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The Key Factor Determining The Performance of carbon Fibers - Protofilament 

1. Raw silk is a key semi-finished product of carbon fiber: it determines the final performance of carbon fiber and accounts for over 50% of the cost

(1) The carbon fiber performance of the raw silk is decisive: during the production stage of the raw silk, there will be certain degrees of gaps, fuzz, and uneven silk threads due to differences in process details and material ratios. These defects of the raw silk will not disappear during the carbonization stage. Good carbon fiber should have the characteristic of uniformity. After inheriting the defects of the original fiber, the uniformity of each segment of carbon fiber will vary, resulting in insufficient tensile strength of carbon fiber. So the quality of finished carbon fiber directly depends on the quality of the precursor used for producing carbon fiber. Although carbon fibers can be combined with resin materials and their properties can be controlled by changing the impregnation method, temperature, steps, and other procedures, various characteristics can also be created for different downstream applications in the manufacturing process of carbon fibers by adding gases, liquids, and other materials. So starting from the raw silk stage can generate customized carbon fiber products with better specific performance.

(2) The cost proportion of raw materials is high: from the perspective of carbon fiber cost, the proportion of raw materials in carbon fiber has reached 51%, which is more than twice the second cost proportion of carbonization. Therefore, the cost of carbon fiber directly depends on the cost of raw materials, and semi-finished raw materials are also the channel for carbon fiber to achieve the highest cost reduction target and revenue in the future.

(3) The performance of PAN precursor mainly depends on the structure and arrangement of PAN molecules, where the control of PAN molecular structure is mainly concentrated in the polymerization process, while the arrangement of PAN molecules is mainly formed in the spinning process. So for high-performance carbon fibers, the first step is to have PAN precursor fibers with excellent performance.

2. The two key steps of raw silk polymerization: one-step method has strong stability and slow speed; two-step method has high production capacity elasticity and fast speed

(1) The polymerization process is mainly divided into two types: homogeneous solution polymerization (one-step method) and heterogeneous solution polymerization (two-step method): the one-step method refers to the solvent used that can dissolve both monomers and the resulting polymer. Therefore, after the reaction is completed, the polymerization solution can be directly used for spinning. The two-step method refers to the process where a solvent can only dissolve monomers and cannot dissolve the resulting polymer (usually using water as a medium, also known as aqueous precipitation polymerization). Before spinning, the polymer is re dissolved in a solvent to produce a spinning solution.

(2) From the perspective of production elasticity and speed, the two-step method is easier to master and can achieve intermittent production, resulting in stronger production capacity elasticity; Secondly, the two-step method allows for continuous filling during the production process, resulting in faster production speed. However, if we consider factors such as polymerization heat, polymer quality, and operational difficulty, the one-step method is more conducive to controlling production processes and ensuring product quality stability for large-scale industrial production. At present, major domestic and foreign manufacturers adopt a one-step production process route, while overseas Dow and Monte, as well as domestic Jilin Carbon Valley, adopt a two-step method.

(3) In terms of solvent usage, DMAC, DMSO, and NaSCN are the mainstream: the main solvents used in the one-step method include DMSO, DMF, DMAC, NaSCN, and ZnCl2; The solvents used in the two-step method mainly include DMSO, DMF, and DMAC. From the perspective of manufacturers at home and abroad, the mainstream solvents are DMAC, DMSO, and NaSCN. Among them, DMAC and NaSCN are more suitable for large fiber bundle products, while DMSO is more suitable for small fiber bundle products. Therefore, even though the price of DMSO is much higher than other solvents, most manufacturers focusing on small fiber bundles still choose DMSO

3. Two key steps of raw silk - spinning: good performance of dry process products and low cost of wet process

(1) Dry spinning: The spinning solution is formed into a fine flow through the spinneret, and the solvent is evaporated and carried away by the hot air in the spinning channel. At the same time, the polymer is concentrated and solidified to form primary fibers. The method is simple to operate and has a high solvent recovery rate. The cross-section of the raw silk produced by dry spinning is more regular, so there are fewer defects inherited from the subsequent pre oxidation and carbonization processes, which is conducive to the production of high-performance raw silk. Carbon fiber production processes above T700 mostly use dry spinning.

(2) Wet spinning: The spinning solution is filtered and defoaming, and extruded from the spinneret through a metering pump. Under the action of a coagulation bath, the solution solidifies in a fine flow to form a layer of film. The coagulant diffuses into the fine flow, and the solvent in the fine flow also diffuses into the coagulation bath through the cortex. After double diffusion molding, the appropriate spinneret is stretched to form primary fibers. Wet spinning has lower costs but slower spinning speeds.

(3) Dry wet spinning (dry spray wet spinning): Dry spray wet spinning combines the technical advantages of dry and wet methods, and is more similar in operation process to wet methods. After the spinning solution is sprayed out through the spinneret hole, it does not immediately enter the coagulation bath, but first passes through the air layer and then enters the coagulation bath for double diffusion, phase separation, and the formation of filaments

4. Comparison of different spinning process routes: Wet and dry wet methods 

(1) The process flow is different. Wet spinning involves pressing the spinning solution obtained by the solution method out of the fine pores of the spinneret to form a fine flow, and then solidifying it into silk in the solidified solution; Dry spinning involves pressing the spinning solution prepared by solution spinning out of the spinneret, forming a fine flow and solidifying into silk in hot air due to the rapid evaporation of the solvent.

(2) Different fiber qualities. The fiber structure obtained by wet spinning is loose, with poor physical and mechanical properties and lower fiber quality; The fiber structure obtained by dry spinning is compact, with good physical, mechanical and dyeing properties, and high fiber quality.

(3) Different spinning speeds. The spinning speed of wet spinning is lower, while the number of holes on the spinneret is higher than that of melt spinning; Dry spinning has high spinning speed and high yield.

(4) Different applications. Wet spinning is commonly used to produce fiber materials with special properties such as silk, artificial silk, and carbon fibers; Dry spinning is mainly used for producing fibers of materials such as polyester and nylon, especially for spinning polymer solutions with high molecular weight, high concentration, and high viscosity.

(5) Different advantages and disadvantages. Wet spinning can produce softer and more hygroscopic fiber products, but the production process requires a large amount of water and chemical agents, resulting in low production efficiency; Dry spinning technology is mature, with high production efficiency, suitable for producing stable fiber products, but requires high temperature treatment and high processing conditions for certain polymer compounds.


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