Effect pigments - Pearlescent pigments (Part 1)

Excerpt: Pearlescent pigment is a type of effect pigment. It is obtained by coating metal oxide on laminar flat surfaces such as natural mica. Different metal oxides are used to prepare pearlescent pigments.

Yash Melkeri and Dr. Tipanna Melkeri 

Chemtip Laboratories

8

 Shams Industrial Estate, Near Parmar Ind. Estate,

Vasai Phata, Vasai (E).

ABSTRACT

Pearlescent  pigment is a type of effect pigment. It is obtained by coating metal oxide on laminar flat surfaces such as natural mica. Different metal oxides are used to prepare pearlescent pigments, such as titanium dioxide, iron oxide, zirconium oxide, chromium oxide etc. Substrate selected can be either natural or synthetic. Between the two, natural mica is widely used substrate to prepare pearlescent pigments. But nowadays synthetic mica and borosilicateglass are also used to prepare pearlescent which provide brilliant sparkling effect. Pearlescent effect is observed as the result of multiple reflection, refraction and diffraction phenomenon. When mica is coated with titanium dioxide, a silver colour pearlescent pigment is obtained. As the thickness of TiO2 coating increases, because of the interference effect the colour changes from silver to yellow to pink to blue to green. The coating of iron oxide on mica results into bronze, copper and wine red colours as the thickness of the coating increases. When mica is coated with titanium dioxide and iron oxide together, a gold colour pearl pigment is obtained. Pearl pigments are available in the various grades depending upon the particle size. Pearlescent pigments are used in paints, plastics, papers, and automotive industry. One must follow some basic guidelines to use pearl pigment in plastics. In contrast to absorption colours, there is a trick in mixing colours to get desired colour in pearl pigment.

EFFECT pigments are typically defined as flake or platy or laminar structures that imparts a directional light reflectance, scattering, absorption, or optically variable appearance to the substrate in or on which they are applied. The mechanisms, however, by which all effect materials manipulate light, and thus color, still revolve around three basic optical principles, “reflection”, “interference” and “diffraction.”

Pearlescent pigment is one of the type of effect pigments. Pearlescent pigment is manufactured by coating a metal oxide onto a laminar substrate. The resulting pigment is semitransparent, and has some unique optical properties. The substrate used can be either natural or synthetic. Various examples of substrates used are natural mica, alumina, borosilicate, synthetic mica, etc.

Natural mica

Natural mica is a widely-used substrate in manufacturing of the pearlescent pigments. India is a major producer of mica in the world. According to British Geological Survey, the world's largest deposit of mica is at Koderma district in the state of Jharkhand, India. About 95% of India's mica is distributed in just three states of Jharkhand, Andhra Pradesh and Rajasthan. It is a natural mineral made up of potassium aluminum silicate and is mined from various deposits. The density is 2.7-3g/cm3 with the chemical formula, KAl3Si3O10(OH)2. They are available in 37 grades. The most common include: Purple lepidolite, black biotite, brown phlogopite and clear muscovite. For pearlescent pigment, muscovite grade mica is preferred. The chemical composition of muscovite mica is give in the Table 1.

The quality of muscovite mica is determined by visual inspection per ASTM D351- 571 from the highest quality V1 to the V12 (V-10A). V-1 indicates Clear - Hard, of uniform colour, nearly flat, free of all stains, foreign inclusion, cracks, and other similar defects where V-10A indicates Densely Black and Red Stained - Hard, may contain heavy waves, air inclusion, cloudy stains, slight black and red dots (mineral), red stains (mineral), black and red stains (mineral), green stains (vegetable type), and sand blast, very dense black and red stains (mineral), and slight clay stains. V-1 grade is preferred for silver coloured pearl pigment.

Fig. 1: Muscovite grade mica flakes.

Other substrates nowadays common are Calcium-Aluminum Borosilicate (Borosilicate), Fluorophlogophite (synthetic mica) and alumina.

  • Calcium-Aluminum Borosilicate: Brilliant sparkle effect can be achievedbecause of its high transparency. This also suits clear formulations where effects are desired with less opacity. But the cost is typically more than traditional mica.

