Bioplastic from olive tree pruning residues

A research team from the University of Jaén (Jaén, Spain) and the Andaltec Foundation (Jaén, Spain) have obtained a plastic from olive tree waste, with characteristics similar to those of any other non-waste material, suitable for food packaging. The model designed by the researchers has improved the process, simplifying stages and optimising times, with a more economical and sustainable treatment. The work has been financed by the ‘BioNanocel’ project of the Regional Ministry of University, Research and Innovation through FEDER funds.

In addition, the researchers have confirmed the thermal stability of the product, which makes it a candidate for the manufacture of transparent biofilm and for processing with conventional technologies in other plastic products. This is explained by the experts in the article ‘Production and characterization of cellulose acetate using olive tree pruning biomass as feedstock’, published in the journal Biofuels, Bioproducts and Biorefining, in which they demonstrate how the polymer can be obtained from pruning waste, which could be applied to multiple uses.

According to the National Statistics Institute (INE), olive trees in Jaén account for 85 % of the cultivated land and are the main economic engine of the province. So far, the by-products obtained from pruning have acquired value as compost, for the generation of biomass energy or even as ground cover to retain humidity and control weeds.

Its use as a material for plastic manufacturing would be more profitable for both farmers and manufacturers. Moreover, it is a material that is constantly available, as it is a practice that must be carried out every year and which produces tonnes that can feed the production demand. “The Andalusian olive grove could thus become the main supplier of raw material for the manufacture of bioplastics in Spain”, says University of Jaén researcher María Dolores La Rubia.

The farm-industry alliance
The key lies in cellulose, the material that gives consistency to the cell walls of plants and is widely used in the manufacture of paper and cardboard, among many other uses. Thus, the experts shredded the pruning material and optimised the purification process with an acid solution to extract the cellulose components, obtaining a yellowish pulp.

Subsequently, to remove all non-cellulosic compounds, the product was treated with sodium hydroxide (caustic soda) in a process known as alkaline hydrolysis, in which the molecular bonds are broken. The cellulose pulp then reacts in an acidic medium with a compound called acetic anhydride, causing a reaction called acetylation. This results in cellulose acetate, a white base with a concentration of 95 % cellulose, after treatment with hydrogen peroxide.

The product can be processed with conventional polymer processing technologies, such as injection moulding and extrusion, to obtain different plastics. By injection moulding, the acetate is introduced into a mould under high pressure to obtain the desired shape. After cooling, the moulded part is extruded. In extrusion, it is passed through a die with a specific shape to produce a continuous product, such as sheets or tubes. These can be filaments for the production of textile fibres, moulded into films for coatings or packaging, or used in other industrial applications.

The experts are continuing their work with different plasticiser additives to provide new options for other uses that demand particular qualities, such as flexibility, colour or strength. AT

www.ujaen.es
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Source: Regional Government of Andalusia (Junta de Andalucía)
Translated from Spanish.