Cellulose Chemistry and Technology, 40 (5), 313-317 (2006) FORMATION NANO-STRUCTURE OF MICROCRYSTALLINE CELLULOSE Received October 6, 2005 MICHAEL IOELOVICH and ALEX LEYKIN NanoAdd Ltd, Migdal HaEmek 23100, Israel Structural characteristics of the microcrystalline celluloses (MCC) obtaining by heterogeneous acid hydrolysis of cotton cellulose at various conditions with the following mechanical disintegration of non-dried samples in a high-pressure homogenizer have been studied. In order to obtain nano-scale MCC having C1- structure the narrow concentration interval of the mineral acids (7-9 M) together with mechanical disintegration should be used. The correlation between dispersity degree of MCC-particles and the acidity function was established. Structural transformations of cellulose fibers at the hydrolysis and mechanical disintegration are discussed Key words: microcrystalline cellulose, nano-cellulose, structure. INTRODUCTION Microcrystalline cellulose (MCC) is produced by depolymerization of cellulose materials with solutions of mineral acids at increased temperatures up to level-off degree of polymerization (LODP) from 120 to 250 that approximately corresponding to average length of individual nano-crystallites of cellulose. MCC is used mainly as an inactive ingredient of tablets, cosmetic formulation and food products, as well as a filler and special additive for some technical applications. Despite that process of MCC-forming was studied during about 60 years beginning with early investigations of Rånby, Sharples, Battista, et al., 1-3 certain substantial problems were not solved. One from these problems is connected with contradiction between nano-structure of individual crystallites and micron sizes of MCC-particles. As it is known, cellulose fibers are built from superfine fibrils having nano-diameter and each such nanofibril contains ordered crystallites and low ordered non-crystalline (mesomorphous and amorphous) domains. 4-6 Cellulose chains pass through lots of crystallites and non-crystalline domains and bind them with chemical 1,4-β-glycoside bonds. In cellulose materials of various origins, 1
length of crystallites is 60-120 nm and width 4-10 nm, i.e. they are nano-scale objects. The crystallites are strong and inaccessible structural elements, while non-crystalline domains are weak and accessible places of nanofibrils. Therefore, the acid catalyst destroys glycoside bonds mainly in non-crystalline domains. The breaking process of these bonds in non-crystalline domains of cellulose nanofibrils under effect of acid catalysts should facilitate releasing of individual crystallites and forming free nano-crystalline particles. In the fact, acid depolymerization of the cellulose even to LODP leads to forming micron-scale particles. For example, the average particle size of typical MCC Vivapur 105 is 25 µm, of Avicel PH-101 is 50 µm, while the average size of Avicel PH-102 particles is about 100 µm. Thus, contradiction between the nano-crystalline structure and large sizes of the real MCC particles was not solved and it needs to explain. The main purpose of the present paper is to perform a detailed analysis of structural modifications of cellulose fibers at the MCC formation process. EXPERIMENTAL The pure Whatman's cotton cellulose (DP=2500, 98.5% α-cellulose) was used as initial cellulose materials. The initial cellulose was treated with water solutions of the hydrochloric or sulfuric acids at increased temperatures. In order to prevent aggregation of the MCCparticles, all experiments were carried out using never dried or non-water dried samples. The hydrolyzed cellulose was washed and squeezed on vacuum filter. Then, a non-dried wet cake was diluted with water and dispersed by high-pressure homogenizer APV-2000 (at 100 MPa). The particle size distribution and the average particle size (D 50 ) of aqueous MCCsuspensions were tested by a method of the laser-light scattering using Malvern's Mastersizer-2000 apparatus. Samples for scanning electron microscopy and X-ray were prepared by squeezing of MCC-suspensions, washing of the wet-cake with ethanol, acetone and pentane, vacuum drying and comminuting of cellulose powders in the agate mortar. Scanning electron micrographs (SEM) were obtained with a Hitachi S-430 apparatus; the cellulose particles were preliminary evaporated with carbon and coated with gold. Diffractometer Rigaku- 2
