The road to intelligent coloured coatings

The road to intelligent coloured coatings

Coloured coatings can cover more than purely appearance properties of a product. Many established products have added functionalities related to their coloured surfaces, which are well conceived and designed parts of the product. This has been so for many decades. For example, painted metals for construction or transport have a thick multilayered organic coating often containing inhibitors to protect the metallic structure against corrosion. Electrocoloured aluminium used for façade panels in architecture has a 20–25 μm thick porous anodised layer containing metal deposits in the pores that interplay with the visible light to produce a spectrum of possible colours; this product has a guaranteed colour lifetime of 30 years, as well as a good corrosion resistance, hardness and wear resistance due to the hard oxide covered surface. These added functionalities are obtained by dedicated surface processing of the metal product, where the actual colour is only one of many desired properties.

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Diversifying such functionalities to obtain multifunctional coloured surfaces is a definite trend. New developments are flourishing and exciting R&D lies ahead of us. It can be envisioned that when coloured coatings become intelligent coatings, many products in as many industrial sectors will benefit.

In this editorial, I do not aim to give a complete overview of the current state-of-the-art, as this is too vast and too complex. Also the reference list that I have added consists of a selection of studies, but is by far not an exhaustive list, as many more research groups worldwide are doing excellent and ground breaking work in colour coating technology. My only aim is to give some striking examples – some old, some new and some even a bit curious – of how colour can change the perception in technology.

Colour changing sensors

For the development of sensors induced visible changes in colour offer easy recognition. Colour changes in materials can occur under various stimuli. Surfaces that change colour by thermal stimuli are produced, for example, on coffee mugs or infant feeding spoons to indicate the temperature of the food or beverage. These colour changes happen due to so called thermochromism. This phenomenon is based on either liquid crystals or leuco dyes. In both cases, the temperature rearranges the material at the molecular level, and as such the interaction with the visible light is affected: the material now absorbs and emits light at different wavelengths, and hence a different colour is observed. These colour changing surfaces are used for various applications going from cute gadgets and toys, to thermometers, battery testers and more advanced adaptive solar protection devices.

Chemical stimuli are the basis for colour change, for example, in glucose sensors,2 using nanoporous silicon films with switching colour characteristics when contacted with glucose. These are small, portable and electric free sensors with higher sensitivity, then the electronic sensors. Also a fast and sensitive sensor for the detection of Gram negative and Gram positive bacteria, and for screening specimens of physiological fluids (blood and urine) and foodstuffs (meat) for bacterial contaminations is based on agarose embedded chromatic films which produce colour changes and fluorescence transformations in response to bacterial growth.3

Also coloured surfaces that indicate corrosion threats are in development.4 Or imagine that colour changes on a gas or oil pipe can indicate minute levels of increased stress related to microcracks in the metal, which when left undetected will surely result in a leak. Smart monitoring devices could be envisioned.

Obviously the sky is the limit for coloured sensor concepts …

Cool colour coatings with high IR reflection

To decrease the heat build-up of surfaces as roofs, facades, fuel tanks, coolers and walls in warehouses, the application of pigmented organic coatings containing complex inorganic colour pigments with high IR reflectance properties is being studied.5 It is found that coatings containing (near) infrared reflective pigments have solar reflectance values higher than those of standard coatings.6 Such cool coloured coatings maintain lower surface temperatures. The use of cool coloured coatings can improve building comfort and reduce cooling energy use, and at city scale, it can contribute to the reduction in the air temperature due to the heat transfer phenomena and improve outdoor thermal comfort. This functionality combined with actual ‘cool’ colours opens the path to attractive design in building development.

Nature in colours… Lotus effect, hydrophobic–hydrophilic coatings, biofouling

The structural blue colour of the Morpho butterfly originates from microstructure elements on its wings creating diffraction of light and interference effects. In addition to the colour, the wing has superhydrophobic properties. Inspired by this natural material, in Ref. 7, a technique to prepare such structural colour films from colloidal solution is developed. By doping an appropriate dye in the colloidal particles to absorb the scattering light, colours are obtained. The hydrophobic and hydrophilic properties are enhanced when the roughness of the respective areas is increased. As such, structural colour films with superhydrophobic properties, as well as with superhydrophilic properties are possible. Similarly in Ref. 8, superhydrophobic coatings are prepared by a pigment nanoparticle suspension, where the type of nanoparticle controls the colours of the coating, and the particle size determines the surface structure and level of hydrophobicity.

