Manufacturing Novel Structured Plastic Materials for Strength and Functionality. Dave Zumbrunnen

Transcription

1 Manufacturing Novel Structured Plastic Materials for Strength and Functionality Dave Zumbrunnen Warren H. Owen Duke Energy Professor Department of Mechanical Engineering, Clemson University Tel A Advanced Transportation Workshop of the Global Climate & Energy Project, Stanford University, October 10, 2005

2 Acknowledgements: NSF, US DOC, DARPA, 3M Company, ILC Dover, Inc., Dow Chemical Company Research Assistants: V. Chougule, R. Danescu, A. Dhoble, B. Gomillion, S. Inamdar, A. Joshi, P. Keener, Y. Liu, C. Mahesha, K. Miles, B. Nagarajan, Y. Parulekar, R. Subrahmanian, P. Verma, W. Wright, D. Zhang Post-doctoral Research Associates: R. Danescu, O. Kwon, B. Kulshreshtha Faculty Colleagues: N. Coutris, M. Ellison, R. Kimmel

3 Outline Historical context of present-day polymer processing Smart blending: A new plastics technology Examples of smart blending applications: Toughening (interior and exterior panels) Reductions in permeation (fuel tank, fluid reservoirs) Nanocomposites (flame resistant and structural plastics) Electrically conducting plastics (RF shields, wires, sensors) A 10% reduction in vehicle weight gives 5 to 7% reduction in fuel usage.

4 General types of plastics: Thermoplastics: Materials that soften and become fluid upon heating. They are often sold as 2 3 mm pellets containing additives to tailor processability. A wide variety of manufacturing processes are used (e.g., die extrusion, blow molding, thermoforming, injection molding). Thermosets: Materials that initially flow but undergo irreversible chemical reactions which transform them to permanent solids. These are generally less susceptible to heat distortion. Inter-related, multi-scale influences on properties Blend morphology Crystallinity Molecular interactions Size Molecular structure This talk will focus on improving a plastic s properties by combining two or more polymers or by adding solids to polymers.

5 Thermoplastics are most frequently used.

6 Historical Perspective Present-day polymer processing equipment is derived from 1900 s technology intended for mixing or conveying. Because mixing instead of structuring has been the focus, the properties and costs of plastic composites (i.e., polymer blends, plastics with solid additives) are not necessarily optimized. The potential of new plastic materials such as nanocomposites may not be met without new processes and equipment. Sturges apparatus for cooling and mixing soap (1871) Higbie s grain conveyor and drier (1877) Gray s wire coating extruder (1879) Reference: J. L. White, Twin Screw Extrusion: Technology and Principles, Hanser, Munich, 1990.

7 How to draw a tree (or make an advanced polymer blend or composite) - Based on drawing method by Leonardo Da Vinci

8 In the photos above, wood chips are converted to saw dust. When plastics of different types are combined, polymer resin pellets undergo a similar process in screw extruders of various types to yield a mixture with limited structure variety. Property-structurecomposition optimization may not be possible. Often, only droplet dispersions result especially where a polymer type is present at lower concentrations.

9 Mixing-based processing: Structure is broken down with goal of achieving well-mixed condition. = = Smart blending and progressive structure (morphology) development: Processing conditions promote in situ structuring to increase the variety of structures attainable at a fixed composition.

10 Progressive structure (morphology) development Processing conditions where fine-scale shapes among melt components or arrangements among solid additives are formed progressively in situ. A variety of blend morphologies are obtained via sequential morphology transitions. Chaotic advection is enabling to progressive structure development. New processes Better plastics >>Mixing versus in situ structuring

11 Chaotic advection: What is it? Chaotic motion of passive particles in a fluid. What is its relevance to smart blending? The collective chaotic motions cause initially large melt bodies to become stretched and folded in a so-called baker s transformation. Stretching and folding occur recursively as depicted on the right. As such, a multi-layer structure of increasingly finer scale emerges in the melt. This multilayer structure is parent to many derivative structures, all of which can have important applications.

12 Internal surface areas can be in the 10 s of millions of cm 2 /ml. In polymer combinations where very thin layers can be formed, characteristic dimensions of derivative morphology features can be similarly small since these are derived from morphology transitions in the parent layers. In the example above, very thin layers with thicknesses of about 5-10 nm were formed by chaotic advection in a blend of 85% LDPE / 15% HDPE.

13 The application of chaotic advection to materials science is one of three recent developments that may hold particular promise. [H. Aref, The Development of Chaotic Advection, Physics of Fluids, Vol. 14, pp , 2002.] Predominant Focus Mixing & Turbulence (Chaotic Mixing) Chaotic Advection (Aref) Heat Transfer Enhancement Smart Blending Smart Blending Research Featured

14 LAPM&T smart blender test bed In some following slides, N is indicated. This parameter is specified by operators and reflects the amount of structuring imposed by the smart blender as melts flow toward the extrusion point. Because specific structures can be specified by operators, the blender is denoted as smart.

