lesson7.pptx

High performance fibres Carbon fibres

High performance fibres Carbon fibresHandbook of tensile properties of textile and technical fibres, Woodhead Publishing, 2009High performance fibres, Woodhead Publishing, 2001Carbon fibers are defined by the International Union of Pure and Applied Chemistry (IUPAC) as fibers (filaments, tows, yarns, rovings) consisting of at least 92% (mass fraction) carbon, usually in a non-graphitic state.Thomas Edison was the first to use carbon fibres when he employed charred cotton threads to conduct electricity in a light bulb (he patented it in 1879).Continuous regenerated cellulose fibers began to be developed and used in the 1950s by researchers in the USA to produce the first continuous carbon fibers. The incentive for this research was the requirement of light and stiff materials for the aerospace industry. The carbon-carbon bond is the strongest in nature and it was expected that such fibres would provide a valuable structural material.Today, carbon fibers continue to be the main reinforcement materials in advanced composites.21/10/20142MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013The ultimate properties of the carbon fiber directly depend on the selection and processing of the precursor materials, the formation of the fiber, and subsequent processing of the fiber.The carbon fibres made from cellulose have a modulus disappointedly low because of their poorly organised atomic structures. This was due to there being only approximately 24% by weight of carbon available in cellulose from which fibres could be formed. The removal of the other elements left a fibre with poor atomic organization. 21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20133The commercial production of carbon fibres made from polyacrylonitrile (PAN) began in the 1960s. They have become the most widely used form of carbon fibres.The aerospace industry has adopted carbon fiber composites as a major structural material. The latest generation of planes, such as the Airbus A380 and Boing 787, use around 35 tonnes of carbon fibre reinforced plastic (CFRP) for each airplane to reduce weight. Carbon fibres also have an important role in reducing greenhouse gases. Huge windmills of around 100 meters in diameter are being installed thanks to the lightweight and very rigid blades made with CFRP, providing clean power generation.Moreover, reducing the weight of airplanes, automobiles, trains and all transportation vehicles is crucial in reducing CO2 emissions during the service life of these vehicles.

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20134Carbon fibers can be categorised into two groups: as carbon fibres made by carbonizing precursor fibers and carbon fibres synthetized directly from hydrocarbon gas, such as methane. Carbon fibres made by carbonizing precursor fibres are categorised according to the type of precursor fibre used, such as PAN-based carbon fibres, pitch-based carbon fibres and rayon-based carbon fibres.Of these fibers, PAN-based carbon fibers have become the most widely used form of carbon fibers.More than 90% of commercial carbon fibers produced globally are made from PAN precursor fibers.

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20135Carbon fibers synthesized in the gas phase have been made in the form of whiskers, with diameters in the range of 0.5-1.5 micron.Finer carbon nanotubes can be categorized as one of the gas phase grown carbon fibers.The performance of carbon fibers is mainly determined by the structure of graphite crystallites in their microstructure. The carbon nanotube, which has perfect graphite crystallites, is the ideal material for obtaining high performance as a single reinforcement element. However, mass production or mass application, such as in airplane or satellites, is still a very difficult target for carbon nanotubes.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20136

Carbon fibers are also categorized by performance according to their tensile moduli, into:- low modulus carbon fibers (350 GPa)- ultra high modulus (>600 GPa)

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20137Carbon fibers are also categorized according to their tow size. Historically, tows of 3.000 or 6.000 filaments were standard.However, in order to reduce the cost of carbon fibers, the standard tow size has been increased to 12.000 filaments but 24.000 filaments are also available.Tows of 24.000 or fewer are called regular tows.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20138

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The production process for making carbon fibers from PAN precursor fibers consists of three main steps, namely a polymerization process to produce PAN polymer from acrylonitrile monomer a spinning process to produce PAN precursor fibers and a carbonization process to carbonize the PAN precursor fibers.Performance and cost of PAN-based carbon fibers are dominated by all three processes.

