Rachel Rivenc

Application of chemical and thermal analysis methods for studying cellulose ester plastics.

Cellulose acetate, developed about 100 years ago as a versatile, semisynthetic plastic material, is used in a variety of applications and is perhaps best known as the basis of photographic film stock. Objects made wholly or partly from cellulose acetate are an important part of modern and contemporary cultural heritage, particularly in museum collections. Given the potential instability of the material, however, it is imperative to understand the aging mechanisms and deterioration pathways of cellulose ester plastics to mitigate decomposition and formulate guidelines for storage, exhibition, and conservation. One important aspect of this process is the ability to fully characterize the plastic, because variations in composition affect its aging properties and ultimate stability. In this Account, we assess the potential of a range of analytical techniques for plastics made from cellulose acetate, cellulose propionate, and cellulose butyrate. Comprehensive characterization of cellulose ester plastics is best achieved by applying several complementary analytical techniques. Fourier-transform IR (FTIR) and Raman spectroscopy provide rapid means for basic characterization of plastic objects, which can be useful for quick, noninvasive screening of museum collections with portable instruments. Pyrolysis GC/MS is capable of differentiating the main types of cellulose ester polymers but also permits a richly detailed compositional analysis of additives. Thermal analysis techniques provide a wealth of compositional information and thermal behavior. Thermogravimetry (TG) allows for quantitative analysis of thermally stable volatile additives, and weight-difference curves offer a novel means for assessing oxidative stability. The mechanical response to temperature, such as the glass transition, can be measured with dynamic mechanical analysis (DMA), but results from other thermal analysis techniques such as TG, differential scanning calorimetry (DSC), and dynamic load thermomechanical analysis (DLTMA) are often required to more accurately interpret the results. The analytical results from this study form the basis for in-depth studies of works of art fabricated from cellulose acetate. These objects, which are particularly at risk when stored in tightly sealed containers (as is often the case with photographic film), warrant particular attention for conservation given their susceptibility toward sudden onset of deterioration.

Polyurethane coatings used in twentieth century outdoor painted sculptures. Part I: comparative study of various systems by means of ATR-FTIR spectroscopy

Like acrylics and alkyds, polyurethanes (PUs) represent an important class of industrial paints adopted by 20th and 21st artists; primarily by those creating outdoor painted sculptures (OPS). Because PU coatings offer a compromise between aesthetic and performance expectations, unachievable with other types of paints, they are commonly recognized as the most appropriate option for painted artworks intended for an outdoor setting. However, the PU class includes various systems and subgroups possessing very different properties. Through the FTIR-ATR study of 137 PU coating references from the Getty Conservation Institute (GCI) reference collection, this paper outlines the differences and the similarities existing, in terms of the composition, of two package solvent-borne, two package water-borne, one package water-borne and fluoropolymer polyurethanes. The comparison of the obtained FTIR-ATR results allowed determining markers helpful to discriminate specific PU subgroups by means of ATR-FTIR spectroscopy. The goals of this paper are to provide to the conservation professionals a better understanding of the versatility and diversity of PU coatings and to facilitate the identification of the various types initially used by the artists.

Polyurethane coatings in twentieth century outdoor painted sculptures. Part II: comparative study of four systems by means of Py-GC/MS

Because PU coatings offer a compromise between aesthetic and performance expectations, unachievable with other types of industrial paints, they are currently recognized as the most appropriate option to coat sculptures intended for an outdoor setting. However, the PU class includes various systems, such as two package solvent-borne, two package water-borne, one package water-borne and fluoropolymer polyurethanes, which possess very different properties. 115 reference samples of PU coatings were investigated by means of Py-GC/MS, in order to outline the differences and the similarities existing, in terms of composition, between the major PU systems used for creating as well as for conserving modern painted outdoor sculptures. The Py-GC/MS study of an extended number of reference samples showed that the composition of equivalent PU systems strongly varies depending on the product line and the manufacturer. Furthermore the comparison of all the produced pyrograms allowed defining characteristic marker compounds helpful to discriminate specific PU paint systems.

Coat of arms: Dovetailing the needs of outdoor sculpture and military assets to develop more durable and adaptable paint systems

The requirements for paints used on outdoor sculpture and military vehicles are similar: overall protection for the underlying substrate, and the ability to withstand prolonged outdoor exposure and recurring physical contact. Aesthetic requirements are different but stringent for both categories of coatings and include maintaining appearance over time. Building on prior work undertaken by the United States Department of Defense (DoD) Army Research Laboratory (ARL) to modify military coating systems into a matte black paint suitable for outdoor sculpture, a long-term interdisciplinary collaboration is under way between the ARL, the Getty Conservation Institute, Mack Art Conservation, and NCP Coatings, that aims to develop and evaluate a wider range of robust paint systems with novel attributes for both the DoD and outdoor painted sculptures. For the latter, this includes coatings with a broader range of gloss, formulated with a much more varied palette. New low-gloss coatings used on military assets were used to paint sculptures by Alexander Calder, Tony Smith, and Louise Nevelson, designed to an aesthetic standard based on approved colour and gloss levels by the relevant artists’ foundations and estates, and formulated to enhance durability. These new coatings offer the ability to match or meet a range of artists’ aesthetic preferences, and to use a variety of polymeric flattening agents to control gloss levels, reduce marring, and eliminate volatile and hazardous pollutants. The use of low molecular weight (LMW) resins provides 4–8 hour pot life to assist in the application process. Compared to current resins, the LMW systems reduce organic solvent levels, are low viscosity, and permit far easier application. In addition, the replacement of typical pigments with low solar absorbing (LSA) pigmentation provides higher reflection which ‘shields’ the resin/binder system and reduces degradation. A particular advantage to conservators from this collaboration is that the coatings on DoD assets provide a convenient indicator of the paints performance and durability. The outcome will be a unique coating system to provide enhanced durability and maintain the original coating properties for a period of 6–10 years.