Mechanical pulp
Mechanical pulp is pulp produced by grinding or refining wood physically rather than dissolving the lignin with chemicals. It retains nearly all of the wood's mass — high yield, low cost — at the price of shorter fibres and remaining lignin.
Three main processes: stone groundwood (SGW, the original method), thermomechanical pulp (TMP, the modern standard), and chemi-thermomechanical pulp (CTMP, a hybrid with a light chemical pre-treatment). All keep the wood's lignin in place rather than dissolving it.
Yield is the headline advantage. Chemical pulping (kraft, sulphite) loses about half the wood mass as dissolved lignin. Mechanical pulping retains 90-95% — far more paper per cubic metre of wood, far cheaper per ton.
The trade-offs: shorter fibres mean lower strength, and remaining lignin means the paper yellows in sunlight over weeks. Mechanical pulp dominates newsprint and mid-grade printing papers but is uncompetitive in packaging and bag papers where strength matters.
Related terms
- Kraft paperKraft paper is strong, typically brown paper produced by the kraft pulping process — a chemical method that preserves long cellulose fibres. It is the dominant fibre source for linerboard, shipping sacks, and packaging.
- NewsprintNewsprint is a lightweight, low-cost paper made primarily from mechanical pulp, used for newspapers, freebie publications, and increasingly as protective packaging void fill. It is typically 40 to 52 gsm.
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