EFFECTS OF DILUTE CALCIUM WASHING TREATMENTS ON PAPER
JOHN BOGAARD, & PAUL M. WHITMORE
It is evident from these results that brief contact with very dilute calcium salt solutions is sufficient to introduce small amounts of calcium into the paper sheets, and in general these quantities have a beneficial effect on the chemical stability of the cellulose in the sheet. As we reported earlier (Whitmore and Bogaard 1994), the product distribution in the thermal aging of untreated filter paper is consistent with hydrolysis being the major cellulose degradation chemistry, and the present study demonstrates that the same holds true for the thermal aging of treated sheets. The stabilizing effects of calcium washing treatments must then derive from the effect of these calcium salt additions on the cellulose hydrolysis reaction.
It is well established (see, for example, Sharples 1971) that cellulose hydrolysis is an acid-catalyzed reaction. As it is supplied, Whatman no. 42 filter paper contains a small amount of cellulose carboxyl groups, and the acid washing during manufacture that removes the trace metals from the paper leaves these carboxyl groups in their acid form (bearing an H+). These are weak acids, so at equilibrium only a small fraction of the carboxyls have dissociated, freeing a proton (H+) that can act as a catalyst for the cellulose hydrolysis. Slowing the hydrolysis reaction is most likely achieved by reducing the amount of this free acidity in the sheet, and this reduction is the objective of neutralizing and deacidifying treatments.
The reduction of the free acid content of the sheet during the washing treatment involves a number of processes that can be collectively described as ion exchange (Helfferich 1962). First, free protons (liberated from their carboxyls) can be leached from the paper fiber into the treatment bath. As this happens, previously undissociated carboxyl groups will release their protons in an attempt to reestablish the equilibrium concentration of free acid. This process will continue until all of the bound acid has been freed and removed into the bath. Simultaneously with this process, cations from the treatment solution will enter the fiber in order to maintain a charge balance with the negative charges on the dissociated cellulose carboxyls. The rates of these ion transport processes are increased by an increased difference between ion concentrations in the fiber wall and the treatment bath, and for very dilute solutions the rates can be very slow. One can witness the slow withdrawal of paper acidity when performing a conventional cold extraction pH measurement of the paper with deionized water: it often takes an exceedingly long time to achieve a stable pH reading, for the acidity is only slowly drawn out of the fibers. It is clear that the use of very dilute conservation treatment baths, while intended to avoid risks, will suffer from the slow rates of the transport processes at these low concentrations.
As just noted, the transport processes can be accelerated by increasing the concentration difference between the fiber and the treatment solution. More alkaline solutions will have much smaller H+concentrations, and the driving force will be greater for releasing free acids from the fiber. Of course, alkaline solutions also may react with the paper acids within the fibers, creating neutralization products that may either diffuse out of the fiber or be retained in the sheet.
Increasing the ionic strength of the solution will also increase the rate of cation transport into the fiber, which will displace the free acidity into the solution more rapidly. This behavior is in fact the rationale for the recently revised practice for measuring cold extraction pH of a paper (Scallan 1990). Instead of using deionized water for extracting the paper acid, a relatively strong (0.1 N) solution of sodium chloride is used. The sodium ions in this salt bath displace the free and bound protons into the solution, thus allowing for much faster establishment of stable pH readings.
In the case of calcium salt treatment solutions, one other chemical process will occur: the reaction of calcium directly with the cellulose carboxyl groups. Unlike ions such as sodium, calcium ions bind relatively strongly to carboxyl anions. This characteristic is the origin of the comparatively poor water solubility of calcium salts of organic acids and the principle underlying water-softening ion exchange resins. In this case, the calcium cations that are introduced into the fiber from the treatment bath probably bind rapidly to dissociated carboxyl groups and displace the proton from undissociated carboxyls, speeding the reduction of paper acidity.
