EDIT: I made an update to this with more info to add to this More info about brittle filament due to loss of plasticizers & wetness
I wanted to look into this in depth because I’ve been eating through a bunch of old filament, and given colors all of it no matter the brand is brittle where others are fine. In this case, all my black filament while stored in a box, vacuum bag, and in room temp (so no high heat and no UV). It is all brittle.
The common thing for most is “it is wet”. And during my test, it isn’t. Not only it isn’t showing any signs of wetness outside of the brittle. Drying it is not changing the weight of the filament no matter how many times I tested it. In fact, one test I actually left a spool out to see if it will weigh more, and it did. Drying it did bring it back down to the prior weight. And I couldn’t get it below that. Note when the weight went up it didn’t help or hurt the brittleness.
And note there was some filaments like a few blue and a few white that I had mix results. One blue was brittle and acted as the above. The other blue and a few white they were wet even if they were in a bag.
Keep in mind all of these filaments, they have been in the box since 2015 timeframe (10 years).
Note my focus is far more on PLA. I haven’t had these problems with PETG, but my PETG isn’t as old.
The Research
Summary:
Polylactic acid (PLA), a persistent challenge encountered by users is the material’s propensity to become brittle over time, particularly during long-term storage. This degradation frequently manifests as broken filament strands, leading to compromised print quality and outright print failures. The inherent brittleness of PLA is a well-documented characteristic, with the material typically exhibiting less than 10% elongation at break, which inherently limits its applicability in scenarios demanding significant plastic deformation or high stress levels.
My first theory on my problem centers on mechanical stress , positing that the continuous strain imposed by long-term storage on a spool, especially when the filament is subsequently straightened from its coiled configuration, induces microfractures within the polymer structure. This mechanism is considered particularly problematic for 3D printers employing Bowden extrusion systems, where the filament undergoes extended and repeated bending. The second theory suggests that PLA loses its “natural oils” over time, a concept that, in material science, aligns with the migration or volatilization of plasticizers, thereby contributing to the observed brittleness.
Mechanical Stress and Microfracture Formation
PLA filament is inherently subjected to significant mechanical stresses during its manufacturing process. The extrusion of molten polymer, followed by drawing to achieve the precise diameter, rapid quenching in a water bath for cooling, and subsequent winding onto a spool, all contribute to “baking in” residual stresses within the filament’s polymer chains. These internal stresses are a foundational element that can predispose the material to degradation over time.
When stored on a spool for extended periods, the filament is held in a continuously bent, stressed state. This sustained deformation, particularly the tighter coiling towards the inner core of the spool, perpetually maintains these internal stresses. The filament, therefore, is not in a relaxed, unstressed condition but rather under constant tension and compression along its length. A critical aspect of PLA’s mechanical behavior is its unique “creep to rupture” or “creep-to/then fracture” characteristic under sustained stress, even at ambient temperatures. This viscoelastic phenomenon describes how a polymer slowly deforms over time when subjected to a constant load. For spooled PLA, this means that the continuous bending stress, over long durations, causes the polymer chains to gradually rearrange and deform. This long-term creep renders the material highly susceptible to brittle failure when its stress state is abruptly altered, such as during the process of unspooling and straightening the filament for printing. This time-dependent deformation makes long-term coiled storage inherently challenging for PLA, as the material’s fundamental properties lead to a gradual weakening under the very conditions of its storage.
The act of straightening the filament from its coiled state, especially when it has already undergone some degree of physical aging or other forms of degradation, applies a mechanical strain that can exceed the material’s diminished flexibility. This external stress, interacting with the pre-existing internal stresses and any accumulated material changes, can lead to the formation of microscopic cracks or “microfractures” on the filament’s surface. These microfractures then propagate through the filament’s cross-section, ultimately causing it to snap. This mechanism is particularly relevant to Bowden extrusion systems. In such setups, the filament is subjected to continuous bending and unbending as it traverses the PTFE (polytetrafluoroethylene) tube. This constant flexing, combined with any pre-existing microfractures or inherent brittleness stemming from long-term stress and aging, significantly exacerbates the degradation. The repetitive stress cycles can accelerate the propagation of microfractures, leading to frequent filament breakages within the Bowden tube. Furthermore, thicker filaments, such as 3mm compared to 1.75mm, may exhibit increased susceptibility to this form of mechanical degradation due to the higher surface stresses experienced for the same radius of bend.
