Carbon fiber reinforced plastic Composites

ABSTRACT

            Composites are the 21^st century material to meet the stringent demands of light weight, high strength, corrosion resistance & near-net shapes. Carbon Fiber Reinforced Plastic Composite is new class of materials has recently emerged as a leading contender for numerous application in automobile, aerospace, electronic and wear industries. in brake disks of aircraft or Formula 1 / Indy race cars. These type of composite can operate at temperatures up to approx. 900ºC or higher. Useful properties of carbon fibres are their durability, resistance to fatigue and that they are chemically inert and still exhibit high  strength at high temperatures.Although they were know to mankind since prehistoric times, the concept and technology have undergone a sea change with better understanding of the basics like the bonding mechanism between the matrix and fiber. Technologically composites are artificially produced multiphase materials having desirable combination of best properties of the constituent phases. Since carbon is a high performance fiber material that is most commonly used reinforcement in advanced Polymer- matrix composites known as CFRP.

Keywords: - CFRP, Carbon fiber, PMC.

INTRODUCTION

             Many of our modern technologies require material with unusual combination of properties that can not be met by the conventional metal alloys, ceramics & polymeric materials. Composites are one of the most widely used material because of their adaptability to different situations & the relative ease of combination with other material to serve specific purposes & exhibit desire properties. “A composite material is a combination of two or more chemically distinct & insoluble phases.” Its properties & structural performance are superior to those of the constituents acting independently. The plastic posses mechanical properties that are generally inferior to those of metal & alloys-in particular low strength, stiffness & creep resistance. These properties can be improved by imbedding reinforcement of various types (such as glass or graphite fibers) to produce reinforced plastic.

Many composite materials are composed of just two phases ; one is termed the matrix, which is continuous & surrounds the other phase often called dispersed phase another is discontinuous phase & termed as fiber. The properties of composites are a function of the properties of the constituent phase, their relative amounts, & the geometry of fibers.

FIBERS:-

In a continuous fiber reinforced composite, the fibers provide virtually all of the load carrying characteristics of the composites, the most important of which are strength and stiffness. The multiple fibers in a composite make it a very redundant material because the failure of even several fibers results in the redistribution of load on to other fibers rather than a catastrophic failure of the part.

On the basis of diameter & character, fibers are grouped into three different classifications; whiskers, fibers, & wires.

Fig.  Common forms of fiber reinforcement

Matrices:-

The purpose of the matrix is to bind the reinforcements together by virtue of its cohesive and adhesive characteristics, to transfer load to and between reinforcements, and to protect the reinforcements from environments and handling. The matrix also provides a solid form to the composite, which aids handling during manufacture and is typically required in a finished part. This is particularly necessary in discontinuously reinforced composites, because the reinforcements are not of sufficient length to provide a handle able form. Because the reinforcements are typically stronger and stiffer, the matrix is often the “weak link” in the composite, from a structural perspective. As a continuous phase, the matrix therefore controls the transverse properties, interlaminar strength, and elevated-temperature strength of the composite. However, the matrix allows the strength of the reinforcements to be used to their full potential by providing effective load transfer from external forces to the reinforcement.

                                                             Fig: Types of matrices
 

CARBON FIBER RAINFORCED PLASTIC (CFRP)

In this type of composite carbon/graphite fiber is embedded in polymer matrix. Carbon is a high performance fiber material that is most common in used reinforcement in advanced (i.e. non-fiber glass) polymer- matrix composites. The reason for this is as follows:

1. Carbon fibers have the highest specific modulus & specific strength of all reinforcing fiber materials.
2. They retain their high tensile modulus & high strength at elevated temperatures: high temp. Oxidation however may be a problem.
3. At room temp. Carbon fibers are not affected by moisture or a wide variety of solvents, acids, bases.
4. These fibers exhibit a diversity of physical & mechanical characteristics, allowing composites in corporating these fibers to have specific engineered properties.
5. Fiber & composite manufacturing processes have been developed that are relatively inexpensive & cost effective.

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. Carbon fibers classified as "low modulus" have a tensile modulus below 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 500 million-1.0 billion kPa. As a comparison, steel has a tensile modulus of about 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.

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.

MANUFACTURING PROCESS

           The manufacturing process for producing carbon fibers involved highly controlled steps of heat treatment and tension to form the appropriately ordered carbon structure. Rayon, Pitch has been largely supplanted as a precursor by Polyacrylonitrile (PAN). Polyacrylonitrile precursors produce much more economical fibers because the carbon yield is higher and because PAN-based fibers do not intrinsically require a final high-temperature “graphitization” step. Polyacrylonitrile-based fibers having intermediate- modulus values of about 240 to 310 GPa (35 to 45 _ 106 psi), combined with strengths ranging from 3515 to 6380 MPa (510 to 925 ksi), are now commercially available. Because carbon fibers display linear stress-strain behavior to failure, the increase in strength also means an increase in the elongation-to-failure. The commercial fibers thus display elongations of up to 2.2%, which means that they exceed the strain capabilities of conventional organic matrices. The diameter of carbon fibers typically ranges from 8 to 10 lm (0.3 to 0.4 mils).

