While plastics are very useful on their own, they can be much stronger when reinforced and mixed with a range of fibers. Not surprisingly, this includes the thermoplastic polymers which are commonly used with FDM 3D printing, such as polylactic acid (PLA) and polyamide (PA, also known as nylon). Although the most well-known fibers used for this purpose are probably glass fiber (GF) and carbon fiber (CF), these come with a range of issues, including their high abrasiveness when printing and potential carcinogenic properties in the case of carbon fiber.
So what other reinforcing fiber options are there? As it turns out, cellulose is one of these, along with basalt. The former has received a lot of attention currently, as the addition of cellulose and similar elements to thermopolymers such as PLA can create so-called biocomposites that create plastics without the brittleness of PLA, while also being made fully out of plant-based materials.
Regardless of the chosen composite, the goal is to enhance the properties of the base polymer matrix with the reinforcement material. Is cellulose the best material here?
Cellulose Nanofibers
Plastic objects created by fused deposition modeling (FDM) 3D printing are quite different from their injection-molding counterparts. In the case of FDM objects, the relatively poor layer adhesion and presence of voids means that 3D-printed PLA parts only have a fraction of the strength of the molded part, while also affecting the way that any fiber reinforcement can be integrated into the plastic. This latter aspect can also be observed with the commonly sold CF-containing FDM filaments, where small fragments of CF are used rather than long strands.
According to a study by Tushar Ambone et al. (2020) as published (PDF) in Polymer Engineering and Science, FDM-printed PLA has a 49% lower tensile strength and 41% lower modulus compared to compression molded PLA samples. The addition of a small amount of sisal-based cellulose nanofiber (CNF) at 1% by weight to the PLA subsequently improved these parameters by 84% and 63% respectively, with X-ray microtomography showing a reduction in voids compared to the plain PLA. Here the addition of CNF appears to significantly improve the crystallization of the PLA with corresponding improvement in its properties.
Fibers Everywhere
Incidentally a related study by Chuanchom Aumnate et al. (2021) as published in Cellulose used locally (India) sourced kenaf cellulose fibers to reinforce PLA, coming to similar results. This meshes well with the findings by Usha Kiran Sanivada et al. (2020) as published in Polymers, who mixed flax and jute fibers into PLA. Although since they used fairly long fibers in compression and injection molded samples a direct comparison with the FDM results in the Aumnate et al. study is somewhat complicated.
Meanwhile the use of basalt fibers (BF) is already quite well-established alongside glass fibers (GF) in insulation, where it replaced asbestos due to the latter’s rather unpleasant reputation. BF has some advantages over GF in composite materials, as per e.g. Li Yan et al. (2020) including better chemical stability and lower moisture absorption rates. As basalt is primarily composed of silicate, this does raise the specter of it being another potential cause of silicosis and related health risks.
With the primary health risk of mineral fibers like asbestos coming from the jagged, respirable fragments that these can create when damaged in some way, this is probably a very pertinent issue to consider before putting certain fibers quite literally everywhere.
A 2018 review by Seung-Hyun Park in Saf Health Work titled “Types and Health Hazards of Fibrous Materials Used as Asbestos Substitutes” provides a good overview of the relative risks of a range of asbestos-replacements, including BF (mineral wool) and cellulose. Here mineral wool fibers got rated as IARC Group 3 (insufficient evidence of carcinogenicity) except for the more biopersistent types (Group 2B, possibly carcinogenic), while cellulose is considered to be completely safe.
Finally, related to cellulose, there is also ongoing research on using lignin (present in plants next to cellulose as cell reinforcement) to improve the properties of PLA in combination with cellulose. An example is found in a 2021 study by Diana Gregor-Svetec et al. as published in Polymers. PLA composites created with lignin and surface-modified nanofibrillated (nanofiber) cellulose (NFC). A 2023 study by Sofia P. Makri et al. (also in Polymers) examined methods to improve the dispersion of the lignin nanoparticles. The benefit of lignin in a PLA/NFC composite appears to be in UV stabilization most of all, which should make objects FDM printed using this material last significantly longer when placed outside.
End Of Life
Another major question with plastic polymers is what happens with them once they inevitably end up discarded in the environment. There should be little doubt about what happens with cellulose and lignin in this case, as every day many tons of cellulose and lignin are happily devoured by countless microorganisms around the globe. This means that the only consideration for cellulose-reinforced plastics in an end-of-life scenario is that of the biodegradability of PLA and other base polymers one might use for the polymer composite.
