Carbon Fibre Report

44
Light weight material- Carbon Fibre Presented by Nebcin Varghese Roll No: 35 7 th Semester Department of Mechanical Engineering

Transcript of Carbon Fibre Report

Page 1: Carbon Fibre Report

Light weight material-Carbon Fibre

Presented by

Nebcin VargheseRoll No: 357th Semester

Department of Mechanical EngineeringRajiv Gandhi Institute of Technology

Kottayam

Abstract

Page 2: Carbon Fibre Report

Steel and other hard construction materials have revolutionized the field of industry. Now, a stage has come that there is a need of a better material to catch up with the growing needs and demands of the modern society. This need has bought up a newer material to the field which is now known as Carbon Fibres.

Carbon fibre is one of the latest reinforcement materials used in composites. It's a real hi-tech material, which provides very good structural properties, better than those of any metal. Carbon fibre has a tensile strength almost 3 times greater than that of steel, yet is 4.5 times less dense. Carbon fibers are carbon fibres with values of Young’s modulus between 150 and 275 to 300 GPa.

Introduction

2

Page 3: Carbon Fibre Report

When you go to a sports shop you are inundated with new "graphite" based materials for sports equipment: golf clubs, tennis rackets, bicycles (frames and wheel disks), ultra light airframes feature these new lightweight materials. But, we are also familiar with graphite as being a very common and mundane substance. Graphite has long been a component of pencil lead, and is used as a basic lubricant. How is it that graphite is both a hi-tech and low-tech material? Could we take a bunch of pencil leads and epoxy them together into a cutting edge tennis racquet? Anyone who has used mechanical pencils knows that the leads break far too easily to provide a strong frame. It would seem as if there are two different kinds of graphite. In fact, this is true. When vendors market "graphite fibre" products they are usually selling a "carbon fibre" product. The correct name for the fibres used in all strengthening and reinforcing applications is carbon fibres. But, there is more to the story than just a general misconception over the term "graphite fibres." Surprisingly, if we look at a small section of graphite and carbon fibres on the atomic level they appear to be identical.

What is Carbon Fibre?

3

Page 4: Carbon Fibre Report

Carbon Fibre is one of the most recent developments in the field of composite materials and is one of the strongest fibers known to man. It is usually the first choice of fibre if something very strong and very light is required. Carbon fibre was originally developed in space technology, but has now been adopted in many other areas of manufacture. Racing car monocoques and aero plane wings are usually constructed of carbon. Generally the term "carbon fibre" is used to refer to carbon filament thread. Carbon fibre is one of the latest reinforcement materials used in composites. It's a real hi-tech material, which provides very good structural properties, better than those of any metal. This material is known for its high specific stiffness and strength. The material has an advantageous combination of good mechanical properties and low weight. With the decrease in its cost over recent years, it is fast becoming one of the leading materials in many areas, including performance sport equipment, transport, scientific experiments and even wallets and watches!

Key BenefitsProperty Fine Grained

Graphite Unidirectional

Fibres 3-D Fibres

Elastic Modulus (GPa) 10-15 120-150 40-100

4

Page 5: Carbon Fibre Report

Tensile Strength (MPa) 40-60 600-700 200-350 Compressive Strength (MPa) 110-200 500-800 150-200

Fracture Energy (kJm-2) 0.07-0.09 1.4-2.0 5-10

Oxidation resistance Very low poor better than graphite

Carbon fibre has a tensile strength almost 3 times greater than that of steel, yet is 4.5 times less dense.Some other properties of carbon fibre are: high tensile strength low thermal expansion Resistance to corrosion and fire High stress tolerance levels electrically and thermally conductive Chemical inertness light weight and low density very hard and brittle high abrasion and wear resistance

PRODUCTION PROCESSES – Carbon fibre

Carbon fibres are long bundles of linked graphite plates, forming a crystal structure layered parallel to the fiber axis. This crystal structure makes the fibers highly anisotropic,

TENSILE STRENGTH

DENSITY SPECIFIC STRENGTH

CARBON FIBRE 3.50 1.75 2.00

STEEL 1.30 7.90 0.17

5

Page 6: Carbon Fibre Report

with an elastic modulus of up to 5000GPa. Fibres can be made from several different precursor materials, and the method of production is essentially the same for each precursor: a polymer fibre undergoes pyrolysis under well-controlled heat, timing and atmospheric conditions, and at some point in the process it is subjected to tension. The resulting fiber can have a wide range of properties, based on the orientation, spacing, and size of the graphite chains produced by varying these process conditions.

Precursor material is drawn or spun into a thin filament. The filament is then heated slowly in air to stabilize it and prevent it from melting at the high temperatures used in the following steps. The stabilized fibre is placed in an inert atmosphere and heated to approximately 1500°C to drive out the non-carbon constituents of the precursor material. This pyrolysis process, known as carbonization, changes the fibre from a bundle of polymer chains into a bundle of "ribbons" of linked hexagonal graphite plates, oriented somewhat randomly through the fibre. The length of the ribbons can be increased and their axial orientation improved through further heating steps up to 3000°C, a process called graphitization. Because the graphite ribbons are bonded to each other perpendicular to the fibres only by weak Van der Waals bonds, the ribbons must be reoriented to increase the tensile strength of the fibre to a useful level. This is accomplished through the application of tension at

6

Page 7: Carbon Fibre Report

some point in the stabilization or pyrolysis phases, the exact time depending on the precursor material. Increased axial orientation increases the fibre's tensile strength by making better use of the strong covalent bonds along the ribbons of graphite plates.

