Volume 4, Number 4
Nov 1991

The Nature of Lignin

Lignin is found in all vascular plants, mostly between the cells, but also within the cells, and in the cell walls. It makes vegetables firm and crunchy, and gives us what we call "fiber" in our food. It functions to regulate the transport of liquid in the living plant (partly by reinforcing cell walls and keeping them from collapsing, partly by regulating the flow of liquid), and it enables trees to grow taller and compete for sunshine. and researchers see it as a disposal mechanism for metabolic wastes.

In nature it is very resistant to degradation, being held together with strong chemical bonds; it also appears to have a lot of internal H bonds. It is bonded in complex and various ways to carbohydrates (hemicelluloses) in wood. This picture of usefulness and stability presents quite a contrast to the familiar lignin in groundwood paper, which is so unstable and so troublesome in books and records of value. The contrast can be explained by the radical effect of pulping and bleaching on the lignin as it is separated from the fibers.

Lignin is actually not one compound but many. All are complex, amorphous, three-dimensional polymers that have in common a phenylpropane structure, that is, a benzene ring with a tail of three carbons. In their natural unprocessed form, they are so complex that none of them has ever been completely described, and they have molecular weights that my reach 15,000 or more.

Lignins are not acids, though most of them contain certain carboxylic acids, and wood gives off acids as it deteriorates, as do paper and board that contain lignin. This deteriorates cellulose and other sensitive materials nearby, as well as the cellulose fibers within the lignin-containing paper itself. This is why permanent paper standards in the past have always specified that no groundwood or unbleached fiber should be used to make the paper. Recently, though, it has been recognized that a calcium carbonate filler would effectively cancel this destructive effect of lignin for an indefinite period of time, and the current draft of the ANSI Z39.48 standard permits as much as 7.5% lignin. This specification is controversial, and raises several questions having to do with measurement: How reliably and easily can the purchaser test paper and board to see how much lignin it contains? What research has been done on the permanence of paper that contains both lignin and calcium carbonate? Are the forms of lignin in different types of pulp equivalent, as far as their effect on permanence is concerned?

These questions are important. More and more lignin will be included in paper as time goes on, because of the increasing use of recycled postconsumer fiber (from which it is hard to exclude groundwood and other mechanical pulp papers) and the growing international pulp shortage (which provides incentive for use of high-yield pulps).

Lignins as they occur in nature (protolignins) have been grouped into several types, characteristic of hardwoods, softwoods and grasses. Within each type there is a lot of variation: lignins differ from species to species, and from one tissue to the next in the same plant--even within different parts of the same cell. The process of removing them from the plant changes their form and chemical makeup to a greater or lesser extent, which makes then hard to study and way account for the large and growing number of analytical techniques in use. No one method is ideal for all cases, and the limitations of each method have to be borne in mind when results are interpreted.

(Science, experience and mythology are full of instances like this, in which the very act of observing something causes it to change into something else, if not to disappear altogether. The anthropologist inevitably affects the primitive societies he or she observes, because it is impossible for them to be a fly on the wall; a photograph or water color which is exhibited for too long may fade away to nothing; and Euridice had to return to Hades when Orpheus looked back to see if she was still following him. According to the uncertainty principle in quantum mechanics, you can calculate either the position or the velocity of an electron, but not both at once. Of course seeing the electron directly is out of the question.)

Besides protolignins, there are lignin preparations or model compounds, which are removed from the wood matrix by mild, relatively nondestructive means for lab study and research. They retain the characteristic phenylpropane structure of lignins . The lignin compounds that occur in paper, on the other hand, have been treated rather roughly and do not retain that structure, so they are not what a purist would call true lignins.

Some pulping processes are designed to remove as much lignin as possible, while others are designed merely to separate the fibers so as to allow banding. The more lignin is removed, up to a certain point, the more bonding can take place, and the stronger the paper. (The amount of lignin in a sheet, by the way, cannot be estimated by how brown it is: high-yield bleached chemi-thermomechanical pulp is white, but kraft grocery bags, which contain only 3 1/2 %to 8% lignin, are brown.) High-yield pulp, from which a minimum of lignin is removed, may be prepared by a mechanical, semichemical or chemical process. Lignin is left in the pulp to bulk up the paper, increasing the yield from a given amount of wood.

Pulp intended for use in fine papers, on the other hand, is prepared by cooking to remove the lignin, as well as to separate the fibers. The process ordinarily has to be stopped before all of the lignin is removed, however, because the lignin gets more resistant to removal as pulping proceeds, and the cellulose becomes more vulnerable to the chemicals used. If pulping proceeded until all the lignin was gone, most of the cellulose would be gone too. Most of the remaining lignin is removed by bleaching.

Because of the flap about dioxin, mills are using as little chlorine in bleaching as possible, and some are using other bleaching chemicals instead. Some of the residual lignin that used to be removed by bleaching is now being removed in some pulp mills by extended delignification, which Smook defines as "Kraft pulping modification within a continuous digester which utilizes a two-stage, partially counter-current cooking sequence to achieve lower kappa number and improved overall washing efficiency." This can bring the percent of lignin down to about 1% on the weight of the paper, though Wolfgang Glasser says, on p. 66 in his chapter in Casey's Pulp and Paper (1980), that complete delignification is not possible. Bleaching is still necessary, whether or not extended delignification has been used.

Products from. kraft pulping are loosely referred to as "kraft lignins" and products resulting from chlorine bleaching as "chlorolignins," but we know much less about them than we do about model compounds and lignin preparations. Lignosulfonates, byproducts of sulfite pulping, which can also be produced from kraft black liquor, have commercial value because they can be used for so many different purposes. These include use to reduce road dust, and as an additive to concrete mixtures, textile dyes, brick clay, animal feed and paperboard. An industry group called the Lignin Institute was formed recently. Its membership includes both producers and users, and its activities will include research and dissemination of information. The Lignin Institute Technical Committee has started work on test methods used to analyze lignin products. The Institute is in Atlanta, and its number is 404/252-3663.

Each radical or end group of the lignin polymer reacts in characteristic ways with other chemicals. The coniferylaldehyde groups, for instance, are the ones that give the magenta color with the phloroglucinol/HCl mix. However, as Wolfgang Glasser says in the work cited above, "A positive reaction is positive only with those lignin preparations that contain coniferylaldehyde end groups; this constraint seriously limits the application of this test with chemical and high-yield chemical pulps."

Two fairly comprehensive guides to the use of spot tests for the detection of lignin are 1) TAPPI Official Test Method 401, "Fiber Analysis of Paper and Paperboard" and 2) Paper Conservation Catalog, Part 10, "Spot Tests." The Paper Conservation Catalog is available from the American Institute for Conservation (202/232-6636); it gives sources of supply for each stain. It refers frequently to relevant TAPPI standards and has a 41-itern bibliography, which overlaps partly with the TAPPI 401 bibliography.

A succinct review of 45 methods for chemical analysis is on p. 17-20 of New Methods of ring Wood and Fiber Properties in Small Samples (TAPPI Press, 1987; now on sale for $10; a steal; buy it). This is followed by reviews of 12 studies on preparative lignins, of 25 studies on characterization of ligins, and of i3 on chemical modification and degradation. In the section on pulps (p. 41) the accuracy of lab tests for lignin content is discussed: "Accurate analytical determination of the lignin content of pulps has constituted a particular problem.... The kappa number-lignin content relationship differs with pulping method and species; therefore it must be established for each combination of these factors." However, for any given combination, there is a straight-line relationship between kappa number and lignin content.