SUMMER, 1998
GROUP VI: Kathleen Dombink, Tim Pieprzyca, Joe Sens, Rick Wolf
COMPOSITES
Content
One of the most enduring images of Operation Desert Storm was the CNN picture of the sky over Baghdad during the initial hours of engagement. In an apparent blind rage, the Iraqi forces lit up the sky with antiaircraft tracers and fire. The world soon learned that the Coalition air forces had used stealth technology to defy detection by conventional radar-directed weapons. Through some magic, planes were made invisible! The resounding success of the Coalition air forces was attributable in large part to the technological advantages they possessed over the out-dated Iraqi military. A new generation of materials, capable of rendering conventional radar useless, made its debut in dramatic fashion. The media identified the highly unconventional materials as "composites." Because of the critical role they play in the fabrication of the "skins" of jets, the rotors of helicopters and the armor of an arsenal, ranging from tanks to the individual foot soldiers, composites was added to the vernacular as a buzzword for an advance in materials that played a major role in the superiority of the Coalition forces in Operation Desert Storm.
The use of advanced composites fills a need for structural materials that are stronger, stiffer, lighter in weight and more resistant to hostile environments than traditional materials. This radically novel class of materials has ushered in a new age as copper, bronze, and iron did in earlier times. By blending two different materials into a single substance, composites possess capabilities uniquely superior to the properties of the individual constituent materials. The performance of such composite material systems in designs that demand exceptional strength, stiffness and low density surpass the capacities of systems composed of a single (monolithic) components. In addition, composite materials present engineers with a greater variety of options since the composition of the material can be tailored to meet particular structural needs. The greater strength-to-weight ratio of advanced composites will ensure that they will be much in demand as an energy-conscious society searches for building materials to use in the twenty-first century.
In spite of the fact that advanced composites represent the latest trend in structural technology, the mixing of two uniquely distinct materials to produce a structure of superior strength and stiffness predates the arrival of humans on the planet. Composites appeared on Earth when plants embedded long fibers in a softer matrix material as when stems and trunks used fibrous chains of cellulose in a matrix of lignin. Wood is an original, natural composite. The biological world offers other examples of composites in bone and teeth which are essentially composed of hard inorganic crystals in a matrix of tough organic collagen.
Whether considering primordial biological composites or the most futuristic composite material to spring from the drawing board of a contemporary design engineer, most composites possess the quality of anisotropy (their properties vary when measured in different directions). This comes about because the harder component is usually fibrous in nature and structural strength runs along the fibers’ axis.
The use of advanced composite materials is expanding at an exponential rate. When performance is the controlling factor, the greater cost of composite materials can be disregarded. As other environmental and economic factors come into play, the initial expense of composites may count for less in the long run. At the present time, world annual production of composites is over ten million tons. The market has been growing 5-10% annually.
DEFINITION OF COMPOSITES
In the broadest sense, composites are mixtures of chemicals. They are solids that are composed of two or more materials. Usually, the result of embedding fibers, particles, or layers of one material in a matrix of another material, composites are designed to exploit the best properties of both components to produce a material that surpasses the performance of the individual parts.
The fibers can be individual strands as thin as a human hair or they can be multiple fibers braided in the form of a yarn (or tow). The fibers in advanced composites are usually non-metallic in nature, consisting of such materials as glass, carbon, silicon carbide, boron, alumina or Kevlar. Fibers can be produced on a continuous basis or they can be fabricated in a variety of shortened forms as whiskers, platelets or particulates. For this reason, advanced composites can be categorized as continuous reinforced composites or discontinuous reinforced composites characterized by the nature of the reinforcing material.
Fibers imbue material with great strength and stiffness. They are the load-bearing element of the composite. The role of the matrix is to act as a medium to keep the fibers properly oriented and to protect them from the environment. Significantly, both fibers and matrix retain their individual physical and chemical properties; yet together, they produce a combination of qualities either constituent would be incapable of producing alone. Fibers are most commonly used in structural applications called laminates. Here the reinforcing material is positioned as layers within the matrix.
