Using unconventional plant fibres, such as okra, as the filler for conventional thermosetting matrices can be considered a preliminary step towards the fabrication of fully biodegradable composites. The use of thermosets enables a reflection on a

Table 11.2 Decomposition temperatures for selected natural fibres (De Rosa et al. 2010a)

number of aspects. These include, for example, the possible maximum amount of fibres leading to an improvement of the composite properties, before effective impregnation gets hindered by an excessive amount of filler, and the evaluation of effectiveness of chemical treatment to provide a sounder fibre-matrix interface. It needs to be noted that plant fibre composites including kenaf fibres, which are botanically similar to okra, in thermosetting matrices are quite diffuse and a number of studies have been performed. In particular, it was demonstrated that alkali treat­ment with NaOH has some positive effect on fibre density and assists in improving the mechanical interlocking and chemical bonding between polyester resin and the

fibre, resulting in superior flexural and impact properties (Aziz and Ansell 2004). Another significant observation is that kenaf is very suitable to manufacture fabrics, therefore producing composites with a sufficiently high amount of fibres, compati­ble, e. g. with the requirements of the automotive industry as regards the envisaged substitution of fibreglass (Na and Cho 2010). It is not surprising, therefore, that fibres such as okra are having some minor interest for application in thermoset matrix composites.

In particular, unsaturated polyester matrix was reinforced with a maximum of 36 vol% okra fibres: the fibres were defined as “woven” in that the whole usable length of the stem, equal to around 60 cm, is employed to reinforce the composites (Srinivasababu et al. 2009). The fibres were introduced either without treatment or subjected to two different procedures, both including an alkali pretreatment with sodium hydroxide, followed by a long-time (14 h) or short-time (5 min) treatment with a very diluted acid solution of potassium permanganate (Srinivasababu and Rao 2009; Srinivasababu et al. 2009). Both treatments brought to some improve­ment of tensile modulus and especially to a more consistent increase of it with the introduction of a higher amount of fibres, which may depend on their more uniform geometry after treatment. In contrast, tensile strength only shows some improve­ment for the highest volume of fibres introduced (36 %), being reduced at lower volumes of reinforcement with respect to untreated okra fibre composites (Srinivasababu et al. 2009). Another study was performed on the introduction of a limited amount, up to 20 wt%, of okra fibres bleached with sodium hypochlorite, in a Bakelite matrix. Here, the introduction of fibres did not result in an increase of tensile and flexural strength with respect to the pure matrix, even for the maximum amount of fibre introduced and the situation was not substantially improved by bleaching, although this treatment did lead to an increased strength of the fibres alone (Moniruzzaman et al. 2009).

To summarize these results, the problems linked to the reduced interface strength and to the anomalous section of the okra fibres, as extracted from the bast, appear still limiting factors for possible applications as reinforcement for semi-structural components.