J Mater Sci (2011) 46:479–489
DOI 10.1007/s10853-010-4935-0
The dynamic water vapour sorption behaviour of natural fibres
and kinetic analysis using the parallel exponential kinetics model
Yanjun Xie • Callum A. S. Hill • Zaihan Jalaludin
Simon F. Curling • Rajesh D. Anandjiwala •
Andrew J. Norton • Gary Newman
•
Received: 20 April 2010 / Accepted: 17 September 2010 / Published online: 2 October 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Hygroscopic behaviour is an inherent characteristic of natural fibres which can influence their applications as textile fabrics and composite reinforcements. In
this study, the water vapour sorption kinetic properties of
cotton, filter paper, flax, hemp, jute, and sisal fibres were
determined using a dynamic vapour sorption apparatus and
the results were analyzed by use of a parallel exponential
kinetics (PEK) model. With all of the fibres tested, the
magnitude of the sorption hysteresis observed varied, but it
was always greatest at the higher end of the hygroscopic
range. Flax and sisal fibres displayed the lowest and highest
total hysteresis, respectively. The PEK model, which is
comprised of fast and slow sorption components, exhibited
hysteresis in terms of mass for both processes between the
adsorption and desorption isotherm. The hysteresis derived
from the slow sorption process was less than from the fast
process for all tested fibres. The fast processes for cotton
and filter paper dominated the isotherm process; however,
the hemp and sisal fibres displayed a dominant slow
Natural fibres are extensively used in the textile industries
for fabric making and are also used as fillers or reinforcements in composite materials. These fibres can be classified
according to where in the plant they originate: leaf such as
sisal, banana, palm, pineapple; seed such as cotton; bast
such as jute, hemp, flax; fruit such as coir, oil palm, kapok;
grass such as bamboo, bagasse; and stalk such as straw [1].
The fibres most commonly used are from the bast and leaf,
and are mainly produced in Asia, Africa, and America.
Y. Xie
Key Laboratory of Bio-based Material Science and Technology,
Ministry of Education, Northeast Forestry University, 26 Hexing
Road, Harbin 150040, People’s Republic of China
R. D. Anandjiwala
CSIR Materials Science and Manufacturing, Nonwovens and
Composites Group, Polymers and Composites Competence
Area, P.O. Box 1124, Port Elizabeth 6000, South Africa
C. A. S. Hill (&) Z. Jalaludin
Forest Products Research Institute, Joint Research Institute
for Civil and Environmental Engineering, Edinburgh Napier
University, 10 Colinton Road, Edinburgh EH10 5DT, UK
e-mail: c.hill@napier.ac.uk
R. D. Anandjiwala
Faculty of Science, Department of Textile Science,
Nelson Mandela Metropolitan University, Port Elizabeth 6031,
South Africa
C. A. S. Hill
JCH Industrial Ecology Ltd, Llandegfan, Anglesey, UK
S. F. Curling
The BioComposites Centre, Bangor University, Bangor,
Gwynedd LL57 2UN, UK
process in the isotherm run. The characteristic time for the
fast sorption process did not vary between adsorption and
desorption, except at the top end of the hygroscopic range.
The characteristic time for the slow process was invariably
larger for the desorption process. The physical interpretation of the PEK model is discussed.
Introduction
A. J. Norton
Renuables, Llanllechid, Gwynedd, UK
G. Newman
Plant Fibre Technology Ltd, Bangor, Gwynedd, UK
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480
Natural fibres are hygroscopic materials because their
cell walls contain abundant water sorption sites (hydroxyl
groups) and because they can swell to accommodate the
sorbed water in the cell wall. In the textile industry, fabrics
made from natural fibres vary in their ability to absorb
perspiration, transport moisture, and adjust the relative
humidity (RH) in the clothing microclimate. The sorption
behaviour of garments is closely related to the perception of
comfort [2–5]. Used as fillers or reinforcements in polymer
matrix composites, these renewable and biodegradable
natural fibres can offer the resulting composites many
claimed advantages due to high specific strength, ease of
disposal, and potentially reduced environmental impact [6].
These hydrophilic fibres can be blended well with many
thermoset matrices, but they are much less compatible with
non-polar thermoplastics thereby resulting in insufficient
interfacial adhesion [7, 8]. Although encapsulation of natural fibres by polymer matrices can decelerate the rate of
water sorption compared with natural fibres alone, longterm outdoor exposure can also produce severe warping,
mould growth, fungal decay, and strength loss [9–15].
These issues are mostly associated with the moisture
adsorbed by the hygroscopic natural fibres and subsequently
entrapped by the polymer matrix. Consequently, it is of
importance to understand the moisture sorption behaviour
of various natural fibres, thereby predicting the properties of
materials containing them. Finally, there is considerable
interest in using natural materials in the built environment,
impelled by initiatives such as the UK Government’s
aspiration to have zero carbon homes being built in the UK
by 2016. Natural fibres possess advantages in this respect,
not only because they contain sequestered atmospheric
carbon dioxide, but also because their hygroscopic behaviour can be advantageously exploited in moderating
climatic extremes in the interior environment of buildings.
