PA6/PA66 - properties, experiences and useful links
The members of the polyamide family are coded with cryptic extensions like 6, 11, 12, 66, 46 and 69. The numbers describe the type and number of polymer chains in their chemical structure. This approach, the normal practice in the industry, is not very user-friendly from the product designer’s perspective.
The two most commonly used polyamide grades by far are PA6 and PA66. The remaining members of the polyamide family are primarily used in applications that require unusually high service temperatures or tolerance of moist conditions. Bio-based and transparent (amorphous) PA grades are also available. In machine design purposes PA is often reinforced with glass or - in case of highly demanding applications - with carbon-fiber. Reinforcement changes the characteristics of the material and its applications considerably and therefore I’ll write a separate article on them later. This article focuses on unfilled PA6 and PA66.
What is the difference between PA6 and PA66 then?
Here is a summary:
PA66 compared to PA6
- absorbs slightly less moisture
- higher modulus
- better wear resistance
- better short term heat resistance
PA6 compared to PA66
- better ductility (due to higher toughness)
- better long term heat resistance
- better impact resistance in low temperatures
- better surface quality
- better creep resistance
- better UV-resistance (depends on modification)
- lower cost
The differences are small, however, and in some circumstances some of the items on the list above may be invalid, depending on the exact grade in use.
One of PA’s basic characteristics is its tendency to absorb water. This decreases its strength and modulus, but makes it tougher. More on PA water absorption can be read in Dry vs. Conditioned Polyamide Nylon Explained by UL-IDES
High gloss and good fatigue-, wear- and impact resistance makes PA a good option for plastic high end chairs.
Polyamide is commonly called nylon, which is also a trademark of Dupont. Here is a story about how nylon got its name.
Practical experiences of unfilled PA6/PA66:
- Water absorption is a well-known property of PA and worth taking into account. The difference can be felt when a product that has been stored outdoors over winter is taken indoors for a few days: its stiffness invariably increases.
- High quality cable ties are made of PA66. This illustrates well the strength and toughness of the material. It also reveals that unfilled PA is suitable for applications that require high flexibility or even integral hinge. Apart from cable ties, products made of unfilled PA66 are not very common. PA6 appears to be much more popular choice.
- Because of its good surface appearance PA6 is also often used (rather than amorphous plastics ABS, PC, PC/ABS) on housings of some outdoor applications like this tractor. Read the full article on Basf site.
- Even though parts made of PA can have a glossy surface, replacing amorphous grades like ABS with PA might cause problems because of the higher shrinkage and therefore warpage.
- If you are trying to determine if a product is made of PA, cut a little piece off and burn it. The odor of PA is similar to burning hair.
I’ll add more links to the page every time I find something useful. If you have a good link, case or a story in mind, please email me at firstname.lastname@example.org or add it in comments below.
To fully grasp the look and feel of PA you can order the Plasticprop Essentials sample kit including 20 of the most common plastics. It has been carefully designed to convey and explain the characteristics of each material. Read here what the sample tells about unfilled PA6.