Urushiol Processing

The recovery of the sap from the tree Rhus vemicifera to obtain urushiol has been described in the classical process (ref. 184, 185) and with regard to recent developments (ref. 186). It is a totally different operation from that with Anacardium occidentale, and not unlike that in the process for natural rubber. The name derives from kiurushi, the Japanese word for sap which is a primary product of the tree. Much less of this very ancient material is now processed in the original region of Sendai, Japan and the main industry is based in China with some in Korea and Japan. In northern Vietnam and Taiwan, a source of lac is Rhus succedanea and in Thailand and Burma, Melanorrhoea usitata. 

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A crude process of extraction of urushiol from the tree Rhus vernicifera was used by the Chinese during the Chou dynasty of 1122-249 BC, and the process was systematised by the Japanese. The tree is tapped at about the 10th year of cultivation by a lateral sloping incision into the bark during June to September. The sap is white to grayish in color, but on exposure to air turns yellow-brown and then black. The crude sap contains approximately 70% urushiol, 4% gum, 2% albuminous materials, and 24% water. It is stirred and filtered and heated to reduce the moisture level [138] and finally stored in air-tight containers. 

Kalergis, A.M., et al., Modulation of fatty acid oxidation alters contact hypersensitivity to urushiols Role of aliphatic chain B-oxidation in processing and activation of urushiols. Journal of Investigative Dermatology, 108, 57, 1997. 

Raw lacquer is called urushi. For our knowledge of the composition of urushi and the complex hardening process of the thin film layers, we now rely primarily on the recent work of Kumanotani and his coworkers (1-7). The sap of the Japanese lacquer tree is a latex containing 20-25% water, 65-70% urushic acid (urushiol), approximately 10% gummy sub-... 

Kalish RS, Wood JA, LaPorte A. Processing of urushiol (poison ivy) hapten by both endogenous and exogenous pathways for presentation to T cells in vitro. / Clin Invest. 1994 93 2039-2047. 

The phenolic lipids of Anacardieum occidentale have been commercially exploited (ref. 174) and those in Rhus vernicifera to a lesser extent. Most of the technical cashew nut shell liquid (CNSL) which results from industrial processing is and has been employed as a phenolic source for formaldehyde polymerisation the products from which in compounded form have been the basis for friction dusts widely used throughout the world in vehicle brake and clutch linings (ref.175). Urushiol has had use over many centuries in the art of Japanese lacquering (ref. 176) and in more recent years has been sometimes supplemented with CNSL. Chemical uses are referred to later. 

A number of applications of commercial lacs and of separated urushiol have been referred to (ref. 2). As with the phenolic lipids of Anacardium occidentale a great deal of work has been carried out particularly in Japan and China to diversify the uses of lacs from Rhus vemicifera. It is widely employed in artistic decoration, building materials, textile equipment and furniture. The industrial utilisation of polyketide natural products including the phenolic lipid urushiol has been reviewed (ref. 314). The great number of uses largely comprise polymerisation reactions and some non-polymeric processes, some of both of which are described in the next sections. 

Plant phenolics of biphenyl type but of different biogenetic origin to those of the aucuparins are constituents of the sap of the lac tree (Rhus vernicifera, Anacardiaceae). The sap is the raw material of the Japan lacquer. It dries into a tough and brilliant film and has been used in the Orient as a coating material for thousands of years (75) (see also Chap. 1.1). The major constituent of the sap is urushiol (24). The drying process is believed to be an enzymatic oxidative coupling of urushiol under the influence of oxidoreductases - e.g. laccase to form biphenyls (e.g. 25), dibenzofurans (e.g. 26), oligomers, and polymers (88). 

Fig. 10 shows the presence of grains in the sap film in the process of drying. These support the previous assumption that the sap film is composed of polymerized urushiol and plant gum domains. 

In order to elucidate the formation of aggregated cell structure made from plant gum and urushiol in Japanese lacquer films proposed above, it is most important to know the interaction between urushiol and plant gum in the sap being treated or in the lacquer in the film formation process. 

