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
– 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
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
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
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
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.
vf 
vf 
 d 
 
4 s
2
Max(Vf) where s=d:
Vf = 0.785
 d 
2
 
2 3 s
Max(Vf) where s=d:
Vf = 0.907
In
,
max(Vf) = 0.5 – 0.8
Much of the composites volume can be occupied by voids because of
inefficient packing.
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
Product Comparison
Product
Constituent
Adhesive
/Matrix
Density
(g/cm^3)
Modulus
(GPa)
Strength
(MPa)
Particleboard Particles
UF
0.56 – 0.83
2.4
16.5
MDF
Fibers
UF
0.64 – 0.96
2.4
24.0
Hardboard
Fibers
None
.80 – 1.12
NA
31.0
WPC
Flour
HDPE,
LDPE, PP,
PVC
1.2
3.0
18.0
The mechanical properties are based on bending values for representative materials.
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.
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
Source: United Nations FAO – FAOSTAT
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
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.
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 58% 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.
Composite Panel Production
Source: Wood Handbook
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
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
Chips undergo
thermomechanical
pulping
Process closely
resembles
particleboard
manufacturing
from this point
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.
Wood
chips
Pulping
Process
Steam 165-175°C
Chips are refined
using
Board
Formation
PrePress
thermomechanical
pulping
Resin Added
Hardboard
HotPress
Wet and Dry Process Hardboard
Wood Pulp mixed with water
Wood-fiber mixture is
metered onto a wire screen
Water drained by suction
from underneath wire screen
The fiber mat is moved to a
pre-press; water is squeezed
out
High pressure and heat form
ligneous bonds, squeeze out
water and dry the mat
Wood pulp mixed with air
Fiber is dried, resin added
Furnish is introduced into a forming
device that creates a “snowstorm” =
dry, fluffy fiber
The loosely piled fibers form a
blanket which is hot-pressed
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
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
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.
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.
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.
Heat and Mass Transfer in Pressing
• Numerous models: Suschland, Lang and
Wolcott, Humphrey, Wang, etc.
• Conductive transfer
• Convection
• Heat generation/RF pressing
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
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
Bulk
Interphase
Wood
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 twinscrew extruder.
Source: polymerprocessing.com
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
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
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
Additives
•
•
•
•
Talc may be added to improve the stiffness
Borates are often used as a fungicide
Fire retardants
UV stabilizers
– Tannins
– Others
Injection Molding
• Injection molding can be
used to produce small
parts quickly from precompounded 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
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.
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.
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 Woodplastic Composite Deck Boards and Guardrail Systems Guards or Handrails
– ASTM D 6662-01 Specification for Polyolefin-Based Plastic Lumber Decking
Boards
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/se
veral_pubs.php?grouping_id=100&header_id=p