Cardanol applications

Although urushiol possesses an interesting structure for transformation into speciality polymers, no attempt has been reported. Notwithstanding its applications in a specified area, it appears that it is not properly put to use as it can be converted to polymers with better properties. The possibilities for such conversions into high-performance polymers are illustrated by cardanol, a phenolic lipid of related structure obtained from Ana-cardium occidentale. 

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 10.3 Cardanol as a potential material for various applications.

Kim YH, Suk An E, Keun Song B et al (2003) Polymerization of cardanol using soybean peroxidase and its potential application as anti-biofilm coating material. Biotechnol Lett 25 1521-1524... 

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. 

The properties of phenolic lipids have tended to be dominated by technological aspects and it is only comparatively recently that potential biological usefulness has come to the fore. For example, products derived from the Anacardiacae occidentale, notably cardanol obtained by semisynthesis through thermal decarboxylation were all directed to polymeric and technical applications and the vast industrial literature [246] contains only two references to insecticidal uses of chlorinated cardanol [247]. 

Historically the main interest in cardanol, in the form of the semisynthetic, technical cashew nutshell liquid, obtained by thermal decarboxylation of natural cashew nutshell liquid, has been in a wide range of technological applications such as in friction dusts and in polymer chemistry many of which have been described in a number of reviews [1,2,11]. Remarkably, very new biological applications have been found for this versatile raw material. 

The finding (ref. 126) that methoxyarenes complexed with chromium hexacarbonyl, thus forming arene chromium tricarbonyls, then undergo 3- rather than the anticipated 2-/4- substitution with certain nucleophiles has a potential application in the synthesis of cardanols and cardols. The method has a number of variants (refs. 127, 128) and is shown in the following scheme. Salicylates formed two isomers. 

Since cardanols can be obtained from anacardic acids and cardols from orsellinic acids, the methods outlined have a general applicability to a range of phenolic lipids. Reference has been made largely to the phenols of the Anacardiacae but the methods are likely to be applicable to other phenolic systems, and those with methylene-interrupted structures at different side-chain positions. Alkynes and phosphorans have both proved invaluable in synthetic studies but attention should be drawn to the very elegant use of ailenic compounds in the polyethenoid (arachidonic) series (ref. 168) which has a potential application with phenolic lipids. Methods for the synthesis of leukotrienes are also relevant for the methylene group-interrupted structures of phenolic lipids (169). 

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). 

There has been great interest in thermally stable materials having ablative and fire-retardant applications. Phosphorylated CNSL has been prepared many years ago (ref.246) but phosphorylated cardanol prepolymers, containing the dihydrogenphosphate group, have been obtained from CNSL by phosphorylation with orthophosphoric acid and dimeric products by simultaneous phosphorylation and oligomerisation (ref. 247). 

Reactions specificaly involving the side chain to increase the functionality of cardanol have been examined in recent years. For example, CNSL and cardanol have found an application in polysiloxane chemistry and a significant development has been the hydrosilation of cardanol with MeHSiCl2 followed by hydrolysis leading to the incorporation of 0-Si-C bonds (ref.255). Other double bonds may be involved than the terminal one. 

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. 

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. 

Cardanol, a component of the agricultural by-product cashew nut shell liquid, also has a phenolic nature and can react with aldehydes under several conditions (Raqueza et al, 2010). Phenolic-type resins based on cardanol have been investigated for such applications as civil engineering structures (Cardona et al, 2010). 

Rao B S and Palanisamy A (2011), Monofunctional benzoxazine from cardanol for biocomposite applications . React Fund Polym, 71, 148-154, and references therein. 

CNSL is obtained as a by-product of the cashew nut industry, mainly containing anacardic acid 80.9%, cardol 10-15%, cardanol, and 2-methyl cardol (Fig. 10). CNSL occurs as a brown viscous fluid in the shell of cashewnut, a plantation product obtained from the cashew tree, Anacardium oxidentale (Bhunia, et al., 2000). CNSL is used in the manufacture of industrially important materials such as cement, primers, specialty coatings, p)aints, varnishes, adhesives, foundry core oils, automotive brake lining industry, laminating and rubber compounding resins, epoxy resins, and in the manufacture of anionic and non-ionic surface active agents. CNSL modified phenolic resins are suitable for many applications and perform improved corrosion and insulation resistance. 

Cardanol from anacardium occidentale gave vinyl resins used as additives, sealants, in brake linings and in coatings applications.25 26... 

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