Polyacrylonitrile (PAN) and rayon are the most commonly used precursors. PAN is stretched during the stabilization phase, and heated to 250°C in air. The tension is then removed, and the fibre is heated slowly in an inert nitrogen atmosphere to 1000-1500°C. Slow heating maintains the molecular ordering applied by tension during the stabilization phase. Graphitization at temperatures up to 3000°C then follows. Applying tension at 2000°C further increases the proper ordering of graphite ribbons. Rayon, a cellulose-based fibre made from wood pulp, is spun into a filament from a melt, and stabilized without tension up to 400°C. It is then carbonized without tension up to 1500°C, and is stretched in the graphitization phase up to 2500°C

PRODUCTION PROCESSES – Carbon Matrix

7

Page 8: Carbon Fibre Report

Manufacturing of Carbon fibre partsA wide range of different processes have developed for moulding of composites parts ranging from very simple manual processes such as hand lay to very sophisticated highly industrialized processes Each process has its own particular benefits and limitations making it applicable for

8

Page 9: Carbon Fibre Report

particular applications. The choice of process is important in order to achieve the required technical performance at an economic cost

The main technical factors that govern the choice of process are the size and shape of the part, the mechanical and environmental performance and aesthetics. The main economic factor is the number of identical parts required. Most processes will have an initial investment or set up cost. This is a major factor in the choice of process. Some of the common methods are:

Open moulding - hand and spray lamination Vacuum Infusion

Resin injection

Vacuum Bag and Press Moulding Pultrusion

Advantages

Very low weight High impact tolerance Insensitive to climate and temperature changes Reduced maintenance costs

9

Page 10: Carbon Fibre Report

Long service life

Shortcomings The chief drawback of carbon fibre composites is that they oxidize readily at temperatures between 600-700°C, especially in the presence of atomic oxygen. A protective coating (usually silicon carbide) must be applied to prevent high-temperature oxidation, adding an additional manufacturing step and additional cost to the production

10

Page 11: Carbon Fibre Report

process. The high electrical conductivity of airborne graphite particles creates an unhealthy environment for electrical equipment near machining areas. Carbon fibre composites are currently very expensive and complicated to produce, which limits their use mostly to aerospace and defense applications.

Applications

Carbon fibres are cutting edges in:

Aerospace and aircraft industry Sports equipment Automotive parts Small consumer goods like laptops, watches etc.

11

Page 12: Carbon Fibre Report

Air filtration Fishing rods and tripods Acoustics As a microelectrode in extracellular recording in medicine

Conclusion

Carbon Fibre is now an engineering material that must be designed, engineered and manufactured to the same standards of precision and quality control as any other engineering material. Carbon fibre thus has revolutionized the field of light weight materials. This can be used as a

12

Page 13: Carbon Fibre Report

substitute for steel without the most of latter’s difficulties like high weight, lack of corrosion resistance etc. This is thus one of the future manufacturing materials.

Bibliography

www.chemitry/carbon.com

www.grandprix.com

www.germancarfans.com

www.quoromtech.com

13

Page 14: Carbon Fibre Report

Material Science and Engineering, van Black

A carbon fiber is a long, thin strand of material about 0.0002-0.0004 in (0.005-0.010 mm) in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment makes the fiber incredibly strong for its size. Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is combined with epoxy and wound or molded into shape to form various composite materials. Carbon fiber-reinforced composite materials are used to make aircraft and spacecraft parts, racing car bodies, golf club shafts, bicycle frames, fishing rods, automobile springs, sailboat masts, and many other components where light weight and high strength are needed.

Carbon fibers were developed in the 1950s as a reinforcement for high-temperature molded plastic components on missiles. The first fibers were manufactured by heating strands of rayon until they carbonized. This process proved to be inefficient, as the resulting fibers contained

14

Page 15: Carbon Fibre Report

only about 20% carbon and had low strength and stiffness properties. In the early 1960s, a process was developed using polyacrylonitrile as a raw material. This produced a carbon fiber that contained about 55% carbon and had much better properties. The polyacrylonitrile conversion process quickly became the primary method for producing carbon fibers.

During the 1970s, experimental work to find alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch derived from oil processing. These fibers contained about 85% carbon and had excellent flexural strength. Unfortunately, they had only limited compression strength and were not widely accepted.

Today, carbon fibers are an important part of many products, and new applications are being developed every year. The United States, Japan, and Western Europe are the leading producers of carbon fibers.

