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LIGNIN - Permanent Problem or Manufacturing Solution?

Reprinted from Paper Conservation News, Sept. 1995. Iris Winston is staff writer at the National Library of Canada.

Yellowing is a sign of deterioration in paper. Or is it? Lignin—a naturally binding agent in wood—is known to cause discoloration, if there is sufficient exposure to light. But there is considerable disagreement about whether or not lignin is the enemy to paper permanence that it was previously assumed to be.

The uncertainty about the effects of lignin exists because virtually all the paper manufactured between 1850 and 1950 was manufactured through an acid-based process and contained acidic alum as well as lignin. Any of these factors could have affected paper stability and contributed to the embrittlement and deterioration of the paper.

Sections of the pulp and paper industry maintain that lignin is no villain and can safely remain part of the permanent paper-manufacturing process, pointing out that there is no scientific evidence to support the belief that lignin is detrimental to paper longevity. In recent years, scientific research has demonstrated that physical deterioration and yellowing are different and separate mechanisms. Some scientists say that the presence of lignin may actually contribute to mechanical permanence, and several published research papers have shown that, under controlled conditions, alkaline, lignin-containing papers are as permanent as those containing mechanical pulps. In some cases, they have been shown to perform even better. But those concerned with the preservation of print information want to ensure that no risks are taken in trying to preserve Canada’s cultural heritage on paper that might not last the course.

Paper producers and advocates of permanent-paper use agree that the alkaline/acid content of paper and the presence of an alkaline reserve are critical in determining whether or not a paper may be classified as permanent. The American National Standards Institute (ANSI) standard of 1992 and the International Standard Organization (ISO) standard of 1993 require paper to have a pH content of at least 7.5 and a minimum alkaline reserve equivalent to two percent calcium carbonate. The preambles of both standards acknowledge that some papers containing lignin meet the strength-testing requirements for permanence. However, both indicated that because of possible negative effects of airborne-pollutants on lignin, lignin-containing pulps should be excluded from the standards. Research at the Institut für Papierfabrikation in Germany and some recent newspaper reports have indicated that lignin is not negatively affected by airborne pollutants during an accelerated aging process. However, paper-containing lignin appears to absorb sulphur dioxide from the atmosphere and also discolors more rapidly than paper without a lignin content. The questions to be considered are whether an alkaline reserve such as calcium carbonate counters the effect of such a pollutant and whether lignin is detrimental to paper permanence.

These questions will be clarified through a unique, Canadian cooperative research project now in progress. Jointly developed and funded by several federal and provincial government departments including the Canadian Conservation Institute—Industry Canada, the National Archives of Canada and the National Library of Canada and members of the pulp and paper industry. This initiative is supporting scientific research to determine the impact and effects of lignin on paper permanence. The work, by CCI and the Pulp ad Paper Research Institute of Canada (Paprican), is expected to take two years and will be the basis for development of a Canadian standard for paper permanence. It may also lead to a review of the ANSI and ISO standards, as both bodies are committed to reviewing their standards in the light of new research findings.

‘This new initiative is very exciting,’ says Marianne Scott, National Librarian of Canada. ‘It places Canadian efforts to promote the use of permanent paper in an international context and emphasizes cooperation between industry and government. It is in all our interests to ensure that the broadest range of papers lasts for the longest possible time.’

Experiments are to be conducted on a variety of mechanical, chemical and rag pulps with and without alkaline reserves. Assorted paper samples will be exposed to controlled amounts of air pollutants and monitored during accelerated aging at various intervals. Data analysis will concentrate on the effect of lignin under various conditions over the experimental period. In addition to dealing with the primary question of lignin’s impact on paper permanence, the research program may go on to consider the feasibility of having paper standards based on paper function rather than composition. This is of particular relevance in view of the increased use of recycled materials and changes in wood-pulping technologies.

The research work co-leaders are Dr. Helen Burgess of CCI and Dr. Norayr Gurnagul of Paprican.

‘It represents a unique opportunity to combine the knowledge, experience and expertise of two of Canada’s most respected cultural and industry-technical organizations,’ says Alan Bird, Marketing Manager of DuPont Canada Inc. ‘Canada is the world leader in production of the new generation of high-tech, chemi-thermo mechanical pulp (CTMP) with more than 70 percent of the world’s capacity. CTMP is ideally suited to spruce and aspen fiber and needs low-cost electricity. We have these elements in abundance in Canada.’