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Paper appears simple, yet its manufacture relies on sophisticated chemistry. By transforming brown wood chips into bright, versatile sheets, the paper industry showcases a series of precise chemical reactions and physical processes.

Kraft Pulping

Wood is primarily cellulose bound by lignin. The Kraft process removes lignin by treating chips with a mixture of sodium hydroxide and sodium sulfide under high temperature and pressure. The strongly basic environment cleaves lignin chains, liberating cellulose fibers for further processing.

Alternative Pulping Methods

While Kraft pulping dominates, other strategies exist:

• Acid sulfite pulping uses sulfurous acid and a bisulfite salt (sodium, magnesium, calcium, or ammonium) to dissolve lignin.
• Neutral sulfite semichemical pulping combines sodium sulfite and sodium carbonate; it removes only part of the lignin, necessitating mechanical shredding afterward.

Bleaching Chemistry

Residual lignin often stains pulp brown. Bleaching employs oxidizing agents that selectively oxidize lignin while preserving cellulose. This step is more precise than pulping, which can inadvertently damage fibers.

Common Bleaching Agents

The industry relies on several oxidants, each with unique properties:

• Chlorine and chlorine dioxide offer high selectivity and excellent dirt removal.
• Hydrogen peroxide is a powerful, environmentally friendly option.
• Oxygen, ozone, and sodium hypochlorite (household bleach) also contribute to lignin removal.

Post‑Pulping Chemical Treatments

After pulping and bleaching, paper mills apply additional chemical processes to tailor product attributes:

• Sizing improves moisture resistance.
• Retention controls additive distribution during sheet formation.
• Wet‑strength systems, such as polyamido‑amine‑epichlorohydrin (PAEM‑ECH) resins, cross‑link cellulose fibers, preventing disintegration in wet conditions.

These treatments, combined with physical sheet‑forming operations, produce the high‑quality paper we rely on daily.