EMPLOYED MATERIALS - CARBON FIBER

A composite, non-metallic polymeric material is called 'carbon fiber'. It is composed of a matrix (called dispersing phase or resin) that shapes the piece, which contains a reinforcement, or dispersed phase, based on carbon fibers (whose raw material is polyacrylonitrile). It is a very expensive material, with high mechanical properties and lightweight. Like fiberglass, it is a common case of metonymia, in which the whole name of a part is given, in this case the name of the fibers that reinforce it.

Being a composite material, in most cases - approximately 75% - thermosetting polymers are used. The polymer is usually epoxy resin, of the thermosetting type although other polymers, such as polyester or vinyl ester are also used as the basis for carbon fiber although they are falling into disuse.

Main Properties:

• High mechanical resistance, with a high elastic modulus.

• Low density, compared to other elements such as steel.

• High production price.

• Resistance to external agents.

• Great thermal insulation capacity.

• Resistance to temperature variations, retaining its shape, only if thermoset matrix is ​​used.

• Good flame retardant properties.

 Carbon fiber (FC) was initially developed for the space industry, but now, as the price drops, it has been extended to other fields: the transport industry, aeronautics, high competition sports and, lately we find FC even in pocket wallets and watches.

The FC is composed of many strands of carbon threads. There are many kinds of FC with diverse properties, adapted to many applications.

 To get an idea, just compare the FC with the steel:

Characteristics F. Carbon Steel
Mod. Tensile strength 3.5 1.3
Specific resistance 2.0 0.17
Density 1.75 7.9
 

Its resistance is almost 3 times higher than that of steel, and its density is 4.5 times lower. In terms of modulus of elasticity there is a wide range of HR from 240 to 400.

Other very appreciable properties in carbon fiber are resistance to corrosion, fire and chemical inertia and electrical conductivity. With variations in temperature, it retains its shape.

Carbon fiber is a polymer converted to fiber. In most cases, FC remain as non-graphite carbon. The term graphite fiber is only justified, when the FCs have been subjected to a graphite heat treatment (2000-3000 ° C), which gives them a three-dimensional crystalline order, observable by X-rays.

Synthesis of Carbon Fiber

A common method of obtaining carbon filaments is the oxidation and thermal pyrolysis of PAN (polyacrylonitrile), a polymer used to create many synthetic materials. Like all polymers, PAN forms long chains of molecules, aligned to make the filament continuous. When the PAN is heated in correct temperature conditions, the PAN chains are joined side by side, to form graphene ribbons.

The PAN or its copolymer is spun using the wet spinning technique. The technique of sometimes molten spinning is also used. The first step is to stretch the polymer so that it is parallel to what will be the axis of the fiber and oxidizes at 200-300 ° C in air, a process, which adds oxygen to the PAN molecule and creates the hexagonal structure. The polymer that was once white is now black.

PAN-based fibers have diameters ranging from 5 to 7 microns. And those of tar 10-12 microns. The FC is classified by the number of filaments, in thousands, of which the thread consists. A 3k FC (3000 filaments) is 3 times stronger than one of only 1k, but also weighs 3 times more.

With that strand an FC fabric is woven

Carbonization

To achieve a high resistance fiber, the carbonization heat treatment is used: the PAN is heated to 2000-2500 ° C in an oxygen-free atmosphere, the polymer chains are aligned to form graphene sheets, very thin, two-dimensional ribbons, and a tensile strength of 5,650 N / mm2.

Graffiti

In the heat treatment of graphitization, if we heat the PAN at 2500-3000 ° C we get the maximum resistance of the FC: 531,000 N / mm2.

Now is the time to weave the fiber, to form sheets and tubes, which will then be impregnated in an epoxy resin in a mold. Once the cured resin, hardened, it must be mechanically shaped, to get the finished product, for example: the blade of a propeller. There are several types of fibers, from treatment temperatures:

The high modulus fiber: It is the most rigid and requires a higher treatment temperature. Its modulus of elasticity exceeds 300 and even 500 GPa. Better yet, the single crystal "graphite" has a 1050 GPa module. The modulus of elasticity 390 GPa is 70 times higher than that of aluminum alloys.

The fiber of high tensile strength: It is carbonized at the temperature that gives the highest tensile strength, with values ​​greater than 300 GPa.

The standard fiber: It is the most economical and isotropic structure. The stiffness is lower than in the previous ones; The treatment temperature is lower. It is marketed as short fibers.

Activated carbon fiber: It has an adsorption speed 100 times higher than that of activated classic coals. It is obtained through carbonization and physical and chemical activation of different precursors: breas, rayon, polyacetates, etc. It has a large specific surface area and very uniform pore size. The fiber comes in the form of felts or fabrics.

