Chapter 9
The Structure and Mechanical Properties of Cellulose Nanocomposites Prepared by Twin Screw Extrusion 1
2
1
Aji P. Mathew , Ayan Chakraborty , Kristiina Oksman , and Mohini Sain
Downloaded by UNIV OF GUELPH LIBRARY on August 9, 2012 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch009
3
1
Department of Engineering Design and Materials, Norwegian University of Science and Technology, Trondheim, Norway Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada Centre for Biocomposites and Biomaterials Processing, Faculty of Forestry and Chemical Engineering, University of Toronto, Toronto, Ontario, Canada 2
3
The goal of this work has been to prepare cellulose nanocomposites of polylactic acid (PLA), cellulose nano whiskers (CNW) and microfibers (MF). Nanocomposites were prepared by pumping an aqueous dispersion of MF and CNW into the PLA during extrusion. The prepared materials were studied using different microscopy methods (TEM, AFM, SEM), X-ray, dynamic mechanic thermal analysis (DMTA) and conventional mechanical testing. The MF was shown to form a network of fibrils while CNW existed as needle shaped crystallites after the isolation process. DMTA and tensile tests indicated no significant improvement in mechanical properties of the composites. This may be attributed to poor dispersion of microfibres and nanowhiskers in PLA.
114
© 2006 American Chemical Society
In Cellulose Nanocomposites; Oksman, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
115
Downloaded by UNIV OF GUELPH LIBRARY on August 9, 2012 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch009
Introduction Cellulose is the most abundant, renewable and biodegradable natural polymer on earth. Cellulose fibers are present in plant cell walls in combination with hemicelluloses, lignin, waxes etc (/). These fibers consists of bundles of microfibrils where the cellulose chains are stabilized laterally by hydrogen bond between hydroxyl groups. Cellulose microfibrils can be separated from various sources by chemical and mechanical treatments. This cellulose fibril can be about 5-10 nm in diameter and the length varies from 100 nm to several micrometers depending on the source (1-3). Each microfibril consists of monocrystalline cellulose domains linked by amorphous domains. On acid hydrolysis the microfibrils undergo transverse cleavage along the amorphous regions into microcrystalline cellulose or whiskers. Due to the near perfect crystalline arrangement of whiskers they have high modulus and act as efficient reinforcing materials (4). Properties of cellulose crystallites from earlier reports are shown in Table I (2,3). However, the high reinforcing potential of the microfibrils and the whiskers has not been fully exploited and utilized in a commercial scale yet, even though attempts are being made in this direction.
Table I. Properties of Cellulose Whiskers Property Length (nm) Diameter (nm) Aspect Ratio (I/d) Tensile Strength (MPa)* Ε-Modulus (GPa)*
Cellulose crystallites 300-600 5-10 20-60 10000 150-250
•ref. (2,3)
Cellulosic reinforcements when combined with polymers from renewable resources are the potential and effective way to produce the so-called green materials with improved performance. Biopolymers including cellulosic plastics obtained from wood (CA, CAB), starch, polylactic acid (PLA) derived from corn and poly hydroxyl alkanoates (PHAs) produced by bacteria are very interesting matrix polymers in this context. (5-9). There are several works on cellulose based nanocomposites and have reported exceptional properties. Dufresne and coworkers have prepared nanocomposites by solvent casting of various water soluble polymers using wheat straw, tunicin, chitin and sugar beet as reinforcements (10-14). They have reported the tangling effect of microfibrils
In Cellulose Nanocomposites; Oksman, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
116
Downloaded by UNIV OF GUELPH LIBRARY on August 9, 2012 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch009
from sugar beet on poly (styrene-co-butyl acrylate) (15). Nakagaito et al. have separated microfibrils from kraft pulp by a homogenizing process and used it as reinforcement in PF resin (16, 17). Cellulose nanocomposites based on cellulose nanocrystals from bacterial cellulose and cellulose acetate butyrate prepared by solution casting was reported by Grunert and Winter (18). Wu et al. have reported about elastomeric PU/cellulose nanocomposites prepared by in-situ polymerization (19). However, all these studies on cellulose nanocomposite processing have been limited to laboratory scale, and focused on solvent casting. In this work, melt extrusion using a twin screw extruder was explored as a technique of preparing cellulose nanocomposites. The advantage of melt extrusion lies in the fact that unlike solvent casting, this method can be scaled up to an industrial level. For this purpose, two different cellulose reinforcements, cellulose nanowhiskers (CNW) and microfibres (MF), were considered in a polylactic acid (PLA) matrix. PLA is a versatile polymer made from renewable agricultural raw materials and is fully biodegradable. Furthermore, PLA possesses excellent processibility, good stiffness and strength (20, 21). The main drawbacks with PLA are low toughness and thermal stability. The reinforcements used in this study are based on wood products, viz, wood pulp and microcrystalline cellulose (MCC), which are commercially available in bulk. We have recently reported the preparation of polylactic acid based cellulose nanocomposites by melt extrusion. In this study DMAc/LiCl solution was used as the dispersion medium for cellulose whiskers. However, this system showed degradation during high temperature processing (22). CNW was isolated from MCC by acid hydrolysis and dispersed in aqueous medium. Cellulose whiskers are stiff, individual, crystalline needle shaped entities. The microfibers were separated from wood pulp by cryocrushing and filtration. Microfiber (MF) in this context refers to fibers of cellulose which are 1 tm and less in diameter. The structures of the nanoreinforcements and the nanocomposites were studied using microscopic methods and X-ray diffraction. The material performance was evaluated by dynamic mechanical thermal analysis (DMTA) and conventional tensile testing.
Experimental
Materials Matrix: Poly Lactic Acid (PLA), Nature Works™ 4031 D, was supplied by Cargill Dow LLC, Minneapolis, USA. The density, glass transition temperature (T ) and melting point are 1.25 g/cm , 58 °C and 160 °C respectively. PLA has a 3
g
In Cellulose Nanocomposites; Oksman, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
117 molecular weight (Mw) ranging between 195,000 and 205,000 g/mol, and a melt flow index (MFI) of 2-5 g/min.
Downloaded by UNIV OF GUELPH LIBRARY on August 9, 2012 | http://pubs.acs.org Publication Date: July 13, 2006 | doi: 10.1021/bk-2006-0938.ch009
Reinforcements. Micro Crystalline Cellulose (MCC), VIVAPUR® 105, supplied by J. Rettenmaier & SÒHNE GMBH + CO (Rosenberg, Germany), is commercially available and was used as raw material for the isolation of the whiskers. It is > 93 % pure microcrystalline cellulose and the particle size is between 10-15 μιη. Bleached softwood kraft pulp for generating the MFs was supplied by Kimberly-Clark Forest Products Inc., (Terrace Bay, Ontario, Canada). It has the following composition, cellulose 88 %, hemicellulose 11%, Lignin
