Pulps for section 1
Thermo-mechanical pulp (TMP)
The TMP sample used here was obtained from the SCA pulp mill at Örtviken, Sundsvall, Sweden; it had a Canadian Standard Freeness (CSF) value of approximately 65 mL and a dry solid content of 34 %. The wood type used was fresh Norway spruce (Picea abies).
Chemi-thermomechanical pulp (CTMP)
The CTMP sample used in this investigation was obtained from the SCA Östrand pulp mill, Sundsvall, Sweden. The wood material used was solely never-dried Norway spruce (Picea abies) with a high CSF value of approximately 619 mL.
A 1 % diethylenetriamine penta-acetic acid (DTPA) solution was used to wash 150 g of o.d. pulp of approximately 5 % consistency. The pulp slurry was placed in a plastic bag and sealed. The bag was then put into a hot water bath at approximately 70 °C for 1 h for the reaction to be effective. After that, the DTPA solution was washed out with distilled water using a Buchner funnel. A pulp dry solids content of approximately 18 % was achieved for all the samples, which were stored in a refrigerator at 4 °C before further pretreatment in a wing mill refiner.
Wing mill refiner (mixer)
This equipment allows certain process parameters, such as temperature, residence time, and rotor mixing speed (rpm), to be controlled. Although the main purpose of the equipment is to mimic refining conditions, in this work, the wing mill refiner was used solely as a mixer (Fig. 1). Before sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) pretreatment of the pulp fibres, the temperature of the wing mill refiner was set to 90 °C; 150 g of o.d. pulp was placed in the vessel and preheated for 10 min at a rotor speed of 60 rpm. After this, the two chemicals were mixed together and charged via the nozzle at a pressure of 3 bars; immediately after the chemical dosage, the rotor speed was increased to 750 rpm for 1 min to achieve effective mixing. The rotor speed was later reduced to the usual 60 rpm for the rest of the 15 min reaction time. The samples were collected and stored in the refrigerator before further treatment. The reaction of the chemicals was stopped by diluting the mixture with distilled water and then subjecting it to a double fractionation process using a Britt Dynamic Drainage Jar (BDDJ or BJ) using both the 30- and 100-mesh wire screens.
Britt Dynamic Drainage Jar (BDDJ) fractionation
In this paper, the pulps were fractionated using a specially designed 8-L BDDJ at the SCA Research Centre in Sundsvall. Pulps were disintegrated for 30,000 revolutions and diluted with 8 L of tap water. Thirty grams of o.d. pulp was used for each fractionation. The fine fractions were obtained from the pulp samples using the BDDJ fractionation technique in which fractions passing through the 30-mesh wire screen were regarded as “fines” while the fractions retained on the 30-mesh wire screen were discarded; a 100-mesh wire screen was also used for the fractionation process. The cut-off or pore size of the BDDJ 30-mesh screen was approximately 600 µm. Fibre fractionation was conducted to obtain small, short fibres that could be easily homogenised.
A Niro Soavi homogeniser (ARIETE, model NS2006H; GEA Group, Parma, Italy) was used to produce nano-ligno-cellulose. The high-pressure homogeniser subjects the pulp slurries to high impact forces and high shear rates due to the reciprocating action within the valves. The pulp suspension undergoes high-pressure micronisation, which reduces the fibre size to obtain a stable suspension. The homogenisation experiments in this study were performed using thermomechanical pulp and chemi-thermomechanical pulp fines. The pulp slurries were treated in the homogenising equipment at approximately 1 % consistency for 120 min at a homogenisation pressure in the range of 200–300 bars for approximately 9 and 18 passes (homogenisation cycles). Before homogenisation, the pulp fibres were diluted with water to a solids content of approximately 1 % Table 1 shows the process variables, Table 2 represents the chemical dosages used, Table 3 shows the codes names of various pulp samples and lastly Table 4 shows the code names of MFC samples.
Scanning electron microscopy (SEM)
To freeze-dry the MFC, pulp suspensions were first placed in moulds and then plunged into liquid propane for rapid freezing. Moulds containing frozen pulp specimens were transferred to a freeze dryer and allowed to dry for 24 h. Freeze-dried pulp fibres and MFC were mounted on aluminium studs using double-sided carbon tape. Fibre specimens were sputtered with Au/Pd particles in an Emitech K100X sputterer (Emitech, Fall River, MA, USA) for 1.5 min and then examined using a Zeiss Sigma VP scanning electron microscope (Zeiss, Oberkocken, Germany).
Transmission electron microscopy (TEM)
For transmission electron microscopy, a 20-µL drop of pulp and MFC dispersion was placed on a QUANTIFOIL 400-mesh copper grid with holey carbon film (EMS, Hatfield, PA, USA). The excess solution was blotted with a filter paper and the dispersion was allowed to dry. Grids with pulp specimens were examined using an FEI Tecnai 12 transmission electron microscope (FEI, Hillsboro, OR, USA) at an accelerating voltage of 120 kV.
Pulps for section 2
The NFC was produced from a chemi-thermomechanical pulp (CTMP) (SCA Östrand Pulp Mill, Sundsvall, Sweden) and from a commercial sulphite softwood dissolving pulp (SP) (Domsjö Fabriker AB, Örnsköldsvik, Sweden) with very low contents of hemicellulose (<5 %) and lignin (<1 %).
MFC processing for section 2
TEMPO-mediated oxidation was conducted using never-dried CTMP and SP according to the method described by Saito et al. (2006). The chemical oxidations were conducted using NaClO, NaBr, and TEMPO catalyst. The dosages of NaClO used in these trials were as follows: 0, 3, 5, 7, and 10 mmol NaClO g−1 of cellulose. The pH was kept at 9–10 by adjustment with NaOH or HCl and the reaction time for the chemical oxidation was approximately 2 h. After the TEMPO-mediated oxidation, the pulp suspensions were thoroughly washed with distilled water and mechanically treated using the T-25 ULTRA-TURRAX high-speed homogeniser (IKA, Wilmington, NC, USA) to produce NFC. The homogenising equipment was set at 20,000 rpm for 30, 60, and 90 min.
It is worth noting that the ability of the crill analyser to measure nano-size particle is limited. However, main objective of the crill qualitative study was to lay a foundation for developing a rapid and robust method for a continuous online and/or offline monitoring system during the processing of nanocellulose. The method analyses the interaction between fibres and microfibrils (crill) and is based on the optical response of a suspension at two wavelengths of light, UV (this is capable of detecting tiny particles in range of approximately 100–250 nm and IR (this is capable of detecting coarse particles above 900 nm). In this work a PulpEye Analyser (PulpEye, Örnsköldsvik, Sweden) was used in evaluating the degree of fibrillation of the cellulosic nanoparticles. For all crill value measurements, we used 1 g (dry solid content) of the NFC suspension. The PulpEye analyser gives a qualitative idea of the fibrillation efficiency of an NFC suspension. More on crill methodology can be read in Osong et al. (2014), Pettersson (2010), and Steenberg et al. (1960). The crill method possesses the inherent ability to characterise particle size of a microfibril/nanofibril within a very short time frame (i.e., a few seconds) and without any damage to the nanocellulose suspension under investigation, making it a non-destructive method. It should be noted that using this method to evaluate nanocellulose makes it possible to reduce the use of time-consuming microscopy techniques used in evaluating different nanocellulose qualities. Figure 2a, b shows schematics of the crill measurement principle. The crill value is dependent on the transmittance of light, and the transmittance is a function of absorption, light scattering and concentration of the particles in the suspension. Higher crill value would means more particles per volume, thereby increasing the signal in the UV wavelength of light.