Today, many PLA products end up discarded in landfills or polluting the environment, where PLA’s biodegradability is consistently shown to be poor, similar to other plastics, as it requires an industrial composting process involving microbial and hydrolytic treatments. Although incinerating PLA is not a terrible option due to its chemical composition, it is perhaps an ironic thought that the PLA in cellulose-reinforced PLA might actually be the most durable component in such a composite.
That said, if PLA is properly recycled or composted, it seems to pose few issues compared to other plastics, and any cellulose components would likely not interfere with the process, unlike CF-reinforced PLA, where incinerating it is probably the easiest option.
Do you print with hybrid or fiber-mixed plastics yet?
Reinventing HPL?
But the resins used there are not very friendly for the environment (which also makes it long lasting). I sort of like the idea of natural fiber reinforced PLA.
Recently I also read some snippets about bacteria breaking down plastics. Apparently lot of plastic waste in the environment encourages bacteria to adjust and eat it.
Oooo, neat, maybe this bacteria will get into underground petroleum reserves.
Just remember the gadget in your hands, needed crude oil to manufacture it.
Your ear buds. Your cruelty free leather free plastic shoes. Our usb thumb drives
Gets a bit mind boggling when you and I take a slow look around and begin to realize how hard it is to find anything in our house/apartment that could be produced now without oil or natural gas.
Ever had an injection or a vaccine?
Was the syringe a disposable plastic?
Ever had any thoughts of trying 3d printing?
I’m mostly saying, be careful what ya wish for.
Nevermind the ” petrochemicals are damn useful” bit… If bacteria were to metabolize the geological reserves, what exactly would you expect to happen? You know, other than a rather bizarre World War III (balancing ‘wasting’ fuel with “well, this superbacteria is going to metabolize it anyway”), you’ve suddenly metabolized, and thus released all that carbon. An oil-consuming bacteria that could functionally destroy geological carbon reserves would make the industrial revolution look like a single cow farting.
This is “well shit, the wish ran away from the genie” territory.
For bacteria plastics are a high concentration of a single molecule they can break down and get energy from. Those polymers are not part of crude oil and in underground crude the oxygen to generate energy from the hydrocarbons is missing
Fibers small enough to not pose a nozzle clogging risk but strong enough to make a difference…
Most of this stuff, when it causes cancer does so because of it’s mechanical properties not it’s chemical ones right? It’s small enough to get into the lungs tiniest places and pokey enough to stick through cellular membranes and scramble the nucleus.
I suspect people will flock to these natural sounding solutions (hemp solves everything right man?) only to find that they run into the same problem time and time again because it wasn’t the chemical, it was physical nature of what is being asked for.
It’s a combination, yes, there is a mechanical component of the tinyness and the pokeyness, but it’s also a chemical issue; the tiny pokey thing can’t be broken down (at all, or quickly enough, or fully). Hemp, for instance, might be different in this. More research is possibly needed.
A few years ago I was involved in a project to add sapphire fibers to ceramic parts to make them stronger but not brittle. We grew sapphire 0.005″ diameter and up in long lengths. It was then chopped to 0.01″ and shorter and mixed with the ceramic slurry. The end use was to make small engines for spacecraft use.
It worked but very expensive so the project was eventually dropped.
Fun times.k
Composite materials are defined by three regions: the matrix, the filler, and most importantly, the interface between the two.
Chuanchom Aumnate et al. (link above) for example use a functionalization step where the cellulose fibers are reacted with tetraethyl orthosilicate (TEOS). Leave them in the TEOS solution (usually TEOS in ethanol) for a while and a few hundred nm thick silica shell has formed around the fibers, and it would lead us to question whether such a filler would produce ultra-fine particles when fractured or sanded. But even a few nm will already be plenty to establish the desired interfacial bonding. It doesn’t take much and it mostly comes down to complete surface coverage without the fibers starting to cake together.
“the main challenge is to promote the homogeneous dispersion of cellulose in polymer
matrixes. Thus, improvements of interfacial adhesion between cellulose and polymer matrixes are required.
Interestingly, the cellulose surface modification with tetraethyl orthosilicate (TEOS) has shown various advantages, including low-temperature processing, controllability of composition, resultant structure, and easy scale-up.”
She’s my main thing
She makes me feel alright
She makes my heart sing
– Rick james
use all the parts. It’s been used for a long time to make stuff