The research team, led by dr. Joris Sprakel, developed a new measurement technique that allows them to look beneath the surface of paint films, where the human eye cannot see. The method, based on a medical imaging technique, is known as Laser Speckle Imaging, LSI. It uses a laser beam to make processes within the paint film visible. The method is very sensitive and can determine how paints dry or form cracks at scales of only one micrometer, one hundredths of the thickness of a human hair. Co-researcher Hanne van der Kooij, who carried out the work says: “This new method gives us completely new insight into what happens in a drying paint film right at the moment cracks start to form. It makes us see things that we did not even know were happening, such as the unexpectedly slow growth of fractures or how entire pieces of a paint film suddenly peel away from the surface within a fraction of a second.”
In their publication in the journal Scientific Reports, the team demonstrates how this new method guides the development of more sustainable paints by shedding light on a wide range of common problems, such as the visibility of brush mark in the dried coating or the formation of wrinkles in the paint film, encountered during the formulation and use of water-based paints.
The formation of small cracks in paint surfaces is a major threat to the conservation of famous works of art. These cracks, that form the characteristic craquelure pattern seen in many old oil paintings, can lead to the paint detaching from the canvas underneath and the destruction of the painting. The novel technique not only allows the researcher to see how, when and where cracks form, but also to study the most suitable approaches to repair this damage.
Together with MSc students of Wageningen University, the team of researchers is now working on transforming the laboratory method into a practical and portable instrument. “Our dream is to develop a handheld and low-cost version of this technique so that it is available not only to researchers in industry and academia for the development of green paints”, Joris Sprakel says. “It should also be an instrument for art conservators across the world to help preserve our cultural heritage”.
Paints form a protective layer on most materials we use in daily life, ranging from the walls and windows to food containers, and the cars and airplanes. Traditionally, these paints are based on organic solvents, such as turpentines, that are released into the atmosphere during drying. In the past decades it has become clear that these volatile organic compounds, VOCs, contribute to the greenhouse effect. These compounds are now known to cause brain damage to professional painters exposed to their fumes in their daily work, leading to severe health problems known as painter’s disease.
Coatings based on water, such as the latex paints used for in-house wall painting, are both environmentally and consumer-friendly, but do not yet offer the same performance and longevity as their VOC-rich counterparts. Especially for demanding applications where the coating is exposed to extreme conditions, such as for cars, airplanes or ships, VOC-based paints are still the standard. Developing water-based paints with excellent properties is one of the foremost sustainability challenges in the coating industry. This is a difficult endeavour since the drying process of water-based paints is very complex and not well-understood, leading to paint films that crack or detach from their surface after some time.