Eliyahu Mirlis – Designer

Acrylic Paint Properties

Here, Eliyahu Mirlis shares the properties of acrylic paint.

Thermoplasticity

As Eliyahu Mirlis indicates, the temperature sensitivity of acrylics can be problematic, especially when paints are in storage or in transit. Their low Tg makes them rubbery at room temperature, attracting dirt and air pollution. (The Tg is the temperature above which the polymers cease to have stiffness and brittleness characteristics to become plastic solids, that is, deformable). High temperatures and RH can even cause packaging materials to stick to the surface.

Low temperatures and relative humidity

According to Eliyahu Mirlis, low temperatures and relative humidity are particularly damaging as they can cause a significant decrease in the elasticity of the acrylic paint film and consequent cracking of the paint upon bending. In various studies, emulsion films were exposed to various combinations of temperature and relative humidity (RH). Strength, modulus, and elongation at break were measured. The warning was clear: low temperatures and low RH, conditions that are feasible in winter, make acrylic paint brittle and susceptible to cracking.

In contrast, a warm and humid environment can encourage mold growth. In this case, loose mold and grime were removed with a dry brush and vacuum, then distilled, non-ionic water. However, this treatment was further complicated by the matte and powdery nature of the paint; mold left black spots in some areas of the paint.

Aging Properties of Acrylic Artworks

Here, Eliyahu Mirlis will share some of the aging properties of acrylic artworks.

Discoloration

According to Eliyahu Mirlis, clear acrylic paint media were observed to be yellowish or slightly discolored in three studies. In the first, the acrylic media were subjected to natural and accelerated aging. Discoloration was observed in samples applied to cotton and linen substrates, more than in those applied to glass.

Discoloration appeared shortly after the samples had dried naturally; then, accelerated aging does not mean an increase in yellowing. The discoloration was attributed to the migration of components from the support to the medium during drying. The water in the media moistened the backing and internal components and, as it evaporated through the surface of the film, it introduced the bleaches into the media.

Yellowing

As Eliyahu Mirlis specifies, the yellowing of acrylic emulsion media was also noted during another study. Clear acrylic media samples, supported on glass plates, showed slight yellowing (and increased UV fluorescence) after natural aging in the dark, after light aging, and after oven. During the aging process, the films were subjected to periodic solvent extraction to monitor changes in MW through viscosity. The increase in yellowing coincided with a decrease in solubility. The yellowing was attributed to slight crosslinking of the film.

Finally, in a further study, yellowing of clear acrylic films in the dark was explored, providing evidence that they can be bleached in light, to varying degrees, just like oil paint; SID films are less susceptible to light bleaching.

Crosslinking and oxidation

Eliyahu Mirlis claims that crosslinking in water-based acrylic emulsions can occur in three stages: during polymerization / production of the crude polymer resin; during drying / coalescence of the paint film; and during aging (both natural and accelerated) of the dry film. Acrylic emulsions can be formulated to undergo varying degrees of crosslinking during drying, depending on the end use of the product, using additives called crosslinkers. It has been reported that during aging, a film can crosslink and oxidize as a result of photodegradation and the effect of residual surfactant.

The main consequences of crosslinking are an increase in brittleness and toughness, which can actually improve the film's resistance to dirt and abrasion.

Methods for collecting dirt

Frequently mentioned in the literature, although rarely discussed, is the tendency of an acrylic paint film to absorb dirt through the surface. Dirt can come into contact with paint through airborne contaminants, tampering (like fingerprints), and accidents or vandalism. It has been suggested that indoor air pollution, which gradually builds up on a paint surface, can take approximately 50 years to become noticeable to the human eye. The main factors believed to affect the attraction of dirt to acrylic paints include:

If all are low, then the resulting film exhibits low hardness at room temperature, forming a sticky "trap" for incidental dirt.

Here, Eliyahu Mirlis will share more about acrylic painting conservation.

Static charge

Acrylic paint films are susceptible to building up a static charge, attracting dust from the air.

Pigment concentration

As Eliyahu Mirlis indicates, It has been suggested that a high pigment load can block dirt build-up. However, the uneven surface resulting from a high pigment loading paint is likely to mechanically trap dirt and thus significantly increase the difficulty of dirt removal.

Sensitivity to solvents

Eliyahu Mirlis claims that the sensitivity of acrylic emulsion paint films to organic solvents clearly limits the conservator's choice of cleaning techniques, paint materials, and varnish and varnish removal options. There are difficulties in removing embedded dirt without altering the texture, color and gloss of the surface. Mechanical cleaning such as eraser crumbs are sometimes used before trying wet cleaning techniques.

Sensitivity to water

Eliyahu Mirlis specifies that even water or water-based cleaning methods can affect the surface of the paint. Acrylic emulsion films can remain water soluble for up to a week or more after application. As they dry, they become less soluble in water. However, it is widely known among modern paint conservators that acrylic emulsion films are still sensitive to water swelling.