Components of CNSL

Cashewnut Shell Liquid (CNSL) is an agricultural product, which as such, qualifies its inclusion in the category of renewable resource. Major components of CNSL have been characterized by various researchers using different techniques such as ultraviolet (UV), infrared (IR), nuclear magnetic resonance (NMR) spectroscopy. 

This is one of the most widely distributed plants cultivated to obtain cashew nut. The phenolic lipid is only a by-product known as cashew nut shell liquid (CNSL). The nut, attached to the base of the cashew nut apple consists of an ivory-colored kernel covered by a thin brown membrane (testa) and enclosed by an outer porous shell, the mesocarp which is about 3 mm thick with a honey-comb structure where the reddish brown liquid (CNSL) is stored [91]. The major components of CNSL are a phenolic acid, anacardic acid, a dihydric phenol, cardol with traces of mono hydric phenol, cardanol, and 2-methyl cardol [92-95]. 

Peroxidase (CiP) oxidatively polymerized cardol, a major component of CNSL [105]. The efficient polymer production was achieved in an equivolume mixture... 

Cardanol, a main component obtained by thermal treatment of cashew nut shell liquid (CNSL), is a phenol derivative having mainly the meta substituent of a C15 unsaturated hydrocarbon chain with one to three double bonds as the major. Since CNSL is nearly one-third of the total nut weight, a great amount of CNSL is obtained as byproducts from mechanical processes for the edible use of the cashew kernel. Only a small part of cardanol obtained in the production of cashew kernel is used in industrial fields, though it has various potential industrial utilizations such as resins, friction-lining materials, and surface coatings. Therefore, development of new applications for cardanol is very attractive. 

Figure 4. Structure of four major components of cashewnut shell liquid (CNSL).

AR occur in cashew nut shells as a fraction of other oil components like cardols, cardanols and anacardic acid. A comparative study on the extraction of cashew nut shell liquid (CNSL) was presented by Shobha and Ravindranath (50). The study involved the extraction of the cashew nut shell by supercritical CO2 or pentane. The pentane extraction was carried out on 50g steamed or fresh cashew nut shells in lOOmL solvent. Supercritical CO2 extraction was performed on 300g freshly broken cashew nut shells at 25 MPa and 40 C with the CO2 flow kept at 4-5Kg/h for 17,5h with extract collection every 2.5h. The resorcinolic lipid fraction obtained by supercritical CO2 represented 82% of the equivalent obtained by pentane extraction of fresh cashew nut shells and 70% of the extraction of steamed material. Despite this appreciable variation on the ratio of the total cardols and cardanols from one mediod to other, the relative proportion of the enomers in each group was very similar (50). Generally, the extraction yield obtained by supercritical CO2 was lower (= 60%) than that obtained by the classical solvent extraction methods (50), however, the product was nearly colorless. One of the major problems in the industrial application of CNSL is the very dark brown color of the solvent extracted product. 

An inherent problem in the usage of phenolic lipids, particularly in surface coatings, is the discolouration which can impair products. Apart from colourants arising from the solvent action of CNSL on the shell in the industrial process, the dihydric phenols In CNSL notably the minor component 2-methylcardol (ref. 200) more than cardol appear to be the cause of this deterioration rather than the monohydric member, cardanol. The usage of purer cardanol, or the less unsaturated material by semi-hydrogenation or chemical reduction, as well as the Incorporation of an antioxidant are methods for colour stabilisation (ref. 277). Antioxidant applications and pharmaceutical uses of CNSL and its component phenols are referred to in the next section. 

Cardanol is a main component of thermally treated cashew nut shell liquid (CNSL), and is a phenolic compound with a long unsaturated hydrocarbon chain substituted in the meta position (Figure 2.17a). Urushiol, which is obtained from lacquer tree, poison ivy, poison oak, and poison sumac (Toxicodendron), and used for a raw material of a lacquer (urushi) in East Asia, is also a phenolic compound of catechol with a long unsaturated or saturated hydrocarbon chain (Figure 2.17b). Cardanol-based polymers have been reported very often, while there are a few research reports on urushiol-based polymers. Research on polymers synthesized from cardanol or CNSL are reviewed elsewhere.In the late 1980s, cardanol or CNSL-based polymers began to be reported as novel phenol-formaldehyde type resins and novel epoxy resins.Thereafter, Pillai and his co-workers have vigorously studied synthesis of various type of cardanol-based polymers polymers obtained... 

