MEDIACIÓN

One of the key features of good nanocrystals the ability to disperse well in the medium/solvent/polymer used. Dispersibility of CNC was investigated by dispersing them in deionised water and/or dimethylformamide (DMF) and observing the stability of dispersion over some period of time. Photographs were taken immediately after 2 minutes of ultrasonication, and at 1 h, 24 h and 10 days. Even though it was difficult to differentiate the dispersion level between the samples, as shown in Fig. 4.11, all CNC dispersed in deionised water appeared to be stable for 10 days. Meanwhile, CNC dispersed in DMF showed good dispersion, but sedimented within 24 h. The very hydrophilic CNC and water molecules tend to associate through strong hydrogen bonding which contributes to the stability of aqueous suspension, whereas CNC in DMF suggest a major aggregation of the suspended particles38 in the less polar organic solvent suspension. Further investigation of suspension stability in deionised water was determined by the zeta potential value (Table 4.5). In agreement with the physical appearance of the suspension, the zeta potential for all CNC suspensions recorded almost similar values from 33 – 38 mV. This also indicates the

57 nanoparticle has moderate dispersibility in water.39, 40 No element of P and S has been traced due to their low levels and their inability to be detected by the instrument (minimum sensitivity is 1%). The dispersion ability of CNCs was also supported by the charge (sulphate and phosphate) concentration. CNC samples hydrolysed by phosphoric acid first show higher charge concentration as phosphate has stronger inhibition than sulphate anion.41, 42

Figure 4.11 Photographs of CNC-PS, CNC-SP, CNC-PH and CNC-HP isolated via mixed acid hydrolysis system were dispersed in deionised water and DMF, taken after ultrasonication, 1 hour, 24 hours and 10 days.

In deionised water In DMF After Ultrasonication 1 hour 24 hours 10 days

58 Table 4.5 Characterisation of CNC on elemental analysis, zeta potential and amount of charges.

4.4 Conclusion

Cellulose nanocrystals from microcrystalline cellulose (MCC) were isolated via acid hydrolysis using either single acid or mixed acid systems. In the single acid system, isolation of CNC using cotton via sulphuric and phosphoric acid was successfully obtained in terms of shape, size and dimension in accordance with previous studies.18, 23 The disadvantage of isolation via sulphuric acid hydrolysis was related to the low thermal stability of CNC produced due to sulphates which tend to promote the dehydration process. CNC obtained via acid hydrolysis process using phosphoric acid (according to the reported protocol) have shown enhanced thermal stability. However, this hydrolysis process used a higher total solid to liquid ratio (1:185) compared to sulphuric acid hydrolysis (1:75) and consumed a large volume of solvent.

To reduce the solid to liquid ratio, MCC was chosen as the cellulose source; as the initial smaller particles appear to have distinctively more favourable characteristics for the production of CNCs at low solid: liquid ratio. At the solid to liquid ratio of 1: 77, under optimised conditions, the CNC (CNC-PB1) hydrolysed at 70°C, 30 min have exhibited morphology and dimensions almost the same as typical CNC isolated from cotton via sulphuric acid hydrolysis. Significantly, based on the production yield, the acid consumption per gram of CNC has been reduced almost 71% in comparison of reported protocol for phosphoric acid hydrolysis using cotton. In addition, this was achieved at lower temperature and less time.

In a mixed acid hydrolysis system, CNC with enhanced thermal stability and dispersibility were successfully produced using phosphoric acid in combination with hydrochloric acid. By using

Sample Elemental Analysis Zeta Potential (mV) Charge concentration (mmol/kg cellulose) C (%) H (%) CNC-PS 41.63 ±0.03 6.30 ±0.09 33.24 ±1 98.0 CNC-SP 41.59 ±0.07 6.42 ±0.01 34.46 ±2 53.4 CNC-PH 41.53 ±0.01 6.27 ±0.05 38.86 ±4 102.0 CNC-HP 41.50 ±0.05 6.29 ±0.13 35.28 ±4 46.0

59 phosphoric acid as a basis, the acid consumption was successfully reduced by approximately 52% compared to our optimised hydrolysis process to produce CNC-PB1.

Overall, if the comparison was made between the volumes of commercially available concentrated phosphoric acid required for producing per gram of CNCs, via reported protocol for CNCs from cotton using phosphoric acid and our mixed acid method, acid consumption was reduced up to 86%. The summary of CNCs (the best CNC obtained from single acid (CNC-PB1) and mixed acid (CNC- PH) produced from MCC based on their properties as shown in Table 4.6. Indeed, less acid consumption in producing CNC with good thermal stability and dispersibility makes a remarkable contribution to an environmentally friendly and economical process.

Table 4.6 Properties evaluation of CNC produced from MCC via acid hydrolysis.

Rating * 1 2 3 Thermal stability CNC-PB1 CNC-PH Dispersibility (deionised water) CNC-PB1 CNC-PH Environmental impact (Acid consumption) CNC-PB1 CNC-PH

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This chapter based on Amin et al. RSC Adv.2015, 5, 57133-57140. Chapter 5