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201310PAN-based carbon fibresPolyacrylonitrile (PAN) is a long-chain linear polymer composed of a carbon backbone with attached carbonitrile groups.

PAN does not melt under elevated temperature so melt spinning cannot be applied for the production of PAN fibers.Solution wet spinning is normally used. One of the reason why the carbon fiber share of three Japanese companies is around 70% in the world, is that those three companies were originally producers of regenerated cellulose fibers, which are wet spun.21/10/201411MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013

Spinning of PAN fibersA concentrated solution containing 9395 wt% polyacrylonitrile is dissolved in a solvent (typically dimethylacetamide) to form a concentrated polymer solution that is fed to a storage tank. The solution is filtered to minimize impurities and passed through a spinneret.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201312

The fibers emerge through the small capillary holes directly into a coagulation bath containing ethylene glycol that extracts the solvent from the fiber.The development of internal voids or flaws in the fiber as well as the shape and texture of the fiber are controlled by the solvent removal rate.The fiber that emerges from the coagulation bath undergoes a series of post-spinning steps including washing, drawing, and drying during which the fiber solidifies into its final form. These processes play a crucial role in developing the internal morphology of the carbon fibers.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201313In the wet spinning process, polymer comes out of the solution into the bath and forms a gel-state solid, and then exchange takes place between the polymer solvent and the non-solvent (the coagulation bath).The relative rates of diffusion of solvent and non-solvent in the coagulation bath control the fiber structure.Two important features of the morphology of the fibers are the shape of cross-section and the presence of macrovoids.If there is less volume of non-solvent diffusing in than solvent diffusing out, the shape will become not-circular and tend towards a kidney-bean shape.The rupture of skin, followed by penetration of the non-solvent into the interior, is responsible for developing macrovoids, which act as internal flaws.

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Production of PAN-based carbon fibresPAN-based carbon fibres carbonization processThe figure above shows a schematic representation of the PAN carbonization process. This consists of an oxidation step, during which the oxidized fibers are heat treated in an oxidative atmosphere, such as air, in the temperature range between 200 and 300C, followed by a carbonization process, during which the oxidized fibers are heat treated in a non-oxidizing atmosphere, such as nitrogen, at temperature higher than 1000C.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201316PAN consists of around 67% by weight of carbon. This value is obtained from the ratio of the atomic weight of the three carbon atoms to the sum of the atomic weight of all species in the pure PAN monomer. In the carbonization process, hydrogen, nitrogen and oxygen, which is introduced in the oxidation process, are removed from the fibers, and carbon fibers, consisting of 90% or higher of carbon by weight, are produced.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201317

Throughout the processing of the PAN precursors to make carbon fibers, it is vital that the fibers are held under tension.If not, the alignment of the molecular structure, induced in the PAN precursor by drawing, is lost and the carbon fibers have low modulus.

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201318The first critical step in making carbon fibre from PAN fibre is causing the pendant nitrile groups to cyclise. This process is thermally activated and is highly exothermic.The next step is to make the fibre infusible: this is accomplished by adding oxygen atoms to the polymer, again by heating in air. The reaction is diffusion limited, requiring exposure times of tens of minutes. When about 8% oxygen by weight has been added, the fibre can be heated above 600C without melting.21/10/201419MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013Cyclization and oxidation

21/10/201420MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013The stabilized fiber is carbonized in an inert atmosphere at temperatures ranging from 1000 to 3000C, depending on the desired properties. During the pyrolysis, nearly all inorganic (N, H, O) and non-aromatic carbons are driven off. Primary off-gases include hydrogen cyanide, carbon dioxide, water, ammonia, methane, and hydrogen.A decrease in tensile strength is generally observed at temperatures above 1500C , which corresponds to the final major release of nitrogen. As a result, many of the highest strength PAN-based carbon fibers contain residual nitrogen.

