Automotive carbon fiber composite materials (CFRP) can be said to be the star material in the development of automotive lightweighting. In recent years, the industry's research on the application of this "black gold" material has also continued to develop. With the debut of two domestic mass-produced carbon fiber composite parts models, Qiantu K50 and NIO ES6, independent brands have entered the era of mass production of carbon fiber applications! At the same time, the application scope of carbon fiber composite materials is also expanding. It extends from the body and interior and exterior decoration systems to the chassis and powertrain system; it extends from the outer cover material to the structural material or structural reinforcement material. However, the high cost of carbon fiber is still an important factor limiting its development. At present, commercial-grade automotive carbon fiber is mainly PAN-based carbon fiber, and its high cost problems mainly focus on the higher production cost of PAN precursor and long production process. Therefore, the main way to reduce the cost of automotive CFRP is to reduce the cost of automotive carbon fiber precursors and seek low-cost fiber production processes and low-cost CFRP preparation processes.
1. Large tow production process
Generally, carbon fiber above 48K is called large tow carbon fiber. The performance advantages of large tow carbon fiber are mainly in the following two aspects: (1) Large tow Tow carbon fiber has lower quality requirements for PAN raw filaments than small tows, so civilian PAN filaments can be used. (2) The manufacturing cost of large tow carbon fiber is about 60% of that of small tow.
However, the difficulty in the production of large tow carbon fiber lies in the accumulation of large tow fibers, poor yarn spreading effect, difficulty in uniform infiltration of gauze sheets, and difficulty in meeting the requirements of product structure design in thickness and quality of gauze sheets; Yarn wool often appears during the yarn spreading process, resulting in yarn chaos and yarn breakage, affecting production efficiency and product appearance. Material properties cannot be effectively converted, and product performance is unstable.
Japan's Mitsubishi and Toray are typical representatives of the early masters of low-cost manufacturing technology of large tow carbon fiber. In recent years, domestic companies such as Shanghai Petrochemical, Geely Petrochemical, Guangwei Composite Materials, Lanzhou Fiber, etc. have also successively developed this front-end technology, and have now achieved the localization of large-tow carbon fiber.
2. Development of low-cost carbon fiber precursors
Relevant data shows that the price of PAN accounts for approximately 50% of the cost of carbon fiber production. Therefore, domestic and foreign carbon fiber manufacturers have also begun to seek lower-cost raw materials other than PAN to prepare carbon fibers. The United States, Japan and other major automobile carbon fiber manufacturing countries have developed low-cost alternative materials including polyolefin polymers, lignocellulose, electrospun phenolic fibers, radiation acrylic textiles, etc. For example, the Oak Ridge National Laboratory (ORNL) in the United States extracted lignin from pulp waste liquid and made low-cost carbon fiber through melt spinning and carbonization. The production cost can be controlled at 4~5$/kg. Dow Chemical "steams" and carbonizes polyethylene and other fibers in an oxygen-free state, arranges or weaves the carbon fibers into sheets on a flat surface, and then reinforces them with resin to become CFRP. Swedish research institutes Innventia and Swerea SICOMP also claim to be able to create woven CFRP laminates weighing around 1.8g based on 100% softwood lignin precursors.
3. Hybrid carbon fiber technology
Mixing carbon fiber with other fibers can complement each other in performance and effectively reduce production costs. For example, mixing carbon fiber with glass fiber, aramid fiber, etc., through reasonable structural design can reduce production costs while maintaining the original high performance of the material.
4. Pre-oxidation process
The long pre-oxidation time in the carbon fiber production process leads to a long production cycle and is also an important reason for the high cost of carbon fiber production. At present, there have been studies on using physical treatments such as ultraviolet rays and X-rays on PAN precursors, or using chemical treatments such as KMnO4 and C6H5COOH to reduce the cyclization temperature and shorten the pre-oxidation time. In terms of technology, process parameters such as temperature, time, and gas atmosphere can be changed to improve the performance of carbon fiber.
The manufacturing cost of carbon fiber composite materials mainly consists of two aspects. The first reason is that molding equipment such as autoclave and automatic lamination are expensive, and the second reason is that the long molding time of composite materials results in the consumption of manpower and material resources. Therefore, resin materials and new molding processes based on efficient molding will be an important way to optimize the low cost of carbon fiber composite materials.
Epoxy resin is the first choice for carbon fiber composites due to its excellent bonding strength and modulus, creep resistance, high toughness and good fatigue resistance. American Hexion and Dow Automotive Systems have successively launched two types of epoxy resins that can be "instantly cured" in 60s. Among them, Hexion launched EPIKOTE TRAC06170 epoxy resin and EPIKURE TRAC06170 curing agent for resin transfer molding (RTM) and liquid compression molding (LCM) processes, which only require 20s of resin injection time (RTM or LCM) and 40s of curing time to complete. Composite molding.
The VORAFORCE resin launched by Dow for LCM process can directly apply the resin evenly on the dry fiber preform, and use pressure to evenly infiltrate the resin fabric in the thickness direction.
Gurit UK has also launched "instant cure" epoxy resin. Its resin formula is mainly used for complete sets of prepregs. and hot-in/hot-out stamping and forming processes. Although the curing cycle of this process takes 5 minutes, it is reported that the surface of the parts manufactured can reach Grade A without the need for mold post-processing.
Huntsman Advanced Materials also announced the launch of a fast-curing epoxy resin. According to Huntsman, the resin can be cured in only 30 seconds at 140°C, making the composite molding process possible within 1 minute. To this end, Huntsman has also developed dynamic fluid compression molding (DFCM) to match the resin, which can eliminate the need for high-pressure injection molding and, in many cases, the fiber prepreg process. Compared with conventional wet compression molding (WCM), one of the main advantages of this process is that it can reduce the gap between laminate layers, the composite porosity is less than 1%, the performance is comparable to the high-pressure RTM process, and up to 66% fiber Volume content (FVC) composites can be achieved without special processing conditions.
The recycling and reuse of carbon fiber composite materials is an effective method to reduce the cost of carbon fiber use and increase its economic added value. At present, research on carbon fiber recycling methods is also constantly updated, such as high-temperature thermal cracking, oxidation fluidized bed method, supercritical fluid technology, etc.
In terms of application, Ford used recycled carbon fiber reinforced polypropylene PP composite materials in its 2018 Explorer sports utility vehicle SUV for the rigid part of the A-pillar bracket, replacing the original ASA material.