The performance comparison between thermoplastic carbon fiber and thermosetting carbon fiber for aerospace applications.
Since the new millennium, significant achievements have been made in the research and exploration of various new composite materials, such as the currently popular glass fiber, carbon fiber, and aramid fiber composites. This article will introduce carbon fiber and its composites, known as "black gold." Carbon fiber has been around for over a century, and with continuous development, it has gradually found applications in sports equipment and Formula 1 racing cars. Currently, the mainstream material is thermosetting carbon fiber composites, which include thermosetting resins such as epoxy resin, phenolic resin, and bismaleimide resin.
Thermoplastic carbon fiber composites are more suitable for aerospace applications.
With the increasing research on carbon fiber and various plastics, it has been discovered that using specialty plastics as a matrix in combination with carbon fiber can better leverage the high-performance characteristics of carbon fiber. If continuous carbon fiber reinforced thermoplastic composites can be mass-produced, the entire industrial sector will benefit, and high-end industries such as aerospace and medical fields will experience significant growth. Currently, the advantages of carbon fiber epoxy resin composites-such as high strength, low creep, high modulus, and low cost-have been proven applicable to the aerospace field. However, their weaknesses are also quite evident, including high brittleness, susceptibility to splitting, and high moisture absorption rates, which pose certain application risks. The incorporation of thermoplastic matrix materials can address these performance deficiencies and open up new possibilities for carbon fiber composites.
There are many high-performance specialty plastics, such as Polyether Ether Ketone (PEEK), Polyether Ketone Ketone (PEKK), Polyether Ketone Ether Ketone Ketone (PEKEKK), Polyether Imide (PEI), Polyphenylene Sulfide (PPS), and Polyamide (PA). These thermoplastic matrix resins can provide better physical structure and chemical properties for carbon fiber. Taking Polyether Ether Ketone (PEEK) as an example, it has a glass transition temperature (Tg) of around 150°C and a melting point of about 370°C, which significantly enhances the high-temperature resistance of carbon fiber composites. Additionally, it better maintains the inherent properties of carbon fiber, ensuring good strength, toughness, chemical resistance, and solvent resistance. PEEK also possesses excellent thermal stability, flame retardancy, and low dielectric constant, making it one of the highly sought-after materials for future aerospace applications.
Performance Comparison of Thermoplastic and Thermosetting Carbon Fiber for Aerospace Applications
Research teams have conducted in-depth studies on thermosetting and thermoplastic carbon fiber composites for aerospace applications, comparing carbon fiber reinforced polyether ketone (PEK) composites with carbon fiber reinforced epoxy resin composites.
1.Carbon Fiber Reinforced Polyether Ketone Plate: This composite consists of a laminate made from 60% carbon fiber and 40% polyether ketone (PEK). It features ten layers of bidirectional carbon fiber placed between eleven layers of PEK, with PEK film on both the top and bottom. The stacked CF/PEK is pressed at 410°C under 10 Bar pressure for 30 minutes.
2.Carbon Fiber Epoxy Resin Plate: This composite uses LY556 epoxy resin as the matrix material, reinforced with bidirectional carbon fabric. At room temperature, the HY951 curing agent is added to the epoxy resin, mixed in a ratio of 100:12. The carbon fiber reinforcement is maintained at 60 wt%, resulting in an approximately 3 mm thick carbon fiber epoxy resin laminate using ten layers of fabric.
3.Testing Methodology: Mechanical performance tests were conducted on the two types of carbon fiber plates mentioned above, including tensile testing, hardness testing, and fracture toughness testing. Additionally, thermal performance tests were performed on both carbon fiber plates, including differential scanning calorimetry (DSC) and limiting oxygen index (LOI) tests.
4.Performance Testing Results Show:
A. Tensile Strength and Modulus: The average tensile strength and modulus of carbon fiber reinforced polyether ketone (PEK) composites are 425 MPa and 7.8 GPa, respectively, while the average tensile strength and modulus of carbon fiber reinforced epoxy resin composites are 311 MPa and 5.2 GPa, respectively. The elongation at break for carbon fiber reinforced PEK composites is 9.43%, whereas that for carbon fiber reinforced epoxy resin composites is 11.32%.
B. Hardness: When carbon fiber is added to the matrix, the overall hardness of the composite increases, indicating that the filler enhances resistance to plastic deformation. The hardness values for PEK and epoxy resin are 87 and 85, respectively, with corresponding composite hardness values of 94 and 89, showing no significant difference.
C. Fracture Toughness: Due to the brittleness of epoxy resin, the fracture toughness of carbon fiber reinforced epoxy resin composites decreases as the matrix toughness decreases. In contrast, the PEK matrix exhibits better toughness, leading to improved toughness in carbon fiber reinforced PEK composites. The maximum load considered when calculating fracture toughness is the maximum load the material can withstand before fracture in the SENB test; a higher stress intensity factor (Kic) corresponds to higher toughness. The results show that the Kic of carbon fiber reinforced PEK composites is 13.71 MPa·√m, while for carbon fiber reinforced epoxy resin composites it is 11.53 MPa·√m, indicating better performance for the former.
D. Thermal Behavior During Heating and Cooling: The thermotransitions of polymer composites during heating and cooling were studied using DSC. The melting temperature and crystallization temperature of the matrix were compared, revealing the melting temperature (Tm), crystallization temperature (Tc), and glass transition temperature (Tg) of the sample materials.
E. Limiting Oxygen Index: Testing of the limiting oxygen index (LOI) shows that incorporating carbon fiber into both matrix materials significantly improves the LOI. Data indicate that the LOI for epoxy resin and PEK are 25 and 35, respectively, while the corresponding LOIs for the carbon fiber composites are 32 and 47, with carbon fiber reinforced PEK composites showing notable improvement.
Through testing, researchers found that thermoplastic carbon fiber composites with PEK as the matrix outperform thermosetting carbon fiber composites with epoxy resin across various performance metrics. The substantial differences in data highlight the fundamental performance disparities between thermosetting and thermoplastic carbon fiber composites, suggesting vast application potential for thermoplastic carbon fiber composites, especially in advanced fields like aerospace.
However, why is the adoption of thermoplastic carbon fiber composites far less widespread than that of thermosetting composites? This is closely related to their respective processing techniques. Thermoplastic carbon fiber composites require high processing temperatures, and the molten thermoplastic resin often struggles to fully impregnate the carbon fiber bundles. If this step is not executed perfectly, the mechanical performance of the resulting thermoplastic carbon fiber composites may even fall short of the current mainstream thermosetting carbon fiber composites.
