Both Nylon 6 and Nylon 66, two of the most popular polyamides, have different molecular compositions, so the properties they have are very different physically, chemically, and mechanically, and affect the uses for them in different industries. The differences discussed in this paper are fully described, from molecular architecture to thermal and mechanical stability, chemical resistance, and application areas.
Nylon 6 and nylon 66 are very different in their molecular compositions. Nylon 6 is formed by the polymerisation of caprolactam monomers, molecules that have repeated caprolactam units in their chains and are semi-crystalline with linear chains split into crystalline and amorphous regions, the crystalline side granting strength and stiffness and the amorphous side granting flexibility and elasticity. This structure is why nylon 6 is so good at resisting mechanical fatigue and offering the right blend of properties for fibers, engineering plastics, and molded parts.
Figure 1: Chemical structure of nylon 6 and nylon 66[1].
By contrast, nylon 66 has two monomers - adipic acid and hexanediamine - each having 12 carbon atoms, which gives it an extremely dense molecular structure that is more strong, stiff, heat-resistant, and chemically resistant. As Nylon 66's molecule is symmetrical and there can be hydrogen bonding to all the carbon and nitrogen bonds, as Nylon 6's molecule is not symmetrical and some of the carbon and nitrogen bonds cannot have hydrogen bonding between them, the Nylon 66 fibers are stronger and more machine-friendly than Nylon 6 fibers. The crystallization property and crystallization percentage of nylon 66 are higher than those of nylon 6 (around 20 times and 12 times, respectively), and it can be used in high-speed direct spinning process to achieve better mechanical properties and strength of the product and is usually applied for premium tires.
They are also affected by the water absorption of Nylon 6 and Nylon 66. The amine and carbonyl groups on nylon products are so different in their molecular structure that they react with water molecules to form hydrogen bonds, which cause volume growth, modulus loss and creep under tension as it soaks up water. The water absorption impacts on the properties of nylon are dramatic; they include loss of hardness, modulus, tensile strength, yield point, and impact strength. Nylon 6/66's glass transition temperature (Tg) is water sensitive, so Tg drops drastically upon water absorption and is gradual, i.e., drops rapidly in the beginning, then slowly thereafter, once there is a certain threshold of water absorption.
It is easier to work with because Nylon 6 is lower in melting point and density than Nylon 66, which has a higher melting point and wear resistance. This makes Nylon 6 suitable for applications that need more stretch and Nylon 66 appropriate for applications that need to be strong under mechanical stress.
| Property | Nylon 6 | Nylon 6,6 |
| Melting Point (℃) | ~220 | ~260 |
| Density (g/cm3) | 1.13 | 1.14 |
| Moisture Absorption | Higher (3.5% equilibrium) | Lower (2.5% equilibrium) |
| Hardness | Lower | Higher |
| Wear Resistance | Moderate | Superior (33% higher) |
Nylon 6 is weaker and more easily deformed at high temperatures due to its lower crystallinity and heat deflection temperature, whereas the higher crystallinity and strong acid resistance of Nylon 66 enhance its chemical resistance and longevity.
| Property | Nylon 6 | Nylon 6,6 |
| Chemical Structure | Polycaprolactam (Single Monomer) | Hexamethylene Diamine + Adipic Acid |
| Crystallinity | Lower | Higher |
| Acid Resistance | Weaker | Stronger |
| Heat Deflection Temp | Lower | Higher |
Nylon 6 has greater impact resistance, which is particularly favourable for low-temperature applications, while Nylon 66 has greater tensile and creep resistance and hence is a better choice for use when carrying loads in high stress conditions.
| Property | Nylon 6 | Nylon 6,6 |
| Tensile Strength | Lower | Higher |
| Flexural Modulus | Moderate | High |
| Impact Resistance | Superior at low temperatures | Moderate |
| Creep Resistance | Lower | Higher |
Nylon 66 is superior to Nylon 6 in thermal resistance and high-temperature strength retention, so it is used for premium automobile and industrial components. Nylon 6 is easy to mold and manufacture due to its lower processing temperature and shrinkage, while Nylon 66 can be better crafted using more complex techniques for dealing with higher shrinkages and temperatures during manufacturing.
| Property | Nylon 6 | Nylon 6,6 |
| Short-Term Max Temp (℃) | ~200 | ~240 |
| Heat Aging Strength | Lower | Higher |
| Processing Temp (℃) | Lower | Higher |
| Shrinkage | Lower | Higher |
These molecular differences in Nylon 6 and Nylon 66 make them quite different from one another in terms of thermal, mechanical, and chemical properties, which determine their specific uses. Nylon 6 is the perfect material for high-temperature flexibility and processing, while Nylon 66 is the material of choice for ultra-high-performance applications that require superior strength and thermal resistance. Knowing these differences is the key to choosing the right nylon for the right applications.
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Reference
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