Amorphous cellulose and hemicellulose are mostly responsible for the high water uptake of natural fibers, since they contain numerous easily accessible hydroxyl groups which give a high level of hydrophilic character to fibers.Although UV light makes up only about 5% of sunlight, it is responsible for most of the sunlight damage to the materials, especially polymers, exposed outdoors. This is because photochemical effectiveness of light increases with decreasing wavelength. This rupture causes changes in molecular weight, formation of cross-links, and reaction with oxygen. These structural changes lead to gross physical changes such as chalking, cracking, surface embrittlement,discolouring and loss of tensile and impact strength .In one study on the effect of weathering on polyester resin, the resin was exposed to actinic rays in a twin arcfadeometer . An exposure of 400 – 700 hours was considered equivalent to one year of outdoor exposure.
Initially there was no visible change after several hundred hours of exposure. Then the yellowing of the samples suddenly became noticeable. The tensile strength of the resin deteriorated from 66 MPa to 47 MPa after 2400 hours of exposure. Similarly the flexural strength deteriorated from 90 MPa to 82 MPa after same exposure period.To simulate the physical damage caused by sunlight it is not necessary to reproduce the entire spectrum of sunlight. In many cases it is only necessary to simulate the short wavelength. To overcome the problem of UV light, ultraviolet absorbers, such as carbon black or aromatic ketones, are added to polymers.The amount of moisture absorbed by the polymer matrix composite depends on the matrix type, exposure time, component geometry, relative humidity, temperature, and exposure conditions. Typical consequences of exposure of composite materials to these environments are: plasticisation of matrix, resulting in reduction in glass transition temperature and usable range, changes in dimensions and fibre/matrix debonding due to matrixand fibre swelling, enhanced creep and stress relaxation, resulting in increased ductility, change in coefficient of expansion, reduction in ultimate strength and stiffness, matrix cracking, and chain scission .
In case of natural fibre reinforced composites, additional problems are encountered because of hydrophilic nature of natural fibres. The moisture content of natural fibres can vary between 5% and 10%. This can lead to dimensional variations in composites and also affects mechanical properties of composites. It can cause fibres to swell and ultimately rot through attack by fungi. A possible solution is to improve fibre-matrix interface by using compatibilizers and adhesion promoters. With better adhesion, the moisture sensitivity is usually reduced .Also surface treatments of fibres with silanes can make the fibres more hydrophobic.The study of weathering properties of natural fibre composites is a new and growing field. The use of accelerated weathering testing is a preferred method for this purpose whereby the outdoor exposure conditions are simulated in laboratory in controlled atmosphere and in accelerated conditions which can reproduce the damage to the material in few days or weeks that actually occurs over months or years of outdoor exposure.A number of studies have recently been carried out which have examined the effects of accelerated weathering on the properties of natural fibre composites. Generally, all types of natural fibre composites show fading of colour and reduction in mechanical properties following exposure to natural and accelerated weathering conditions.
The use of coupling agents, fibre surface treatments, ultraviolet absorbers and pigments to improve the weathering resistance has been reported to have positive effects on composites. The effect of UV radiation only and UV radiation plus condensation on weight of these composites is shown in Figure 3. It was observed that the composites started to lose weight soon after exposure to these conditions. The increase in weight loss for UV exposed composites was greater than for UV plus condensation exposed composites. Whereas UV exposed composites lost nearly 3.2% of their original weight following 250 hours of exposure,UV plus condensation exposed composites lost only about 0.2% of their original weight following same exposure time. It has been shown that when unsaturated polyester resins are heated, the polymer chain begins to dissociate chemically and that polyesters have maximum photochemical sensitivity at wavelength of 325 nm. Since the UV radiation used in this study had a wavelength of 340 nm, it is expected that this resulted indegradation of the upper polyester layer of the composites resulting in loss of weight. This degradation is a manifestation of chain scission and breaking of bonds between polymer molecules following exposure to UV radiation.However following increased exposure times, as the UV radiation interacted with the hemp fibres, the fibres offered more resistance to UV radiation.