When dealing with composites, the properties of special interest are stiffness (Young’s modulus), strength and toughness. Density is of great significance in certain applications. Composites possess high internal damping. Increased absorption of vibrational energy results in reduced transmission of noise and vibration. Thermal expansion, generally lower than in metals, and thermal conductivity may also play important roles when considering the suitability of composite materials.
The proper selection of a matrix can supply the composite with ductility and toughness, properties lacking in most fibers. The matrix may be a polymer, a metal or a ceramic. Most of the composites used in industry today are based on polymer matrices. These polymers can be thermosetting or thermoplastic. Lately there has been a great deal of interest in metal matrix composites (MMC’s) such as aluminum reinforced with ceramic particles on short fibers and titanium containing long, large diameter fibers.
The production of composites requires the balancing of many factors. Not the least of these are performance, fabrication methods, speed of production and total cost. It is only natural that overwhelming use of advanced composites is done by the military where a premium is placed on performance.
HISTORY OF COMPOSITES
One of the earliest recorded examples of a composite can be found in the Bible. It explains the mixing of straw with mud to fashion bricks. The use of concrete, mortar, wood and other ancient composites goes back just as far.
Advanced composites arrived on the scene in 1935 with the use of fiberglass filaments in fiber reinforced plastics. These primitive plastics were put to use in the war effort by the Army Air Corps. Fiber-reinforced plastics were made translucent in the early ‘50’s and were used to make plastic boat hulls, car bodies and truck cabs.
In the late ‘60’s and early ‘70’s, strong aramid, glass and carbon fibers set the stage for the composites in use today.
The development of resins began in 1939 with epoxies and in 1969 with phenolics. Both types of resins are thermosetting.
Carbon fibers found their first use about one hundred years ago as filaments in electric lights. These filaments were not suitable as structural materials, lacking the strength for support. But in 1963 improvements were made to allow carbon fibers’ use in special applications where cost is not critical such as aircraft design and sporting goods.
Now in the final decade of the century, boron, silicon carbide and aluminum are joining the ranks of the advanced fibers that endow composites with an extremely high modulus of stress.
ADVANTAGES OF COMPOSITES
DISADVANTAGES OF COMPOSITES
MATRICES
The binding agent in a composite is of critical importance. The four major types of matrices are: polymer, metallic, ceramic and carbon.
Polymer resins are divided broadly into two categories: thermosetting and thermoplastic. In thermosetting polymers, resin starts as a liquid and is changed to a hard, rigid solid when cross-links are formed at the molecular level. The mechanical properties of the matrix depend on the degree of cross-linking. The curing process is critical because it determines in large part the amount of cross-linking. Thermoset materials tend to be brittle . The primary thermoset resins include epoxies: (used in aerospace and aircraft); polyester and vinyl esters (used in automotive, marine, chemical and electrical applications); phenolics (used in bulk molding compounds) ; polyimides, polybenzimidazoles (for high temperature aerospace applications).
Thermoplastic resins do not form cross-links. They derive their strength from their particular properties which are determined by their monomer units and their high molecular weight. In general, thermoplastics are impermeable to chemicals and moisture. Because of the absence of cross-linking, thermoplastics will undergo large deformation before fracturing. Thermoplastic resins include: nylon, polyethylene terepththalate (PET), polycarbonate (used in injection molded articles); polyamide-imide, polyether ether ketone (PEEK) . Thermoplastic processing is more difficult in general than thermoset processing. Thermoplastic resins tend to be more viscous than thermoset resins. This viscosity makes the resins difficult to impregnate into the reinforcing fibers.
The primary metallic matrices under current investigation are those that incorporate aluminum, magnesium or titanium.
There are four classes of ceramic matrices: glass (easy to fabricate because of low softening temperatures, they include borosilicates and aluminosilicates); conventional ceramics (silicon carbide, silicon nitride, aluminum oxide and zirconium oxide are fully crystalline); cement and concrete; carbon carbon components.
REINFORCING FIBERS
For thousands of years, humans have turned to fibers for strength and lighter weight . Cotton and wool were followed later by other fibers. The alignment of the fibers along an axis gives the material additional strength and stiffness that it would lack without this special arrangement.