Plant gum, separated from a sap as an acetone insoluble part, showed an IR spectrum characteristic of the polysaccharides in a salt form. However, the aceone insoluble part of the lacquer showed an IR spectrum having characteristic peaks of the plant gum and urushiol. Furthermore, though sap is partly soluble in toluene, it becomes completely soluble in toluene when made into lacquer. These results may suggest that the water soluble gum becomes soluble or dispersible stable in the oil(urushiol)phase as a result of grafting of urushiol on the plant gum(polysaccharides) in the lacquer-making process. 

Polymerization of Urushlol 25 Since urushiol is a major component of the sap or lacquer, participation of urushiol, in the lacquermaking process from the sap or in the film formation process from the sap or lacquer is significant. 

The catechol nucleus of these dimeric urushiol undergoes enzymatic oxidation into the corresponding quinones following the same type C-C and C-0 coupling reaction with urushiol or with each other. Thus urushiol grows in its polymerization up to of 20,000-30,000 in the lacquer-making process from sap. 

So far, few modeling studies of urushi have been attempted, mainly due to difficulties in the chemical synthesis of the urushiol. New urushiol analogues have been developed by a convenient synthetic process for the preparation of... 

The urushi lacquer has been used for more than 5000 years in China " and it is known as a highly durable material. Polymerization of urushiol, the major component of the lacquer, involves laccase-catalyzed dimerization and aerobic oxidative polymerization, " and the drying process takes a very long time. Several studies on shortening of this time have been carried out UV curing " " and hybridizing with other reactive polymers or monomers. " " Cardanol has a similar structure to urushiol, and the enzymatic oxidative polymerization of cardanol were reported by three research groups. " The development of the polymerization process leads to artificial urushi . 

The oriental lacquer is prepared from the sap of the varnish tree" Rhus verni-ciflua, which is an emulsion of an aqueous phase and an organic phase called urushi-ol. The urushi fraction consists of a mixture of catechol derivatives substituted in 3-po-sition with aliphatic C- 5 or C- 7 side chains. About 60% of these side chains are trie-nes. The structure of the trienes is similar to that in linseed or tung oil and is essential for the effectiveness of the oxidative curing of oriental lacquer. We have utilized new techniques and used combinations of modern techniques to analyze the urushiol mixtures and to characterize and identify each individual compound. We have also developed ultraviolet stabilizers for oriental lacquer, stabilizers that could be incorporated into the polymerizing mixture during the curing process to result in ultraviolet stabilized oriental lacquer. 

To prepare the lacquer sap for the actual coating material, the sap is stirred in an open vessel at room temperature for about 1.5 hours, and then at a temperature increasing from 20 to 45 C where it is kept for 2 to 4 hours until the water content is reduced to 2-4% ("sugurome" process). The temperature cycle must be carefully controlled to retain the activity of the enzymes. At this point the sap has become clear, has changed in color and has increased in viscosity. This liquid, known as "raw lacquer", consists of urushiol and oligo-urushiol, small amounts of water and other components and is ready for application. Urushiol is cured by oxidation of the... 

In addition, especially in the processed urushi samples, peaks with masses [M]K" =665 daltons and [M]K" =667 daltons which are the expected products of the dimerization reaction by oxidation of the phenol part of the catechol derivatives of urushiol. Supercritical fluid chromatography in combination with electron bombardment mass spectrometry was then used for the identification of some of the major components of the urushiol samples. Most prominently were found the trienes with [M]=314 daltons, and [M]=317 daltons and the monoenes. Again the compound with the mass of [M]=332 daltons was noticed which was the water adduct of the triene [M]=314 daltons. 

A most careful study was now carried out by NMR spectroscopy, not only by regular Ih 500 MHz and 13C but also by 1H COSY NMR spectroscopy [4,5). Individual compounds were isolated by preparative HPLC of the mixture in urushiol, particularly of kuro-urushi and all the isomers, including the cis-trans isomers were identified and characterized [Figure 4] [5). Particular care was taken to identify the water addition product [M]K" =371 daltons. It was found that water had indeed been added to the triene to form a C10 hydroxylated diene [Formula] [5]. This water addition is catalyzed by some additives such as ferric chloride which are used in the sugurome process to optimize the lacquer properties. The details of the process is a closely guarded secret and is slightly different from manufacturer to manufacturer. 

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