Spectroscopic techniques and the conservation of artists’ acrylic emulsion paints
© Willneff et al.; licensee Springer. 2014
Received: 3 June 2014
Accepted: 17 October 2014
Published: 12 November 2014
Artists’ acrylic emulsion paints are used in many contexts such as paintings, murals, sculptures, works on paper and mixed media; and are forming increasing proportions of modern and contemporary art collections. Although acrylic emulsion paints have been the focus of museum-led research over the past decade, the impact of artists’ technique and conservation treatment on the upper-most surface of these paints remains essentially unexplored.
This paper summarises previous studies using vibrational (FTIR) spectroscopy and presents initial assessments of paint surfaces using X-ray spectroscopies (XPS and NEXAFS) aimed at characterising artists’ acrylic paint film surfaces after natural ageing and wet surface cleaning treatment. Both techniques were found to be well suited for surface-sensitive investigations of the organic materials associated with artists’ acrylic paints, including explorations into: (A) cleaning system residues, (B) surfactant extraction from paint surfaces, (C) the identification of migrated surfactant, and (D) monitoring pigment changes at the paint/air interface of paint films.
It has been shown is that these X-ray spectroscopic techniques can be used for the analysis of almost purely organic materials in a way that complements mass spectroscopic techniques, FTIR and XRF. This investigation forms part of broader, currently ongoing, multi-technique investigation into the properties of artists’ acrylic paints and development of conservation treatments for works-of-art made with these materials.
KeywordsAcrylic emulsion ATR-FTIR NEXAFS XPS Microemulsion Residues Pigment Surfactant Heritage Science Conservation
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 -. 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 . 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 .
Surfactants are added to these paints for several purposes. For example, they can act as pigment dispersants, defoamers and emulsion stabilisers . 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 . 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) ,. 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 .
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 . 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.
The popularity of FTIR spectroscopy in conservation science lies in its convenience, relative affordability and long history as an analytical tool . 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 . 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 . 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 ,,,,.
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 ,.
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 .
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 ,,. 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 .
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 ). 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 .
Results and discussion
As the bulk of the FTIR spectroscopy-based research exploring acrylic emulsion paint behaviour has been published ,,, the next section focuses on information gained through the use of XPS and NEXAFS spectroscopies. Several aspects of ongoing research are highlighted including (a) residues from applied cleaning treatments, (b) surfactant extraction from the paint surface, (c) surfactant migration following natural ageing and, (d) response of pigment to cleaning treatments.
A. Exploring wet-cleaning system residues
Ideally, wet cleaning treatments used to remove accumulated grime and soiling will leave minimal residues on the cleaned surfaces of works of art. However, with some of the newer cleaning treatments developed recently, the extent to which residues remain on unvarnished paint surfaces is unknown. MS, FTIR, and fluorescence detection of labelled materials have been used to investigate residues from applied cleaning systems, particularly surfactant residues .
In general, because of less surface sensitivity (and hence less influence of oxygen-containing contaminants) oxygen NEXAFS spectra offered a clearer distinction between the surfactant and binder than the O1s XPS, which highlights a possible benefit of the technique. However, this needs to be balanced against the ease of accessing a lab based (XPS) vs. an exclusively synchrotron based (NEXAFS) technique.
B. Surfactant extraction (from the paint film surface)
C. Surfactant migration
D. Pigment behaviour in response to cleaning treatment
One further aspect of paint film chemistry being investigated includes changes in pigment behaviour in response to wet cleaning treatments. Pigment loss (transfer of pigment to swabs/cleaning materials and/or solubility of pigments) during cleaning treatments is undesirable and cleaning treatments are designed to minimise this.
The increased PY3 pigment mobility could have several origins. The solubility of pure PY3 pigment in petroleum spirits is not high and is therefore unlikely to be a major contributing factor. Although swelling data shows that these types of paints do not swell extensively in non-polar solvents , the response of the uppermost paint surface to solvent exposure and mechanical action may result in pigment separation from the binder, which requires further exploration.
We have presented initial investigations of acrylic paint surfaces using two X-ray spectroscopic techniques that are hitherto not commonly used for examining organic materials in the context of heritage science. If samples are compatible with the measurement requirements (most notably ultra-high vacuum), XPS provides chemical state information for elements across the entire periodic table, while NEXAFS provides chemical and molecular bonding information for elements with absorption K or L edges between 200 – 1200 eV i.e. in the soft X-ray range. Both techniques were found to be well suited for surface-sensitive investigations of the organic materials associated with artists’ acrylic paints, including explorations into: (A) cleaning system residues, (B) surfactant extraction from paint surfaces, (C) the identification of migrated surfactant, and (D) monitoring pigment changes at the paint/air interface of paint films. At this early stage it is not possible to identify direct implications for applied conservation practice other than reiterating the need to keep solvent exposure to a minimum during surface cleaning treatment. It has been shown that these soft X-ray spectroscopic techniques can be used for the analysis of almost purely organic materials, paints in this case but also plastics and related materials, being investigated in the context of cultural heritage in a way that complements mass spectroscopic techniques, FTIR and XRF. This investigation forms part of broader, currently ongoing, multi-technique investigation into the properties of artists’ acrylic paints and development of conservation treatments for works-of-art made with these materials.
