Lignin
Biopolymer copolymer
Off-white solid
Fixed
Lignins (Latin lignum "wood" ) are a group of phenolic macromolecules that are composed of different monomer units. There are solid biopolymers that can be stored in the plant cell wall lignification of the cell and thereby cause ( lignification ). Approximately 20 % to 30 % of the dry mass of woody plants consist of lignins, so that they are next to the cellulose and chitin, the most common organic compound on earth. The total production of lignins is estimated at about 20 billion tonnes per year.
Because lignins are essential to the strength of plant tissues, the evolution of terrestrial plants and especially the trees is very closely linked to the formation of lignin. Only with lignin plants can form consolidating elements which ensure the stability of larger plant body out of the water - the water of buoyancy for the necessary stability.
- 3.1 Biosynthesis of Ligninvorstufen
- 3.2 Ligninsynthese
- 3.3 Genetic modification of lignin biosynthesis
- 4.1 Biodegradation
- 4.2 Technical degradation 4.2.1 pulp production in sulfate and sulfite
- 4.2.2 saccharification of wood
- 4.2.3 lignin for the biorefinery
- 5.1 lignin in the energy use
- 5.2 Use of lignosulfonates
- 5.3 lignin as a Biomaterial
- 5.4 lignin in the chemical industry and bio-refinery
Function
Lignin has as support material and hardened polymer has a number of important tasks for the plant. Lignins are essential to the strength of plant tissues, where they are mainly for the compressive strength of central importance, while the embedded cellulose fibers provide the tensile strength. So there is a penetration of tear-resistant, flexible fibers ( cellulose) with a dense and rigid polymer as a filler material ( lignin). As analogies and technical materials such as reinforced concrete or natural fiber reinforced plastics are constructed.
Plants without lignin can through the cellulose Although withstand considerable tensile forces to pressure the other hand, they are sensitive. Without lignin no strength members can be trained to ensure in the absence of buoyancy through the water, the stability of larger plant body and build appropriate support structures against the pressure effect by the gravitational force. Also, the formation of branches and Astsystemen for creating great photosynthetically effective surfaces can only be performed by a stabilization of the branches.
In addition, lignin serves as a cementing material for the cell assembly of the middle lamella. It provides protection against ingress of water into the cell wall material and holds it thus in the Leitgefäßen ( xylem and phloem ) and inside the cells. Further protection is against UV light as well as mechanical damage and the ingress of pests. Finally, lignin can be difficult to distinguish from bacteria or fungi are reduced and consequently inhibits the growth of pathogenic micro-organisms passively and actively by building Wundlignin in areas of mechanical damage. A similar structure with a similar structure represents the suberin is found mainly in the cell walls of Phellems ( cork).
The evolution of terrestrial plants and especially the trees is very closely linked to the lignin biosynthesis. The lignin is accordingly only to be found as real lignin with the appearance of these plants, while only the blocks or lignin -like polymers are present in more primitive plants like green algae. The current assumption is that lignin represents a new development and thus a gruppenbegründendes feature ( autapomorphy ) of vascular plants. Probably it could first establish as a defense substance against fungal infections in the form of Wundlignin and took on this basis a central role as a stabilizing material. However, 2009 could also be detected in the red algae species Calliarthron cheilosporioides lignin. This raises the question of whether it is caused either convergent both in the higher plants as well as in the red algae, or perhaps already appeared early in the evolution of eukaryotes and has disappeared in other organisms again.
Properties
Lignin is very firm to brittle and light to dark brown in color. It is optically isotropic, UV light is almost completely absorbed by the material, the visible light to some extent.
However, lignin is not a single substance, but a group of phenolic macromolecules that are composed of different monomer units. Here, a densely crosslinked, amorphous mass is built up by combining similar basic molecules. The structure has, in comparison to polysaccharides substantially less polar groups which are hydrophobic and therefore lignin insoluble in water and many other solvents. For this reason, they are biologically and chemically degradable heavier than other natural substances.
