Volume 19, Number 6-7
Dec 1995

Paprican's Dry Deacidification Method

U.S. Patent Number 5,433,827, entitled "Method for the Deacidification of Papers and Books," was issued on July 18, 1995. The inventors, who assigned the patent to the Pulp and Paper Research Institute of Canada (Paprican), are Derek H. Page, Anthony M. Scallan, Steven R. Middleton, and Xuejun Zou.

This deacidification method consists essentially of conditioning books and documents to a high relative humidity (97% works very well), interleaving them with paper containing calcium carbonate, and putting them under pressure for a few days or longer, up to three months or so. The carbonate sheets can be no more than six pages apart; the closer together they are, the more quickly the papers or book will be deacidified. The pressure can be anywhere from 0.1 psi to 50 psi. The greater the pressure, the speedier the process.

The process by which the carbonate paper affects the paper undergoing this nongaseous, nonliquid treatment is described as follows:

It has been known for some time that free acid in paper can migrate to paper in contact with it, under air dry conditions.... This occurs even when the acid is non-volatile, for example sulphuric acid.

The migration of ions in air-dry paper is also known from evidence of electrical conductivity. At 50% relative humidity, paper with a moisture content of about 6% can have an electrical conductivity several orders of magnitude higher than that of bone dry paper... this is attributed to the freedom of cations such as calcium, magnesium or sodium to migrate through the anionic, water-swollen fibers.

In a sheet of mechanical or chemical pulp, deacidification cannot be achieved simply by the migration of free acid. These pulps always contain acidic groups bound within the cell walls of the pulp, with counter ions associated with them. For deacidification to be achieved, and to meet the condition of electrical neutrality, hydrogen counter-ions must be replaced by other cations such as calcium, magnesium or sodium which must migrate into the sheet....

It is probable that migration of ions across the interface between papers is facilitated by the formation of a continuous pathway resulting from the condensation of water in small capillaries at the contact regions....

Since this transfer of ions does not take place at lower relative humidities, water obviously plays an important part.

The patent does not claim that an alkaline reserve is left in the paper, only that the acid content of the paper is reduced to an innocuous level, and that treated papers age much more slowly both under natural conditions (two months) and in the aging oven (e.g., 20 days at 80°C and 75% RH). However, among the 16 "examples" (brief reports of experiments using variations of the method) there is one that records a quintupling of calcium ions to a level of 94 milliequivalents per kilogram.

EXAMPLE 12

Samples of the unbleached kraft paper deacidified by contact with a commercial paper containing calcium carbonate and added calcium chloride as described in Example 8 [85% RH, 1 psi] were analyzed for sodium and calcium ions. As shown in Table II, below, deacidification was accompanied by an increase in the concentration of calcium ions thus demonstrating the migration of ions from one sheet of paper to another.

TABLE II

All of the papers in the examples are newsprint, unbleached kraft (like grocery bag paper) or CTMP--in other words, papers with a significant lignin content. This patent may be a logical outgrowth of the paper Xuejun Zou gave at the ARSAG Conference in Paris in May 1994: "The Role of Lignin in the Mechanical Permanence of Paper: Part II. Effect of Acid Groups." In this paper he and coauthor Norayr Gurnagul showed how paper with a high acid group and high lignin content can be stabilized for strength (though not necessarily for brightness) if the acidic groups are neutralized by treating the paper with sodium hydroxide, or washing the paper in tap water, or making the paper with calcium carbonate in the first place. This replaces the hydrogen ion in the acidic groups by a more benign metal ion, e.g., sodium or calcium. (Additional acid groups did form during aging, though.)

Comment

The obvious advantage of this method is that it is low-tech and inexpensive. It may be a good way to deacidify newspapers, if the original is to be kept after microfilming. It may prove useful for archival materials, especially in underdeveloped countries, or wherever labor is cheap, because it is a labor-intensive method. It is also a method that calls for a great deal of storage space, because so much material will be in the process of treatment for such a long time.

