Taking nature's course: What the world can teach us about colour

Structural colour is a common occurrence in nature and is responsible for the colour of some birds, butterflies, beetles and other animals, including chameleons. Sophisticated processing techniques have been devised to replicate the structures and produce the same effects.

Our investigation considers how the visual effect relates to pigments with different colours depending on the viewing angle. These structural colours work on the basis of light interference in the layers of material that are dispersed in the finish. This paper describes the newest developments in this field and how they are applied in media especially for leather. It discusses paint coating and pigments, which have cross-linking moieties with a liquid-crystalline structure.

Colour is essential. It has a significant place in our lives. On the one hand there are psychological associations related to colour and this can vary from culture to culture. In physics, colour is described through electromagnetic radiation with its wavelength (or frequency) and its intensity.

To produce colour, three components are required: a light source, an object to bounce the light source off and something to view that process, usually eyes. When one of these parameters is altered the colour stimulus changes.

With leather, the application of light absorbing dyestuffs and pigments is well known and has been described in many books, as well as in technical and commercial papers. However, structural dyestuffs for leather are a new and an innovative technique. Some objects not only reflect light but also transmit light or emit light themselves in order to contribute to the colour. At present, researchers are working with a variety of these materials, particularly in the security sector on credit cards and banknotes. However, more recently work has been done on coatings and in the consumer industry.

Structural effect colours in nature

Colour in nature is undoubtedly fascinating; a rainbow is caused by light being reflected in droplets of water. Also animals are capable of producing colour with the help of optical refraction. The colour alteration in chameleons is carried out by pigment dilution and concentration in the cells and by guanine in order to break and reflect the incoming light. The optical appearance of effect pigments based on reflection and interference in nature and is already known by butterflies, beetles and others.

In biology, a colour created by an optical effect is called a schemochrome. Structural colours is the general term for an object capable of producing colour by interacting with electromagnetic radiation, pressure, pH, electricity, friction, heat and other physical forces. If this effect is caused by pigments, they are called "Effect Pigments".

The colour of an object like leather depends on the light leaving the surface, which normally depends on the wavelength within the visible spectrum of approximately 390 nm to 750 nm, and the reflectance properties of the surface. The physical basis of a standard absorption pigment is based on the selective reflection and absorption of certain wavelengths of visible light. As an example, the blue cobalt pigment absorbs red and green light and reflects blue, creating the blue viewing colour.

Metallic pigments comprise of small platelets of aluminium, copper, zinc or other metal. They reflect the light almost completely and thus produce a gloss and mirror effect, so called a metallic look. The conventional technique of colour design is based on mixtures of absorption pigments with metal effect pigments. This produces a bathochromic glossing shift of the reflected colour.

Pearlescent pigments are similar to natural pearls. The refractive index difference by pearls is caused through alternating transparent layers: calcium carbonate and proteins. The requirement for this effect is the specific relation of the thickness of each layer to the wavelength of incident light. Pearlescent is defined by multiple reflections. Natural pearl essence consists of 75-97% guanine and 3-25% hypoxanthine.

Even though the pearlescent effect has been known since the 1650s, it is only in the last 50 years that significant development in the pearlescent effect pigments has taken place. The most important class of pearlescent pigments is composed of mica flakes, which are coated with a thin transparent metal oxide like titanium, iron or other metallic components. Depending on the thickness of the metal oxides different colours can be created. In the technique are two different principles to consider: Silver white pigments and interference pigments. Silver white pigments are covered with a thin metal oxide surface of ca. 40-60 nm and reflect the whole wavelength spectrum. Depending on the particle size, the gloss effects can be adjusted from silk matt to high glittering. Thicker metal oxide layers create the interference phenomena. In physics, interference is the addition of two or more waves which result in a new wave pattern. Countless varieties of colour effects are possible by combining different metals and controlling the metal layer thickness of the mica flakes.

Effect pigments based on liquid crystal polymers (LCP)

A new category of structural colours is based on liquid crystal structures. In 1888, the Austrian botanical physiologist Friedrich Reinitzer observed interesting colour effects when cooling cholesterol derivatives to just above the freezing point. They had two melting points. His academic friend Otto Lehmann reported him seeing liquid crystallites. This liquid crystal shows interaction with polarized light. Today, Liquid Crystal Displays (LCD) are widely used.

In order to produce finishes with liquid crystal they should have a solid form corresponding to pigments. Therefore, it is necessary to fix the liquid crystalline phase or stabilize it mechanically. This can be done by incorporating this liquid crystal in solid polymers. However, we have to consider that the appearance of the liquid crystalline phase often depends on temperature, ie as the liquid crystalline phase is heated or cooled, different wavelengths are reflected.

Therefore, LCP must avoid this effect to retain the same colour. Appropriate chemicals connected to a solid matrix polymer are the crucial point for this type of effect pigments.

The key point for LCPs is their chirality. This is a property possessed by a molecule that makes it non-superimposable on its mirror image. Molecules that are chiral are optically active and furthermore, if these molecules have a twisted structure and if the pitch corresponds to the wavelength of the light, then the light is reflected selectively and is angle-dependent. This is called the "Flip-Flop Effect". As a result, the reflected colours depend on the viewing angle.

Many of the LCPs with angle reflection properties are based on polysiloxanes. The core of the siloxane oligomer consists of a silicone-oxygen eight membered rings with some silicone having reactive groups, often an acrylic type, to form stable polymer chains that can be processed to thin solid films. The standard method for continuous foil production is the roll-coater technique. After the formation of the film they are ground to small transparent platelets. The resulting flakes have a thickness of ca. 6 µm and a particle size of ca. µm. Thus they have the same properties as a pigment but are colourless. These designed LCPs are able to achieve angle-dependent viewing effects.

There is no substantial application difference between the conventional pigments and the use of effects pigments. The incorporation of these LCP pigments into a coat is based on the tanner's knowledge of his finishing processes.

These pigments have been successfully applied and tested on cars. Also the application on leather could be made with a clear top coat system. However, a black dyeing or black base coat is highly recommended. It is required to avoid unwanted reflection of scattered light on the bottom, which results in a reduction of the colour efficiency. Careful dispersion and a parallel orientation of the LCP particles in the layer are also recommended.

Conclusion and Outlook

In general, the final colour design for all the effect pigments are influenced by the length / thickness ratio of the individual particles, the concentration and the optic properties of the matrix (refractive index, opacity). Interesting new bright, shiny and glossy colour effects as in nature can be achieved.

Of course, like all other new developments, many detailed questions need to be answered before the new finish will be suitable for mass production application. Not surprisingly, these unusual finishes focus on the luxury and fashion consumer goods. Nevertheless, this technique is already used for credit cards, banknotes and electronics and has been successfully tested and used on cars. One can be sure that luxury leather goods will be an attractive target to be finished using the same natural principle as is seen with rainbows, butterflies and the chameleon.