Confusingly, wood free paper is not free of wood. It’s just free of part of the wood fiber that gets included when you chew up a tree. “Wood free” means that the lignin has been taken out of the paper pulp. Wikipedia tells us “Lignins are one of the main classes of structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically lignins are cross-linked phenol polymers.” Lignins left in the pulp are what causes the yellowing of newsprint.
Here’s Glatfelter’s explanation:
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Those who know the brown paper used to wrap parcels as Kraft paper may be puzzled to hear the Glatfelter commentator refer to the process used as the Kraft process. But although it is brown, Kraft paper does indeed not have any lignin content, which would weaken it significantly. The Kraft process was invented in Germany by Carl Dahl in 1879. It’s name is a reference to the strength of the paper thus made. Kraft paper is just made from unbleached pulp.