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bioplasticsMAGAZINE_1101

Basics Basics of Lignin

Basics Basics of Lignin Article contributed by Hans-Peter Fink Johannes Ganster Gunnar Engelmann Fraunhofer Institute for Applied Polymer Research Potsdam-Golm, Germany Lignin is one of the most frequently occurring natural polymers in the world and the main one with aromatic rings. Nature uses lignin as the glue to build its sophisticated strong and yet flexible composite structures found in tree trunks and grass stalks. The elongated wood cell walls, mainly consisting of strong cellulose fibrils and hemi celluloses, are glued together by lignin which contributes to the compression strength of the composite. Moreover, the rather hydrophobic lignin is known to protect the structure from adverse environmental influences such as fungal attack. Industrially, lignin figures mainly in the pulp and paper industry. There, however, processes are optimized for extracting cellulose, and lignin is basically used for generating heat for the pulping process. This situation is clearly unsatisfactory in a sustainable economy and serious attempts have been, and are being made, to utilize lignin for various alternative applications. Figure 1: Phenylpropane-based monomers of lignin, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol Figure 2: Proposed model structure of spruce lignin according to Freudenberg [2]. Structure of lignin Lignin is built up of the three phenylpropane derivatives: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (s. Fig. 1). These constituents are irregularly linked at various positions in the molecules, resulting in an extended network. The resulting linking patterns and the monomer ratios dominate the properties of natural lignin in general and depend on the lignin source. The main sources are softwood, hardwood and grasses [1]. Softwood lignins are almost exclusively made from coniferyl alcohol. Typical raw materials are cedar, cypress, fir, hemlock, larch, pine, redwood, spruce, and thuja; containing between 25 and 35 % of lignin. Hardwood lignins are dominated by mixtures of coniferyl and sinapyl alcohols in varying amounts. Sources are ash, aspen, beech, birch, elm, eucalyptus, hickory, maple, oak, poplar or walnut, with lignin contents of about 20 to 25%. The lignin composition of grasses is characterized by p- coumaryl- and coniferyl alcohols. The lignin content ranges between 15 and 20%. Further influences working on the lignin structure are the growing conditions of the plants, i.e. the climate, the place of growing and last but not least the part of the plants. The lignin structure of a crown is not the same as the lignin structure of the stock of the tree, for instance. During the industrial pulping processes the natural lignins, having huge molecular weights at the start, are degraded into smaller fragments - in many cases down to ranges of 3000 to 4000 Daltons. A proposed model structure for degraded spruce lignin is presented in Fig. 2. Obviously, lignin is a substance of high complexity and, moreover, high structural variability. 54 bioplastics MAGAZINE [01/11] Vol. 6

Basics However, lignin-based products with defined properties can only be made from lignins with reproducible characteristics (e.g. solubility, glass transition temperature) and, ideally, reproducible average composition and purity, hydroxyl number, and molecular weight. Therefore, structure characterization plays an important role in lignin product design. To elucidate the composition, spectroscopic methods can be advantageously applied. As an example, solid state CP/MAS 13 C-NMR spectra of a softwood and a hardwood lignin are presented in Fig. 3. The differences are clearly visible, in particular in the range between 160 and 100 ppm chemical shift displaying the electronic environment of the ring carbons. In such a way, softwood and hardwood species can be differentiated. Lignin extraction Up to now lignin is commonly known for being a by-product of cellulose pulping processes which are run on a scale estimated to be 175 million tons of pulp per year worldwide. To separate and isolate cellulose from wood as the main product, different pulping processes were developed over the past almost 150 years. The two classical pulping processes, sulphite and sulphate (Kraft) pulping, work with H 2 SO 3 and Na 2 S, respectively, lignin ending up in the so-called brown and black liquors, respectively. Lignins produced in sulphite pulping are known as lignosulphonates which are water soluble and typically contain 5 – 9 % sulphur, while in the sulphate process water insoluble Kraft lignins with 2 – 3 % sulphur are formed. The percentages of the classical pulping processes of industrial relevance are given in Figure 4 [3] showing the overwhelming dominance of the Kraft process. Figure 3: 13 C-NMR spectra of a hardwood and softwood lignin sample While lignosulphonates are marketed for various applications (see below), Kraft black liquor is locally combusted in the pulp mill’s recovery plant to produce heat for the process and for sale. An increase in pulp production in connection with optimized processes can bring the capacity of the recovery plant to its limit and alternative lignin uses can become of interest. First, however, lignin must be isolated from the black liquor. For this purpose, the so called LignoBoost process, now owned by Metso [4], was developed in the last decade and uses pressurized CO 2 for lignin precipitation. The demonstration plant in Bäckhammar, Sweden, is run by Innventia and has a capacity of 8,000 tons per year [5]. 89% sulphate pulping sulphite pulping others Sulphur-free lignins are of interest for applications in the materials sector. Several methods have been developed [6]. A classification can be made with respect to the liquid medium used in the pulping processes. The Soda-Anthrachinon procedure works with aqueous sodium hydroxide solution and uses Anthrachinon to stabilise the cellulose during pulping. Alcell processes use only organic solvents such as methanol or ethanol. The Organocell procedure was developed as a Figure 4: Global pulp production by category [3]. 5% 6% bioplastics MAGAZINE [01/11] Vol. 6 55

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