The world’s supply of bio-fibres i.e., natural polymers is being depleted, the demand for renewable raw materials continues to rise. So responsible use of available bio-fibres has become an inevitable task for scientists. Okra bast fibre is a lignocellulosic plant fibre which obtained from okra plant that grows everywhere abundantly in Bangladesh. Now-a-days, it is rejected as an agricultural waste products. Therefore, okra bast fibre can play an important role in the field of our national economy in finding various applications. They are generally biodegradable but do not possess the necessary and sufficient properties desirable for engineering or commodity plastics. Besides, like other vegetable fibres okra bast fibre possess few weak points i.e., rub resistance, colour fastness, wash and wear properties and very much prone to creasing, possibly because of high degree of orientation of cellulose in the fibre. This defect of creasing of cellulosic fibre may be remedied remarkably by the crease-resisting process in which resins are synthesized within the cellulosic materials in different proportions. In order to improve the textile properties of okra bast fibre it is an urgent need to improve several properties such as whiteness, softness, washing, dyeing behavior, colour fastness, light resistance, thermal resistance, etc. So, an attempt has been made to improve those characteristics of okra bast fibre through chemical modification and dyeing.
The constituents of okra bast fibre are 59.2% α-cellulose, 19.0% hemicellulose, 9.8% lignin, 3.7% pectin, 5.3% fatty and waxy matters and 3.0% aqueous extract.
The modification has been carried out in aqueous medium using potassium persulphate (K2S2O8) as an initiator under the catalytic influence of ferrous sulphate (FeSO4) in presence of air. To attain the maximum graft level, optimum conditions, viz., monomer concentration, initiator concentration, catalyst concentration, as well as reaction time and temperature have been determined. The percentage of grafting increases with the increase of monomer concentration, initiator concentration, catalyst concentration, time and temperature up to certain limits and thereafter it decreases.
The optimum conditions of modification for acrylonitrile are 0.03 M monomer, 0.005 M initiator, 0.005 M catalyst, reaction time 90 min and temperature 70ºC. The maximum percentage of grafting at optimum condition is 7.38%. Again, the formation of composite by the incorporation of monomer with okra bast fibre is confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) measurements.
The bleached and modified okra bast fibres have been dyed with Direct Green 27 and Direct Red 28 in presence of sodium sulphate as an electrolyte. The conditions of dyeing are 5% dye, 5% electrolyte, time 60 min and temperature 70ºC for both Direct Green 27 and Direct Red 28. The dye absorption is decreased with the increase of grafting due to the increases of hydrophobicity of fibres.
The absorption of dyes and colour strength (K/S value) by bleached fibre is comparatively higher than that of modified fibre. Due to the hydrophobic nature of modified fibre absorb the less amount of dye from the dyebath.
The colourfastness onto sunlight of modified dyed fibre is comparatively higher than that of bleached dyed fibre, this is perhaps, modification and dyeing together inhibit the UV-irradiation save the fibre from photo-oxidation. The colourfastness tests to spotting of okra bast fibre with acids and alkalis are remarkable. Sulphuric acid and sodium hydroxide give unsatisfactory results by changing colour but in most of the cases, such as acetic acid, sodium carbonate and ammonium hydroxide give satisfactory results.
The tensile strength of bleached fibre is comparatively lower than that of modified fibre. On the other hand, the percentage of tensile strength losses lower in case of modified fibre than bleached fibre on exposure to sunlight and heat. The percentage loss of tensile strength of bleached fibre increases than modified fibre.
The effect of temperature on the tensile strength of bleached fibre is more pronounced than that of modified fibre. This means that modified fibre is more heat resistant than the bleached fibre.
It can be concluded considering all the parameters used in the present investigation that depending upon the effect of various external influences on the bleached and modified okra bast fibres, the modified fibre is more resistant than bleached fibre. Between the dyes Direct Red 28 exhibits better results than Direct Green 27 in most of the cases.
Keywords: Okra Bast Fibre; Chemical Modification; Acrylonitrile monomer; Fourier Transform Infrared Spectroscopy (FTIR); Scanning Electron Microscopy (SEM); Dyeing Behaviour; Colour Strength; Degradation Behaviour; Colourfastness; Tensile Strength.
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