A long-term challenge facing the advanced composites industry is to find a way to produce low-cost carbon fiber so that all industries that want to use this valuable and effective composite reinforcement can use it. Thanks to decades of efforts to increase productivity through precursor chemistry, mechanical innovation, and process improvements, the cost of carbon fiber has been greatly reduced.

One of the promising areas is the use of precursors to replace traditional aerospace grade polyacrylonitrile (PAN). For example, the U.S. Department of Energy (DOE, Washington, DC, U.S.) and Oak Ridge National Laboratory (ORNL, Oak Ridge, Tennessee, U.S.) have developed methods to produce carbon fiber from alternative precursors such as polymers, lignin, and coal.

Alternative precursors

One of the most promising, and one of the precursors closest to commercialization, is textile-grade polyacrylonitrile (PAN), which is similar to the fibers used to produce acrylic sweaters. Like other alternative raw yarns researched by ORNL, textile grade PAN carbon fiber (TCF) differs from special aviation PAN carbon fiber in several ways.

One of the differences is the textile-grade PAN precursor, because it is used in the textile industry (such as curtains, clothes, furniture fabrics) and is supplied in the form of wide tow, and its cost is inherently lower than that of the specialized aerospace-grade PAN. wire. In the carbon fiber production process, compared with traditional PAN fiber, this has the effect of increasing output and reducing conversion costs. It is also suitable for the production of larger tow fibers-in ORNL's carbon fiber technology factory, the K number of products on the production line is about 300K to 450K, while the traditional PAN carbon fiber is usually 3K to 50K. In addition, although it takes longer to process the TCF precursor, it does not contain reaction promoters like conventional PAN and usually runs at a lower temperature (this depends on the balance of residence time and temperature requirements), so More fibers can be processed in a given time, and the energy usage is lower in terms of weight or volume, which helps reduce costs. In fact, ORNL estimates that fiber conversion can save about 60% of the total energy and save about 50% of the cost.

This in turn provides an opportunity to mass-produce lower-cost fibers and use them to produce products with smaller carbon footprints. In industries such as automotive/ground transportation, consumer electronics, sporting goods, construction/construction and wind energy, TCF is an attractive reinforcement material compared to strength-driven applications in aerospace, because its application is often stiffness Driven. It is worth noting that the non-aerospace industry may consume a lot of fiber.

However, TCF is still a product different from traditional PAN carbon fiber, so it is not only necessary to better characterize this fiber, but also to find a way to transform and utilize it. Because this precursor can be processed in an ultra-wide tow belt, the processing equipment needs to be modified throughout the fiber production process. It also changes the surface modification and packaging after production. Of course, it will affect the conversion process from manufacturing fiber tapes and fabrics to prepregs and preforms.

It must be clearly recognized that on the one hand, TCF can solve the problem of carbon fiber cost/availability, on the other hand, it also creates new problems in how to handle, convert and package this material. In the past three years, the Advanced Composites Manufacturing and Innovation Institute (IACMI, Knoxville, Tennessee, USA) has also been carrying out many member-supported research projects to solve these problems. One of the interesting projects studied the method of converting ultra-wide tow TCF into thermoplastic prepreg.

reduce costs

There is growing interest in thermoplastic composite prepreg tapes in many industries, but these products are often expensive because it is necessary to successfully impregnate any kind of fiber with pre-polymerized high molecular weight and high viscosity thermoplastic resin (rather than unreacted low viscosity Thermosetting resin) requires specialized equipment and technical knowledge. It is too easy to produce a prepreg tape with many voids and poor fiber moisture permeability. The final product not only looks bad, but also has the risk of premature failure.

University of Tennessee Knoxville (UTK, Knoxville, Tennessee), professor and director of Advanced Composites Manufacturing (Advanced Composites Manufacturing), and IACMI Chief Technology Officer (CTO) Dr. Uday Vaidya since the early 2000s Since then, he has cooperated with George Husman, president of Husman Consulting Inc. (Cape Coral, Florida, USA) and retired director and chief technology officer of Zoltek Co. Inc. (St. Louis, Missouri, USA) to develop thermoplastic composite materials projects. The many mutual exchanges between them promoted the idea of in-line prepreg traditional large-tow carbon fiber (24K to 50K) to produce thermoplastic composite prepreg tape at the back end of the fiber production line. The production of prepreg tape immediately after fiber production will save a separate intermediate process step and all transportation and handling work. It is hoped that this will help reduce the cost of prepreg tapes and parts made from these prepreg tapes.

To realize this concept, it is necessary to develop processes and equipment to manufacture carbon fiber reinforced thermoplastic prepreg tapes with different tow sizes. In 2018, Vaidya and Husman filed a joint application with the University of Tennessee Research Foundation (UTRF, Knoxville, Tennessee) for a provisional patent related to the online production of thermoplastic prepregs reinforced with carbon fiber tow of up to 50K. . The following year, Vaidya and his UTK team extended this work to the second application, including thermoplastic prepreg for ultra-wide TCF (300K to 450K tow).