In Ref. 7, also tuneable structural nature colours, such as the colour change of a damselfish, are mimicked. Phototunable photonic crystals using photoresponsive azobenzene derivatives are presented.

A curious and probably initially unintended colour effect is seen for the adhesion strength of organisms to coloured fouling release coatings. The complex effect of colour on barnacle adhesion may be because of physicochemical surface characteristics varying with pigments, and their interactions with local environmental conditions, as well as interactions with the settling barnacle larvae.9

Solar energy in colour

As the use of solar energy is becoming more and more established in building projects, building integrated photovoltaic (BIPV) systems are an interesting alternative to the standard solar panels for increasing the available area for electricity production.10 In BIPV, the visual appearance becomes important, including its colour given by the antireflection coating (ARC). The ARC is a thin film structure that increases the amount of current and, hence, the efficiency of a solar cell. The deposition of silicon nitride single layer ARCs with a dark blue colour is the most common process today. However, efficient, but differently coloured solar cells are important for the further development of BIPV. For example, in Ref. 10, it is shown that the use of multilayer ARC structures can allow solar cells in a range of different colours with very high efficiencies. In Ref. 11, coatings based on titanium alloy nitride show a variety of different colours for solar–thermal integration in buildings. In Ref. 12, an adjusted colour of amorphous silicon thin film solar cells is illustrated by using a multilayer film consisting of silver and gallium doped zinc oxide (GZO). The colour can be adjusted from dark blue to blue red by playing with the GZO layer thickness.

Expensive colour

Diamond-like carbon films have been developed as colour coatings for a range of substrates.13 They can be used in a wide variety of applications, including jewellery and other applications, where colours need to be combined in a tough, hard wearing, aesthetically appealing and biocompatible application. An example of the jewellery application is the UK Millennium Medal.

Optical modelling and prediction

In more and more material science disciplines, there is the emergence of system modelling as a research tool to predict or simulate a material's property. This is a well established practice for mechanical properties of metals, starting from the metal grain size, grain boundary distribution function, defect analysis and chemical composition. A more recent development is the electrochemical modelling and prediction of metal corrosion; the development of corrosion models is encouraged in large scale corrosion projects, like in the FP6-EU project SICOM for aerospace aluminium AA 2024.14

In the same context, system modelling of the optical properties, such as colour and gloss, shows high potential. The appearance of complex surfaces can be predicted based on the physical properties of the material and its interaction with light. Hence, as complementary research tool, all developments in the field of optical modelling are highly welcomed. In a PhD at the Vrije Universiteit Brussel,15 ^– 18 the colour and gloss of steel surfaces covered by thermal oxides and thin organic films were optically modelled, taking both the geometrical (surface roughness resulting in more matt or more specular reflection) and the spectral appearance factors into account. Later, we used a similar modelling approach to predict and simulate the colour of electrocoloured anodised aluminium with a highly complex multilayered metal oxide structure for the generation of interference colours.19

Thin film optical stacks are also used to model and simulate the colour changing ability of chameleons.20 The skin of a chameleon has four layers, which together produce various colours. The layers have various types of pigment cells that can rapidly relocate, thereby influencing the colour of the chameleon. The modelling approach developed to model the chameleon's skin is also applied to simulation of hair and fur camouflage, which does not exist in nature.

The modelling approach has proven its merit, and can be extended to additional properties. For example, interesting research is ongoing on the self-cleaning photocatalytic properties of anodised titanium. Optical modelling, extending to the UV light range, can surely assist in this work. The same is valid for optical modelling of IR reflection performance of cool colour coatings. Combining the optical properties with the added functional properties in a modelling approach is the ultimate goal.

As said in the introduction, the research on coloured surfaces and coatings spans worldwide and the topics are extremely diverse. In the hope that this short introduction may have triggered some new research ideas for the future, I wish you all good luck in the search for intelligent coloured coatings!