Nylon is a synthetic polymer and a foundational material in modern industry. This manufactured material belongs to the class of polyamides, characterized by a repeating molecular structure. Since its invention, nylon has demonstrated a combination of properties that make it suitable for applications far beyond its original use as a silk substitute. Its adaptability allows it to be processed into fine fibers, durable sheets, and complex molded parts.
The Polyamide Structure and Synthesis
The defining chemical feature of nylon is the presence of amide linkages, which connect the molecular building blocks into long polymer chains. The term “polyamide” describes this structure, indicating many amide bonds along the backbone. These robust linkages are created through condensation polymerization, a reaction that joins two different types of functional groups.
The synthesis of common nylons, such as Nylon 6,6, involves reacting two distinct monomers: a diamine (containing two amino groups) and a dicarboxylic acid (containing two carboxyl groups). When these groups react, they form the characteristic amide bond while releasing a small byproduct molecule, typically water. This release of a small molecule defines the reaction as condensation polymerization.
Nylon 6,6 is named because it is formed from two monomers, hexamethylenediamine and adipic acid, each contributing six carbon atoms. The resulting long polymer chains are held together by hydrogen bonds that form between the amide linkages of adjacent chains. These strong intermolecular forces give nylon its semi-crystalline structure and high mechanical integrity. The ordered arrangement translates directly into the material’s overall strength and stability.
Defining Physical Characteristics
The inherent strength of nylon stems from the powerful forces between its polymer chains, resulting in high tensile strength, which is the resistance to being pulled apart. This makes the material highly durable and able to withstand significant mechanical stress. Nylon also exhibits exceptional abrasion resistance, holding up well against repeated rubbing and wear.
Nylon possesses a high melting point, providing good thermal stability. Common varieties, such as Nylon 6,6, melt around 260°C, retaining their structural shape at elevated temperatures. This thermal resistance, combined with a low coefficient of friction, makes nylon suitable for moving parts where heat generation is a concern.
When spun into a fiber, nylon gains elasticity, allowing it to stretch and recover its original shape. However, nylon is hygroscopic, meaning it absorbs moisture from the environment. This water absorption can temporarily reduce its stiffness and strength, affecting the material’s dimensional stability and mechanical performance in humid conditions.
Common Commercial Varieties and Uses
The most widely used commercial variants are Nylon 6 and Nylon 6,6. The numbers indicate the carbon atom count of the monomers used in their creation. Nylon 6,6 uses two six-carbon monomers, while Nylon 6 is synthesized from a single six-carbon monomer called caprolactam via ring-opening polymerization. Despite this structural difference, both share the fundamental polyamide chemistry.
Nylon 6 tends to be slightly more flexible and possesses better impact resistance, making it ideal for applications requiring strength and toughness. It is widely used in the textile industry for clothing, carpets, and tire cords due to its good dyeability and finish. The more rigid Nylon 6,6 offers better heat resistance and stiffness, leading to its use in demanding engineering applications.
As an engineering plastic, Nylon 6,6 is selected for molded components such as gears, bearings, and automotive parts (including engine covers and radiator end tanks) where superior thermal and mechanical performance is required. Other nylon types, like Nylon 11 and Nylon 12, are used for flexible tubes and cables due to their lower moisture absorption and excellent chemical resistance.