By Ben Howe, UL Prospector
Prospector datasheets display both dry and conditions plastics information and many of our members ask about the difference between dry and conditioned data for polyamide (nylon) plastic materials. In a nutshell, “dry” refers to data that is obtained from a sample of material with equivalent moisture content as when it was molded (typically <0.2%). “Conditioned” on the other hand, refers to data obtained from a sample of material that has absorbed some environmental moisture at 50% relative humidity prior to testing. The vast majority of dry and conditioned data is seen with polyamide (Nylon) materials. To understand the need for two sets of data it’s important to understand a little about the structure of Polyamide.
Polyamide / Nylon
Polyamide is classified as a crystalline polymer, but it is only mostly crystalline; some amorphous regions do exist. The crystalline regions form because of the amide group of the polymer being polar. The electrons shared between some atoms aren’t shared equally resulting in regions of slight positive and slight negative charges in the polymer chain. These charged regions are attracted to one another causing the polymer chain to fold over itself again and again. These crystalline regions are illustrated in Figure 1.
Figure 1. Polyamide Crystalline Regions
It is from these crystalline regions that polyamide receives much of its strength. These crystalline regions resist being pulled apart. Add more polymer chains to the mix, and you have a lot of semi-crystalline polymer chains attracted to one another. Although these regions haven’t technically experienced cross-linking, the result is similar, higher strength and higher stiffness.
The polar nature of the amide group in the polymer is not only polyamide’s strength, but is a weakness as well. Water is also a polar molecule, and water is everywhere. As mentioned previously some atoms don’t share electrons equally. In this case the selfish atoms are Oxygen and Nitrogen. When a water molecule comes into contact with polyamide, weak bonds form between the two (see Figure 2). As this process continues, the water molecules diffuse though the material seeking out any charged areas and forcing polymer chains apart along the way. This is why polyamide parts swell after being exposed to moisture. The separation of the polymer chains reduces the polar attraction between chains and allows for increased chain mobility, resulting in diminished mechanical properties. It is within the amorphous regions that the water bonds to the polymer chain. Luckily the crystalline regions of polyamide resist being pulled apart by the water because the bonds between the amide groups are stronger than the attraction to water. Were this not the case polyamide would dissolve in water. Instead, water acts as a plasticizer rather than an out right solvent.
Figure 2. Polyamide Interaction with Water
Plasticizers cause a polymer to swell and soften. Both of these effects are exhibited by polyamides when they are exposed to moisture and must be taken into account when designing a part. This is why both dry and conditioned data for polyamides are given. The tensile strength and stiffness of the material will diminish, while the flexibility and impact toughness will increase with moisture absorption. The extent to which the different properties change depends a great deal on the chemistry of the polymer itself. Polyamide 12, for example, doesn’t absorb as much moisture as Polyamide 6, so Polyamide 12’s properties don’t fluctuate as much with moisture.
About the Author
|Ben Howe is the Content Development Lead for UL IDES and is responsible for the management of 83627 unique grades of material datasheets from more than 883 resin suppliers. Ben holds a degree in Mechanical Engineering from the University of Wyoming (2001). Ben has a knack for tracking down hard to find materials information and authored the Property Descriptions and Test Method Standards library.|
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