Physical aging, often referred to as natural aging, is an intrinsic phenomenon observed in many polymers, including PLA. During the rapid cooling phase following extrusion in manufacturing, the polymer chains within the amorphous regions of PLA are effectively “frozen” into a non-equilibrium microstructure characterized by high enthalpy values. Over time, even when stored at temperatures below its glass transition temperature (Tg), these macromolecular chains slowly rearrange into more stable, lower-energy configurations. This process is driven by thermal motions and results in a gradual loss of the excess enthalpy, a change that can be quantitatively assessed using techniques such as differential scanning calorimetry (DSC). This molecular rearrangement directly alters the material’s properties, including its glass transition temperature, yield strength, and overall mechanical behavior. Given that PLA’s Tg typically falls within the range of 50-63°C, which is relatively close to common room temperatures, this aging process is noticeably active even under normal storage conditions. Its rate accelerates as the ambient temperature approaches the material’s Tg. This uncontrolled physical aging leads to an increase in material stiffness and a corresponding reduction in ductility, making the filament more prone to snapping. Studies on 3D-printed PLA specimens have demonstrated a decrease in ductility over time due to room temperature aging, with properties stabilizing after approximately five days post-printing, indicating a relatively rapid initial aging phase.
Source Aging effects at room temperature and process parameters on 3D-printed poly (lactic acid) (PLA) tensile properties, Natural Ageing of PLA Filaments, Can It Be Frozen?
Plasticizers “natural oils”
Plasticizers are low-molecular-weight chemical compounds intentionally incorporated into polymers like PLA during their formulation. Their primary function is to enhance the material’s flexibility, ductility, and overall workability. This is achieved by increasing the “free volume” within the polymer matrix, which, in turn, boosts the mobility of the polymer chains and reduces the intermolecular forces between them. The net result is a more pliable material with improved elongation at break and enhanced impact strength.
My theory regarding the loss of “natural oils” over time directly corresponds to the scientific phenomenon of plasticizer migration or volatilization. Research indicates that plasticizers are often more volatile than the base polymer itself. Consequently, over extended periods, even under seemingly optimal storage conditions such as cool and dark environments, these plasticizers can gradually “cook off” (volatilize) or migrate out of the filament. This depletion of plasticizers leads to a progressive loss of flexibility and a corresponding increase in brittleness in the PLA filament. The direct consequence of plasticizer depletion is a reduction in the free volume and molecular chain mobility within the PLA matrix. This structural change manifests as a significant decrease in the filament’s elongation at break and an increase in its stiffness, rendering the material more prone to snapping during handling or printing.
Manufacturers face considerable challenges in formulating PLA with plasticizers that are both effective and stable, meaning they do not readily migrate or leach out of the material. The difficulty in preventing plasticizer migration is a known concern that directly impacts the material’s long-term performance, reinforcing that the loss of these additives is a distinct and significant mechanism contributing to PLA brittleness.
Something to note is I can’t get a hard timeframe on how long it sticks around. But everything is saying in a filament (not printed) it loses it extremely fast compared to printed. This could be due to the continued bending stress on the coiled filament (creep) and possibly a difference in the crystalline structure formed during printing.
Now as far as how long until you pretty much hit a no point of return. I’m not really able to find this info that easily. It does appear additives like color additives can mess with this length. Which might explain why virtually all my black filament was having problems, and looking into this it appears many others report the same with their black filament. Also many report this happening to cheaper filament. But there isn’t enough data. But from what it looks like is there is a noticeable loss over a year, but the data I ran across focus more on wetness than anything. One place said up to 15 years, but I couldn’t find where they came up with that number. Others are indicating after a few years. If you can find a study then let me know.
