New Work Tackles Challenges of Producing Thermoplastic Tapes Directly from Textile-Based PAN Carbon Fiber.

One of the biggest challenges the advanced composites industry faces is that there is insufficient supply of low-cost carbon fiber to meet demand in industries that would use the reinforcement if they could both afford it and get enough of it in a usable form. This is a long-standing challenge and one that many IACMI members and members of the global advanced-composites supply chain have long worked on.

Illustration of polymer flow exiting the flooding die

For example, IACMI member, Oak Ridge National Laboratory (ORNL) and the U.S. Dept. of Energy (DOE) have invested decades of work to develop low-cost carbon fiber from alternative precursors, including polyethylene, lignin, pitch, and a textile-form of polyacrylonitrile (PAN) that is similar to the fiber used to make sweaters. The textile-PAN precursor produces fiber that would be perfect for market segments like automotive, wind energy, sporting goods, consumer electronics, and building/construction in that it has higher modulus but lower ultimate strength than typical carbon fiber produced from conventional aerospace-grade PAN precursor. Additionally, estimated production costs are reduced by 50 percent and energy usage by 60 percent, making the textile-based carbon fiber (TCF) both more affordable and giving it a lower, greener energy footprint.

Ongoing TCF Projects

Ongoing work on process and property enhancements for the alternative precursors continues at ORNL’s Carbon Fiber Technology Facility (CFTF), a semi-production scale, 42,000 square foot/3,900 square meter carbon fiber production facility specifically designed for the development and demonstration of high-potential, low-cost precursor materials to produce industrial-grade carbon fiber. The facility is managed by Dr. Merlin Theodore, CFTF Director and also IACMI Materials and Processing Director.

Under the direction of IACMI Chief Technology Officer Dr. Uday Vaidya interesting and complementary work with TCF has been underway on a number of IACMI projects. Vaidya is also the University of Tennessee-Oak Ridge National Laboratory Governor’s Chair for Advanced Composite Manufacturing.

The aim of this research is to bridge the data gap between those who may eventually be involved in producing, finishing, and packaging TCF and those who might be involved in converting and using the fiber. Topics include both characterization of the fiber in its various forms and also finding methods to directly use the ultrawide tow band (~300-600K) that is a characteristic of the TCF production process. The wide tow textile grade precursor enables higher throughput through the furnace(s) in turn helping to lower production costs and make more fiber available for markets that want it. However, that wide tow band also makes handling, converting, and packaging the as-produced fiber a big challenge for converters, molders, and end-use customers.

One project that Vaidya has been directly involved with is finding methods to make thermoplastic composite tapes directly from the TCF.  This work has taken three paths. First, Vaidya, George Husman, president, Husman Consulting, Inc. and retired director and CTO of Zoltek Companies Inc. (St. Louis, Mo.), and the University of Tennessee Research Foundation have filed a provisional patent covering a process to produce thermoplastic composite tapes in-line with the production process for both TCF and conventional carbon fiber (12k to 50k). As part of the provisional patent, University of Tennessee (UT) and Husman developed the process to impregnate narrower 12K, 24K, and 50K tows with thermoplastic resins. In recent months, the UT team has advanced to making tapes with the wider TCF tow bands.

TCF Thermoplastic Tapes: Opportunities & Challenges

Thermoplastic composite tape is an intermediate and growing form factor for carbon composites not only in the aerospace industry—where thermoset composite tapes have been used for some time—but also in industries like automotive, ground transportation, and energy storage (compressed-natural gas (CNG) containers). Not only can thermoplastic composite tapes be fed into automated tape layup (ATL) equipment to produce tape-based composite structures with fiber layers in nearly any orientation, but the tapes also can be cut into locally-reinforcing patches that are then insert molded/overmolded with discontinuous fiber composite to enhance the performance of compression and injection molded parts. Still other processes that can use thermoplastic tapes include pultrusion (another IACMI project) and filament winding. Additionally, the tapes themselves can be chopped into discontinuous long-fiber flakes and used in thermoplastic compression or injection molding or possibly even in hybrid molding combining thermoplastic and thermoset composites. The ability to produce thermoplastic composite tapes using TCF could help reduce the costs of such products and the parts made with them. However, there are many technical challenges that must be addressed before hot-melt impregnation of TCF’s wide tow band is commercially viable.