The interaction of water vapour with a natural fibre
involves a dynamic exchange of water molecules between
the atmospheric water vapour and water molecules located
within the cell wall internal nanopores [16–18]. The water
sorption behaviour of natural fibres is complicated due to
the complex internal geometry of the cell wall and also the
continuous nano-structural changes associated with the
dynamic behaviour of the cell wall macromolecular components. Determining the equilibrium moisture content
(EMC) of natural fibres by the gravimetric method at a
given RH invariably uses saturated salt solutions as a means
of evaluating the sorption properties of fibres [19–21]. More
recently, a dynamic vapour sorption (DVS) technique has
been used to investigate the sorption properties of different
natural or regenerated cellulosic materials [22–26]. This
DVS technique yields highly reproducible data and can
provide accurate isotherms over a wide RH range and at
different pre-set isotherm temperatures [22].
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The ability of this technique to gather ‘real time’ data
additionally allows for the analysis of the sorption kinetic
behaviour of natural fibres. Previous attempts to model
sorption kinetic processes have invariably utilized Fick’s
law on the assumption that the rate limiting step is a diffusion process, which is likely to be true for samples where
the volume to surface ratio is high. But in studies of cell
wall sorption processes, where single fibres or fibre bundles
are used, the kinetics may not be diffusion limited and
indeed it has been shown unequivocally that the sorption
kinetics fits a so-called parallel exponential kinetics (PEK)
model for natural fibres, regenerated cellulose fibres,
microcrystalline cellulose, and some foodstuffs [26–35].
However, whilst it has been established that the PEK model
is applicable, there is as yet no definitive explanation as to
what the model may represent in terms of the physical
behaviour of the cell wall undergoing sorption.
We have already reported on the isotherm sorption
properties of cotton, flax, hemp, jute fibres using a DVS
apparatus under conditions of equilibrium [22]. The DVS
kinetic behaviour of flax fibre has also been presented
recently [27]. In this study, for the first time a comparative
study of the DVS kinetic behaviour of a wide range of
natural fibres is presented and discussed. The sorption
kinetics were analyzed by fitting the dynamic moisture
content data using a PEK model; the fast and slow sorption
processes and their separate hysteresis behaviour was also
calculated and compared with those from experimental
isotherms. A discussion of the physical interpretation of the
PEK model is also presented.
Materials and methods
Natural fibres
The natural agrofibres, cotton (Gossypium barbadense),
flax (Linum usitatissimum), hemp (Cannabis sativa), and
jute (Corchorus capsularis), were supplied courtesy of the
BioComposites centre, Bangor, Gwynedd, UK. Sisal
(Agave sisalana) was supplied by CSIR Materials Science
and Manufacturing, Nonwovens and Composites Group,
Port Elizabeth, South Africa. Filter paper (Whatman Grade
No. 4) purchased from Fisher Scientific UK Ltd (Leicestershire, UK) and was used as a ‘pure’ cellulosic fibre
reference (referred to as ‘‘filpa’’ in the context). All the
fibres were used in an as-supplied state.
Determination of dynamic water vapour sorption
Sorption isotherms of natural fibres were determined using
a DVS Intrinsic apparatus (DVS, Surface Measurement
Systems Ltd, London, United Kingdom) as previously
J Mater Sci (2011) 46:479–489
481
reported [22]. The natural fibre was cut with a pair of
scissors to short pieces measuring approximately 5 mm
long and placed in a conical steel wire gauze (sample
holder) which was hung in the thermostatically controlled
cabinet where the pre-set RH was increased from 0 to 95%
in steps of 5% in a pre-programmed sequence, before
decreasing to 0% RH in the reverse order. The mass of
fibres was strictly set at 5 mg in order to remove any effect
of fibre mass on the sorption kinetics between the different
types of fibres. The sorption processes were run at a constant temperature of 25 ±0.1 °C over the full RH range.
The instrument maintained a constant target RH until the
rate of sample moisture content change (dm/dt) was less
than 0.002% per minute over a 10 min period. This
pseudo-equilibrium condition had been previously established as giving moisture content data within 0.1% of the
EMC [22]. The running time, isotherm temperature, target
RH, actual RH, and sample weight were recorded
throughout the isotherm run. We have previously found
that the EMCs in the fibre replicates obtained with DVS
apparatus are highly reproducible in the RH range of
0–80% and exhibit only minor variation in the RH range of
80–95% [22]. Accordingly, in this study only one measurement is reported for each type of natural fibre.
Fig. 1 Example of PEK curve fitting to experimental adsorption data
(open triangles) of hemp fibre at 65% target RH, the fitted curves
(lines) showing the slow and fast parallel exponential kinetic
processes, and the sum of fast and slow processes
be removed from the fitting process. This has been discussed in some detail by Hill et al. [24, 27].