The method of paint film preparation, application of artificial soiling, and wet cleaning treatments used to prepare the samples as well as details of the experimental techniques have previously been published  and are summarised here.
Paint film preparation
Artists paints, Golden Heavy Body Acrylics Hansa Yellow Light and Talens Rembrandt Azo Yellow Lemon, both containing PY3 azo yellow organic synthetic pigment, were applied to a triple primed canvas with a draw-down technique on a Sheen Instruments film caster, to a wet thickness of approximately 800 μm and dry thickness of 200 – 250 μm, as measured with a digital caliper. The resin for the Golden paint was a pn(BA/MMA copolymer while the resin for the Talens paint was a p(EA/MMA copolymer with detectable amounts of a chalk (CaCO3) extender.
Samples discussed here underwent simulated cleaning treatments with the following wet cleaning agents. The water (W) was deionized (DI) (Purite, D700 deionizer). A 100% aliphatic petroleum spirit (PS) (VWR International) with a boiling point of 120 – 160°C was used as received. The novel Dow microemulsion (ME) was a water-in-oil microemulsion comprised of proportions of lauryl ammonium sulphate (LAS), low molecular weight alcohol–based cosolvents, a Shellsol D38 mineral spirits solvent continuous phase and deionized water  To assess cleaning efficacy, an artificial soiling mixture  approximating typical indoor particulate soiling which might accumulate passively over many years was brushed onto some samples (referred to as ‘soiled’) and allowed to dry before cleaning treatments.
Each cleaning agent was applied to an approximately 1 cm2 square area of the paint film by dipping a pre-rolled cotton swab into the solution once, and then rolling the swab back and forth (1 roll) across the paint film 20 times without application of a subsequent clearance step (which would be standard conservation practice) and dried in ambient conditions. Samples for spectroscopic analysis were prepared using a single hole punch, resulting in circular disks with a diameter of 6 mm.
To obtain reference spectra (labelled ‘surfactant’) of the type of PEO surfactant often found in these types of paints , films of dried surfactant on paint films were prepared for the NEXAFS and XPS measurements by brushing aqueous solutions (50% v/v) of Triton™ X-405 (70 wt% in water, Aldrich) onto an unsoiled Talens PY3 yellow paint film and the triple primed canvas respectively. The films were allowed to dry under ambient conditions.
X-ray Photoelectron Spectroscopy (XPS)
XP spectra were collected from samples mounted on double sided adhesive tape with a Kratos Axis Ultra spectrometer operating with a monochromatic Al Kα X-ray anode (1486.69 eV) at 180 watts (15 kV, 12 mA), a hemispherical analyser in electrostatic mode (p <10−7 mbar) and charge neutralisation. Survey XP spectra were acquired in a single sweep with a pass energy of 80 eV, in steps of 0.35 eV and dwell time of 150 ms, giving collection times of approx. 9 min per spectrum. High resolution XP spectra were acquired in a single sweep with a pass energy of 20 eV, in steps of 0.1 eV with a dwell time of 200 ms, giving a collection time of 1 min per spectrum. Data analysis was carried out with CasaXPS. Binding energies were referenced to a primary hydrocarbon peak set to 285.0 eV, which required a correction of approximately +3 – 3.5 eV to the experimental data. XP spectra acquired of chlorine present at low concentration were noisy and smoothed for better clarity.
Near-edge X-ray Absorption Fine Structure (NEXAFS)
NEXAFS measurements were performed at the U7a beamline of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, NY . Spectra were collected in partial electron yield (PEY) mode via a channeltron electron multiplier. The entrance grid bias (EGB), which permits tuning of the surface selectivity of the spectra, was −50 V for all spectra except for several spectra in Figure 7 acquired at −200 V (more surface selective). The monochromator grating had 600 l/mm at the C and 1200 l/mm at the O K-edges respectively. The energy scale was calibrated at the C K-edge by setting the first maximum in the spectrum of the amorphous carbon grid to 285.1 eV. At the N K-edge the first maximum of the TiN reference spectrum was set to 400.6 eV while at the O edge the I0 first minimum was aligned to 531.2 eV. Spectra were normalised in intensity via the Autobk subroutine in Athena .
BAO developed the intellectual background to the work which is summarized in the introduction and presented in previous publications and also presented the conference paper on which this article is based. EAW and SLMS carried out XPS measurements. SLMS carried out NEXAFS measurements. EAW performed the analyses with advice on interpretation provided by SLMS and BAO. All authors conceived of this study applying X-ray spectroscopies, participated in its design and coordination, and drafted the manuscript.
Attenuated Total Reflection-Fourier Transform Infrared (spectroscopy)
Atomic Force Microscopy
Entrance grid bias
Linear alkyl sulfonate
Near-Edge X-ray Absorption Fine Structure
Partial electron yield
X-ray Photoelectron Spectroscopy
EAW thanks the AHRC/EPSRC Science and Heritage Programme for funding her Postdoctoral Research Fellowship. We are grateful for support in NEXAFS measurements by Dr Cherno Jaye and Dr Daniel Fischer (NSLS), as well as Dr Joanna Stevens and Mr Adrian Gainar (University of Manchester). Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
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