Isolation of Cellulose Nanocrystals via a Scalable Mechanical Method

The production of rigid rod-like cellulose nanocrystals (CNC) via more scalable methods is necessitated by an increasing demand for CNC in various industrial sectors over the last few years. Contemporary protocols involve a consumption of large amounts of strong acids, enzymatic treatments, ultra-sonication and combinations thereof. In an attempt to address this scalability challenge, we aimed to isolate CNC via a scalable mechanical method i.e. high energy bead milling (HEBM). An aqueous dispersion of commercially available microcrystalline cellulose (MCC) was micronized through a HEBM process. This process was optimised by varying the concentration (0.5-2 wt. %) and time (15-60 min) parameters, in order to obtain a high yield of well-separated CNC as characterised by transmission electron microscopy (TEM). Micronisation of cellulose via the HEBM method under mild conditions resulted in cellulose nanocrystals with an average aspect ratio in the range of 20 to 26. The nanocrystals also retained both their crystallinity index (ICr) (85 to 95 %) and

thermal stability described in terms of onset degradation temperature (Tonset) (230 – 263 °C). The production yield of CNC from MCC via this process ranged between 57 to 76 %. In addition, we found that micronisation of the MCC in presence of dilute phosphoric acid also resulted in CNC with an average aspect ratio ranging from 21 to 33, high crystallinity (88-90 %) and good thermal stability (Tonset 250 °C). In this study, we demonstrate the micronisation of commercially available MCC into CNC and describe their dimensions and properties after acid treatment and HEBM. Furthermore, we are able to recommend the use of this scalable milling process to produce rod-like cellulose nanocrystals having a thermal stability suitable to withstand the melt processing temperatures of most common thermoplastics.

63 5 .1 Introduction

Recently, nanoscale cellulose particles have gained much interest from both academia and industry as a new class of renewable nanomaterials due to their useful mechanical properties (eg. high specific strength and stiffness and thermodimensional stability) l combined with their nanoscale crystalline and fibrous morphology, chemically tuneable surface functionalities, an ability to be obtained in various dimensions (esp. aspect ratio) and renewability.1, 2

Depending on the processing methods and the sources, they can be obtained as rod-like low- aspect ratio nanocrystals, filament-like long nanofibrils and sphere-like nanocrystals.2-5 The properties of individual rod-like or rice like cellulose nanocrystals (CNC) are drawing great interest when compared with other engineering materials like aluminium,6-8 steel7 and glass fibre.9 These properties greatly recommend them as advanced reinforcing or functional fillers for thermoplastics, thermosets and elastomers. They have been explored as fillers and rheology modifiers in various fields like thermoplastics,10-12 foams,13 aerogels14 and polymer electrolytes.15, 16 Isolation of CNC via sulphuric acid hydrolysis is not ideal as it requires a considerable consumption of solvent (acid) and time. Furthermore, the resulting product also exhibits poor thermal stability (attributed to the dehydration mechanism facilitated by the sulphate groups) 17 and is therefore sometimes unsuitable for further processing at the elevated temperatures (180-250 °C) typically employed in the manufacturing of many thermoplastic polymers. Hence, it becomes critical to produce CNC with enhanced thermal stability. To date, alternative approaches have included the use of some mild mineral and organic acids18, 19 or enzymatic methods in combination with ultrasonic or microwave irradiation.20

Recently, highly thermally stable CNC have been produced via mild acid hydrolysis (phosphoric acid) and hydrothermal treatment (hydrochloric acid) at laboratory scale.19, 21 However, these methods are severely limited by low yields and poor scalability due to the high consumption of mild acids (solvents) and time, respectively. Mechanical methods have also been explored for the preparation of nanoscale cellulose particles, either as part of the production process using combinations of acid hydrolytic, oxidative, and enzymatic treatments, or directly. These mechanical methods include ultrasonication20, 22, microfluidisation,23 high pressure homogenisation24 or ball milling.5, 25-27 Among these methods, ball milling has many advantages such as the possibility of both dry and wet

64 milling, temperature control and scalability in both batch and continuous formats. For micronising cellulose, wet conditions are preferred as the fluid can create a buffer between bead and cellulose particles, thereby minimising the damage to the crystalline structure.25 Recently, ball milling methods have been successfully employed for isolating nanoscale cellulose particles whereby water28, and organic solvents29 were employed as the milling media resulting in the retention of the inherent crystallinity of the particles. However, most of the mechanical fibrillation methods reported (including wet ball-milling methods), generated a final product of filament like nanofibrils, called nanofibrillated and microfibrillated cellulose (NFC/MFC) or cellulose nanofibres (CNF), which can be characterized with a diameter in nanometers or tens of nanometers and a length of up to several microns.1, 30, 31

These filament-like nanoparticles have been explored in several applications where fibre entanglements are required. However, in polymer reinforcements, cellulose nanofibrils can raise an issue in terms of dispersibility and viscosity increases in a polymer matrix, especially with an aspect ratio above 100.2 Thus, in this study, we aimed at producing short ‘rod-like’ or ‘rice-like’ cellulose nanocrystals with an aspect ratio below 100 primarily using aqueous high-energy bead milling (HEBM), and while also investigating various mild chemical processing conditions in parallel.

As shown in Figure 5.1(a), HEBM operates via an impact mechanism to micronise the particles. When centrifugal force is generated in the mill, beads are forced to rotate around the mill wall. The reverse rotation of disc also applies centrifugal force in the opposite direction leading to the transition of balls to the opposite walls of mill providing an impact effect to micronise the materials in between the beads32. Furthermore, HEBM was compared with ultrasonication as one of the mechanical methods explored. As shown in Figure 5.1(b), ultra-sonication operates on a well-known high-speed shear mechanism. Upon ultrasonication, high-speed liquid jets are created. These liquid jets pressurize the liquid locally between the particles at high speed (1000 km/hr) to separate or fibrillate them and to promote additional particle collisions, so as to result in very small particles.20 By evaluating the resulting dimensions, crystallinity and the properties of CNC produced from both methods, we hereby demonstrate the potential of HEBM to cost-effectively produce CNC with good thermal stability in a scalable quantity.

65 Figure 5.1(a) High energy bead milling33 and (b) ultrasonication working mechanism. Reproduction of image from 20 with permission from Elsevier.

5.2 Experimental