Results and discussion
Kinetic analysis using PEK model
The experimental moisture content data obtained at each
target RH using the DVS apparatus were curve-fitted using
OriginPro 8.0 software (OriginLab Corporation, USA) to
the function ‘expassoc’ which has a double exponential
form (Eq. 1):
MC ¼ MC0 þ MC1 ð1 et=t1 Þ þ MC2 ð1 et=t2 Þ
s
ð1Þ
where MC is the moisture content time t of exposure of the
sample to a constant RH, MC0 is the moisture content of
the sample at time zero. The sorption kinetic curve is
composed of two exponential terms which represent a fast
[MC1 ð1 et=t1 Þ] and a slow [MC2 ð1 et=t2 Þ] process
having characteristic times of t1 and t2, respectively. The
terms MC1 and MC2 are the moisture contents at infinite
time associated with the fast and slow processes, respectively. The experimental data of dynamic moisture content
at each RH level was curve-fitted (R2 [ 0.99) and the
parameters of MC0, MC1, MC2, t1, and t2 obtained as
described in detail previously [22, 24, 25, 27–29]. Utilizing
these given parameters, the fitted curve can be deconvoluted according to Eq. 1 into a fast process associated with
moisture content (MC1) and a slow process associated with
moisture content (MC2) at infinite time t (Fig. 1). It must
be noted that the RH does not change instantaneously in the
apparatus and that accordingly the first few (normally two)
data points corresponding to this period of RH change must
Sorption dynamics of natural fibres
The response of the moisture content of a fibre sample to a
change in RH in the sample chamber produced an
asymptotic curve when plotted as moisture content against
time, approaching the EMC after infinite time of exposure
at a given RH (Fig. 2). The moisture content of the fibre
gradually increased in the adsorption process or decreased
Fig. 2 Changes in moisture content of natural fibres with the variable
RH levels over the time profile during isotherm runs
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Table 1 Compositions and cell wall structures of natural fibres
Fibre
Classification
Main compositions (%)
Cellulose
Hemicellulose/pectin
Microfibril angle (°)
Fibre geometry
Reference
[62–64]
Lignin
Cotton
Seed
94
6
0
20–34
Single cell
Filpa
Wood
100
0
0
–
Single cell
[64]
Flax
Bast
81
16
3
5–7
Cell bundle
[62–67]
Hemp
Bast
74
19
4
6.2
Cell bundle
[62–64, 68, 69]
Jute
Bast
72
14
13
7–9
Cell bundle
[62–64, 70]
Sisal
Leaf
73
13
11
10–22
Cell bundle
[62, 64, 71, 72]
in the desorption process as a new target RH started, until
the moisture content change was less than the defined value
(0.002% min-1) for 10 min, and then the RH changed to
the next preset value (Fig. 2). The filter paper took the least
time to run through the full isotherm sorption process;
however, it took the sisal and jute fibres nearly double the
time compared to the filter paper to complete the isotherm
run (Fig. 2). The total running time for fibres may depend
on several factors such as fibre geometrical morphology,
chemical compositions, and total mass. In this study the
sample mass tested has been set at the same level (5 mg)
and so is not a variable in these experiments. The longer
running time for sisal and jute may possibly be attributed to
their larger diameter of fibre bundles and higher content of
lignin, which could slow the diffusion of water molecules
into the bundle interior. However, it has been clearly
demonstrated here and previously reported [26–34] that the
sorption kinetics is not diffusion limited and that any
explanation for these differences has to be found elsewhere. In a study of the rate of water vapour sorption into
wood single cell walls, Christensen found that the kinetic
response was the same irrespective of the cell wall thickness [36].
Cotton fibre is a single cellular botanical entity and the
cell often exhibits ribbon-like, twisted, and flattened tube
being composed of the interwoven primary wall, helical
winding layer and secondary wall with a closely packed
parallel microfibril arrangement [37, 38]. Filter paper
(Filpa) in this study is made of highly cellulosic wooden
pulp. The single fibres in these pieces are arranged randomly in a plane with a thickness of 50 lm. The flax,
hemp, jute, and sisal fibres are multi-cellular fibre strand
(bundle) containing 10–40 elementary fibres (cells) with a
diameter of approximately 100 lm. The architecture of the
single cell in these fibres includes a primary and secondary
cell wall network microstructure. The cotton and sisal
exhibit considerably greater microfibril angles (about 20°)
than the other fibres (about 6°). Pectin (above 15%) and
lignin (about 3%) constitute the major non-cellulosic
composition of flax and hemp fibres; however, the jute
and sisal fibres present a similar amount of pectin
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(hemicelluloses for sisal fibre) and lignin (about 13% each)
as summarized in Table 1. A small quantity of wax (around
1%) is distributed in the primary wall of the fibre cells
except for filter paper. It is to be appreciated that there are
considerable chemical and morphological differences
between the fibre types studied making definitive interpretations of the sorption data difficult.