Vapor grown in vapor phase: This fiber is obtained by a catalytic process of chemical surface deposition in vapor phase (in English: VGCF steam ground carbon fibers). Because of their variety of sizes, they are a bridge between conventional FC and nanofiber.

The Manufacturing of Composite Material 

The choice of the matrix profoundly affects the properties of the finished product.

Thin FC sheets adhere to the mold, which take the desired piece shape. We align the fibers of the tissue in the most convenient direction, because the fibers are anisotropic. We impregnate the FC fabric with resin.

On the resin we place another FC fabric impregnated with resin, and so on overlapping FC fabrics and resin layers.

It is easy to see that there is a lot of skilled labor. The more intense the loads that the product will bear, for example: a helicopter blade, the more careful we will be to align the fiber direction correctly.

Finally heat the piece, or cure it in the air. Exposed to water will not suffer corrosion, and is very strong compared to how little it weighs.

If there are air bubbles in the mold, the final resistance will be reduced.

Matrices or resins

The matrices are thermostable or thermoplastic.

The fiber is not used by itself, but to reinforce matrices, for example: the aforementioned epoxy resin or other thermosetting plastics. In some applications the matrix is ​​thermoplastic.

The Thermosets

These polymers are plastics that heat cured, or other means, are transformed into an infusible and insoluble product. They are the most used (90 percent) in structural composites.

65 percent of thermostable matrices are unsaturated polyesters.

The biggest advantage of the thermostable is that they have a very low viscosity, and can be introduced into the fibers at low pressure.

The impregnation of the fibers initiates the chemical curing, which produces a solid structure, is a process carried out isothermally. Recycling, in practice, is not possible.

Thermoplastics

The thermoplastic is capable of being softened repeatedly by heat, and hardened by cooling. It can be easily recycled, which is very important in the automobile sector. Its impact resistance is excellent.

Thermoplastics provide the advantage that the molding is not isothermal, that is: hot and molten plastic is introduced into the cold mold, and thus very short cycles are achieved in time.

But molten polymerized thermoplastics usually have viscosities between 500 and 1000 times higher than thermosets. The process therefore requires high pressures and increased costs.

Lately there is the process of liquid monomer. The advantage of the thermoplastic liquid monomer (for example, Cyclics PBT) is that it is processed isothermally (injection, polymerization, crystallization and demolding at the same temperature), as if it were a thermostable.

"Hybrid thread" is the last method of processing thermoplastics: the polymer is introduced in solid form, such as powder or fiber and is achieved to mix with the carbon fibers. The "hybrid thread" becomes fabric, or other textile forms, sufficient heat and pressure is applied, the thermoplastic melts and fills the short distance that separates it from the carbon fiber. Then the impregnated piece is cooled and we achieve the solid composite.

The pressure mold or “The vacuum bag” is excellent for quality products: the sail regatta mold, with its impregnated FC fabrics, is introduced into a bag of waterproof walls and we remove the vacuum. The flexible walls of the bag strongly press the helmet, and we eliminate air bubbles. The FC fabric interface and the resin is also improved.

The Miraculous Fiber

The Japanese Association of FC manufacturers call it: "Light in weight, strong and durable." It undoubtedly has a great industrial future, even outside the aeronautical-space area. It is the technological material of the 21st century, precursor of nanomaterials. High price, but with a tendency to fall.

Low density, exquisite mechanical properties, electrically conductive, high elastic and tensile modulus, heat resistant, low thermal expansion, chemical stability, thermally conductive and also permeable to X-rays, an important property in medical equipment.

The transport industry, especially aerospace, has been looking for composite materials (C / C) for decades to replace metal. The goal is to reduce vehicle weight and increase efficiency.

The satellite and military aircraft industry takes the lead; The high price of C / C is not an inconvenience. The X-32A, from Boeing, is an excellent example.

Commercial aircraft have already reached 10-25 percent of the total weight of the aircraft. For the first time Boeing now offers us the 787, for 250 seats, with 50 percent of the weight in C / C, mainly carbon fiber (FC).

In sporting goods: fishing rods, snowshoes, bicycles, formula 1 cars, carbon fiber is already popular, although high priced.

Penetration will continue to increase until series cars are reached.

Outside of transport, in construction, a sector where the weight is somewhat secondary, carbon fiber is already used in bridges and walkways. It even provides economic advantages over traditional methods.

Carbon fiber is generally stronger than Kevlar, that is, it can withstand more force without breaking. But the Kevlar tends to be harder. This means that it can absorb more energy without breaking, even more than carbon fiber.

Some types of Carbon Fiber 

1x1 MESH: Also called Flat Carbon Fiber or Taffeta

2x2 MESH: Also called Twill Carbon Fiber or Twill