Manufacture and characterisation of polylactic acid biocomposites with high-purity cellulose isolated from olive pruning waste

A two-step chemical process was carried out on olive pruning residues according to an optimised sequence that led to the isolation of natural fibre with a high cellulose content. Reaction time, temperature and HNO3 concentration in the acid hydrolysis stage were optimised by means of the Response Surface Methodology to achieve the highest removal of hemicellulose and lignin and the highest crystallinity index, minimising cellulose hydrolysis. Subsequent hydrolysis with NaOH allowed to obtain a pulp enriched in cellulose (83.28 wt.%). Analysis revealed that the cellulose isolated had a high crystallinity index (70.06%) and thermal stability (T (max) = 357 degrees C). The cellulose obtained was finally used for the manufacture of polymer biocomposites and to evaluate its viability as a filler for polymeric materials. The selected polymer matrix used was polylactic acid (PLA) and the amount of filler was 5 and 15% by weight, respectively. In general, the fibres did not improve the mechanical properties of PLA, and maintained unchanged its melting temperature. Microscopic analysis revealed that PLA/fibre adhesion was stronger for treated fibres. Contradictorily, the composites with untreated fibres presented slightly higher thermal stability. Water uptake increased with the concentration of fibres, being higher in those materials with untreated fibre.

Automated summary

A two-step chemical process was used to isolate natural fibre with high cellulose content from olive pruning residues. The optimum reaction time, temperature, and HNO3 concentration were determined using the Response Surface Methodology. The obtained cellulose had high crystallinity index and thermal stability. The cellulose was used to manufacture polymer biocomposites and showed potential as a filler for polymeric materials. The mechanical properties and thermal stability of the composites varied depending on the treatment and concentration of the fibres.