Pulp: Where paper begins

Mechanical vs. chemical pulp

The two primary methods of producing pulp are mechanical and chemical pulping, with chemi-mechanical processes combining the two. Mechanical pulping separates fibers through physical processes and includes refiner-mechanical pulping (RMP) and thermomechanical pulping (TMP). RMP uses disk refiners to mechanically separate fibers, while TMP heats chips under pressure before refining to soften the lignin. Mechanical pulp typically uses 90–95% of the wood and produces paper with good bulk, opacity, resilience and printability. Chemical pulping removes lignin through cooking, producing stronger, purer cellulose fibers with greater brightness and permanence.

Chemi-mechanical pulping uses chemical treatment to partially loosen the lignin before mechanical separation, providing a balance between yield and fiber strength. Each method offers different advantages, and many papers combine mechanical and chemical pulps to achieve the desired combination of strength, bulk, opacity, brightness and printing performance.

The kraft process

The kraft process is the most widely used chemical pulping method in the world. During this process, wood chips are cooked in a chemical solution that dissolves lignin—the natural material that binds wood fibers together—while preserving the cellulose fibers needed for papermaking.

The resulting pulp is strong, versatile and well-suited for a wide range of paper and packaging applications. The kraft process also allows many of the processing chemicals to be recovered and reused, making it both efficient and sustainable.

Bleaching

Pure cellulose is naturally white, but fibers from the pulping process are brown because they retain color-producing materials. This brown stock can be used to make strong kraft paper, but producing white paper requires bleaching to increase the fiber’s reflectance, particularly in the blue range of visible light.

Bleaching is a multi-stage process that uses several chemicals, and modern mills have eliminated elemental chlorine to reduce the formation of undesirable compounds such as dioxins. The process must balance brightness with paper quality because excessive bleaching can reduce opacity and fiber strength.