Apart from differences in the sorption rate, there were
also differences in the EMC values exhibited by the fibres
in the isotherms. At the highest RH (95%) cotton reached
an EMC of 14.4%, which was lower than with the other
fibres (Fig. 3). This may be explained by the fact that pure
cotton is mainly composed of cellulose (about 94% by
mass) with a high degree of crystallinity [39]. This results
in a lower number of accessible OH groups per unit volume. The low level of inter-microfibrillar matrix material
can also limit the cell wall swelling therefore reducing the
moisture adsorption amount. The major constituent of the
commercial filter paper (filpa) is also cellulose; however, it
absorbed slightly more water vapour (16.7%) at 95% RH
than the cotton fibres. A possible explanation is that the
crystalline segments in filpa fibres are partly destroyed
during the preparation of pulp [40–42] resulting in a
(a)
(b)
Fig. 3 Equilibrium moisture content of natural fibres in the full set
RH range during the adsorption (a) and desorption (b) processes in
the isothermal sorption run
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483
relatively lower crystallinity compared to cotton fibre and
hence a higher level of accessible OH and amorphous
polymeric content. The EMC exhibited an ascending
sequence for flax (19.4%), jute (24.6%), hemp (25.0%),
and sisal (25.4%), respectively. All natural fibres in this
study showed typical sigmoidal isotherm curves (IUPAC
Type II), which is different from the Type III isotherms
exhibited by certain polysaccharide films such as high
amylase corn starch film, which due to a lack of crosslinking can swell significantly at high RH (above 60%)
accordingly resulting in an exponential increase in EMC of
40% or higher at RH values of 85% [43].
Moisture sorption rate of natural fibres
With each step change in RH, there is a response in the
material which establishes a new equilibrium condition
within a specific time period. Pragmatically, the instrument
is programmed to change RH when a pseudo-equilibrium is
established (a rate of mass change of lower than 0.002%
per minute over a 10 min period). It has been shown previously that this condition provides EMC data within 0.1%
of the moisture content at infinite time of exposure (true
EMC condition) [24, 25, 27, 28] and this is also demonstrated for the materials in this study in a later part of this
paper. Dividing the change in EMC in response to the RH
change by the time taken to reach this new EMC gives
what is herein termed a ‘sorption rate’.
There are differences in sorption rate behaviour at different final RH between the various fibres investigated in
this study, as is shown in Fig. 4. Whilst cotton (Fig. 4a)
and filter paper (Fig. 4b) are both cellulose-rich substrates,
they show quite different behaviour. With the filter paper,
the time to establish an equilibrium condition is longer and
especially so at the lower and upper end of the hygroscopic
range. Jute (Fig. 4e) also exhibits an increase in sorption
rate at the lower and upper ends of the RH range. Of the
remainder, flax (Fig. 4c), hemp (Fig. 4d), and sisal
(Fig. 4f) have an approximately constant sorption rate until
the RH reaches 70%, where it then increases.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4 Average sorption rate within a set RH during the adsorption processes (solid legends) and desorption processes (empty legends) of cotton
(a), filpa (b), flax (c), hemp (d), jute (e), and sisal (f)
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Sorption hysteresis of natural fibres
In this study, the sorption hysteresis was analyzed using
three means: total hysteresis characterized by the area of
isotherm loop, EMC difference through the full RH range
by subtracting the EMC in the adsorption isotherm from
the EMC in the desorption isotherm at given target RH, and
hysteresis coefficient which is a ratio of the EMC for
adsorption to that at desorption for any given RH as shown
in Eq. 2 [44, 45]:
H ¼ EMCa =EMCd
ð2Þ
where H is the hysteresis coefficient, EMCa is the equilibrium moisture content for adsorption run, and EMCd is
the equilibrium moisture content for desorption run.
The cotton, flax, and hemp showed similar magnitudes
in isotherm loop area (Fig. 5a), suggesting that the total
amount of hysteresis exhibited by these fibres is similar,
although they exhibited different EMC’s through the full
RH range (Fig. 3a, b). The loop area of sisal fibre was
approximately 60% higher than that of cotton, flax, and
hemp (Fig. 5a), which means a higher total sorption hysteresis. Some comment of the differences in the total
hysteresis reported in Fig. 5a and b is required. As noted in
‘‘Materials and methods’’ section, it is not possible to
obtain an instantaneous change in RH and that there is a
transition period during which the RH moves from one set
value to the next and this has been fully discussed elsewhere [24–29]. The net result is to introduce an effective
‘induction curve’ in the mass change kinetic data and these
data points must be removed in order to obtain a satisfactory curve fit. The result is that the values for MC1 and
MC2 are slightly inaccurate and this inaccuracy increases
(a)
when the cumulative values are calculated. Hence the
heights of the columns of Fig. 5b do not correspond with
those of Fig. 5a. The partitioning of the water between the
fast and slow processes in Fig. 5b is accurate, however.
