Mechanical characterization of a cross-sectional TiO₂ acrylic-based paint by nano-indentation

ABSTRACT

The mechanical response of an embedded acrylic-based paint was analyzed using quasi-static and dynamic nano-indentation. The aim of this study was to propose and evaluate a methodology for selecting indentation test parameters for characterizing the mechanical properties of cross-sectional paint samples, particularly with respect to the inherent issues of embedding materials. The experimental methodology measured the reduced modulus (E_r), dynamic reduced storage modulus ($E_{\mathrm{r}}^{\mathrm{\prime }}$), and creep (C_IT) of cross-sectional paint samples and compared the results with those obtained for a reference free-film paint sample. Although nano-indentation testing on cross-sectional paint samples has been conducted previously, a systematic approach focusing on the effects of sample size and compliance of the embedding material has yet to be introduced to the conservation field. Nano-indentation was performed on two cross-sectional samples of TiO₂ acrylic paint of varying size to demonstrate the practical application and constraints of the technique.

RÉSUMÉ

La réponse mécanique d’une coupe transversale d’un film de peinture acrylique monté dans une résine a été analysée à l’aide de la nanoindentation quasi statique et dynamique. L’objectif de cette étude était de proposer et d’évaluer une méthodologie de sélection des paramètres d’essai d’indentation afin de caractériser les propriétés mécaniques de coupes transversales d’échantillons de peinture, en particulier en ce qui concerne les questions inhérentes aux matériaux dans lesquels les échantillons sont montés. Pour la méthodologie expérimentale, on a mesuré le module réduit (E_r), le module de conservation réduit en mode dynamique ($E_{\mathrm{r}}^{\mathrm{\prime }}$) et le fluage (C_IT) de coupes transversales d’échantillons de peinture, et on a comparé les résultats avec ceux obtenus pour un échantillon témoin d’un film de peinture non monté. Bien que des essais de nanoindentation sur des coupes transversales d’échantillons de peinture aient été effectués dans le passé, une approche systématique, axée sur les effets de la taille de l’échantillon et la conformité du matériel de montage, n’a pas encore été mise en œuvre dans le domaine de la conservation. La nanoindentation a été réalisée sur deux coupes transversales d’échantillons de peinture acrylique au blanc de titane (TiO₂) de taille différente pour démontrer l’application pratique et les contraintes de cette technique. Traduit par Élisabeth Forest.

RESUMO

A resposta mecânica de uma tinta à base de acrílico incorporada foi analisada usando nanoindentação dinâmica e quase-estática. O objetivo deste estudo foi propor e avaliar uma metodologia para a seleção de parâmetros de teste de indentação para a caracterização das propriedades mecânicas de amostras de tintas de seções transversais, particularmente no que diz respeito às questões inerentes à incorporação de materiais. A metodologia experimental mediu o módulo reduzido (E _r), módulo de armazenamento dinâmico reduzido ($E_{\mathrm{r}}^{\mathrm{\prime }}$) e fluência (C_IT) de amostras de seções transversais de tintas e comparou os resultados com os obtidos de uma amostra de tinta de referência sem película. Embora os testes de nanoindentação em amostras de seções transversais de tintas tenham sido realizados anteriormente, uma abordagem sistemática com foco nos efeitos do tamanho da amostra e na conformidade do material incorporado ainda não foi introduzida no campo de conservação. Nanoindentação foi realizada em duas amostras de seções transversais de tinta acrílica de TiO₂ de tamanho variável para demonstrar a aplicação prática e as restrições da técnica. Traduzido por Millar Schisler.

RESUMEN

La respuesta mecánica de una pintura con base acrílica incrustada se analizó mediante nano-indentación cuasi-estática y dinámica. El objetivo de este estudio fue proponer y evaluar una metodología para seleccionar los parámetros de prueba de indentación para caracterizar las propiedades mecánicas de las muestras de pintura de sección transversal, en particular con respecto a los problemas inherentes a la inclusión de materiales. La metodología experimental midió el módulo reducido (E _r), el módulo de almacenamiento dinámico reducido ($E_{\mathrm{r}}^{\mathrm{\prime }}$) y la fluencia (C_IT) de muestras de pintura de sección transversal y comparó los resultados con los obtenidos para una muestra de pintura de película de referencia libre. Aunque las pruebas de nano-indentación en muestras de pintura de sección transversal se han realizado anteriormente, aún no se ha introducido en el campo de la conservación un enfoque sistemático que se centre en los efectos del tamaño de la muestra y el cumplimiento del material de incrustación. La nano-indentación se realizó en dos muestras transversales de pintura acrílica TiO₂ de tamaño variable para demostrar la aplicación práctica y las limitaciones de la técnica. Traducción Amparo Rueda.

KEYWORDS:

• Nano-indentation
• acrylic
• viscoelastic
• cross-section
• mechanical properties

Acknowledgments

The authors would like to acknowledge the support of The Getty Conservation Institute’s Managing Collections Environments Initiative.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Ashley Freeman

Ashley Freeman is currently working on her PhD at the Norwegian University of Science and Technology (NTNU-Trondheim). Her research is part of the Sustainable Management of heritage Buildings in a Long-term perspective (SyMBoL) project and focuses on the physical behavior of distemper paint from Norwegian stave churches. Prior to her studies at NTNU, Freeman was the research lab associate (2016–2018) and graduate intern (2015–2016) for the Managing Collection Environments Initiative, at the Getty Conservation Institute. She holds a MAC in Conservation Science from Queen's University, a study certificate for restoration and conservation from the Lorenzo de’ Medici, and a MS in Chemistry from Loyola University Chicago. Address: The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049-1684, USA. Email: [email protected]

Michal Łukomski

Michal Łukomski is head of Preventive Conservation research, which assesses the effects of environmental conditions and lighting on museum objects. He received his PhD in physics from the Jagiellonian University in Krakow, Poland, in 2003 and completed his postdoctorate fellowship at the University of Windsor in Canada. For the last several years, he has worked on describing quantitatively the response of hygroscopic materials relevant to collections of fine and decorative art, in particular wood, textiles, animal glue, gesso and paints to variations of climate conditions, using several scientific methods. His current area of research focuses on the mechanical characterization of historic materials and their response to changes of environmental parameters, as well as investigation of painted surfaces by advanced optical techniques. Address: As for Freeman. Email: [email protected]

Vincent Beltran

Vincent Beltran joined GCI Science in 2002. He has been an active participant in a range of projects including the mechanical characterization of historic materials, evaluations of packing case performance during transport, the advancement of microfading tester practice in the conservation field, and assessments of environmental management systems in hot and humid climates. He is also a co-author of Environmental Management for Collections: Alternative Conservation Strategies for Hot and Humid Climates. He holds a BS in General Chemistry from the University of California, Los Angeles and an MS in Oceanography (Geochemistry) from the University of Hawaii at Manoa. Address: As for Freeman. Email: [email protected]