 2 layer hemp reinforced samples, the tensile stress is increased by 22% after immersion in water. This increase in tensile
Fig. 6. Degradation of composite showing (a) crack development (b) lost of resin particles due to high accelerated ageing at 100 C.
60 50 40 30 20 10 0 0.1
0.15
0.21
0.26
UPE
Fibre volume fraction Fig. 7. Tensile stress versus fibre volume fraction.
Strain for samples without moisture absorption
14
Strain for samples with moisture absorption
12 10
Strain (%)
at RT. Higher fibre loaded samples, as would be expected, contain a greater diffusivity due to higher cellulose content. The moisture uptake at elevated temperatures compared to RT seems to obey non-Fickian behaviour showing a 23% higher moisture uptake for 5 layer hemp fibre reinforced composites. The moisture uptake results in this study show Fickian behaviour at room temperature and non-Fickian at boiling temperature. This is attributed due to the moist, high temperature environment, and microcracks developed on the surface and inside the materials [29]. As the cracks develops material is actually lost, most likely in the form of resin particles [30,31] as can be seen in Fig. 6a and b. After the occurrence of damage in the composites water transport mechanisms become more active [32]. The deviation from Fickian water uptake behaviour at 100 C is attributed to the development of micro cracks in the composites [33].
Stress for samples with moisture absorption
80
Tensile stress (MPa)
Composite fibre (vol%)
1679
8 6 4 2 0 0.1
0.15
0.21
0.26
UPE
Fibre volume fraction Fig. 8. Tensile strain versus fibre volume fraction.
stress for unreinforced and 2 layer hemp reinforced sample implies that further crosslinking or other mechanisms are taking place enhancing the material strength. The tensile stress however, drops by 38 and 15%, respectively, for 3 and 4 layer hemp reinforced specimens. Generally, for higher fibre volume composites samples immersed in water, it is expected that the relative extent of decrease in tensile properties is greater compared to dry samples. However, it is interesting to note that for 5 layer hemp reinforced samples, the ultimate tensile stress of wet samples is higher than that for dry samples. This could be due to the fact that high amounts of water causes swelling of the fibres, which could fill the gaps between the fibre and the polymer–matrix and eventually could lead to an increase in the mechanical properties of the composites [34]. Similar observations have been reported for jute fibre reinforced polymer composites where after 24 h of soaking in water the flexural strength increased
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by 28% and after 72 h of water immersion, and flexural strength was increased by 45% [35]. The failure tensile strain value for all water-immersed specimens was found to increase compared to dry specimens. The increase in failure strain upon exposure of the samples to a wet environment can be attributed to the plasticisation of hemp samples caused by moisture absorption. Fig. 9a shows a SEM picture of hemp fibre. At regular intervals along the fibre surface, kinks or nodes can be clearly seen. Fig. 9b shows a HFRUPE composite where the effect of kinks or nodes on the surface of the composite laminate reflect the misalignment of fibres. When these irregularly shaped fibres are placed in composites they do not seem aligned properly leading to fibre entanglement. Fibre alignment factors play a crucial role in the overall properties of composites. There is always a chance of fibre entanglement with randomly oriented fibre reinforced composites. The random orientation of fibres produces lower mechanical properties compared to long unidirectionally orientated fibres. This fibre entanglement can create resin rich areas, which can contribute to the formation of voids and porosity (Fig. 10). Voids and porosity can act as stress concentrators leading to failure of composite samples. Hence, the void content for 2, 3, 4 and 5 layers of hemp fibre composites specimen was found to be 12.56, 14.46, 16.60 and 18.64%, respectively. The void content of
Fig. 9. SEM micrograph of hemp fibre (a) showing kinks or nodes (b) showing fibre misalignment and entanglement.
composite was calculated using the following standard formula: wf wm V v ¼ 1 qc ð6Þ þ qf qm where Vv is the volume fraction of voids, qc the density of composite, wf the weight percent of fibre (%), wm the weight percent of matrix (%), qf the density of fibre g/cm3 and qm is the density of matrix g/cm3. As far as voids content in natural fibre composites is concern, the fabrication techniques are not yet fully developed and the natural origin of the fibre component necessarily induces an element of variation in to the composites; both factors contribute in creation of voids which affects to the overall composite properties. It is evident in this study that as the fibre volume fraction of hemp reinforced composite sample increases the void content also increases. 3.2.2. Flexural properties The flexural stress–strain versus fibre volume fraction results for dry and water immersed (exposure time 888 h at RT) HFRUPE composites are shown in Figs. 11 and 12. The observations made earlier for the effect of water absorption on tensile stress/strain properties are also relevant here. The flexural stress drops incrementally as the fibre volume fraction increases hence increased moisture uptake percentage. The decrease in flexural properties after water immersion can be related to the weak fibre–matrix interface due to water absorption. Flexural strain for water-immersed samples has increases dramatically compared to dry samples. Flexural strain for 5 layer hemp reinforced dry samples is 8% whereas after 888 h of water immersion the strain is almost doubled. HFRUPE composites become more rigid due to the lower flexibility of the unsaturated polyester chain. After water aging for 888 h, strain is almost doubled compared to dry specimens since natural fibre reinforced composites tend to be ductile once the loss of cellulose and integrity has taken place [36]. It has been reported that water molecules act as a plasticiser agent in the composite material, which normally leads to an
Fig. 10. Micrograph of water immersed hemp/UPE samples showing effects of voids (a) voids, (b) voids acting as reservoirs and (c) matrix cracking and delamination after 888 h of immersion.
