Biodegradable Nanocomposites Reinforced with Cellulose Fibrils Qingzheng Cheng Dr. Siqun Wang Dr. Timothy G Rials Tennessee Forest Products Center University of Tennessee June 15, 2007 Outline Introduction Materials Experimental Results Conclusions Acknowledgement 1
Introduction Cellulose microfibril in the cell wall is the basic structural unit generated during plant photosynthesis. It has very high strength and potential reinforcement for polymer materials. The high strength and elasticity of wood come from the composite structure of its cell walls. Kretschmann et al, 2006 Introduction Cellulose is a perfect natural composite: cellulose fibrils embed in lignin matrix. Challenging task: Isolate cellulose fibrils and embed them in a polymer matrix. Four models of polymeric wood components in wood cell wall Fengel, D., and G. Wegener. 1984 2
Introduction Two ways to make cellulose fibrils: Chemical treatments: mainly by acid hydrolysis to remove the amorphous regions, which produce nano-size fibrils called cellulosic whisker or cellulose nanocrystal. Physical treatment: such as high pressure homogenizer and grinder treatment, using high mechanical shear force to generate bundles of microfibrils called cellulose microfibril or microfibrillated cellulose with diameters from tens of nm to µm. Introduction However, the existing procedures produce low yields, severely degrade cellulose, and are not environmental friendly or energy efficient. A novel process using high-intensity ultrasonication was used to isolate fibrils from natural cellulose fibers. High intensity ultrasound can produce very strong mechanical oscillating power, so the separation of cellulose fibrils from biomass is possible by the action of hydrodynamic forces of ultrasound. 3
Introduction Nanocomposites Nanocomposite: Composite that at least onedimensional of one component is at the nanometer scale (1-100 nm). Tow biodegradable polymers (PLA, PVA) were reinforced with cellulose fibrils to make biodegradable nanocomposites. Materials Raw Fibers: Regenerated cellulose fiber (Lyocell fiber) Pure cellulose fiber Pulp fiber Microfibrillated cellulose (MFC, Japan) Polymers: Poly(vinyl alcohol) (PVA) Poly(lactic Acid) (PLA) 4
Experimental Isolate fibrils from cellulose fibers: High intensity ultrasound used to generate cellulose fibrils Centrifuge used to separate fibrils from the treated materials Experimental Composite from PVA and fibrils by casting: PVA (10%) water solution Fibril (or mixture of fibril & fiber) aquatic suspension + Stir and ultrasonic Heat treatment (~70 ºC) > 4 h Room temperature evaporation 5
Experimental Composite from PLA and fiber and fibrils mixture by filtration system followed by compressive molding (Sandwich); Sandwich structure formation for compressive molding Experimental Water retention value (WRV): measure the degree of homogenization or microfibrillation, which is related to fibril and microfibril surface and volumetric phenomena WRV = W Water W Sample 100% where W Water is the weight of water in the sample after centrifugation, and W Sample is the dry weight of the sample According to TAPPI UM 256 by centrifuge with the force of 900 g and 30 min. 6
Polarized light microscopy (PLM, Olympus-BX51): Observe the morphology of the fibrils, the surfaces and crosssections of the reinforced composites. Experimental http://www.olympusamerica.com/seg_section/seg_product.asp?product=665 Experimental Scanning electron microscopy (SEM, LEO 1525): Observe the morphology of the fibrils and the crosssections of the reinforced composites after tensile test. http://www.gcemarket.com/gcemarket/index.nsf/odwf?readform&id=gcem-6hlgk8&gclid=cn6y2qj58yscfqungqodaq9vbw 7
Atomic force microscopy (AFM, PASI XE-100 ): Observe the morphology of the fibrils and the crosssections of the reinforced composites. Experimental Experimental Composites characterization by tensile test (ASTM D1708): Speed: 1 mm/min Specimens: (dogbone shapes) width: 5 mm Length: 20 mm (total ~40 mm) Instron testing machine (model 5567) 8
