After working on many nylon material selection projects, we’ve found that most mistakes come down to the same issue: failing to understand the fundamental molecular differences between PA6 and PA66, and making the decision based simply on price or experience.

A typical example: PA6 was used instead of PA66 for an under-the-hood automotive component, resulting in softening and deformation at high temperatures—and an entire batch being rejected. On the other hand, PA66 was selected where PA6 would have been more suitable for an impact-resistant application, causing parts to crack under impact in cold weather and ultimately leading the customer to switch suppliers.

The price difference may be only a few thousand RMB per ton, but choosing the wrong material can cost far more than the material itself.

PA6 and PA66 differ by just one “6” in their names, and their pellets may look almost identical. But their molecular chain structure, crystallinity, and melting points are fundamentally different—and these differences ultimately determine their strength, toughness, heat resistance, moisture absorption, and cost.

Keep reading, and let’s break down the differences in just three minutes.

Molecular Chain Structure Determines Performance

PA6 is produced by the ring-opening polymerization of a single monomer, caprolactam, meaning its molecular chain consists of repeating units of the same type. PA66, on the other hand, is produced through the condensation polymerization of two monomers, adipic acid and hexamethylenediamine, with two different units arranged alternately along the chain.

Although both are called “PA6,” their molecular backbones are different. PA66 has a higher density of amide groups, stronger intermolecular hydrogen bonding, and inherently higher crystallinity.