Woodpulp is a fibrous material derived from wood, serving as a fundamental raw material in the production of countless everyday items. It forms the basis for various paper products, textiles, and hygiene goods that are deeply integrated into modern life. Woodpulp provides the structural foundation for many materials, making it a globally significant commodity.
What Woodpulp Is
Woodpulp primarily consists of cellulose fibers, which are long chains of glucose molecules that provide structural support to plant cell walls. These fibers are extracted from both hardwood and softwood trees. Softwoods, such as pine and spruce, yield longer, stronger fibers that provide tear resistance and tensile strength. Hardwoods, like oak and eucalyptus, contain shorter, finer fibers that contribute to smoothness, opacity, and printability.
Beyond cellulose, wood also contains hemicellulose, lignin, and various extractives like resins and fats. Lignin acts as a natural glue, binding the cellulose fibers together and providing rigidity to the wood structure. The pulping process separates these cellulose fibers from the lignin and other non-fibrous components, allowing them to be further processed and reformed into new materials.
How Woodpulp is Made
The transformation of wood into pulp involves several distinct manufacturing processes, each designed to achieve specific fiber characteristics. Mechanical pulping is one method where wood logs are ground against rotating stones or refined between rotating discs. This process yields a high percentage of the original wood material, around 90-95%, because it retains most of the lignin. The resulting mechanical pulp has shorter, weaker fibers and tends to yellow over time due to the presence of lignin, making it suitable for products like newsprint and cardboard.
Chemical pulping, particularly the Kraft (sulfate) process, is the predominant method globally. In this process, wood chips are cooked under pressure in a solution containing sodium hydroxide and sodium sulfide. These chemicals dissolve the lignin, separating it from the cellulose fibers, which results in a stronger, purer pulp with a lower yield, between 45-55% of the original wood mass. Kraft pulp is widely used for producing strong packaging materials, fine writing papers, and absorbent products due to its superior fiber strength and brightness after bleaching. The Kraft process also recovers and reuses its pulping chemicals, and the dissolved lignin can be burned to generate energy for the mill.
Semi-chemical pulping combines aspects of both mechanical and chemical methods to achieve intermediate properties. Wood chips are first treated with chemicals to soften the lignin, followed by mechanical refining to separate the fibers. This hybrid approach yields 60-80% of the wood, producing a pulp with good stiffness and strength. Semi-chemical pulp is frequently used in the production of corrugated medium, the fluted layer in cardboard, where rigidity and crush resistance are desired.
Recycled pulping involves processing waste paper to recover usable fibers for new products. This method begins with sorting and repulping the collected paper in water to create a slurry. Ink particles are then removed through a process called de-inking, and the pulp is cleaned to remove contaminants like plastics and staples. Recycled fibers can degrade with each processing cycle, leading to shorter and weaker fibers over time.
Types and Properties of Woodpulp
The distinct manufacturing processes and original wood sources significantly influence the properties of the resulting woodpulp. Softwood pulps, derived from trees like pine, contain long fibers, ranging from 2.5 to 4.0 millimeters in length. These longer fibers interlock effectively, imparting high tear strength and tensile strength to paper products, making them suitable for packaging and durable papers. Conversely, hardwood pulps, from trees such as eucalyptus, feature shorter fibers, between 0.7 to 1.5 millimeters. These shorter fibers contribute to paper’s smoothness, opacity, and excellent printability, making them preferred for fine printing and writing papers.
Chemical pulps, such as Kraft pulp, exhibit superior strength compared to mechanical pulps because lignin has been largely removed. This high purity and lack of lignin also allow chemical pulps to achieve high brightness levels through bleaching processes. Bleaching involves treating the pulp with chemicals like chlorine dioxide or hydrogen peroxide to remove residual lignin and chromophores, resulting in a whiter appearance. Mechanical pulps, retaining much of their lignin, are less strong and have a lower natural brightness, yellowing upon exposure to light and air.
The purity of pulp, the extent of lignin and other non-cellulose component removal, also impacts its absorbency. Highly purified chemical pulps, with their increased cellulose content and open fiber structure, can be engineered to be highly absorbent. This property is important for products like tissue paper and fluff pulp used in hygiene products.
Applications of Woodpulp
Woodpulp forms the foundation for an extensive array of paper products. Newsprint, made from mechanical pulp, serves the rapid printing of newspapers. Writing and printing papers, requiring higher strength and brightness, incorporate chemical pulps from both hardwood and softwood. Tissue paper, known for its softness and absorbency, relies on a blend of chemical pulps. Packaging materials like cardboard and paperboard leverage the strength and rigidity provided by Kraft pulps.
Beyond traditional paper, woodpulp is a raw material for various textiles and hygiene products. Cellulose-based textile fibers, such as rayon, viscose, and lyocell, are regenerated from woodpulp, offering breathable and comfortable alternatives to synthetic fabrics. In the hygiene sector, highly absorbent fluff pulp is a main component in disposable diapers, sanitary pads, and wipes, designed to wick and retain liquids.
Woodpulp also finds its way into numerous specialty products. It is used in various filtration media. Cellulose derivatives extracted from pulp serve as thickeners, stabilizers, and binders in food products, pharmaceuticals, and cosmetics. Newer applications include the development of bio-based plastics and composites, where cellulose fibers can act as reinforcing agents or form the primary polymer matrix.