The method may be risky because it calls for operating conditions that invite mold growth. Any time paper is held in high humidity, with access to surrounding air, for longer than about 48 hours, mold can be expected. Some of the "examples" describe contact times of 40, 60 or 100 days at room temperature and elevated RH. Still, no mention of mold is made in the text.

Another drawback is that if this method is used for books, they may become distorted by the combination of inserted sheets and the pressure. Spine adhesive and sewing may break, even if the spine is allowed to go concave.

To reduce the thickness of the book, the patent says that one can dust the surface of the pages with precipitated calcium carbonate instead of inserting papers between them. Dusting them with sodium bicarbonate works even more quickly--24 hours, as opposed to several days. The authors may not have known that sodium compounds are not favored for stabilizing cellulose, perhaps because of their high pH. Richard D. Smith reports in his 1970 doctoral dissertation at the University of Chicago, The Nonaqueous Deacidification of Paper and Books, that he surveyed 20 American patents to see which compounds and chemicals were used for neutralizing and stabilizing cellulose esters. Sodium compounds were most often judged "unsatisfactory," while seven other compounds and chemicals--magnesium, calcium, barium, strontium, potassium, aluminum and zinc--were judged more satisfactory than sodium.

Paper that is pressed when its moisture content is high will be compacted and flattened. If the paper is coated, the pages may stick together. Colors may offset.

Neutral Salts as Facilitators

The section on the description of the invention has a couple of paragraphs in which the use of a neutral salt is advocated to facilitate measuring surface pH. (A chemistry professor who was an early subscriber to the Abbey Newsletter, the APA's sister publication, reported in April 1979 how adding 0.1 N KCl to the distilled water speeded up surface pH readings. The following two paragraphs explain this procedure at greater length.)

The progress of deacidification is evaluated in the following way. A sample of paper is dispersed in deionized water and the pH of the paper is measured according to the standard test of TAPPI (T 509 OM-88). Solid sodium chloride is then added so as to raise the average concentration of the liquor to 0.1 molar and the pH is remeasured (hereafter referred to as the "salt pH"). The acid content of the paper is then measured by titration of the same suspension with 0.01 molar sodium hydroxide. The amount of alkali required to reach pH 7.5 is used to calculate the acid content of the paper in the units of milliequivalents per kilogram of dry paper.

On occasion the surface pH of a paper is measured using the TAPPI test (T 529 OM-88) but using 0.1M sodium chloride instead of deionized water. However, even in the presence of salt, this measurement reads somewhat high and less reproducibly than the other tests--probably due to inadequate mixing of the liquid with the fibers. Nevertheless, it is a non-destructive test and is therefore frequently used by conservationists.

The presence of a neutral salt, such as sodium chloride as described above, is essential for accurate evaluation of the acidity of cellulosic fibers. In aqueous suspensions of fibers of low ionic strength, the hydrogen ions are much more concentrated inside the fiber walls than in the external liquor and the pH of the suspension as measured in the external liquor is erroneously high as a measure of the total acid present. The addition of sufficient salt makes the hydrogen ions more evenly distributed between the fiber walls and the external solution and this, in turn, leads to a realistic evaluation of acid content as determined by pH measurement and by titration. The use of salt in titrations of cellulose fibers was first suggested by Neale and Stringfellow in "The Determination of the Carboxylic Acid Group in Oxycelluloses" [1937], and it has been subsequently adopted by most workers. The importance of salt during titration or pH measurement is still not appreciated by all workers on the conservation of papers.

Electrolytes and soluble salts can also be put into the carbonate papers, where they speed up the transfer of ions. Figure 5 in the patent shows how calcium chloride can cut treatment time by 2/3, and sodium chloride and sodium bicarbonate can cut it by 5/6.

(From the Oct. 1995 Alkaline Paper Advocate)