Cellulose microfibres produced from banana plant wastes: Isolation and characterization

Abstract

Cellulose microfibres from banana fibre waste have been isolated and characterised. Bleached banana waste fibres were hydrolysed, under different conditions, to study the effects of temperature, reaction time, and acid concentration on the properties of the resultant cellulose microfibres. As the concentration of acid used in the hydrolysis was increased, more stable aqueous suspensions of the cellulose product were obtained and the dimensions of the resulting cellulose microfibres were reduced. XRD studies reveal that cellulose prepared by such hydrolysis was more crystalline than the banana fibres. The effect of initial dimension of banana fibres on the size of cellulose microfibres was also examined.

Introduction

Over the past decades there has been a growing interest in the use of natural fibres as reinforcing fillers in polymer matrices (Mohanty et al., 2000, Samir et al., 2005). The fibres can be utilised for reinforcement in the long form, short form or in a derived form.

The banana plant fibres are fibrous residue of pseudo-stems and leaves left over after banana cultivation. India is one of the larger producers of banana plants. Banana cultivation generates a considerable amount of cellulosic-based waste. The banana fibre wastes generated is lignocellulosic in nature. The comestible part, the fruit, constitutes only 12% by weight of the plant. The remaining parts become agricultural waste, causing environmental problems in banana farming regions. This residual resource is rich in cellulose, a feature that has attracted much interest due to the potential use of such materials as a reinforcing component in high performance composite materials.

The use of nanoparticles and microparticles as reinforcements in high performance composites and other structural materials has attracted interest (Gacitua, Ballerini, & Zhang, 2005). In the last decade, effort has been placed on developing nanofillers from various natural resources. Cellulose has emerged as a strong candidate for providing nanoparticles for use as a reinforcing agent.

Microcrystalline cellulose (MCC) is widely used, especially in the food, cosmetic and medical product industries. Such applications involve MCC acting as one or more of the following ways: as a water-retainer, as a suspension stabilizer, as a flow characteristics controller in the systems used for final products and as a reinforcing agent for products such as medical tablets. MCC is obtained on an industrial scale through the hydrolysis of wood cellulose and of cotton cellulose using dilute mineral acids. The preparation of MCC from wheat and cereal straws (Alemdar and Sain, 2008, Jain et al., 1983), jute (Abdullah, 1991), soybean husk (Nelson, Edgardo, & Ana, 2000), flax fibres and flax straw (Bochek, Shevchuk, & Lavrentev, 2003), sugar cane bagasse (Bhattacharya, Grminario, & Winter, 2008), mulberry barks (Li et al., 2009) and peel of pear fruits (Habibi, Mahrouz, & Vignon, 2009) has been studied.

The controlled acid hydrolysis of native cellulose fibres yields highly crystalline, rod-like particles through the selective degradation of the more accessible material. Depending on their origin, these microfibrils differ in lateral size. Upon the action of strong acids, these fibres break down into short crystalline rods or cellulose microcrystallites, having shorter lengths, ranging from a few hundreds of nanometers to a few microns.

Acidic hydrolytic cleavage is dependent on the acid species, the acid concentration, the time for hydrolysis and the temperature of the hydrolysis reaction. Under controlled conditions, cellulose microcrystals can be obtained using sulphuric acid hydrolysis. This process induces the grafting of the sulphate groups, randomly distributed on the cellulose microfibril surface, providing a negative electrostatic layer, covering the microfibrils. The different treatments of these charged microcrystallites, such as mechanical dispersion or ultrasonication, permits the dispersion of the aggregates and finally produces colloidal suspensions. Because of their stiffness, thickness, thickness distribution, length and length distribution, these rod particles are commonly called ‘‘whiskers’’ (de Souza Lima & Borsali, 2004). The controlled hydrolysis conditions with sulphuric acid (temperature, time, and acid content) lead to whisker suspensions that neither precipitate nor flocculate. This effect is mainly caused by electrostatic repulsion between the negatively charged particles on their surfaces.

Zhang et al., have proposed a method for the synthesis of cellulose nanospheres from cotton fibres by using mixed acid treatment (6:1:3 = water:HCl:H₂SO₄) (Zhang, Elder, Pu, & Ragauskas, 2007). It was observed that the product mixture consisted of two different particle size species, averaging approximately 500 nm and 70–200 nm. The study revealed that cellulose nanoparticles of smaller sizes could be obtained by a further acidic sonication of the initially sonicated cellulose fibres. The authors pointed out that there was a linear relationship between the size of the cellulose nanoparticles and the treatment time. Another observation was that the initial cellulose sample was cellulose I, whereas the obtained cellulose spherical particles were of cellulose II polymorphic character. Bhattacharya et al. have reported the isolation of cellulose microfibrils from bagasse, a sugar cane by-product by using 60 wt.% sulphuric acid (Bhattacharya et al., 2008). The authors reported that bagasse was more resistant to hydrolysis than were the tunicate, bacterial and wood celluloses. Dong et al., examined the effect of the preparation conditions (time, temperature and ultrasound treatment) on the resulting microcrystalline cellulose structure from the sulphuric acid hydrolysis of cotton fibres (Dong, Revol, & Gray, 1998). The authors found a decrease in microcrystalline cellulose fibre length and an increase in the surface charge of the particles with prolonged hydrolysis times.

It has been reported that cellulose whiskers prepared by sulphuric acid hydrolysis are more stable than those prepared using hydrochloric acid (de Rodriguez, Thielemans, & Dufresne, 2006). Indeed, the sulphuric acid prepared whiskers present a negatively charged surface. The hydrochloric acid prepared whiskers are not charged. Another way to achieve charged whiskers consists of the oxidation of the surface of the whiskers (Araki et al., 2001, Isogai and Kato, 1998) or the post-sulphonation of the hydrochloric acid prepared microcrystalline cellulose (Araki, Wada, Kuga, & Okano, 1999).

Zuluaga et al. have isolated cellulose microfibrils from banana rachis, using a combination of chemical and mechanical treatments (Zuluaga, Putaux, Restrepo, Mondragon, & Ganan, 2007). The chemical treatment involved treatment of bleached banana rachis residues with a mixture of 80% acetic acid solution and 70% nitric acid solution at 120 °C for 15 min. The washed and purified cellulose was sonicated for 15 min. In the mechanical process, a bleached residue was suspended in water and homogenized. It was noted that an acidic treatment resulted in shorter aggregates of parallel cellulose microcrystallites. Individualised or bundled microfibrils were obtained by homogenization. In another report, banana pseudo-stem fibres have been used for the preparation of sodium carboxymethylcellulose (Mario, Marseno, & Haryadi, 2005).

The objective of the present study was to examine the possibility of using sulphuric acid hydrolysis method for the preparation of cellulose microfibres from banana fibres and to characterise the resultant microfibres. The effect of preparation conditions such as time, temperature, acid concentration and initial dimension of the banana fibres, on the cellulose microfibre structure and characteristics has also been investigated.