Modern paints
The complexities of twentieth century synthetic polymer-based paints such as those based on acrylic emulsion copolymers render them a challenging research topic. Paints created for the industrial, house and artists’ paint markets include a wide range of pigments, binders, additives and diluents whose exact composition is proprietary. As a result, understanding the relationships between paint formulation and preservation issues is not particularly straightforward. Not only do formulations vary depending on their intended commercial market or application, but they also change in response to market demands influenced by improvements in technology as well as health and/or environmental legislation. As a consequence, changes are made to formulations on a regular basis.
The historical development, properties and conservation-related concerns about the use of acrylic emulsion paints by artists have been published in detail elsewhere [1]-[4]. Briefly, the polymeric composition of artists’ acrylic emulsion paints has evolved since their introduction to the market. The early formulations were based on a poly ethyl acrylate/methyl methactylate (pEA/MMA) copolymer and some Talens acrylic paints remain based on this [5]. In the 1980s, poly n-butyl methacrylate/methyl methacylate (pnBA/MMA) was widely substituted for pEA/MMA. In the late 2000s, a further change the base emulsion formulation took place with the incorporation of terpolymers and copolymers of 2-EHA by some artists’ paint manufacturers [6].
Surfactants are added to these paints for several purposes. For example, they can act as pigment dispersants, defoamers and emulsion stabilisers [7]. As films mature polyethoxylate type (PEO) non-ionic surfactants (primarily used as emulsion stabilisers) have been shown to segregate to the paint film /air interface [7]. The extent of segregation depends on a number of factors including: paint brand (base emulsion type), sample substrate, paint film thickness, pigment type, artists’ technique (including dilution and or additions of mediums and other materials), previous conservation treatments, display history (exposure to UV and visible light), environmental history (e.g. exposure to high temperatures), and humidity (particularly high relative humidity) [8],[9]. Surfactant levels can also vary due to the pigment types used. It has been noted that cadmium pigmented passages are less likely to have migrated surfactant layers on the paint surface, which may be due to an acceleration of photo-oxidative degradation [10].
Acrylic emulsion paints respond differently to traditional (oil-based) paints with respect to optical properties (such as gloss), physical properties (such as flexibility and softness), ageing, environmental conditions and conservation treatments [2]. Acrylic and other modern and contemporary paintings can also be more vulnerable because they are often large, unframed, unglazed and fewer coatings/varnishes are used by artists and conservators for the protection of these works. Current conservation practice requires sensitivity to the paint surface and properties, an understanding of the artists’ intention, awareness of the fact that these relatively young works of art may not yet have undergone conservation treatment, and that the removal/disturbance of original paint material should be minimised through the use of preventive conservation measures, appropriate conservation materials and minimal intervention where possible.
FTIR spectroscopy
The popularity of FTIR spectroscopy in conservation science lies in its convenience, relative affordability and long history as an analytical tool [11]. However, although some portable instruments have recently become available, most FTIR equipment requires removing samples from a work of art. Once acquired the samples can be reused, thereby rendering the technique ‘semi-destructive’. FTIR is suitable for bulk and surface analysis typically via transmission and ATR configurations, respectively. The surface sensitivity of ATR detection is on the order of micron(s).
FTIR spectroscopy has played a vital role in the routine analysis (identification) and research on modern paints [12]. It has proven useful in understanding the chemistry of artists’ acrylic emulsion paints for: identifying base polymer compositions; pigments and filler materials present in paints; evidence for migrated surfactant on various films; the impact of ageing and cleaning treatments on migrated surfactant; the conditions required for surfactant extraction and/or removal and in assessing the efficacy of soiling removal [13]. This has resulted in a significant body of information that has contributed to our understanding of the behaviour and potential vulnerability of acrylic paints. This in turn has led to the development and modification of lower-risk wet-cleaning systems for the removal of accumulated soiling from these paint films [4],[7],[11],[14],[15].
Two significant limitations of FTIR spectroscopy include the beam penetration depth and detection limits, which have thus far hindered the exploration the upper-most surfaces of these paint films. Characterising the more subtle changes in the surface of these paint films is essential, as this represents the most vulnerable portion of these paints, i.e., the interaction zone with the surrounding environment as well as being directly affected by conservation treatments.
XPS and NEXAFS
Both XPS (X-ray photoelectron spectroscopy) and NEXAFS (Near Edge X-Ray Absorption Fine Structure spectroscopy) have had much less application in Heritage Science than X-ray fluorescence (XRF), which is an X-ray spectroscopic tool particularly suitable for inorganic elemental analysis. In the context of modern and contemporary paint research, XPS and NEXAFS offer complementary benefits by broadening the scope of elemental analysis to include organic materials and by providing additional elemental chemical state information covering both the organic and inorganic regions of the periodic table. A summary of the typical sample requirements and analytical capabilities of the techniques is presented here and the reader is referred to excellent textbooks covering the techniques in detail [16],[17].