The structure and composition of the lignin
Lignins are three-dimensional and amorphous networks (polymers) of aromatic building blocks, which are in various forms linked to each other. In addition to aromatic bonds they contain many more carbon -carbon single and double bonds, also many phenolic groups are present.
These are high molecular weight ( relative molecular mass of approximately 5000-10000 ) derivatives of phenylpropanoids as substituents of the benzene ring next to a propane chain one OH or hydroxyl group, one or two OCH3 - or methoxy - as well as various residual chains ( alkoxy or aryloxy groups). Since the macromolecules grow, however, in all spatial directions, especially the middle lamella enable a strong expansion, and are also linked secondarily to each other, corresponding to the Ligninmasse in a full blown tree last probably a single lignin polymer molecule whose mass then amounts to several tons.
Depending on the type of wood it is made up of structures, which on the basis of p- coumaryl alcohol blocks, coniferyl alcohol and sinapyl alcohol ( monolignols ) are due (see biosynthesis ). Since the lignin produced in a free-radical process in which the radical formation is enzymatic, but not their further reaction, the composition and proportions of the individual components are highly variable; a directed link for an ever same schema does not exist. In addition to the variability of each lignin molecule itself also distinguishes the different lignin wood or plant species by the percentages of alcohols and the derived phenyl radicals: Softwood lignin contains predominantly coniferyl units ( about 90 % ) that a Guajacylrest ( 3 - methoxy- 4-hydroxy- phenyl ) have and will therefore be referred to as G - lignin. Hardwood lignin contains varying proportions of Guajacylresten and Sinapyl elements that contain a Syringylrest (3,5- methoxy -4-hydroxy - phenyl residue ). The Syringylanteil may be 5-65 %, the resulting lignin are referred to as GS- lignin. Lignin semiwoody grasses and other monocot is characterized by a high proportion of about 15 to 35% Cumaryl elements that form the para -hydroxy- phenylpropane and together with a Syringylanteil the same height and a Guajacylanteil from 50 to 70 % form the HGS lignins. Add small amounts of cinnamic acids and cinnamaldehydes also be ( the starting materials of the base alcohols) integrated into the matrix.
In analytical detection reactions for several lignin are also known which are based on the structure of the fabric. To determine the presence of lignin leads to a red color by hydrochloric acid phloroglucin. This reaction is due to the embedded in the Ligninmatrix cinnamaldehydes. Due to the different composition of the lignins in softwoods and hardwoods can be achieved by different coloration, which is achieved with this evidence, to distinguish between the two types of wood. Conifers stain this cherry red, red-violet hardwoods. In addition, a yellow coloration with aniline / sulfuric acid and a violet coloration with Schiff reagent is possible. For qualitative analysis, especially gas chromatography is employed. For the determination of the lignin fraction the Klason method for use, in which the polysaccharides decomposed by a two-step acid hydrolysis and the remaining Ligninrest is then weighed ( Klasen lignin ). For GS lignins UV spectroscopy of the acid solution is then necessary since this acid-soluble fractions of the lignin contains.
Lignin
The adjacent table shows the amounts of lignin, cellulose and hemicellulose at different biomass raw materials dar. This is especially relevant to commercial wood and lignocellulosic residues.
Lignin as cell wall reinforcement
Plant cell walls consist of cellulose fibrils that are embedded in a matrix of pectins, hemicelluloses, proteins and lignin. Here, the cellulose molecules from each about 100 individual molecules lie parallel together to form so-called elementary fibrils or Micellarsträngen, which are stabilized by hydrogen bonds. Each of these 20 Micellarstränge together form a microfibril with a diameter of about 20 to 30 nm microfibrils in turn can combine to form macrofibrils with a diameter of about 400 nm, thereby arise gaps of about 10 nm, which remain as interfibrillar spaces. The interfibrillar spaces serve, among others, the water transport in the cell wall, as well as larger molecules such as hemicelluloses, pectins and lignin are incorporated to strengthen the cellulose structure ( incrustation ) in these spaces.
