MACROMOLECULAR STRUCTURE OF SISAL FIBER AND ITS POTENTIAL APPLICATIONS

Abstract

Among all the natural fibers, sisal is widely used as yarns, ropes, twines, carpets etc. because of its high cellulose content and tensile strength. Several researches have been conducted on this fiber and it is found to be more suitable for the reinforcement applications in polymer composites because of its superior properties such as, low density, high specific strength, environment friendly nature and low cost. As it is the fiber extracted from leaves of a drought resistant plant hence, it can easily be commercially exploited. However, the hydrophilic nature of the fiber hinders its application in the composite field due to the lack of compatibility between hydrophilic fiber and hydrophobic thermosetting matrix. The present study focuses on various surface treatment methods on the sisal fiber to enhance the adhesion, compatibility between the fiber and the matrix in order to improve the mechanical strength as well as electrical properties of the composite for attracting industrial interests for its potential use. Macromolecular structural parameters are evaluated to know the effect of modification on the fiber properties.
Various surface modifications such as Dewaxing, KMnO4, Microwave and Alkali treatments were conducted on the sisal fiber. For dewaxing, the fiber were immerged in ethanol and benzene solution for 12 hr with 2 hr of intermediate heating and cooling of solution in which fibers were soaked. For KMnO4 treatment, the dewaxed fibers were soaked in 0.01%, 0.05% and 0.1% concentered KMnO4 –acetone dewaxed fibers were soaked in 0.01%, 0.05% and 0.1% concentered KMnO4 –acetone solution for 1, 2 and 3 mins. Physical modification of the fiber was done by microwave irradiation at 160W, 320W, 640W for 2, 4 and 8 mins. Alkali treatment on the fiber was carried out for 2hrs at 2%, 4%, 6% and 8% NaOH concentrations. The soaking period of fiber in 6% NaOH solution was varied from 2 hr to 8 hr with a step of 2hr.
The short sisal fiber reinforced polymer composites were fabricated for different fiber volume percentage (5, 10, 15 and 20 %) by hand layup technique. The composite having 15% fiber volume fraction was found to have better mechanical strength and lower loss tangent in comparison to others. Hence, 15% volume fraction of the fiber was kept constant for the fabrication of all composites in the entire study.
Small angle X-ray scattering (SAXS) of the treated fiber showed the change in macromolecular parameters in comparison to the untreated fiber. The enhancement of the macromolecular structural parameters like average periodicity transverse to the layer, volume fraction of matter phase etc. and the reduction of disorderliness of the structure at nano order were observed in each treatment for a particular combination of concentration, soaking period, irradiation power and duration of treatment.
Density of the fibers was determined by Archimedes’ principle and varied after the various treatments. It was observed that the density of the surface modified fiber was higher than the untreated one at the optimum condition, which may be due to the removal of natural wax and impurities present in the fiber.
For KMnO4 and Alkali treatment, the observed increment in the structural parameters like periodicity transverse to the layers, volume fraction of matter phase,of the fiber may be due to the partial removal and reduction of hemicelluloses, lignin, pectin, wax etc. Microwave irradiation helps in reduction of residual stress in the fiber leading to the increased value of crystallinity of the fiber, confirmed from the XRD studies of the fiber.
Surface roughness of the fiber was achieved after almost all the treatments at optimum treatment condition. It was ascribed to the removal of surface impurities like natural wax, lignin, hemicelluloses and removal of internal constraint from the fiber. Degradation of the fiber was confirmed from the SEM micrograph at higher level of concentration, duration of chemical treatments and irradiation power of microwave.
FTIR study of the fiber helps to assess the structural changes of the fiber by change in absorbed intensity ratio of the different functional groups. The reduction of OH group, hemicellulose was observed for the fiber modified by suitable treatment parameters.
Improved flexural strength and tensile strength was observed after all the treatment at optimized condition. Better wetting of the fiber with the matrix was due to enhanced surface roughness of the fiber as observed in SEM micrographs of the fractured surface of the composites. This may be the possible cause of the improved mechanical strength of the fiber. This may be the possible cause of the improved mechanical strength of the fiber. Degradation of the mechanical strength of the composite was ascribed to the reinforcement of degraded fiber and poor adheison between fiber and matrix. Electrical study of the short sisal fiber reinforced epoxy composite revealed that the loss tangent and dielectric constant of the composite were reduced for the suitably modified fiber reinforced composite. However, significant increase in loss tangent for the composite reinforced with degraded fiber was observed. The modulus formalism had been adopted to study the relaxation mechanism in the composite. The advantage of complex electric modulus formalism is that it suppresses the electrode effect. The shifting of relaxation peaks in the temperature dependent modulus plot confirmed the presence of temperature dependent relaxation process in the materials.