Paper is often considered as a subject of study in forensic document examination since it remains as an integral part despite living in digital age. To distinguish between paper of the same type but of a different age, it is essential to know whether the degradation process of paper influences the possibility of differentiation between paper samples. It is known that the aging changes are visible after long time. There is still widespread use of paper for writing, record keeping or notes of reminders.
Artificial aging has been used to reproduce natural aging processes in an accelerated pace. Questioned documents were exposed to certain temperature in a well-defined manner in order to simulate an increased age. This may be used to study the aging processes or to date documents by reproducing their aging curve.
The study aimed to analyse degraded papers by spectroscopic techniques and evaluate the possibility of differentiation between artificially aged paper samples. Infrared and Raman spectroscopy are analytical tools applied for the structural analysis of paper and can also be used for studying the paper ageing process and its products as they play an important role in the kinetic examination of autocatalytic depolymerisation. Deterioration of paper is mostly due to the degradation of cellulose.
The most common reaction is considered to be the hydrolytic degradation of cellulose molecules, which is determined by the temperature, the acidity and the amount of moisture present in the paper. The second common deterioration process is the oxidative degradation of cellulose, followed by the third mechanism which is the process of thermal degradation.
During this ageing process of paper, crystallinity might be expected to change and, as a result of oxidation, carbonyl and carboxyl groups were formed, which were visible in the mid infrared. Certain peak intensities were reported to change in the opposite direction with changed in crystallinity and could therefore be rationed and used as a crystallinity index. Discrete changes in IR and Raman spectra of examined aged paper samples were visible.
Water molecules played an important role as a reactant. The oxidative degradation of cellulose was primarily induced by the presence of oxygen in the air. Oxidation of the cellulose polymer made the molecule more easily hydrolyzed. Free radicals generated by oxidation or by light could also cut the cellulose chain.
The oxidation of cellulose contributed to raising the concentration of acid in paper since photo-oxidation resulted in the oxidation of primary alcohol groups in the cellulose polymer to aldehydes and later to carboxylic acid species. Oxidation during thermally accelerated ageing had been brought about by the action of oxygen called auto oxidation. Oxidized cellulose usually degrades fast by acid hydrolysis or under alkaline conditions.
The technique of 2D-IR correlation analysis was employed to obtain spectral data which enabled to show aging profiles of paper samples. The degradation process was found to be systematically related to the temperature, assuming that other environmental conditions were constant. It allowed differentiation of the contribution of paper components, cellulose, and fillers like carbonates and kaolinite, to the paper aging process.
Zięba-Palus, J., Wesełucha-Birczyńska, A., Trzcińska, B., Kowalski, R., & Moskal, P. (2017). Analysis of degraded papers by infrared and Raman spectroscopy for forensic purposes. Journal of molecular structure, 1140, 154-162.