Meaning
Degradation reactions involve the chemical cleavage of amide bonds in the polymer backbone when the material is heated in the presence of water. Polyamide hydrolysis results in a rapid reduction of molecular weight and a corresponding drop in the physical properties of the moulded part. This process is the primary reason why nylon resins must be dried to very low moisture levels before they are processed in an extruder or injection press.
Moisture Impact
Water molecules act as a reactant that breaks the long chains of the polymer into smaller fragments. The rate of polyamide hydrolysis increases with both the temperature and the amount of moisture present in the resin pellets. Even a small amount of residual water can cause a notable loss in toughness, making the finished components brittle and prone to failure under load.
Chain Scission
Molecular breakdown occurs at the nitrogen carbon bond that forms the link between the repeating units of the polymer. During polyamide hydrolysis, this bond is replaced by a carboxyl group and an amine group, effectively ending the chain at that point. This change is permanent and cannot be reversed by simply drying the material after the damage has been done.
Mechanical Loss
Performance of the final product is compromised when the internal structure of the material is weakened by these chemical changes. A part affected by polyamide hydrolysis will show a lower elongation at break and a reduced resistance to fatigue. This is hazardous in automotive or industrial applications where the polymer is expected to withstand constant vibration or repeated mechanical stress over many years.
Because the loss of strength is not always visible on the surface, parts may pass a visual inspection but fail immediately once they are placed under a functional load in the field.
