372 9th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING – 24-26 April 2014, Tallinn, Estonia LIGNIN AND OUTER CELL WALL REMOVAL FROM ASPEN PULP FIBRES BY USING SUPERCRITICAL CO 2 EXTRACTION Kärner, K.; Elomaa, M. & Kallavus, U. Abstract: Supercritical carbon dioxide extraction (scCO 2 ) is chosen to a treatment to remove lignin and to peel off outer cell wall layers to expose S2 layer of aspen wood fibres. The aim is to find an effective and environmentally friendly method to advance the fibrillation of the BCTMP pulp. The effects of the treatment are analysed by using scanning electron microscopy (SEM). The chemicals used together with scCO 2 extraction include 1:1 ethanol: water co-solvent, isopropyl alcohol, and dimethyl sulfoxide (DMSO) with urea. The results show that supercritical CO 2 extraction helps to peel the fibres of mechanical pulp. Best results are gained with 1:1 ethanol: water co- solvent. Key words: Cellulose; Lignin; Super- critical CO 2 ; Aspen; BCTMP, Microfibrils, 1. INTRODUCTION Cellulose is one of the most abundant biopolymers on earth; it has attractive characteristics of being both recyclable and renewable. It consists of microfibrils that are bundles of elementary crystallites bridged by amorphous phases. Because a complete purification of cellulose demands a lot of energy and chemicals, there are many products in which cellulose is used only partly purified. For example, products like bleached chemi-thermo mechanical pulp (BCTMP) have been developed to be used as a component in paper making. A characteristic property of mechanical pulp is that it retains most of the wood lignin. Fig.1. Wood cell wall layers The structure of fibre consists of several different layers; three main layers are the middle lamella, the primary wall and the secondary wall. Secondary wall is divided into sub layers: S1, S2 and S3. Different cell wall layers can be identified by the orientation of cellulose microfibrils; it is specific to each layer, see Figure 1. Lignin and cellulose contents are also different in different layers; the lignin content is highest in the middle lamella and the primary wall, whereas cellulose content is highest in the S2 layer. Therefore, in order to get pure cellulose from wood fibre, it is necessary to remove outer cell wall layers. The proof of successful treatment is a clearly visible S2 layers with parallel orientation of cellulose microfibrils. Lignin that is present on the surface of BCTMP forms a physical barrier against inter fibre bonding, moreover, it hinders the inter-fibre hydrogen bonding due to the hydrophobic nature of lignin [ 1 ]. Therefore, it is essential to remove the lignin-rich material in order to fibrillate fibres and to produce e.g. nanocellulose. The