Polyurethane coatings in 20th century outdoor painted sculptures: discrimination of major subgroups 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, for instance two package solvent-borne, two package water-borne, one package water-borne and fluoropolymer polyurethanes. The present research aims to provide to the conservation professionals a better understanding of the versatility and diversity of PU coatings through compositional information and to outline markers helpful to differentiate the major PU subgroups from OPS by means of ATR-FTIR spectroscopy. The ATR-FTIR study conducted on a wide range of PU reference materials from the Getty Conservation Institute (GCI) reference collection highlights the relevance of this routine analytical method to discriminate certain subgroups of PU coatings. Indeed, by investigating well-known specimen it was possible to outline diagnostic FTIR features for three specific systems; fluoropolymer PU, one package water-borne PU made from acrylic latexes and two package water-borne PU prepared with PU dispersions. Furthermore, the FTIR measurements performed on various activators and co-reactants emphasizedthesignificantcontributionofthepolyisocyanateabsorptionsinthespectraofthe activated two package PU systems. However, the results obtained for various unmodified and water dispersible HDI polyisocyanate activators showed that the FTIR-ATR technique does not allow the discrimination within both types

Polyurethane coatings in 20th century outdoor painted sculptures. Part I: Comparative study of various systems by means of ATR-FTIR spectroscopy

peer reviewedLike 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

Local Treatment for Monochrome Outdoor Painted Metal Sculptures: Assessing the suitability of conservation paints for retouching

peer reviewedWhen outdoor painted sculptures get chipped, scratched or abraded, conservators might consider local retouching treatments as an option that would protect the exposed metal substrate and restore the aesthetic integrity, thus postponing a very costly and invasive overall repainting. Unfortunately, matching colour gloss and texture on large monochrome surfaces is always challenging. This paper reports on research undertaken to investigate some of the materials and application techniques that could be used to improve the matching of local areas of inpainting as part of a broader maintenance strategy for painted works in sculpture parks, and discusses some of the advantages and disadvantages of working with both industrial products and conservation materials. Previous work by the lead author investigated the use of industrial paints to retouch monochrome painted metal sculptures, using Claes Oldenburg’s Trowel (1971) from the Kröller-Müller Museum as a case study. Several industrial products were tested and for Trowel it was found that using a paint of the same type as the original, manipulated to modify its gloss and colour, gave the best results. However, the original paint is not always available on the market because of product discontinuation or paint formulations changing over the years, and so research on alternative products was carried out at the Getty Conservation Institute. Instead of investigating industrial paint systems, a range of conservation paints and products that are usually employed in indoor applications were tested. Although these materials are unlikely to match the durability of industrial products in outdoor settings, they offer a number of advantages that might make them appropriate for temporary treatments, including a better workability for colour and gloss adjustments, a generally easier application procedure, better availability in small quantities, and a lower cost than their industrial counterparts. For the present study, a matte industrial paint that was recently developed as a coating for Alexander Calder sculptures was used as the target surface. Various conservation retouching paints were selected and used to retouch mock-ups prepared with these matte industrial paints, which were damaged with scratches and other mechanical means to reproduce typical damage to painted sculpture. The simulated scratches and losses were first primed and filled using materials that were selected in consultation with conservators and the paint industry. Since the composition of some of these products was unclear they were analysed and determined. The retouching was then carried out as a last step, applying the paint with an airbrush and using an ‘over-the-edge spray technique’. Some of the retouching paints were modified following the advice of the manufacturer to improve workability or durability. The colour retention of the various products is currently being tested with artificial aging in a weatherometer; the adhesion of the local treatment and the compatibility with the overall paint will be tested with the mock-ups placed outdoors for an extended period of time. Preliminary results show that the workability of some of the products tested is satisfying and that the retouching, when carried out properly, reintegrates both the paint coherence and the visual aspect of the artwork

Polyurethane coatings in 20th 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

A Carbon Nanotube Optical Sensor Reports Nuclear Entry via a Noncanonical Pathway

