Mechanical Properties of Paints Research
In this research universal testing machine (UTM), dynamic mechanical analysis (DMA), micro- and nano-indentation techniques are deployed for the characterization of paint samples of different complexities, shapes and dimensions. By using a range of techniques, team members are able to perform a combination of static and dynamic testing to characterize time-dependent properties of paint materials, conditions of glass transitions, and residual stresses in surface layers.
With micro- and nano-indentation techniques, measurements on historical samples can be achieved—essential to understand the ageing processes of artistic materials. While macro techniques (UTM and DMA) will help to not only validate the results from small-scale engineering methods but also contribute to developing a more robust characterization of artists' materials.
Macro Testing Techniques: DMA and UTM
Mechanical testing is used to reveal information about a material's mechanical properties under dynamic or static forces. In the conservation field, mechanical tests are often used to determine whether a material is suitable for its intended purpose or whether specific environmental conditions have an impact on an object. This is done by determining different properties such as elasticity, tensile strength, elongation, hardness, and fracture toughness.
Conventional mechanical characterization methods such as the DMA or UTM typically require the destruction of large samples. Having access to sufficient numbers of historical materials that satisfy this sample size requirement is impractical if not impossible to obtain when working with museum collections.
Accordingly, the generally accepted method for material characterization is to determine the physical behaviors of macro-sized mock-ups, which are prepared to mimic the composition and manufacturing process of historical materials and are often subjected to artificial aging. However, the appropriateness of recommendations for exhibition and storage conditions based on such research is often questioned, as the behavior of fresh materials can be an inaccurate representation of historical materials and artificial aging may not reflect the same complexity of interconnected physical and chemical processes as those occurring under natural aging. Also, mock-up systems can be simplified so the material in question can be treated as a lone entity, for example, varnish applied to a non-reactive substrate when there is known interaction with the paint layer such as movement of acids.
Micro- and Nano-Indentation Testing
To bridge this research gap, mechanical properties need to be determined directly from the historical materials in question. This can be done with micro- and nano-indentation. These techniques provide the means for mechanical analysis of conservator-prepared cross-sections from historical objects. In addition, these small-scale engineering methods can potentially address two significant gaps in the mechanical characterization process of museum objects: the distribution of mechanical properties across a typical collection of a certain age, and the aging factor which alters the mechanical properties of different artists' materials.
Nano-indentation has become a well-established technique for mechanical characterization of materials. Although most successful for homogeneous, non-viscous media, this technique can also provide useful information about a diverse range of materials. Recently, nano-indentation has been successfully applied in the characterization of historical painting materials, demonstrating its capability of consistent measurements on sub-millimeter paint samples (Salvant, Barthel, and Menu 2011; Wright et al., 2014). These results signal the potential of nano-indentation to mechanically characterize a wider set of historical materials, which is essential for supporting and furthering conservation practice and the development of predictive models.
As micro- and nano-indentation technology have only recently begun to be used for researching historical materials, research at the GCI is focused on the selection and optimization of the measurement protocol.
As a first step, the mechanical properties of cross-sectional and free-film TiO2 acrylic-based paint samples were compared. Systematic measurements at the micro- and nano-scales were performed to evaluate the influence of sample size, compliance of the embedding resin (edge effect), and sample polishing and cleaning procedures on the sample.
Mechanical parameters such as reduced modulus and hardness were obtained for paint cross-sections and compared with results obtained with non-embedded material. The tests clearly demonstrate the feasibility for the routine application of micro- and nano-indentation techniques to characterize preexisting cross-sectional samples of historical paints, binding media, and polymer coatings and films. But a proper protocol must be elaborated and the geometrical effects of the sample must be taken into consideration. Restrictions for mechanical characterization using nano-indentation results from the properties of existing samples (e.g., material type, embedding method, surface polishing) rather than limitations of the measuring technique itself.
Subsequent research at the GCI seeks to extend the measurement program to investigate correlations between the nano-, micro- and macro-properties for specific classes of artistic materials. Establishing such a relationship would make accessible the use of nano-indentation data within existing predictive models of material behavior and predictive tools built specifically for the conservation field.
Page updated: June 2017