Impact of accumulation of organic acids on the degradation of cellulose in historic paper

The acidity of historic paper, a property crucial for its preservation, is thought to mainly depend on the type of sizing. However, this research shows that during its degradation, paper acidity increases mainly due to the formation of non-volatile carboxylic acids, which accelerate acid-catalysed hydrolysis. Whether and how this accumulation depends on paper composition has not been studied systematically so far. A collection of 89 European paper samples, dated between 1844 and 1990 was analysed for organic acids content and other conservation-relevant properties. Oxalic acid was the acid determined in the highest concentrations and with the strongest influence on paper pH. Multivariate data analysis revealed correlations between lignin content and oxalic and formic acid contents, highlighting the dominant influence of lignin on acid production. The results also indicate that the content of oxalic acid in paper increases by approximately 0.01 mmol/g per year, resulting in a decrease of pH at 0.008 per year (assuming other variables remain constant). To assist in paper conservation, a portable, non-invasive Raman spectroscopic method was developed to determine the lignin content of historic paper, using partial least squares regression. The research results are significant as they demonstrate the dominant impact of oxalic acid on acidic paper conservation.