Divalent cations such as calcium may also provide a secondary benefit beyond their function during the acid transport processes. Calcium ions that bind to carboxyl ions will have only one of their two positive charges neutralized by the carboxyl group. In carboxyl-rich ion exchange resins, each calcium would bind two carboxyls, but in cellulose, even relatively oxidized cellulose, it is unlikely that many carboxyls will be in close enough proximity to each other for two carboxyls to bind a single calcium ion. Consequently, it is likely that the charge balance of the treated sheet is maintained by anions from the treatment solution binding to the calcium-carboxyl pairs. In this way, the treatment introduces a “reserve” of anions—alkaline species if the treatment bath was a solution of an alkaline calcium salt—that can act to neutralize a small amount of additional acidity.
The experimental results presented in this study are consistent with a description of the treatments working in this way. Despite the low concentrations of calcium salts and relatively brief contact times used, the treatments seemed to deliver a measurable small amount of calcium into the papers. However, the presence of the calcium alone is insufficient to stabilize the cellulose, for the calcium chloride—treated sheet was stabilized only to a very small degree, indicating only partial neutralization of the total acidity in the sheet. This inference is also supported by the pH data in table 1. The more alkaline bicarbonate and hydroxide treatments resulted in increased stability of the cellulose, but the effect of the bicarbonate treatment was not as long lasting as the hydroxide treatment, suggesting that the neutralization was more thorough in the latter. The long period of very slow hydrolysis for the hydroxide-treated papers is evidence of both its more efficient neutralization and the residual alkalinity that presumably accompanies calcium reaction with the carboxyls. Acid generation chemistries appear to be unaffected by these treatments, since the rate of scission production in treated sheets eventually matches that of the untreated papers. For the hydroxide-treated sheets, small “alkaline reserves,” which in this case would be on the order of parts per million concen-trations, are apparently sufficient to neutralize the additional acids produced for a period of oven aging. Hydroxide treatment of more acidic photo-oxidized filter paper appears to demonstrate only incomplete neutralization of paper acidity, probably due to the treatment times being insufficient for transporting such large amounts of material into and out of the fibers.
The case of the borohydride-calcium hydroxide two-step treatment is interesting because of the significant stabilization of the treated sheet. The boro-hydride reduction was intended primarily to reduce carbonyl groups that could be prone to reacting with the alkaline treatment bath, thus causing cellulose DP lowering during the treatment. However, oxidation on the cellulose chain is also known to increase the rate of hydrolysis (Whitmore and Bogaard 1995), which is why wood pulp hydrolyzes faster than cotton, and chemical reduction can decrease the rate back to that of an unoxidized cellulose (Rånby and Marchessault 1959). The data indicate that chemical reduction succeeded in reducing these risks of cellulose damage, but the resulting very slow hydrolysis rate is considerably lower than that of an unoxidized, untreated paper sheet, such as was shown in figure 1. It is possible that much of the observed effect is the result of contact with a high ionic strength alkaline bath (the sodium borohydride) followed by the exchange of residual sodium ions with calcium and its attendant alkaline anions.
Finally, the calcium hydroxide treatment seems to offer an unexpected additional benefit of slowing down the photochemical deterioration of the cellulose. The mechanism by which the alkaline calcium addition interferes with this degradation is not clear. Since calcium chloride treatments do not seem to affect the photochemical reaction rate, it is not likely that some purely physical effect, such as salt deposits that might reduce oxygen availability or increase the opacity, is at work. Some research (Arney and Chapdelaine 1981) has shown that the rate of atmospheric oxidation in a paper sheet can be slowed by reducing its acidity, and this result may be a similar effect. The product distributions of the untreated and treated sheets clearly demonstrate that oxidation of the cellulose is the dominant path for both. As was discussed in prior work (Whitmore and Bogaard 1994), cellulose oxidation proceeds via hydrogen atom abstraction from one of the carbon atoms on the glucose ring, and the observed product ratios probably reflect only the statistical nature of oxidant attack at various ring positions. Consequently, the observation of identical product ratios for untreated and treated sheets indicates merely that whatever its mode of action, the calcium addition is not affecting the relative reactivity of any particular ring position. Without a more detailed knowledge of the chemical species involved in the photochemical degradation reactions, the role of alkaline calcium salts in slowing those reactions will remain speculative. Nevertheless, the fact remains that the calcium hydroxide treatment seems to slow the photodegradation of the cellulose significantly.