In most plant tissues, the lignin is only about 1% in lignified plant parts due to pressure stresses can be more than 30% of the total mass; we speak in these cases of lignocellulose. In addition to lignin, various mineral substances for the incrustation may be responsible, including some silicates of grasses, sedges and horsetails or calcium carbonate in calcareous algae.
In a cell wall lignification, the original matrix is replaced by the lignin polymer with embedded cellulose fibers. The cellulose fibers are packed so tightly into the polymer, that they can no longer move against each other and also lose their swelling capacity. A special form of lignification takes place in the so-called reaction wood: areas that are exposed to particularly high loads, continuously strengthen. However, this reaction falls out differently, with horizontally growing branches of conifers occur for example due to the pressure load to a reinforced lignification of Astunterseite by ligninreiches compression wood. Deciduous trees on the other hand increase at the same stress especially the Astoberseite with cellulose- rich tension wood without Liginanteile.
Xylemverholzung
Druckbeanpruchung arises not only in structural elements, but also in the areas of the plant that have to withstand a high internal pressure. This is the case especially in the channels for water transport in the stem and in the roots, as here, the water is transported against gravity and exerts pressure on the surrounding tissue. Accordingly, also form Verholzungen that lead to cell-wall tubes with high lignin content. The water-repellent (hydrophobic ) character of this function is an essential property because it prevents leakage of water from the channels into the surrounding tissue and thus allows the transport of water over long distances.
These water- conducting elements of the xylem, which are distinguished in tracheae and tracheids due to their size and structure, can be counted together with the xylemverstärkenden sclerenchyma to the main load-bearing structures in land plants.
Biosynthesis
Biosynthesis of Ligninvorstufen
The lignin is a descendant of the phenylpropanoids, in turn, derived itself from L -phenylalanine. By elimination of ammonia by a phenylalanine ammonia-lyase (PAL ) (EC 4.3.1.5 ) produced from the phenylalanine a cinnamic acid. This is converted by other enzymes to p- Coumaryl -coenzyme A. This compound is the starting material for further modifications, such as hydroxylation on the aromatic ring and subsequent methylation. In the last step, the coenzyme A bound to intermediates are reduced by a zinc-containing Cinnamalkohol dehydrogenase (CAD ) (EC 1.1.1.195 ) to the monolignols, always NADPH is used as a reducing agent. This leads to the biosynthesis of lignin alcohols are p- coumaryl alcohol (H- unit ), coniferyl alcohol (G- Unit) and sinapyl alcohol ( S unit ).
The composition of the lignins depends mainly on the proportions of the different monolignols and the reaction conditions. In angiosperms, lignin particular Sinapyl and coniferyl alcohol is constructed in Nacktsamigen plants and grasses dominated coniferyl alcohol use all three monolignols. A key function is played CAD to, which is probably responsible for the different proportions of the alcohols in the different plant groups by their different substrate specificity: CAD from angiosperms and grasses reduces all three cinnamaldehydes while for the CAD of Nacktsamigen plants Sinapylaldehyd only a poor substrate represents and is less strongly implemented accordingly.
Ligninsynthese
1848 could produce Karl Freudenberg and employees an artificial lignin ( Dehydrisierungspolymerisat ) from coniferyl alcohol and an extract of Agaricus campesterol (field mushroom ). A later isolated from spruce wood lignin showed similar chemical and physical properties such as the artificial lignin. In this respect, was indirectly demonstrated that coniferyl alcohol plays an essential building block in the Fichtenligninbildung. Further in vivo studies with radiolabeled Coniferylakolohol or coniferin confirmed that these modules play an essential role in the biogenesis of lignin. With these studies, a breakthrough in Ligninforschung was achieved.
Due to the composition of the individual modules and the various Polymerisierungsmöglichkeiten lignins can have different structures and form accordingly an entire class of compounds. They are built only in the interfibrillar spaces of serving as precursors alcohols. But the monolignols are exported from the cell, is not fully understood. Probably these are called glucosides - Glucocumarylalkohol, coniferin and Syringin - transported to the outside. In this case, the alcohols are bound via their phenolic OH group β - glycoside of sugar ( glucose), and in this form are more soluble in water. Thus, the molecules can be transported through the plasma membrane of the cell and the apoplast, and infiltrate into the interstices of the cellulose. Finally, be cleaved by β - glycosidases of the cell wall, the sugar molecules. Such β - glycosidases have been identified in some plants. Whether the monolignols passively diffuse through the cell wall or passing through the transport system to the outside, is still the subject of research.
