Paper Degradation from Cellulose Chains to Cellulose Fibres: an experimental study

Degradation of paper initiates at the molecular level and propagates along several length scales, from the fibre and the network levels towards the macroscopic, sheet scale. In particular, the scission of cellulose molecular chains and the consequent reduction of Degree of Polymerisation (DP) is a key phenomenon influencing the decay of the mechanical properties of the material. From a conservation perspective, this decay in mechanical properties is a critical factor for the handleability of paper artefacts. By considering naturally-aged paper samples made between 1840 and 1960, the current study focuses on experimentally establishing the relation between the mechanical properties of cellulose fibres, DP and age. In paper degradation studies it is common to perform zero-span tensile tests to obtain an estimation for the tensile strength of paper fibres. This method is based on the extrapolation of a few small-span tensile tests to attain a theoretical zero-span tensile strength, under the assumption that the fibre-fibre bonds take a negligible part of the load with respect to the fibres. Despite that this is an intelligent strategy, it is based on over-simplifying assumptions. Therefore, the authors recently proposed a novel methodology for the direct measurement of both strength and elastic properties of single cellulose fibres, based on optical profilometry and Digital Image Correlation (DIC) [1]. This method is also employed in the current contribution in order to accurately assess the mechanical properties of single cellulose fibres extracted from historical paper samples. Additionally, the viscosity-average degree of polymerisation of the historical papers is measured. To this aim, small pieces of each naturally aged sample are dissolved in cupriethylenediamine (CED) and the viscosity of the dilute solution is measured using a rheometer. This rheometric measurement results in dynamic viscosity of the solution from which the intrinsic viscosity of cellulose and consequently the viscosity average DP is evaluated. Such accurate measurements of both the mechanical and chemical properties of naturally aged paper samples shed light on the changes of these properties with age, and allow for directly identifying the contributions of the underlying micro-scale constituents. The obtained insights can be useful for kinetic studies and multi-scale numerical models for paper degradation. [1] An in-depth insight on the mechanical response of cellulose fibres by means of optical profilometry techniques