PA6 and PA6.6 in nylon yarns: which differences really matter
When we talk about nylon yarn we are almost always talking about two polyamides: PA6 (polyamide 6) and PA6.6 (polyamide 6.6, also written PA66). They are related materials, but with differences that have a real impact on how the yarn behaves — above all in dyeing and under thermal stress.
Structure and melting point
The starting difference is chemical: PA6.6 has a more regular molecular structure, which translates into a higher melting point (around 255–265 °C) compared to PA6 (about 215–220 °C). In practice PA6.6 tolerates high temperatures better, an advantage in technical applications where the yarn has to withstand stretching, setting or heat during downstream processing.
Behaviour in dyeing
This is where the difference becomes operational for a dyehouse. PA6 has a more amorphous structure and tends to absorb dye more quickly and with a higher yield, often at lower temperatures. PA6.6, being more crystalline, generally requires harder conditions (temperature, time) to reach the same degree of bath exhaustion.
The practical consequence: the same color recipe does not give the same result on the two materials. Recipes, temperature curves and auxiliaries must be calibrated for each polyamide.
For us, the choice follows the customer
In our dyehouse we dye both PA6 and PA6.6: technically, for us, they are equivalent. Which of the two polyamides to dye is not our decision — it is set by the customer's request and by the needs of the application. Today market demand leans mainly towards PA6, but this is a matter of the market, not a technical limit.
The counts we work with most frequently are 78/24/2 and 78/24/3 (78 dtex over 24 filaments).
Color fastness and yield
With equally good execution, PA6.6 tends to offer slightly higher fastness under certain conditions — in particular to heat — thanks to its greater structural stability. PA6 remains an excellent choice for most clothing and hosiery applications, where brightness and color consistency are what count.
What really changes by sector
In practice, for the buyer, the difference between sectors lies less in the choice between PA6 and PA6.6 — which, as said, follows the customer's specification — and more in the count and construction of the yarn, which we adapt to the needs of each application. A few concrete examples:
- Sportswear hosiery: 78/24/3 Flexy, a bulkier and more elastic construction. The yarn is "fuller" and fills the sock better, giving body and fit to the sports garment.
- Narrow-fabric (tape) manufacturers: 78/24/2 package-dyed (tinto in rocca), for a uniform, clean color on the elastic tapes used in underwear.
- Technical and medical: 44/34/1 or 44/34/2, generally in PA6.6 and in Flexy construction, to push elasticity and performance even further — the key requirements of this segment.
In summary
PA6 and PA6.6 are not interchangeable at process level: they change the behaviour in dyeing, the heat resistance and, consequently, the color recipe. But for the finished product what makes the difference, alongside the polyamide, is the count and construction of the yarn: that is where we choose, sector by sector, what is really needed.