Degradation phenomena

water sensitivity

Causes of Water Sensitivity in Acrylic Paint
Acrylic paints have been popular with artists since their invention after the second World War. Their ability to be used with extreme flexibility, from transparent washes or ‘stains’ to thick, heavy bodied paint, excited artists and lead to visual effects previously not possible with traditional media. From a conservators point of view, acrylic paint tends to be more flexible and has less cracking than traditional (stiff, brittle) oil paint. Acrylic paintings also often tend to be large, durable but simultaneously fragile works (Doménech Carbó M.T. et al. 2013, Stavroudis et al. 2013).

Water and solvent sensitivity in acrylic and acrylic emulsion paint is a well-known phenomenon that has been studied extensively. Like oil paints acrylic paints have a tendency to form a ‘skin’ or film on the surface as it dries and hardens. The two main ways in which water and solvents affect an acrylic paint film is by swelling the film and extraction of paint film components. These can lead to various changes in the appearance of the paint, including changes to colour, gloss and adhesion (Wolbers et al. 2013, Jablonski et al. 2003, Jablonski et al. 2010).

A number of different factors can influence the extent to which an acrylic paint film can be affected by water or other solvents. The concentration of pigment, added fillers, age hardening at the surface, polymer elasticity, film drying rate, film thickness, prior treatments and environmental factors (such as Relative Humidity, temperature and environmental conditions) all affect the aging properties of an acrylic film, and by extension its swelling and extraction properties (Wolbers et al. 2013). Surfactants are added as stabilizers and pigment wetting agents to acrylic and acrylic dispersion paints. They are often found to be concentrated on the surface of acrylic paint films, having migrated from the bulk of the paint, although they are also found in the bulk of the paint film too. These surface surfactants can be degraded by light, and can have various optical effects on the paint layer, from matte spots to a mottled effect or the appearance of a surface haze over a section or entire paint film (Ormsby et al. 2009, Ormsby et al. 2013).
Acrylic paint films have a low Glass Transition temperature (Tg) at ambient temperatures, which is caused by surfactants acting as a plasticiser to the paint film. A low Tg indicates a softness of the paint, and combined with the separation and migration of surfactants to the surface of the paint films, acrylic paints are particularly vulnerable to dirt attraction, which becomes imbibed in the surface. Often finger marks and scuffs from handling are found in their surfaces, along with accumulated surface and imbibed grime (Stavroudis et al. 2013).

Treatment of Acrylic Paintings
As mentioned, the two ways in which water and other solvents affect paint films is by swelling of the film and the leaching of components. These processes often occur simultaneously.

Swelling by water and (hydrocarbon) solvents can occur at macro and micro scale. Exposure to water can leave surfaces with increases of pores or perforations, surface erosion, roughness or scratches. Cleaning with low polar solvents can lead to the solubilisation of polymer chains and additives in the paint film, leaving it more brittle and sensitive (Wolbers et al. 2013).

The leaching of components such as surfactants from the paint film will leave the paint film more brittle. The migration of surfactants to the surface is exacerbated by aqueous cleaning, and may actually risk embedding the dirt or soiling more thoroughly by solubilising the film and then leaving it to harden (Doménech Carbó M.T. et al. 2013).

As dirt is often imbibed or well-adhered to acrylic paint films, dry cleaning methods are not always effective, and can leave their own specific types of damage on these films (residues, changes to surface gloss, streakiness, for example). This means that, despite the risks, ‘wet’ solutions such as aqueous systems or those based on solvents are required to remove dirt. Aqueous solutions are known to have the greatest effect for the removal of surface dirt (Ormsby et al. 2009).

The use of deionised water, as is often used for old oil paint films, draws out film materials, like surfactants, as it is a hypotonic solution. Hypertonic solutions, on the other hand, with high conductivity, are less likely to draw out materials or swell the films (Wolbers et al. 2013, Dillon et al. 2012). Solutions with a high pH (of around 4-5) are also recommended for dirt removal, leaving the paint film intact. The ionic strength of a film has also been shown to be of importance, with Stavroudis (2013) recommending around 29 mM concentration. He recommends using carbonated distilled water, which naturally has a pH of around 6.3 with the addition of 1% of ethanol as this lowers the effective detergency of the cleaning solution. The use of gels, for example Agar, is also recommended for the removal of surface dirt from acrylic paintings, as is the use of a micro-aspirator (Cremonesi 2016, Ormsby et al. 2009, Ormsby et al. 2013, Lagalante et al. 2016, Stavroudis et al. 2013).