Understanding Nylon and Moisture

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Nylon is one of the oldest and most commonly used thermoplastics but there still seems to be a lot of confusion about properly drying nylon and in general about how nylon is affected by moisture. Moisture in the raw material causes many processing issues and part failures and the affects that moisture has on molded parts seems to confound people as well. I hope in this article to clear some of this up.

A Few Basic Concepts to Understand

Nylon is hygroscopic, meaning that it absorbs moisture right out of the air surrounding the material or directly from water if it is submersed. The higher the humidity, the faster it will absorb moisture. Immersing nylon in water will cause it to absorb moisture extremely fast. However, it only absorbs moisture until it is saturated. At that point no amount of humidity will make it absorb more. Similar to a sponge, once the sponge has been soaked in water to saturation, it will not hold more water. Also like a sponge, when nylon is exposed to dryer air, it will give that moisture up and dry out. Nylon does not have a shelf life; there is not a “point of no return” in which the material cannot be dried.

Under normal conditions, say 20-80% RH, nylon will reach saturation in about 24 hours. This means that the same amount of drying time will be required to dry material that has sat open for 24 hours or 24 years. The only exception to this would be if the material was compounded in such a way that a large amount of water actually gets encapsulated in a small hollow spot in the pellets. This is called encapsulated moisture. No amount of drying time or temperature will get that water to evaporate. Encapsulated moisture is pretty rare though. I have seen it once or twice in over 25 years in the materials business.

Processing

The problem with processing nylon or any other material that is wet is that the water will reach boiling point at the processing temperature of the material. That liquid water expanding into steam is very destructive to the polymer causing loss of properties and the “splay” appearance that we are all familiar with.

Nylon 6 and 6/6 typically need to be dried for 40 hours at 175F. If the material is packaged in bags or Gaylords that are lined with foil, you can probably cut that time in half. I have even heard of some people skipping drying altogether. However, you have to use it quick once you open the bag and if you do not use it all, the remaining material in the bag will have to be dried for the full amount of time. It is important to note that once the material is dried that it must be run immediately. If the material sits open, it will absorb the moisture right back and in my experience, it seems that when the material is warm, it absorbs the moisture even faster. It is also very important that all hose fittings on the dryer be secure. A loose fitting can draw in moist air from the shop and ruin the dry material.

It should also be noted that moisture in nylon acts as a lubricant which increases the flow of the material. The higher the moisture content, the higher the flow. This is why you never see melt flow reported on a nylon data sheet. The moisture content cannot be controlled well enough to insure anything resembling accuracy. Most nylon manufacturers will recommend drying nylon to below 0.2% moisture content. This will typically yield acceptable parts but reducing the moisture even further will improve the properties still more.

A lot of people speak of “over drying” nylon but you cannot really dry it too much. It would be great if you could reduce the moisture to zero. What happens when you dry the material to extremely low moisture content is that the flow decreases which can then make it difficult to fill the mold cavity. Sometimes people alter their processing parameters to try and fill the cavity and end up degrading the material with shear heat. This is then blamed on the material being too dry. This is a misunderstanding.

Trouble Shooting Defective Nylon Parts

Once the material has been run wet, the moisture has burned away. It will come out of the barrel as steam. The part molded from wet nylon will be dry; therefore you cannot check the moisture content of the part to determine if it was run from wet material. The effects of running wet nylon can look very similar to the effects from running the material too hot or even running contaminated material. A thorough investigation will be required to determine the root cause.

Moisture in Molded Parts

Once a part has been molded or extruded, the part will immediately start absorbing moisture from the atmosphere. This moisture absorption has two effects on the part.

First, it will affect the physical properties of the part. The part will get less brittle and more flexible. These properties will change dramatically. Looking at a typical unfilled Nylon 66, here are some properties for Dry as molded specimens and after the specimens have been conditioned at room temperature and 50% relative humidity:

Property                                  Dry as Molded                        50% RH

Tensile Strength at Yield        12,000 psi                    8,000 psi

Flexural Modulus                      420,000 psi                  180,000 psi

Izod Impact                                 1.0 ft-lb/in                   3.0 ft-lb/in

Keep in mind that if the part is exposed to intermittent periods of moist and dry environments, it will continue to absorb and give up moisture and the properties will change accordingly for the life of the part. It will never reach any type of equilibrium.

The second change will be dimensional. The dimensions of nylon parts are dramatically affected by temperature and humidity. This can severely affect the functionality of long parts. It is important to take these issues into account when designing a Nylon part. If you cannot afford to have this much change in dimensions, I would recommend that you look at PBT or PET as alternative materials.

Water Packing

Many processors will pack nylon parts in water to improve the impact strength or flexibility. Nylon parts are dry coming out of the molding machine so adding moisture will change the properties.

There is confusion about how this works. The property changes that are caused by the moisture absorption are not permanent. Once the parts are removed from the water, they will start to give up their moisture and the properties will adjust accordingly. Water packing is a good solution for temporarily adding impact strength and flexibility. This might aid in part assembly.

For instance, if you have a problem with tabs on a part cracking during assembly, water packing might be a good solution. Packing the parts in water will make them more flexible to ease assembly and then the parts will stiffen back up as the dry. If the part that you are molding needs more flexibility for ongoing use, you might want to switch to a more flexible grade as water packing is not going to help.

Just remember that it is not a permanent change in the properties.

Conclusion

Many processors have been plagued by problems with Nylon and moisture. Many people in the material business have told me that this is one of the most common causes of customer complaints and material returns. I think that this may be because of some general misunderstandings about how moisture affects Nylon.

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