Meaning
Synthetic thermoplastic polymers containing amide linkages in their main chain exhibit high mechanical strength and chemical resistance. These polyamides are highly hygroscopic and absorb moisture from the surrounding environment. This behavior necessitates thorough drying of the resin before melt processing operations.
Chemical Structure
Amide groups along the polymer backbone form strong hydrogen bonds with water molecules. This polar interaction allows the polymer to absorb water from the atmosphere, which disrupts the polymer-to-polymer hydrogen bonds. This chemical behavior varies depending on the density of the amide groups in the specific grade, with nylon six absorbing more water than nylon twelve.
Absorption Trajectory
Exposure to humid air leads to a gradual increase in moisture content until the polymer reaches an equilibrium state. The rate of this absorption depends on the temperature, the relative humidity and the thickness of the material. This moisture uptake causes the polymer to swell, which alters the dimensional stability of the extruded or molded parts.
Processing Rule
Dryness limits must be strictly enforced before the material enters the injection molding machine. Processing these resins with a moisture content above the specified threshold, typically below one-tenth of a percent, leads to hydrolytic degradation and severe loss of mechanical properties. Implementing dry air hopper dryers is the standard method used to reach and maintain these required dryness levels in the factory, preventing processing failures and securing consistent product output.
