10.1.1 Background on the moisture sorption of natural fibres

The major constituents of natural fibres are cellulose, hemicellulose and lignin. Natural fibres are inherently hydrophilic in nature owing to the presence of a large number of hydroxyl (− OH) groups available, particularly in cellulose and hemicellulose. However, not all constituents contribute to the absorption of moisture. Cellulose, which forms the major constituent of natural fibres, is hydrophilic in nature and it absorbs water molecules. Even though cellulose has a high –OH to C ratio, not all –OH groups are exposed or accessible as cellulose is semicrystalline (Pott 2004). The highly crystalline region of cellulose is virtually inaccessible to water molecules but the water molecules are able to penetrate and gain access into the amorphous region of cellulose. Hemicellulose, on the other hand, is predominantly amorphous (Hansen and Plackett 2008). It is highly accessible to water molecules as it has a high –OH to carbon (C) ratio. Lignin, however, has a low –OH to C ratio and it is hydrophobic in nature (Bismarck et al. 2005). As natural fibres absorb water molecules, they swell up owing to water molecules occupying the space in between the microfibrils. This space that water molecules occupy is termed the transient microcapillary network (Hill et al. 2009b). The water molecules within natural fibres can either form a monolayer, which associate closely with the available –OH groups, or form a multilayer at which not all water molecules are in intimate contact with available –OH groups. It has been shown that the water molecules in a monolayer are more mobile than the water molecules located in a multilayer (Walker 2006).

It was found that the moisture sorption isotherm of natural fibres follows an International Union of Pure and Applied Chemistry (IUPAC) type II isotherm. Water sorption hysteresis (the difference between the sorption and desorption loop) has also been observed (Pott 2004; Hill et al. 2009a; 2009b). The sigmoidal nature of the type II isotherm can be further categorised into three regions. Region 1 is typically assigned to relative humidity (RH) of between 0 and 15%, where the dominant moisture sorption mechanism is monolayer adsorption of water molecules onto the cell wall surface of natural fibres. In region 2, the water molecules form a multilayer in the transient microcapillary network between 15 and 70% RH. Beyond 70% RH (region 3), capillary condensation is the dominant mechanism for water molecules absorption. It must be noted that at low RH, adsorption is the dominant mechanism but at higher RH, absorption is the dominant mechanism. In this context, absorption refers to the uptake of water by natural fibres owing to surface tension forces. The heat required to evaporate absorbed liquid is only slightly higher than the energy required to evaporate a drop of liquid from a flat surface (Walker 2006). Adsorption, on the other hand, is based on the attractive forces between the water molecules and natural fibres as a result of hydrogen bonding or van der Waals forces (Walker 2006).

When discussing the moisture uptake of natural fibres, the concept of the fibre saturation point must be introduced. The fibre saturation point is the moisture content at which absorbed water molecules are removed but the cell walls of the fibres are still saturated (Tiemann 1906) by adsorbed water molecules. This point usually occurs around a moisture content of between 25 and 35% RH (Walker 2006). It can also be calculated by fitting the water sorption isotherm to 100% RH (Hill et al. 2009b). However, the fibre saturation point has been shown to be difficult to determine and does not exist experimentally in practice (Hernandez and Pontin 2006; Almeida and Hernandez 2006a; 2006b). The main reason for this lies in the concept of the fibre saturation point. It is assumed that there exist a distinct cut-off point between a fully saturated cell wall and the total removal of absorbed water molecules (Hill et al. 2009b). However, at high RH, capillary condensation begins to fill the lumen of natural fibres, causing a sharp increase in the water sorption curve (Walker 2006).

10.2.1 Simple weight gain determination of the moisture content of natural fibres

Weight gain measurements utilise humidity chambers or desiccators set up at specified RH using distilled water at room temperature (Bismarck et al. 2002; Baltazar-y-Jimenez and Bismarck 2007). The RH of the humidity chamber can be adjusted using salt solutions. In order to obtain more accurate results, the fibre bundles are dried overnight in an oven before placing them in the chambers. The weight difference before and after exposure to the specified RH were then measured at different time intervals and the moisture content calculated using the equation shown:

[10.1]

where MC = moisture content, m = mass of the sample after exposing to specified RH and m₀ = dry mass of natural fibres.