Unfortunately, the increased tensile strength is effective only in the direction of the fiber axis. The tight microstructure of the fiber can also cause the composite to be brittle. Nevertheless, fibers will, in general, double the strength of the resin alone with a load of fibers that is 20-40% of the composite composition.
TYPES OF COMPOSITES
The variety of composites reflects the endless search for materials that can withstand ever greater loads in increasingly hostile environments. For the sake of simplicity however, composites can be grouped into categories on the nature of the matrix each type possesses. Methods of fabrication also vary according to the physical and chemical properties of the matrices and reinforcing fibers.
Polymer Matrix Composites (PMC’s)
Polymer matrix composites are the most commonly used of the advanced composite materials. These materials can be fashioned into a variety of shapes and sizes. They provide great strength and stiffness along with a resistance to corrosion. The organic polymer matrix can be thermosetting or thermoplastic and gains its strength from fibers of carbon or boron. Although the goal is to ensure a microstructure where the fibers are well-wetted, uniformly distributed and correctly aligned, manufacturing methods often depend on the type of matrix being used. For thermosetting matrices, the preferred methods of fabrication include: liquid resin impregnation (The resin is rolled or sprayed on the fibers. The fibers can be in a mat or distributed in a mold. The curing agent is mixed with the resin immediately before application.); filament winding (Bundles of fibers (tows) are drawn through a resin bath and wound into a mold.); resin transfer molding (Fibers are placed in a die and mixed with a precatalyzed resin.); pressurized consolidation of resin prepregs (Ready-to-mold fiber-reinforced polymer sheets--prepregs--are stacked in pre-determined directions and heated to cure.); consolidation of resin molding compounds (Resins are mixed with chopped fibers and hot press molded.).
Thermoplastic resins are fashioned into composites through: injection molding (Pellets of polymer are fed into a heated barrel and injected into a mold.); hot press molding of thermoplastics (Prepreg sheets are stacked and hot pressed to form laminated structures.).
Metal Matrix Composites (MMC’s)
Although heavier than PMC’s, metal matrix composites possess greater tensile strength, higher melting points, smaller coefficients of expansion, higher ductility and increased toughness. At the present time the focus seems to be on the development of matrices of aluminum, magnesium and titanium. Methods of producing these composites include: squeeze infiltration (Liquid metal is injected into a mat of short fibers.); stir casting (Liquid metal is stirred with ceramic particles and allowed to cool.); spray deposition (Droplets of metal are sprayed onto a substrate.); powder blending and consolidation (Metallic powder is mixed with ceramic fibers or particles.); diffusion bonding of foils (Titanium foil is placed along a fiber array. The fibers are then wound and hot pressed.).
Ceramic Matrix Composites (CMC’s)
Naturally resistant to high temperatures, ceramic materials have a tendency to be brittle and fracture. Composites successfully made with ceramic matrices are reinforced with silicon carbide fibers. These composites offer the same high temperature tolerance of superalloys but without such a high density. The brittle nature of ceramics makes composite fabrication difficult. Usually most CMC production procedures involve starting materials in powder form.
Carbon Carbon Composites
Coveted for superior strength-to-density ratios at temperatures in excess of 1200K, carbon carbon composites are excellent structural materials. Primary methods of production include: chemical vapor impregnation (An appropriate hydrocarbon gas is injected with hydrogen and nitrogen on fibers . The gases are heated and cooled repeatedly until a carbon layer deposits on the fibers.); infiltration (A carbon bearing fluid is poured over the fibers.); liquid impregnation (A pitch or resin is injected and heated so that it decomposes to leave a carbon deposit on the carbon filaments.).
APPLICATIONS
The uses to which composites can be put are as limitless as the types of composites themselves. The chemical and physical properties of composites can be tailored to meet the needs of each individual application. The vast potential of composites will never be completely exploited until a time comes when it is no longer possible to design revolutionary new composite materials.
Such properties as low density, absence of magnetism, ease of molding, resistance to corrosion, fatigue, stress, and marine fouling make composites the ideal structural medium for the production of minesweeper hulls. The high torsional stiffness, low density, good thermal stability, inherent damping and good surface finish that make composites the suitable material for newsprint rollers also promote their use in the fabrication of such diverse objects as helicopter rotor blades, automobile parts and athletic equipment.
