Lignin and its Properties

Glossary of Lignin Nomenclature

What is lignin? Even lignin chemists have trouble agreeing to an answer. There is general agreement, however, that the term lignin refers to a group of phenolic polymers that confer strength and rigidity to the woody cell wall of plants. These compounds make up the second most abundant class of chemicals found in wood. Cellulose is first. A small section of a lignin polymer is presented below illustrating some typical chemical linkages seen in Lignin.

The term lignin defines only the polymeric material located in the woody cell wall. Various processes can be used to remove and isolate lignin from these cells. Each process, however, produces material of different composition and properties. Because of these differences, unique names are applied to each lignin preparation and should be used when describing these materials. There are four important factors to take into account when working with lignin:

• Source (woody plant) of the lignin.

• Method used to remove lignin from the plant.

• Method(s) used to purify the lignin.

• Nature of the chemical modification of the lignin after isolation.

These factors influence the properties of the lignin preparations and will affect how they function. Important properties of these formulations include:

• Chemical composition (%)

  • Modification of Lignin structure

  • Carbohydrate degradation products

  • Plant extractive materials

  • Inorganic materials

• Lignin modification

  • Molecular weight

  • Functional groups

  • Molecular Linkages

This newsletter provides a short glossary of various lignin preparations. It is separated into two parts. The first, deals with very pure analytical lignin materials. Procedures used limit these lignin modifications to structural studies. The second part describes highly modified lignin materials produced using various industrial procedures.

Analytical Lignin Preparations

Brauns Lignin

Is obtained by the solvent extraction of wood meal. Lignin yield in this procedure is very low (2-3%). Often referred to in early publications as Native Lignin.

Cellulolytic Enzyme Lignin

A Lignin preparation isolated by cellulolytic enzyme treatment of finely ground wood meal followed by solvent extraction. Although a tedious procedure, relatively high yields of lignin are obtained.

Dioxane Acidolysis Lignin

A Lignin preparation isolated by the treatment of woody material with dioxane/dilute HCl. Certain linkages are cleaved by this procedure thus altering the lignin structure.

Milled Wood Lignin

Is isolated by solvent extraction and purification of finely ground wood meal. Yield is ~ 25% of the total lignin. Of the lignin preparations available, this is thought to best represent lignin found in wood. Also known as Bjorkman Lignin.

Klason Lignin

Lignin preparation isolated through the strong acid degradation of woody materials. This procedure is used to quantify the amount of lignin in woody materials. The lignin structure is markedly changed by the drastic conditions used in this treatment.

Periodate Lignin

Is Lignin isolated through successive treatments of woody material with sodium periodate followed by boiling water. This removes carbohydrates leaving lignin. Some degradation of the lignin occurs with this method.

Industrial Lignin Preparations

These lignins are co-products from processes designed to break down woody materials into fibrous materials and/or chemicals. Often, these materials are burned to produce energy or disposed of through biological treatment systems. Certain of these products are marketed commercially. All of these lignin products contain various amounts of non-lignin chemicals, and most have been purified to some extent. Hydrolysis Lignin (Acid)

Is a water insoluble product produced by the strong acid hydrolysis of woody material to produce sugars for fermentation to alcohol. The resulting lignin is highly altered structurally and contains a large amount of sugar degradation products, wood extractives, and inorganic compounds. Much effort has been applied in developing useful products from this material particularly in the countries of the former USSR.

Kraft Lignin

The vast majority of chemical pulp produced worldwide is generated through kraft pulping. Lignin is made from woody material in reaction with NaOH and Na2S at temperatures of 155-175°C. Although almost all of the lignin generated from this process is burned to produce electricity and steam and to regenerate cooking chemicals, a small amount is isolated through acidification or ultrafiltration and sold. This water insoluble product is almost entirely lignin although highly modified. A large percentage of the kraft lignin market is as a sulfomethylated, water-soluble form.

Lignosulfonates: (From Acid Sulfite Pulping)

Woody material is reacted with sulfur dioxide and a metal bisulfite at pH 1-2 and a temperature between 125-145°C. The water-soluble lignosulfonates produced contain sulfonated lignin polymers, sugars, sugar acids, and small amounts of wood extractives and inorganic compounds. Approximately a million tons of lignosulfonates made by this process are sold each year, making it easily the largest source of commercial lignin products. A large percentage of these products have been modified through various purification procedures and/or chemical reactions.

Lignosulfonates are produced as sodium, calcium, magnesium or ammonium salts and can be converted to other salts through base-exchange procedures. Changing the base of a lignosulfonate product will often affect how it performs and have significant affect on its physical properties. The two most common forms, sodium and calcium salts, have many different governmental approvals including use in food and feed, as antiscaling agents, as boiler water additives and as binding agents for animal feeds. Specifications for identity and purity have been published in the U.S. Food Chemicals Codex for these two lignosulfonate salts.

The properties of lignosulfonates are also affected by whether they are produced from softwoods, hardwoods, or grasses. Hardwood and grass lignosulfonates for example, generally have lower molecular weight than softwood lignosulfonates. Additionally, the composition of non-lignin chemicals is significantly different between lignosulfonate products produced from these materials.

Lignosulfonates: (From Bisulfite Pulping)

Woody material is cooked at a pH of 3-5 at 150-175°C with a metal bisulfite salt. The water-soluble material produced contains 40-50% sulfonated lignin. The remaining material consists of sugar polymers, sugars, sugar acids, wood extractives and a large amount of inorganic compounds. Lignosulfonates from this process are currently not sold commercially.

Lignosulfonates (From the Neutral Sulfite Semi Chemical Process)

Woody material is treated with salts of bisulfite/sulfite at pH 6-9 prior to mechanical refining. The water soluble material that is produced (in very low yields) contains less than 50% sulfonated lignin and a variety of sugar degradation products, wood extractives and a large amount of inorganic compounds. A small amount of this material is sold commercially.

Lignosulfonates from Alkaline Sulfite - Anthraquinone Pulping

Woody material is reacted with sodium sulfite and a catalytic amount of anthraquinone at pH 9-13 and 160-180° C. The resulting water-soluble lignosulfonates contain less than 50% sulfonated lignin, a variable amount of wood extractives, and a large amount of both sugar acids and inorganic compounds. These are not currently commercial products.

Organosolv Lignin

Lignin products produced from a number of different organic solvent-based systems are referred to as organosolv lignins. The water insoluble products produced in these systems are usually purified and contain a very high percentage of lignin. Many different modified lignin products have been developed from organosolv lignins including water-soluble sulfonated materials. These lignin products are currently not marketed because of unfavorable economics associated with the production of pulp using these systems. The following are the four major organosolv processes:

Alcell: Ethanol/water pulping.
ASAM: Alkaline sulfite anthraquinone methanol pulping.
Organocell: Methanol pulping followed by methanol, NaOH, and anthraquinone pulping.
Acetosolv: Acetic acid/HCl pulping.

Steam Explosion Lignin

Woody material is separated into fibers through high temperature/high pressure treatment with steam. The lignin material that is obtained contains low level of carbohydrate and wood extractive impurities. The water insoluble lignin itself has somewhat reduced molecular weight caused by acid hydrolysis reactions. This type of system is often used along with enzymatic hydrolysis to produce sugars for fermentation.

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