The sorption hysteresis presented as the EMC difference
through the full RH range varied with the RH (Fig. 6a).
The sorption hysteresis first increased with the RH up to
80%, after which it decreased. In the RH range of 0 to ca.
50% the hysteresis value of hemp was higher than the flax,
followed by cotton. Over 50% and up to 80% RH, the
hysteresis sequence changed in a descending order of
cotton, hemp, and flax fibres (Fig. 6a). The sisal exhibited a
significantly higher sorption hysteresis compared with the
other five types of fibres over most of the RH range,
especially in the high RH range. This further confirms the
results of total hysteresis as shown in Fig. 6a. The hysteresis coefficient, which has been used in previous studies
[e.g., 46, 47], exhibited a considerable variation ranging
from about 0.63 to 0.93, depending upon the type of fibres
(Fig. 6b). In previous studies, this hysteresis coefficient
supposedly takes account that a material with higher levels
of sorption would be expected to exhibit greater hysteresis
[47]; however, such hysteresis coefficients for these natural
fibres tested in this study did not exhibit any significant
correlation with the EMC (Fig. 6a).
The extent of hysteresis can be influenced by the nature
and composition of cellulosic materials, relating to their
potential for structural and conformational rearrangement,
which change the accessibility of sorption sites [48].
Increasing the environmental temperature can also reduce
the sorption hysteresis of cellulosic and lignocellulosic
materials [22]. In order to further an understanding of the
source of sorption hysteresis, the increment/decrement of
(b)
(a)
Fig. 5 Mathematical area of the isotherm loop for different fibres,
which is calculated by using an empirical 4th order polynomial fitting
to the adsorption and desorption isotherm curves, integrating the fitted
curves and determining the area difference between the adsorption
and desorption isotherm run (a) and the area difference of integrated
fast and slow process curves which are polynomially fitted with 4th
order during the adsorption and desorption runs (b)
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(b)
Fig. 6 Sorption hysteresis of natural fibres obtained by subtracting
the values of equilibrium moisture contents of the adsorption isotherm
by the desorption isotherm at a same RH level (a) and hysteresis
coefficient which is a ratio of the EMC for adsorption to that at
desorption for any given relative humidity (b)
J Mater Sci (2011) 46:479–489
485
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 7 Moisture increment within a set RH in the adsorption run (solid arrow direction, RH from 5 to 95%) and moisture decrement in the
desorption run (dot arrow direction, RH from 95 to 5%) of cotton (a), filter paper (b), flax (c), hemp (d), jute (e), and sisal (f)
moisture content at a given RH in the adsorption and
desorption run were plotted in Fig. 7a–f.
It is notable that at a specific RH, the MC decrement in
the desorption isotherm was slightly greater than the MC
increment in the adsorption isotherm in the RH range of
5–85%. Only at the RH levels of 90 and 95% were the MC
increments significantly higher than the MC decrements,
especially at the 95% RH level. In the desorption run from
95 to 90%, the MC does not decrease as much as it gains
from 90 to 95%. This ‘residual’ MC is stored in the fibre
cell wall acting as a reservoir and is subsequently released
gradually in the desorption run. The hysteresis of the natural fibres in the isotherm run thus primarily resulted from
the difference in MC increment and decrement resulting
from the change in RH from 95 to 90%. Comparison of the
difference between the MC increment and decrement at the
95% RH level with the total hysteresis (Fig. 5a) also
showed that the greater difference corresponded to a higher
total hysteresis. It is, however, wrong to interpret the origin
of sorption hysteresis solely to any changes occurring at the
top end of the hygroscopic range. It is well established that
hysteresis is displayed when desorbing from any RH, with
the desorption line initially scanning across the space
occupied by the hysteresis loop until it reaches the
desorption boundary curve [49, 50]. This type of behaviour
with scanning curves running through the space delineated
by the boundary adsorption and desorption curves is a
characteristic feature of the independent domain theory for
explaining hysteresis phenomena [51–54] and this theory
has recently been applied to the study of the hysteresis
associated with the sorption of water vapour into wood [49,
50]. It is central to the independent domain theory that each
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of the domains is able to display bistable behaviour for
hysteresis to be observed. A suitable model that provides a
basis for these domains to exist in two possible states has
been developed for explaining hysteresis in humic soils
[55–57] and glassy polymers [58] and this theory has
recently been applied to cellulosic and lignocellulosic
materials [27, 29]. This ‘dual mode’ model considers the
response of a glassy solid below the glass transition temperature to the ingress or egress of sorbate molecules under
adsorption or desorption conditions. The structure of plant
fibres is one of crystalline microfibrils embedded within an
amorphous matrix. Adsorption of water vapour into the
fibre wall results in the creation of nanopores between the
microfibrils and within the matrix substances. The nature of
the matrix substances varies, depending on the fibre type
(Table 1). In the dual mode model description, the creation
of nanopores in response to the incoming sorbate molecules is inelastic on the time scale of molecular diffusion.
