Analysis

The Development of New Effect Pigments That Add More to Paint Than Just Color

Turkchem 08 Jun 2017 39 9 dk okuma

TURKCHEM

The activities of pigment manufacturers are guided by the market needs of the paint industries.

Special Effects

The activities of pigment manufacturers are directed according to the market needs of the paints industry. The main driver of innovation in the field of effect pigments is the desire of industrial paint designers to obtain the best tools for attractive colour designs. Innovation in pearl pigments based on synthetic mica and in aluminium pigments coated with iron oxide is providing visually striking whites and other exciting colours for paints. The external design of a product is largely determined by the impression of shape, colour and touch. Customers usually decide whether they are interested in a product at first sight. For this reason, inspiring colours, many of which have special effects, are needed across all areas. Designers frequently require effect pigments that support extreme requests such as the transparency of pearl tones, more natural flops, deeper blacks, cleaner whites, more vibrant and clean colours. They also want effects that shimmer under direct light and have a liquid appearance. These demands can be optically described as liquid metal appearances, almost like mercury. If everything inside a store looks new, a product can stand out with a completely different appearance from others. For example, a "vintage look" is sometimes used to increase the value of products. In the industrial paints sector, similar effects are achieved with paints having a cracked, patina or rusty appearance. Another topic of interest to designers is colours that change with angle, temperature, sound, electricity or magnetic fields. In the paints field, gonio-chromatic colours and thermochromic colours are encountered. Only an attractive colour that can be easily applied is acceptable by demanding sectors such as the automotive industry. The main demands from the technical side of paint production are about the following properties of effect pigments: • Increased opacity of pigments, • More uniform particle size distribution, • Higher mechanical strength, • Low abrasiveness, • Higher gloss, • Resistance to outdoor weathering, • Better moisture resistance, • Quality consistency, • Colour equivalence in water-based and solvent-based systems. A final and important factor in new developments is the "environmentally friendly" image of products and demands resulting from the ban on heavy metals, reduction of volatile organic compounds (VOCs) and sustainable production processes.

Technology Trends in the Production of Effect Pigments

The requirements of the paints sector mentioned above play a role in the development of new effect pigments. The main development activities are grouped under three categories: 1. Ways of producing new substrates/effect pigments:

Crystallisation

As an alternative to natural mica (muscovite), more transparent synthetic mica (fluorophlogopite) has been developed as a substrate for pearl pigments [1]. Alternatively, aluminium flakes created using a melting and crystallisation process provide substrates for pigments with attractive gloss [2].

Mesh Technologies

Coating a moving mesh with silica layers, aluminium films or multi-layer systems and then subdividing this coating film into flake form provides attractive substrates for pigments: Silica flakes provide substrates for subsequent oxide coatings in order to create gonio-chromatic colours. Similarly prepared aluminium films (fine Al flakes) can be used to produce vacuum-metallised pigments [3-5] that provide liquid metal effects. Multi-layer pigments produced for the first time with a patented mesh system, consisting of semi-transparent Al films, transparent SiO2 films and opaque Al layers, have enabled the production of effect pigments with strong interference properties [6]. However, these have never been produced on a commercial scale. Multi-layer effect pigments containing Cr/MgF2/Al/MgF2/Cr are used in security inks [7].

Advancing Processes in Glass Processing

Recent developments in the field of glass processing have made it possible to reach borosilicate glass flakes with flake thickness < 1 μm. Fine glass flakes are now produced using a spinning vessel into which molten glass is injected, replacing the brittle glass bubbles previously used [8]. 2. Better colour properties such as high colour gloss, gonio-chromaticity, shimmer, liquid metal appearance and high transparency can be achieved through the use of the following: • Finer flakes with narrower particle size distribution, • More uniform pigment particles, • Smoother flake surfaces, • Use of multi-layer systems (chemical synthesis [9] or physical vapourisation [7]). 3. New surface treatments are being developed to remove heavy metals from effect pigments for use in water-based systems, to eliminate chromium from aluminium and micas with surface treatment. The main technologies used in place of Cr surface treatments are silica/silane surface treatments.

Conclusions

• One of the main demands of designers is effect pigments that support the desire to go to "extremes" in colour matters, particularly in terms of colour vibrancy, brightness of colour, cleaner whites and darker, deeper blacks. • The requirements of the paints sector are enabling the development of new effect pigments through developments entering three main areas: crystallisation, mesh technologies or improved glass processing. To produce new substrates/effect pigments. Better colour properties are achieved through the use of more uniformly shaped pigment particles, smoother flake surfaces or new surface treatments for use in water-based systems. • A series of bluish white pearl pigments based on synthetic mica substrates, clean white pearl tones and when combined with carbon black provide attractive greys and silky or shimmering lead blacks.

