Lignosulfonates

Lignosulfonates, or sulfonated lignin, (CAS number 8062-15-5) are water-soluble anionic polyelectrolyte polymers: they are byproducts from the production of wood pulp using sulfite pulping.[1]

Most delignification in sulfite pulping involves acidic cleavage of ether bonds, which connect many of the constituents of lignin.[2] The electrophilic carbocations produced during ether cleavage react with bisulfite ions (HSO3

-) to give sulfonates.

R-O-R' + H+ → R+ + R'OH R+ + HSO3

- → R-SO3H

The primary site for ether cleavage is the α-carbon (carbon atom attached to the aromatic ring) of the propyl (linear three carbon) side chain. The following structures do not specify the structure since lignin and its derivatives are complex mixtures: the purpose is to give a general idea of the structure of lignosulfonates. The groups labeled "Q" can be a wide variety of groups found in the structure of lignin. Sulfonation occurs on the side chains, not on the aromatic rings, like in p-toluenesulfonic acid.

Lignosulfonate have very broad ranges of molecular mass (they are very polydisperse). A range of from 1000–140,000 da has been reported for softwood lignosulfonates with lower values reported for hardwoods.[1]

Preparation

Lignosulfonates are recovered from the spent pulping liquids (red or brown liquor) from sulfite pulping. The most widely used industrial process is the Howard process, in which 90–95% yields of calcium lignosulfonates (CAS number 904-76-3), are precipitated by adding of excess calcium hydroxide. Ultrafiltration and ion-exchange can also be used to separate lignosulfonates from the spent pulping liquid.[1] A list of CAS numbers for the various metal salts of lignosulfonate is available.[3]

Uses

Lignosulfonates have a wide variety of applications.[4]

The single largest use for lignosulfonates is as plasticizers in making concrete,[1] where they allow concrete to be made with less water (giving stronger concrete) while maintaining the ability of the concrete to flow. Lignosulfonates are also used during the production of cement, where they act as grinding aids in the cement mill and as a rawmix slurry deflocculant (that reduces the viscosity of the slurry).

Lignosulfonates are also used for the production of plasterboard to reduce the amount of water to, make the stucco flow, and form the layer between to sheets of paper. This allows lower kiln temperatures to dry the plasterboard and to save energy.

The ability of lignosulfonates to reduce the viscosity of mineral slurries is used to advantage in oil drilling mud, where it replaced tannic acids from quebracho (a tropical tree).

Lignosulfonates are used to disperse pesticides, dyes,[5] carbon black,[6] and other insoluble solids and liquids into water. They are used in tanning leather. They are also used to suppress dust on unpaved roads.[7]

Oxidation of lignosulfonates from softwood trees produced vanillin (artificial vanilla flavor), but this is not a current use.[8]

Dimethyl sulfide and dimethyl sulfoxide (an important organic solvent) are produced from lignosulfonates. The first step involves heating lignosulfonates with sulfides or elemental sulfur to produce dimethyl sulfide. The methyl groups come from methyl ethers present in the lignin. Oxidation of dimethyl sulfide with nitrogen dioxide produces dimethyl sulfoxide (DMSO). [1]

Dyes

Lignins are primarily derived from trees--a renewable forest resource. Lignin is the tree's natural binding agent and is released during the production of pulp. Lignosulfonates, or sulfite lignins, are hydrophilic and are products of sulfite pulping. Kraft or sulfate lignins are hydrophobic and are obtained from the kraft pulping process.

Since the early 1900's, the textile dyeing industry has known the dispersant properties of lignin. Since many dyeing processes involve the application of insoluble

dyes in a dispersed form, lignsulfonates are needed to keep the dye particles in a finely divided state by preventing agglomeration. These are usually disperse dyes which are applied to polyester and acrylic fibers or vat dyes which are particularly suited for use on cotton and rayon.

Early requirements for lignin were not stringent and only simple modifications of the spent sulfite liquors were needed to manufacture acceptable dispersants. The advent of modern technology brought the development of synthetic fibers and dyes which required high temperatures during high speed dyeing operations. Various lignins have evolved and have been modified for high performance. Today not only do these products provide a strong dispersing effect but also good solubility and high temperature stability of the dispersion throughout the dyeing process.

