Some polymers have a natural tendency to absorb water from the environment, including the air. These materials are called hygroscopic polymers. This phenomenon is often unknown or overlooked by many manufacturers and it can cause printing defects and failures.
Filament that has absorbed a high percentage of water may experience dimensional changes or, when melted in the nozzle, may produce more undesirable effects such as hydrolysis. However, the good news is that the process is reversible and it can be corrected before printing by drying out the material.
In this article, we will go through how this process works, why it is undesirable when 3D printing components, and some options to fix it.
Understanding Hygroscopic Materials
Some polymers such as nylon, ABS, and polycarbonate have the ability to absorb moisture from the environment. The degree of absorption depends mainly on the material properties, temperature, exposure time, and relative humidity in the air.
Disclaimer: This section may get a little hard to read, but I think it is important to describe how this entire process works. Feel free to skip it, but I think it may be very helpful! 😜
Polymers are made up of many long, repeating chains of molecules called polymer chains. Some polymers such as polyamides (nylon) or polyurethanes (TPU) have a 'polar' structure at the end of the chains that tend to attract moisture due to a force called hydrogen bonding. Basically, a hydrogen atom wants to bond with an oxygen atom.
Water molecules (H20) that come into contact with hygroscopic materials want to take part in this hydrogen bonding. A hydrogen atom contained in the water bonds to an oxygen atom in a polymer chain. At the same time, the oxygen atom contained in the water bonds to a hydrogen atom in a polymer chain. This causes the water molecule to attract to the polymer.
The molecular structure of the polymer is also important in determining the absorption rate of the material. Polymers can have crystalline regions and amorphous regions. A crystalline region is an area of the material where the polymer chains have aligned, allowing them to sit closer to each other. An amorphous region is an area where polymer chains are held loosely together and in no orderly manner.
Polymers with higher levels of crystalline regions absorb less water than their counterparts with more amorphous regions. Crystalline regions are pulled together and held together by a force called a van der Walls force, making it more difficult to fit water molecules within these regions. Therefore, polymers with more amorphous regions tend to absorb more water. There is more space for water molecules to be absorbed!
We can verify this by looking at polyamides (nylon). They are mainly made up of amorphous regions. Nylon is capable of a very high moisture content of almost 10% when submerged in water!
Relative humidity is the driving environmental factor that affects the amount of water absorbed in a polymer. Relative humidity is usually shown as a percentage and describes the amount of water vapor in the air in relation to the maximum amount of water vapor that can be held in the air. You can think of it as the concentration of water in the air.
The moisture content of hygroscopic polymers depends on this concentration. Remember in the last section that we discussed the different molecular regions within a polymer? Let's see if we can describe the relationship between relative humidity and a polymer's moisture content by using water and a sponge as an example.
We will represent the amorphous regions within a polymer as the space or "holes" within a sponge. Relative humidity will be represented by the amount of water we have available.
The more holes you have in your sponge, the more space you have to hold and absorb water. Similarly, the more amorphous the structure, more water molecules are absorbed.
If you drop a sponge into a bowl with 1 ounce of water, it may absorb all of the water. However, if you drop it into a bowl with 20 ounces of water, it may only absorb 3 ounces of the water and leave the remaining water in the bowl. There is only so much space to hold the water in the sponge. The sponge will absorb water until it reaches an equilibrium with its environment.
When storing polymers at your facility, the moisture content in the material will find a balance with the moisture content in the air. This is called the equilibrium moisture content (EMC). It is the point where the material is neither gaining nor losing moisture. For example, nylon 6 may have an equilibrium moisture content of 3% by weight when stored in a 50% relative humidity environment at 73F. However, when it is submerged completely in water at the same temperature, the moisture content could reach almost 10% by weight!
Many filaments are shipped in sealed bags with desiccant to keep them as dry as possible. As soon as you open the bag, the material will start to equalize with its surrounding environment and absorb moisture. It will try to find its EMC point where the moisture content is balanced. How fast this occurs will be discussed in a few moments!
Temperature will affect the vapor pressure of water. Vapor pressure is a measure of the tendency of a material to change into the gaseous state. An example of this is boiling water. When the temperature of water reaches its boiling point, the vapor pressure of the water increases to a point where it equals the vapor pressure of the surrounding air. This results in some of the water molecules transitioning to the gaseous state and evaporating into the air.
If we assume a constant relative humidity in the air and increase the environmental temperature, the amount of water content in the material is typically reduced by a small amount. This indicates that the vapor pressure in the material must increase at a rate slightly faster than the surrounding air when increasing the temperature. Overall, the environmental temperature by itself does not play a large role in the amount of moisture stored in the material, but it does affect how fast moisture is absorbed or released!