              The process for making carbon fibers is part chemical and part mechanical 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. This process is called carbonization and leaves a fiber composed of long, tightly.

Fig: Manufacturing of carbon fibers.

PYROLYSIS PROCESS:-

Fig:Pyrolysis processes for PAN precursors

Pyrolysis is the processes of inducing chemical changes by heat-for a instance, by burning a length of yarn & causing the material to carbonize & become black in color. The temperature of carbonizing range up to about 1500°C; for graphitizing to 3000°C,    Here is a typical sequence of operations used to form carbon fibers from polyacrylonitrile.

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 polycyclic 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 provide 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 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.

Fig: sizing

. Capabilities:-

• Excellent air flow uniformity
• Easy internal access to facilitate maintenance
• Electric or gas fired
• Optimal temperature uniformity
• Operator isolation from effluent

Features

• Multiple temperature control zones
• Proven alternating cross flow design
• Adjustable louvers and diffuser plates for precise temperature adjustment
• Excellent float end seals for positive sealing, minimized infiltration of ambient atmosphere and improved temperature uniformity
• Aluminized steel construction
• Plug fans to facilitate maintenance.

TYPICAL PROPERTIES OF REINFORCING FIBER

1. The advantages of carbon fibre products over different woods and metals come to      prominence when a rigid, strong but also lightweight material is needed

2. Further useful properties of carbon fibres are their durability, resistance to fatigue   and that they are chemically inert and still exhibit high strength at high temperatures.

3. It is often preferred to other fibre composites due to the tensile strength and modulus of the high quality fibres which perform better than fibreglass or Kevlar.

TYPE	TENSILE  STRENGTH        (MPa)	ELASTIC MODULUS    (GPa)	DENSITY      ( kg /  m3 )		RELATIVE COST
Boron	3500	380	2600		Highest
Carbon
High strength	3000	275		1900		Low
High modulus	2000	415	1900		Low
Glass
E  type	3500	73	2480		Lowest
S  type	4600	85	2540		Lowest
Kevlar
29	2800	62	1440		High
49	2800	117	1440		High

POLYMER MATRIX COMPOSITES

        It consists of a polymer resin as the matrix, with the fiber as the reinforcement medium. Polymer makes ideal matrix materials as they can be processed easily possess lightweight, inlight of their room temperature properties, & cost. The various types of PMCs are classified according to reinforcement type (i.e. glass, carbon, aramid & boron)

The two main kinds of polymers are Thermosets & Thermoplastics.

Thermosets:-

Thermosets have qualities such as a well bonded three dimensional molecular structure. The most common resins of these types are epoxies, phenolics, polyimides & cyanate esters. The epoxies are more expensive & in addition to commercial application are also utilized extensively in PMCs for aerospace application. For high temperature applications polyimide resins are employed.

Thermoplastics:-

Thermoplastics have one or two dimensional molecular structure & they tend to soften at an elevated temperature & show exaggerated melting point. High temperature thermoplastic resins offer the potential to be used in future aerospace application; such material include Polyetheretherketone (PEEK), Polyphenylene sulfide (PPS) & Polyethereimide (PEI).

APPLICATIONS

1. . Most sports now use carbon fibres in their equipment whether it is to reinforce the traditional materials or create new ones. Golf clubs, tennis rackets and bicycles all use the strength and durability of modern composites to enhance the quality of the sport. Formula One and other motor sports use carbon fibre composites in order to protect the driver in the event of a crash.

2. Reinforced plastic are used for automobile parts. The fatigue properties of the materials & low weight, ability to sustain strains from the engine heat & low frequency road vibrations are features that favors composites in truck & other vehicles.
3. Carbon/graphite is also used as a moderator in both reactor & non-reactor system & exhibit good moderating characteristics. They are also used in space application due to thermal conductivity & strength.
4. Fiber epoxy composites have been used in aircraft engine to enhance the performance of the system
5. The aerospace and automotive industry also use the excellent properties to their advantage and are always researching and developing further improvements in the quality of the fibres.

 

Fig: The Boeing 7E7

CONCLUSION

            There are many fibers which embedded in PMCs & to increase the strength & desired properties of material. Out of which carbon & boron fibers are mostly used but economical point of view carbon has low cost as compared to boron. Polymer matrix are used in large quantities, in light of there room temperature properties, ease of fabrication & cost. Carbon fibers are produced by many processes by using Reyon, Pitch or PAN as a precursor. Pyrolysis of PAN produces fibers of high strength & stiffness. As CFRP is light in weight they are used in aerospace & space application