Acid-free Paper - Overview

Overview

Paper made from wood-based pulp that has not had its lignin removed turns yellow, becomes brittle, and deteriorates over time. When exposed to light and/or heat, the molecules in the acidic paper will break down even faster. It was only in the 1930s that the effects of wood-pulp paper became known, when William Barrow (a librarian) published a report about the deterioration of acidic paper in the libraries. For fear of the gradual disintegration of written materials, measures have been taken to improve the quality of paper.

During production, acid-free paper may be treated with a mild base (usually calcium or magnesium bicarbonate) to neutralize the natural acids occurring in wood pulp, and it may also be buffered to prevent the formation of additional acids (as may develop from the application of sizing).

The bicarbonate is added in excess, to supply the paper with an alkaline reserve to provide protection from further attack by acids remaining in the paper or supplied by the environment (e.g. atmospheric sulfur dioxide). The bicarbonate during drying loses carbon dioxide and water and is converted to calcium carbonate or magnesium carbonate. In order for paper to last at least 100 years it must have an alkaline reserve of 2% or more.

Today, much of the commercially produced paper is acid-free, but this is largely the result of a shift from china clay to (cheaper) chalk as the main filler material in the pulp: chalk reacts with acids, and therefore requires the pulp to be chemically neutral or alkaline. The sizing additives mixed into the pulp and/or applied to the surface of the paper must also be acid-free.

Alkaline paper has a life expectancy of over 1,000 years for the best paper and 500 years for average grades. The making of alkaline paper has several other advantages in addition to the preservation benefits afforded to the publications and documents printed on it. Because there are fewer corrosive chemicals used in making alkaline paper, the process is much easier on the machinery, reducing downtime and maintenance, and extending the machinery's useful life. The process is also significantly more environmentally friendly. Waste water and byproducts of the papermaking process can be recycled; energy can be saved in the drying and refining process; and alkaline paper can be more easily recycled.