Preparation and characterization of cellulose hydrogel derived from pineapple leaf fiber

Abstract

In this study, hydrogel was prepared by using cellulose extracted from pineapple leaf fiber (PALF). PALF has higher cellulose content and better tensile properties compared to other cellulosic agro wastes such as sugarcane bagasse. However, up to date most of the researches are only focusing on the effect of alkali treatment on the properties of fiber regenerated from PALF and none has reported on the preparation of hydrogel from PALF. Besides, studies also showed that the cellulose concentration affected the properties of hydrogel. In this study, the effects of alkaline treatment on the lignin removal to obtain the PALF cellulose and the properties of PALF cellulose hydrogel were investigated. Then, the effect of cellulose concentration using the selected cellulose obtained from the treatment on the properties of the resultant PALF cellulose hydrogel were investigated. For the alkali treatment, PALF has been treated with different concentrations of sodium hydroxide (NaOH) (2-10 wt%) at two different temperatures (room temperature and 80°C). Then, PALF was dissolved in N,N’-dimethyl acetamide and lithium chloride solvent and underwent a phase inversion to change from liquid into a solid cellulose hydrogel. Later, after the optimum concentration and temperature for alkaline treatment were obtained, the cellulose hydrogel was prepared with different cellulose concentrations (0.5–2.0 wt%). From the Fourier transform infrared results, lignin was removed starting at the concentrations of 6 wt% NaOH at room temperature and 4 wt% NaOH at 80°C, respectively. The thermal stability of fiber was increased at higher NaOH concentration and the crystallinity of cellulose was also increased (71-79%) after the treatment. Higher NaOH concentration and treatment temperature have increased the swelling equilibrium of the hydrogel but adversely affected the gel fraction. Besides, the ultraviolet-visible spectra showed that the highest transmittance (87.8 %) and the lowest intensity of absorbance (at 280 nm corresponding to lignin) were obtained by the cellulose hydrogel prepared with cellulose treated at 8 wt% NaOH and 80°C. Later, the designated cellulose was chosen to prepare the hydrogel at different cellulose concentrations. It was found that, the swelling and the transparency of hydrogel decreased with increasing of cellulose concentration. On the other hand, the viscosity of the solution, the gel fraction and the tensile strength of the hydrogel increased as the cellulose concentration increased. The highest tensile strength (1 MPa) was obtained by the hydrogel at 2.0 wt% cellulose concentration. The increase in these properties were most probably due to the increased of entanglement of the cellulosic chains due to the formation of the hydrogen bonding at higher cellulose concentration. From the rheological measurement, the elastic modulus (G') of all the hydrogels were higher than the loss modulus (G'') regardless of the cellulose concentration and both parameters were independent to the measured frequency. This showed that the PALF hydrogels possessed the ideal rubber characteristics. The mesh sizes of the hydrogel decreased with increasing cellulose concentrations. Clearly, this revealed that the condition of the alkaline treatment affected the cellulose extraction and the hydrogel properties. However, the cellulose concentration has greater effect towards the physical and tensile properties of the hydrogel. Ultimately, cellulose hydrogels obtained from this study have promising potential to be used as biomaterials in many applications.