21/10/201421MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013Carbonization21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201322

The upper diagram is the oxidising stage and the lower is the carbonising stage. The long residence times needed for the reactions to occur and for volatile by-products to exit from the interior of the fibre mean that: (a) very large ovens are needed;(b) process speeds are slow.21/10/201423MSc in Textile Engineering - Ada Ferri - A.Y.2012-201323Although heat treatments of PAN-based carbon fibers above 1700C are often referred to as graphitization, these fibers are not graphitizable and do not develop highly ordered graphene planes. They maintain a highly turbostratic organization of the graphene layer planes, even when exposed to very high treatment temperatures.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201324Fiber structure and morphologyThe word graphite is much misused in carbon fibre literature. The word refers to a very specific structure, in which adjacent aromatic sheets overlap with one carbon atom at the centre of each hexagon, as shown in the figure.This structure appears very rarely in carbon fibres, especially in PAN-based fibres, even though they are conventionally called graphite fibres.

21/10/201425MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013The large aromatic sheets are randomly oriented to each other, and are described as turbostratic (turbulent and stratified), as shown in the figure.

21/10/201426MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013Real carbon fibers cannot achieve the graphitic ideal. Instead, the graphene planes are uneven and wavy, with a mean interplanar spacing significantly greater than the 0.335 nm of graphite.

The mean interlayer distance is a good indication of the degree of structural defects in a carbon fiber. The Figure shows the mean interlayer distances in as-received commercial fibers, and the reduction of the interlayer distance value as a result offurther heat treatment, whichheals structural defects.

The weight loss experienced in the production of carbon fibres from PAN precursor is approximately 50%. This leads to a structure containing many longitudinal voids, and a density of ~1.8 g/cm3, compared with 2.28 g/cm3 for pure graphite, and 2.1 g/m3 for pitch-based carbon fibres.

21/10/201429MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013Carbonized PAN fibers have a fibrillar microstructure as shown in figure. This microstructure may be viewed as undulating ribbons and is highly resistant to premature tensile failure because of its flexibility.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201330

The carbon fiber surface, just after the carbonization process, consists of planar microcrystallites aligned parallel to the fiber surface and along the fiber axis direction.There are no pendant bonds available so that these crystals are quite inert.Carbon fibers are therefore surface treated using an oxidation process or other methods to make the fiber surface more reactive so as to improve adhesion between the fiber surface and matrix resin.After the surface treatment, carbon fibers are normally coated with a sizing agent to improve processability in the proceeding fabrication process.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201331Carbon fibers are normally used as reinforcement in composite materials with various kind of resins as matrix materials. Therefore, surface treatment and the nature of the sizing agent are very important in controlling the ultimate performance of CFRP.For this reason the surface treatment and the sizing agents are designed and optimised according to the fibers end uses and fabrication process.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-20133221/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201333

The high tensile strength and modulus are the main characteristics which made carbon fiber materials of choice to replace conventional materials.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201334

