Following the journey of a nanomaterial from wheat to plastics

Following the journey of a nanomaterial from wheat to plastics

Rigorous chemical treatments of pure bleached pulp are the most common approaches used to obtain cellulose nanofibrils. These processes generate significant costs and are harmful to the environment.

Study: Production of lignocellulosic nanomaterials from wheat straw by pretreatment with peracetic acid and its application in plastic composites. Image Credit: OskarsK / Shutterstock.con

An article published in the journal Carbohydrate Polymers reports on the synthesis of lignocellulosic nanofibrils (LCNF) using wheat straw (WS) feedstock. LCNF shows great promise in bioplastic applications.

Cellulose nanofibrils: an ecological alternative to plastics

Products made from green and renewable sources are in high demand. Cellulose, an abundant natural resource, is renewable, non-toxic and biodegradable.

Plastic waste has accumulated due to the regular dumping of non-biodegradable plastics, causing serious pollution of land and marine environments. Cellulose Nanofibrils (CNF) are a greener alternative to plastics in a variety of applications.

The small manufacturing volume and the significant production costs of cellulose nanofibrils prevent them from reaching their true potential in different industries. Therefore, economical and efficient production techniques must be developed to encourage the use of cellulose nanofibrils as viable alternatives to plastics.

How are cellulose nanofibrils synthesized?

Cellulose nanofibrils are typically made from bleached kraft fibers, a quality raw material that provides almost pure cellulose. The kraft fibers are mechanically fibrillated to extract the cellulose nanofibrils.

Mechanical fibrillation processes require a considerable amount of energy. To advance the fibrillation phase and minimize energy needs, pretreatment procedures can be used.

A widely used pretreatment process to synthesize cellulose nanofibrils is the TEMPO (2,2,6,6 Tetramethylpiperidin-1-oxyl) oxidation. This technique stimulates the oxidation of the cellulose fibers, which increases the charge density on their surface. Ultimately, this leads to electrostatic repulsion between the cellulose nanofibrils and produces significant fibrillation output.

Disadvantages of using TEMPO

Disadvantages of employing the TEMPO oxidation technique include substantial chemical expense and the toxic nature of TEMPO. Furthermore, expensive removal methods such as dialysis are required to remove small amounts of leftover TEMPO from the cellulose nanofibrils, making scalable application of the approach quite difficult.

The fabrication of cellulose nanofibrils needs a cheaper conversion technique if the scalable implementation of CNF is to be achieved.

Development of lignocellulosic nanofibrils with wheat straw

To avoid the drawbacks of TEMPO oxidation, LCNF manufacturing may be the key. LCNF production requires less expensive lignin-containing feedstock rather than fully bleached pulp. Furthermore, mild treatment procedures may suffice for LCNF, resulting in high-yield products composed of lignin, cellulose, and hemicellulose components.

Wheat production creates a by-product known as wheat straw. WS contains the residual stalk that remains after the harvest of the wheat grains.

Wheat straw acts as a suitable raw material for the manufacture of LCNF because it is available in abundance. Being an agricultural surplus, the costs of using wheat straw are much lower than compared to other raw materials.

Therefore, wheat straw provides an abundant and low-cost non-wood fiber resource to produce LCNF.

Advantages of peracetic acid treatment

Peracetic acid (PAA) is a biodegradable reagent that can participate in reactions with lignocellulosic biomass at temperatures below 100 °C. Therefore, the use of PAA for the pre-processing of LCNF appears to be a viable strategy.

PAA treatment produces higher yields due to its high oxidizing capacity, which selectively removes lignin while preventing carbohydrate solubilization.

Peracetic acid can trigger oxidation of the reducing parts of carbohydrates, resulting in a negative charge on the surface. This could help in the nanoscale fibrillation process and produce stable colloidal suspensions.

Compared to the traditional TEMPO oxidation process for the synthesis of cellulose nanofibrils, peracetic acid treatment provides several benefits. PAA is less toxic and more environmentally friendly than TEMPO, and allows greater control over the removal of hemicellulose and lignin from the pulp material.

The architecture and content of the nanofibrils generated by peracetic acid treatment differ significantly from the cellulose nanofibrils produced by traditional TEMPO oxidation.

Applications of cellulose nanofibrils in plastic composites

Cellulose nanofibrils can be used as reinforcements in plastic compounds to minimize the amount of petroleum components and at the same time improve the characteristics of plastic compounds.

Hydrophilic and biodegradable polymers, such as polyvinyl alcohol (PVA), show weaker mechanical characteristics than their synthetic alternatives, which requires the addition of different additives to improve their qualities.

Higher aspect ratios and better interface interactions with the polyvinyl alcohol matrix allow cellulose nanofibrils to improve the mechanical properties of PVA nanocomposites when introduced in small amounts (<5 wt%).

Study results

The team showed that it is possible to manufacture lignocellulosic nanofibrils with excellent plastic reinforcement characteristics using low-cost agricultural waste. They achieved this by alkaline peroxide pulping, after which the LCNF was treated with PAA.

The lignin and hemicellulose parts of the nanofibrils, formed with wheat straw, remained, which was essential for improving productivity.

Although the PAA treatment generated nanomaterials with a lower surface charge density than the TEMPO oxidation, the material characteristics were not affected. All specimens demonstrated high colloidal stability under aqueous conditions.

PAA-treated materials improved thermal stability due to their lower charge density and increased lignin level. Furthermore, regardless of the charge density, all the nanofibrils were well dispersed in the polyvinyl alcohol matrix. This improved the tensile strength and Young’s modulus of the composites, showing a unique case of simultaneous strengthening and hardening.

The study established a new way to produce lignocellulosic nanofibrils from agricultural waste. This economically feasible approach has the potential to enable scalable production of nanomaterials at commodity costs, allowing them to be used in high-volume industries such as bioplastics.

Reference

Pascoli, DU, Dichiara, A., Roumeli, E., Gustafson, R., & Bura, R. (2022). Production of lignocellulosic nanomaterials from wheat straw by pretreatment with peracetic acid and its application in plastic composites. carbohydrate polymers. Available at: https://doi.org/10.1016/j.carbpol.2022.119857

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