The exported monolignols are then linked via an enzymatic oxidation polymerization reaction spontaneously to an amorphous three-dimensional structure. Here, the Lignifikation at corners and the middle lamella of xylem cells begins. Catalyze the polymerization extracellular peroxidases or laccases of hydrogen peroxide from oxygen, thereby phenoxy radicals are formed. Where, however, comes the hydrogen peroxide, is still unclear. The single electron is delocalized and stabilized over the entire molecule. This allows for the formation of various Knüpfungspunkte reticulated lignin. Lignin contains chiral centers, with conventional methods, however, no optical activity could be detected.
If the cross-linking can be controlled, is still the subject of research. You may be able extracellular glycoproteins conductors proteins that cause a certain specificity in networking.
The Lignineinlagerung proceeds in three phases. In the first phase, supporting the macromolecule in the cell corners and the middle lamella, after the Pektineinlagerung is completed in the primary wall. This is followed by a progressive lignification of the S2 layer of the secondary cell wall. The Hauptlignifizierung occurs after formation of the cellulose microfibrils in the S3 layer. Within the three phases, and thus varies in the different layers, the composition of lignin.
Genetic modification of lignin biosynthesis
Since the removal of lignin from the wood for pulp production and in perspective, especially for the production of biofuels ( cellulosic ethanol ) is one of the most complex stages of production, there are several efforts to reduce the amount of lignin in the wood already by green genetic engineering. This is done primarily by interfering with the genes necessary for the synthesis of monomers, including by " turning off " the Cinnamalkohol dehydrogenase ( CAD) and caffeic acid O-methyltransferase ( COMT) by antisense RNA.
The corresponding techniques are currently mainly on poplars and willows still in the research for growing short rotation plantations and had not been realized for the technical implementation, but a more effective delignification in the pulp process has already been demonstrated. However, it was also found that the effect of Ligninreduktion is not uniform, and environmental factors probably have a greater impact on the Ligninproduktion than the genetic modification.
Lignin
Lignin can be both biodegraded as well as by various chemical engineering processes. The biological degradation of lignin represents a process of wood degradation by bacteria and especially fungi ( decomposers ) dar. higher organized beings are not capable of lignin degradation. The technical lignin decomposition, however, is to separate part of processes with the aim of lignin and cellulose in the wood and to send different utilizations. He plays accordingly especially in pulp production, wood saccharification and perspective in the use of lignocellulosic biorefinery in a major role. The conventional thermochemical methods of technical lignin degradation are very energy-intensive, polluting the environment and produce toxins.
Is processed and bare wood exposed for a long period of time ultraviolet radiation, so it is damaged the surface, whereby especially the lignin is denatured. In the case of direct weathering it is subsequently washed out by rain water. The surface then acts dirty gray. Failing the action of rain water, the wood gets as a result of the UV effect a silvery- white color.
Biodegradation
Lignin is a persistent natural products through its complex network and can be decomposed only very slowly by decomposers. The humus of the soil is thereby largely promoted by lignin degradation. The wood decomposition takes place in two partially parallel running fractions: The degradation of the cellulose in the form of brown rot in the wood as a result of the residual lignin turns brown, while the degradation of lignin takes place in the form of white rot, in accordance with the wood brightly colored.
