A diagnostic tool for assessing the conservation condition of cellulose nitrate and acetate in heritage collections: quantifying the degree of substitution by infrared spectroscopy

Abstract

Cellulose nitrate and acetate are materials at risk in heritage collections because it is not possible to predict the evolution of their conservation state over time. Knowing that the degree of substitution (DS) of these materials correlates with their state of conservation because the fundamental degradation mechanism is hydrolysis, in this work, DS was measured in historical objects and artworks. Infrared spectra were used to develop and optimize calibration curves for cellulose nitrate and acetate references that were next applied to calculate DS values of heritage objects. The extent of hydrolysis measured, with this tool, correlated well with the physical deterioration assessed through the sample hardness (Shore A) which was measured with a Durometer. Calibration curves were optimized in reference materials by Attenuated Total Reflectance (ATR-FTIR) and Micro Fourier Transform Infrared Spectroscopy (μFTIR). The DS values of the AC reference materials was previously calculated by nuclear magnetic resonance spectroscopy. The calibration curves were obtained plotting DS as a function of the ratio between a reference peak (which does not suffer relevant changes during degradation) and selected peaks that monitor the degradation for cellulose acetate and nitrate polymers (avoiding the interference of plasticizers). The reference peak for both was the COC stretching mode (νCOC). The probe peak was, for cellulose nitrate, the NO 2 asymmetric stretching (ν a NO 2) and, for cellulose acetate, the OH stretching mode (νOH). This ratio was then applied to calculate DS values of historical materials, in good and poor conservation condition; in situ by ATR, and in micro-samples collected from artworks by μFTIR. This selection comprises cinematographic and photographic films dated from the 1890s to the 1960s, and contemporary works of art made with cellulose acetate sheets by Portuguese artist José Escada dated from the 1960s. Finally, by comparison with the original estimated DS values, we show how this tool permits to define the state of degradation of these complex polymer matrixes. Thus, establishing the quantification of the DS as a novel tool to monitor the degradation of cellulose ester plastics, contributing in this way for a sustainable preservation of an irreplaceable heritage.

Novel Markers to Map and Quantify Degradation on Cellulose Nitrate-Based Heritage: at the Submicrometer Level Using Synchrotron UV-Visible Multispectral Luminescence

“…To slow down degradation, low-temperature storage is accepted by the conservation community 6 , but these solutions prohibit public access, are price-sensitive, have high energy costs and there are concerns about its effects on the physical stability and material lifetime 7,8 . Important insights on its degradation were achieved over the last decade , but a full elucidation of its mechanisms is still required to develop sustainable conservation strategies as in the NEMOSINE project, by developing a smart modular package with the main goal of energy-saving and extent conservation time [33][34][35] . Figure 2.…”

“…An 1899 celluloid year calendar with an advertisement to The Packer Manufacturing company in New York. § Degree of substitution calculated by µFTIR using the calibration curve described in Nunes et al 33 Synchrotron deep UV photoluminescence micro spectral-imaging to safeguard celluloid heritage. Celluloid nitrate absorbs and can be excited in the UV (Supplementary Fig.…”

“…In this work, the level of degradation assessed by the luminescent markers is compared with the degree of substitution (DS) of CN calculated by infrared spectroscopy, since DS decreases with ageing 20,26,28,29,33,41 . This decrease in DS in CN-based polymers, is a consequence of the main degradation mechanism, which occurs through homolytic scission of the nitrate groups and the glycosidic main chain 19,26 .…”

“…This decrease in DS in CN-based polymers, is a consequence of the main degradation mechanism, which occurs through homolytic scission of the nitrate groups and the glycosidic main chain 19,26 . In a pristine matrix used as support for cinematographic lms, the DS is expected to be ca 2.26 33,42 , and its decrease is a consequence of the scission of the nitrate group and its substitution by hydroxyl groups as depicted in Fig. 3.…”

“…3. This leads to an increase and broadening of the carbonyl band at 1740 cm − 1 alongside the increase of the OH absorption between 3700 and 3100 cm − 1 , in the infrared spectrum 33 .…”