Moisture absorption

Components used in building constructions are exposed to moisture and humidity during their lifetime; hence information on moisture absorption and weathering is important in the building sector. Natural fibres from plants are cellulose based and swell when they come into contact with water or organic liquids, resulting in poor mechanical properties and dimensional stability [94]. The swelling of natural fibres is dependent on parameters of the wetting liquid (including hydrogen bonding capability, molecular size, cohesive energy, viscosity, density, and basicity), parameters of fibres (including surface coating and treatment, density, porosity, and crystalline structure of the fibres), the ambient temperature and the steric effects [95]. Natural fibres can absorb and desorb moisture from the

Page | 35 atmosphere and equilibrate with the surrounding environment. Hence, natural fibres undergo swelling in humid and dry environments affecting their dimensional stability. Water absorption in natural fibre composites is governed by mechanisms such as [96]:

1. Hydrogen bonding of the water molecules to the free OH-groups present in the cellulose materials

2. Diffusion of water molecules into the fibre-matrix interface caused by the capillary action

To reduce water absorption and swelling of natural fibres, a number of studies have been conducted including the use of coupling agents [97] to bond with OH-groups in the cellulose material and the use of chemical treatments [98].

In polymer composites, the transport of water is facilitated by three mechanisms [93, 99]. The first involves the diffusion of water molecules into the micro-gaps between polymer chains. The second mechanism involves capillary transport into the gaps and flaws at the interfaces between the fibre and polymer, resulting in poor wettability and impregnation (during composite manufacturing). The third involves the transport of water molecules by micro- cracks in the polymer as a result of fibre swelling (particularly in natural fibre composites).

Of these three mechanisms, diffusion mechanism majority influences the water absorption in polymer composites. Diffusion behaviour is further classified according to the relative mobility of the penetrant of the polymer segments which is known as Fickian diffusion model, non-Fickian or anomalous, and an intermediate behaviour between Fickian and non- Fickian. However, the water absorption pattern of natural fibre composites at room temperature tend to follow the Fickian behaviour whereas at higher temperatures the absorption of water is non-Fickian [100]. In Fickian diffusion, the rate of diffusion is much less than that of the polymer segment mobility. Figure 2.13 present the Fickian and non- Fickian diffusion.

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Figure 2.13 (a) Fickian diffusion at room temperature and (b) non-Fickian diffusion at elevated temperature [101]

Composite materials absorb water when they are exposed to humid environments. The water absorption continues until saturation. The water absorbed in composites consists of free water and bound water [102]. Free water are water molecules that are able to move freely through the micro voids and holes whereas bound water are dispersed water molecules that are bound to the hydrophilic groups of the polymer matrix. At high humidity levels, water penetrates and attaches onto the hydrophilic groups of the fibre, establishing intermolecular hydrogen bonding with fibres and hence, reducing the interfacial adhesion of fibre-matrix [103]. The degradation process occurs when swelling of cellulose fibres develops stress at interface regions causing micro-cracking of the matrix around swollen fibres and this promotes capillary and transport of water via micro-cracks. Water soluble substances start leaching from the fibres and eventually lead to ultimate de-bonding between the fibre and the matrix. After long periods of water exposure, biological activities such as fungi growth degrade the natural fibres [101].

The water absorption behaviour of a composite depends on the type of fibre and matrix, the fibre content, fibre orientation, atmospheric conditions like temperature and humidity, water distribution within the composite and reaction between water and matrix, permeability nature of the fibre, and characteristics of the surfaces exposed to water [99, 104].

Page | 37 Several researchers have studied the effects of water absorption on properties of natural fibre reinforced polymer composites. They also reported on the effects of fibre content, fibre type and matrix type on moisture absorption properties.

Venkateshwaran and Elayaperumal [105] studied the effect of layering sequence on the mechanical (tensile, flexural and impact) properties of woven jute/banana hybrid composites fabricated by hand lay-up. Epoxy resin LY556 with hardener HY951 were used to produce composites reinforced with jute, banana, jute/banana/jute (JBJ) and banana/jute/banana (BJB) fabrics. It was observed that the composite reinforced with BJB achieved higher tensile and flexural properties followed by composite reinforced with JBJ and the least by the composite reinforced by only banana fibre fabric. This shows that the addition of banana fibre as skin layer increases the strength and stiffness. Hybridisation improves the properties of the composite; the layering sequence has much more effect on the flexural properties of the composite. The moisture absorption were 4.6%, 6.4%, 6.8% and 7.4%, respectively; for composites reinforced with banana fibres, jute fibres, BJB, JBJ fabrics. The results showed that the composite reinforced with three-layers JBJ has the highest moisture absorption of 7.4% because of the moisture affinity of jute fibres. Jute fibre showed the highest sorption, diffusion and permeability coefficients than banana fibre resulting in high moisture absorption capability of the composite reinforced JBJ. Hybridisation of jute fibres with banana fibres slightly decreased the moisture uptake.

In another study [104], composites produced from thermoset matrices (epoxy and polyester) reinforced with carpet waste jute yarns were investigated. Waste jute yarns were treated with 10 and 25 wt% alkali solutions. The composites were compression moulded as single-ply (4/1 lea) and double-ply (4/2 lea) yarn composites. For flexural and impact tests, the samples were subjected to water submersion to investigate the effects of water absorption on the mechanical properties. The percentage weight gain of samples was measured at different intervals and the magnitude of diffusion parameters such as n and K was evaluated to assess the mode of water transport. It was found that the composites follow Fickian diffusion behaviour. Both the water content (Mmax) and effective diffusion coefficient (Deff) values

increase with the increase in fibre content. The K value was also found to increase with increasing temperatures, hence the Deff value increased remarkably. This indicated that the

Page | 38 increasing the mobility of polymer segments and creating additional voids for water to penetrate.

The flexural strength and modulus decreased with water uptake for all composites. Nonetheless, impact strength values increased with water absorption for all composite samples and this was caused by the plasticizing effect of water on the matrix and hence, the effects of water on fibre-matrix interface. The composites reinforced with double-ply yarns showed higher water uptake than that of single-ply yarns.