The identification of synthetic organic red pigments in historical plastics: Developing an in situ analytical protocol based on Raman microscopy

Abstract

The identification of colorants in historic plastics is a methodological and analytical challenge. Although deformulation is performed by the plastics industry, in the case of historical objects, sampling is often impossible, and in situ protocols are needed. The accurate identification of colorants provides insights into historical plastic formulations, supports planning of conservation studies, and critical data for objects that already exhibit color change (e.g. fading). Indeed, colorants may degrade following exposure to light, and color changes have been reported for plastic objects. The analysis of colorants remains a challenge, and microsampling is usually required. Successful identification of red pigments from thirty historical plastics is reported following a new in situ analytical protocol based on Raman microscopy. Information about other pigments, fillers and plastic polymers are reported. Fading has been observed in the historical plastic objects containing Pigment Red 53.

Discoloration of Historical Plastic Objects: New Insight into the Degradation of β-Naphthol Pigment Lakes

“…Nevertheless, it is important to mention that titanium dioxide, zinc sulfide, iron oxide-based pigments, and inorganic additives such as fillers (i.e., calcium carbonate, barium sulphate, silica) could have played a role in the plastic discoloration (Supplementary Table S1) [41]. Extensive literature is available concerning the photosensitizing and photoprotective influence of the two crystalline modifications of titanium dioxide (rutile and anatase) on the polymer degradation, including polyolefins [43,44,[119][120][121][122], while little research was conducted on the photoactivity of the other inorganic compounds in polymeric systems [43,44,121].…”

“…The photodegradation of polymer resins and its contribution to discoloration (yellowing and darkening) has been widely studied [ 34 , 35 , 36 , 37 , 38 ], while only recently, the fading of organic pigments has been evaluated in conservation science, focusing mainly on the identification of the faded organic pigments in historical plastic objects [ 39 , 40 , 41 ]. Although few key research studies from the polymer science community discussed the degradation of pigmented polymers [ 42 , 43 , 44 ], they were not comprehensive enough in describing the photo-fading mechanisms of the organic pigments.…”

“…The study of the organic pigments and their fading phenomena in historical plastics represents a methodological and analytical challenge, firstly because of technical difficulties in their identification [ 41 ], secondly due to the complexity of their degradation pathways [ 43 ], and thirdly due to the often-unknown exposure (duration and intensity) to the agents of degradation (environmental factors). Once the decay mechanism starts, it is usually irreversible, and the pigment modification results in a chemical change of the molecule’s pigment structure that causes a color shift or loss.…”

“…They were one of the first pigment categories used in the coloring of plastics [ 54 , 55 ] and the subject of relevant publications [ 55 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 ]. In historical collections, problems related to their fading in printing inks [ 68 ], modern and contemporary paintings [ 69 ], as well as plastic artifacts [ 39 , 41 ] are well-documented. Nevertheless, little is known about their photochemistry in cultural heritage collections because few studies have focused on their degradation [ 70 , 71 , 72 ].…”

Applications of Raman spectroscopy in art and archaeology

“…Portuguese everyday plastic objects presumably produced between the 1950s and the 1970s were studied by Angelin et al [ 22 ] identifying their red colourants. Polyethylene, polystyrene and polypropylene objects were studied using non‐destructive optical microscopy, confocal μ‐Raman spectroscopy (633‐ and 785‐nm excitations), ATR‐FTIR spectroscopy and energy‐dispersive XRF microspectroscopy.…”