The economical significance of dispersing agents to the textile industry is clear when we consider that 20-60% of every disperse and vat dye consists of the dispersing agent. Worldwide, 25-50 thousand tons of lignin dispersants are used each year by the dye industry.

Lignin contains two functional groups, adsorbant and hydrophilic. Each group performs a different function. The adsorbent groups attach to the dye crystals during milling operations and the hydrophilic groups interact with water molecules to form a double layer that prevents dye particles from reaglomerating

Lignins- Products With Many Uses

 The usefulness of commercial lignosulfonates products comes from their dispersing, binding, complexing and emulsifying properties. Industry

Lignins are derived from an abundant and renew

first began to use lignins in the 1880s when lignosulfonates were used in leather tanning and dye baths. Since then, lignosulfonates have even found applications in food products, serving as emulsifiers in animal feed and as raw material in the production of vanillin. (Vanillin is widely used as an ingredient in food flavors, in pharmaceuticals and as a fragrance in perfumes and odor-masking products.) Lignin uses have expanded into literally hundreds of applications - impacting on many facets of our daily lives.

Lignins can serve the following purposes:

 BinderDispersantEmulsifierSequestrant

Researching Lignin Analysis and Use:

1. K.V. Sarkanen and C.H. Ludwig, eds., Lignins: Occurrence, Formation, Strucutre and Reactions, Wiley-Interscience, New York, 1971.

2. E. Adler, Lignin chemistry-Past, Present and Future, Wood Sci. Technol., 11, 169-218, 1977.

3. R.A. Northey, Low-Cost Uses of Lignin, Emerging Technology if Materials and Chemicals from Biomass, ACS Symposium Series 476, Washington, D.C., 1992.

4. S.Y. Lin and S.E. Lebo, Jr., Lignin, Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 15, 268-289, 1995.

5. S.Y. Lin and S.E. Lebo, Jr., Lignin, Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 15, 268-289, 1995.

able resource: trees, plants, and agricultural crops. Lignins

are nontoxic and extremely versatile in performance, qualiti

es that have made them increasingly important in many industr

ial applications.

Commercial lignin is currently produced a

s a co-product of the paper industry, separated from trees by

a chemical pulping process. Lignosulfonates (also called lig

nin sulfonates and sulfite lignins) are products of sulfite p

ulping. Kraft lignins (also called sulfate lignins) are obtai

ned from the kraft pulping process. Other delignification tec

hnologies use an organic solvent or a high pressure steam tre

atment to remove lignins from plants.

Because lignins are ve

ry complex natural polymers with many random couplings, the e

xact chemical structure is not known. Physical and chemical p

roperties differ depending on the extraction technology. For

example, lignosulfonates are hydrophilic (will dissolve in wa

ter) and kraft lignins are hydrophobic (will not dissolve in

water).

Lignin as a Binder

Lignosulfonates are a very effective and economical adhesive, acting as a binding agent or "glue" in pellets or compressed materials. Lignosulfonates used on unpaved roads reduce environmental concerns from airborne dust particles and stabilize the road surface. This binding ability makes it a useful component of:

coal briquettes plywood & particle board

ceramics animal feed pellets

carbon black fiberglass insulation

fertilizers & herbicides linoleum paste

dust suppressants soil stabilizers

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Lignin as a Dispersant

Lignosulfonate prevents the clumping and settling of undissolved particles in suspensions. By attaching to the particle surface, it keeps the particle from being attracted to other particles and reduces the amount of water needed to use the product effectively. The dispersing property makes lignosulfonate useful in:

cement mixes leather tanning

clay & ceramics concrete admixtures

dyes & pigments gypsum board

oil drilling muds pesticides & insecticides

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Lignin as an Emulsifier

Lignosulfonate stabilizes emulsions of immiscible liquids, such as oil and water, making them highly resistant to breaking. Lignosulfonates are at work as emulsifiers in:

asphalt emulsions pesticidespigments & dyes wax emulsions

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Lignin as a Sequestrant

Lignosulfonates can tie up metal ions, preventing them from reacting with other compounds and becoming insoluble. Metal ions sequestered with lignosulfonates stay dissolved in solution, keeping them available to plants and preventing scaly deposits in water systems. As a result, they are used in:

micronutrient systemscleaning compounds