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Composites Science and Technology xxx (2006) xxx–xxx
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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
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).
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.
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
3
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
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
d2
p2 t70
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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16
UPE only
UPE/3 Layer hemp
UPE/4 Layer hemp
UPE/5 Layer hemp
14
12
Weight gain (%)
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
5
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.
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Table 3
Moisture uptake of hemp fibre composites immersed in water at RT and
100 C
0 (UPE only)
10 (2L hemp)
15 (3L hemp)
21 (4L hemp)
26 (5L hemp)
Saturation
moisture uptake
Mm (%)
Initial slope of
plot (k) M(t)
versus t1/2
Diffusion
coefficient, D,
·103 (m2/s)
RT
100 C
RT
100 C
RT
100 C
0.879
3.441
5.639
8.161
10.972
1.947
–
7.366
9.125
13.533
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
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%)
Stress for samples without moisture absorption
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.
7
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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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-
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.
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