In general phenolic lipids have been separated from natural sources for compositional studies and structural determination by solvent extraction and chromatographic techniques. The individual component phenols of the major phenolic lipid (CNSL) from Anacardium occidentale have assumed some significance in certain chemical applications and detailed purification processes... 

With the aid of gradient elution, a reversed phase partition method and an internal standard, considerable progress has been made in the quantitative determination of each constituent of the main component phenols present in technical CNSL and in natural cashew-nut shell liquid provided that relative molar response (RMR) factors are used. A typical quantitative HPLC analysis of the technical material indicated cardanol (67.8%), cardol (18.2%), 2-methylcardol (3.3%), minor constituents (3.3%) and polymeric material (7.4%). 

For natural CNSL from different regions the % composition of the components, anacardic acid, cardol and 2-methylcardol by HPLC has been found to vary although all were produced from Anacardium occidentale (ref. 221). 

Although HPLC analysis affords by virtue of its partition nature a very detailed analysis of natural and technical CNSL, it is of interest to compare compositional results obtained by other methods. By contrast, GLC is less detailed and TLC as an adsorption process, operating through functional groups, affords a very direct uncomplicated indication of the main components present. Not all areas in the world producing CNSL have access to HPLC or even GLC analytical procedures and other chromatographic techniques may in those circumstances be obligatory. Tables 7 and 8 show respectively the % composition of the main components and of the unsaturated constituents of anacardic acid in natural CNSL by several different techniques. 

Thus while HPLC gives a detailed picture of the composition of both natural and of technical CNSL, these complex products contains many minor materials of a homologous or different structural type which would be most easily identified by HPLC/MS or HPLC combined with another spectroscopic technique. Developments in H and C high resolution NMR spectroscopy and in FTIR may enable the analysis of the constituents of each component phenol to be extended to the technical and natural products. Observations have been made on the mixed unsaturated constituents by H NMR and C studies (refe.201, 222). 

The cardol present in CNSL, if in high proportion can lead to an exothermic reaction with formaldehyde and also It appears desirable for the phenolic components to have a high proportion of triene in order for the first acid-catalysed side chain oligomerisation stage to proceed. CNSL-formaldehyde polymers have greater flexibility than those from phenol-formaldehyde, due to internal plasticising, they are also more soluble in solvents, and due to their hydrophobicity they have resistance to water penetration, and hence acidic and alkaline media. For some applications highly methylolated cardanol is useful and this can be formed with formaldehyde, by the use of adipic or succinic acid as catalysts, and subsequently rapidly cured with hexamine (ref. 245). 

The development of improved separational processes for obtaining pure (mixed) cardanol and cardol from technical CNSL has encouraged experimentation in chemical instead of polymer uses for these component phenols as well as for anacardic acid, by extraction from natural CNSL. Some of the earlier chemistry has been reviewed (ref. 2). Most of the more recent uses particularly for cardanol, but also cardol and anacardic add, stem from the conception of their semi-synthetic applications as readily available replenishable resources (refs. 278, 279). As with CNSL, the reactions considered in this section are concerned with the hydroxyl group of the side chain and substitution in the ring. 

Toxic 5-alkatrienyl, 5-alkadienyl, 5-alkenyl and 5-alkyl resorcinols, collectively known as cardol, occur in the cashew nutshell liquid (CNSL), also known as cashew shell oil, a natural resin found in the honeycomb structure of the cashew nutshell. It is a byproduct of cashew nuts processing (see Section 8.2.6.1.6). A typical solvent-extracted CNSL contains 15-20% of cardol. The main components are (8Z,llZ)-pentadeca-8,ll,14-trien-l-yl (10-1), (8Z,llZ)-pentadeca-8,ll-dien-l-yl, (8Z)-pentadeca-8-en-1-yl and pentadecyl derivatives. 

Resins. The phenolic resins used are varied. They are novolak, resole, novolak/resole combinations, cresol resins, and rubber-modified and oil-modified novolaks. Tung oil, cashew nut shell liquid (CNSL), linseed oil, and soya bean oil are reacted with phenolics. The value of oil-modified phenolics is the quieter performance of the braking action, and the reduction of cracking in components. Oil-modified novolak resins are supplied in viscous liquid or powdered forms. The flexibility in the cured resin can be controlled by the percentage and type of oil introduced into the resin. 

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