TCF TP prepreg tape

As expected, there are many technical challenges that need to be resolved in this process. Fiber feeding and processing require major equipment modifications, because TCF tends to be wider and involve more fibers than traditional carbon fibers. In the fiber prepreg step of prepreg tape production, it is also prone to catenary behavior. This means that when the tow enters the prepreg mold, it will form a sine wave, causing the tow to split unevenly and enter the mold under different tensions, resulting in distortion and deformation of the prepreg tape, and poor fiber wetting.

"Finding a way to maintain a balance between fiber tension and flexibility is really challenging and requires a lot of work," Vaidya explained. "Our team has gradually realized how important it is to maintain the integrity of the tow in order to disperse the tow and obtain a high degree of wettability. Of course, this is essential for the production of high-quality prepreg tapes."

Then there is the issue of size. The size of the TCF tow is large, which can help the fiber to move smoothly from the creel to the impregnation die, and then wet it with resin in the impregnation die to make a prepreg tape. However, once it enters the mold, previous studies have shown that to use high-viscosity thermoplastics to obtain good wettability, the presence of rubber will actually hinder impregnation, so it is highly desirable to remove the rubber. The trial and error eventually led the team to develop a technique that can burn off the glue before the tow enters the dipping mold.

In order to accurately predict the rheology of the resin and the amount of polymer fed through the impregnation die and make a good prepreg tape, new simulation and verification work is required. The team is committed to producing TCF prepreg tapes made of polypropylene (PP) and polyamide 6 (PA6), two thermoplastics that are widely used in automobiles due to their toughness and affordability. The team used the PolyXtrue extrusion die design software of Plastic Flow LLC (Hancock, Michigan, USA), which was based on the Williams-Landel-Ferry (WLF) model and provided results with measured rheology and shear rate. Excellent correlation.

Mold design itself is another important research area, especially when the team switched from standard 12K tow to 50K tow, and then to ultra-wideband TCF tow. At this stage, the mold must be completely redesigned and the two-stage process adjusted. In the first stage, the fiber is pre-impregnated. In the second stage, the optimal break angle is set for the tension/dipping roll to control the tensile tension, fiber weight fraction (FWF) and quality control of the carbon fiber to achieve the ideal tensile properties of the prepreg tape. Currently, the team has produced 30-50% FWF prepreg tapes with PP and PA6, even if the tow is larger.

At the same time, in order to quickly cool the prepreg tape after leaving the mold, a post-immersion air cooling system was developed. Due to the width of the product and the challenges posed by the downstream application of these prepreg tapes, the team even had to devise a method to tighten the complete prepreg tape on the creel/reel.

Applications of thermoplastic prepreg tapes include automobiles, trucks, wind blades, infrastructure (bridges), construction, sporting goods, ships and marine products.

Vaidya said the team is currently focusing on electronic integration, which includes building formal graphical user interfaces and developing programmable logic controller (PLC)-based systems. Ultimately, the team’s goal is to develop a thermoplastic prepreg production module that can be added to the back end of TCF or conventional carbon fiber production lines to facilitate the production of secondary/intermediate thermoplastic prepregs.

Vaidya explained: "Our team faces many technical challenges, but we have also achieved some success." "Processing such a wide tow and successfully and quickly impregnating the fibers to achieve high-quality, void-free thermoplastic prepreg It is difficult. However, our team has explored many process parameters, including multiple iterations of mold design; polymer flow simulation through the die; and various aspects of fiber feeding, tensioning and preheating. We have not only reached According to the standard requirements in our patent, and while producing 30% FWF polypropylene and PA6 tape, the impregnation line speed has reached 12 feet/minute (3.7 meters/minute).

Next step plan

Vaidya said that it has submitted intellectual property protection around this prepreg technology to the UT Research Foundation, one of which uses traditional 24K and 50K fibers, and the other uses wide tow fibers. He said that commercialization will focus on online prepreg in the carbon fiber production line. He said: "This will greatly reduce post-processing and ease of adaptation, thereby further reducing the total cost of intermediates."

How to use these prepreg tapes? Vaidya said that there are many options: "For example, the material can be cut into long fiber form, used for pultrusion of raw materials, filament winding of large cans, secondary molding in compression molding (similar to organic sheet), and sheet material in mixed processes. Reserve-for example, the use of LFT (long fiber thermoplastic), SMC (sheet molding compound) and other synergistic process materials. "This opens up a range of product types that will benefit from improved stiffness, high impact toughness and improved processing properties , Such as shape, stretching and bending, recycling and reversible chemical properties.

He said that applications include automobiles, trucks, wind power blades, infrastructure (bridges), construction, sporting goods, ships and marine products. "Now the wide-filament carbon fiber thermoplastic intermediate may provide one more possibility for the adoption of carbon fiber, which was too expensive before," Vaidya concluded.

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