Investigation into the use of spectrophotocolorimetry to quantify colour changes of passive samplers for the detection of acetic acid: an application to cellulose triacetate films affected by Vinegar Syndrome

Vinegar Syndrome (VS) is one of the first signs of degradation of cellulose triacetate (CTA) films [1], resulting in the release of acetic acid (AA) with a vinegar-like odour [2]. This process is spontaneous and autocatalytic: the deterioration of CTA films may also be extremely rapid, accelerating under certain thermo-hygrometric conditions. Currently, the most widely adopted strategy in CTA film archives is the periodic inspection of AA emissions from materials using colour-change passive samplers (e.g., Acid Detection (AD) strips® [3] or Dancheck strips® [4]). These passive samplers are usually placed in situ for short exposures (i.e., at least 24 hours). Then, by direct reading of the colour change compared to a standard scale, it is possible to isolate CTA films affected by VS from the unaffected ones. Although colour-change passive samplers are low-cost and user-friendly, they provide qualitative measures only, are sensitive to light and may present interferences with other acidic compounds. This contribution aims to objectively quantify the AA released from CTA films under VS as detected by means of colour-change passive samplers. Specifically, the UV-visible spectrophotocolorimetry (Konica Minolta spectrophotometer model CM-2600d) was used to measure the colorimetric coordinates (i.e., L = luminance, a = redness, b = yellowness) of the passive samplers before and after the exposure to different concentrations of AA in a controlled environment. The colour change in passive samplers, expressed in terms of ΔL, Δa, Δb, was associated with the known AA concentrations, and a calibration curve was derived. Thereafter, colour-change passive samplers were placed in a climate chamber together with artificially aged CTA films. The colour change of passive samplers was measured, and the outcomes were compared with the calibration curve to derive the AA concentration. In this way, ΔL, Δa, and Δb may be associated with different degradation stages, tracking the impact of environmental factors on CTA films. Finally, the same method was applied to assess the conservation state of several CTA films, stored in the cellars of the Centro Sperimentale di Cinematografia (CSC) in Rome. The quantitative measurement of AA concentrations in CTA film archives may be used to fine-tune the selection of collections in the early stages of VS, without losing the ease and speed of screening, even if many films coexist in the same place. This approach improves the assignment of conservation priorities for CTA films in relation to the VS occurrence. [1] Ahmad, I.R., et al., 2020. DOI: 10.1016/j.polymdegradstab.2019.109050. [2] Allen, N.S., et al., 1987. DOI: 10.1016/0141-3910(87)90038-3. [3] Image Permanence Institute. https://filmcare.org/ad_strips. [4] Dancan Cinema Services. https://dancan.dk/?page_id=1072.