In the accelerated aging of paper at various temperatures and relative humidity values, a variable almost universally overlooked, or ignored, is that the vapor pressure of oxygen in the aging atmosphere is not constant. A simple example will illustrate this situation. Let us assume that we wish to age paper at 90°C at relative humidity values of 2, 17, 58, 78 and 100%. The data are given in Table 1.
The vapor pressure of water at 90°C is 525mm Hg. The relative humidity in the aging vessel is given in Column 1, and the vapor pressure of water at that relative humidity at 90°C is given in the second column. In the third column, we have subtracted the vapor pressure of water in the second column from 760, the nominal atmospheric pressure at sea level, to give the vapor pressure of oxygen, nitrogen and other gases in the atmosphere. The approximate composition of dry air is 21% oxygen, 78% nitrogen and one percent other gases. The data in columns 4, 5 and 6 in the table show the distribution of oxygen, nitrogen and other gases at the indicated relative humidity values.
The data show that the oxygen content of the aging atmosphere decreases substantially as the relative humidity increases. If one wishes to compare aging at room temperature with aging at higher temperatures, the oxygen content of the atmosphere must be adjusted. Otherwise, one is likely to obtain data that provide nice smooth curves for the next generation of scientists to discredit.
Data obtained in the Paper Section at the National Bureau of Standards (now NIST) showed that oxidation played a substantial role in the aging of lignin-free wood pulp paper. My opinion is that if lignin were present, the role of oxidation would be still greater. I am aware that phenolic groups in lignin may act as antioxidants in the degradation of cellulose. But what happens to the lignin? What degradation products are produced? And what might be the role of these degradation products in the long-term aging of paper?
Vapor pressurea of water, oxygen and nitrogen in aging atmospheres at 90°C
|Col. 1||Col. 2||Col. 3||Col. 4 Col. 5 Col. 6|
|RH in aging vessel||V.P. water in aging vessel||760 minus Column 2b||Vapor pressure of|
a. These are ball-park figures. Atmospheric pressure seldom is exactly 760 mm and the influence of temperature is ignored.
b. Total vapor pressure of oxygen, nitrogen and other gases in aging vessel.