De-acidification refers to chemical treatments meant to slow down the acid hydrolysis and embrittlement of books and paper documents that had been printed on acidic paper. From the early 1800s up to about 1990, papermakers used aluminum sulfate, an acidic compound, in most printing papers. Certain deacidification methods use non-aqueous media to distribute alkaline mineral particles such as MgO within the pages of the treated books. Evidence is considered here as to whether or not the proximity of alkaline particles within such documents is sufficient to neutralize the acidic species present. Because much evidence suggests incomplete neutralization, a second focus concerns what to do next in cases where books already have been treated with a non-aqueous dispersion system. Based on the literature, the neutralization of acidic species within such paper can be completed by partial moistening, by high humidity and pressure, by water condensation, as well as by optional treatments to enhance paper strength and a final drying step.
A large proportion of books and other paper documents stored in libraries, museums, and private collections have become so brittle over the course of time that they no longer can be checked out to readers. Though there are a number of contributing factors, it is well known that one of the dominant mechanisms tending to shorten the useful life of paper documents is acid-catalyzed hydrolysis, a process that attacks the cellulose chains in the fibers that make up paper. Millions of tons of acidic paper had been produced worldwide for printing applications during the previous two centuries, and it is still being produced in Asia and other continents. Starting in about 1980, the paper industry went through a major shift in its manufacturing practices, such that most books nowadays are printed on paper that was made under near-neutral to weakly alkaline conditio. As a consequence, the books are much less likely to suffer from embrittlement during ordinary storage. Meanwhile, it has been estimated that 20 to 40% of Indian publications and 2 to 3% of papers from the United States require deacidification, since some publishers source papers from outside the United States.
Available methods and technology for individual sheets of paper (archive documents) using water-based processes can assure high efficacy, longevity, and usability through both completeness of neutralization and multifunctional preservation completeness. The aqueous processes can assure (1) homogeneous distribution of alkali in the cellulose micro- and nano-structure, as well as (2) completeness in terms of the preservation functions that are necessary for a particular type of acid documents, such as information, color, ink or print letters, characters or images fixation, strengthening brittle paper, biocidic protection, etc. The preservation (“deacidification”) water bath also can contain fixation, strengthening, and other conservation compounds.
Challenges of De-acidification
The deacidification of bound books presents additional challenges. The soaking of a book in an aqueous solution swells the paper sheets. This can cause some bindings to or the pages to become wavy. In addition, the drying of a bound volume, while preserving its flatness, is more difficult and time-consuming. Accordingly, some libraries and deacidification services have instituted rebinding of books as a routine practice associated with acidification. Such practices increase the complexity and cost of deacidification of books. Thus, the most widely used deacidification methods that have been applied to whole books have been based on non-aqueous treatments, of which there are several competing systems. These have become known as mass-deacidification programs.
Acidic Paper and the More Rapid Degradation of Books
People have been concerned about the premature degradation of stored books and the poor quality of the paper for a long time. Gradually it became known that acidity is a key factor in shortening the useful life of books. The same period, the typical pH values of manufactured books became lower than 6 and continued to decrease to a range between 4.5 and about 5. A correlation between pH and the rate of strength loss was found for both accelerated and natural aging tests of the same books. Books from previous centuries have been found to remain in relatively good condition, and such findings were generally correlated to higher pH values Especially low values of pH happened to be measured for samples manufactured in the 1960s and 1970s. But the situation was very different for the samples that had been manufactured in the 1980s and 1990s, for which the average pH of the paper was near neutral and the number of double-folds before breakage was dramatically higher. Not only were those relatively recent samples newer, but they also were not degrading as fast. What happened, in order to bring about that change, was almost certainly the emergence of alkaline papermaking practices, which in just a few years has come to be dominant for the production of printing papers.
In addition to acidity present in paper at the time of its manufacture, acids are formed continually both in acidic and alkaline cellulose material due to hydrolysis or oxidation reactions, which will be described later. Also, some acidity accumulates through absorption of pollutants from the air, particularly in urban areas. However, the detailed effects are different, depending on whether the paper’s microstructure is acidic or alkaline: The acids arising in the alkaline part of the microstructure, which contains alkaline reserve, presumably can be continually neutralized. The degradation in the alkaline paper continues very slowly, and most probably the effects of the likely predominant peeling reaction can be neglected. Such a process would result in a negligible difference of the DP, longevity, brittleness, folding endurance, usability, and other mechanical and chemical quantities of paper or books. Another alkaline degradation mechanism, the beta-elimination reaction, could be detected in the deacidified book papers. However, it had little influence on the molar mass of cellulose in the paper compared to the beneficial effects obtained by deacidification treatment.
It is also important to keep in mind that acids are formed continually in paper by oxidation with oxygen, ozone, nitrogen, sulfur oxides, and other oxidants, in both acidic and alkaline cellulosic material, including paper and books during the whole paper lifetime . Acids arising from oxidation presumably can be continually neutralized by alkaline reserve, assuming that contact is achieved locally between the alkaline and acidic substances. However, the neutralization of acids does not stop their formation. Continuing generation of acidic species in paper might help to explain observations of a gradual drift to lower surface pH values over time.
To provide context for later discussion concerning non-aqueous dispersion-type deacidification, some points about deacidification processes in general will be summarized here. Such processes will be roughly divided as aqueous systems, non-aqueous systems involving dissolved alkali species, and non-aqueous systems involving suspensions of particles, i.e. non-aqueous dispersion systems.
In addition to direct neutralization of acidic species present in the paper, aqueous treatments (as in the case of non-aqueous treatments) can be used to leave behind an alkaline reserve. In other words, the paper is provided with a capacity to consume newly-formed acidity or acidic compounds that may reach the paper through the air or due to reactions within the material. As an example, a compound such as sodium bicarbonate may precipitate out of solution and remain in the paper when it is dried after the treatment. Another potential advantage of aqueous treatments is that they can be used to wash the paper free of dirt, as well to remove products of hydrolytic decomposition, which may include acidic compounds .
Certain potentially unfavorable aspects of aqueous-based deacidification appear to have provided the main motivation for widespread adoption of non-aqueous deacidification systems. For instance, if a book is dropped into a sink of water, the paper can become wavy and a lengthy drying period may be need. Each individual page tends to expand when it is wetted by water, increasing its thickness. Such swelling may exceed the binder’s capacity, causing the book to break . When deacidifying individual sheets of paper documents, such development of waviness can be avoided by drying the sheets individually against a smooth surface or by other means of restraint during drying . In the case of books, freeze-drying has been used successfully to avoid deformation of pages . To overcome these issues concerning the aqueous treatment of bound books, it is common practice to remove and replace the original binder for the book, allowing the pages to be treated and dried individually. Advantages of this approach include optional washing of the pages, the ability to directly neutralize acidic groups present in the material, the ability to apply pH-buffering agents and/or alkaline particles, the ability to treat the paper with reducing agents such as borohydride, and the opportunity to apply dissolved starch or other hydrophilic polymers able to increase the strength of the paper. The down-side of such an approach is that it requires a considerable expense and time to disassemble and reassemble each treated book.
Other concerns about aqueous treatment include possible bleeding or dissolution of water-susceptible print colorants a tendency of paper to become more bulky (thicker pages) after a cycle of wetting and drying and the sticking together of pages after redrying of bound volumes, especially if the paper had been coated during its manufacture . For all of these reasons, when bound volumes need to be acidified, non-aqueous deacidification programs or treatments involving freeze drying processes generally have been selected.