Just to clarify this point, typically in real life the relative humidity in the air decreases when temperature is increased. This decrease in relative humidity reduces the moisture content in the material as described in the relative humidity section. However, in the temperature example we held the relative humidity constant to help describe why temperature affects the rate of absorption.
Time also affects the amount of moisture content in a polymer. When a material is exposed to a new environment with higher or lower humidity levels, the material does not immediately reach the EMC point.
Let's go back to our example of opening a new bag of nylon filament. When you open the bag, the material will begin to absorb moisture and move towards the EMC point. This happens at a rate that we can call the diffusion index. It signifies how easy it is for water molecules to move within the polymer. It is different for each polymer, and as we discovered above, the temperature can also play a part in increasing the rate of absorption.
To understand this we need to look at how temperature affects the molecular structure of the material. Let's start with the concept of diffusion. Diffusion is the random movement of molecules within a substance. It occurs from a higher concentration of molecules to a lower concentration of molecules. An example of this would be a plug in scent diffuser in your house. The scented molecules are more concentrated at the diffuser. These molecules move towards a lower concentration of molecules (the air) and disperse throughout the room. It is the same concept with water absorption and polymers. Water molecules at a higher concentration in the air disperse within the polymer chains of the material.
How fast this dispersion occurs is related to the space between the polymer chains. One factor is how many amorphous regions the polymer has. The second is temperature.
Solids, liquids, and gasses expand when they are exposed to higher temperatures. The atoms within the material do not expand, but the volume that they take up does expand. The space between atoms and molecules increases.
In a polymer, molecules are held close together to form a solid. When the material is heated to a higher temperature, the atoms vibrate faster and they push neighboring atoms and molecules apart from each other. This pushing slightly increases the distance between the molecules and an overall volume increase occurs. More space within the molecular structure equals more space for water molecules to move. Therefore, increasing the ambient temperature can cause a polymer to absorb (or eliminate) moisture faster!
Why is Moisture Content Bad for 3D Printing?
There are three reasons why excessive moisture content is bad for 3D printing. Let's take a look at them.
The first is that excess moisture content can dimensionally change the diameter of your filament. When polymers absorb moisture, their overall volume increases. Inconsistency in the diameter of the filament can cause the nozzle to jam and can be a bummer when printing very long projects!
The second is the vaporization of water in the nozzle. When the water molecules are heated to their boiling temperature, they are released in the form of a gas. This can create bubbles in your 3D print, cause inconsistent material flow, or reduce the quality of your surface finish!
The third reason is another process called hydrolysis. I am definitely not a chemist (😁) and I am not able to prove this with testing, but there is reason to believe that this process could occur during 3D printing. If anyone has looked into this, I would love to get a message from you!
Hydrolysis is the chemical breakdown of a substance due to a reaction with water. It is time dependent and can be affected by temperature. An example of a material that exhibits this property is polyvinyl alcohol (PVA). It has a high degree of hydrolysis between 80% to 90% and is soluble in water.
Hydrolysis occurs when a bond is broken within the polymer chain and a new one is formed with the water, creating two more more new substances. It basically degrades or breaks down the polymer material by splitting molecules from the polymer chain. The expected result would be a negative effect to the mechanical properties of a material. It seems as though melting polymers at a high temperature in the nozzle could speed up this process and cause it to occur during processing. Again, if anyone has done any studies on this, I would love to hear from you!
How to Reduce the Moisture Content of Filaments
The main way to reduce the moisture content of filaments is by drying them out before processing! Polymers begin to release moisture as soon as they are exposed to an environment with lower relative humidity. Most injection molding manufacturers include this as a standard part of their process because they know that processing polymers with high moisture content can cause defects and poor mechanical performance.
One way to dry filament is by storing it in a dry environment with a low relative humidity (possibly below 10% RH). However, remember that this process is time dependent. Depending on the molecular structure of the polymer, it could take a a long time and it is difficult to tell if enough time has passed to allow the material to reach its EMC point.
The best way is to dry the material is in an elevated temperature environment with low relative humidity and with good air flow. There are products on the market that do this, or I have seen other 3D printing experts make their own out of food dehydrators! To summarize why, here are the reasons.
The low relative humidity creates a differential in the vapor pressure causing the release of moisture
The higher temperature increases the space between polymer chains speeding up the time required to release moisture
Adding or improving the airflow within the drier increases the speed at which moisture is carried away from the filament
Many filament dryers on the market seem to run at around 50 degrees Celsius for 6 hours. However, recommendations for each material type may be different. If you research online for your specific material, there may be guidelines already that are used by the injection molding industry.
One last reminder, make sure that you process or store the material in a low humidity environment immediately after drying it. The material will begin to absorb moisture as soon as you remove it from the drier. You don't want all of your time and hard work to be wasted!
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