There is no known practical method for consumers to reintroduce lost plasticizers into brittle PLA filament to restore its original properties. While thermal treatments (like baking or using a filament dryer) can often “fix” brittle filament, this primarily works by removing absorbed moisture and relaxing internal stresses from physical aging, not by replenishing lost plasticizers. In fact, it appears that over-drying or using excessively high temperatures can further degrade plasticizers.
Many assumed it was due to the filament being too dry. But the more I’m looking at it, the more dry the filament the better. It is this degradation that’s the problem.
This is likely THE issue. This explains why during drying the filament can go through a Bowden system with almost no problem. But as soon as the filament stops printing and sits there for a few minutes in the tube away from the drying it breaks. You’re taking it out of it’s natural state and it lost pretty much all the ability to bend without help.
Thermal Treatment (Drying/Low-Temperature Baking)
Gentle heating of the filament, typically to temperatures near or just below its glass transition temperature (Tg), allows the polymer molecules to regain some mobility. This thermal energy enables the relaxation of internal stresses that were “baked in” during manufacturing or accumulated from long-term coiling on the spool. This process is akin to a low-temperature annealing for the filament itself. It can effectively reverse the brittleness caused by uncontrolled changes in the crystal structure due to physical aging, allowing the polymer chains to rearrange into a less brittle state. This dual action of heat explains why drying often appears to “fix” brittle filament, even when users may not initially attribute the issue solely to moisture. The heat simultaneously addresses both moisture-related issues and the physical aging effects, which are frequently intertwined.
Aim for 45°C to 50°C for 2-6 hours. Some go as high as 55°C.
As mention prior, this really isn’t a “fix” in situations I’m talking about. Low-quality or inconsistently manufactured filament may possess structural flaws that predispose it to more severe and permanent degradation over time. Similarly, certain color additives can make a filament more prone to irreversible brittleness. Therefore, while drying can often improve printability and flexibility, it is not a universal panacea, and users should manage their expectations for heavily degraded filament, prioritizing prevention over reliance on restoration for severely compromised material.
If you do find yourself with material that you believe is beyond the point of no return. Avoid using things like the AMS, and just use a direct drive or external feed. For example, when I print with the x1c, I can have my bad filament sit in a drying box at 50°C. I’m not worried about the degrade as mention prior because the filament is already at no point of return. So it stays at that temp through the print. Sometimes it works even if I don’t do this. Anyways, I feed it in using the external method using a tube going to the thing.
If the print does stop for a few minutes, then it is possible the filament will break. More often than not it is in connection parts. I take what I can out, but then push the rest to the extruder and let it extrude it. This letting me use my old filament without having to take apart the AMS or given parts again and again.
Prevention
I can’t find good storage options outside of the obvious ones on this (dry place, away from UV, etc). There is some theories a freezer might help, but there isn’t a lot of info on this, and this isn’t realistic for most. In fact, it could make things worse. If you want to try this out and test it, then please do so.
The best thing I can recommend for this is track the age, and use the oldest filament first. Note low quality filaments are more likely to have this problem. Same with some colors.
TLDR
Plasticizers are fundamental to PLA’s initial flexibility. The type, quantity, and stability of these plasticizers directly influence the filament’s long-term resistance to brittleness. They go away over time even if you keep your filament in a safe place for a long time. There isn’t a lot of info on how long you have before no point of return, but once it is blow a given amount then there is largely nothing you can do to reverse the damage in the long term. More than less, it is a ticking time bomb and you need to use the filament prior to this point. If you are beyond it, then use a direct drive if possible. Avoid a AMS if all possible.
Because the filament loses the stuff that makes it flexible, and the natural state is it being on the spool. When it is bent in a new way or straighten out this causes microfractures or complete breaks. And this is why filament just sitting in an AMS or in a printer extruder can break.
Note that your filament might be wet, and largely drying it is all you can do to reverse any damage to any filament. And if drying doesn’t work, then it is likely your filament loss given additives it needed to be flexible.
Note over drying can cause this to damage your filament and shorten the life of it.
