Synthetic fibers are manufactured fibers created through chemical synthesis, primarily using polymers derived from petrochemicals or minerals. They are distinct from natural fibers like cotton or wool, and also from semi-synthetic fibers like rayon, which are made from naturally occurring polymers such as cellulose. A standardized classification system is necessary to ensure consistency in global manufacturing practices, facilitate international trade, and provide consumers with predictable product performance. This organization allows manufacturers to select the appropriate polymer chemistry and production technique needed to achieve desired characteristics.
Classification Based on Chemical Composition
The most fundamental way to categorize synthetic fibers is based on the chemical structure of the repeating molecular unit, or polymer backbone. Synthetic fibers are created through polymerization, where monomers link together to form long polymer chains. This chemical identity dictates the fiber’s inherent properties, such as thermal behavior, moisture absorption, and durability.
Polyester, one of the most widely produced synthetic fibers, is chemically defined by the presence of an ester functional group in its main chain. The most common form, Polyethylene Terephthalate (PET), is synthesized from ethylene glycol and terephthalic acid monomers. This linkage provides the fiber with high strength, resilience, and excellent wrinkle resistance.
Polyamides, commonly known as Nylon, are identified by the repeating amide linkages that connect their monomer units. For example, Nylon 6,6 is formed by the reaction between adipic acid and hexamethylene diamine. This robust chemical structure makes polyamides highly abrasion-resistant and strong, which is why they are often used in carpets and technical apparel.
Acrylic fibers are polymers composed of at least 85% by weight of the monomer acrylonitrile. The resulting structure gives the final fiber a soft, warm feel, often used to simulate wool. Their chemical composition requires specialized spinning techniques due to their high melting point.
Polyolefins, including polypropylene and polyethylene, are characterized by their simple, long carbon-chain backbones. These fibers are exceptionally lightweight and hydrophobic, meaning they resist absorbing water. Their structure makes them highly resistant to many chemicals and allows them to be used extensively in industrial fabrics, ropes, and thermal clothing.
Classification Based on Manufacturing Method
Synthetic fibers are also classified by the technique used to convert the raw polymer into a continuous thread, a process known as spinning. This classification is determined by the polymer’s thermal properties and solubility. The three main spinning methods—melt, dry, and wet—each use a different physical mechanism to solidify the fiber.
Melt spinning is used for polymers that can be heated and melted without chemical degradation, such as polyester and nylon. The polymer chips are melted into a viscous liquid and then extruded through a device called a spinneret. The extruded filaments solidify almost instantaneously as they are cooled by a blast of ambient or chilled air. This method produces a high volume of fiber efficiently.
Dry spinning is employed for polymers that dissolve readily in a volatile solvent but cannot be easily melted without breaking down. The polymer is first dissolved to create a thick solution, sometimes referred to as a “dope.” This solution is then extruded into a spinning chamber where hot air is circulated to evaporate the solvent, leaving the solidified filament behind. Acrylic fibers and some elastomeric fibers are often formed using this process.
Wet spinning is necessary for polymers that have poor thermal stability and require a solvent that is not easily evaporated. In this method, the polymer solution is extruded directly into a liquid coagulation bath. The chemical interaction between the bath liquid and the polymer solution causes the filament to precipitate and solidify. This technique is used for fibers like aramid and certain high-performance acrylics.
Classification Based on Final Physical Form
The final classification system organizes synthetic fibers based on their physical presentation, which directly influences how they are processed into yarn and fabric. This categorization simplifies the selection of raw material for textile manufacturing applications. Fibers are primarily categorized by their length and structural arrangement.
Filament fibers are continuous, long strands that can measure thousands of meters in length. When extruded from the spinneret, the fiber is kept in its continuous form, resulting in smooth, strong yarns with a lustrous appearance. Most synthetic fibers, such as nylon and polyester, are initially produced as continuous filaments.
Staple fibers are short, discrete lengths of fiber, typically ranging from 35 to 150 millimeters. Synthetic filament is intentionally cut into these short lengths to mimic the feel and spinning behavior of natural fibers like cotton or wool. These short fibers must be twisted together to form a yarn, which usually results in a softer, bulkier texture than filament yarn.
Tow represents an intermediate stage in the production of staple fibers. It is a large, rope-like bundle containing thousands of continuous filaments collected after the spinning process. This bundle is then subjected to further processing, such as crimping or drawing, before being cut to the desired staple length for use in textile spinning machinery.