During the biological degradation of lignin, a distinction between the recycling already dissolved lignin fragments and the actual degradation of the natural product. The former can already be utilized by many bacteria, especially actinomycetes and streptomycetes. White rot fungi such as the tinder fungus ( Fomes fomentarius ), the gray fire sponge ( Phellinus igniarius ), the Turkey tail ( Trametes versicolor) and Phanerochaete chrysosporium, however enzymatically destroy the lignin content of the wood, in order to realize their true substrate, cellulose and hemicelluloses. Accordingly, the wood turns white in the white rot and fibrous. Most of these fungi while building the lignin and carbohydrates from ( Simultanfäule ), the degradation rates are also similar high. Other fungi build the lignin initially faster, and it comes to a cellulose enrichment ( Successive white rot ). This can be found for example in the mosaic layer mushroom ( Xylobolus frustulatus ) or the root fungus ( Heterobasidion annosum ), which caused the red rot in spruce.
The lignin degradation takes place here always under aerobic conditions and is very energy intensive. He can not serve as the sole carbon and energy source accordingly. Therefore, it is always in white rot fungi by a cometabolism in conjunction with other carbon sources. For the degradation of the fungi form threadlike hyphae that penetrate the lignin. For the lignin different enzymes are used, which are emitted by the fungus by exocytosis into the medium and diffuse into the lignin. The degradation of lignin is de facto a depolymerization and peroxidases and laccases requires that behave synergistically in their effect. In addition, oxygen, coenzymes, metals and complexing agents are needed.
The mushrooms sit initially free glyoxal is oxidized by oxygen through a glyoxal oxidase to oxalic acid and hydrogen peroxide ( H2O2). H2O2 is then of a manganese peroxidase ( MnP ) (EC 1.11.1.13 ) reduced to water, while manganese ( II) (Mn 2 ) is oxidized to Mn3 . Mn3 is chelated and penetrates as a small active oxidant easily into the lignin. Mn3 there can snatch the phenolic components of lignin single electrons, so that a radical cation is formed. This is split into multiple fragments, often in benzaldehyde derivatives.
The radical cation can also be formed by a lignin peroxidase ( LiP ) (EC 1.11.1.14 ). LiPs are heme -containing enzymes, the substituted aromatics, the main ingredient in the lignin, can directly oxidize. However, not all white-rot fungi encode LiPs. The peroxidase uses hydrogen peroxide as the oxidant.
Meanwhile, it has been discovered, so-called hybrid enzymes in Pleurotus, Bjerkandera and other fungi, known as "versatile peroxidases " (VP ) (EC 1.11.1.16 ). These have both manganese peroxidase and lignin peroxidase via an activity.
Laccases ( EC 1.10.3.2 ) eventually oxidize mainly low molecular weight fragments of the lignin. While you can attack generally phenolic components of lignin enzymatically. However, since those components account for only 10% of the lignin macromolecule is primarily utilized by peroxidases above.
When lignin many other enzymes ( oxidoreductases, dehydrogenases ) are also involved yet.
Technical breakdown
Pulp production in sulfate and sulfite
The technical lignin plays an important role, especially in pulp production. For the production of pulp, the lignin from the lignocellulose must be dissolved and removed from the process. Different methods for cellulose digestion as well as for the subsequent pulp bleaching exist.
In about 80 percent of all pulp mills the information on the so-called sulphate process takes place, also known as kraft process. In this case the lignin is carried out by hydrogen sulfide ions ( HS- ) in a basic medium at a pH of about 13 by the use of sodium sulfide ( Na2S ) and sodium hydroxide (NaOH) or sodium hydroxide. The process takes about two hours at temperatures of about 170 ° C, however, the ions attack the cellulose and hemicelluloses to, whereby only a part of information is possible. The liquor of this process contains in its solid substance in the use of softwoods and about 45% for hardwood about 38 % of the so-called kraft lignin.
An alternative is the cellulose digestion is the sulphite, wherein the lignin degradation is carried out by a sulfonation. As chemically not precisely defined reaction product of lignin with sulfur dioxide arising lignosulfonates, the salts of lignin sulfonic acid. Calcium salts of lignin sulfonic acid produced during digestion of the wood with Calciumhydrogensulfit solutions. Here, the liquor in their solid substance in the use of conifers contains about 55 % and about 42 % for hardwood in the form of lignin.