Innovative Effect Pigments in Paint Design

For luxury products, the lustrous and noble gloss of pearl effect pigments in paints is extremely popular. Traditional pearl pigments based on titanium dioxide coated natural mica flakes typically show a yellowish colour tone due to iron impurities in the natural mica. Brand owners generally want to avoid a yellowish appearance in white colours and prefer more bluish "ice" whites. A series of pure, slightly bluish pearl pigments based on synthetic mica, introduced recently, is now available. Unlike natural micas, synthetic mica substrates are, as shown in Figure 1, absolutely transparent and do not contain colour impurities. These fine pigments allow formulation of neutral to slightly bluish white pearl tones. Formulations optimised with three layers provide more pronounced whites when viewed from all angles. When combined with carbon black, it is possible to obtain lead greys with light bluish tones and noble blacks. Depending on particle size distribution, silky, glossy and even shimmering effects can be achieved with these new pearl pigments. "Silk white" is a bluish pearl pigment with a smooth, velvety appearance. Despite its small particle size of 12 μm, the lightness flop is pronounced. "Frost white" pigment is a glossy-looking bluish pearl pigment. With a mid-level particle size of approximately 19 μm, "Frost white" provides a good balance between lightness flop and gloss, providing suitability for demanding industries such as automotive base coats. "Crystal white" pigment is designed to provide strong shimmer effects not only in terms of gloss but also at more oblique viewing angles. The typical application area for this 30 μm large, glossy pigment is industrial coatings where a good balance between strong shimmer and technical suitability is required. Larger flakes generally provide stronger effects, but in some applications technical suitability is lower.

Measuring the appearance of pearl tones with the liquid metal factor

Flop index (Fi) and liquid metal factor (LMF) are tools used in characterising modern pearl tones. The formula for measuring angle-dependent lightness called "flop index" was published by Dupont in 1992 [10]. "Glacier Silk White" is a powerful tool when paint designers request a smooth pearl appearance along with lightness flop. While high lightness is desired from the front angle, maximum dark appearance is targeted in the low flop. In scattered reflected light, the smoothest appearance is usually required in colour design. In terms of colour measurement, a low value of roughness (G) is targeted. A mercury-like liquid metal surface has extremely low roughness in scattered reflected light and a strong lightness flop in directed light. LMF converts this effect into a number.

Figure 3. Colour performance of newly developed effect pigments compared to state-of-the-art

Figure 4. Iron oxide coated aluminium particles (Paliochrom Orange)

Figure 5. Iron oxide coated flake-shaped aluminium particles (Paliochrom Brilliant Orange)

Figure 6. Colour performance comparison of orange/red metallic pigments with iron oxide coated aluminium pigments, in combinations with full shade and transparent organic pigments

The higher the flop index and the lower the roughness, the better the liquid metal character of a design will be. It is important to compare LMF values only for pigments with similar particle size. When comparing effect pigments with very different particle sizes (for example; d50 = 12 μm and d50 = 25 μm), the LMF values are meaningless. "Glacier Silk White" pigment was designed as a bluish pearl pigment with an optimised LMF (see Table 1). Typical applications of fine-sized pearl pigments are three-coat paint systems where, as shown in Figure 2, an opaque white base coat is covered with a second coat containing pearl pigment and then with a conventional transparent topcoat. Depending on the design, it is possible to create bluish white tones or yellowish white pearl tones. Mixtures of natural mica-based pearls and synthetic mica-based pearls provide access to tones between them. Larger types of pearl pigments are used when a design requires an appearance with striking gloss. Usually at face angle, these paints exhibit strong shimmer effects. At more oblique angles, the shimmer effect is usually less pronounced. Particularly on large surfaces, having good gloss only at face angle view and low shimmer effect at all other angles can be a disadvantage. Recently introduced, based on synthetic mica, a new shimmer pigment – glacier crystal white – offers high shimmer intensity also at higher angles, as also shown in Figure 3. Whereas the shimmer area value (S_a) is related to the number of light reflections in a given surface area, shimmer intensity (S_i) measures how concentrated the relevant light reflections are. For normal thickness micas, the red line is characteristic. At a 15° illumination angle, both S_a and S_i values are high. At higher illumination angles, both values are lower. "Crystal white" pigment behaves differently. Higher illumination angles related to the low flop exhibit higher shimmer intensity. This makes the new pigment attractive for applications where strong shimmer intensity is required from every angle, such as decorative wall paints or powder coats for exterior façades.

High Gloss Iron Oxide Coated Flake Aluminium

More than 20 years ago, classic iron oxide coated aluminium pigments entered the paints market, as shown in Figure 4. Depending on the thickness of the iron oxide film on the aluminium surface, gold, orange and red colours are possible [11-13]. The unique combination of high colour intensity, gloss and hiding power provided by iron oxides has enabled opaque automotive effect shades unprecedented to date [14]. To further enhance the performance of automotive paints, flake-type iron oxide coated aluminium pigments were recently introduced [15]. The excellent light-reflecting surfaces of iron oxide coated flake-type substrates have further increased the gloss of metallic red colour, as shown in Figure 5.

The colour potential of iron oxide coated aluminium flakes can be increased by combining them with transparent perylene or DPP pigments (diketopyrrolopyrrole) as shown in Figure 6.

Iron oxide coated flake-type aluminium pigments show stronger shimmer effects compared to classic aluminium pigment types. Classic aluminium substrates are thinner and therefore their appearance in paint is flatter. Having both pigment types in the same colour allows paint designers to easily adjust the desired shimmer angle as shown in Figure 7.

Summary

The main requirements of the paints sector are effect pigments that provide more vibrant colours, strong shimmer, clean white and black tones. In addition to improved colour properties, there are continuing demands, particularly for more opaque base coats. The innovative pigments recently introduced consist of synthetic mica-based bluish pearl pigments and pigments from iron oxide coated aluminium flakes. This article emphasises the requirements of the paints sector, technology trends in effect pigment synthesis and effect pigment innovations based on synthetic mica and iron oxide coated aluminium.