Better Living Through Chemicals: Treating Hemp Fibers To Improve Performance of Green Composites

        We live in a time where there is a lot of push to get back to the use of natural materials in the production of as many things as possible. From the need for greater availability of materials, cleaner methods of disposal, to overall greater sustainability in all steps of material production, fast growing sources such as hemp present myriad advantages for producing composite materials. However, the “green” ideology of utilizing biobased materials currently has limitations which many hope to overcome by blending natural fibers, such as hemp, into composites with other materials such as epoxies and other resins, known as thermosets, with preference for thermosets which are also derived from biobased sources (such as polybenzoxazine). Doing so gives nearly endless possibilities of changing the mechanical properties of the natural materials in ways that transcend their current limitations and allow these materials to be used in applications which may currently seem pretty far fetched. For instance, how often do you find yourself pondering whether or not you’ll one day own a car, home, or fly in an airplane made from hemp? 

In this entry I am discussing a journal paper entitled The influence of different chemical treatments on the hemp fiber/polybenzoxazine based green composites: Mechanical, thermal and water absorption properties from the journal Materials Chemistry and Physics Volume 217, from 15 September, 2018, the authors Abdul Qadeer Dayo, Abdeldjalil Zegaoui, Adnan Aftab Nizamani, Sadia Kiran, Jun Wang, Mehdi Derradji, Wan-an Cai and Wen-bin Liu describe a few ways these hemp composites are being treated with various chemicals to produce better results and more specific mechanical properties.

This research consisted of using three different techniques for treating short help fibers; a wash with cyclohexane/ethanol alone (WHF= Washed Hemp Fiber), and treatment with either alkaline (AHF) or silane (SHF) coupling agent after a cyclohexane/ethanol wash. The treated hemp fibers were reinforced in a polybenzoxazine matrix at 25% by volume, resulting in what they refer to as “green composites.” The mechanical, thermomechanical, thermal, and water uptake properties were then tested for the impact each of the three techniques had on the green composite.

A key aspect of this research was to improve the bonding between the resin and the natural fibers. The fibers contain moisture, while the resins are hydrophobic, meaning they repel moisture. I’m sure you can see how those traits can be problematic when trying to get the two materials to bond. Therefore, the chemical treatments were used to modify the surface of the natural fibers in order to facilitate adhesion, not unlike priming an innertube by roughing up the surface before applying a patch; the roughness gives the adhesive something to weave itself through to create a much stronger bond. In this case, the rough surface was achieved not by physically abrading the surface, but from free space created from the removal of lignin and hemicellulose compounds via the alkali and silane treatment. The benefit in this was clear from the results of tensile stress-strain, flexural, and impact tests all showing vast improvements in performance, especially in the SHF composite.

The issue of moisture content of the fibers doesn’t end with simply affecting the bonding process of the composite to the fibers. The absorption and release of moisture plays a role all on its own. If the fibers are fully saturated with moisture during the formation of the composite, time and heat can lead to the fibers losing moisture. The shrinkage affiliated with the lower moisture content creates gaps in the composite, to the detriment of its stability. If the fibers are not fully saturated with moisture during the formation of the composite, the fibers start off smaller (in their dried, shrunken form), but moisture absorption would cause the fibers to swell and lead the composite to crack from the internal pressure.

The results were unanimously positive for the alkali and silane treated hemp in every means tested, although it should be noted that, alone, the cyclohexane/ethanol wash created some issues such as a smooth surface (harder to bond to the resin) and greater hydrophilic properties (tendency to attract and absorb moisture). Also,  thermal stability of the green composites was lower than that of neat resin (resin without the hemp fiber), even after the improvements demonstrated after chemical treatment. This was likely expected due to the nature of the hemp fibers having significantly lower thermal stability, which moisture plays a large role in, but thermal stability of the green composites was still increased after silane and alkali treatments, so progress was made, nonetheless. Fiber reinforced composites treated with silane showed the most improvement overall, and Dayo, et. al., determined that it could be used as a “high performance composite,” though no further details were given on what “high performance” means in this instance.

Does this mean your next flight to Honolulu will be in a hemp composite aircraft, or the new home being raised in your neighborhood will be out of the same material? Definitely not. It does, however, give us a glimpse of the potential for more sustainable material development that could lead to significant changes in the future, even if we’re still decades away from seeing these results in action.

Follow the doi in the work cited for the fun technical details of this research.

Work Cited

Dayo, A. Q., Zegaoui, A., Nizamani, A. A., Kiran, S., Wang, J., Derradji, M., Cai, W.-an, & Liu, W.-bin. (2018). The influence of different chemical treatments on the hemp fiber/polybenzoxazine based Green Composites: Mechanical, thermal and water absorption properties. Materials Chemistry and Physics, 217, 270–277. https://doi.org/10.1016/j.matchemphys.2018.06.040