Nylon, a revolutionary material, was born 80 years ago and was invented by Dr. Wallace Carothers of DuPont in 1935. It is the earliest synthetic fiber. Afterwards, DuPont led development of materials science and applied nylon as an engineering polymer, enabling it to achieve exponential development. Emergence of nylon has given a new look to textiles. Its synthesis is a major breakthrough in synthetic fiber industry and a very important milestone in polymer chemistry.

 

Main varieties and representative companies of nylon family are shown in following table:

Among large nylon family, PA6 and PA66 have the largest amount. In recent years, with increase of running enthusiasts, midsole running shoes based on Pebax nylon elastomer foam have become popular, PA12 and PA11 manufacturers headed by Arkema in France have made a lot of money. In general, PA6 and PA66 still occupy most of nylon family market.

 

Nike uses Pebax nylon foam midsole, which is lighter and has better resilience compared to traditional EVA and TPU midsoles.
PA6 and PA66 are mostly used for clothing in civil yarns, cord fabrics, fishing nets, conveyor belts in industrial yarns, high wear-resistant insulator parts in injection molding plastics, daily necessities and packaging films due to characteristics of their thermoplasticity, light weight, good toughness, chemical resistance and durability. But at the same time, there are subtle differences between the two products, and there are also different application areas in terms of application due to difference in characteristics.

You want to analyze difference between them, you must start with their structure. As everyone knows, PA6 is made by ring-opening polymerization of caprolactam, while nylon PA66 is made by condensation polymer of hexamethylene diamine and adipic acid. PA6 and PA66 have same molecular formula, but structural formula is different.

 

It is precisely because of this difference that its properties are different, such as different intermolecular hydrogen bond forces. Number of hydrogen bonds in PA66 is more than that in PA6. Intermolecular force of PA66 is stronger than that of PA6, so PA66 is better than PA6 in thermal properties (so processing temperature is higher), PA66 has better rigidity than PA6, PA6 has better toughness than PA66, and PA6 has a faster water absorption rate than PA66. Difference in properties between PA6 and PA66 is mainly caused by above hydrogen bonding factors.

Both PA6 and PA66 are translucent or opaque milky white crystalline polymers. However, there are big differences in production raw materials: raw material of PA6 is caprolactam, which is obtained by ring-opening polymerization of caprolactam; raw material is mainly made of petroleum benzene, some manufacturers use hydrogenated benzene due to insufficient supply of petroleum benzene, but quantity is very small. PA66 is made by polycondensation of hexamethylene diamine and adipic acid.
Compared with PA66, PA6 has a lower melting point and a wide process temperature range. Its impact resistance and resistance to solubility are better than PA66, but it is also more hygroscopic. Because many quality characteristics of plastic parts are affected by hygroscopicity, more attention must be paid during use. PA66 molecular arrangement is symmetrically distributed, all flag groups and amino groups can form hydrogen bonds. In addition, during spinning process, its dynamic crystallization ability is about 20 times higher than that of PA6, so cracking strength of PA66 industrial knot has reached 9.7 g/d, while strength of PA6 industrial yarn is only about 9.0g/d under same conditions.

 

As PA6 easily absorbs moisture, special attention should be paid to drying before processing. If material is supplied in waterproof packaging, container should be kept tightly closed. If humidity is greater than 0.2%, it is recommended to dry it in hot air above 80C for 16 hours. If material has been exposed to air for more than 8 hours, it is recommended to perform vacuum drying at 105C for more than 8 hours.

PA6: 80~90℃. Mold temperature significantly affects crystallinity, which in turn affects mechanical properties of plastic parts.
For thin-walled, longer-flow plastic parts, it is also recommended to apply a higher mold temperature. Increasing mold temperature can increase strength and rigidity of plastic part, but it reduces toughness. If wall thickness is greater than 3mm, it is recommended to use a low temperature mold of 20~40℃. For glass reinforced materials, mold temperature should be greater than 80℃.
PA66: 80℃ is recommended. Mold temperature will affect crystallinity, and crystallinity will affect physical properties of product.

PA6: 230~280℃, 250~280℃ for enhanced varieties.
PA66: 260~290℃. For glass additive products, temperature is 275~280℃.

They are generally between 750 and 1250 bar (depending on material and product design).

All are high speed (reduced slightly for reinforced materials).

Since solidification time of PA6 and PA66 is very short, location of gate is very important. Gate aperture should not be less than 0.5*t (where t is thickness of plastic part). If you use a hot runner, gate size should be smaller than using a conventional runner, because hot runner can help prevent premature solidification of material. If a submerged gate is used, minimum diameter of gate should be 0.75mm.

PA6 engineering plastic has high tensile strength, good impact resistance, excellent wear resistance, chemical resistance and low friction coefficient. Through glass fiber modification, mineral filling modification, and addition of flame retardants, it can make its comprehensive performance more excellent. It is mainly used in automotive industry and electronic appliances.

 

PA66 has good overall performance, high strength, good rigidity, impact resistance, oil and chemical resistance, wear resistance and self-lubricating advantages, especially hardness, rigidity, heat resistance and creep performance. Because PA66 has higher strength than PA6, it is more used in production of industrial yarns such as cords.

 

PA6 and PA66 are materials that are very suitable for lightweight design, and they are also very similar in terms of characteristics. However, price of PA66 continues to rise, increasing production costs. Due to its cost advantage, PA6 has become a better alternative to PA66 in many scenarios.