Characteristics of wood and papermaking fibers

Wood fibers are the base upon which most of the world’s paper is made. But as a pulp or paper maker, how much do you really know about the fine details of wood fibers and their structure? ABB’s Papermaking Fiber Guide contains a wealth of material and information covering this subject matter – and much more.

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Wood used for pulp making can be classified into two main groups: Softwoods (coniferous trees, such as pine, fir, spruce and hemlock) and Hardwoods (deciduous trees, such as birch, aspen, eucalyptus, acacia and oak). The biggest difference between hardwood and softwood fibers is that the hardwood fiber is considerably shorter and thinner than the softwood fiber. Generally, hardwood contains more cellulose and hemi cellulose and less lignin than softwood, while softwood has a higher proportion of extractives, i.e. resin.

Softwood

Fibers from softwoods are long and strong, and therefore the strongest paper grades are generally manufactured from chemical pulp made of softwood. Typical important end uses include cardboard boxes and milk cartons, which require high strength to function correctly.

Softwood is composed of up to 90% long, tapering cells called tracheids, which function both for support and water transportation. The hole in the middle of the tracheid is called lumen. The tracheids are connected to each other through pores. Apart from the tracheids, there are other cell types present, for example resin channels storing resin, and ray cells. These cells are principally connected with lignin, forming a stable support structure that holds the fibers in place.

A sketch exemplifying the structure of softwood. Earlywood and latewood fibers are evident. A resin channel can be seen at the top of the picture.

Hardwood

Hardwood fibers are short and thin, giving better paper formation than softwood fibers. They also give paper a smooth printing surface and high opacity. In addition, because there is somewhat less lignin in hardwood compared to softwood, it is also easier to bleach the hardwood pulp to a high brightness.

These qualities make hardwood very suitable for use in printing papers, although these grades generally consist of a blend of hardwood and softwood pulps to meet the combination of strength and the printing surface demands of the customer.

Hardwoods have a more complex structure than softwoods, with different cells for water transport and support. Elongated libriform fibers function for support and are thick-walled in proportion to the diameter. Shorter, wider cells called “vessels” are responsible for water transportation.

Geographical differences

In Scandinavia, the growth period is short but intense, particularly in the most northern areas. It lasts from May to September-October, and then growth is idle during wintertime. It takes about 75 years for a Scandinavian spruce or pine to be ready to harvest, compared to a pine in the southern U.S., where it only takes 25 years for the tree to be ready to harvest. This is due to the warm climate all year round. And in Brazil it takes only about seven years for a eucalyptus tree to be ready to harvest when grown on a plantation.

Pulp fibers can be extracted from almost any vascular plant found in nature, but a high yield of fibers is necessary to make the process economical. Today, globally about 90% of the fibers used for papermaking are wood fibers. The rest is produced from non-wood fibers like bagasse (sugarcane), straw and bamboo.

Chemical composition

Wood mainly consists of three types of materials: Cellulose, hemi-cellulose and lignin. The relative composition in the wood varies in different species of trees. 

Cellulose is the main component of the fiber. It is a straight-chain carbohydrate polymer composed of glucose units and is the structural material on which the fiber is built. Cellulose is insoluble in most solvents, and it is resistant to the action of most chemicals except strong acids. Cellulose is also very important to paper properties because the attraction between cellulose molecules on different fiber surfaces is the principal source of fiber-to-fiber bonding in paper to give strength.

Hemi-cellulose is the second major component in a wood fiber. It is also a polymer built up of branched molecular chains of glucose and other monosaccharides, and it can be removed by mild chemical action. The molecular chains are much shorter than in cellulose. Hemi celluloses are very important in papermaking since they promote the development of fiber-to-fiber bonding, because they can help the fibers to take up water during processing, thus directly participating in the bonding.

The third major component of a wood fiber is lignin, the glue which holds fibers together to form the wood structure. It also occurs within the fiber wall. Lignin is very complex non-uniform molecule and does not dissolve in water or other common solvents, but it can be made soluble by chemical action. The purpose of all chemical pulping processes is to dissolve and remove lignin. However, there is no practical method for complete lignin removal by pulping. The residual lignin gives the unbleached pulp a brown tint, and if white paper is to be made, the remaining lignin must be removed by bleaching. Besides reducing the natural whiteness of pure cellulose fibers, lignin also prevents the formation of fiber-to-fiber bonds in paper, thus reducing the sheet strength.

This article on the Characteristics of Wood and Papermaking fibers is a highly condensed version of 3 of the 20 chapters included in the ABB Papermaking Fiber Guide, which contains valuable information for any professional working with wood fiber processing in pulp or paper mills.

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