By 2025, the average fuel economy must increase to 22.3 km/l (52.5 mpg). One method automakers will use to achieve this goal is to use lightweight materials such as carbon fiber. If the three major car manufacturers use 8.2 kilograms of carbon fiber in each car they produce, then global carbon fiber production capacity will need to be at least doubled to meet demand.
The cost of carbon fiber must be cut in half to reach an acceptable price level in the automotive industry. The use of cheap precursors is one way to reduce the cost of carbon fiber.
The demand for lighter-weight carbon fiber composite materials in automobiles is increasing, which is driving the development of the carbon fiber industry toward high-quality and cheap carbon fiber. Most carbon fibers produced today use PAN as a precursor, which does not help to produce low-cost carbon fibers.
Using other carbon fiber precursors, such as lignin, has been explored for more than forty-five years, but with only partial success. The main obstacles include that the lignin precursor cannot support its own weight during most of the heat treatment process, the amount of raw materials required for the melt spinning process, and the modulus, strength and brittleness of the carbon fiber produced.
With the advancement of process and mechanical technology, asphalt precursors have become popular again, and because the carbon content of asphalt precursors is as high as 85% or more, it does not require the same depth of stabilization as PAN precursors, making carbon fiber costs lower.
This article outlines the steps required to convert pitch precursors to carbon fibers, as well as the key factors and difficulties associated with these steps.
Carbon fiber compared with current automotive materials
In order to better understand the impact of the automotive industry's possible demand on the carbon fiber production industry, it is necessary to understand the number of conventional materials used in the global automotive industry. In the automotive industry, aluminum and magnesium have become the fastest growing materials due to their light weight. They are increasingly used in the manufacture of automotive components, power transmission systems, and body structures.
In 2018, 86 million cars were sold worldwide, with an average weight of 1815 kg. Using a reasonable estimation method, half of the average vehicle is made of a certain metal, then the metal consumption of the automotive industry exceeds 78,000,000 metric tons. Compared with the global carbon fiber production capacity in 2018 (approximately 149,000 metric tons) [5], it is clear that if carbon fiber is to become a common material in the automotive industry, its manufacturing capacity must be greatly improved.
Increasing the world's carbon fiber production capacity is not the only issue that needs to be considered, and price is also an important consideration. Now, 50K industrial-grade carbon fiber can achieve a price of 15 to 18 US dollars / kg, the cost of low-carbon steel is 1.10 US dollars / kg, the average cost of recycled aluminum in 2018 is 2.27 US dollars / kg.
Many research teams have put a lot of effort into reducing the cost of carbon fiber, but developing "low-cost carbon fiber" is a great challenge. The use of inexpensive precursors, such as pitch, is an effective way to reduce the cost of carbon fiber.
Background introduction of carbon fiber
Carbon fiber was developed in the 1950s as a reinforcement material for high temperature resistant components of missiles. The original carbon fiber was produced by heating the rayon to carbonization, but because the carbon yield was only about 20%, and the resulting fiber had low rigidity and strength, polyacrylonitrile (PAN) was used as a precursor of carbon fiber. . Compared with the carbon fiber with rayon as the precursor, the carbonization yield of the PAN precursor reached 55%, and the physical and mechanical properties of the PAN-based carbon fiber were significantly improved.
In the 1970s, people continued to look for other raw materials, which led to the introduction of carbon fiber made from petroleum pitch, which originated from petroleum processing. The carbon yield of petroleum pitch precursors is higher than 85%, and the resulting carbon fiber has a high modulus and strength, which meets the needs of many applications. Due to its excellent flexural modulus and ease of graphitization, pitch-based carbon fiber is considered an excellent choice for some special applications.
PAN-based and pitch-based carbon fiber
The output of PAN-based carbon fiber accounts for 90% of the total carbon fiber and is the most common type of carbon fiber. The conversion rate of carbon fiber is 50% to 55%. It has a medium modulus and a high tensile strength, and is mainly used for structural reinforcement. Asphalt is a polycyclic aromatic hydrocarbon extracted from petroleum pitch, coal tar or polyvinyl chloride. The conversion rate of carbon fiber is about 85%. Compared with PAN, it can reach a higher modulus and has higher electrical conductivity and thermal conductivity.
Compared with pitch-based carbon fiber, the PAN-based carbon fiber production line has a low carbonization yield and a large amount of exhaust emissions, so a larger exhaust emission treatment system is required. PAN-based carbon fiber and pitch-based carbon fiber also have very different exhaust gas components to be treated in the production process. The largest waste gas component in the production of PAN-based carbon fiber is HCN. Others include NH3, CO, CO2 and H2. The pitch-based carbon fiber production process will not release any detectable HCN. The main components of exhaust gas are CO, CO2 and H2. Both will produce some tar and coke.
Sulfur residues in pitch-based carbon fibers are usually naturally occurring. For PAN-based carbon fibers, the sulfur residue is usually due to the use of DMSO as a solvent, and the spinning desulfurization process is not fully removed. Sulfur is undesirable in PAN because it can cause metal dust and corrosion in high and low temperature furnaces. For pitch-based carbon fibers, unless extremely high temperatures are reached, such as above 2500°C, sulfur in the pitch is unlikely to be a problem, so the impact on the equipment is much smaller.