H.N. Dhakal et al. / Composites Science and Technology 67 (2007) 1674–1683 140
Dry sample
Water immersed sample
Flexural stress (MPa)
120 100 80 60 40 20 0 UPE
0.1
0.15
0.21
0.26
Fibre volume fraction Fig. 11. Flexural stress versus fibre volume fraction.
30
Dry sample
water immersed sample
Flexural strain (%)
25 20 15 10 5 0 UPE
0.1
0.15
0.21
0.26
Fibre volume fraction Fig. 12. Flexural strain versus fibre volume fraction.
increase of the maximum strain for the composites after water absorption [37]. The decrease in mechanical properties with increase in moisture content is may be caused by the formation of hydrogen bonding between the water molecules and cellulose fibre. Natural fibres are hydrophilic with many hydroxyl groups (–OH) in the fibre structure forming a large number of hydrogen bonds between the macromolecules of the cellulose and polymer [38]. With the presence of a high –OH group percentage, natural fibres such as hemp tend to show low moisture resistance. This leads to dimensional variation of composites products and poor interfacial bonding between the fibre and matrix, causes a decrease in the mechanical properties [39]. Water absorbed in polymers is generally divided into free water and bound water. Water molecules (which are contained in the free volume of polymer and are relatively free to travel through the micro voids and holes) are identified as free water. Water molecules that are dispersed in the polymer–matrix and attached to the polar groups of the polymer are designated as bound water [40]. The char-
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acteristics of water immersed specimens are influenced not only by the nature of the fibre and matrix materials but also by the relative humidity and manufacturing technique, which determines factors such as porosity and volume fraction of fibres. Water uptake can be advantageous for some natural fibres (such as Duralin fibre) at 66% relative humidity as can fibre plasticising effect as a result of from the presence of free water [41]. Excessive water absorption, however, leads to an increase in the absorbed bound water and a decrease in free water. In this situation, water can penetrate into the cellulose network of the fibre and into the capillaries and spaces between the fibrils and less bound areas of the fibrils. Water may attach itself by chemical links to groups in the cellulose molecules. 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. Consequently the mass of the cellulose is softened and can change the dimensions of the fibre easily with the application of forces. Observation of the fracture surface from the flexural test sample further emphasises the importance of fibre–matrix adhesion on flexural strength. 3.2.3. Influence of moisture on the modulus Table 4 represents the results of tensile modulus and flexural modulus for both dry and water-immersed specimens at RT. It can be seen that moisture absorption causes change in the modulus as determined by tensile and flexural tests. The tensile modulus decreases for all hemp reinforced samples. The reduction in tensile modulus for 3, 4 and 5 layer hemp reinforced specimens compared to dry specimens is 61, 97 and 87%, respectively. A plausible explanation for this would be that, the elastic modulus is a fibresensitive property in composites and is affected as a result of moisture absorption. This effect is particularly greater for the composites with higher fibre content, in which stress transfer capability between fibre and matrix interface gets sharply reduced due to moisture content. The flexural modulus, however, is not adversely affected by moisture absorption. The increase in flexural modulus is more pronounced with higher fibre content specimens, hence higher moisture content. It would be intuitive to assume that the effect of fibre reinforcement to be less critical for the flexural failure stress than in tensile failure Table 4 Tensile and flexural modulus for dry and wet samples Specimens
Fibre volume (%)
Tensile modulus (GPa)
Flexural modulus (GPa)
Dry
Wet
Dry
Wet
UPE only 2 Layer hemp 3 Layer hemp 4 Layer hemp 5 Layer hemp
0 10 15 21 26
0.56 0.72 1.0 1.22 1.27
0.60 0.64 0.62 0.62 0.68
5.51 4.20 5.34 7.30 6.49
5.81 5.76 6.08 6.06 8.05
Data in table are means with a sample size of 5 for dry and 3 for wet for each specimen group.
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