C. Santulli, F. Sarasini, E. Fortunati, D. Puglia, and J. M. Kenny

Contents

11.1 Introduction………………………………………………………………………………………………….. 176

11.2 Okra Fibre as the Reinforcement for Thermosetting Polymers………………………………………… 178

11.3 Okra Fibre Based Thermoplastic Composites…………………………………………………………… 181

11.4 Okra Fibres as a Possible Source of Nanocrystalline Cellulose……………………………………….. 184

11.5 Conclusions and Future Perspectives…………………………………………………………………….. 187

References………………………………………………………………………………………………………….. 187

Abstract The need to find environmentally friendly alternatives to traditional synthetic fibres such as glass fibres to be used as reinforcement in polymer matrix composites has attracted a growing interest in natural plant fibres in the last decade. In this regard, this chapter provides a comprehensive overview on a less common, but promising, natural fibre known botanically as Abelmoschus esculentus. It focuses on the origin, history and use of this plant with a particular emphasis on the fibres extracted from the stem of this plant, also known as okra fibres. A comprehensive mechanical, morphological and thermal characterization of the fibres is addressed in this work aiming at investigating their possible use as reinforcement in polymer matrix composites. The addition of okra fibres in thermoplastic and thermosetting

C. Santulli (*)

Universita degli Studi di Camerino, School of Architecture and Design, viale della Rimembranza, Ascoli Piceno 63100, Italy e-mail: carlo. santulli@unicam. it

F. Sarasini

Department of Chemical Engineering Materials Environment, Sapienza—Universita di Roma, Via Eudossiana 18, Rome 00184, Italy

E. Fortunati • D. Puglia • J. M. Kenny

Materials Science and Technology, Civil and Environmental Engineering Department, Universita di Perugia, Strada di Pentima 4, Terni 05100, Italy

K. R. Hakeem et al. (eds.), Biomass and Bioenergy: Processing and Properties,

DOI 10.1007/978-3-319-07641-6_11, © Springer International Publishing Switzerland 2014 matrices is reviewed, while the last part of the chapter is devoted to the development of cellulose-based nanocomposites, which is unanimously perceived as one of the most promising research fields related to plant-based products. The extraction of cellulose nanocrystals from okra fibres and their incorporation in thermoplastic composites is described. The problems that appear as limiting factors for possible application of okra fibres as reinforcement for semi-structural components are high­lighted and discussed.

Keywords Okra fibres • Thermoset composites • Thermoplastic composites • Nanocellulose extraction

11.1 Introduction

Okra (Abelmoschus esculentus (L.) Moench), also known as Hibiscus esculentus L., is a member of the mallow (Malvaceae) family, which includes also hibiscus and cotton among other species, and can be found as a tall-growing, warm-season annual (primarily in the United States) or perennial (in India and Africa) that is well suited to a wide range of soil types. It represents the only vegetable crop in the Malvaceae family, whose products have significant use in the food sector. In several parts of the world it is known also as Okra, Quingumbo, Lady’s finger, Gombo, Gumbo, Bamia, Bhendi and Bhindi. The origin of Okra is disputable, but it seems to be native to the so-called Abyssinian centre of origin of cultivated plants, an area that includes Ethiopia, Eritrea and the eastern part of the Anglo-Egyptian Sudan. It is currently grown throughout tropical Asia, Africa, the Caribbean and southern United States.