Single-walled carbon nanotubes are of interest in biomedicine for imaging and molecular sensing applications and as shuttles for various cargos such as chemotherapeutic drugs, peptides, proteins, and oligonucleotides. Carbon nanotube surface chemistry can be modulated for subcellular targeting while preserving photoluminescence for label-free visualization in complex biological environments, making them attractive materials for such studies. The cell nucleus is a potential target for many pathologies including cancer and infectious diseases. Understanding mechanisms of nanomaterial delivery to the nucleus may facilitate diagnostics, drug development, and gene-editing tools. Currently, there are no systematic studies to understand how these nanomaterials gain access to the nucleus. Herein, we developed a carbon nanotube based hybrid material that elucidate a distinct mechanism of nuclear translocation of a nanomaterial in cultured cells. We developed a nuclear-targeted probe via cloaking photoluminescent single-walled carbon nanotubes in a guanidinium-functionalized helical polycarbodiimide. We found that the nuclear entry of the nanotubes was mediated by the import receptor importin β without the aid of importin α and not by the more common importin α/β pathway. Additionally, the nanotube photoluminescence exhibited distinct red-shifting upon entry to the nucleus, potentially functioning as a reporter of the importin β-mediated nuclear transport process. This work delineates a noncanonical mechanism for nanomaterial delivery to the nucleus and provides a reporter for the study of nucleus-related pathologies

Single-Chirality Near-Infrared Carbon Nanotube Sub-Cellular Imaging and FRET Probes

Applications of single-walled carbon nanotubes (SWCNTs) in bioimaging and biosensing have been limited by difficulties with isolating single-chirality nanotube preparations with desired functionalities. Unique optical properties, such as multiple narrow near-infrared bands and several modes of signal transduction, including solvatochromism and FRET, are ideal for live cell/organism imaging and sensing applications. However, internanotube FRET has not been investigated in biological contexts. We developed single-chirality subcellular SWCNT imaging probes and investigated their internanotube FRET capabilities in live cells. To functionalize SWCNTs, we replaced the surfactant coating of aqueous two-phase extraction-sorted single-chirality nanotubes with helical polycarbodiimide polymers containing different functionalities. We achieved single-chirality SWCNT targeting of different subcellular structures, including the nucleus, to enable multiplexed imaging. We also targeted purified (6,5) and (7,6) chiralities to the same structures and observed internanotube FRET within these organelles. This work portends the use of single-chirality carbon nanotube optical probes for applications in biomedical research

A Carbon Nanotube Optical Sensor Reports Nuclear Entry <i>via</i> a Noncanonical Pathway

Single-walled carbon nanotubes are of interest in biomedicine for imaging and molecular sensing applications and as shuttles for various cargos such as chemotherapeutic drugs, peptides, proteins, and oligonucleotides. Carbon nanotube surface chemistry can be modulated for subcellular targeting while preserving photoluminescence for label-free visualization in complex biological environments, making them attractive materials for such studies. The cell nucleus is a potential target for many pathologies including cancer and infectious diseases. Understanding mechanisms of nanomaterial delivery to the nucleus may facilitate diagnostics, drug development, and gene-editing tools. Currently, there are no systematic studies to understand how these nanomaterials gain access to the nucleus. Herein, we developed a carbon nanotube based hybrid material that elucidate a distinct mechanism of nuclear translocation of a nanomaterial in cultured cells. We developed a nuclear-targeted probe <i>via</i> cloaking photoluminescent single-walled carbon nanotubes in a guanidinium-functionalized helical polycarbodiimide. We found that the nuclear entry of the nanotubes was mediated by the import receptor importin β without the aid of importin α and not by the more common importin α/β pathway. Additionally, the nanotube photoluminescence exhibited distinct red-shifting upon entry to the nucleus, potentially functioning as a reporter of the importin β-mediated nuclear transport process. This work delineates a noncanonical mechanism for nanomaterial delivery to the nucleus and provides a reporter for the study of nucleus-related pathologies