Tow fiber is an industrial precursor material consisting of a large, continuous bundle of thousands of individual filaments running parallel to each other without any significant twist. This semi-finished product is engineered for efficient handling and processing into fabrics, chopped fibers, or reinforced plastics. Its continuous, untwisted nature makes it suited for high-volume manufacturing of high-performance materials like carbon fiber and glass fiber.
Defining Tow Fiber and Its Structure
Tow is distinguished from other textile forms, such as staple fiber and yarn, by its structure as a continuous, untwisted bundle of parallel filaments. Staple fibers are short, cut lengths, while yarn involves twisting filaments together for textile applications. The parallel nature of tow allows for maximum strength transfer in composite materials.
The size of a tow is quantified by its filament count, known as the “k-count,” where ‘K’ denotes thousands. For example, a 12K tow contains 12,000 filaments, while heavy industrial tows can range up to 320K. Lower k-counts, such as 3K, are used for parts requiring flexibility, while larger tows are favored for high-volume production where material deposition speed is prioritized.
A chemical coating known as “sizing” is applied immediately after spinning to maintain the tow’s structural integrity. This layer protects the filaments from abrasion during handling and processing. The sizing is chemically formulated to ensure optimal adhesion with the final matrix material, such as epoxy or vinyl ester resin. This chemical affinity facilitates the effective transfer of mechanical load in the finished composite structure.
Common Materials Used in Tow Production
Tow fibers are manufactured from distinct chemical compositions that determine the final material’s performance characteristics, such as stiffness, heat resistance, or tensile strength. These fibers begin as precursor materials that are chemically or thermally processed into continuous filaments.
Carbon fiber tow is predominantly derived from Polyacrylonitrile (PAN), which serves as the primary precursor material for nearly 90% of all carbon fiber. The PAN co-polymer is spun into continuous filaments and then subjected to a multi-stage thermal process. The first stage is low-temperature oxidation (200°C to 400°C), which stabilizes the polymer chains.
Following stabilization, the material undergoes carbonization in an inert atmosphere, typically at temperatures around 1000°C, which expels non-carbon elements and leaves behind a high-purity carbon structure. For ultra-high-performance fibers, a final stage of graphitization, reaching temperatures up to 3000°C, aligns the internal structure and enhances stiffness. This precise thermal treatment controls the fiber’s final mechanical properties, including tensile strength and modulus.
Another widely used material is glass fiber tow, also known as fiberglass roving, created from silica-based raw materials, including silica sand, limestone, and various oxides. These materials are melted in a furnace, often exceeding 1700°C, to form molten glass. The molten glass is then extruded through tiny orifices to draw out the continuous glass filaments.
The most common variant is E-glass (electrical glass), an alkali-free borosilicate glass known for its insulating properties and high strength-to-weight ratio. High-strength applications utilize S-glass, which offers superior mechanical performance due to higher amounts of silica and alumina. Other specialized synthetic tows, such as aramid, are produced for applications requiring exceptional chemical resistance or impact absorption.
Primary Industrial Applications
Tow fiber is a foundational component in advanced structural materials because its continuous, untwisted form is optimized for mechanical reinforcement. A major application is in composite manufacturing, where tow acts as the reinforcement embedded within a polymer matrix. It is the preferred form for creating high-strength, low-weight components used in aerospace, high-performance automotive, and sporting goods.
A common intermediate product is the “prepreg,” which is tow pre-impregnated with a thermoset resin, such as epoxy, and partially cured. This pre-impregnated tow is laid up in specific orientations to build complex shapes, ensuring precise fiber volume and resin content. The parallel nature of the filaments allows for efficient saturation by the resin, a process known as wet-out.
Tow is also processed through weaving and braiding techniques to create structural fabrics. Woven fabrics made from tow are used as ply layers that provide reinforcement in multiple directions within a composite structure. This process is valuable in creating complex, near-net-shape parts requiring a tailored balance of strength and stiffness.
Beyond structural reinforcement, the continuous nature of tow is leveraged in specialized industrial fields. The bundled filaments are used in the production of filtration media, benefiting from the material’s large surface area and chemical resistance. Additionally, continuous filaments are chopped into short lengths and utilized in insulation products or as reinforcement fillers in plastics and concrete.