Electrical and mechanical properties of banana fiber reinforced polyester composites

Abstract

Banana fiber is a one of the natural fibers with growing interest as a reinforcing agent in thermosetting polyester resin. Banana fibers were collected from local area and extracted from banana tree by rotting. Raw banana fibers and fibers treated with 5% NaOH solution were used to prepare composites as reinforcement in polyester resin matrix with different weight percentage (wt%) of fiber loading by a simple hand lay-up press molding process. The effects of NaOH treatment and fiber adhesion in composites on its physical, mechanical, electrical, thermal and structural properties were investigated. It is found that the bulk density of untreated (raw) banana fiber-polyester resin composites decreases with the increase of fiber content. The density of treated samples is higher than that of untreated sample and always improved on treatment of fiber with 5% NaOH solution at longer times. The water absorption of the composites is increased with the increase of fiber loading indicating diffusion phenomenon. But the water absorption rate is decreased with the increase of NaOH treatment time. The treatment of fiber is thought to have changed fiber morphology and removed hemicellulose and other soluble matters from its surface and might have increased the crystallinity index percentage of cellulose. It is observed that the tensile strength of banana fiber-polyester resin composites decreases during the initial stage of fiber loading which indicates that the load is not properly transmitted to the fibers and increases with the increase of fiber loading from 5%. The Young’s modulus of composites increases with the increase of fiber addition. The flexural strength of these composites is decreased with the increase of fiber addition up to 10% and then increases with composition. At lower addition of fiber, the interfacial bonding is poor due to presence of lignin, fatty, and waxy materials. In further addition of fiber (20%), both the fiber and matrix bear the load and make good interfacial bonding between the fiber and matrix. The tensile strength, Young’s modulus, flexural strength, and flexural modulus of the composites are improved with the increase of NaOH treatment time. This suggests that due to treatment of fibers with alkali, the interfacial strength might have slightly increased and aided mechanical interlocking between the fiber and matrix. AC conductivity (σac) is increased linearly with the frequency of the applied signal with lower slope in the low frequency region (up to 105 Hz) and higher slope in the high frequency region (>105 Hz). The observed frequency dependent of σac indicates that the mechanism of carrier conduction is due to hopping of charge carriers between the localized states. The dielectric constant (έ) is decreased with increasing frequency and increased with increasing temperature. This constant is decreased sharply at low frequency (100 – 900 Hz) region and smoothly decreased at high frequency region. At lower frequency the value of dielectric constant is much high as compare to higher frequency indicating ionic effect. All specimens exhibit a normal dielectric behavior of materials, which may be due to the interfacial polarization. There is a big dip observed in the loss tangent (tanδ) behavior. The dip is found to maximum at around 100 kHz. The loss tangent gradually decreases with increasing frequency up to 100 kHz and after that it increases sharply with increasing frequency. The σac, έ, and tanδ are decreased with the increasing fiber content and NaOH treatment time. It may happen because of the oxygen vacancy, hydroxyl ion motion or interfaces between the fiber and matrix. Mass loss profile of TG/DTA/DTG of banana fibers changes considerably when they are treated with NaOH solution. On treatment with alkali, mass loss (%) in major degradation region is decreased with an increase in amount (%) of final residual mass (carbonaceous char). Major degradation stage, maximum degradation rate, and endothermic peak of composites shifts towards higher temperature as compare to resin sheet and fibers. The banana fibers-polyester resin composites are thermally more stable than that of fibers and polyester resin. The observed FTIR spectra of banana fiber and composites are characterized of lignocellulosic fiber, which consists of the primary components: cellulose, hemicellulose, and lignin. The vibrational modes after NaOH treatment did not suffer significant changes and the main bands appeared approximately in the same range of the wave numbers.