For example, fiber feed and handling must be significantly modified as TCF’s wide tow band is challenging to handle owing to the large number of filaments involved (that require equipment modification) as well as the tendency to form a catenary wave across the tow band during tape-making. This catenary behavior can cause tows to split unevenly and hence to enter the impregnation die under different tension, which, in turn, can cause the tape to twist, deform, and adversely affect wetout. Finding a way to maintain balance between tension and flexibility of the fiber during tape impregnation proved to be a challenging and iterative process. Researchers came to understand the importance of maintaining tow integrity in order to spread the filaments to achieve a high degree of filament wetout, which is critical for producing quality tapes without voids.

Sizing was another area of work. As would be expected, the wide-tow fiber is heavily sized and that treatment is necessary to help fiber travel smoothly between feed and die. However, previous studies indicated that the best fiber wetout by higher-viscosity thermoplastic resins was achieved using fiber without sizing.  Hence, the team developed a technique to take advantage of sizing at the start of the process, but then to burn it off just prior to entry of the tow band into the impregnation die to produce tapes.

Die design was also an important area of research. In moving from 50K tows to TCF’s wider tow bands, the die had to be completely redesigned with a two-stage design eventually being adapted. In Stage 1, the fiber is impregnated; in Stage 2, optimized brake angles for tensioning/impregnation pins are configured to set the desired fiber weight fraction (FWF) exiting the die. The team currently is producing 30 percent to 50 percent FWF impregnated thermoplastic tapes.

Rheology and polymer feed are another area the team has been evaluating. The work has focused on producing TCF tapes using polypropylene (PP) as well as polyamide 6 (PA 6 / nylon) matrices—two polymers heavily used in the automotive industry. To achieve good polymer flow and fiber impregnation across the die, it was important to be able to accurately simulate and then validate the models. The first figure below shows the close correlation achieved between predicted and measured rheology and shear rates for a thermoplastic polymer at different temperatures. The team used PolyXtrue extrusion die-design software (from Plastic Flow, LLC of Hancock, Mich.). This code is based on the Williams-Landel-Ferry (WLF) model. Below that is a photo of polymer exiting the flooding die. The red lines highlight flow of the polymer (which is not otherwise visible owing to resin transparency).

Representative rheology and shear rates of generic homopolymer polypropylene at different temperatures – comparison of predicted vs. measured data

More recent work has focused on equipment. Because water baths proved ineffective in rapidly cooling tapes as they exited the extrusion die, a system of post-impregnation air cooling was developed. Another project evaluated methods for take-up of completed tapes onto creels/spools after production. Currently, attention is being given to electronic integration, including development of a formal graphical user interface and a programmable-logic controller (PLC)-based system. The ultimate goal of the work is for a TCF-capable thermoplastic tape production module to be added to the back end of a TCF carbon fiber production line to eliminate the need to produce secondary/intermediate products.

“Our team has faced many technical challenges, but we’ve also had some important accomplishments,” Vaidya explains. “Handling such a wide tow band and successfully—and quickly—impregnating the fibers to achieve quality thermoplastic tapes without voids has been difficult. However, our team has explored a number of process parameters including multiple iterations of die design; polymer flow simulations through the die; and various aspects of fiber feed, tensioning, and preheating. Not only has the team proven out some of the claims in our patent, but they have also achieved impregnation line speeds of 12 feet [3.7 meters) per minute to produce 30 percent FWF polypropylene and polyamide 6 tapes. Now they’re working to scale up the process to make tapes with the ultrawide tow band TCF.”

Contributions

Key members of the TCF thermoplastic tapes team include:  Dr. Merlin Theodore, who has been producing various version of the TCF fiber at the CFTF, as well as Saurabh Pethe (UT M.S. student in Mechanical Engineering), Mohamed Sager (UT M.S. student in Electrical Engineering), Benjamin Schwartz (UT graduate with a B.S. in Mechanical Engineering), and Alex Fleish (Purdue University graduate with a B.S. in Mechanical Engineering who is currently working at UT and FCMF).