Lignin is also responsible for the yellowing of paper that can be bleached with so ligninabbauenden enzymes such as laccase. Bleaching is done technically but especially a chlorine bleach or nowadays mostly " chlorine-free " with a bleach with oxygen, chlorine dioxide, or, less commonly, hydrogen peroxide or ozone. In both cases, the residual lignin in the pulp and existing dyes are degraded by oxidation. This is relevant, less in " wood-free " paper, especially with so -called " mechanical pulp " paper. The terms containing wood and wood-free indeed are commercially available and colloquially common, but technically meaningless, since paper contains from wood in any case wood components. In wood-free paper, these are just only the cellulose and hemicelluloses, lignin in wood pulp paper the pulp. Correctly, therefore, the terms would ligninhaltig and lignin free.
Saccharification of wood
For the transformation of wood into usable sugars ( saccharification of wood ) a number of different processes are applied that remove the lignin from the wood and thus make the cellulose available. This involves a series of chemical, hydrothermal and enzymatic processes.
Historically important are mainly technical applications using acids, especially hydrochloric acid (HCl) or dilute sulfuric acid (H2SO4), in which the shredded wood is cooked. In the process, water molecules attach to the cellulose to form oligosaccharides, especially di-or trisaccharides, including glucose content. Due to the present in the wood next to the cellulose hemicellulose and lignin caused by-products or impurities which make the result almost exclusively for fermentation to alcohol or as a growing medium for the fermentation of yeast used. At times, so the wood spirits was generated. For use in the chemical industry, the solution must be laboriously cleaned and desalted.
Lignin for the biorefinery
In the context of the discussion about the development of the biorefinery to the saccharification of special enzymes, cellulases, held biotechnologically. As a result, one hopes as pure as possible fractions of cellulose for further saccharification of hemicelluloses and lignin in order to perform all three components of the wood to further utilization can.
To get the individual fractions as pure as possible and without prejudice from the wood, it requires a special treatment. This is done differently depending on the technical way and can be based for example on the treatment with solvents such as ethanol ( organosolv processes ) or ionic liquids, the use of enzymes or the steam treatment ( Aquasolv method).
Use
Lignin is - you can see the use in the form of wood from - especially as a by- product of the paper and pulp industry used. Currently about 50 million tons of lignin are produced globally each year in this way. The resulting amounts of kraft lignin and lignosulfonates lie in dissolved form in the waste liquors available and can be extracted from these. The main use for both lignin types currently consists of the energy use, other uses are mainly for lignosulfonates from the sulfite before.
Basically, the various technical lignins differ in several properties that can affect their use. The essential difference lies in the molecular size: Kraft lignin has a molecular mass of 2000 to 3000 g / mol, molar mass while achieving lignosulfonates 20000-50000 g / mol. Organosolv lignin is 1000 to 2000 g / mol. Lignosulfonates also contain a sulfur content of 4 % to 8 % and few phenolic hydroxyl groups (-OH), compared with 1 % to 1.5 % sulfur content and many phenolic hydroxyl groups in the Kraft lignin and many phenolic hydroxyl ions (OH - ) without sulfur content in the organosolv lignin.
Intensive study the properties of the modified by oxidative ammonolysis of lignin as humus substitute. The nitrogen-containing lignins are similar in structure to the humic substances and are useful as slow release fertilizers. The N- lignins are also suitable for the recultivation of post-mining landscapes.
The direct use of technical lignins as raw products is very limited, as there are a number of drawbacks that prevent this. Lignin is then very limited extent for applications due to its very complex structure and the related non-uniformity, as more precisely defined properties of the raw material are generally required. In addition, the high degree of impurities in the waste liquors and the high sulfur content of the lignin types, make complex purification steps required. The resulting very elaborate production from the waste liquors will cause unpurified technical lignin is used until now mainly for low range applications such as the production of energy or as a nonspecific adhesive component and dispersants. Material uses that go beyond, are either in the direct use of lignosulfonates or in the chemical modification by the use of pyrolysis, hydrolysis or hydrogenolysis to produce various chemicals. These trails are also complex and are accordingly only comparatively rarely.