Samples must typically be between 0.5 – 1 cm in any direction and compatible with ultra-high vacuum (<10−7 mbar) although there are microfocusing or imaging systems that can accommodate smaller samples on the order of 100s of microns and instruments that permit higher pressures up to 10−4 mbar. Apart from the possibility of slight local damage by the X-ray beam, the techniques are non-destructive to the bulk of the sample, enabling samples to be reused for further research.
XPS provides elemental and chemical state (bonding) information by measuring subtle variations in the binding energy of a strongly bound core shell (as opposed to weakly bound valence shell) photoelectron ejected from an atom after absorption of an X-ray photon. The binding energy of the relevant core electrons is element specific, but differences in chemical bonding vary in the binding energy value by +/− 0.1 eV to a few eV. These chemical shifts depend primarily on the charge of the element in question, which is determined by the number and electronegativity of the atoms it is bound to and by its oxidation state. In practice, low resolution spectra of all elements with atomic number Z = 3-92 can be rapidly acquired via broadband survey spectra that are suitable for quantitative elemental analysis, but do not resolve all features associated with the chemical environment. For more detailed investigations of the chemical state, high resolution spectra are acquired. This makes XPS analysis suitable for a wide range of inorganic and organic materials.
The elements accessible with NEXAFS are restricted by the X-ray photon energies available at monochromated synchrotron radiation source end-stations. The NEXAFS spectra occur in the vicinity of the core electron binding energies probed by XPS. All elements have at least one K or L absorption edge suitable for NEXAFS in the photon energy range between 200 eV and 50,000 eV. The photon energy range is conventionally divided into the ‘soft’ and the ‘hard’ X-ray range. Soft photon energies, below 2000 eV, require the use of vacuum chambers and monochromatization technology based on gratings, while hard X-rays (energies above a few 1000 eV) penetrate air and are obtained using double crystal monochromators. The transition energy range between 1000 eV and 4000 eV is sometimes referred to as the ‘tender’ X-ray range.
Acrylic paints contain mostly light elements, including C, N, O, Ca, and Na, which have their NEXAFS spectra in the soft X-ray range. X-ray absorption at higher X-ray energies probes heavier elements in the period table. The acronym XANES (X-ray Absorption Near Edge Structure) is often used instead of NEXAFS to describe the spectra for these elements. Moreover, at higher X-ray energies, the photoelectron interference patterns known as EXAFS (Extended X-ray Absorption fine Structure) can be used to obtain structural information such as bond lengths. XANES and EXAFS are increasingly used in cultural heritage research as well [18].
NEXAFS is particularly sensitive to the bonding environment (coordination and molecular bonds) of organic materials by exploiting significant differences in the spectral fingerprint of their pi vs. sigma orbitals. The benefit of NEXAFS for acrylic paints parallels those observed in other applications of polymer science/coatings, via using the carbon K edge absorption spectra as a surface sensitive probe for identifying organic materials based on the different bonding environments of their carbon atoms [17],[19],[20]. This is a useful diagnostic tool for distinguishing aliphatic, aromatic and carbonyl carbon species which are commonly found in the context of acrylic paints and also in some of the more advanced cleaning treatments being developed for these materials e.g. microemulsions, gels [4].
It is important to remind the reader that one spectroscopic method in isolation is unlikely to fully reveal the multifaceted properties of paints. As with other spectroscopies, NEXAFS and XPS should ideally be supported by a suite of analytical tools including mass spectrometry (ESI-MS, DESI-MS), microscopy (optical, SEM, AFM), and thermal analysis techniques (DSC, TGA, DMA).
The results presented below are a subset of a broader ongoing investigation into the response of artists’ acrylic paint films to wet-surface cleaning treatments used for the removal of accumulated soiling. To explore this, selected representative wet-cleaning systems, some established and some novel, were applied to prepared acrylic paint films from paints made by Golden Artist Colors and Talens (detailed elsewhere [13]). In some cases a commonly executed clearance step (using deionised water or a low aromatic content hydrocarbon solvent) was applied with the aim of removing system residues. In other cases the clearance step was omitted to explore instrumental detection limits. Several established and novel wet cleaning systems are under investigation. Here we discuss those based on: deionised water, a low aromatic content petroleum spirit (Shellsol D40), and a mineral spirits continuous phase water-in-oil microemulsion system containing a sodium (Na) sulfonate (LAS) anionic surfactant, which has been described in detail elsewhere [4].