15 Smart blender test bed Various dies can be installed to produce structured materials of essentially any profile. Above, a film die is used.

16 In experiments, the appearance of holes (i.e., ruptures) in melt layers suggests that hole formation and growth, which can occur interactively among the layers, have central importance to structure development. PP/ 20% EPDM

17 Dual phase continuous morphologies from multi-layer melts (micro-sponges) Computer model-generated movie Nano-scale layers may give nano-sponges.

18 Fibrous morphologies Computer model simulation For low minor component compositions, hole growth in layers occurs with less layer interaction. Fibers form as holes enlarge and grow preferentially in the shear direction.

19 Progressive morphology development in PP/LDPE 70/30% blends: (The preceding morphology is the parent to the following morphology.)

20 Substantial enhancement in impact toughness of polypropylene (PP) was obtained by adding a synthetic rubber (PP/EPDM 80/20 blends). Impact Strength (J) PP (0.49J) (a) % Increase in Impact Strength (J) (b) N N Exposed interconnections Interconnected multilayer morphology subsequent to cryogenic fracture and solvent removal of EPDM. N=10

21 Similar to conventional plastics Multilayer, N=8 Thin, interconnected Droplet, N=16 layers, N=10 In addition to toughness increases, the impact failure of polypropylene- EPDM films was qualitatively changed by blend morphology. Crack propagation was suppressed. Conventional plastics (right) have catastrophic failure whereas the optimal plastic (center) resists deformation and crack propagation.

22 Thinner, compliant layers N=8 N=10 Impact toughness enhancements were due to interconnections among numerous thin layers and a more distributed tendon formation among very thin, compliant PP and EPDM layers. Energy absorption was more volumetrically distributed as N was increased from 8 to 10.

23 Barrier films: EVOH/LLDPE films N /20 70/30 4 P (cc.mm/m 2.day.atm) N 8 Oxygen permeation was reduced by 100x to 400x using 10% to 30% EVOH. Optimal oxygen barrier was obtained with a mechanically interlocked layer-platelet morphology (5 < N < 6). 20% EVOH 30% EVOH (cast film, unstretched)

24 Nanocomposites Permeants move readily around unaligned platelets. Permeant pathways are reduced or blocked. Inefficient structure: unoriented and randomly located platelets obtained by conventional blending. Effective placement in layers and alignment of platelets by chaotic advection in a smart blender. In addition to making plastics less permeable, platelets also can improve thermal stability, flame retardency, and mechanical and dielectric properties.

25 PA-6 / 2% Clay Nano-composites On the left, nano-platelets are disoriented due to conventional mixing. On the right, the platelets have been aligned in the smart blender. Alignment occurs volumetrically and before extrusion steps. PA-6 nano-layers Multiple nanoscales are present. l L

26 Aligned nanotubes in PP matrix (2002) Note: This result is preliminary. Research in ongoing. As with nano-platelets, new opportunities are available to manipulate nanotubes so they can be more effective in enhancing properties. In the micrograph above, the ends of nanotubes are shown in a fracture surface. As with all fiber reinforced composites, fiber orientation is an important factor in determining physical properties.

27 Electrically conducting plastics (solid additives) Initial Particle Cluster Smart blending / chaotic advection: Percolating networks are constructed in situ. Mixing: Percolating networks result from chance encounters.

28 Electrically conducting films (carbon black) 50 um Increasing chaotic advection Conducting networks formed at 3 wt% carbon black in LLDPE. 50 um

29 A: Random distribution A B: Unidirectional conductivity C: Bidirectional conductivity B Notably, it was possible with the smart blender to create pathways to form to lesser or greater extents at low carbon black loadings so as to impart directional electrical properties to extruded films. C

30 Concluding Remarks: 1. Mixing-based processing may limit plastics usage due to non-optimal properties that can result. Deficient properties may include toughness, permeation resistance, stiffness, electrical charge dissipation, and others. 2. Structure-property-composition relations for many plastics that consist of two or more polymer types or polymers and solid additives are still not known due to current limitations of processing equipment designed to mix and instead of create fine-scale structures in melts. 3. New processing methods that controllably form structures in plastics at the micro- and nano-scales may give new opportunities for weight reduction via improved physical properties, lower material costs, and multi-functionality (e.g., plastics with multiple, desirable properties such as low permeation, high toughness, and electrical conductivity). For more information, please contact Prof. Dave Zumbrunnen, Dept. of Mechanical Engineering, Clemson University, Tel , A list of publications is available on the Web at