CONCLUSION
Composites blend the desirable properties of two or more types of materials into a single material system. The uses for these high performance structural materials are limited only by the imaginations of their designers. New generations of composites that possess smart skins and smart structures containing sensors and electronic circuitry are already on the drawing board. The extreme strength and stiffness of liquid crystal ordered polymers are being developed into ultralight composites and will soon find applications to utilize their unique properties. Indeed, when speaking of uses of composites in the production of everything from athletic equipment to aerospace applications, it is inaccurate to say that the sky is the limit --rather, it is only the beginning!
References:
Hull, Derek. Clyne, T.W., An Introduction to Composite Materials. Cambridge University Press. Cambridge. 1996
Schwartz, Mel M., Composite Materials Volumes I & II. Prentice Hall, Upper Saddle Creek, N.J. 1997
Chawla, Krishan K., Composite Materials: Science and Engineering. Springer-Verlag. New York. 1987
Jacobs, James A. "High Performance Composites." The Technology Teacher. February, 1994.
CONNECTIONS TO HIGH SCHOOL CHEMISTRY CURRICULUM:
1. Condensed states of matter: structures of solids
2. Descriptions of matter: Mixtures versus pure substances
3. Chemical reactions
4. Chemical bonding
5. Polymers
6. Thermodynamics and reaction rates
7. Science and technology
OBJECTIVES:
After completing this unit on composites, students should be able to:
ACTIVITIES:
(1) Introductory Activity
At the beginning of the school year when discussing the classification of matter--and heterogeneous mixtures specifically--give each student a length of a Kit-Kat bar. Have them break it into two pieces to expose its components. Use this to illustrate the nature of a composite in general and a laminate in particular.
(2) Experiment: Design Your Own Composite
Introduction Polydimethylsiloxane (PDMS) is a colorless, transparent elastic polymer.
It can be used to coat circuit boards to protect them from the environment. PDMS can be synthesized from a kit containing two liquids--a "base" and a "curing agent".
Aim: To prepare and test the strength of a composite consisting of PDMS as the matrix.
Materials PDMS kit, mold for curing, burlap, various strips of cloth, fiber glass, saw dust
Safety Concerns The chemicals in the kit are messy and mild irritants. Wear gloves and safety goggles!
Procedure
Part I: Preparation of the Control Matrix
1. Using a plastic spoon measure out 10.0 grams of "base" in a weighing boat.
2. Place the weighing boat of "base" on the balance, tare, and add the "curing agent" dropwise with a Beral pipet until 1.0 gram has been added.
3. Stir the mixture in the weighing pan, counting 100 strokes, to ensure thorough mixing. Allow to set for about 10 minutes for the bubbles to come to the surface. Note: You may blow gently on the surface to remove the bubbles.
4. Transfer the mixture into one of the spaces in the curing mold. This will be your control.
Part II: Preparation of the Composite
5. Repeat steps #1-3. Set the mixture aside.
6. Choose a substance to be used as the constituent for your composite. You may choose one of the items in the list of materials, request a substance that may be available in the classroom, or bring a substance from home with your teacher’s permission.
7. Describe your constituent under data, including the mass, and any treatment that you performed on this material (eg. shredding, cutting into strips).
8. Add the constituent to the matrix in the weighing boat and site to mix thoroughly (100 strokes). Allow to set for about 10 minutes for bubble to rise to the surface as before.
9. Transfer the mixture into a second space in the curing mold.
Note: You may choose to layer the constituent to produce a LAMINATE if you desire. Describe your procedure under data.
10. Give the curing mold to the teacher who will set it aside for 2 days OR heat it at 130 degrees Celsius for 20 minutes. Consult teacher for specific directions.
11. At the end of the curing time, remove each substance from the curing mold. Measure and record the mass of each substance.
Part III: Testing the Strength of the Composite
12. Design a set-up to test the maximum mass that the matrix by
itself (the control) and the composite can each support without
breaking. Consult the teacher concerning available standard
masses.
