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What Is Paper Pulp?

Recycled paper, which is made from paper pulp.
A bag made of recycled pulp.
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  • Written By: Paul Scott
  • Edited By: E. E. Hubbard
  • Last Modified Date: 10 August 2014
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Paper pulp is a term used to describe various slurry preparations used to manufacture paper and paper products. The pulps are either produced from wood or cotton fiber, and are made by “cooking” the wood chips or cotton fibers in a solution of water and various chemicals to reduce them to a consistency suitable for the rolling of the various end products. Both hard and soft woods harvested from sustainable sources are used to produce wood-based paper pulp. In the case of cotton-based pulps, raw cotton linters or recycled rag off-cuts are used to produce higher quality papers than those made with wood pulp. After production, the pulp may be used immediately, stored in vats, or dried and packaged for later use.

Most commercial paper products are made from pulped wood or cotton fiber. With both raw material sources, the paper pulp is generally manufactured by “cooking” wood chips or cotton fibers at high temperatures and pressures in a digester to reduce, or break down, the material to form a viscous suspension. The raw material is cooked in a solution of water and chemicals such as sodium hydroxide, sodium sulphide, and calcium carbonate, known as white liquor. Once the cooking process is complete, the pulp typically undergoes several washing and bleaching stages before it is dried and processed.

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Wood-based paper pulp processes start with the collection of suitable wood stocks from sawmill waste sources, forestry harvesting, or thinning operations. These stocks include both hard and soft wood varieties, including pine, spruce, and birch. The wood is then cleared of bark and debris and chipped to produce a fine aggregate. The chips are introduced into a heated pressure vessel known as a digester along with a solution of water, sodium hydroxide, and sodium sulphide, where they are cooked under pressure for approximately 90 minutes. This process breaks down the lignin, or cell-binding agents, in the wood, reducing the chips to a thick pulp.

Cotton paper pulp is produced from one of two raw material sources — raw cotton linters and cotton rags. Cotton linters are long, fine fibers that surround the seed on a cotton boll, while rags are simple garment and fabric manufacturing off-cuts. These cotton fibers are cellulose-rich and have no lignin, resulting in a whiter and stronger end product requiring less bleaching. Cotton pulp is made using a similar process to wood pulp with the fibers being cooked under pressure in a solution of water and chemicals such as calcium carbonate and hydrogen peroxide. Due to their strength and longevity, cotton-based papers are commonly used as archival supports for artwork and currency manufacture.

In both wood and cotton paper pulp processes, the pulp undergoes several washing stages after the initial cooking is complete. In addition to washing, wood pulp is typically also bleached to neutralize any remaining lignin coloring in the pulp. The finished pulp is then either used immediately or may be stored wet in vats, or dried, cut, and packaged for distribution to paper producers.

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