* Fig. 2: Borosilicate flakes.*

  • Fluorophlogophite: Referred to as synthetic mica, fluorophlogophites are one of the newest innovations in substrate. They are used in cosmetic applications because of their high purity, cleanliness and transparency unlike mica which carries natural impurities along with it. This results into more brilliant sparkles effect. They have high aspect ratio and well controlled particle size. 

Fig. 3: Fluorophlogophite (synthetic mica).

Aluminum oxide (Alumina) Al2O3 flakes have very smooth surface and corundum structure (α-Al2O3). The flakes are produced using the crystal growth process. One can get the color purity and transparency of the effect pigments by coating the Al2O3 flakes with metal oxides.

The pearlescent effect is based on a directed reflection/refraction of light. Thus substrate should be transparent, a completely flat, perfectly circular disc. All scattering centers, like edges and steps that are part of the substrate flakes, should be avoided. Substrate must be with high aspect ratio (diameter: thickness). For higher particle diameters, i.e. coarser pearlescent pigments, the edge-to-area ratio must be lower. 

Metal Oxides

Most common Metal oxide used for coating the substrate are titanium dioxide (TiO2), Iron oxide (FeO, Fe2O3), zirconium oxide, aluminum oxide, silica, etc. Oxidation states of these metal must be +3, +4 or +5 as the divalent cation will not coat. Refractive index of metal oxide used to coat substrate is an important criterion to decide the type and quality of pearlescent pigments. Higher refractive indices are preferred for better pearlescent effect. For example, TiO2, Fe2O3, Zirconium oxide are having high refractive indices whereas silica, tin oxide are having lower refractive indices. 

Pearlescent effect 

Pearlescent effect is observed when a light fall on metal oxide coated flat, transparent substrate such as mica. When light is incidented on the metal oxide coated substrate, there is a multiple reflection, refraction and diffraction. Due to this phenomenon, the pearlescent effect is observed. The same can be depicted as shown in Fig. 4. 

Fig. 4: Pearlescent effect due to reflection, refraction and interference of light.

The most widely used metal oxide for coating mica is Titanium dioxide (TiO2). Titanium dioxide has a very high refractive index. Titanium is in tetravalent state in TiO2. When mica is coated with Titanium dioxide (TiO2), a pearlescent pigment is obtained with silver colour. 

Interference Effect

An optical thickness of substrate must equal to a whole number multiple of one-quarter of the wave-length at which interference is expected. Such construction of the so-called quarter-wave stack is a widely accepted and implemented condition. When more and more TiO2 is coated, the thickness of coating increases, the interference patternchanges and hence interference colour of the pigment changes from silver to gold, to pink and finally green. Further increase in thickness of TiO2 coating, colour sequence once again repeat in same order. Fig.5 depict the interference behavior of TiO2 coated mica pigments.We can see in Fig.5that transmission colour is the opposite of the reflected colour.

Fig. 5: Interference of TiO2 coated Mica.

TiO2 coating produces a white silvery powder when viewed from certain angle, however, when same is viewed from another angle, gold, pink, violet, blue or green colour can be observed depending upon the thickness of the TiO2 coating. Hence, they are called irradiance colours or two tone pearlescent pigments. Fig.6 shows this irradiance colours.  

Fig. 6: Interference colours of mica based pearlescent pigment.

When TiO2 is coated on mica it produces an anatase grade Titanium Dioxide unless modified with some modifying agent to convert to the rutile grade. The Modifier used to convert anatase grade TiO2 into the rutile grade TiO2 in the coating are Antimony, Tin, Iron or Chromium.

When Iron Oxide alone is coated on mica different coloured pearlescent pigments are obtained. Depending upon the thickness of Iron oxide coating the colour of the pigments obtained are Bronze, copper and wine red.

Fig. 7: Iron oxide coated mica, bronze pearl pigment.

When TiO2 and iron oxide are coated in combination, the pearl pigment obtained is of Gold colour.

In Combination with TiO2, other metal oxide coated are Zirconium, chromium, aluminum, silica etc., depending upon their applications.

Fig. 8: Titanium dioxide and iron oxide in combination coated Gold pearl Pigment.

Different grades of pearlescent pigments are manufactured in same colour with varying particle size and distribution. With increase particle size the gloss and luster increases but there is decrease in hiding power of the pigment. The widely-used grade has a particle size of 10 -60 microns. Their applications vary depending upon their particle size.

