Automobile carbon fiber composite material (CFRP) can be said to be a star material in the lightweight development of automobiles. In recent years, the automotive industry's application research on this "black gold" material is also constantly developing. With the debut of China's promising K50 and NIO ES6 mass-produced carbon fiber composite material models, Chinese independent brands have entered the era of mass production of carbon fiber applications! At the same time, the scope of application of carbon fiber composite materials has also continued to expand. It extends from the car body, the interior and exterior trim system to the chassis and the powertrain system; it extends from the material of the outer cover to the material of the structural part or structural reinforcement. However, the high cost of carbon fiber is still an important factor limiting its development. At present, the commercial grade automotive carbon fiber is mainly PAN-based carbon fiber, and its high cost problem is mainly concentrated in the higher PAN filament production cost and longer production process. Therefore, the main way to reduce the cost of automotive CFRP is to reduce the cost of automotive carbon fiber strands, and seek low-cost fiber production processes and low-cost CFRP preparation processes.
Low-cost carbon fiber composite technology
1. Large tow production process
Generally, the carbon fiber with a length of more than 48K is called large tow carbon fiber. The performance advantages of large tow carbon fiber are mainly in the following two aspects: (1) The quality requirements of large tow carbon fiber for PAN original yarn are lower than that of small tow. wire. (2) The manufacturing cost of large tow carbon fiber is about 60% of that of small tow.
However, the difficulties in the production of large tow carbon fiber are: the accumulation of large tow fibers, the effect of yarn spreading is not good, the yarn piece is difficult to evenly infiltrate, the thickness and quality of the yarn piece is difficult to meet the requirements of product structure design; Wool causes messy yarn breaks and yarn breakage, which affects production efficiency and product appearance. Material properties cannot be effectively converted and product performance is unstable.
Mitsubishi and Toray of Japan are typical representatives of mastering the low-cost manufacturing technology of large tow carbon fiber earlier. In recent years, domestic SHSH, JLSH, WHGW Carbon Fiber Technology Group, LZ Carbon Fiber and other companies have also developed this front-end technology in succession, and now have achieved the localization of large tow carbon fiber.
2. Low-cost carbon fiber precursor development
Relevant data show that the price of PAN accounts for about 50% of carbon fiber production costs. Therefore, domestic and foreign carbon fiber manufacturers have also begun to look for lower cost raw materials other than PAN to prepare carbon fiber. Major automobile carbon fiber manufacturing countries such as the United States and Japan have developed low-cost alternative materials including polyolefin polymers, lignocellulose, electrospun phenolic fibers, and radiant acrylic textiles. For example, the lignin extracted from the pulp waste liquid of the Oak Ridge National Laboratory (ORNL) in the United States has been made into low-cost carbon fibers by melt spinning and carbonization. The production cost can be controlled at 4~5$/kg. Dow Chemical charcoalizes carbon fibers such as polyethylene in an anaerobic state, arranges or weaves carbon fibers on a flat surface, and then reinforces them with resin to become CFRP. Swedish research institutions Innventia and Swerea SICOMP also claim to be able to manufacture woven CFRP laminates weighing approximately 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 can effectively reduce production costs. For example, by mixing carbon fiber with glass fiber, aramid fiber, etc., through reasonable structural design, the production cost can be reduced on the basis of maintaining the original high performance of the material.
4. Pre-oxidation process
The long pre-oxidation time in the production process of carbon fiber, leading to a long production cycle is also an important reason for the high cost of carbon fiber production. At present, there have been studies on the physical treatment of ultraviolet rays, X-rays, etc., or the chemical treatment of KMnO4, C6H5COOH, etc. to reduce the cyclization temperature and shorten the preoxidation time. In terms of process, the process parameters such as temperature, time and gas atmosphere can be changed to improve the performance of carbon fiber.
High efficiency molding resin and molding process
The manufacturing cost of carbon fiber composite materials is mainly composed of two aspects. The first is that the molding equipment such as autoclave and automatic lamination is expensive, and the second is that the longer molding time of the composite material causes the consumption of manpower and material resources. Therefore, resin materials based on efficient molding and a new molding process will be an important way for low-cost optimization of carbon fiber composite materials.
Epoxy resin is the first choice for carbon fiber composite materials because of its excellent adhesive strength and modulus, creep resistance, high toughness and good fatigue resistance. American Hexion (Hansen) and Dow Automotive Systems have successively introduced two types of epoxy resins that can be “instantly cured” within 60 seconds. Among them, Hexion launched EPIKOTE TRAC06170 epoxy resin and EPIKURE TRAC06170 curing agent for resin transfer molding (RTM) and liquid compression molding (LCM) processes. It only takes 20s resin injection time (RTM or LCM) and 40s curing time to complete Composite molding.
The VORAFORCE resin for the LCM process launched by Dow can directly apply the resin evenly on the dry fiber preforms, and evenly infiltrate the resin fabric in the thickness direction by pressure.
Gurit UK has also launched an "instant curing" epoxy resin whose resin formulation is mainly used for complete prepreg and hot-in/hot-out stamping forming processes. Although the curing cycle of this process requires 5 minutes, it is reported that the surface of the parts it manufactures can reach grade A without post-processing of the mold.
Huntsman Advanced Materials also announced the launch of a fast-curing epoxy resin. According to Huntsman, the resin can be cured at 140°C for only 30 seconds, which makes the composite molding process possible within 1 minute. To this end, Huntsman has also developed Dynamic Fluid Compression Molding (DFCM), which is compatible with the resin. This process can eliminate the high-pressure injection molding process, and in many cases, the fiber prepreg process can also be omitted. Compared with conventional wet hair compression molding (WCM), one of the main advantages of this process is that it can reduce the gap between the laminate layers, the porosity of the composite material is less than 1%, the performance is comparable to the high-pressure RTM process, and up to 66% fiber The volume content (FVC) of the composite material can be achieved without special treatment.
Carbon fiber recycling technology
The recovery and reuse of carbon fiber composite materials is an effective method to reduce the use cost of carbon fiber and enhance its economic added value. At present, the research on carbon fiber recovery methods is also constantly updated, such as high temperature thermal cracking, oxidized fluidized bed method, supercritical fluid technology, etc.
In terms of applications, Ford used recycled carbon fiber reinforced polypropylene PP composite material in its 2018 Explorer SUV, which is used for the rigid part of the A-pillar bracket to replace the original ASA material.