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Pulp consists primarily of cellulose fibers, supplemented by variable proportions of hemicellulose, lignin, extractives (e.g., resins and pigments), pectin, and inorganic ash. The relative abundance of these components significantly influences pulp properties-and consequently, its suitability for specific papermaking applications.
1. Mechanical Wood Pulp
Also referred to as groundwood pulp, mechanical wood pulp is produced by physically refining fibrous wood raw materials-typically without chemical delignification. It occupies a prominent role in the global paper industry due to its low production cost, straightforward manufacturing process, and favorable functional properties: notably high ink receptivity, excellent opacity, and a soft, smooth sheet formation.
Mechanical wood pulp is broadly categorized into two variants: white (bleached) and brown (unbleached). White mechanical pulp is predominantly employed in newsprint production and is frequently blended with other pulps to manufacture writing and printing papers. Brown mechanical pulp is chiefly utilized in packaging grades-particularly industrial corrugating medium and solid fiberboard-where high bulk and stiffness are prioritized over brightness or strength.
2. Sulfate (Kraft) Wood Pulp
Sulfate pulp-commonly known as kraft pulp-is manufactured using an alkaline cooking liquor composed of sodium hydroxide and sodium sulfide. This process enables selective delignification while preserving fiber integrity, yielding pulp with superior tensile, burst, and tear strength. Kraft pulp is available in both bleached and unbleached forms.
Unbleached kraft pulp serves as the primary raw material for packaging grades requiring high strength and durability, including kraft paper, sack paper, kraft linerboard, and general-purpose corrugated board. Bleached kraft pulp, by contrast, is used in the production of premium graphic papers-such as coated and uncoated offset papers, high-brightness newsprint, and fine writing papers-where whiteness, uniformity, and printability are critical performance criteria.
3. Sulfite Wood Pulp
Sulfite pulp is produced via acidic pulping using cooking liquors based on sulfur dioxide and a counterion (e.g., calcium, magnesium, sodium, or ammonium bisulfite). This method yields long, flexible fibers characterized by high toughness, excellent fibrillation potential, and superior inter-fiber bonding capacity. Sulfite pulps are readily bleachable and are classified according to degree of bleaching: unbleached, semi-bleached, and fully bleached.
Fully bleached sulfite pulp exhibits high brightness, purity, and softness; however, oxidative and hydrolytic degradation during bleaching slightly reduces fiber strength relative to unbleached counterparts. Consequently, it is reserved for high-value specialty papers-including tissue, cigarette paper, filter paper, and premium writing and printing grades-where optical and tactile qualities outweigh absolute mechanical strength requirements.
4. Recycled (Deinked) Pulp
Recycled pulp is derived from post-consumer waste paper or pre-consumer trimmings generated during printing and converting operations. Processing involves sorting, pulping, screening, cleaning, deinking (via flotation or washing), and optional bleaching.
Fiber properties-including length, strength, and flexibility-are inherently constrained by prior processing history: repeated drying–wetting cycles, mechanical refining, and exposure to chemicals progressively reduce fiber swelling capacity and bonding potential. As a result, recycled pulp generally exhibits lower tensile strength, reduced tear resistance, and diminished formation uniformity compared with virgin pulps. Its application is therefore stratified by feedstock quality: higher-grade sorted office waste supports production of printing and writing papers; mixed or unsorted waste is typically directed toward packaging board, newsprint, or lower-tier hygiene products.