Tensile strength is mainly dominated by defects. A carbon fiber is an elastic material and tensile failure is initiated at a defect.If a defect is big, tensile failure occurs at low stress level. Therefore, decreasing the size and number of defects is the key in order to improve tensile strength.Tensile modulus is mainly dominated by the size and alignment of the crystals along the fiber direction.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201335Another characteristic of carbon fibers is their elasticity, which is determined by the covalent bonds linking the carbon atoms.In contrast to metal, carbon fibers are perfectly elastic and fatigue deformation does not occur by loading-unloading, although large cyclic loading of CFRP can induce damage by provoking failure in the matrix or at the fiber-matrix interface.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201336CreepThere is a property of high-performance carbon fibres, both PAN and mesophase pitch-based, which sets them apart from other materials.They are not subject to creep or fatigue failure. These are important characteristics for critical applications. In a comparison of materials for tension members of tension leg platforms for deep sea oil production, carbon fibre strand survived 2 000 000 stress cycles between 296 and 861MPa. In comparison, steel pipe stressed between 21 and 220 MPa failed after 300.000 cycles.Creep studies on PAN and pitch-based carbon fibres were conducted at 2300 C and stresses of the order of 800MPa. Projections of the data obtained to ambient temperatures indicates that creep deformations will be infinitesimally small.21/10/201437MSc in Textile Engineering - Ada Ferri - A.Y.2012-2013Another beneficial characteristics of carbon fibers is their low coefficient of thermal expansion, which is one order lower than that of metals. Therefore the effect of temperature on structural dimensional stability of CFRP is one-tenth of that of metal structures.It is beneficial in reliability for space and other structural applications which are subjected to varying temperatures.Low Coefficient of Thermal expansion makes carbon fiber suitable for applications where small movements can be critical. Telescope and other optical machinery is one such application.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201338Compressive strength The dominant factor or failure mechanism for compressive strength is more complex than for tensile strength.The presence of defects is an important factor for the compressive strength. Another important factor for compressive strength is crystallite size: the larger the crystallite size, the lower the compressive strength.Therefore, reducing crystallite size and keeping crystallite alignment is effective in improving the compressive strength.

21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201339The adhesion between fiber and resin is very important in determining the performance of CFRP. Compressive strength is affected by the fiber support to the matrix resins. Adhesion between fiber and matrix resin is important in transferring the compressive strength of single filaments to that of CFRP.On the other hand, in the case of tensile strength, better adhesion between fiber and matrix resin results in a lower tensile strength of the CFRP.Tensile failure can initiate from the tensile failure of a single filament. If adhesion between fiber and matrix is too high, the crack, initiated from the single filament failure, propagates in the transverse direction without the hindrance and without being hindered by the interface between fibers and matrix.Therefore, the adhesion between fiber and matrix needs to be optimized so as to achieve the best overall performance of CFRP.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201340Surface-treated carbon fibers contain solid oxygen complexes on the surface. Such solid surface oxide complexes improve the adhesion of carbon fibers to polymer matrices in composites.The Figure shows the various types of surface oxides and their affinity to polar groups in the resins, which initiates at least chemisorptive adhesion and perhaps even formation of extensive chemical bonding between fiber and resin.

Applications of PAN-based carbon fibersCommercial production of PAN-based carbon fibers started in the UK around 1967 and in Japan in the early 1970s.The initial development was for aerospace applications but their potential for improving sports goods was rapidly recognized.Such sporting goods were golf club shafts, fishing rods, tennis rackets, archery equipment, skis, racing cars, yachts and many more.

Nowadays, in space, satellites are another application in which about 80% of the weight can be attributed to CF composites.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201342Historically, the next large market was for industrial applications, which cover pressure vessels, off-shore oil applications, robot arms, PC cases, wind-generation blades,Nowadays, half of all applications in volume are industrial applications, expansion will continue or become more accelerated in near future.

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The next big market was for transportation.After accumulating experience and demonstrating the reliability of carbon fibers with secondary structures, nowadays the fuselage, main wing and many other structures are made with carbon fibers reinforced plastics.

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Environmental consideration is another area of interest in developing new applications for CFRP.Carbon fibers need high temperature in the production process. Therefore carbon fibers need much energy for the production. In other words, the environmental impact of the production of carbon fiber is larger than that for conventional metals, such as iron.However, if the life cycle of the products (Life Cycle Assessment) with CF is considered, CFs are very environmental friendly material.If carbon fibers are used for 50% of the structure of the airplane (Boing 787), one tonne of carbon fibers allows 1400 tonnes of CO2 to be reduced during the lifetime of the airplane. CO2 emission for carbon fibers production is around 20 tonnes per one tonne of carbon fibers.21/10/2014MSc in Textile Engineering - Ada Ferri - A.Y.2012-201348

The carbon fiber manufacturing market is very concentrated; six companies produce nearly 93% of the world's supply of carbon fiber.