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Chapter 6: Particulate and Natural Fiber Composites

Chapter 6: Particulate and Natural Fiber Composites. Particulate and Fiber Composites. Advanced composite – composed of two or more chemically different materials In most cases the fiber is a reinforcing material separated by an interface from the matrix

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Chapter 6: Particulate and Natural Fiber Composites

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  1. Chapter 6: Particulate and Natural Fiber Composites

  2. Particulate and Fiber Composites • Advanced composite – composed of two or more chemically different materials • In most cases the fiber is a reinforcing material separated by an interface from the matrix • Natural fibers have a structural hierarchy in addition to that of the composite • Natural fiber and particulate composites are almost exclusively discontinuous composites (composites where the fibers do not extend the full length, usually random in orientation) • Plywood, lvl, and glulams can be made out of continuous elements • Most natural fibers are short elements • Regenerated cellulose fibers can be made continuous (the fibers extend the full length of the composite product and are usually highly ordered) • The composite will have properties that are a hybrid of the parent materials

  3. Advantages of Composites • Low density, high specific properties (many natural, and biological materials are composites) • Some composites made from natural fibers will actually have much higher bulk densities than that of the parent material • Use of extremely high property (strength and modulus) constituents • Discontinuities in composites limit fracture propagation • Design flexibility: the “rule-of-mixtures” - an additional design degree of freedom • Synergistic effects: role of the interface, of heterogeneity/anisotropy/hierarchy • Anisotropy: property directionality • Heterogeneity: chemical variability

  4. Historical Perspective • Used in ancient Egypt, Americas, and China • Straw was used to reinforce bricks • Many natural materials are composites • Wood, grasses, bones, fingernails, bee hives, bird nests, deer antlers, etc. • Composites have been used in aircraft since WWII

  5. Types of Natural Fiber Composites • Thermoset composites • Particle board • Fiberboard (MDF, HDF, cardboard, hardboard) • Paper and hardboard – in general does not rely on an adhesive, but relies on hydrogen bonding from fiber to fiber • Thermoplastic • Wood-natural fiber composites (WPC) • Non-wovens

  6. Volume Fraction of Fibers or Particles in a Composite Assuming that the fiber diameter (d) and the spacing between fibers (s) does not change, the maximum fiber loading is the case where s = d. Max(Vf) where s=d: Vf = 0.907 Max(Vf) where s=d: Vf = 0.785 In practice, max(Vf) = 0.5 – 0.8 Much of the composites volume can be occupied by voids because of inefficient packing.

  7. Applications • Construction • WPC are used in decking and exterior applications • Particleboard may be used as underlayment • HDF may be used in some flooring laminates • Automotive • Consumer • Furniture, paper, packaging

  8. Product Comparison The mechanical properties are based on bending values for representative materials.

  9. Performance Characteristics • Mechanical properties • These products are not ideally suited for structural applications with few exceptions. • WPC are used as decking, but require shorter spans than wood. • WPC outperforms wood in compression perpendicular loading allowing for use in exterior doors, windows, and some load bearing applications. • Physical properties • MDF and hardboard – can be manufactured for exterior/siding applications. This requires reformulating by adding waterproof adhesives and waxes. • Particleboard is almost exclusively for interior use. • WPC are designed for exterior uses, but the ability to produce a variety of shapes and improved thermal stability over unfilled plastics lends itself for use in automotive interiors.

  10. Markets 2000 Particleboard Downstream Markets Household Furniture 23% Kitchen & Bath 20% NEC 15% Office Furniture 8% Custom Laminators 7% Flooring Products 7% Door Core 5% Stocking Distributors 5% All Other Categories 10% 2000 MDF Downstream Markets Household Furniture 20% Other (NEC) 20% Moulding 11% Millwork 10% Stocking Distributors 9% Kitchen & Bath 8% Custom Laminators 7% All Other Categories 15% Source: Composite Panel Association

  11. Source: United Nations FAO – FAOSTAT

  12. Particleboard • Particle board is a panel product made by compressing small particles of wood and bonding them with adhesive. • Particle types used include- shaving, flake, chip, sawdust, silver, excelsior, strand, and wafers are reduced to particles. • It is usually made in three layers. The outer layers will contain more fines and be a higher density to provide a good surface for finishing or laminating. The core of the panel is lower density and contains larger particles to reduce the density of the overall panel. • Resin: urea-formaldehyde (UF-amino based) adhesives are the most commonly used resin to hold the particles together: • Boards with UF are intended for interior use only

  13. Particleboard Manufacture • Raw material is brought to the plant in the form of shavings, chips, mixed mill residue, or sawdust. • Whatever the raw material used – it should not be mixed; either all chips, all sawdust, etc. as this minimizes the process adjustments and assures quality control. • Raw material is bought by weight; dry planer shavings are the best and sawdust is valued least. • Some type of milling is required for any raw material. • Refiners, hammer mills, and flakers grind, cut, tear or otherwise reduce the wood into the range of particle size called furnish.

