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Manufacturing process of carbon fiber

Polymerization process

The process began with polymer raw materials called precursors. Today, about 10% of carbon fibers are made of rayon or asphalt precursors, and most of them are derived from polyacrylonitrile (Pan), which is made of acrylonitrile, which is derived from commercial chemicals propylene and ammonia. This paper mainly introduces the manufacturing process of PAN based carbon fiber.

For a long time, converting pan into carbon fiber has been a challenge for manufacturers, because most of the investment of carbon fiber manufacturers is spent on precursors, and the quality of finished carbon fiber also directly depends on the quality of precursors. Usually, the precursor preparation starts with acrylonitrile monomer, which is combined with plasticized acrylic acid comonomer and catalyst, such as itaconic acid, sulfur dioxide acid, sulfuric acid or methacrylic acid, and continuous stirring will mix the components to ensure viscosity and purity, and trigger the formation of free radicals in the molecular structure of acrylonitrile. This change leads to polymerization, a chemical process that produces long-chain polymers that can form acrylic fibers.

Spinning process

PAN fibers are formed by a method called wet spinning. The concentrated liquid is immersed in the liquid condensation bath and extruded through the holes in the spinneret made of precious metal, which match the required number of filaments of PAN fiber (for example, 12000 holes for 12K carbon fiber). Wet spun fibers are drawn by washing to remove excess coagulant, and then dried and stretched to continue to improve the orientation of Pan polymer.

An alternative process to wet spinning is a hybrid process called dry jet / wet spinning, which has an air section between the fiber and the coagulation bath, resulting in smooth circular PAN fibers. The last process of PAN precursor fiber is oiling, which can prevent the tow from sticking and then winding.

Oxidation

Oxidation treatment is the longest time-consuming process in the carbon fiber preparation process. The temperature range of oxidation furnace is 392 ° f to 572 ° f (200 ° C to 300 ° C). This process combines the oxygen molecules in the air with the PAN fiber and starts the crosslinking of the polymer chain, which increases the fiber density from 1.18 g / cc to up to 1.38 g / cc.

In order to avoid runaway heat release of PAN fiber (the total heat release energy released during oxidation is estimated to be 2000 kJ / kg, which will cause fire hazard), pre oxidation furnace manufacturers use a variety of airflow designs to help heat dissipation and control temperature.

The oxidation time varies according to the chemical reaction of the precursor fibers. Generally speaking, it takes 60 to 120 minutes. Each production line requires 4 to 6 ovens, which can be stacked to provide two heating zones. The oxidized PAN fiber contains about 50% to 65% carbon molecules, and the rest is a mixture of hydrogen, nitrogen and oxygen.

Carbonization

The carbonization reaction is carried out in a specially designed carbonization furnace and requires inert (oxygen free) atmosphere protection. In the absence of oxygen, only non carbon molecules including hydrogen cyanide and other VOCs (generated at a concentration of 40 to 80 ppm during stabilization) and particles are removed and discharged from the high-temperature furnace, followed by post-treatment in an environmentally controlled incinerator.

In the carbonization process, a certain drafting tension must be applied, so that the crystallization of carbon molecules can be optimized to produce carbon fibers with a carbon content of more than 90%. The difference between carbon fiber and high modulus carbon fiber (also known as "graphite fiber") is that the former is a fiber carbonized at about 1315 ° C / 2400 ° F, with a carbon content of 93% to 95%, while the latter is graphitized at 1900-2480 ° C (3450-4500 ° f), with a carbon content of more than 99%.

The relatively high cost of high modulus and ultra-high modulus carbon fibers is partly due to the length of residence time and the temperature that must be reached in the high temperature furnace. The pre oxidation treatment time is in hours, but the carbonization time is one order of magnitude shorter, in minutes. After carbonization, the fiber will significantly reduce its weight and reduce its diameter.

Surface treatment and sizing

In addition to precursor fibers, surface treatment can best distinguish a supplier's products from competitors' products. The adhesion between matrix resin and carbon fiber is very important for strengthening composites, and the purpose of surface treatment is to enhance this adhesion in the process of manufacturing carbon fiber. The manufacturer will use different treatment methods to increase the surface activity of the fiber after treatment, so as to improve the surface area that can be used for interfacial fiber / matrix bonding, and add reactive chemical groups such as carboxyl.

The subsequent process is sizing treatment. Generally, sizing agent accounts for 0.5% to 5% of the weight of carbon fiber, which can protect carbon fiber from becoming an intermediate product in the process of processing and processing (such as weaving). Sizing can also bind the filaments in each tow to reduce fuzzing, improve processability and increase the interfacial shear strength between the fiber and the matrix resin. After sizing and drying, the long carbon fiber preparation process is completed, and the single tow is separated and wound on the bobbin.