Classification of Carbon Fibers

Carbon fibers are classified by the tensile modulus of the fiber. Tensile modulus is a measure of how much pulling force a certain diameter fiber can exert without breaking. The English unit of measurement is pounds of force per square inch of cross-sectional area, or psi. Carbon fibers classified as "low modulus" have a tensile modulus below 34.8 million psi (240 million kPa). Other classifications, in ascending order of tensile modulus, include "standard modulus," "intermediate modulus," "high modulus," and "ultrahigh modulus." Ultrahigh modulus carbon fibers have a tensile modulus of 72.5-145.0 million psi (500 million-1.0 billion kPa). As a comparison, steel has a tensile modulus of about 29 million psi (200 million kPa). Thus, the strongest carbon fiber is about five times stronger than steel.

The term graphite fiber refers to certain ultrahigh modulus fibers made from petroleum pitch. These fibers have an internal structure that closely approximates the three-dimensional crystal alignment that is characteristic of a pure form of carbon known as graphite.

15

Page 16: Carbon Fibre Report

Plastics are drown into long strands or fibers and then heated to a very high temperature without allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled.

Raw Materials

The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile. The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one company to another and is generally considered a trade secret.

During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fiber to achieve a specific effect. Other materials are designed not to react or to prevent certain reactions with the fiber. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.

16

Page 17: Carbon Fibre Report

The Manufacturing Process

The process for making carbon fibers is part chemical and part mechanical. The precursor is drawn into long strands or fibers and then heated to a very high temperature with-out allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly

The fibers are coated to protect them from damage during winding or weaving. The coated fibers are wound onto cylinders called bobbins.

The fibers are coated to protect them from damage during winding or weaving. The coated fibers are wound onto cylinders called bobbins. inter-locked chains of carbon atoms with only a few non-carbon atoms remaining.

Here is a typical sequence of operations used to form carbon fibers from polyacrylonitrile.

17

Page 18: Carbon Fibre Report

Spinning

1 Acrylonitrile plastic powder is mixed with another plastic, like methyl acrylate or methyl methacrylate, and is reacted with a catalyst in a conventional suspension or solution polymerization process to form a polyacrylonitrile plastic.

2 The plastic is then spun into fibers using one of several different methods. In some methods, the plastic is mixed with certain chemicals and pumped through tiny jets into a chemical bath or quench chamber where the plastic coagulates and solidifies into fibers. This is similar to the process used to form polyacrylic textile fibers. In other methods, the plastic mixture is heated and pumped through tiny jets into a chamber where the solvents evaporate, leaving a solid fiber. The spinning step is important because the internal atomic structure of the fiber is formed during this process.

3 The fibers are then washed and stretched to the desired fiber diameter. The stretching helps align the molecules within the fiber and provides the basis for the formation of the tightly bonded carbon crystals after carbonization.

Stabilizing

4 Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

Carbonizing

5 Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not

18

Page 19: Carbon Fibre Report

contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate de heating during carbonization.

Treating the surface

6 After carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. The addition of oxygen atoms to the surface provides better chemical bonding properties and also etches and roughens the surface for better mechanical bonding properties. Oxidation can be achieved by immersing the fibers in various gases such as air, carbon dioxide, or ozone; or in various liquids such as sodium hypochlorite or nitric acid. The fibers can also be coated electrolytically by making the fibers the positive terminal in a bath filled with various electrically conductive materials. The surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as pits, which could cause fiber failure.

Sizing

7 After the surface treatment, the fibers are coated to protect them from damage during winding or weaving. This process is called sizing. Coating materials are chosen to be compatible with the adhesive used to form composite materials. Typical coating materials include epoxy, polyester, nylon, urethane, and others.

8 The coated fibers are wound onto cylinders called bobbins. The bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

19

Page 20: Carbon Fibre Report

Quality Control

The very small size of carbon fibers does not allow visual inspection as a quality control method. Instead, producing consistent precursor fibers and closely controlling the manufacturing process used to turn them into carbon fibers controls the quality. Process variables such as time, temperature, gas flow, and chemical composition are closely monitored during each stage of the production.

The carbon fibers, as well as the finished composite materials, are also subject to rigorous testing. Common fiber tests include density, strength, amount of sizing, and others. In 1990, the Suppliers of Advanced Composite Materials Association established standards for carbon fiber testing methods, which are now used throughout the industry.

Health and Safety Concerns

There are three areas of concern in the production and handling of carbon fibers: dust inhalation, skin irritation, and the effect of fibers on electrical equipment.

During processing, pieces of carbon fibers can break off and circulate in the air in the form of a fine dust. Industrial health studies have shown that, unlike some asbestos fibers, carbon fibers are too large to be a health hazard when inhaled. They can be an irritant, however, and people working in the area should wear protective masks.

The carbon fibers can also cause skin irritation, especially on the back of hands and wrists. Protective clothing or the use of barrier skin creams is recommended for people in an area where carbon fiber dust is present. The sizing materials used to coat the fibers often contain chemicals that can cause severe skin reactions, which also requires protection.

In addition to being strong, carbon fibers are also good conductors of electricity. As a result, carbon fiber dust can cause arcing and shorts in electrical equipment. If electrical equipment cannot be relocated from the area where carbon dust is present, the equipment is sealed in a cabinet or other enclosure.

