THE DIFFUSION OF SULFUR DIOXIDE IN AIR THROUGH STACKED LAYERS OF PAPER
Michael A. Dimitroff, & James W. Lacksonen
The diffusion, absorption and reaction of various gases in paper can be important to both the degradation and preservation of paper objects. Some objects, such as books, can be thick compared to a single sheet of paper. The rate of penetration (by diffusion) of gases into thick paper objects has apparently not been studied as completely as absorption studies with single sheets, although early work by Jarrell, Hankins and Veitch1 did show the significance of different kinds of papers, humidity and metal ion additives in a 15 sheet stacked sample simulating a book.
Other researchers2, 3 used radioactive tracer techniques to measure low concentration absorption of sulfur dioxide in single sheets of 100% rag paper and 6% groundwood, 40% chemical wood printing paper. Sulfur dioxide concentrations from 5000 to 0.75 ppm were used. The conclusion was that humidity affected the short term rate of absorption but not the long term rate. A reversible equilibrium followed by irreversible effects was postulated to explain the results. A number of investigations3, 4, 5 have been made which studied the effects of catalytic and inhibitor additives. The rate of sulfur dioxide fixation was increased by copper and manganese; iron showed a low catalytic effect, perhaps because it was not present as a soluble salt. In a study4 of the combined effects of metallic catalysts and inhibitors (EDTA, sodium phosphates, 8-hydroxyquinoline sodium citrate and ammonium oxalate), the catalytic effects were greater than the inhibitor influence.
Studies made on the transport of water vapor through paper suggest7 that water vapor permeabilities are exponentially related to the relative humidity. The adsorption-desorption of water vapor was modelled as pseudo-first order kinetics.8 The rates were dependent on the history of the sample.
It appears that the diffusion phenomena have not been specifically studied, with perhaps the exception of water vapor. The degradation of paper by acids is well-documented. With the present problems of air pollution, acid rain, etc., it seemed that a study of the acid gas sulfur dioxide would be of general interest, particularly to those concerned with large collections of thick paper objects often housed in metropolitan areas where air pollution can be most severe. Therefore, the sulfur dioxide-air system was selected for study.
The use of 0.5% to 1% sulfur dioxide in air is considered a “dilute” gas, although it is many times more concentrated than the few ppm typical of sulfur dioxide concentrations in city air. The higher concentration levels were dictated by limited sensitivity of the available gas chromatographic technique. Generally, low concentration processes are linearly dependent on concentration. This permits convenient extrapolation to lower values. In a sense, the “higher than normal” sulfur dioxide concentration used is akin to accelerated aging tests done on paper at elevated temperatures. Also, the objective of this phase of the work was to evaluate the effective diffusivity of sulfur dioxide at steady-state conditions. From previous studies in the area of gas diffusion,11 there is evidence to suggest that a 1% gas phase concentration can be considered as “dilute” and any further reduction in concentration should not provide any new or unusual diffusion phenomena.