Homogeneous isolation of nanocellulose from eucalyptus pulp by high pressure homogenization

Highlights

• Intra-molecular hydrogen bonds of eucalyptus pulp nanocellulose are entirely maintained.

• Effects of supramolecular structure of three kinds of cellulose were different by HPH.

• Eucalyptus pulp chains could be interrupted easily, which was suited for HPH.

Abstract

Nanocellulose from eucalyptus (Eucalyptus robusta Smith) pulp was extracted by simply disrupting the hydrogen bond network of celluloses with high pressure homogenization (HPH). It was found that nanocellulose was 20–100 nm in diameter, and presented a narrower molecular weight distribution, lower thermal stability and crystallinity index. Fourier transform infrared (FT-IR) and solid state cross polarization magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS ¹³C NMR) were used to confirm the physicochemical properties of nanocellulose, suggesting that intra-molecular hydrogen bonds are entirely maintained. Meanwhile, the feasibility of high pressure homogenization for different cellulosic biomass materials was investigated using comparison of eucalyptus pulp nanocellulose, sugarcane (Saccharum officinarum) bagasse nanocellulose and cotton (Gossypium spp) nanocellulose. Results showed that eucalyptus pulp chains could be interrupted easily by the shearing forces for its harder texture, which was suited for high pressure homogenization. Other kinds of cellulose could also be well suited through controlling key parameters such as mechanical forces and treatment temperature in the process.

Introduction

Eucalyptus is genus of tall, evergreen and magnificent trees cultivated all over the world. It is well famous for its various extracts, such as essential oils (Xiong et al., 2004), insecticidal constituent, ethanol (Castro et al., 2013) and xylose (Canettieri et al., 2007), which are extensively used in the cosmetics (Yang et al., 2004), perfumery, food and pharmaceutical industry. Besides the extractives, the main components of the plants are celluloses (the holocellulose content is about 74.2%) (Casas et al., 2013), which can be employed in many other applications capable of upgrading the value of eucalyptus utilization. For example, several authors have been reported that the extraction of eucalyptus globulus bark and a concentration strategy to obtain a phenolic-rich extract for application in the leather tanning industry (Pinto et al., 2013). Among the applications, the development of nanocellulose have been attracted much attention due to its super functionalities, such as its high aspect ratio, high surface area and good mechanical properties (Brinchi et al., 2013). It may provide value-added materials with superior performance and extensive applications, such as in paper, polymer, textile, pharmaceutical, biomedical applications and food industries (Chen et al., 2014).

Extensive investigations indicated that nanocellulose are normally isolated by means of physical treatments (Ikeda et al., 2002), chemical treatments (Peng et al., 2011) and biological treatments (Penttilä et al., 2013), so that the bonds of the cellulose–hemicellulose–lignin complex can be broken. There are some disadvantages among these methods that restrict the applications of nanocellulose. For example, by chemical treatments, due to the harsh reaction conditions used and the resulting acid degradation of the amorphous region, the production is time consuming and the yield is low (Conte et al., 2009). The enzymatic hydrolysis is an expensive technique and therefore requires more elaborate components (Brinchi et al., 2013). The mechanical treatments is the most well-known and widely used (Avolio et al., 2012, Hu et al., 2011). However, due to the highly crystalline of celluloses and the presence of strong intermolecular hydrogen bonds between chains and intra-molecular hydrogen bonds within a single chain, it need extensive energies to overcome the resistance of celluloses chains in the solid phase. In previous studies, liquid homogenous nanotechnology (First, cellulose is dissolved in a homogeneous solution. Then it is restructured through the high pressure shearing during the whole homogeneous solution, And thus the nanocellulose was obtained after regeneration) is proved to have a significant effect on structure of fibers of several different sources (sugarcane bagasse celluloses and cotton celluloses) (Li et al., 2012, Li et al., 2014, Wang et al., 2015), which can be considered a promising and sustainable alternative for its simplicity, high efficiency and conquering huge resistance between celluloses chains (Saelee et al., 2016). However, the applicability about liquid homogenous nanotechnology is unclear now.

As we known that fiber clusters of sugarcane bagasse cellulose (SCB) have characteristics including high stiffness, short fibers clusters and weak interlacing of fibers bundles (Oliveira et al., 2016). Furthermore, interior of SCB is filled with pores or vessels or holes. On the contrary, the ones of cotton cellulose possess opposite characteristics, covering high softness, long fibers clusters and strong interlacing of fibers bundles (Rahbar Shamskar et al., 2016). Experimental results show that the nanocellulose from cotton cellulose aggregated to a far greater extent than the ones from SCB (Li et al., 2012, Wang et al., 2015), which could be that chemical composition, refining treatments, structure, and properties of fibers influenced the nanocellulose, mean particle size and so on. Therefore, nanocellulose from eucalyptus pulp was extracted by HPH in order to study the suitability of liquid homogenous nanotechnology for fibers with different structure.

Up to now, there is a little information available on structure and properties of fibrous raw material and their impact on the subsequent HPH. As a consequence, the current research is specifically focused on this aspect. In the paper, nanocellulose from eucalyptus pulp was isolated by HPH associated with ILs (liquid homogenous nanotechnology), which could overcome enormous resistance of celluloses chains in the solid state (Cerruti et al., 2008). The performance of the nanocellulose was characterized using gel permeation chromatography (GPC), X-ray diffraction (XRD) and thermal gravimetric (TG) analysis. Furthermore, we present a quantitative analysis of the mechanism of fabricating nanocellulose by FT-IR spectra and CP/MAS ¹³C NMR measurements. The present work was also focused on assessment of liquid homogenous nanotechnology for the applicability from eucalyptus.