The Future

The latest development in carbon fiber technology is tiny carbon tubes called nanotubes.

20

Page 21: Carbon Fibre Report

These hollow tubes, some as small as 0.00004 in (0.001 mm) in diameter, have unique mechanical and electrical properties that may be useful in making new high-strength fibers, submicroscopic test tubes, or possibly new semiconductor materials for integrated circuits

Read more: How carbon fiber is made - material, making, used, processing, parts, components, composition, structure, steps, industry, machine, Classification of Carbon Fibers, Raw Materials http://www.madehow.com/Volume-4/Carbon-Fiber.html#ixzz1oFxCYJBO

21

Page 22: Carbon Fibre Report

What is Carbon Fiber?Carbon fiber is composed of carbon atoms bonded together to form a long chain. The fibers are extremely stiff, strong, and light, and are used in many processes to create excellent building materials. Carbon fiber material comes in a variety of "raw" building-blocks, including yarns, uni-directional, weaves, braids, and several others, which are in turn used to create composite parts.

Plain Carbon Fiber Weave

 

Carbon Fiber Twill Weave

Within each of these categories are many sub-categories of further refinement. For example, different types of carbon fiber weaves result in different properties for the composite part, both in fabrication, as well as final product. In order to create a composite part, the carbon fibers, which are stiff in tension and compression, need a stable matrix to reside in and maintain their shape. Epoxy resin is an excellent plastic with good compressive and shear properties, and is often used to form this matrix, whereby the carbon fibers provide the reinforcement. Since the epoxy is low density, one is able to create a part that is light weight, but very strong. When fabricating a composite part, a multitude of different processes can be utilized, including wet-layup, vacuum bagging, resin transfer, matched tooling, insert molding, pultrusion, and many other methods. In addition, the selection of the resin allows tailoring for specific properties.

Carbon fibers reinforcing a stable matrix of epoxy

Strength, Stiffness, and Comparisons With Other MaterialsCarbon fiber is extremely strong. It is typical in engineering to measure the benefit of a material in terms of strength to weight ratio and stiffness to weight ratio, particularly in

22

Page 23: Carbon Fibre Report

structural design, where added weight may translate into increased lifecycle costs or unsatisfactory performance. The stiffness of a material is measured by its modulus of elasticity. The modulus of carbon fiber is typically 20 msi (138 Gpa) and its ultimate tensile strength is typically 500 ksi (3.5 Gpa). High stiffness and strength carbon fiber materials are also available through specialized heat treatment processes with much higher values. Compare this with 2024-T3 Aluminum, which has a modulus of only 10 msi and ultimate tensile strength of 65 ksi, and 4130 Steel, which has a modulus of 30 msi and ultimate tensile strength of 125 ksi.

As an example, a plain-weave carbon fiber reinforced laminate has an elastic modulus of approximately 6 msi and a volumetric density of about 83 lbs/ft3. Thus the stiffness to weight for this material is 107 ft. By comparison, the density of aluminum is 169 lbs/ft3, which yields a stiffness to weight of 8.5 x 106 ft, and the density of 4130 steel is 489 lbs/ft3, which yields a stiffness to weight of 8.8 x 106 ft. Hence even a basic plain-weave carbon fiber panel has a stiffness to weight ratio 18% greater than aluminum and 14% greater than steel. When one considers the possibility of customized carbon fiber panel stiffness through strategic laminate placement, as well as the potentially massive increase in both strength and stiffness possible with lightweight core materials, is it obvious the impact advanced carbon fiber composites can make on a wide variety of applications.

Pros and ConsCarbon fiber reinforced composites have several highly desirable traits that can be exploited in the design of advanced materials and systems. The two most common uses for carbon fiber are in applications where high strength to weight and high stiffness to weight are desirable. These include aerospace, military structures, robotics, wind turbines, manufacturing fixtures, sports equipment, and many others. High toughness can be accomplished when combined with other materials. Certain applications also exploit carbon fiber's electrical conductivity, as well as high thermal conductivity in the case of specialized carbon fiber. Finally, in addition to the basic mechanical properties, carbon fiber creates a unique and beautiful surface finish.

Although carbon fiber has many significant benefits over other materials, there are also tradeoffs one must weigh against. First, solid carbon fiber will not yield. Under load carbon fiber bends but will not remain permanently deformed. Instead, once the ultimate strength of the material is exceeded, carbon fiber will fail suddenly and catastrophically. In the design process it is critical that the engineer understand and account for this behavior, particularly in terms of design safety factors. Carbon fiber composites are also significantly more expensive than traditional materials. Working with carbon fiber requires a high skill level and many intricate processes to produce high quality building materials (for example, solid carbon sheets, sandwich laminates, tubes, etc). Very high skill level and specialized tooling and machinery are required to create custom-fabricated, highly optimized parts and assemblies.

Carbon Fiber vs. Metals

23

Page 24: Carbon Fibre Report

When designing composite parts, one cannot simply compare properties of carbon fiber versus steel, aluminum, or plastic, since these materials are in general homogeneous (properties are the same at all points in the part), and have isotropic properties throughout (properties are the same along all axes). By comparison, in a carbon fiber part the strength resides along the axis of the fibers, and thus fiber properties and orientation greatly impact mechanical properties. Carbon fiber parts are in general neither homogeneous nor isotropic.