13. Draw a diagram of your testing apparatus.
14. Record the mass supported by each sample under data.
15. Calculate the ratio of supported mass/mass of matrix (or
composite)
Data (to be completed by student)
Record specific data on the spreadsheet provided by your teacher. Use this information, along with observations of the composites of other groups, to answer the following questions.
Questions
1. Which composite was the strongest? the weakest?
2. If you were to prepare another sample of your constituent, how
would you change your procedure? Would you use a different mass?
position it differently in the matrix?
3. If the composite was to be used in an aerospace application which
required the composite to be light in mass, which composite would you
choose? Explain your choice.
4. Suggest a use for your composite.
TEACHER NOTES
References
Beckman, K.J., Calderon, C.E., Doolan, P.W., Yuelke, S, Campbell, D.J. Composites and Surfaces, laboratory handout for Chemistry 109, Fall, 1997
Mathras, M.S., "Comparing Composites" The Science Teacher, Vol 60, Issue 5, (May 1993) p. 18.
Source of PDMS Kit: Sylard Elastomer (1.1 lb kit)--Dow Corning
184
Cost ~ $39/kit
Ellsworth Adhesive Systems
N117 W18711 Fulton Drive
P.O. Box 1002
Germantown, WI 53022-8202
414/253-8600
Fax: 414/253-8619
(3) Experiment: Making a Refrigerator Magnet.
CONNECTIONS TO THE COMMUNITY:
Composite materials are used in the following industries:
ASSESSMENT:
Traditional
1. (Give a diagram of a composite.) Label the matrix and the constituent. Identify the type of composite as a particulate a laminate or a fiber.
2. Choose a specific item that is made of a composite. Discuss the properties of the composite which makes it a better choice than the original component alone.
3. List three advantages of composites over the use of traditional materials.
4. Name four products that make use of high performance composites.
5. A natural polymeric composite material, wood, has anisotropic properties. Explain what is meant by this. Cite another application of composites that provides anisotropic properties.
6. Identify a possible use for a laminated composite consisting of two bonded sheets of different materials each having a different coefficient of thermal expansion.
7. List four classes of composites based on the matrices they use.
Alternative
(1) Poster Session Divide the class into groups. Each group chooses a different product made of a composite. Each group designs a poster depicting the product, its components, and the advantages of the composite over previously used materials for the product. .
(2) Oral Presentation Each student (or group of students) brings an item to school which is a composite. The presentation focuses on the advantages of this item over early designs, including a discussion of the components. Whenever possible, an early version of the item could be demonstrated.
(3) Research Activity (5-paragraph essay) Choose one the following topics. Determine how the development of composites improved performance.
1. Stealth Bomber vs. B-52
2. Tennis racquets
3. Water (or snow)skis
4. Golf clubs
5. Prosthetic devices
6. Grand Princess vs. Titanic
7. F-18 vs. P-51
8. Bullet-proof vests
(4) Take-Home Activity Give each student a miniature Snickers bar. Have the student take a bite and observe the interior of the bar. The student then writes a description of Snickers as a composite and how its properties (flexibility and strength) are related to the components.
(5) Classroom Display Create a display of composite materials manufactured or marketed in the community.
(6) Computer Search Use a computer to find examples of structural failure to illustrate the importance of choosing the best material for the job.
(7) Design Project: Use the background gained from the study of composites in class to have the students design of composite of their own. They can use reinforcing sheets or particles such as woven glass fiber, plastic shrink wrap and peanut shells. Matrix resins can be polyester, epoxy, portland cement or other products available at a local hardware store.
WEB REFERENCES:
1. University of Delaware Center for Composite Materials: info@ccm.udel.edu.
2. Composites Institute of Australia Inc.: caiinfo@compinst.asn.au
3. Marion Composites Marion, Va.: http://www.marion.thomasregister.com/olc/marion/
5. http://www.isc.tamu.edu/~clint/html/CCcomposite.html
6. Composite materials website: http://CMLwww.ecn.purdue.edu/CML/Index.html
7. Owens Corning glass fiber composites:
http://www.owenscorning.com/owens/composites
KEYWORDS:
matrix, laminate, particulate, fiber, constituent
ITEMS FOR A KIT:
PDMS kit, curing mold, magnetic array, square mold, refrigerator magnet