The Presence and Role of Paramagnetic Impurities in the Degradation of Paper
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ABSTRACT Electron Spin Resonance spectroscopy has been used to carry out a systematic investigation of the paramagnetic impurities present in antique and modern paper. Up to eleven different paramagnetic centres, which include different transition ions (Fe+, Mn2รท,Cu2l), organic free radicals, and paramagnetic point defects have been detected, inconcentrations ranging from few ppm to several hundreds. Experiments performed on new and deteriorated samples strongly indicate that some of these impurities participate directly or are produced during the aging and degradation processes. They can be used as quality indicators or probes for quantitative monitoring of the degradation of paper. INTRODUCTION
Paper is a composite material which, beside cellulose and water, may contain substantial amounts of hemicellulose and lignin plus various organic and inorganic impurities and additives. Previous work1-3 has shown that paramagnetic impurities, including transition metal ions, organic free radicals and paramagnetic defect centers, are commonly present in paper and may act as catalysts and initiators, playing a key role inthe degradation process.4 5 This latter is a complex phenomenon which consists, essentially, in the more or less extended reactions. 67 depolymerization of the cellulose chain, caused by hydrolytic and/or oxidative 2 The degradation is always accompanied by a severe loss of bound water and leads to the decay of the mechanical properties of the original material.The details of the degradation
reactions sequence may follow many different paths and depend also on environmental factors such as pH, humidity, temperature, exposure to daylight or artificial irradiation. For these reasons we have carried out a systematic Electron Spin Resonance (ESR) investigation on the nature and origin of the paramagnetic impurities in modern and antique paper, both well preserved and naturally or artificially aged. The purpose of this work is threefold. The first is to give a phenomenological description of the origin and nature of all the possible paramagnetic centres which can be found in paper. The second is to correlate the quality or conservation state of the sample to the presence of specific paramagnetic centres. The third and long term aim of the work is to formulate effective and feasible procedures for slowing down the degradation process and improve the conservation procedures. EXPERIMENTAL Samples of antique paper in various states of conservation and dating from the XV to the XVIII century have been obtained from old books coming from various areas in Europe. Modern paper is of commercial origin or has been prepared on purpose from bleached pine kraft, cotton linters, and groundwood spruce commercial pulps subjected to neutral or acidic sizing (hereafter these samples are referred to as KA, KN, LA, LN, WPA, and WPN). Artificial aging has been carried out under controlled conditions of temperature, relative humidity and irradiation. The spectra were recorded at ca. 100 Kusing a Varian X-band E-9 spectrometer. All s
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