The Impact of Humidity in the Thermal Ageing of Celluloid: An Inter-Scale Investigation

Abstract

The role of humidity on the ageing of celluloid is investigated by performing accelerated ageing tests on mock-up samples. At 70 °C, three levels of relative humidity (RH) are selected: 30%, 50%, and 70%. Samples are monitored for the macro- and micro- changes occurring through ageing to relate the visible modifications to the molecular ones. Infrared and Raman spectroscopy, microscopy, mass and contact angle measurements, profilometry, and colourimetry are combined for this purpose. While the ageing test at 30% RH results in a slight embrittlement of the samples and small spectral changes, the one at 50% RH induces significant modifications at the molecular level and the formation of cracks, while the one at 70% RH causes a fast deformation of the samples and the development of bubbles. Although quite diverse, such results prove to be related to the same chemical processes: denitration, chain scission, and oxidation. These occur more promptly or extensively based on humidity level, resulting in different outcomes. Beyond morphology and brittleness, macroscopic effects also involve mass loss, surface roughening, and yellowing. A possible correlation between the macro and micro modifications is present, highlighting the influence humidity has on the degradation process of celluloid.

Keywords: ATR-FTIR spectroscopy; Raman spectroscopy; ageing; celluloid; cultural heritage; electron microscopy.

Figure 1

For celluloid samples aged at 70 °C and 30%, 50%, and 70% RH, the percentage mass loss (a) and b* colourimetric values (b) are plotted versus the time of ageing. No colourimetric measurements were performed for the ageing at 70% RH. In (b), the b* value collected on the 41st day of 50% RH ageing is indicated with a star (*), since it refers to the sample used for contact angle measurements. In (c), optical micrographs of the most significant morphological changes are displayed. The days of ageing are reported in dark yellow, while the RH% used for the ageing is written in light blue. For the 50% RH ageing, a detail of the cracks is shown for the 27th day, while for the ageing at a higher humidity, samples of the unaged material and that aged for one week are also shown on the edge.

Figure 2

(a) SEM micrographs acquired on the section of a sample aged at 70 °C and 50% RH for 27 days (lower magnification on the left and zoomed detail of the red framed area on the right); (b) schematic of the bubbles formed upon ageing at 70 °C and 70% RH for 7 days and SEM micrographs of the inner and external surfaces; (c) profilometry maps acquired on the unaged material and on celluloid aged at 70 °C and 50% RH for 13 days; (d) plots of arithmetic mean roughness (Ra) and arithmetic mean waviness (Wa) values registered after different time intervals for samples subjected to the ageing experiments at 70 °C and 30%, 50%, and 70% RH.

Figure 3

ATR-FTIR (a) and µ-Raman (b) spectra collected on the pristine material (black) and on samples aged for 41 days at 70 °C and 50% RH (purple). The wavenumbers of the discussed signals are reported in black. For the ATR-FTIR spectra, the signals considered for intensity ratios are coloured according to the corresponding functional groups highlighted in the chemical structure of cellulose nitrate and camphor. Moreover, a detailed view of the low-frequency region is shown in a box.

Figure 4

µ-ATR-FTIR (a) and µ-Raman (c) representative spectra collected through the section of bubbles formed on celluloid after 7 days of ageing at 70 °C and 70% RH. The approximate position of the analyses on the bubble section is shown in (b), with colours and numbers corresponding to those of the related spectra.

Figure 5

Intensity ratios calculated for representative ATR-FTIR signals on the surfaces of samples aged at 70 °C and 30%, 50%, and 70% RH over time (a) and through the section of bubbles formed after 7 days of ageing at 70 °C and 70% RH based on the arbitrary units assigned to depth (see Figure 4b) (b).