A variation of this method can be performed by immersing the fibres in distilled water instead of a humidity chamber at specified RH (Sreekala and Thomas 2003). The increase in the weight of the fibres was recorded at specific time intervals. The authors found that the values obtained from this process are highly reproducible. The water uptake by the fibres (in mol percent of water uptake per mass of fibres) can be evaluated using:

[10.2]

where X(t) = moles of water absorbed per unit mass of fibres, m_water(t) = equilibrium mass of water at time t and m_initial = initial mass of the fibres. This analysis can be extended further by fitting it into the empirical equation [10.3] (Khinnavar and Aminabhavi 1991; Sreekala and Thomas 2003):

[10.3]

where k and n are parameters fitted through linear regression, k is a parameter that depends on the interaction between the solid substrate and the water molecules, and n gives an indication about the types of mass transport phenomena (Fickian or Knudsen).

10.2.2 Dynamic vapour sorption in measurement of the water sorption isotherm of natural fibres

Dynamic vapour sorption (DVS) is very useful in the accurate measurement of sorption isotherms at various temperatures and relative humidity. The vapour phase can also be replaced; instead of water, other organic solvents can be used as long as Antoine’s equation (see for instance: Perry and Green 1997) for the particular solvent is available. Antoine’s equation describes the pure saturated vapour pressure of a solvent as a function of temperature. With this equation, the partial pressure of a solvent in the vapour phase can be predicted accurately. Using different organic solvents proves to be useful when studying the sorption isotherm of (modified) natural fibres. The use of DVS to study the moisture uptake of natural fibres has been studied extensively (Gouanve et al. 2006; Bessadok et al. 2007; Bessadok et al. 2008; Collet et al. 2008; Alix et al. 2009; Hill et al. 2009b). Such extensive use of DVS is not surprising as this technique is well established. In addition to this, natural fibres can be exposed to varying RH (as a function of time and temperature) to study the effect of different RH in situ on the moisture uptake of natural fibres.

The apparatus setup for a DVS instrument is shown in Fig. 10.1. The setup contains two measuring pans (one reference pan and one sample pan) suspended from the arm of an ultra-sensitive microbalance (up to 0.05 μg resolution). The measuring pans are connected to the microbalance by hanging wires and both the pans are situated in their respective chambers. These chambers are located in a temperature-controlled environment. A flow of dry gas along with the correct amount of water vapour is then passed through the chambers to maintain the required RH.

10.3.1 Moisture content of natural fibres based on weight gain measurements

Table 10.1 shows the moisture content of various natural fibres evaluated using simple weight gain measurements. The equilibrium moisture content of different natural fibres is around 7 wt% at 100% RH. It is possible to increase the moisture content of sisal fibres from 5.8 wt% to 6.5 wt% by dewaxing the fibres. This can be done by either heating the fibres in a mixture of ethanol and benzene (a ratio of 1:2) for 72 h at 50 °C (Bismarck et al. 2001) or Soxhlet extraction in acetone for 1 h (Mukherjee and Satyanarayana 1984; Juntaro et al. 2008) to remove waxy substances. It seems, however, that dewaxing of coir fibres reduces its moisture content. Unfortunately, no explanation was given by the authors. Alkaline treatment is known to remove fatty acids from the surfaces of natural fibres (Geethamma et al. 1995). As a result, the moisture uptake of alkaline treated sisal and coir fibres increased when compared with neat fibres. Bismarck et al. (2001) also observed an increase in the thermal degradation temperature of sisal and coir fibres. This is thought to be due to the removal of compounds, such as wax or fatty acids, that decompose earlier than the major constituent in natural fibres.

Bismarck et al. (2002) investigated the moisture uptake behaviour of flax fibres extracted from flax stems by various treatments; namely green flax, dew-retted flax and Duralin flax. Dew-retting is a fibre extraction process that relies on the natural colonisation of aerobic fungi in the stalks and stems of fibres just after harvesting. The process to obtain Duralin flax is developed by CERES B.V., which uses deseeded straw of flax fibres as starting material. It turns out that the use of the straw of flax fibres is beneficial for both strength and reproducibility (Stamboulis et al. 2000). More importantly, no retting process is required to extract natural fibres from the straw. A description of the Duralin process can be found in the literature (Stamboulis et al. 2000; Stamboulis et al. 2001; Bismarck et al. 2002). Green flax takes up 40% more moisture than dew-retted flax. Duralin flax, on the other hand, shows the lowest moisture uptake. This is a result of the Duralin process, which depolymerises hemicellulose and lignin into lower molecular weight compounds. These compounds subsequently cure into a water resistant resin (Stamboulis et al. 2000), which leads to the lower moisture uptake of Duralin flax than green flax.

