Defence of doctoral thesis: Angelica Avella – Reactive extrusion of lignocellulose-polyester biocomposites

The development of biodegradable and recyclable composites based on renewable resources can mitigate the effects of plastic pollution and the depletion of fossil fuels. A biocomposite consists of a matrix strengthened with fibres, and in this work, biodegradable polyesters have been blended with lignocellulosic derivatives, and reactive processing strategies have been developed to tackle the drawbacks of poor lignocellulose dispersion and poor adhesion of the lignocellulose to the polymer matrix. Reactive melt processing combines melt compounding with chemical reactions, and herein it has been explored to tune the interface of biocomposites and improve their performance. Three different ways of strengthening the polymer-lignocellulose interface have been investigated involving (a) modification of the polymer matrix, (b) modification of the lignocellulose, and (c) the addition of a third component.

The first approach was a peroxide-initiated branching/crosslinking carried out with water-assisted feeding of the lignocellulose. Crosslinking led to the formation of a uniform hybrid polymer-lignocellulose network that developed creep resistance and heat-shrinkage in the matrix. The mechanical recycling and industrial composting of crosslinked poly(butylene adipate-co-terephthalate) (PBAT)-pulp fibre biocomposites were successfully verified.

In the second category, the grafting of epoxidized bio-sourced oils onto industrial lignin was investigated as a way to plasticize the lignin and promote its miscibility with polyesters. Deformable and tough PBAT-modified lignin blends were prepared and shaped by film-blowing, to be subsequently mechanically recycled or industrially composted. The cellulose was also modified by in-situ polymerization of bio-sourced ethylene brassylate to graft the polymer from the cellulose surface. Ring-opening polymerization was achieved by organic and enzymatic catalysis, which showed that grafting from is an effective method of achieving nanocellulose dispersion and consequent stress transfer with the matrix.

In the third approach, amphiphilic diblock copolymers with two different tail lengths were designed to mediate the interface between cellulose nanofibrils and PBAT. In an aquatic environment, the cationic anchor block was effectively adsorbed onto the negatively charged nanofibrils, promoting their dispersion, while the longer tail block favoured entanglement with the matrix and deformation of the biocomposites.

This thesis contributes to the understanding of biocomposite interfaces, paving the way for future investigations, and proposes sustainable alternatives for the industrial replacement of commodity plastics.