**5.1 Biocomposites using tannins**

Tannins are a group of polyhydroxy phenolic compounds and exhibit good alternatives to synthetic adhesives for green chemistry in developing composites. They are found abundantly in nature. Their functions are to protect the plants against predation and might help in regulating the plant growth. Tannins are heterogeneous in nature and chemically classified into two main groups viz. hydrolysable and condensed tannins. Hydrolysable tannins are small molecular weight (30-3000D) compounds, heterogeneous in nature and hydrolysed by water, acidic or alkaline conditions into smaller water soluble molecules such as gallic acid and ellagic acid (**Figure 2a** and **b**) constituting gallotannins and ellagitannins. The gallotannins and ellagitannins comprise of a central sugar unit esterified with several molecules of

#### **Figure 2.**

*(a) Gallic acid (b) Ellagic acid (c) 5,7,3,4- tetrahydroxyflavan– 3– ol (d) 5,7,3,4- tetrahydroxyflavan– 3,4– diol (e) ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (f) HBT (1-hydroxybenzotriazole).*

#### *Opportunity of Non-Wood Forest Products in Biocomposites DOI: http://dx.doi.org/10.5772/intechopen.97825*

gallic acid and a dimer of gallic acid as the basic phenolic unit known as ellagic acid respectively. Gallotannins, or commonly tannic acid, is the acknowledged source of the hydrolyzable tannins produced by extraction with water or organic solvents from the galls of certain trees, *Quercus infectoria* and *Rhus chinensis*, and pods from *Caesalpinia spinosa*. The European chestnut tree (*Castanea sativa*) and the oak species especially *Quercus montana* also produce hydrolyzable tannins in sufficient amount which are used in leather manufacture. Ellagitannins, a class of hydrolysable tannins produce ellagic acid on hydrolysis under acidic or alkaline conditions. Mostly these are present in angiosperms extractives. Ellagic acid and their derivatives have extensive applications as antioxidants, chelators, technical and biomedical applications. The possible applications viz. antibacterial, antifungal, antiviral, anti-inflammatory, hepato- and cardioprotective, chemopreventive, neuroprotective, anti-diabetic, gastroprotective, antihyperlipidemic, and antidepressant-like activities, among others have gained interest to researchers and reviewed for commercial exploitation [108, 109].

Condensed tannins (nonhydrolyzable tannin or proanthocyanidine) the larger polyphenol groups with high molecular weight upto 30000D compounds, form insoluble precipitates in aqueous solution and are the polymerization products of monomeric flavan–3-ol or flavan-3,4-diol precursors (**Figure 2c** and **d**) [110] which are joined through stable C-C bonds between C-4 and C-8 and between C-4 and C-6. Tannins are dynamically used in tanning of animal hides in the leather processing industry since 1960s, the beginning of the industry due to interaction and precipitation of the proteins [29], adhesive making (especially wood adhesives), fisheries, beverages manufacturing, animal feed, biosourced foams, wood preservatives, corrosion inhibitors, polyurethane surface coatings, epoxy adhesives, binders for Teflon coatings, as mineral absorption and protein, as iron gall ink production, adhesive production in wood-based industry, anti-corrosive chemical production, uranium recovering from seawater, and removal of mercury and methylmercury from solution. In continuum, tannins are also used as bioactive molecules in nutrition science, 3D printing and biomedical devices [109, 111]. Their presence in natural vegetable material has prompted scientific community for their industrial applications in many different ways. Since historical times their traditional use has allowed their further use after diverse chemical modification for various end use functionalized properties. The main inherent feature of the tannins is due to the presence of phenolic structure similar to synthetic phenols. Mostly the condensed tannins are polymers composed of falvan-3-ol monomers and are mainly extracted from bark and wood for commericial purposes. Structural diversity and functions of varied range of tannins are very well described elsewhere in the literature [112–116]. Tannins are extracted from plant material by simple methods. Nevertheless, there are various extraction processes were developed to isolate the tannins for diverse applications. However, the extraction process remains a challenge due to their heterogeneity character and compositions. Recently, various extraction processes, technological applications and their pros and cons were reviewed and appeared in leading scientific reports [117–119]. Due to similar tannin structural properties as that of synthetic phenols, the basis of wood adhesives was started in the middle of 1940s. The world first commercial wood adhesive credit goes to Australia in the 1960s using *Acacia mearnsii* (black wattle) bark tannins. The successes stories of producing tannin wood adhesive are continued till date with the advancements. The other species used in producing commercial taanins are *Pinus radiate* (radiate pine) bark [120].

Condensed tannins have been in industrial use for nearly 60 years as replacement for phenolic resins for wood based panels with high resistance against moisture and water as well as for boards with very low subsequent formaldehyde emission. Mimosa tannins, obtained from mimosa bark are usually well appreciated for its functional properties for wood adhesives. A wider industrial usage of tannins suffers from the limited availability of raw materials and high transportation costs. Only South Africa is the only actual producer of mimosa tannins on industrial scale [121]. Tannins react with formaldehyde the main crosslinker, and form hardened and crosslinked structures, similar to synthetic phenolic resins. The methylene bridges are formed between two tannin molecules. These methylene bridges are resistant to environmental factors against hydrolysis due to the strong stability of C-C bonds. Tannin-phenolic resin (tannin wt 40%) and sisal fibres (50 wt%) thermoset adhesives were successfully prepared and met all the required standards viz. Izod impact strength increased significatly. Further, it was also observed that sisal fibres and the tannin–phenolic thermosets have close values of the dispersive component and compatible interaction between the sisal fibres and the tannin–phenolic matrix at the interface [122]. A new source of tannin was also reported as a by product during catechin extraction process from a plant leaves (*Uncaria gambier*) extract and the tannin-phenol-formaldehyde wood adhesive was successfully prepared which met all the international standards [110]. Further, a new source of fibres, obtained from *Pinus roxburghii* needles, was also utilized in preparation of composites using phenolic resin adhesives [101]. Since, the early 1990s there are several scientific reports and reviews pertaining to natural fibres and tannin extracts from a natural sources viz. plants, agriculture wastes are available in the literature [122–131].
