Aqueous morpholine pre-treatment in cellulose nanofibril (CNF) production: comparison with carboxymethylation and TEMPO oxidisation pre-treatment methods

Abstract

In this study, pulped cellulose fibres were pre-treated with aqueous morpholine prior to mechanical disruption in the production of cellulose nanofibrils (CNF). The properties of the morpholine pretreated CNF (MCNF) were closely compared with CNF obtained from carboxymethylation (CMCNF) and TEMPO-oxidation (TCNF) pre-treatment methods. An investigation of the swelling behaviours of cellulose in varying concentrations of morpholine revealed that there is a synergistic behaviour between morpholine and water in its ability to swell cellulose. As a result, cellulose pulp dispersed in 1:1 mole ratio of morpholine to water was well swollen and readily fibrillated by mechanical shear. Surface chemistry analyses indicated that the surface of the MCNF remained unmodified, compared to the CMCNF and TCNF which were modified with anionic groups. This resulted in only a slight decrease in crystallinity index and a minimal effect on the thermal stability of MCNF, compared to CMCNF and TCNF which showed marked decreases in crystallinity indices and thermal stabilities. The average widths of MCNF, CMCNF and TCNF, as measured from electron microscopic images, were broadly similar. The higher polydispersity of MCNF widths however led to a differential sedimentation and subsequent lower aspect ratio in comparison with CMCNF and TCNF as estimated using the sedimentation approach. Also, the presence of electrostatic repulsive forces, physical interactions/ entanglements and lower rigidity threshold of the CMCNF and TCNF resulted in higher storage moduli compared to the MCNF, whose elasticity is controlled by physical interactions and entanglements. Aqueous morpholine pre-treatment can potentially be regarded as an ecologically sustainable process for unmodified CNF production, since the chemical reagent is not consumed and can be recovered and reused.

High-shear rate rheometry of micro-nanofibrillated cellulose (CMF/CNF) suspensions using rotational rheometer

“…A separate work has shown that as the number of passes through the Microfluidiser increase, the amount of CMFs is reduced till it vanishes completely at 5 passes (Onyianta et al 2017). Therefore, from the observation of the optical and electron microscopic images, a better description of such material would be CMF/CNF.…”

“…TEMPO mediated oxidation is one of these pre-treatment processes in CNF processing that has become popular in literature because of the ease of preparation and the quality of CNFs, although the cost of reagents has limited its industrial applications. Other pre-treatment routes and different processing methods have been extensively compared (Sacui et al 2014;Onyianta et al 2017) and reviewed (Klemm et al 2011;Lavoine et al 2012;Nechyporchuk et al 2016).…”

“…CNFs and CMFs are used as rheology modifiers in several products which may get exposed to shear rates greater than 10 3 s -1 during the operations such as pumping and filling containers (Barnes et al 1993). Moreover, the processing of CNF using the Microfluidiser involves high-pressure pumping of cellulose suspension through flow channels with diameters generally less than 200 lm (Saarinen et al 2009;Naderi et al 2014aNaderi et al , b, 2015aTaheri and Samyn 2016;Onyianta et al 2017). …”

The effects of morpholine pre-treated and carboxymethylated cellulose nanofibrils on the properties of alginate-based hydrogels

“…Nanocellulose is a nano-sized cellulose material extracted in the form of rod-like crystals (cellulose nanocrystals, CNC) and nanofibrils (cellulose nanofibrils, CNF) using various chemical (Onyianta, Dorris, & Williams, 2018), enzymatic (Beyene et al, 2017) and mechanical pre-treatments followed by mechanical processing (Abdul Khalil et al, 2014). It is a renewable, biocompatible and biodegradable biopolymer with improved mechanical strength, lightweight properties, optical properties, barrier properties and structuring capabilities (Jonoobi et al, 2015;Klemm et al, 2011).…”

“…Some of the chemical pre-treatment methods such as carboxymethylation (Wågberg et al, 2008), 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation (Isogai, Saito, & Fukuzumi, 2011), and quaternisation (Aulin, Johansson, Wågberg, & Lindström, 2010) modify the surface of the cellulose, thereby imparting new properties which are different from the original cellulose material. Onyianta et al (2018) compared the properties of cellulose nanofibrils prepared using three types of chemical pre-treatment methods. The study shows that the presence or absence of surface charge on the CNF is responsible for the changes in crystallinity, viscoelasticity, thermal properties and aspect ratios of these CNF materials.…”