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

• Published in: Journal of Raman spectroscopy Vol. 52; no. 1; pp. 145 - 158
• Main Authors: Angelin, Eva Mariasole, França de Sá, Susana, Picollo, Marcello, Nevin, Austin, Callapez, Maria Elvira, Melo, Maria João
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01-01-2021
• Subjects: Color; colorant identification; Coloring agents; coloring technology; Fading; historical plastic objects; Mathematical analysis; Microscopy; organic red pigments; Pigments; Plastics; Plastics industry; Polymers; Raman microscopy; Red pigments
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.5985

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.
The successful use of Raman microscopy for the in situ identification of organic red pigments in historical plastic objects is demonstrated. An in situ multi‐analytical protocol for the study of organic red pigments is proposed. The novelty of the protocol lies on the analytical sequence of the techniques, combination of the information collected by elemental and vibrational analyses and final molecular identification of the organic red pigments by Raman.