How Is Nylon Prepared? Nylon 66 Production Process Explained

Step by Step Process of How Nylon 66 Is Prepared

Step 1: Formation of Nylon Salt

Hexamethylenediamine and adipic acid are dissolved separately in methanol or water, then mixed together at a controlled temperature to form hexamethylenediammonium adipate, commonly known as nylon salt or "66 salt." This intermediate compound has a sharp melting point near 190 to 192 degrees Celsius, which manufacturers use as a purity check before moving to the next stage. The salt is typically filtered, washed, and dried into a white crystalline powder before it enters the reactor.

Step 2: Concentration and Pre-Polymerization

The nylon salt solution is concentrated by evaporating excess solvent, then heated under moderate pressure, usually around 1.5 to 1.8 megapascals, to begin the condensation reaction. Water is released as the amine and acid groups combine to form amide linkages, and short polymer chains, called oligomers, begin forming at this stage. Pressure is held steady during this phase to prevent premature loss of the volatile diamine before it has fully reacted.

Step 3: High Temperature Polycondensation

The reaction mixture is heated further, typically between 250 and 280 degrees Celsius, while pressure is gradually released to drive off remaining water and push the reaction toward longer chain formation. This melt polycondensation stage determines the final molecular weight of the nylon 66 resin, which in turn controls its mechanical properties. A vacuum stage is often applied near the end of this step, since removing the last traces of water is what allows the chains to keep growing rather than stalling at equilibrium.

Step 4: Extrusion and Chip Formation

Once the target viscosity is reached, the molten polymer is extruded through a die into strands, cooled rapidly in a water bath, then cut into small pellets or chips. These chips are the standard raw material form shipped to downstream manufacturers for spinning into fiber or injection molding into plastic parts. Chip moisture is usually dried down to below 0.1 percent before packaging, since residual moisture can cause hydrolysis and chain breakdown during later melt processing.

Step 5: Melt Spinning or Molding

For textile applications, the chips are re-melted and extruded through spinnerets to form continuous filaments, which are then drawn to align the polymer chains and increase tensile strength. Drawing ratios of four to five times the original filament length are common, since this molecular alignment is what gives nylon 66 fiber its characteristic strength and elasticity. For engineering plastic applications, the same chips are melted and injection molded directly into finished components, sometimes with glass fiber or mineral fillers added to boost stiffness.