Recycling, storage and degradation of polymers.

Generally speaking, plastics (polymers) are thought of as quite stable when compared to other naturally produced materials as they do not rust, rot or otherwise degrade in the traditional ways. However plastics do have their own set of weaknesses which, for better or worse, can lead to their demise a lot sooner than you may expect. So what are a few ways you can ensure your reels of filament and printed objects last as long as possible? A few suggestions are below.

Starting with the obvious, purchasing a quality reel of filament will go a long way to producing strong and long lasting objects. Polymers on their own are very stable, but the monomers they are manufactured from are quite reactive and often carcinogenic. If the ratio of ‘ingredients’ is not perfect during production then left over monomers will remain embedded in your reel of ABS, PLA or HDPE. Over time they will slowly diffuse to the surface in the same process that gives a new car its distinctive (and possibly dangerous) smell. Heating during extrusion will also greatly speed up the migration leading to more possibly dangerous odors. Additionally, if too little or too much plasticisers or fillers are added during production then your will also run into trouble both during extrusion and years after a object has been printed. So quality plastic will not only help in producing structurally long lasting objects but it may also be a positive for your health.

Unfortunately, even if you have purchased a quality plastic, it will still be slowly degrading  before it even reaches you door step. Indeed, thanks to a process known as thermal degradation, your plastic filament is in a state of decline from the second it was synthesized. Thermal degradation is a process where by heat (atomic agitation) or light supplies the energy required to kick off a small part of the polymer chain. This newly exposed part of the polymer is unstable (a free radical) and so there will be a driving force for it to react with other atoms around it. Even room temperature is enough for a very slow level of oxidation to take place. More often than not, this unstable section will come into contact with oxygen and undergo oxidation.

Worse still, this new oxygen containing bond is still unstable and so can form new free radicals around it. This leads to an acceleration of oxidation with time in a process known as autoxidation. Every time the polymer chain is split it becomes shorter and so the materials mechanical and thermal properties as a whole will slowly degrade. Its also worth noting that due to the strong carbon-fluorine bonds in PTFE, it is quite resistant to oxidation.

So how can you slow the process of thermal degradation? Again the answer is fairly obvious. Much like food, storing your plastics away from any light, away from moisture and removed from oxygen and at a constant room temperature will prolong their life. Also be aware that the plasticizers in PVC can interact with other plastics, especially polystyrene, quickly degrading them. I don’t know enough to say if it will be a problem for ABS, PLA or HDPE but best to keep them away from PVC just to be safe.

In addition to how you store your plastic’s, how they are printed will also effect their life span. If your plastic is printed too hot (well above its Tg) then thermal degradation will be accelerated. Also any internal stresses within a printed object will also greatly increase the rate of oxidation. The use of a heated build platform or hot build chamber both aid in decreasing internal stresses.

An example of different degradation rates. The top red line represents a printed object that was 'printed to hot, with high internal stresses and aged in in poor conditions. The blue line represents an object printed without any stresses and stored away from light at at room temperature. These are not based on empirical results and are for demonstration purposes only!

Unfortunately, due to thermal degradation there is a limit to how many times a single plastic object can be recycled. I’m told that in HDPE milk bottle manufacturing the amount of regrind (HDPE that has been extruded once alraedy) is limited to 60% or less to prevent the polymer becoming too reactive and spoiling the milk. So even if a practical and low cost way of regrinding plastic objects at home is developed you will still need a supply of virgin plastic to add to the mix or otherwise end up with a loss of thermal and mechanical properties. The ratio of regrind to virgin would need to be even less if you were to say use a 4th generation plastic (extruded 4 times)  as opposed to a 2nd generation plastic (extruded two times).

I also want to say a few words about polymer ‘creep‘. If your not familiar creep, it’s sumed up as the slow permanent deformation of a material at a force that is lower than its yield stress. This means that if you have a printed object under some stress, no matter how ridged it may feel it will still very slowly deform with time. Unfortunately is inherent problem in plastics and can only be minimised, but not eliminated. However, by keeping your printed objects cool and under low stress (well below their yield stresses) the rate of ‘creep’ is so low that it should not be an issue.

Real empirical results for the creep of injection molded HDPE at different strain load levels. 1 (), 2(), 4(○), 6(•)…18 (♦) MPa

Source for the figure above.

I hope you found this all as interesting to read as it was to write. On a more personal note, my Mendel build and nurdle extruder are both on hold still until I catch up on some university work. I also have a Arduino Mega and 4 Pololu A4983 stepper driver in the post so I can start work on my Mendel electronics. Total cost: $85USD ex tax&shipping. See this list for details.

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About Richard

I am a PhD candidate in Materials Engineering located in Melbourne, Australia.
This entry was posted in Thinking aloud (Theory) and tagged , , , . Bookmark the permalink.

2 Responses to Recycling, storage and degradation of polymers.

  1. nophead says:

    Interesting article as always Richard, thanks.

    “carbon-fluorine bonds in HDPE” did you mean PTFE?

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