The properties of a carbon fiber part are close to that of steel and the weight is close to that of plastic. Thus the strength to weight ratio (as well as stiffness to weight ratio) of a carbon fiber part is much higher than either steel or plastic. The specific details depend on the matter of construction of the part and the application. For instance, a foam-core sandwich has extremely high strength to weight ratio in bending, but not necessarily in compression or crush. In addition, the loading and boundary conditions for any components are unique to the structure within which they reside. Thus it is impossible for us to provide the thickness of carbon fiber plate that would replace the steel plate in your application. It is the customer's responsibility to determine the safety and suitability of any Dragonplate product for a specific purpose. This is accomplished through engineering analysis and experimental validation.

 

COMPARISON CRITERIA

PRODUCTS Stiffness to Weight

Toughness

Crushability

Moisture Resistance

Sound Absorbency

Solid Carbon Fiber GOOD GOOD BEST BEST POOR

Birch Core BETTER BEST BEST GOOD POORBalsa Core BETTER GOOD BETTER POOR GOOD

Polypropylene Honeycomb Core BEST GOOD GOOD BEST BEST

Nomex Honeycomb Core BEST BETTER BETTER BETTER BEST

Depron Foam Core BETTER POOR POOR BETTER BETTER

Airex Foam Core BEST GOOD GOOD BETTER BETTERDivinycell Foam

Core BETTER BEST BETTER BETTER GOOD

 

24

Page 25: Carbon Fibre Report

DragonPlate Glossary3-Point Bending: A condition where both ends of a beam are supported and a load is applied at the mid-span.

Aramid Fiber: A synthetic fiber with exceptional strength and toughness commonly used in applications where high resistance to impacts.

Axial Stress: Stress component along the longitudinal axis of a component.

Brittle Material: A material that does not yield, but instead fails suddenly when the ultimate stress is exceeded.

Carbon Fiber: A high strength, high stiffness material that when combined with a resin matrix creates a composite with exceptional mechanical properties.

CFRP: Abbreviated form of carbon-fiber reinforced plastic

Cantilever: A condition where one end of a beam is fixed and a load is applied to the opposite free end.

Composite Sandwich Core: In a composite sandwich structure, the core is a lower density material placed close to the neutral axis in order to increase the stiffness to weight ratio.

Composite material: A material created by combining two or more materials such that the final construction exploits certain properties from each. In the construction of carbon-fiber reinforced plastics, the high strength, high stiffness of the carbon fibers are combined with a low density stable matrix to create a combined material with desirable material properties.

Density: The weight of a material per unit length, area, or volume (linear density, areal density and volumetric density, respectively).

Epoxy: A polymer resin that hardens when combined with a catalyst. Epoxy is one of the most common materials used to form the matrix in carbon-fiber fabrication.

Fiberglass: A glass fiber reinforced plastic similar to carbon-fiber, but with much lower strength and stiffness, but also much lower cost.

Homogeneous: Defined as having a uniform composition throughout the material.

Isotropic: Defined as having the same properties (mechanical, electrical, thermal, etc) in all directions. Carbon-Fiber laminates are typically highly directional, having high stiffness and strength only along the longitudinal directions of the fibers.

25

Page 26: Carbon Fibre Report

Matrix: In a composite material the matrix comprises the stable "fill" which holds the fiber reinforcement. By itself the matrix is typically much weaker than the fibers, particularly in tension. The matrix's primary function is to transfer the loads between the fibers within the composite material.

Modulus of Elasticity: A measure of the stiffness of a material, defined as the axial stress divided by the axial strain. The higher the modulus, the stiffer the material (i.e. the greater the stress necessary to cause deformation). Also known as Young's Modulus.

Poisson's Ratio: When a material is stretched due to an applied load, it elongates in the axial direction and contracts in the perpendicular, or transverse, direction. The poisson's ratio is defined as the axial strain divided by the transverse strain.

Quasi-Isotropic: In a composite material, the placement of individual laminates, or plies, so that the fibers are directed along multiple directions. The result is a material with approximate isotropy in mechanical properties.

Polyacrylonitrile (PAN): A raw material commonly used to make carbon-fiber.

Pultrusion: A process which creates an extremely stiff rod, tube, or other cross-section whereby all of the carbon fibers are aligned along the longitudinal axis.

Reinforced carbon-carbon (RCC): Carbon-reinforced graphite composite used in high temperature applications.

Shear Modulus: Defined as the shear stress divided by the shear strain. Also known as the Modulus of Rigidity.

Shear Stress: The component of stress parallel to the cross-sectional face of a material.

Shear Strain: Deformation of a material caused by a shear stress. A shear strain causes skewing of a material element.

Strain: The deformation of a material caused by an applied load. The strain is defined as the change in length divided by the original length of a material.

Stress: Defined as the force per unit area. The stresses within a composite are a function of the material properties of the materials, the geometry, and the loading condition.

