Tensile and Flexural Strength Properties of Surface-modified Sisal Fibre Composites

Abstract

Over the last five decades, extensive research has been done to investigate the strengthening effect of several fibre surface-modification techniques on ligno-cellulosic fibres. This has been necessitated by the need to find eco-friendly, sustainable, low-cost alternatives to synthetic, mineral and man-made fibres that are otherwise known to cause serious environmental degradation due to their non-biodegradability. Despite there being large amounts of data on strengthening effect of various fibre surface-modification methods on ligno-cellulosic fibres, no known studies have been carried out to compare the strengthening effect of alkali and thermal fibre surface-modification on high lignin content UG grade Kenyan sisal fibres. This study aimed at determining and comparing the strength properties of mercerised (alkali-treated) and cornified (thermally treated) UG grade Kenyan sisal fibres. The fibres strength properties were determined and analysed using the Weibull Cumulative Distribution function. The untreated, mercerised and cornified fibres, in different volume fractions, were then used to make composites in hydrophilic (Portland cement) and hydrophobic (polyester resin) matrices. Tensile and flexural strength tests were carried out on these composites, and comparisons of the results obtained done. Mercerised sisal fibres displayed the most significant improvement in tensile strength properties with mean fracture strength of 271 MN/m2, which showed a 68.30% increase in tensile strength compared to untreated sisal fibres. Cornified sisal fibres had a mean fracture stress value of 198.57 MN/m2, which was a 23.32% increase compared to untreated sisal fibres. OPC mortar composites of mercerised sisal in uniaxial orientation had a peak Modulus of Rupture of 9.39 MN/m2 at 2.3% fibre volume fraction, which was a 151.07% increase in flexural strength compared to the unreinforced mortar specimens. Chopped, randomly oriented mercerised sisal fibre mortar composites had a peak Modulus of Rupture value of 9.8 MN/m2 at a 4.2% fibre volume viii fraction, which showed a 36.11% increase in flexural strength when compared to the unreinforced mortar specimens. In polyester resin, the untreated, mercerised and cornified fibre reinforcements all exhibited a negative reinforcement, with the composite tensile strength decreasing with increasing fibre volume fractions. The negative reinforcement was attributed to the exothermic curing temperature (about 113°C) of the polyester resin. Upon controlling the curing temperature to some degree, untreated sisal fibre-reinforced polyester resin displayed the most significant gain in flexural strength with a Modulus of Rupture that was 66.93% higher than that of the unreinforced polyester specimens. Finally, in order to ascertain whether the negative reinforcement of the polyester resin was due to the resin’s exothermic curing temperature, the fibres were used as reinforcement in a non-exothermic curing polymeric matrix (epoxy resin) where mercerised sisal fibre-reinforced epoxy resin composites displayed the most significant gain in tensile strength with an ultimate tensile stress value that was 118.52% higher than that of the unreinforced epoxy resin. These findings were in agreement with the results reported in earlier studies.