Experimental setup employed for the thermal behaviour measurements performed with the infrared-emitting lamp.

Researchers have found that the addition of stainless-steel flakes in varying quantities to a water-based wood coating not only results in intense coloration and specific aesthetic characteristics, but also maintains the barrier protection properties of the surface, increases the abrasion resistance of the composite layer and influences the thermal behaviour of the coating.

The purpose of the research was to determine the influence of three different amounts of stainless steel flakes on the aesthetic features and durability of a waterborne wood paint. Colorimetric measurements and optical microscope observations were employed to assess the impact of this novel pigment on the overall appearance of the coatings.

Wood is valued for its abundance in nature, as well as for its specific aesthetic qualities. It is also favourably perceived as a natural and recyclable material, with no carbon-dioxide impact. However, the ligno-cellulose composition of wood renders this material particularly vulnerable to flames and degradation caused by humidity and solar radiation.

To address these practical challenges, wooden surfaces are often covered with organic coatings that safeguard the material from solar radiation, changes in humidity, chemical attacks, mechanical damage, and the proliferation of damaging microorganisms such as fungi.

Additionally, the high hardness, rigidity, and thermal endurance of nanoparticles such as nano-silica, nano-alumina, nano-clay and nano-cellulose have been selected to enhance the physical properties and reduce the water absorption of wood coatings.

Ultimately, the addition of nanomaterials such as copper nano-powders, nano-titanium, and silver promoted the antibacterial and fungicidal activity of wood paints.

Innovative pigments

However, the current wood coatings industry is focusing on the application of vibrant colour layers, obtained through the exploitation of innovative pigments that impart distinct gloss values to the protective coatings.

Stainless-steel flakes serve as an attractive resource for introducing new aesthetic features and multifunctional properties to wood paints. Recent works have highlighted the excellent contribution of this type of additive in organic matrix composite materials, whose mechanical properties have been enhanced by the stainless steel flakes.

Moreover, these fillers have been introduced into organic coatings to improve their protective properties and corrosion resistance performance. Similarly, stainless steel flakes proved to be effective for enamel coatings, which were able to refine the cracking and abrasion resistance of the composite layer.

However, the stainless-steel flakes possess a further aesthetic function, associated with the specific metallic effect they can impart to the coating, due to the particular reflective properties of their structure.

Typically, the metallic appearance in paints is achieved using aluminium-based pigments. Nevertheless, stainless steel flakes present a viable substitute due to their increased durability, superior resistance to abrasion, and enhanced aesthetic qualities.

Moreover, they exhibit greater stability than aluminium-based substances when exposed into aqueous solutions, rendering them a more effective pigment for water-based paints.

Furthermore, optical microscope observations were carried out to examine the emergence of any potential micro defects after of the test. The chemical resistance test was also performed to evaluate the effect of the filler amount on the barrier performance of the acrylic matrix.

Modifying paint

The stainless-steel flakes perform a very distinct function in modifying the appearance of wood paint. Nonetheless, it is crucial to determine whether these additives have an impact on the durability of the paint.

Thus, to evaluate the effects of various amounts of flakes, the four series of samples were subjected to accelerated degradation tests, including exposure to UV-B radiation and drastic temperature variations.

The functional pigment has no effect on the structural morphology of the coating and does not constitute a source of defectiveness in the polymeric matrix.

Thus, the stainless-steel addition can be employed to drastically alter the overall look of the coating, and modifying both its hue and reflective characteristics, offering wood paints new decorative attributes.

The colour change of the coatings caused by UV-B radiation exposure can be mitigated by the flakes, which may partially shield the UV radiation. Furthermore, the stainless-steel flakes display remarkable resilience under thermal stress and do not affect the protective characteristics of the acrylic paint, as well as on the adhesion values of the composite layer.

Certainly, this type of filler could be useful in outdoor coatings employed in environments with significant temperature changes and direct exposure to solar radiation.

Change in the appearance of the coated samples and the wooden reference panel during exposure to UV-B radiation.

High durability

Moreover, the flakes have no major impact on the barrier characteristics of the matrix; nevertheless, they can partially mask the chromatic shifts induced by the potential absorption of solutions.

As a result, the findings of the liquid resistance tests justify the application of this type of flake as a functional pigment for outdoor applications where the coatings may interact with external aggressive agents.

The mechanical tests evidenced that the stainless-steel flakes additionally offer a functional protective contribution, thereby enhancing the overall hardness of the coatings while decreasing the mass loss caused by abrasion processes.

The metallic filler not only serves as an unusual pigment resource, but it can also improve the mechanical characteristics of the acrylic matrix, ensuring the high durability of the composite layer.

Thermal analyses revealed that the flake amount has a significant impact on the thermal properties of the coating, as the behaviour of stainless-steel flakes relies on two competing contributions. The enhanced coating darkness leads to a rise in absorbed heat, while the increased light reflectance results in a reduction of coating temperatures.

Thus, the filler concentration can be properly selected to minimize or increase the heat absorbed by the substrate to which the coating must be applied.  (Source: Journal of Coating Technology and Research; Vol. 21, Issue 1, 03 January 2024).

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