  14. Particleboard Manufacture (cont.) • After drying, the particle are screened to remove fine dust-like particles called fines. If fines are not removed, they absorb much of the resin. • Resin and wax (blending) is added to the particles to provide some water repellency or sizing to the panels. • The process of depositing furnish into a mat is termed forming. (Furnish = grind, cut, tear or otherwise reduce the wood into the range of particle size). • After mats are formed, they are moved into a press loader. • The quantity of resin used is the major factor determining strength and dimensional properties. UF resin level is usually 4-10% on a solids basis. UF resin is a water emulsion that usually contains 55-60% of actual resin (solids). The outer faces usually have a higher resin loading than the core due to the increase in the amount of fines used. • Wax is added at 0.3 – 1% addition level to improve moisture performance. • Pressing is conducted at 140-165°C with a pressure of 1.37 – 3.43 MPa. • Panel moisture contents start at 8-12% moisture entering the press and 5-8% MC exiting the press. A large amount of this moisture is from the resin. • The rate of cure/the press time for 0.5 inch thick UF bonded board is less than 3 minutes.

  15. Composite Panel Production Source: Wood Handbook

  16. Fiber-based Products • More and more “wood” is being combined with various other materials to meet manufacturing demands and the end result is: • Hardboard • Insulation board • Medium-density fiberboard • Insulation/Acoustical Board

  17. MDF • MDF = a panel product from of wood fiber bonded with synthetic resin which has been commercially produced for 40 years and the uses continue to grow. • In contrast to particleboard, MDF requires no edge-banding prior to shaping. • Properties are a uniform density = smooth, tight edges that can be machined almost like solid wood and can be finished to a smooth surface and grain-printed which eliminates the need for surface veneers or laminates. • Also used for wall paneling and wainscot. • Pressing conditions are similar to particleboard except when producing exterior grades. For these grades, PF resins are used with a press temperature of 190°C. Logs, plywood, furniture trim, sawmill cut-off blocks are reduced to chips Process closely resembles particleboard manufacturing from this point Chips undergo thermomechanicalpulping

  18. Hardboard • Hardboard is a high-density wood fiber product. • Manufactured as sheets or shapes in a wet or dry process • In the US hardboard production is decreasing: sensitive to water so interior use only (except for a few siding products); moisture causes linear expansion, swelling and surface blisters. Chips are refined using thermomechanical pulping Board Formation Pre-Press Wood chips Pulping Process Hot-Press Steam 165-175°C Resin Added Hardboard

  19. Wet and Dry Process Hardboard Wood Pulp mixed with water Wood pulp mixed with air Wood-fiber mixture is metered onto a wire screen Fiber is dried, resin added Water drained by suction from underneath wire screen Furnish is introduced into a forming device that creates a “snowstorm” = dry, fluffy fiber The fiber mat is moved to a pre-press; water is squeezed out The loosely piled fibers form a blanket which is hot-pressed High pressure and heat form ligneous bonds, squeeze out water and dry the mat

  20. Green End Processing • Receiving logs • Debarking • Chip and hammer mill (particleboard) • This generates particles and finer flours for WPCs and other products • Refining (MDF and hardboard) • Pressure and atmospheric • Refining produces fibers with a high aspect ratio

  21. Production of Particles • Hammer Milling • Use the natural fracture planes to produce particles • Robust and cheap to maintain • Knife Mills • Use knives to cut particles to a size • More expensive to maintain • Requires less energy than a hammer mill Source: http://www.feedmachinery.com/glossary/images/hammermill_1.jpg Source: http://www.semshred.com/contentmgr/showdetails.php/id/1017

  22. Production of Fibers • Refining – a more in depth look • Large grooved plates, one stationary, one rotating, has a slurry of water and chips move between them under heat and pressure or atmospheric conditions. Source: Premier Pumps PVT. LTD.

  23. Drying • Natural fibers must be dried prior to pressing • This consumes a massive amount of energy • Particles, fibers, and flour are fluidized in an airstream and conveyed pneumatically in an airstream through a steam tube or rotating drum dryer.

  24. Resin Application • Resin is atomized and applied • Blow tubes – resin is applied as fibers are pneumatically conveyed in a pipe to the forming box • Blenders – resin and waxes are applied by disc or air atomization in a rotating drum as particles tumble past one another • Application of waxes and other additives • Waxes, resin, and possibly fire retardants may be added in this manner at levels needed to pass certain standards or codes.

