The result of thickness swelling test was shown in Fig. 2.
Sample UD shows the highest thickness swelling (3.03%)
among all the samples. The woven sample shows moderate
(2.20%) thickness swelling and the mat sample is lower
(2.04%). The figure also showed that the increase in immersion
time will allow water absorption, thus increases the thickness
swelling of the hybrid composites until a constant thickness was
obtained. According to Jawaid et al. [29] the hydrophilic properties
of lignocellulose materials and the capillary action will
cause the intake of water when the samples were soaked inwater
and thus increase the dimension of the composite. The presence of voids also related to the thickness swelling of as the higher
the void contents increase the thickness swelling of composites
[29]. However, the result in swelling thickness is contradictory
to this statement. This may be the effect of the hybridisation of
kenaf with Kevlar, synthetic fibres. According to Ray and Rout
[30], water molecules attract the hydrophilic groups of natural
fibres and react with the hydroxyl groups (—OH) of the cellulose
molecules to form hydrogen bonds. Thickness swelling
occurred as the water molecules penetrate the natural fibrereinforced
composite through micro-cracks and reduce the
interfacial adhesion of fibre with the matrix. Higher Kevlar
content in sample M resulted in higher fibre–matrix interfacial
adhesion, thus lower thickness swelling. Khalil et al. [31]
reported that the water absorption and the thickness swelling of
natural fibre reinforced with polyester composites are improved
by the incorporation of synthetic fibres. The contradiction of
water absorption and thickness swelling in this study may also
be due to the exposure of the lignocellulosic fibre on the surface
of the composite