Introduction to Ecocomposites
By David A. Bainbridge
Associate Professor
United States International College of Business
Alliant International University
San Diego, CA 92131
History is often marked by the materials and technology that reflect
human capability and understanding. Many time scales begin with the Stone Age, which led
to the Bronze, Iron, Steel, Aluminum, and Alloy ages as improvements in refining,
smelting, and science made these more advanced materials possible. In the 1980's the
Composite Age began, represented at its extremes by the Stealth bomber and the sun powered
Solar Challenger. Innovative developments and market forces herald the beginning of the
Ecocomposite Age.
Composite materials combine more than one material or substance, most commonly a matrix
material and high strength fibers. Glass fibers have been one of the most common
reinforcing materials for composites, but pose health and environmental problems and are
energy intensive. Biological fibers and natural or synthetic matrix materials can be used
to make ecocomposites that are equally strong, but environmentally friendly.
Plant cell walls and plant structures are natural composite materials with regular
arrangement of reinforcing materials (Niklas, 1992). Evolution over millions of years has
optimized structural design in many plant structures. [Note: The term biocomposites has
come to be used for biological materials used in medicine, so the term ecocomposites is
used here.]
Wood is a complex natural ecocomposite material, a fiber reinforced structural foam. It is
extensively used and recognized as a structural material, but many other plant components
are excellent raw materials for fabricating materials, structures, tools, and equipment.
These can be combined with natural fibers (kept as long as possible) and natural plastics
and foams to make strong materials with high strength and lightweight. During WWII a linen
composite fuselage for the Spitfire fighter plane was made and apparently passed
structural tests, but the aluminum supply improved and the shift to composites was not
made.
These ecocomposites will become increasingly common as the price of wood continues to
climb, environmental problems with plastics disposal increase, and agricultural fibers
pose increasing costly disposal problems (as field burning of rice straw is curtailed for
example), ecocomposites may be equally well recognized and understood.
Plant fibers are low cost, lightweight, and surprisingly strong, Table 1.
Table 1. Specific tensile strength (strength on a weight basis)
| Material |
MPa |
| Plastic |
|
| Polycarbonate |
28 |
| Polypropylene |
74 |
| Metals |
|
| Mild, low-carbon steel |
42 |
| Annealed aluminum |
21 |
| Steel |
25 |
| Man-made Fibers |
|
| Graphite, intermediate |
142 |
| Aramid |
191 |
| E-glass |
136 |
| Natural fibers |
|
| Spruce |
256 |
| Flax |
160-220 |
| Jute |
245-337 |
| Coir |
107-173 |
Robson and Hague, 1993; Rosato et al., 1991; Venkataswamy et al., 1987;
Niklas, 1992; McClinock and Argon, 1966: Stamm, 1964; Roff and Scott, 1971; Balaguru, P.M.
and S.P. Shah. 1992.
Natural fibers compare favorably with man-made fibers and are attractive
because they have chemically reactive surfaces which make more complete fiber-matrix
bonding possible (Bolton, 1991).
Ecocomposites are also attractive because they are usually safer to handle and work with
and environmentally friendly. Most ecocomposite materials can be recycled (composted or
digested) or burned, without the residues that are left with glass and carbon fiber
composites. Plant fibers can be produced by sustainable agricultural systems (Mitchell and
Bainbridge, 1991), with low embodied energy, with atmospheric carbon rather than mined
"carbon" from petroleum or coal.
Ecocomposites are strong, as anyone who has attempted to open a macadamia (Macadamia
ternifolia) nut can attest. These nuts resist twice the force needed to fracture annealed
aluminum yet have comparable hardness (Niklas, 1992). Many other ecocomposites are very
strong and selections can be made for ultimate strength, elastic modulus, fracture
resistance or impact resistance.
Ecocomposites may be made with natural plastics extracted from plant materials, feather
wastes, or produced in bio-engineered processes using yeasts, bacteria and other organisms
(Comis, 1998). Combinations of plant fibers and recycled plastic may make suitable
composite materials for many uses. Despite glowing promises in the late 1980's the plastic
industry has been unable or unwilling to develop recycling programs for many plastics
(Kleiner and Dutton, 1994). While some progress has been made on PET (24% recycled),
recycling of LD and LLD polyethylene (0.7%), HD polyethylene (5%), PVC (0.2%), PP (3%),
and PS/HIPs (0.8%) is so low it be considered non-existent. More than 11 billion pounds of
LD and LLD polyethylene are produced each year, and 8 billion pounds of HDPE and PVC.
Because the plastic is being recycled into a building material rather than a food contact
product the difficulties in cleaning the waste stream would be minimized.
Composites of biological fiber and recycled plastic may become important building
materials. Capturing even a few percent of available waste materials would provide
sufficient material to manufacture much of the framing used in the U.S. Combining recycled
plastics with slit long straw fiber may provide a useful material for many products and
environments. This composite material would have a density comparable to Douglas fir.
Summary
The age of composites has begun, and with appropriate attention to full cost
accounting and the opportunities in biological fibers and "natural" plastics the
transition to safer and more environmentally friendly ecocomposites can be made.
Agroecological approaches to farming and bioengineering should make it possible to grow
plastic resins and reinforcing materials economically and safely. These materials can be
used to make lighter, stronger and more durable products that save resources and energy.
Long life and eventual recycling can be engineered into these products.
The potential economic benefits include production of biofibers in farming areas beset
with economic and environmental problems. The introduction of straw based building
materials can reduce air pollution problems, absorb some of the plastic waste stream, and
improve the energy efficiency of the homes and commercial buildings.
It is a new frontier and calls upon the talents and skills of engineers, chemists,
botanists, biologists, agronomists, and ecologists. Integrated whole system development
will be essential to recognize the full potential of these materials.
Literature cited:
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McGraw Hill, NY
Comis, D. 1998. Chicken Feathers: Eco-Friendly "Plastics" of the 21st Century?
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http://www.ars.usda.gov/is/pr/1998/980209.htm
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