This time lag in the response of the matrix to incoming or
exiting molecules means that the creation of nanopores is
delayed during the adsorption process and their collapse is
delayed during the desorption process. Hence adsorption
and desorption are occurring in different physical environments. This is a molecular scale process taking place on
a molecular scale time-frame. The dual mode behaviour is
only exhibited if the isotherm temperature is below the
glass transition temperature of the matrix. One consequence is that as the isotherm temperature is increased, the
area bounded by the hysteresis loop decreases, as has been
observed experimentally [22]. The presence of an amorphous cross-linked glassy polymer such as lignin in the cell
wall matrix would also be expected to result in an increase
in the area bounded by the hysteresis loop, as has also been
shown experimentally herein and elsewhere [22]. The
question then arises, that if the hysteresis property is
related to matrix stiffness in the cell wall, is this also
reflected in the sorption kinetic behaviour?
Kinetic analysis using the PEK model
The moisture content associated with fast kinetic process
has been proposed to be related to the fast moisture sorption at the sites of the readily accessible internal surfaces
and ‘amorphous’ regions, while the slow kinetic process
has been related to sorption onto the ‘inner’ surfaces and
‘crystallites’ [59, 60]. This hypothesis has been tested and
reported upon previously and it is concluded that there is
little evidence supporting this idea [22, 24, 25, 27–29]. It is
known that the swelling behaviour of foodstuffs also obeys
a PEK model [33–35] suggesting that the sorption kinetics
is rate limited by the ability of the cell wall matrix to
deform in response to the ingress or egress of water molecules. Krabbenhoft and Dankilde [61] whilst noting the
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J Mater Sci (2011) 46:479–489
general failure of Fickian diffusion models when applied to
dynamic substrates such as wood, consider that the swelling of the material is a crucial factor affecting the sorption
kinetic behaviour. When water vapour is adsorbed into a
plant cell wall, the sorbed water molecules exert an internal
pressure on the interfibrillar elastic gel matrix. The volume
change of this matrix material is a result of the swelling
pressure exerted by the adsorbed water molecules and the
resistance to that swelling pressure offered by the matrix.
During the initial stages of adsorption, the matrix is in a dry
state and the rate of diffusion is dependent upon the rate of
hydrogen bond breaking within the matrix, as is suggested
by recent studies of the activation energy of sorption for
flax [27] and Sitka spruce [29]. The rate of matrix deformation limits the kinetics if this is the rate limiting step,
however, if matrix deformation is rapid then Fickian diffusion would be the limiting factor. The PEK model
accurately fits the sorption data, indicating that matrix
deformation is indeed the rate limiting process. Given that
this is the case, the question then arises as to what the fast
and slow exponential kinetics processes represent in terms
of matrix deformation. This problem is currently being
addressed by the development of a model to describe this
behaviour and which will be reported upon in due course.
The correspondence of the experimental with the
mathematically generated isotherm is shown in Fig. 8. The
sorption isotherm obtained from the PEK fitting was created by summing the cumulative moisture contents associated with the fast and slow kinetic processes and the
moisture content at time zero (EMC = MC0 ? MC1 ?
MC2) in the adsorption and desorption runs. This fitted
isotherm was closely comparable to the experimental isotherm loop for natural fibres (Fig. 8a–f), indicating that the
EMC values as measured by the instrument are very close
to those predicted by the model at infinite exposure time.
Since the isotherm can be generated by summation of
the fast and slow moisture content components, it then
becomes possible to generate pseudo-isotherm curves
based upon the cumulative summation of MC1 or MC2
values (Fig. 9). By so doing, it is possible to provide
information to the following questions: (a) Is there any
difference between the fast and slow processes throughout
the sorption isotherm range? (b) Does hysteresis exist for
both the fast or slow processes between the desorption and
adsorption isotherm? In some cases, closed loop isotherms
are not found, which can in part be related to the inaccuracies associated with determination of the MC1 and MC2
values, as discussed earlier. However, there are also differences due to the allocations of MC1 and MC2 between
the adsorption and desorption isotherms, which is particularly noticeable with flax (Fig. 9c) and hemp (Fig. 9d). In
the case of flax and hemp there virtually no difference in
the MC2 values between the adsorption and desorption
J Mater Sci (2011) 46:479–489
487
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 8 Comparison of sorption isotherm loops derived from experimental data and derived from sums of MC0, MC1, and MC2 obtained from
curve fitting using PEK model for cotton (a), filter paper (b), flax (c), hemp (d), jute (e), and sisal (f), respectively
loops until higher RH values. Where the MC2 values
diverge at the upper end of the hygroscopic range this then
results in an open isotherm loop in the MC1 values. With
the other fibres, there is some hysteresis in the MC2 values,
with a resultant closed (or nearly so) MC1 isotherm. In all
cases the majority of the hysteresis associated with the
mass values is with the fast sorption process. Mathematically derived areas revealing the hysteresis of the fast and/
or the slow processes in the sorption run of natural fibres
were illustrated in Fig. 5b, showing clearly that the hysteresis in the fast sorption process was significantly greater
than this in the slow process.