Ultimate Tensile Strength: The maximum stress a material can withstand in tension, above which failure will occur.

Veneer: A thin, highly flexible sheet of carbon-fiber.

Yield Strength: The stress above which a material with remain permanently deformed even when the applied load is removed.

26

Page 27: Carbon Fibre Report

1. Physical strength, specific toughness, light weight

Aerospace, road and marine transport, sporting goods

2. High dimensional stability, low coefficient of thermal expansion, and low abrasion

Missiles, aircraft brakes, aerospace antenna and support structure, large telescopes, optical benches, waveguides for stable high-frequency (GHz) precision measurement frames

3. Good vibration damping, strength, and toughness

Audio equipment, loudspeakers for Hi-fi equipment, pickup arms, robot arms

4. Electrical conductivity Automobile hoods, novel tooling, casings and bases for electronic equipments, EMI and RF shielding, brushes

5. Biological inertness and x-ray permeability

Medical applications in prostheses, surgery and x-ray equipment, implants, tendon/ligament repair

6. Fatigue resistance, self-lubrication, high damping

Textile machinery, genera engineering

7. Chemical inertness, high corrosion resistance

Chemical industry; nuclear field; valves, seals, and pump components in process plants

8. Electromagnetic properties Large generator retaining rings, radiological equipment

The production of highly effective fibrous carbon adsorbents with low diameter, excluding or minimizing external and intra-diffusion resistance to mass transfer, and therefore, exhibiting high sorption rates is a challenging task. These carbon adsorbents can be converted into a wide variety of textile forms and nonwoven materials [9]. Cheaper and newer versions of carbon fibers are being produced from new raw materials. Newer applications are also being developed for protective clothing (used in various chemical industries for work in extremely hostile environments), electromagnetic shielding and various other novel applications. The use of carbon fibers in Nonwovens is in a new possible application for high temperature fire-retardant insulation (eg: furnace material.)

27

Page 28: Carbon Fibre Report

Although many readers of HPC use carbon fiber, few know much about how it is made. That should surprise no one. Carbon fiber producers are tight-lipped about how their product is manufactured. Each producer’s fiber differs from those of its competitors, and the processing details that give each brand its signature characteristics are considered to be intellectual property. The carbon fiber manufacturing process also is notoriously difficult and expensive. Tool-up of a single world-class production line is capital intensive — $25 million minimum for equipment alone — and can take up to two years to implement. In fact, the cost can be much more.

Tokyo-based Mitsubishi Rayon Co. Ltd.’s (MRC) 9.4 million ft²/874,000m² Otake production facility, for example, is slated for a $100 million, three-year expansion — a production line that could annually produce as much as 20 million lb/9,072 tonnes of carbon fiber (see “Learn More,” at right). This goes a long way toward explaining why, historically, it has been difficult to avoid the imbalances between supply and demand that cause prices to plummet and peak. Little wonder, then, that the current cadre of carbon fiber producers numbers less than a dozen worldwide (see chart at right).

HPC, with the help of several carbon fiber process suppliers, recently peeked behind the veil of secrecy to find this more inclusive, if still incomplete, picture of the process.

A definitive differenceUnlike metals, which are homogeneous and, by design, have properties that conform to established standards, making each producer’s P20 steel, for example, interchangeable with another’s, composites are heterogeneous. Composed of combinations of unlike materials (fiber and resin), their variability, and therefore, tailorability, are central to their appeal. Accordingly, carbon fiber producers make products that are similar but not identical. Carbon fiber varies in tensile modulus (or stiffness determined as deformation under strain) and tensile, compressive and fatigue strength. PAN-based carbon fiber is available today in low modulus (less than 32 million lbf/in² or <32 Msi), standard modulus (33 to 36 Msi), intermediate modulus (40 to 50 Msi), high modulus (50 to 70 Msi) and ultrahigh modulus (70 to 140 Msi). Fiber, which is available in bundles called tow, comes in many sizes, ranging from 1K to 350K (1K equals 1,000 filaments that range from 5 to 10 microns in diameter). Products also vary in the degree of carbon content and type of surface treatment/coating.

“The complexity inherent in carbon fiber composites is the very thing that adds value to structures made from carbon fiber,” says Steven Carmichael, director of sales and

28

Page 29: Carbon Fibre Report

marketing for MRC subsidiary Grafil Inc. (Sacramento, Calif). “Like making fine wine, the right amount of patience, finesse and processing expertise brings out the subtleties in carbon fiber that add value.” That value, of course, is very high: As a metal replacement, carbon fiber composites offer 10 times the strength of steel at half the weight.

In simplest terms, carbon fiber is produced by pyrolysis of an organic precursor fiber in an inert atmosphere at temperatures above 982°C/1800°F. Carbon fiber manufacture, however, is a complex undertaking. Grafil’s 60,000 ft²/5,574m² plant in Sacramento, Calif. — small in comparison to MRC’s Otake plant, even after its 2 million-lb/544-tonne capacity expansion in 2005, with side-by-side production lines — provided the basis for HPC’s walk-through of the primary production phases. These are polymerization and spinning, oxidation (also referred to as stabilization), carbonization (sometimes inaccurately referred to as graphitization), surface treatment and sizing application. Throughout the process, tight tolerances define the fiber’s ultimate utility. “A target coefficient of variation on yield is 1 percent,” says Gordon Shearer, Grafil’s operations director, noting that real-world variation runs about 3 percent for the small tow (1K to 24K) that is used in demanding applications, such as aircraft (hence, its designation as aerospace grade), while large tow (industrial or commercial grade) can vary up to 15 percent.

