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Lignin
Wood contains a natural chemical compound called lignin that binds the wood fibers together to form strong trees, but too much lignin produces weak paper and causes it to yellow with age. Each processing technique strips a certain amount of lignin from the wood pulp, thus producing paper fibers of different strengths, lengths and weights. Since paper is made from a nonuniform web or pattern of wood pulp fibers, mineral fillers and chemicals in the refining determine its quality and brightness.
Chemical Processing
The chemical pulping process uses a combination of sulfate, high temperatures and high pressure that removes nearly all the lignin from the wood, creating the most durable paper. The remaining bit of lignin is stripped entirely from the wood pulp through chemical bleaching. This permanently whitens or brightens the pulp before it's compressed by steam and electricity into paper, and the chemicals are recovered and recycled for future use. Chemically-processed paper is the heaviest, brightest and most expensive stock on the market.
Semi-chemical Processing
Semi-chemical processing is similar to chemical processing, but it uses a combination of chemical and mechanical processes that only remove half the lignin from the wood when it's converted to pulp. During semi-chemical processing, the remaining lignin is decolorized rather than stripped out by bleaching, which is only a semi-permanent effect. This results in lighter-weight, less-bright paper that is also cheaper and flimsier.
Mechanical Processing
Mechanical processing physically separates the pulp from wood chips with no chemical help, and doesn't remove any of the natural lignin. It also uses the semi-permanent process to decolorize the lignin and results in the least-bright, least-durable and least-expensive paper. Mechanical processing accounts for only about 5 percent of U.S. paper manufacturing.
Recycling Process
Recycled processing uses a semi-chemical approach to turn used paper into wet pulp, and recycles chemicals the same way as chemical processing. Recycling uses more water and less electricity, creating lower toxic emissions, and makes a smaller environmental impact. Recycled paper processing is second only to chemical processing in the U.S. and produces durable, inexpensive paper using fewer chemicals and natural resources.