Ultima Plus (CuK α radiation, λ=0.15418 nm) was used for X-ray investigations. Degree of cellulose crystallinity (X) and average lateral size of crystallites (L c ) was calculated according to improved equation methods. 5-7 Transmission electron microscopy (TEM) investigations were carried our by means of a Philips CM200 electron microscope. The cellulose suspension after homogenization step was diluted with ethanol and treated with ultrasound disintegrator. The diluted (0.01-0.05%) suspensions were dropped onto a carbon-coated grid and vacuum-dried. Average degree of polymerization (DP) of the cellulose samples was measured by Cuen-viscosity method. 8 RESULTS AND DISCUSSION Study of cellulose depolymerization showed, hydrolysis of initial cellulose materials with boiled 2-3 M acids leads to decrease of average DP from 2500 up to LODP-value of 190 that corresponding to average length of the nano-crystallites l c = 98 nm. The experiments with non-dried hydrolyzed cellulose evidence that after mechanical treatment of 1-2% MCC suspensions in the high-pressure homogenizer the average particle size reduces up to 2-4 µm (Fig. 1), while the volume part of submicron particles is less 10%. Thus, non-dried cellulose hydrolyzed up to LODP gives mainly the micron-scale particles and only low part of the submicron particles even after hard disintegration conditions. Short-time (15-30 min) treatment of the cellulose with hot 4-9 M acid causes reducing of the LODP-value up to 80-120 corresponding to average length of the nano-crystallites l c = 40-60 nm (Table 1). After the subsequent disintegration of the suspensions in the highpressure homogenizer the particle size decreases. At the acid concentration of 4-5 M, the average particle size after chemical-mechanical treatment reaches about 1 micron, while increasing of the acid concentration to 6-9 M leads to forming nano-scale cellulose suspensions with the average particle size, D 50, about of 200 nm (Fig. 2). 3
Volume (%) 7 6 5 4 3 2 1 Particle Size Distribution 0 0.01 0.1 1 10 100 400 Particle Size (µm) #45-H, Monday, March 08, 2004 5:09:14 PM Fig. 1. - Particle size distribution after cellulose hydrolysis with 3M acid and mechanical disintegration of the non-dried MCC-suspension TABLE 1 Main characteristics of the nano-cellulose Characteristics Value Average particle size (D 50 ), nm 180-200 Degree of polymerization (DP) 80-120 Length of crystallites (l c ), nm 40-60 Lateral size of crystallites (L c ), nm 10-12 Degree of crystallinity (X), % 78-80 Crystalline modification (CM) CI 4
Particle Size Distribution 20 Volume (%) 15 10 5 0 0.01 0.1 1 #51, Thursday, April 01, 2004 4:49:00 PM Particle Size (µm) Fig. 2. - Particle size distribution after cellulose hydrolysis with 8M acid and mechanical disintegration of the non-dried MCC-suspension TEM investigations showed, the obtained nano-cellulose suspension contains anisometric particles having length of 150-300 nm and lateral sizes of 20-40 nm (Fig. 3). The averaged rotating diameter of nano-particle agglomerates corresponding probably to the average particle size, D 50, of 200 nm measured by the laser-light scattering method. Fig.3. - TEM of nano-cellulose particles (Bar s scale is 200 nm) 5
In contrast to existing methods for preparation of the oligomeric nano-products having the CII type of crystalline structure, 9,10 the present method allows obtaining the nanocellulose with keeping of the C1 crystalline modification inherent to natural cellulose (Fig. 4). As it follows from X-ray calculations of hydrolyzed cellulose, the nano-cellulose has crystallinity degree of 78-80% and lateral size of the nano-crystallites is 10-12 nm (Table 1). So, the obtained nano-scale particles consist of lateral aggregates of individual nanocrystallites. Fig. 4. - X-ray pattern of the nano-cellulose To explain the experimental results, the macro-and microstructure of cellulose fibers and their transformation at cellulose isolation, hydrolysis and mechanical disintegration should be discussed in detail. The cotton fibers have length of 5-50 mm, width is 10-30 µm and thickness of the cell wall of 3-6 µm. 11,12 Hollow capillary or lumen extends along the fiber. Cellulose fibers contain various defects or dislocations: pores, cracks, nodes, compression failures, thin places and other damages. These dislocations are weak points for chemical attack and mechanical forces. The wall of the natural cotton fibers is built from external waxy layer - cuticle, primary P and secondary S walls. The thin cuticle and P-wall have nano-size thickness. The S-wall has thickness of 3-5 µm and composed of three layers S1, S2 and S3. The dominating wall s S2-layer of 2-4 µm contains lots of thin lamellas built from fibrillar 6
bundles, having different orientation towards the fiber axis. These bundles consist of elementary fibrils having lateral size 4-10 nm. The elementary and fibrillar bundles of natural fibers are attached with each other by means of the amorphous matrix. Extraction of matrix components, i.e. hemicelluloses, waxes and some other substances, at isolation of pure cellulose from natural cotton fibers causes damage and removing of the external wall layers and obtaining pure cellulose having mainly the S2-layer. Release of the fibril surface from the amorphous matrix permits direct contact of the fibrils with each other and forming lateral aggregates via co-crystallization of adjacent crystallites. 