PolymerizationThe process begins with a polymeric feedstock known as a precursor (“that which comes before”), which provides the fiber’s molecular backbone. Today about 10 percent of produced carbon fiber is made from a rayon- or pitch-based precursor, but the majority is derived from polyacrylonitrile (PAN), made from acrylonitrile, which is derived from the commodity chemicals propylene and ammonia. For that reason, this article depicts production of PAN-based carbon fiber.

Converting PAN into carbon fiber has challenged producers for more than 30 years. Carmichael adds that most of a carbon fiber producer’s investment is spent on precursor, and the quality of the finished fiber is directly dependent on that of the precursor. Specifically, Shearer notes, attention to precursor quality minimizes variation in the yield, or length per unit of fiber weight.

Generally, precursor formulation begins with an acrylonitrile monomer, which is combined in a reactor with plasticized acrylic comonomers and a catalyst, such as itaconic acid, sulfur dioxide acid, sulfuric acid or methylacrylic acid. Continuous stirring blends the ingredients, ensures consistency and purity and initiates the formation of free radicals within the acrylonitrile’s molecular structure. This change leads to polymerization, the chemical process that creates long-chain polymers that can be formed into acrylic fibers.

The details of polymerization, such as temperature, atmosphere, specific comonomers and catalyst are proprietary. According to Peter Morgan, author of Carbon Fibers and Their Composites (CRC Press, 2005, www.crcpress.com), “polymerization should achieve at least 85 percent acrylonitrile content and relative molecular weight of 100,000

29

Page 30: Carbon Fibre Report

g/mole with uniform distribution in order to imbue PAN white fiber with good mechanical properties.” MRC’s precursor as used by Grafil, for example, achieves 94 to 98 percent acrylonitrile content.

After washing and drying, the acrylonitrile, now in powder form, is dissolved in either organic solvents, such as dimethyl sulfoxide (DMSO), dimethyl acetamide (DMAC) or dimethyl formamide (DMF), or aqueous solvents, such as zinc chloride and rhodan salt. Organic solvents help avoid contamination by trace metal ions that could upset thermal oxidative stability during processing and retard high-temperature performance in the finished fiber. At this stage, the powder-and-solvent slurry, or precursor “dope,” is the consistency of maple syrup. The choice of solvent, and the degree to which the dope’s viscosity can be controlled (by means of extensive filtration), are critical to the success of the next phase, fiber formation.

SpinningPAN fibers are formed by a process called wet spinning. The dope is immersed in a liquid coagulation bath and extruded through holes in a spinneret made from precious metals. The spinneret holes match the desired filament count of the PAN fiber (e.g., 12,000 holes for 12K carbon fiber). This wet-spun fiber, relatively gelatinous and fragile, is drawn by rollers through a wash to remove excess coagulant, then dried and stretched to continue the orienting of the PAN polymer. Here, the filament’s external shape and internal cross-section are determined by the degree to which the selected solvent and coagulant have penetrated the precursor fiber, the amount of applied tension and the percentage of filament elongation. The latter is proprietary to each producer, but Morgan asserts that the stretch rate can be up to 12 times the initial pliability of precursor fiber.

An alternative to wet spinning is a hybrid process called dry jet/wet spinning, which uses a vertical air gap between the fiber and coagulate bath. This creates a smooth, round PAN fiber that can enhance the fiber/matrix resin interface in composite materials.

The last step in PAN precursor fiber formation is the application of a finishing oil to prevent the tacky filaments from clumping. The white PAN fiber then is dried again and wound onto bobbins.

OxidationThese bobbins are loaded into a creel that feeds the PAN fiber through a series of specialized ovens during the most time-consuming stage of production, oxidation. Before they enter the first oven, the PAN fibers are spread flat into a tow band or sheet referred to as warp. The oxidation oven temperature ranges from 392°F to 572°F (200°C to 300°C). The process combines oxygen molecules from the air with the PAN fibers in the warp and causes the polymer chains to start crosslinking. This increases the fiber density from ~1.18 g/cc to as high as 1.38 g/cc.

To avoid runaway exotherm (the total exothermic energy released during oxidation, estimated at 2,000 kJ/kg, poses a real fire hazard), oven manufacturers use a variety of airflow designs to help dissipate heat and control temperature (see sidebar, below). Matt

30

Page 31: Carbon Fibre Report

Litzler, president of C.A. Litzler Co. Inc. (Cleveland, Ohio), observes that “every precursor has its own exothermic pattern. Since individual precursor chemistry is fixed, control of temperature and airflow in the oxidation oven is adapted to each precursor and provides stabilization of the exothermic reaction.”

