Pulp explained

What’s the difference between kraft paper pulp and dissolving pulp?

To start, you need to understand that under the bark of the tree, wood is composed of five main constituents — water, cellulose, hemi-cellulose, lignin and other extractives. In making kraft paper pulp, the aim is to

remove the lignin (which is the gluelike substance that holds the fibers in a tree together) and the other extractives while preserving as much of the cellulose and hemi-cellulose as possible. Wood chips are combined with caustic cooking chemicals in a digester, which is very much like a large pressure cooker, to dissolve away the lignin and extractives. The liberated fibers are then washed and bleached to make kraft paper pulp.

But with dissolving pulp, the hemi-cellulose also must be removed to get as close as possible to pure cellulose (essentially chemically identical to

cotton fibers) as the end product. To do this, a step is added before cooking in which steam is added to the chips in the digester before addition of the cooking chemicals. The steam, in essence, strips away the hemi-cellulose from the wood chips. From that point on, the process of making dissolving pulp is nearly identical to that of making kraft paper pulp.

Kraft pulp typically is distributed to make paper and paper products. Dissolving pulp (also called chemical cellulose) is further processed to make more durable substances, whether they are near-plastic-like products such as film and cellophane, or textile products like rayon.

What’s the difference between kraft paper pulp and dissolving pulp?

To start, you need to understand that under the bark of the tree, wood is composed of five main constituents — water, cellulose, hemi-cellulose, lignin and other extractives. In making kraft paper pulp, the aim is to remove the lignin (which is the gluelike substance that holds the fibers in a tree together) and the other extractives while preserving as much of the cellulose and hemi-cellulose as possible. Wood chips are combined with caustic cooking chemicals in a digester, which is very much like a large pressure cooker, to dissolve away the lignin and extractives. The liberated fibers are then washed and bleached to make kraft paper pulp.

But with dissolving pulp, the hemi-cellulose also must be removed to get as close as possible to pure cellulose (essentially chemically identical to

cotton fibers) as the end product. To do this, a step is added before cooking in which steam is added to the chips in the digester before addition of the cooking chemicals. The steam, in essence, strips away the hemi-cellulose from the wood chips. From that point on, the process of making dissolving pulp is nearly identical to that of making kraft paper pulp.

Kraft pulp typically is distributed to make paper and paper products. Dissolving pulp (also called chemical cellulose) is further processed to make more durable substances, whether they are near-plastic-like products such as film and cellophane, or textile products like rayon.

Source: Mike Schultz, Cloquet mill conversion project manager, Sappi Fine Paper

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