Fiber and fibril characterization (PLM) Raw Lyocell fiber: Diameter: 11 µm, Length < 1 mm After 30 min treatment Results Fiber and fibril characterization (SEM, AFM) Raw Lyocell fiber: Diameter: 11 µm, Length < 1 mm After 30 min treatment 9
AFM images: Lyocell fibrils (30 nm to 203 nm) Results Fiber and fibril characterization (SEM, AFM) Pure cellulose: Width: ~20 µm, Length: 200 µm After 30 min treatment 10
Fiber and fibril characterization (PLM, SEM) Pulp fiber: Width: ~30 µm, Length < 1 mm After 30 min treatment Results MFC (AFM): large range of fibril diameters 11
Water retention value (WRV): increasing after treatments indicated that the fibers got smaller (lyocell fiber). 300 Water retention value (%) 250 200 150 100 50 0 71 92 165 230 Control 10min 20min 30min Results Tensile test: Typical stress-strain curve of PVA and its composites 160 140 120 Stress (MPa) 100 80 60 40 20 0 Neat PVA 2% fibril 6% fibril 10% fibril 0 10 20 30 40 Strain (mm/mm) 12
Tensile properties (E Modulus (MOE)): PVA Composites by casting Tensile M odulus (M Pa) 9000 8000 7000 6000 5000 Untreated MFC Treated Fibril 4000 0 2 4 6 8 10 12 Fiber and Fibril Contents (% W/W) Results Tensile properties (Strength (MOR)): PVA Composites by casting Tensile Strength (MPa) 150 130 110 115 Untreated MFC 127 Treated Fibril 122 132 90 0 2 4 6 8 10 12 Fiber and Fibril Contents (% W/W) 13
Tensile test: Typical stress-strain curve of PLA and its composites 70 60 Stress (MPa) 50 40 30 20 10 0 Neat PLA 10% treated fiber 20% treated fiber 0 2 4 6 8 10 12 Strain (mm/mm) Results Tensile properties (MOE &MOR): PLA composites 5000 70 MOE (MPa) 4000 3000 2000 2476 3053 2979 3821 3419 MOR (MPa) 60 50 40 51 48 51 58 42 1000 Control RCF Treated 30min CMNF 0 0 5 10 15 20 25 Fiber and fibrils content 30 Control RCF Treated 30min CMNF 20 0 5 10 15 20 25 Fiber and fibrils content Control: MOE increased 20 & 38%. MOR same, & decreased -18%. Treated: MOE increased 25 & 54%. MOR decreased -6% increased 14%. 14
SEM observations of composites: PVA Composites, mixed fiber reinforced (left), fibril reinforced (right) Results SEM observation of composites: PLA Composites by mixed fibers (treated Lyocell fiber) 15
PLM observations of composites: PVA Lyocell untreated and treated fiber (10%) reinforced composites. Untreated Surface cross-sections Untreated Surface cross-sections Results PLM observations of composites: PVA Lyocell fibril (10%) reinforced composites. Surface (left), cross-sections (right) 16
PLM observations of composites: PVA MFC fibril (10%) reinforced composites. Surface (left), cross-sections (right) Results AFM observation of PVA Lyocell fibril (10%) composites Topography (left), Phase (right) 17
AFM observation of PVA MFC fibril (10%) composites Topography (left), Phase (right) Conclusions 1. Ultrasonication can be used to isolate fibrils from Lyocell fiber, pure cellulose and pulp fibers, obtained a mixture of fiber and fibrils; 2. The mixed fibril and separated fibrils can be used to increase the tensile modulus and strength of biodegradable polymers, such as PLA and PVA; 3. PLM, SEM, and AFM observations show that the size of the fibrils have a wide diameter range from tens of nm to µm; the adhesions between the polymers and fibrils or fibers were poor; 4. The fibrils isolated from the fibers may be the reason that the tensile MOE and MOR of the composites reinforced by treated fibers and separated fibrils were higher than those of the composites reinforced by untreated fibers. 18
Acknowledgement Dr. David Harper at Tennessee Forest Products Center UT for his help about AFM, Dr. Cheng Xing for AFM specimen preparation; Dr. John R. Dunlap of the Division of Biology, UT, for their valuable assistances in SEM experiments ; Lenzing company for their supply of Lyocell fibers. Creafill Fiber Corp. for their providing pure cellulose; Kimberly-Clark Worldwide, Inc. for their providing pulp fiber; USDA Wood Utilization Research Program and Tennessee Agricultural Experiment Station project # 96 for funding. Thank You! June 15, 2007 19