Oxidation time varies, driven by specific precursor chemistry, but Litzler estimates that 24K tow could be oxidized at about 43 ft/13m per minute on a large production line with multiple oxidation ovens. Randy Strop, general manager for oven manufacturer Despatch Industries (Lakeville, Minn.), says an elapsed time of 60 to 120 minutes is typical, as are four to six ovens per production line, with ovens stacked to provide two heating zones that offer 11 to 12 passes of the fiber per oven. In the end, the oxidized (stabilized) PAN fiber contains about 50 to 65 percent carbon molecules, with the balance a mixture of hydrogen, nitrogen and oxygen.

CarbonizationCarbonization occurs in an inert (oxygen-free) atmosphere inside a series of specially designed furnaces that progressively increase the processing temperatures. At the entrance and exit of each furnace, purge chambers prevent oxygen intrusion because every oxygen molecule that is carried through the oven removes a portion of the fiber, explains Robert Blackmon, VP of the Process Systems Div. at furnace source Harper International (Lancaster, N.Y.). This prevents loss of the carbon produced at such high temperatures. In the absence of oxygen, only noncarbon molecules, including hydrogen cyanide elements and other VOCs (generated during stabilization at concentration levels of 40 to 80 ppm) and particulate (such as local buildup of fiber debris), are removed and exhausted from the oven for post-treatment in an en-vironmentally controlled incinerator. At Grafil, carbonization begins in a low-temperature furnace that subjects the fiber to 1292°F to 1472°F (700°C to 800°C) and ends in a high-temperature furnace at 2192°F to 2732°F (1200°C to 1500°C). Fiber tensioning must be continued throughout the production process. Ultimately, crystallization of carbon molecules can be optimized to produce a finished fiber that is more than 90 percent carbon. Although the terms carbon and graphite are often used interchangeably, the former denotes fibers carbonized at about 1315°C/2400°F and that contain 93 to 95 percent carbon. The latter are graphitized at 1900°C to 2480°C (3450°F to 4500°F) and contain more than 99 percent elemental carbon.

The number of furnaces is determined by the modulus desired in the carbon fiber; part of the relatively high cost of high- and ultrahigh-modulus carbon fiber is due to the length of dwell time and temperatures that must be achieved in the high-temperature furnace. While dwell times are proprietary and differ for each grade of carbon fiber, oxidation dwell time is measured in hours, but carbonization is an order of magnitude shorter, measured in minutes. As the fiber is carbonized, it loses weight and volume, contracts by 5 to 10 percent in length and shrinks in diameter. In fact, the demonstrated conversion chemistry ratio of PAN precursor to PAN carbon fiber is about 2:1, with less than 2 percent permutability — that is, considerably less material exits the process than goes into it.

31

Page 32: Carbon Fibre Report

Surface treatment and sizingThe next step is critical to fiber performance and, apart from the precursor, it most differentiates one supplier’s product from its competitors’ product. Adhesion between matrix resin and carbon fiber is crucial in a reinforced composite; during the manufacture of carbon fiber, surface treatment is performed to enhance this adhesion. Producers use different treatments, but a common method involves pulling the fiber through an electrochemical or electrolytic bath that contains solutions, such as sodium hypochlorite or nitric acid. These materials etch or roughen the surface of each filament, which increases the surface area available for interfacial fiber/matrix bonding and adds reactive chemical groups, such as carboxylic acids.

Next, a highly proprietary coating, called sizing, is applied. At 0.5 to 5 percent of the weight of the carbon fiber, sizing protects the carbon fiber during handling and processing (e.g., weaving) into intermediate forms, such as dry fabric and prepreg. Sizing also holds filaments together in individual tows to reduce fuzz, improve processability and increase interfacial shear strength between the fiber and matrix resin. Carbon fiber producers increasingly use a sizing appropriate to the customer’s end use (see sidebar, below and “Sizing and surface treatment: The key to carbon fiber’s future?” in “Learn More,” at right).  At Grafil, Carmichael adds, “we can customize surface treatment and sizing to a particular customer’s resin characteristics, as well as specific properties desired in the composite.”

According to Andy Brink, cofounder of the former Hydrosize Technologies (Raleigh, N.C.), now part of Michelman (Cincinnati, Ohio), which he serves as business development manager, “Polymeric film formers made by the dispersion of particles suspended in water provide a stable chemistry that creates a good coating when dried. The speed of most carbon fiber lines allows for fairly uniform sizing application that minimizes aggregate clumps or bare spots.”

When the sizing dries, the long process is complete. Grafil (as do other suppliers) separates individual tows out of the warp and winds them onto bobbins for shipment to customers, including prepreggers and weavers.

If an industry’s history serves as a precursor of its future, the sheer magnitude of machinery and manufacturing acumen required for the successful transformation of white PAN fiber into black carbon fiber suggests that producing this advanced material is not a business for the faint of heart or the inexperienced. Three decades of processing refinement have brought technology maturity and the ability to translate superior performance and application versatility through the fibers to advanced composites. What has gone before both technologically and economically sets the stage for the potential growth in demand that marks the future

32

Page 33: Carbon Fibre Report

33