Magazine March 2015

Focus on the textile world: from maximum diversity to carbon fibers

In 1879 the American Thomas Edison succeeded in producing carbon strips, which he then used as filaments in electric light bulbs (carbon-filament lamps). He patented the method, which is based on the carbonization of cellulose fibers, during the same year. During the 1960s, carbon fibers in the form of fiber bundles, fiber strands or multifilament yarns found their way into the aerospace industry, but it was only from the mid-1970s onward that they were industrially manufactured and processed into textile reinforcement materials.

What carbon fibers can do

Due to their properties, carbon fibers can be used in all kinds of ways. Relatively lightweight and highly rigid at the same time, they are also largely uninvolved in chemical processes, electrically conductive, thermally stable, infusible, biocompatible and also X-ray permeable. This special combination means they can be used in the most diverse sectors. These practical features stem from the chemical bonds of the carbon atoms within the graphite-like structure of the fibers.

Without carbon fibers, many of the designs used in today's aerospace industry would not have been possible. Carbon-fiber-reinforced plastic also makes aircraft lighter and more economical. Industrial applications of these fibers include rollers for the paper and printing industry, and numerous items in the sports equipment industry (tennis rackets, golf-club shafts, bicycle frames, masts for sailing boats and surfboards, helmets, and fishing rods). Flexible sheets of carbon fiber are also the material of choice for heat conductors, the advantage being that carbon fibers, when heated, have a negative coefficient of expansion.

Most carbon fibers are produced from a polyacronitrile basis. Fibers produced in this way have prevailed in the market, today accounting for about 90% of all carbon fibers. Various methods are available for processing. In addition to weaving, braiding, embroidery work and knitting they are also suitable for the production of multiaxial scrims.

The possible applications of classic carbon fibers are very diverse, and their potential is far from exhausted.

Activated carbon fibers – numerous possible applications

Compared to traditional carbon fibers, activated carbon fibers are relatively young. The base material, activated carbon, is made from hard coal, lignite, wood charcoal, peat charcoal, and coconut fibers. Activated carbon is characterized by its active pore system, created by the removal of the components' volatile constituents using hot water vapor at temperatures of 900 to 1100° C.

Here, an oxidizing reaction between carbon and water takes place on the surface of the carbon, which also removes carbon from the pore walls – a process known as carbonization. The result is amazing: a microscopically fine inner surface structure is formed. One teaspoon of activated carbon therefore has a surface roughly the size of a soccer field. The gas masks that were used in the two World Wars are probably the best-known application area: A layer of activated charcoal was used as a filter.

The production of fibers made from activated carbon has now resulted in an almost endless range of applications. Because of their sometimes very narrow fiber diameter, activated carbon fibers have a very high adsorption rate. This is a perfect prerequisite for protection against biological and chemical pollutants, toxins, and warfare agents. However, housewives (and house-husbands) are also happy about activated carbon fibers: to prevent cleaning rags from smelling unpleasant, an admixture of about 20% activated carbon fiber in relation to the usual fibers normally suffices.

Applicationss in the nonwovens industry

Carbon fibers have gained considerable importance in the automotive industry in recent years. This sharp increase in consumption has been accompanied by an increased amount of fiber waste. In the production of automotive parts made of carbon fiber textiles, even at maximum efficiency, the amount of waste is around 20-30%. The nonwovens industry is likely to play an important role in the future where the recycling of this waste is concerned. This is because recycled fiber waste cannot be recovered in the form of fiber, and cannot therefore be considered for conventional processing methods such as weaving. The use of CFRP components (CFRP stands for "carbon fiber reinforced plastic"), particularly in the automotive industry, will lead to the (further) development and industrialization of processes for the recycling of these components. Here again, the nonwovens industry will be asked to process the resulting textile waste into high-quality nonwovens.

Activated carbon fibers also have applications as nonwovens in numerous areas, e.g. as layers in gas masks, dust masks and masks with odor-inhibiting functions, and also in industrial filters, especially in the production of paints, pesticides and filters for the defense industry.

Carding and needling of carbon fibers

With 100% carbon fibers, the carding process is still in its infancy. Here, the following points need to be borne in mind. Very good extraction must be ensured, since the resulting carbon dust can pose a health risk. The system also has to be specially designed for the purpose, to avoid any electrical short-circuits that may arise from the dust of the conductive carbon fibers. One problem that has not yet been satisfactorily solved is the fact that carbon fibers are very smooth, and a certain degree of crimp is required. There is still no solution here that can be implemented industrially. Experience has shown that a proportion of roughly 30 percent of crimped "transport fibers" is required in order to realize the carding of carbon fibers on an industrial scale.

The needling of carbon fibers, however, has been performed successfully for decades now. A well-known example: brake discs in the aircraft industry. The most diverse needles from Groz-Beckert are used here, always adapted to the specific requirements. On the one hand there are special needle designs developed in collaboration with customers, and on the other there are standard needles from the Groz-Beckert product portfolio (roughly 1,700 needle types) which already match requirements perfectly.

Needling for the production of brake discs made of carbon fibers is divided into pre-needling and main needling. Pre-needling primarily requires the use of standard needles, equipped either with conventional barbs or RF barbs.

Needle examples:

  1. 15x18x36x3 ½ C333 G 1002 / 601731
  2. 15x18x36x3 ½ R333 G 3007 / 607921

In the main needling process, where the most important task is to reorient the fibers from the horizontal to the vertical Z direction, special working part geometries are required. Tri STAR® and Cross STAR® needles are often utilized here.

A suitable needle, for example, is type:

15x16x36x3 C222 G 73012 /609471.

Recent applications of carbon fibers include the needling of recycled carbon fiber composites, which are currently being used primarily in the automotive industry. These applications use stable needles with minimal risk of breakage (needle example: 15x18x3 ½ R333 G 1002/600171).

Among the numerous applications of needled carbon fibers, one product promises to be particularly interesting: needled nonwovens that provide protective and filtering properties against chemical and biological weapons. These are used in the military as linings woven into clothing. Good nonwoven drapability is important here. Standard needles with fine working parts are generally used for producing these:

Needle Example: 15x18x40x3 R222 G 3037/603721

Further areas of application:

  1. Heat protection (clothing for welders, firefighters and racing drivers, as well as flame arresters and insulation for automobiles, trains and airplanes)
  2. Seals (high-temperature applications)
  3. Hot gas filters (e.g. in the cement and steel industry)
  4. Semi-finished products for many composite applications (drapable nonwovens for corners and as intermediate layers in carbon fabrics)
  5. Gas absorption filters made from carbon fiber with a porous surface

Carbon fibers are also frequently used in mixtures (fiber and/or layer mixture) with synthetic fibers and/or mineral fibers – so during needling it is always important to treat the carbon fibers very gently to prevent them from turning into coal dust. Needles similar to those used with other brittle fibers (e.g. glass or ceramic fibers) should thus be employed here.