Lignin in the energy use
The resulting in papermaking, especially in the sulfate process, in large quantities as waste material, lignin is used as a black liquor as a fuel particularly directly into the pulp mills. It has a calorific value of 23.4 MJ / kg and only serves to energy for the factories themselves with a coverage of 80 % to 100 % of energy needs also to optimizing profits from the sale of heat and electricity.
In the production of pellets as a source of energy, the wood 's lignin forms the binder. Finely ground wood is heated during the pressing process, the lignin liquefies and binds the wood particles together upon cooling. Fresh pellets therefore still smell strongly of lignin.
Use of lignosulfonates
Large amounts of lignin sulfonates are used in a wide range of applications in which one uses, especially their properties as a polyelectrolyte, its adsorption, the low viscosity and the dark color. They are physiologically and relatively harmless to the environment, so they are used in sensitive areas. The majority of the production of about 1,000,000 tons per year ( tpy ) is used as a dispersant in concrete and cement (about 100,000 tonnes per year ), as an additive to drilling fluids (about 100,000 tonnes per year ) as well as a binder into pellets for animal food, fertilizers in and other agricultural chemicals, particle board, briquettes and in printing ink and Gießsandkernen. In addition, lignosulfonates are used as a paper additive, as dispersing and emulsifying agents in paints and coatings as well as aggregate in plaster and tannins.
Recent Developments in the Chemistry lignosulfonate use the polyelectrolyte properties of lignin and are aimed at use in medicine, fine chemicals and the improvement of soil water storage.
Lignin as a Biomaterial
Lignin represents a natural substance a highly complex macromolecule (polymer ), a use of this structure as biomaterial offers itself accordingly. However, the kraft lignin from the sulphate papermaking process must first be cleaned, so there is up to now only few approaches to produce lignin- based polymers.
In 1998, a natural biomaterial was developed by the company Tecnaro, who received the name Arboform and is generally referred to as "liquid wood ". It is based on lignin, the natural fibers such as flax or hemp are mixed, and can be processed using established polymer processing forms, especially in injection molding, extrusion, in press method as well as by thermoforming and blow molding.
Both lignin and lignin derivatives can be used as various components in thermosetting plastics or as fillers. Here, they act as a phenolic resin component. By reaction with epichlorohydrin, epoxy resins may be prepared, resulting in a condensation with polyhydric alcohols in alkali lignin. With isocyanates can they be implemented to form polyurethanes. During the reaction of lignin with formaldehyde arise phenolic resins, and the crosslinking with the copolymer, such as urea, melamine and furan resins are formed various formaldehyde ( urea -formaldehyde resins, melamine resins, and furan resins and Syntactics ). In particular ligninbasierte phenolics represent a potential alternative to harmful to health phenols and formaldehyde is as a binder in particleboard and other wood composites; by their high molecular structure, they are less volatile and soluble, in addition they are classified as non-toxic.
Lignin in the chemical industry and bio-refinery
Although today lignin plays no major role for the production of chemicals, the raw material for the future is predicted great potential. Especially in recent years, the research on potential uses of the lignin from the pulp industry and the (still hypothetical ) concentrated biorefinery. The aim of the research is to gain the highest possible quality products from the lignin.
Already lignin is used for the preparation of vanillin, which is used as a nature identical flavor of vanilla. This results in the oxidation of lignin sulfonates, which in turn is obtained by the acid hydrolysis of lignin. About an alkali melt can be produced from lignin various phenols, carboxylic acids, tar and dimethyl sulfide (DMS). The production of DMS is also possible on an alkaline demethylation and can be further oxidized to dimethyl sulfoxide (DMSO ), an important solvent. By hydrogenolysis, in turn, also phenols, tar, benzene and oils can be prepared.
An important option for future use of the lignin also is the pyrolysis, to a process for thermal cracking of organic compounds at high temperatures. As can be obtained by pyrolysis at temperatures of 400 to 500 ° C. Phenols, methane, carbon monoxide and carbon. At temperatures of 700 to 1000 ° C, lignin allows syngas, ethene and benzene columns, and an arc pyrolysis of acetylene is produced.