BALTIC FORESTRY ISOLATION OF SUBERINIC ACIDS FROM EXTRACTED OUTER BIRCH BARK /.../ J. RIÞIKOVS ET AL. Isolation of Suberinic Acids from Extracted Outer Birch Bark Depending on the Application Purposes JÂNIS RIÞIKOVS*, JÂNIS ZANDERSONS, AIGARS PAÞE, AUSMA TARDENAKA AND BAIBA SPINCE Latvian State Institute of Wood Chemistry, Dzerbenes 27, LV-1006 Riga, Latvia. [email protected], Tel. +371 67551552 Riþikovs, J., Zandersons, J., Paþe, A., Tardenaka, A. and Spince, B. 2014. Isolation of Suberinic Acids from Extracted Outer Birch Bark Depending on the Application Purposes. Baltic Forestry 20(1): 98105. Abstract The research applies to the woodworking industry and the utilisation of bark for obtaining of saturated higher fatty acids, containing reactive hydroxyl and epoxy groups, which are promising raw materials for the synthesis of high added value derivatives for cosmetics, emulsion production, the polymer industry, etc. Therefore, the application of the results of our investigation has synergy with material science. The aim of the investigation was to increase the utilization efficiency of birch bark and to inform the specialists of the forestry and wood industry about the wide scope of high added value products, which could be produced from this valuable raw material. The fractionation of suberinic acids by sedimentation in certain ranges of the alkaline-alcohol solution pH 4.7-6.0 made it possible to obtain suberinic acids with the content of 9,10-epoxy-18-hydroxy octadecanoic acids up to 58% oven dry suberinic acids fraction. The mixture of suberinic acids obtained by alkaline-water depolymerisation acts as an excellent binding agent to produce medium and high density formaldehyde free particle boards with low thickness swelling and high bending strength. The biobased rigid polyurethane foams obtained from suberinic acids polyols also demonstrate good mechanical properties. Key words: Birch bark, depolymerisation, suberinic acids, particle boards, polyurethane foams Introduction utes in roots and which is an important in pathogen resistance (Pollard et al. 2008). It is found mainly in the In Latvias plywood factories, along with veneer cell walls of normal and wounded external tissues of shorts, birch bark is burned up in boiler-houses for aerial and/or subterranean parts of many plants, main- steam and heat production. Silver birch (Betula pen- ly in the outer bark of higher plants and in tuber peri- dula Roth.) outer bark comprises up to 3.4% of the derms (Pinto et al. 2009). Figure 1 demonstrates that veneer blocks mass as reported by Jensen (1948). In there are two types of cork cells in outer birch bark: the factories for producing 1 m3 of plywood, on the narrow radial dimension cells, dark in colour due to the average, 2.7 m3 of veneer blocks are necessary, of which polyphenols in their lumena and very broad radial di- 28.6 kg is oven dry (o.d.) outer birch bark. If the ply- mension cells, appearing light in colour due to the dep- wood export was 246.1 thousand m3 in 2012, the total osition of white coloured triterpenes. annual potential of such a valuable readily accessible After extraction of triterpenes, mostly betulin and raw material, concentrated in one place, can reach 7,000 lupeol, which comprise about 30-35% of the o.d. out- t in Latvia. Besides triterpenes, about 40-50% of the er birch bark and about 40-50% of the suberin-con- outer bark mass is polyester suberin, formed by high- taining cork cell residue, are left over (Figure 1). The er saturated oxyfatty acids with a unique and rarely rest 15-20% of outer birch bark components are sug- naturally found location of functional groups in the ars, polyphenols, tannins, cellulose and lignin (Kis- molecule (Pinto et al. 2009). Simultaneously with pro- litsyn 1994, Pinto et al. 2009). ducing 1 m3 of plywood, it is possible to obtain also To obtain suberinic acids in the free form, hydro- about 12.8 kg of suberinic acids from outer birch bark. lytic depolymerisation of suberin in a basic medium It is known from the literature that, in the plant should be carried out, obtaining the corresponding material, suberin is aromatic-aliphatic cross-linked pol- salts. After acidifying, free acids can be isolated. Af- yester, in which suberinic acids are cross-linked with ter depolymerisation, the monomer composition of aromatic polyphenols (Gandini et al. 2006, Ferreira et al. suberin mostly consists of fatty acids, w-hydroxyfat- 2013). It is widespread in the plant kingdom, where it ty acids, a-, w-dicarboxylic acids, aliphatic alcohols plays a key role as a hydrophobic barrier that restricts and aromatic acids depending on the plant material. the paths available for the movement of water and sol- w-hydroxyfatty acids are generally the most abundant 2014, Vol. 20, No. 1 (38) ISSN 2025-9230 98 BALTIC FORESTRY ISOLATION OF SUBERINIC ACIDS FROM EXTRACTED OUTER BIRCH BARK /.../ J. RIÞIKOVS ET AL. A B Figure 