Moisture in Nylon: What's the Right Number?

By Michael Sepe, Materials Analyst

Don’t believe it when the material supplier tells you that filled and unfilled nylon have the same maximum allowable moisture content.

I have been known from time to time to chide the plastics industry for a lack of scientific thinking. But there are some topics that seem so obvious that I never thought I would find myself writing about them. This is one of those.

This article was inspired by a colleague of mine, someone I have worked with for a long time. We have been coworkers in the same company, we have worked for competing molding companies, and now we circulate in overlapping spheres within the industry—he at an OEM that purchases a lot of plastic parts, and I as an independent consultant who works with OEMs that purchase a lot of plastic parts. As such, we both interact with processors and material suppliers.

Plastic material suppliers were once the source of all wisdom and knowledge about plastic materials and processing, and rightly so. They created the technology that produced these wonderful polymers and they had on staff an impressive cadre of professionals who understood design, processing, material properties, and the interactions that enabled users to get the most out of their efforts.

DuPont Engineering Polymers, Dow Plastics, General Electric (now Sabic Innovative Plastics), Celanese (now Ticona Engineering Polymers), to name just a few, had some of the smartest people on the planet working not only in the laboratories where the products were created, but out on the molding floor where the parts were being made. They published design guides and processing manuals that are still some of the best reading you can find in the industry.

It would not be an exaggeration to say that the talent pool has become somewhat diluted. Some of this is because of expansion. More companies are marketing plastic compounds and, like expansion in baseball, the need to fill more positions inevitably means that a few people who would normally languish in the minors are going to get a spot on the roster.

Some of it has to do with the experience of the people entering the business. The folks who used to come into the plant to educate processors about a topic such as maximizing crystallinity and dimensional stability in acetal or nylon were the same people who either had done some of the development work on the polymers or had stood alongside those who had. They possessed an intimate understanding of the connections between composition and processing conditions.

Most of these people have retired, and the new guard has a different skill set more geared toward marketing and less concerned with technical issues. I recently sat through a “technical” presentation on the differences between traditional nylons (the chemists call them aliphatic) and the so-called high-temperature nylons (partially aromatic), where the presenter was unable to answer simple questions about differences in heat resistance between the two families.

You’d think we’d know by now

Given this backdrop, the phone call from my colleague did not come as a huge surprise. He had just come from a meeting with a technical representative of a nylon compound manufacturer (a compounder, not a major) who had tried to convince him that the allowable moisture content specification for a highly filled nylon and an unfilled nylon was the same. Indeed, in the last week I have received certifications for an unfilled and a 40% glass-filled nylon 6/6 that both list moisture content as a specification item. On both sheets the maximum value is the same: 0.20%.

What’s wrong with this picture? If you check the literature published by the original producers of nylon compounds, you find general agreement that nylon should not be molded with a moisture content higher than 0.20%, or 2000 ppm.

The reason is simple: A chemical group in nylons known as the amide group will react with water under the right conditions, causing the nylon chain to break. The right conditions for such a reaction are readily provided by the temperatures in the injection cylinder where the material is heated and melted. Breaking the chain reduces the material’s molecular weight, which is a key to providing the balance of properties in the material, particularly those associated with toughness, abrasion resistance, and long-term attributes such as fatigue resistance. As Wallace Carothers, the inventor of nylon, so eloquently put it, “Useful strength and pliability require a molecular weight of at least 12,000.”

When a material supplier hands a processor a bag of its material, it is providing a compound of an initial average molecular weight. It is the molder’s job to keep it close to that initial value as the material is converted to a molded part. One of the ways to ensure this is to dry the material prior to molding below a certain level. That level is 0.20% by weight of nylon polymer. If you have some time, you can convince yourself of this by running a nylon compound at different moisture levels ranging from very dry (0.04%) to very wet (0.50%) and then measuring either molecular weight or some key property such as impact resistance in the final product. You will see a clear pattern relating moisture content to properties.

Here is the problem. Most nylon compounds are filled with something nonpolymeric. It is usually glass fiber but may also be glass bead, aramid, carbon, or some type of mineral. Commercial compounds are readily available that contain as much as 60% of this other stuff. As these other ingredients displace the nylon, the amount of polymer in the compound drops. This means that there is less polymer to absorb moisture and the inorganic materials generally found in filled nylon materials are not hygroscopic. So what are the implications of this composition for moisture content specification?

We can make the math part of this simple. We will compare an unfilled material to a 50% glass-filled compound. Both materials are found to have a moisture content of 0.20%. Do they have the same moisture content from the standpoint of the polymer?

In the unfilled material, the moisture is distributed across all of the polymer so the moisture content by weight of polymer really is 0.20%. In the 50% glass-filled material, the same 0.20% moisture by weight is contained only in the 50% of the material that is polymer. There is no water in the glass and the glass will not be affected by the moisture once the material is melted. The 0.20% moisture in this material is all concentrated in the half of the compound that is nylon. So the actual moisture content of this polymer is 0.40%. It is wet and unsuitable for processing. The moisture content of this compound needs to be no higher than 0.10% in order to be molded properly.

You can run the same calculation for any level of filler. As the level of filler in the material increases, the allowable moisture content in the compound must be correspondingly lower—0.133% for a 33% filled compound, 0.12% for a 40% filled material, and so on. Given the fact that nylon has been with us as a material for 70 years, it would seem reasonable to suppose that as an industry we would have figured this out by now. But as the discussion between my colleague and the representative for the material supplier suggests, we haven’t.

Yet the myths continue

The other piece of their discussion pertained to the minimum moisture content allowed for the material. Of course, the same faulty reasoning that allows for the conclusion that the maximum moisture content should be the same for both materials is applied to come up with the conclusion that the minimum moisture content of a highly filled and an unfilled nylon should be the same.

The discussion about a minimum moisture content usually includes some mythology about the notion of overdrying. I have written about this topic extensively and will not go through the arguments again here. But for the moment, let’s assume that there is a minimum desired moisture content for molding nylon. We will also assume that the correct number, as supplied by the technical representative sitting across the desk from my colleague, is “about 0.07%.” Now let’s return to the unfilled and 50% glass-filled materials in the previous discussion. What is the real moisture content of the nylon polymer in a 50% glass-filled compound? It is 0.14%, in the upper end of the real specification range for a nylon compound. Here again, our assessment of the real moisture content of the polymer must be indexed to the composition of the formulation. One size does not fit all!

One final word on this subject is in order. The ability to scientifically examine the connection between moisture content, processing, and final properties presumes that the person doing the work is using an instrument capable of accurately measuring moisture. Unfortunately, an estimated 90-95% of processors make no attempt to measure actual moisture content. They rely on that certification sheet that we mentioned at the beginning of the article and the drying instructions provided by the material supplier. If the container instructs the molder to dry for 4-6 hours at 180°F, then it is an article of faith in most molding shops that following those instructions ensures dry material.

Of the 5-10% of processors who do attempt to document an actual moisture content, 90% are using an instrument not suited to the task. Recently I was sent a sealed jar of unfilled nylon 6/6 that was running with splay. The processor had a loss-in-weight moisture analyzer that told him the moisture content was 0.03%. The actual moisture content, when run in the correct instrument, was 0.24%. So before you embark on a study of your own, be sure that the fundamentals of accurate moisture measurement are in place.

May, 2008 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.