Okra can grow up to 2 m tall and has leaves 10-20 cm long and broad, with lobes ranging from 5 to 7. The five white to yellow petals that constitute the flowers of diameters in the range 4-8 cm, are often characterized by a red or purple spot at the base of each petal. The seeds are contained in a capsule up to 18 cm long and that shows from 4 to 10 distinct ribs or ridges (Fig. 11.1). The immature young seed pods are the edible part of this plant. Most okra cultivars produce green pods, but a few varieties produce yellow or dark red pods. These pods are harvested when immature and high in mucilage, generally within 2-6 weeks after flowering. In some countries the most interesting part of okra plant is represented by the seeds, which yield edible oil with a pleasant taste and odour, and high in unsaturated fats such as oleic and linoleic acid. The ripe seeds can also be roasted and ground and used as a substitute for coffee. The last years have witnessed an increasing interest in fibres that can be extracted from the stem of okra plants, which are often considered as an agricultural waste product after the collection (Shamsul Alam and Arifuzzaman Khan 2007; Arifuzzaman Khan et al. 2009; De Rosa et al. 2010a, 2011). This is not surprising since roselle (Hibiscus sabdariffa, L.), a close relative of okra, is traditionally used as a source of fibres (Athijayamani et al. 2009, 2010; Methacanon et al. 2010).

The fibres can be obtained from the stems of okra plants by water retting for about 15-20 days (Arifuzzaman Khan et al. 2009; De Rosa et al. 2010a). From a

morphological point of view, the microstructure of okra fibres is similar to that of other natural fibres, being a hierarchical structure, as confirmed by the investigation of both cross section and longitudinal surface by means of optical and electron microscopy (Fig. 11.2). As observed, okra technical fibre is made up of several elementary fibres (known as ultimate fibres) which appear to overlap each other along the whole length of the fibre while being firmly bonded together by pectin and other non-cellulosic compounds that provide strength to the bundle (De Rosa et al. 2010a; Fuqua et al. 2012). The highly irregular polygonal shape of okra fibre is apparent from Fig. 11.2a, b and its typical diameter is found to be in the range of about 40-180 pm. The shape variability affects the mechanical strength of fibres, being dependent on agricultural factors (stem age and plant variety, for instance) as well as the position of the fibres along the stem height (Ayre et al. 2009). The ultimate fibre appears to be roughly polygonal, with a central hole (lumen) of very variable dimensions. In particular, cell wall thickness and lumen diameter are reported being in the range 1-10 and 0.1-20 pm, respectively (De Rosa et al. 2010a).

The composition of okra fibres in terms of cellulosic and non-cellulosic constitu­ents is reported in Table 11.1 along with the composition of other bast and leaf fibres commonly used as reinforcements in natural fibre composites. It is worth noting that the composition of okra fibres is very similar to that of other bast or leaf fibres. In this regard, comparable thermal behaviour and stability are expected. Okra fibres showed a traditional TG (thermogravimetric) curve characterized by three distinct weight loss steps, with a two-stage decomposition process (De Rosa et al. 2010a). The initial weight loss («8 %) occurring between 30 and 110 °C is due to water vaporization, while the onset of degradation occurs after 220 °C. The first stage (220-310 °C) is well described in terms of thermal depolymerization of hemicellulose, pectin and cleavage of glycosidic linkages of cellulose (weight loss of 16.1 %), while the second stage (310-390 °C) is characterized by the degradation of the a-cellulose that consti­tutes the fibre (Albano et al. 1999). The degradation of lignin is a phenomenon that takes place slowly in the whole temperature range owing to its rather complex struc­ture. It can be concluded that okra fibres experienced a thermal stability comparable with that of other natural fibres, as can be inferred from Table 11.2.

As regards the mechanical behaviour, okra fibres are characterized by a brittle behaviour in single filament tensile test with a wide scattering of data (De Rosa et al. 2010a). This behaviour is common to the other vegetable fibres, being dictated by test parameters, plant characteristics and fibre diameter measurement (Symington et al. 2009; da Silva et al. 2012). The effect of okra fibre diameter can be reasonably described by a two-parameter Weibull distribution (De Rosa et al. 2010a) and the resulting mechanical properties are summarized together with the ones of other veg­etable fibres in Table 11.3, from which the comparable mechanical behaviour with soft bast fibres, such as kenaf, and leaf fibres, such as date palm, henequen and sisal, is evident. From these results, the suitability of okra fibres as reinforcement in poly­mer matrix composites can be easily inferred.