1. Cross section of outer birch bark before (A) and after (B) extraction by SEM group of suberin monomers in the outer birch bark of functional polyesters, which could be also biodegrad- Betula pendula (Gandini et al. 2006), and the main com- able (Yang et al. 2010). pounds in the suberinic acid mixture are epoxy and In the Soviet Union patent SU382657 (Ôåäîðèùåâ hydroxyl groups containing derivatives. If more dras- è Êàëàéêîâ 1973), a method for chemical processing tic alkaline depolymerisation conditions (methanoly- of outer birch bark is shown, with simultaneous ob- sis) are used, alkanedioic and hydroxyalkanoic acids taining of betulin and suberinic acid salts, according (Pinto et al. 2009) are formed (Figure 2). to which outer birch bark is treated with an alkali al- cohol water solution, filtered and treated with direct steam for distillation of alcohol. As a result, a mixture is obtained, which contains betulin, sodium suberinate and lignin, which are then separated from each other. This method is laborious and does not allow obtain- ing high-quality betulin and sodium suberinate due to the complicated isolation from the obtained mixture. For this reason, the isolation of betulin and suberinic acid salts is carried out gradually, firstly isolating betulin, and then suberinic acid salts. Basic depolymerisation is commonly carried out in an organic polar solvents medium, most often us- ing aliphatic alcohols such as methanol, ethanol, iso- propanol, etc. (Cordeiro et al. 1997, Gandini et al. 2006, Figure 2. Main saturated fatty acids derivatives obtained Pinto et al. 2009, Krasutsky et al. 2004). The main rea- by birch bark alkaline depolymerisation: epoxy derivatives son is that, in the organic solvents medium, the suber- (A), alkanedioic acids (B), hydroxyalkanoic acids (C) (Pinto in depolymerisation is more complete, and more indi- et al. 2009) vidual acid salts are formed, acidifying which till pH 5-6, a great amount of suberinic acids, containing epoxy 18-hydroxy-9,10-epoxy-octadecanoic acid (HEPA), acids (mainly 9,10-epoxy-18-hydroxy octadecanoic which comprises 11.9-19.3% of the total suberinic ac- acid), are obtained. ids mass, can be obtained by alkaline-alcohol de- A method of similar trend for hydrolytic depolym- polymerisation of outer birch bark (Ekman 1983, Ek- erisation in basic water medium is recommended by the man and Eckerman 1985, Holloway 1983). HEPA is an World Intellectual Property Organization (WIPO) in the interesting raw material for organic synthesis, because patent application WO/2010/093220 (Iversen et al. it is encountered seldom in nature, and it is very dif- 2010). The advantage of the method is the water me- ficult to obtain it synthetically. These and related dium, but the non-indicated yield makes to doubt monomers are useful to design and synthesise unique whether it would be high. 2014, Vol. 20, No. 1 (38) ISSN 2025-9230 99 BALTIC FORESTRY ISOLATION OF SUBERINIC ACIDS FROM EXTRACTED OUTER BIRCH BARK /.../ J. RIÞIKOVS ET AL. There are several ways to use suberinic acids for Birch bark samples were ground to fine sawdust obtaining of high added value products. Some publi- and passed through different sieves to obtain a pow- cations deal with the potential of the suberinic acids der having a granulometry of 0.2 to 0.5 mm. Elemental in the polyurethane foam synthesis from suberinic analysis was performed on an Elementar Analysensys- acids obtained from the cork of Quercus suber (Cord- teme GmbH vario MACRO CHNS Element Analyser eiro et al. 1997, Cordeiro et al. 1999). Also promising an analyser for the determination of C, H, N and S could be the use of isolated suberinic acids as a binder in solid and liquid samples, using a thermal conduc- for the production of wood board materials to improve tivity detector. the water stability, wherein the effect of the pressing temperature on the physico-mechanical characteristics Depolymerisation of particle board material samples was tested (Sudak- Depolymerisation in alkaline-alcohol solution ova et al. 2008). According to the Krasutsky et al. 2004, the ex- During the present investigation, two main goals tracted outer birch bark was saponified with sodium were advanced. Firstly, we compared and evaluated hydroxide isopropanol solution for 1 h (outer bark - from the technological point of view two methods for isopropanol - sodium hydroxide mass ratio 1:9:0.33), isolation of suberinic acids depolymerisation in al- the solution was discharged, and the outer birch bark kaline-water and alkaline-alcohol media. Secondly, we was boiled with isopropanol for another 20 min. The tried to find another practical solution to the use of solutions were evaporated till a dry state, diluted with suberinic acids depending on the isolation method, and water, and the insoluble 9,10-epoxy-18-hydroxy-octa- to make this isolation procedure as simple as possi- decanoic acid salts were separated.