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?