Fibres from Malvaceae family, in particular the most studied Kenaf fibre (Hibiscus cannabinus) from the Hibiscus family, have been investigated in detail and many reports can be found on the enhancement of composite properties through the incor­poration of this type of short natural fibre in non-biodegradable (Akil et al. 2011) and biodegradable matrices (Russo et al. 2013; Lee et al. 2012). The same approach was considered for the use of another similar fibre coming from the same family, okra (Abelmoschus Esculentus), and the recent study of thermal and mechanical behav­iour of this reinforcement confirmed that okra fibres can be efficiently used for the production of biodegradable composites (De Rosa et al. 2010a; Monteiro et al. 2012).

Up to now, okra fibres in materials have been mainly used in mucilage-based moisture absorbers (Gogus and Maskan 1999) or in its gum form as a source of poly­saccharides (BeMiller et al. 1993), which can be used after an appropriate chemical grafting (e. g. using poly-acrylonitrile) for the synthesis of biodegradable polymers or as a drug delivery system (Avachat et al. 2011) and viscosity modifiers for starches (Alamri et al. 2012). However, the possibility of using agro residuals in polymeric matrices implies, in the specific case of this herbaceous plant, a sound rethinking process, whose principal aim should be the possibility of using not only these fibres just as a waste material, but also ideally offering some reinforcement to a poly­meric matrix. Our previous studies on okra fibres demonstrated that well-known chemical treatments usually applied for natural fibres, such as bleaching or alkaliza­tion, do not significantly improve the fibre properties (Moniruzzaman et al. 2009; De Rosa et al. 2011), so the use of okra reinforced composites in structural applications seems to be difficult to be considered. On a lower profile, which can be recom­mended for materials aimed at large volume applications, where the compostability is a fundamental requirement, the use of easily available biomass, such as herbaceous plants, hardly appropriate for the production of textiles, coupled with the biodegrad­able polymer can be successfully exploited. In this case, the use of short fibres is also recommendable, in particular because the large presence of defects and the uneven fibre diameter result in a rather ineffective stress transfer and as a consequence in lower mechanical performance of the composite for fibres exceeding 5-10 mm length (Kirwan et al. 2007; Juntuek et al. 2012).

The use of okra fibres as reinforcement in thermoplastic biodegradable matrix also belongs to the latter domain. An example of application of okra fibres in ther­moplastic matrices comes from Fortunati et al. (2013c), in which poly(lactic) acid composites containing okra fibres were successfully produced and characterized. This study proved the potential of okra fibres in a context of applications for biode­gradable packaging and also suggested that an alkali treatment on okra fibre can have some positive effect on their use for the fabrication of composites with biopolymer matrix. Specifically, PLA/okra composites were prepared with several amounts of okra fibres (10, 20 and 30 wt%) by using both pristine (UOF) and alkali — treated fibres (ODC, okra derivative cellulose), considering a treatment procedure able to remove the amorphous fraction of the raw fibres. Specifically, okra fibres were firstly treated with 0.7 wt/vol% of sodium chlorite, after that a treatment with sodium bisulphate solution (5 wt/vol%) was carried out. Following this pretreat­ment, holocellulose (a-cellulose+hemicellulose) was obtained by gradual removal of lignin (Chattopadhyay and Sarkar 1946). The obtained holocellulose was treated with 17.5 wt/vol% NaOH solution, then filtered and washed several times with dis­tilled water. After that, the cellulose fibres were dried at 60 °C in a vacuum oven until constant weight. The introduction of these fibres in the polymer always resulted in a higher stiffness of the obtained composite system with an increase of about 30 % in Young’s modulus value with respect to the matrix. Moreover, the addition of okra fibre to the PLA matrix led to a significant nucleation effect, which improved in turn the ability of the polymer to crystallize: this effect was more evident in the composites containing ODC. A disintegration test in composting condition has been

also performed (Fig. 11.3), in order to have useful information about the post-use of the studied composite systems.

The introduction of 10 and 20 wt% of both untreated and treated fibres increased the disintegrability rate of PLA matrix; this behaviour can be explained considering that the hydroxyl groups of the cellulose structure act as catalysers for the hydroly­sis of the ester groups of the polymer. This result suggests the possibility to induce an acceleration of PLA weight loss due to the natural fibre introduction useful for the environmental impact of these composites during their post-use.