  25. Heat and Mass Transfer in Pressing • Numerous models: Suschland, Lang and Wolcott, Humphrey, Wang, etc. • Conductive transfer • Convection • Heat generation/RF pressing

  26. Thermoplastic Natural Fiber Composites • Reinforcement • 12GPa wood vs. 1 GPa PP • Cost • $0.0125/lb wood vs. $0.90/lb PP • Density (specific gravity) • 1.4 cell wall vs. .96 PP vs. 2.6 glass • Durability - slows moisture diffusion • Biological deterioration requires water

  27. Bulk Wood Interphase Internal Structure • Natural flour or fiber • Often wood, straw, jute, etc. • Up to 60% addition level • Hydrophylic • Thermoplastic • Most typically a polyolefin that is semi-crystalline • Needs to process below the thermal degradation temperature of the natural fiber • Hydrophobic and does not adhere readily to hydrophilic fillers • Interphase • Nucleation of plastic crystals on fiber surface • 3 phase morphology

  28. Extrusion • Extruders have a heated barrel with a screw(s) inside to convey, mix, and heat the composite. A die may be place on the end to produce a desired shape. Otherwise, this may simply serve to mix the composite constituents prior to a secondary molding step. • Single – a single screw is rotated in a barrel to melt and convey the polymer melt. Little mixing of the natural fibers occurs in the type of extruder. • Twin • Parallel – the extruder barrel is the same diameter the entire length. Often, a gear pump is needed on the barrel exit to fill a die. • Conical – the extruder barrel is larger on the feeder end than on the outlet. This helps build pressure in the melt and eliminates the need for a gear pump. • Co-rotating – the screws rotate in the same direction to produce high sheer stress for mixing. • Counter-rotating – the screws rotate in opposite directions to convey the material with little shear stress. This method has the least amount of damage to natural fibers. • Conical, counter-rotating extruders are common for natural fiber composite compounding. An image of a parallel co-rotating twin-screw extruder. Source: polymerprocessing.com

  29. Polymer Types • Polyolefins • Polyethylene – this includes high density (HDPE) and low density (LDPE) • Polypropylene (PP) – used in many food storage applications (e.g. Tupperware). Generally, it has higher properties than HDPE, but is more expensive and more susceptible to UV degradation. Also, it processes at a higher temperature. • Polystyrene (PS) – not used extensively in WPC because of its very brittle quality. • Polyvinyl chloride (PVC) – is usually a formulation that can be tailored for a wide range of processing conditions and properties. Have been used in a large extent in windows, doors, and siding. • Acrylonitrile butadiene styrene (ABS) – a copolymer of acrylonitrile and styrene that is toughened to produce a wide range of properties. • Polyesters • Aliphatic – usually not environmentally stable and are degradable such as polyethylene oxide (PEO) • Aromatic – very environmentally stable (e.g. polyethylene terephthalate [PET]), however these materials generally process above the degradation temperature of natural fibers • Natural polyesters (PLA, PHA, PHB) – these are biodegradable and have similar processing properties as polyolefins

  30. Coupling Agents • Try to marry dissimilar materials • Polymer backbone similar to matrix • Polar component similar to adherent • Silanes (thermoset or thermoplastics) • Anhydrides (Polyolefin copolymers) • Hydroxymethylated resorcinol (HMR) • Effective with traditional wood thermosets

  31. Lubricants • Lubricants are added to modify the rheology of the melt. The viscosity of the melt needs to be reduced to aid processing and reduce friction in the screws and extruder barrel. • Waxes • Stearates • Polyesters • Surfactants

  32. Additives • Talc may be added to improve the stiffness • Borates are often used as a fungicide • Fire retardants • UV stabilizers • Tannins • Others

  33. Injection Molding • Injection molding can be used to produce small parts quickly from pre-compounded composite pellets • Pellets are loaded into a hopper and injected into heated mold via a mechanism similar to a single screw extruder. • Many plastic components are manufactured in this manner. Source: http://upload.wikimedia.org/wikipedia/en/2/23/Injection_molding.png

  34. Compression Molding • Material is placed into an open mold (as opposed to a closed mold in injection molding) which is then molded under high heat and pressure. • Advantages • Can mold large parts (e.g. door panels for cars) • Can use continuous fibers with little damage • Little material wasted • Disadvantages • Inconsistent product quality • Throughput • Limited in shapes able to produce compared to other methods.

  35. Other Methods • Pultrusion • Pull the material through a heated die • Good for continuous uni-direction fiber layups • A starting composite lay up needs to have sufficient strength to survive being pulled through the heated die. • Vacuum forming • Vacuum and heat is used to mold composites into a final product • A composite is usually laid-up and placed in a bag where a vacuum is placed on it. The bag is placed in an autoclave where heat and pressure may be added. • This method is usually slow and expensive. • Preserve fiber’s length and complex shapes may be formed.

  36. Standards • Particleboard • American National Standard for Particleboard (ANSI A208.1) • MDF • American National Standard for Medium Density Fiberboard (ANSI A208.2) • Hardboard • American National Standard for Basic Hardboard (ANSI A135.4) • American National Standard for Prefinished Hardboard Paneling (ANSI A135.5)  • American National Standard for Hardboard Siding (ANSI A135.6)  • WPC • ASTM D 7031-04 Guide for Evaluating Mechanical and Physical Properties of Wood-plastic Composite Products • ASTM D 7032-04 Specification for Establishing Performance Ratings for Wood-plastic Composite Deck Boards and Guardrail Systems Guards or Handrails • ASTM D 6662-01 Specification for Polyolefin-Based Plastic Lumber Decking Boards

  37. Resources • Composite Panel Association • http://www.pbmdf.com/index.asp?sid=2 • Wood Plastic Composite Information Center • http://www.wpcinfo.org/ • Wood Handbook • http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

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