Polyamide (PA), commonly known as nylon, is one of the most widely used engineering plastics in the world. It offers excellent mechanical strength, wear resistance, chemical resistance, and processability.
Among them, PA6 (Polyamide 6) and PA66 (Polyamide 66) are the two most common grades. While they belong to the same nylon family, they differ significantly in molecular structure, crystallization behavior, physical properties, and processing characteristics—differences that directly affect their performance in various applications.

1. Molecular Structure & Crystallization Behavior

• PA6: Produced by the ring-opening polymerization of caprolactam. Its molecular chain is more flexible, crystallization rate is slower, and crystallinity is about 50–60%.

• PA66: Produced by the condensation polymerization of hexamethylene diamine and adipic acid. Its molecular chain has higher regularity, faster crystallization rate, and crystallinity is about 60–70%.

Impact of Crystallinity: The higher crystallinity of PA66 gives it better dimensional stability and heat resistance, while PA6 exhibits better toughness and impact resistance.

2. Key Performance Comparison

Technical Insights:

• Heat Resistance: The higher melting point and crystallinity of PA66 offer superior thermal stability and creep resistance.

• Toughness: PA6’s more flexible molecular chain provides better impact performance, especially at low temperatures.

• Moisture Absorption: PA6 absorbs moisture faster and to a slightly higher extent, which can reduce dimensional accuracy and mechanical strength.

• Processability: PA6 has better flowability, making it suitable for thin-walled or complex-shaped parts.

3. Typical Applications

PA6 Applications

• Automotive interior/exterior parts (door panels, dashboard frames)

• Food machinery components (requiring toughness and impact resistance)

• Power tool housings

• Medium-load gears, pulleys

• Textile machinery parts

PA66 Applications

• High-temperature automotive components (intake manifolds, engine brackets)

• High-strength gears, bearing housings

• Industrial fasteners, mechanical structural parts

• Electrical insulation components (terminal blocks, connectors)

• High-wear sliding components

4. Modification & Performance Enhancement

To meet demanding application requirements, PA6 and PA66 are often modified as follows:

1. Glass Fiber Reinforcement (GF)

   • Increases tensile strength, flexural modulus, and dimensional stability

   • PA66-GF30 can achieve HDT above 250℃

2. Flame Retardancy (FR)

   • Halogen-free flame retardants (red phosphorus, metal hydroxides) to meet UL94 V0

   • Suitable for electrical and electronic components

3. Self-Lubricating Wear Resistance

   • Adding PTFE, MoS₂, graphite to reduce friction coefficient

   • Ideal for high-frequency moving parts

4. Impact Modification

   • Adding elastomers such as POE or EPDM to improve low-temperature toughness

5. Material Selection Guidelines

• High temperature, high strength, high dimensional accuracy → PA66 or PA66-GF reinforced grades

• High impact resistance, low temperature toughness, cost-sensitive → PA6 or modified PA6

• High humidity environment → Low-moisture absorption grades (e.g., PA66-GF-LowMoisture)

• Electrical insulation → Flame-retardant PA66 or PA6 grades

6. Processing Considerations

• Drying: Both PA6 and PA66 must be thoroughly dried (<0.12% moisture content) before processing, typically 80–90℃ for 4–6 hours.

• Injection Molding Temperature: PA6 ~230–260℃; PA66 ~260–290℃.

• Mold Temperature: PA6 70–90℃; PA66 80–100℃ to ensure sufficient crystallinity.

• Degradation Prevention: Avoid prolonged residence at high temperatures to prevent discoloration and performance loss.

Conclusion

PA6 and PA66, while both belonging to the nylon family, differ in thermal performance, mechanical strength, moisture absorption, and processing characteristics due to differences in molecular structure and crystallization behavior.
With proper selection and modification, both materials can achieve an optimal balance between cost, performance, and processability. For demanding, high-temperature applications, PA66 is often preferred, while for applications requiring toughness and economic efficiency, PA6 remains a strong choice.