There may be another problem in the production process of PAN-based carbon fiber. When sodium thiocyanate is used as a solvent in the spinning process, sodium will remain in the fiber and react in a high-temperature furnace to form toxic sodium cyanide. For these two precursors, it is critical to understand what residues are generated during the spinning process, the heat treatment phase, and the related reaction kinetics in order to be able to select the correct equipment material and do well in the design for potential problems that may arise ready.
Finally, for fiber filaments that are physically shed during heat treatment, another notable difference between pitch-based and PAN-based is the amount of particles produced. Asphalt-based carbon fibers produce few particulates. In contrast, PAN fibers will generate a large amount of particulates that must be processed due to the use of oil bundling, and may require more processing equipment, such as the need for dust collection at the exhaust chimney To capture these particles. If proper arrangements are not made in the design stage, the accumulation of these particles in the exhaust pipe and equipment may affect the quality uniformity of the product and increase downtime and maintenance time.
In order to increase the modulus of PAN-based carbon fiber as much as possible, it is necessary to use an ultra-high temperature furnace for heat treatment. Because of the high temperature, the internal parts of the furnace body, especially the heating elements, wear out much faster, and the furnace body construction materials and maintenance costs are high. Because pitch fibers are more ordered than PAN fibers, pitch-based carbon fibers can obtain higher modulus than PAN-based carbon fibers without going through this process.
Unlike pitch-based carbon fiber, in order to obtain certain physical and mechanical properties, PAN-based carbon fiber must be drafted in the oxidation and carbonization process. This drafting requires more equipment investment (such as a tension frame), and the price of the equipment is also different. For national manufacturing, special export licenses may be required. Since pitch-based carbon fiber cannot be drawn, the performance of pitch-based carbon fiber is determined during the spinning process.
Throughout the production process, the weight that PAN-based fibers can withstand far exceeds its own weight, so no support is required during processing. PAN is usually spun into fibers and then placed on a spool, which is unfolded and drawn into a stabilizing furnace with a tension stand. The PAN tow K number range is very large, from 1k to 600k (sic), although the PAN spinning process is also difficult to master, it is well known that PAN spinning has now achieved scale.
Asphalt cannot bear its own weight throughout the production process, so it needs support during processing. Usually, the pitch is spun directly onto the mesh belt, and then sent to the stabilizing furnace. PAN-based carbon fiber is usually in the form of tow, while pitch-based carbon fiber is usually in the form of non-woven felt. Asphalt-based carbon fiber can also be processed into tow, but the range of filament number is much smaller than PAN-based carbon fiber, usually <2k. Compared with the PAN-based process, the purification process and spinning process of pitch are very difficult to grasp, and these processes need to be better understood, and the corresponding process scale has not yet reached the development level of PAN-based.
Asphalt precursor
The precursor pitch is a polycyclic aromatic hydrocarbon obtained from petroleum pitch, polyvinyl chloride or coal tar. Compared with PAN, its price is relatively cheap, and the carbon fiber carbonization yield is significantly improved. There are two types of precursor pitch, isotropic and anisotropic (mesophase), but all pitches start from isotropy and become mesophase through treatment. The carbon fibers produced by the two pitches are structural, performance and nanowoven. The structure is different.
Isotropic pitch-based carbon fibers are usually chopped fibers with a diameter of 12 to 18 μm, a density of about 1.6 g/cm 3, a low modulus of about 40 GPa, and a low thermal conductivity, because of its weak structural orientation and graphite crystals Degree is low. The price of isotropic pitch-type carbon fiber is competitive in the market. At the same time, it is widely used in the industrial field because of its light weight, heat resistance, chemical stability and wear resistance.
Mesophase pitch-based carbon fibers are usually continuous fibers (approximately 2k filaments) or sprayed into a nonwoven felt state. The filament diameter is 7-10um, the density is 1.7-2.2 g/cm 3, and the modulus is between 600-965 GPa [10]. Most carbon fibers made with pitch as a precursor use mesophase pitch, because a highly oriented hexagonal plane condensed ring microcrystalline structure can be formed along the fiber axis during the spinning process, thereby eliminating the need for drafting during heat treatment [11] . This highly oriented molecular structure and high crystallinity can make the modulus of pitch-based carbon fibers much larger than traditional PAN fibers, and can approach the theoretical limit of 1000 GPa.
Processing asphalt into carbon fiber
The production process of carbon fiber includes mechanical process and chemical process. Fig. 2 is a schematic diagram of a process of preparing pitch fiber nonwoven felt using pitch as a precursor, and then obtaining carbon fiber nonwoven felt through a carbonization furnace. This schematic diagram shows the process of spinning solvated mesophase pitch into fibers in a melt-blown device and illustrates the complexity of the spinning system compared to the remaining process equipment. The main components of the spinning system include asphalt feeding device, vacuum exhaust port, extruder, spinning pump, pressure pump, spinneret, filter, air jet regulator, fiber collector, temperature and pressure system, As well as the vacuum control system required to form the stabilizing and carbonizing precursor, the precursor eventually forms a carbon fiber product.