What You Need to Know About Drying Specialty Nylons

- May 02, 2017-

Basic chemistry dictates that the strength of molded parts partly derives from the entanglement 

of the long polymer chains that comprise the base resin. Hygroscopic engineering resins, 

such as PBT, PET, PC and nylons, all absorb ambient moisture that can effectively shorten 

their polymeric chains and adversely affect performance in both the mold and the end part.


The aliphatic amine molecule of nylons, in particular, lacks the aromatic ring directly on 

the molecule’s nitrogen atom, which makes them particularly susceptible to picking up moisture. 

If excess moisture is not removed, the presence of heat and pressure during molding can 

cause hydrolysis, which degrades the polymer or causes chain scissions that produce 

excessive outgassing, splay marks, or discoloration and compromise the physical integrity 

of the finished part.


A number of specialty resins, such as polyphthalamide (PPA), have significant amounts of 

aromatic character in their polymeric backbone, which causes them to absorb less moisture 

than aliphatic nylons like nylons 6 and 66, and do so at a slower rate. The diffusion 

coefficient for water in some grades of Solvay’s Amodel PPA, for instance, 

is approximately 20% that of nylon 66 at 73°F (23°C). Put another way, 

PPA generally will not absorb more than 1.5% of its weight in moisture, while nylon 6 can 

hold up to 7%. Yet PPA resins are still considered hygroscopic and require proper drying 

prior to processing.


It is important to emphasize that there is more at stake here than simply getting unwanted 

moisture out. There is such a thing as drying a resin too much. Ultimately, the aim is to 

ensure maximum polymer performance, which is especially important if your design calls for 

a specialty polymer like PPA. This also applies to less hygroscopic specialty resins like 

polyamide-imide (PAI) and even non-hygroscopic polymers, like polyphenylene sulfide (PPS).


Although PPS is hydrolytically stable, pellets may pick up some surface moisture during 

shipping or storage that may result in cosmetic defects, such as surface streaks or splay 

marks on injection molded parts or severe bubbling or streaking in extruded profiles. 

Further, surface moisture can vaporize into steam within the barrel of an injection 

molding machine, creating internal pressures that force nozzle drool. 

Lastly, it is important to remember that many mineral fillers are hygroscopic and can 

make reinforced compounds more susceptible to moisture-driven drool even if the base 

resin is stoutly non-hygroscopic.


ROLE OF DRYING EQUIPMENT & TECHNIQUE

The first step toward optimal drying of hygroscopic resins is to prevent moisture 

absorption in the first place. Amodel PPA, for example, is shipped with less than 0.15% 

moisture in vacuum- sealed, aluminum-lined containers that prevent moisture ingress. 

Once opened, it is critical to reseal packaging containers as quickly and as tightly 

as possible. Many suppliers provide tools to help molders gauge how long they should 

dry a resin based on how long its container has been open.


Equally important is that bags should be opened individually and immediately loaded 

into a hopper dryer. Opening several bags to allow air conveyors to transfer the resin 

to the hopper as needed allows the resin to pick up moisture. When loading resin from 

large containers, it is best to cut just enough of the foil liner to fit an air- conveyor 

wand into the package, and then reseal the foil around the top of the wand. Only dried 

air—as opposed to “shop air”—should convey resin.


More exacting applications may require measurement of resin moisture content prior to 

molding. Karl Fischer titration, which uses a moisture-reactant chemical, offers an 

efficient, rapid,

and highly precise method for determining water content in resins. Gravimetric moisture 

analyzers, while less precise than titration, may be more suitable for a manufacturing 

setting. These devices comprise a sensitive weigh scale embedded within a benchtop drying 

oven to measure how much weight the resin loses during drying. While relatively fast and 

inexpensive, gravimetric analysis is not moisture-specific. So, its measurement may be 

distorted by the evaporation of other volatile components in the resin.


The range of options for drying resin is even more diverse, but they take one of three 

approaches: heat, chemical desiccation, or vacuum pressure. All are suitable for drying 

specialty resins. 


Hot-air dryers are the most effective in removing surface moisture and therefore are 

suitable for non-hygroscopic polymers such as polyolefins, polystyrene, and PVC.


Desiccant dryers have long been the workhorse for drying resins that absorb moisture, 

and generally include some configuration of a moisture-removal filter or bed that recycles 

dry air through the system in a closed loop. Single-bed desiccant systems are adequate 

as long as the desiccant is replaced as necessary, but dual-bed systems allow regeneration 

of one bed while the other is drying. Another variation incorporates a rotor that exposes different 

sections of a rotating desiccant wheel to process air, regeneration or cooling. Rotating or wheel 

designs absorb moisture at a more constant rate than a conventional desiccant bed, 

which helps to minimize spikes in dewpoint or temperature. 

• Low-pressure or vacuum dryers heat pellets just enough to dislodge moisture from the resin’s molecular structure, and then draw it off under vacuum. These systems can dry resins in a fraction of the time it takes other dryers, thereby saving energy and minimizing the prospect of discoloration from heat.

Regardless of the technique, the key metric for effective drying is dewpoint, which equates with ambient moisture. That is, the lower the dewpoint, the less moisture there is in the air inside the dryer. The ideal dewpoint for drying most resins, including PPA, is -40°F/°C. As long as the dryer’s dewpoint remains fixed, however, time and temperature can be used to calculate when the resin is adequately dry for processing. This may vary from polymer to polymer.

For PPAs, the drying time and temperature depends on the moisture content of the resin, the size of the hopper dryer used, and the throughput of the molding process. To determine the proper drying temperature, divide the capacity of the hopper dryer (in lb or kg) by the rate of resin consumption (in lb/hr or kg/hr). This will determine how long the material should remain inside the dryer, which in turn determines the drying temperature. Figure 1 shows recommended drying time and temperature for Solvay’s Amodel PPA resins when their sealed shipping container is opened.

For most uses, the specified dryness is 300 ppm (0.03%) for Amodel PPA resin. However, this specification may be tighter for some applications, such as extruded monofilament, which requires the resin to be extremely dry. 

As previously mentioned, it is possible to overdry PPA and other resins, thereby solid- stating the material or increasing its molecular weight. This typically leads to increased viscosity or lower flow, which may result in short shots, increased pressure required to fill, and inconsistent processing. It is best to find the equilibrium temperature where uniform moisture can be maintained without over drying. The low-end tolerance for Amodel PPA is 100 ppm (0.01%) moisture content. Once a resin has been adequately dried, it is important to maintain relatively constant moisture levels throughout the molding run to ensure process and part uniformity. Melt viscosity is influenced by the amount of moisture in resin. More moisture equates with a lower melt viscosity (see Fig. 2). 

Moisture levels upwards of 0.15% can cause cosmetic problems such as splay or silver streaking on the surface of a molded part. Higher levels can lead to hydrolysis, and result in a significant reduction in mechanical properties. This is clearly an issue for any molder. But it underscores the importance of proper drying techniques when specifying specialty resins like PPA, which are often selected for the most demanding applications.



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