10.3.2 Moisture content of various natural fibres based on DVS

A typical water sorption curve obtained by DVS is shown in Fig. 10.2. The difference between the sorption and desorption loop is termed water sorption hysteresis. Hill et al. (2009b) studied the water sorption behaviour of various natural fibres. From Table 10.2, it can be seen that jute, coir and Sitka spruce (a type of wood fibre) show higher moisture content at 95% RH than hemp, flax and cotton. One of the major differences between these two groups of fibres is their lignin content. Jute, coir and Sitka spruce have a higher lignin content than hemp, flax and cotton (Bismarck et al. 2005). The high moisture uptake of highly lignified natural fibres, such as coir and jute fibres, might be a direct result of the amorphous nature of lignin. Even though amorphous lignin is hydrophobic, it is able to deform to accommodate more water in the cell wall. This argument is also in agreement with the moisture uptake of cotton. Cotton is almost pure cellulose but it has the lowest moisture uptake. The main reason for this observation is that cotton is highly crystalline with low amorphous content. Cotton consists of nearly 90 wt% cellulose molecules with no lignin content (Bismarck et al. 2005) and the crystallinity index of cellulose was found to be 80% (Segal et al. 1959). Therefore, most of the cellulosic –OH groups are not accessible to water molecules, leading to the observed low moisture uptake.

Table 10.2 also shows that the highly lignified fibres have larger water sorption hysteresis than flax, hemp and cotton fibres. Owing to the presence of amorphous lignin, the water molecules follow different pathways during absorption and desorption; that is, water sorption and desorption occur in two different physical states of the natural fibres (Lu and Pignatello 2004). Tvardovski et al. (1997) proposed that the deformation of sorbents is one of the main causes for sorption hysteresis. This concept could be applied to natural fibres. Swelling of natural fibres occurs as a result of the motion of incoming water molecules during water sorption, creating more voids. This swelling effect is confirmed by ζ-potential measurements (Stana-Kleinschek and Ribitsch 1998; Stana-Kleinschek et al. 1999; Bismarck et al. 2000; Bismarck et al. 2001; Bismarck et al. 2002; Baltazar-y-Jimenez and Bismarck 2007). Upon desorption of water molecules, there might be a lag between the water molecules leaving the voids and the relaxation of the sorbent to its original state (Lu and Pignatello 2002), which causes the observed hysteresis because sorption and desorption occur in two different physical environments.

10.3.3 Determination of exposed –OH groups of cellulose by using heavy water

Cellulose is a semicrystalline polymer with ordered regions (lower chemical reactivity) and disordered regions (higher reactivity). Therefore, it is of particular interest to characterise the ‘accessible’ regions of cellulose. This can be done by hydrogen–deuterium exchange. It is well known that deuterium can replace hydrogen in cellulose (Frilette et al. 1948; Sepall and Mason 1961). When cellulose is annealed at 260 °C in 0.1 M NaOD, all the hydrogen atoms (including the crystalline core of cellulose) will be converted to deuterium atoms (Wada et al. 1997). By exposing the deuterium-exchanged cellulose to water under normal condition, the accessible –OD groups will be converted back to –OH, whereas the core of cellulose will stay as –OD groups. By obtaining the IR spectra or measuring the mass increase owing to hydrogen–deuterium exchange of cellulose, the accessibility and crystallite size of cellulose can be obtained (Marrinan and Mann 1954; Horikawa and Sugiyama 2008; Lee et al. 2010). Table 10.3 tabulates the exposed –OH to core –OH ratio of cellulose from various sources.

The same analysis can be extended to determine the exposed –OH groups of cellulose in a DVS setup (Lee et al. 2010). The authors pre-conditioned highly crystalline bacterial cellulose (BC) for 10 h at 0% RH to remove any adsorbed water molecule. The RH of D₂O was then increased to 90% for 2 h and reduced to 0% for 2 h. This cycle was repeated 10 times to enable the adsorption of D₂O molecule on the surface of BC but avoiding bulk sorption of BC by D₂O. BC was then post-conditioned for 10 h at 0% RH to remove any adsorbed D₂O molecules. The amount of exposed –OH groups can be determined from the mass increase of BC (deuterium is 1.67 × 10^− 27 kg heavier than hydrogen, see equation [10.4]). This method could potentially be applied to natural fibres to determine the amount of exposed –OH groups or to study the moisture uptake behaviour of natural fibres (Hill et al. 2009b).

[10.4]

where Δm = mass change after hydrogen–deuterium exchange (mg), DSS = degree of surface substitution, m_i = initial mass of sample (mg), A = Avogadro’s number and m_n = mass of a neutron (mg).

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