Materials

Softwoods are the preferred raw material for ‘strong’ papers, primarily because of the length and slenderness of its fibers. In other papers, it is typically used as a ‘reinforcement’ component. Low-density softwoods (various spruces, firs, scots pine, lodgepole pine, etc.) in which thin-walled fibers normally are predominant are preferred for papers with high demands for bonding-related strength characteristics, such as tensile, burst, and surface strength.

Denser softwoods, such as the southern pines (slash pine, loblolly pine, longleaf pine, and shortleaf pine) and some US western species, such as the Douglas fir, give more tear strength and bulk, which are essential when paper with a high bending stiffness is required. Juvenile wood is characterized by having slightly smaller fibers and a somewhat lower density than the average of a mature tree.

In mechanical pulping, softwoods, especially spruce, are the preferred raw material, since most hardwoods are too dense. Exceptions in this respect are Populus species (aspen, cottonwood, etc.)

Hardwoods are the preferred raw material for pulp used in printing papers. The desirable feature of hardwood pulp in this context is the small dimensions of its fibers. This implies a high number of fibers per unit mass of pulp, which in turn is beneficial for formation characteristics, such as small-scale uniformity, opacity, and surface smoothness, all of which are important for printing paper.

Hardwood fibers should also be slender and not too thick-walled. The short hardwood fibers, especially in eucalyptus pulp, are also preferred in soft tissue, where they improve the tactile softness of the product.

Hardwoods contain more cellulose and extractives, and less lignin, than softwoods; as well, they usually have a higher initial brightness. For pulp purposes, softwood fibers are, on average, more than three times the length of those contained in hardwoods (thus the origin of the terms “long-fiber pulp” and “short-fiber pulp”). And since increasing fiber length usually translates into more inter-fiber bonding, softwood pulps impart greater strength to the products into which they are made, than do hardwood pulps manufactured by the same process. As an example, bleached softwood kraft (BSK) is considerably stronger than bleached hardwood kraft (BHK).

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Wood fibers are natural composite structures in which cellulose fibrils are held together by lignin and hemicellulose. The major constituents of wood fibers are lignin, cellulose, hemicellulose, and extractives.

• Cellulose – long-chained carbohydrates consisting of repeating units of a single simple sugar unit known as glucose;
• Hemicellulose – Shorter chain carbohydrates whose building blocks include five different types of sugars;
• Lignin – A polymeric substance which serves as a cementing agent;
• Extractives – Resins, oils, alcohols, and fatty acids.

Cellulose

Cellulose is the most abundant natural polymer on earth. Cellulose, the major component of papermaking fibers, contributes 40-45% of the wood’s dry weight. Located primarily in the secondary cell wall, cellulose polymers are composed of long linear chains of D-glucose linked by ß-1,4-glycosidic bonds of glucose in a 4C1 chair conformation with equatorially oriented substituents as illustrated in Figure (1).

Although cellulose has four major crystalline forms or polymorphs (cellulose I, II, III, and IV), cellulose I is the main polymorph found in wood; and cellulose I can be converted to cellulose II by treatment in alkali or regeneration (solubilization followed by recrystallization).

Structure in Layers

The dimensions of pulpwood fibers vary among species, geographical locations, and
within individual trees. A model for the layered structure of a typical softwood tracheid is described in Figure (2). The middle lamella and primary cell wall of these fibers are often referred to as the compound middle lamella. The middle lamella contains a high proportion of amorphous material which holds neighboring fibers together. The primary cell wall is approximately 0.03-1.0 μm thick, and also contains a high percentage of lignin. The secondary cell wall consists of three layers, labeled S1 through S3 from the outer to the inner layer. The S1 and S3 layers are thin, at 0.1-0.3 μm, while the middle layer (S2) is thick at 1-5 μm, and is said to be most responsible for the strength properties of individual fibers. The fibrils of secondary cell wall layers are wound helically around the fiber axis, while those of the primary wall are randomly oriented.24 The distribution of fiber constituents throughout the fiber is further described in section 0 of this dissertation.

Here is the following attachment if you wanted to know more about other pulpwood fiber properties:

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Sources

Wielen, L. (n.d). The Structure and Physical Properties of Pulpwood Fibers. Retrieved from: http://biorefinery.utk.edu/technical_reviews/Proerties%20of%20Pulpwood%20Fibers.pdf

Dillen, S. Hamza, M. Dillen, J. (2016). Pulp and Paper: Wood Sources. Elsevier. Retrieved from: file:///C:/Users/Naj1_/Downloads/PulpandPaper_WoodSources%20(1).pdf

EKSTROM, H. (November, 2nd, 2018). Wood fiber costs for the world’s pulp industry fell for the first time in over a year in the 2Q/18. Paper Advance. Retrieved from: https://www.paperadvance.com/blogs/hakan-ekstrom/wood-fiber-costs-for-the-world-s-pulp-industry-fell-for-the-first-time-in-over-a-year-in-the-2q-18.html

Overview of Wood Pulp. (2020). NUMERA ANALYTICS. Retrieved from: https://numeraanalytics.com/overview-wood-pulp/

Hardwood pulpwood prices on the rise. (January 12th, 2017). Global wood markets info. Retrieved from: https://www.globalwoodmarketsinfo.com/hardwood-pulpwood-prices-on-the-rise/

Committed to sustainable forestry and raw material use. (2020). UPMPulp. Retrieved from: https://www.upmpulp.com/sustainable-pulp/sustainability/raw-materials-and-forests/

Softwood Timber. (n.d). Ambika. Retrieved from: https://www.indiamart.com/ambikawood/softwood-timber.html

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