13-15 As a result of the lateral co-crystallization, the primary nano-crystallites irreversible aggregate in more large crystallites (Table 2), while the elementary fibrils form secondary microfibrils and lose their individuality. Hydrolysis of the cellulose with diluted mineral acids promotes an additional lateral co-crystallization of the MCC due to breaking accessible intrafibrillar glycoside bonds and increasing mobility of the crystallites. The secondary microfibrils are cross-linked by means of local crystalline points. Thus, the cell wall of isolated cellulose fibers is bound by three-dimensional fibrillar network. Sample Natural cotton Cotton cellulose isolated by soda cooking MCC of cotton cellulose (hydrolysis with 2.5 M HCl) TABLE 2 Lateral sizes of crystallites L, nm L n = 5-6 8-9 10-12 L/L n 1 1.5 2 Based on the above-described arrangement of isolated cellulose fibers, it can to explain MCC-forming process more detailed. At short-time cellulose hydrolysis with diluted acids (e.g. with 1 M acid for 15-30 min) to DP > LODP and mechanical disintegration, the cellulose fibers are broken first of all in weak dislocation places with forming of rod-like fragments of the fibers having length of 20-100 µm (Fig. 5). 7
Fig.5. - SEM of MCC particles obtained by short-time cellulose hydrolysis with 1M acid (Bar s scale is 50 µm) However, even after hydrolysis up to LODP, e.g. with 2-3 M acid, and the following disintegration of cellulose suspensions in the high-pressure homogenizer, instead of nanoparticles the micron-scale (2-4 µm) fragments of the fiber s wall are obtained, because the damaged cellulose microfibrils are cross-linked with each other by means of relative strong local crystalline joints. When the acid concentration is enough high (6-9 M), it causes not only hydrolysis of non-crystalline domains, but also breaking the local crystalline contacts between partially hydrolyzed microfibrils. After subsequent intensive mechanical disintegration the nanofragments of the microfibrils are released. As it can see from TEM results, these nanofragments are 20-40 nm in the lateral direction and 150-300 nm in the length. Since individual nano-crystallites have diameter of 10-12 nm and length of 40-60 nm, each such nano-particle is built in the lateral direction from aggregates comprising about 2-4 crystallites, while lengthwise the nano-particle consists of about 3-5 crystallites, as it can see from the model represented in Fig. 6. 200 nm Fig. 6. - Structural model of the nano-particle 30 nm 8
The nano-fragments of cellulose microfibrils have defective structure because these contain nano-defects forming as a result of partial removing of non-crystalline domains caused by the hydrolysis. Durability of the nano-particle is provided by means of overlapping these defects with the long nano-crystalline aggregates. The general correlation of the average dispersity of MCC-particles (AD=1/D 50 ) after steps of hydrolysis and homogenization on the acidity function (Ho) was found (Fig. 7). This correlation demonstrates that the average dispersity of the particles increases very low up to boundary value of the acidity function (-H b 2). At -Ho > -H b, a sharp rise in the dispersity value is taken place like to phase transition of the first type caused with "melting" of the local crystalline contacts between nano-fragments of the hydrolyzed microfibrils and forming of the free nano-particles. CONCLUSION Microfibrils of the isolated cellulose fibers form a network bound with local crystalline points. Hydrolysis of cellulose with diluted acids up to LODP causes breaking intrafibrillar 1,4-β-glycoside bonds in the non-crystalline regions that should release the free nanoparticles. However, the following mechanical disintegration of the hydrolyzed cellulose leads to forming micron-scale particles of MCC only, because the nano-scale fragments of microfibrils are cross-linked with crystalline joints hindering their separation at the mechanical treatment. In order to obtain nano-scale cellulose particles, the local crystalline joints should be broken. It can perform by using either very intensive mechanical stresses or increased reagent's concentrations required to etching and removing of the local crystalline points. As it was established in this investigation, the enough concentrated acid having acidity function H from -2 to -4 is able to remove the local crystalline contacts between nanofragments of the hydrolyzed microfibrils without damage in the basic C1 crystalline structure. It promotes release of C1 nano-particles after the following intensive mechanical treatment of the hydrolyzed cellulose. 9
6 5 4 AD 3 2 1 0 0 1 2 3 4 5 -Ho Fig. 7. - Dependence of the average dispersity (AD, µm -1 ) of cellulose particles on the acidity function (Ho) The relative narrow interval of the acidity function should be used to obtain nanocellulose having C1 crystalline modification at the same disintegration conditions. If -Ho < 2, the micron-scale particles mainly are formed. On the other hand, if -Ho > 4, the reagent begins penetrate into the C1 crystalline structure and distorts it due to forming inclusion complex transforming after regeneration in the CII crystalline modification. REFERENCES 1. B.G. Rånby, E.H. Immergut and H.F. Mark, Chem. Berichte, 89, 526 (1956) 2. A. Sharples, Trans. Faraday Society, 54, 913 (1958) 3. O.A. Battista and P.A. Smith, Ind. Eng. Chem., 54, 20 (1962) 4. H. Dolmetsch and H. Dolmetsch, Das Papier, 22, 1 (1968) 5. M. Ioelovich and M. Gordeev, Acta Polymerica, 45, 121 (1994) 10
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