The characteristic time of fibres in the fast process (t1)
was comparable between the adsorption and desorption
processes in most of RH range (not shown here), which is
consistent with the previous findings [26, 29]. Generally,
the characteristic time of the slow process (t2) in the
adsorption run was shorter than in the desorption run. Thus,
whilst the sorbed water can be allocated in terms of mass
between the fast and slow process for both adsorption and
desorption, it is only the slow process characteristic time
that changes under conditions of adsorption and desorption.
This has been universally observed with all analyses of
water vapour sorption kinetics based upon the PEK model.
Conclusions
The different natural fibres exhibited variable dynamic
water vapour sorption behaviour. The maximum sorption
rate at a given RH appeared at the initial stage and the
average sorption rate increased with increasing RH. Sisal
and flax exhibited the highest and lowest hysteresis,
respectively. The PEK model can be used to accurately fit
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J Mater Sci (2011) 46:479–489
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 9 Cumulative moisture contents associated with the fast (MC1) and slow (MC2) exponential kinetic processes in the water vapour
adsorption and desorption runs for cotton (a), filter paper (b), flax (c), hemp (d), jute (e), and sisal (f), respectively
the experimental measurements. The sorption kinetics
resulted from curve fitting showed that both the fast and
slow processes in the fibre cell wall exhibited hysteresis
between the adsorption and desorption isotherm. The reason for the differences in the allocation of moisture to the
fast and slow processes is not understood at present. It is
suggested that the PEK kinetics is related to the swelling
behaviour of the cell wall under conditions of adsorption
and desorption.
References
1.
2.
3.
4.
5.
6.
7.
Acknowledgements The support of the Scottish Funding Council
for the Joint Research Institute on Civil and Environmental Engineering under the auspices of the Edinburgh Research Partnership is
acknowledged. Support from the Carnegie Trust and the Royal
Society for financial support for visits of Callum Hill to South
Africa is gratefully acknowledged. The support from Chinese
National Natural Science Funds (Project No. 30771680) is also
appreciated.
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8.
9.
10.
Kalia S, Kaith BS, Kaur I (2009) Polym Eng Sci 49:1253
Bakkevig MK, Nielsen R (1994) Ergonomics 36:787
Li Y (2005) Ergonomics 48:234
Guo Y, Li Y, Tokura H, Wong T, Chung J, Wong ASW, Gohel
MDI, Leung PHM (2008) Text Res J 78:1057
Hu JY, Li Y, Yeung KW, Wong A, Xu W (2005) Text Res J
75:57
Wambua P, Ivens J, Verpoest I (2003) Compos Sci Technol
63:1259
Bledzki AK, Gassan J, Theis S (1998) Mech Compos Mater
34:563
Cantero G, Arbeliaz A, Liano-Ponte R, Mondargon I (2003)
Compos Sci Technol 63:1247
Lundin T, Falk RH, Felton C (2001) In: Proceedings of the sixth
international conference on wood fiber–plastic composites,
Madison, Wisconsin
Wallenberger FT, Weston N (2004) Natural fibers, plastics and
composites. Kluwer Academic Publishers, Massachusetts, USA
J Mater Sci (2011) 46:479–489
11. Mohanty AK, Misra M, Drzal LT (2005) Natural fibers, biopolymers, and biocomposites. Francis, Taylor
12. Schirp A, Wolcott M (2005) Wood Fiber Sci 37:643
13. Schirp A, Wolcott M (2006) J Appl Polym Sci 99:3138
14. Morris PI, Cooper PA (1997) Forest Prod J 48:86
15. Lomelı́-Ramı́rez MG, Ochoa-Ruiz HG, Fuentes-Talavera FJ,
Garcı́a-Enriquez S, Cerpa-Gallegos MA, Silva-Guzmán JA
(2009) Int Biodeterior Biodegradation 63:1030
16. Carles JE, Scallan AM (1972) J Appl Polym Sci 17:1855
17. Hills BP, Wright KM, Belton PS (1989) Mol Phys 67:1309
18. Ibbett RN, Schuster KC, Fasching M (2008) Polymer 49:5013
19. Hill CAS (2006) Wood modification—chemical, thermal and
other processes. Wiley, Chichester
20. Papadopoulos AN, Hill CAS (2003) Wood Sci Technol 37:221
21. Hernández RE (2007) Wood Fiber Sci 39:132
22. Hill CAS, Norton A, Newman G (2009) J Appl Polym Sci
112:1524
23. Leisen J, Beckham HW, Benham M (2002) Solid State Nucl
Magn Reson 22:409
24. Hill CAS, Norton A, Newman G (2010) Wood Sci Technol
44:497
25. Xie Y, Hill CAS, Xiao Z, Militz H, Mai C (2010) Wood Sci
Technol. doi:10.1007/s00226-010-0311-0
26. Kohler R, Dueck R, Ausperger B, Alex R (2003) Compos
Interface 10:255
27. Hill CAS, Norton A, Newman G (2010) J Appl Polym Sci
116:2166
28. Xie Y, Hill CAS, Xiao Z, Zaihan J, Militz H, Mai C (2010)
J Appl Polym Sci 117:1674
29. Hill CAS, Norton A, Newman G (2010) Holzforschung 64:469
30. Okubayashi S, Griesser UJ, Bechtold T (2005) Cellulose 12:403
31. Okubayashi S, Griesser UJ, Bechtold T (2005) J Appl Polym Sci
97:1621
32. Kachrimanis K, Noisternig MF, Griesser UJ, Malamataris S
(2006) Eur J Pharm Biopharm 64:307
33. Madamba PS, Driscol RH, Buckle KAJ (1996) Food Eng 29:75
34. Tang X, De Rooij MR, Van Duynhoven J, Van Breugel KJ (2008)
J Microsc 230:100
35. Rahman MS, Perera CO, Thebaud C (1998) Food Res Int 30:485
36. Christensen GN (1965) Humidity Moisture 4:279
37. Rollins ML, Tripp VW (1954) Text Res J 24:345
38. Krakhmalev VA, Paiziev AA (2006) Cellulose 13:45
39. Smith CW, Cothren JT (1999) Cotton: origin, history, technology, and protection. Wiley, New York
40. Gümüşkaya E, Kalyoncu EE, Kirci H (2009) Chem Pap 63:670
41. Newman RH, Hemmingson JA, Suckling ID (1993) Holzforschung 47:234
42. Park S, Johnson DK, Ishizawa CI, Parilla PA, Davis MF (2009)
Cellulose 16:641
489
43. Bertuzzi MA, Armada M, Gottifredi JC (2003) Food Sci Technol
Int 9:115
44. Siau JF (1995) Wood: influence of moisture on physical properties. Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Virginia, USA
45. Skaar C (1972) Water in wood. Syracuse University Press, Syracuse
46. Esteban LG, Gril J, de Palacios P, Casasús AG (2005) Ann For
Sci 62:275
47. Shmulsky R, Kadir K, Erickson R (2001) Wood Fiber Sci 33:662
48. Al-Muhtaseb AH, McMinn WAM, Magee TRA (2004) J Food
Eng 6:297
49. Peralta PN (1995) Wood Fiber Sci 27:250
50. Peralta PN (1996) Wood Fiber Sci 28:406
51. Everett DH, Whitton WI (1952) Trans Faraday Soc 48:749
52. Everett DH, Smith FW (1954) Trans Faraday Soc 50:187
53. Everett DH (1954) Trans Faraday Soc 50:1077
54. Everett DH (1955) Trans Faraday Soc 51:1551
55. Lu Y, Pignatello JJ (2004) J Environ Qual 33:1314
56. Lu Y, Pignatello JJ (2002) Environ Sci Technol 36:4553
57. Lu Y, Pignatello JJ (2004) Environ Sci Technol 38:5853
58. Vrentas JS, Vrentas CM (1996) Macromolecules 29:4391
59. Okubayashi S, Griesser UJ, Bechtold T (2004) Carbohydr Polym
58:293
60. Morton WE, Hearle JWS (1997) Physical properties of textile
fibers. The Textile Institute, UK
61. Krabbenhoft K, Damkilde L (2004) Matériaux at Constructions
37:615
62. Mwaikambo LY (2002) Plant-based resources for sustainable
composites. PhD thesis, Department of Engineering and Applied
Science, University of Bath, UK
63. Bolton AJ (1994) Mater Technol 9:12
64. Baillie C (2000) Green composites: polymer composites and the
environment. Woodhead Publishing Limited, New York
65. Van Den Oever MJA, Bos HL, Van Kemenade MJJM (2000)
Appl Compos Mater 7:387
66. Morvan C, Andeme-Onzighi C, Girault R, Himmelsbach DS,
Driouich A, Skin DE (2003) Plant Physiol Biochem 41:935
67. His I, Morvan C, Andème-Onzighi C, Driouich A (2001) J Histochem Cytochem 49:1525
68. Crônier D, Monties B, Chabbert B (2005) J Agric Food Chem
53:8279
69. Vignon MR, Dupeyre D, Garcia-Jaldon C (1996) Bioresour
Technol 58:203
70. Mukhopadhyay AK, Bandyopadhyay SK, Mukhopadhyay U
(1985) Text Res J 55:733
71. Gañan P, Garbizu S, Llano-Ponte R, Mondragon I (2005) Polym
Compos 26:121
72. Martins MA, Kiyohara PK, Joekes I (2004) J Appl Polym Sci
94:2333
123