Pearlescent pigments are employed by various end user industries including Plastics, Paints, Inks, Textiles, Paper, Leather, Cosmetics, Automotive, Fashion etc. Some of the key drivers for the pearlescent pigments market include emerging fashion trends, automotive and the consumer market. In addition, rapid growth in urbanization, the paint and the coating market and building materials and the plastic industry have also been boosting the market for pearlescent pigments.

Pearlescent pigments in plastics 

Pearl pigments have the thermal and chemical stability to survive polymer processing and resist the weathering. They do not bleed or bloom. Because of these inherent properties, pearlescent pigments are widely used in plastic industries. In plastics, the best luster, brightness, and color intensity occurs with particle size of 10 to 40 microns. Smaller platelets impart a smooth, silky luster, and larger ones confer sparkle and glitter. Mica-based pearlescent pigment can be used in nearly all thermoplastics and most processes. It can also be incorporated in many thermosets, including unsaturated polyester, acrylic, urethane, and epoxy.

Guidelines for use of Pearlescent Pigments in Plastics

  • Their effects are most intense in transparent resins like PS, PP, PE, PVC, acrylic, styrene block copolymers, and silicone.

  • l It is also possible to attain pearlescent effect and luster in polymers having little or no transparency--such as nylon 6, ABS, and HIPS--but you may need higher pigment loadings.

  • Pearlescent combined with dark absorption pigments in opaque polymers can yield a strong reflection color and produce pearlescent effects of great richness and depth. But opaque, inorganic, high-coverage pigments should be limited to a small amountto retain any pearlescent effect.

  • For products, such as white HDPE shampoo bottles, some compounders add up to 5% TiO2 to pearlescent color concentrates to enhance opacity.

  • Highly filled plastics are not good candidates for pearlescent because opaque fillers scatter light, eliminating the pearlescent effect.

  • Most users limit fillers to less than 1% in systems containing pearl pigments.

  • Pearlescent pigment powders can be pre-dispersed in most resins by drum tumbling or mixing in twin-shell, ribbon, or high-speed blenders.

  • In injection molding one must follow a three-step process. i) Blend powdered resin and mineral oil for 10 minutes. ii) all colorants - except pearl pigments - and a dispersing aid are added and blended for 10 more minutes. iii) Pearlescent are then added and blended for another 20 minutes.

  • When pearl pigments are used with pelletized polymers, the disparity in size between the pellets and the powdered pigment may cause separation after blending.

  • In compounding it must be use shorter blending times with pellets than with powders because the pellets can fracture the pearlescent particles.

  • It is also observed in compounding that blending works best if pellets are made slightly tacky by pre-blending with mineral oil before adding pigment.

  • In Compounding one must use shorter blending times with pellets than with powders.

  • In compounding it is advisable to avoidpre-blending and potential separation problems altogether by feeding pearl pigments into the molten polymer through a downstream feeder.

  • Dispersion aids help reduce viscosity to improve mixing. Ininjection molding it is often added 1% LMW-PE wax to polyolefins and mix for 20-30 min. With PS they typically use only about 0.1% of a dispersion aid.

  • In case of Pearlescent color concentrates, concentrate commonly contain 25% pearl pigment, though amounts can be as high as 50%. The different pigments are added in a specific order to the mix to optimize color development. Addition usually begins with the harder-to-disperse organic pigments, followed by inorganic pigments, and lastly the pearlescent pigment.

  • Pearlescent need gentle handling during mixing. They should not be grounded or subjected to extended cycles or heavy shear because this can strip off the metal oxide and break the platelets.

  • Banbury-type or continuous mixers are most often used to create pearl concentrates, although two-roll mills, calendars, vertical intensive mixers, and double planetary mixers are also suitable as long as one avoid excessive shear.

  • In liquid colorants, mica-based pearls may settle and pack. It is important to stir pearlescent liquid colors thoroughly before use.

Important Tips for Molding and Extrusion

  • The Intensity and pearlescent effect can be seen effectively when the pearl particle align parallel to the surface of the article. This effect can be achieved by following below given points.

  • One must use higher melt temperature as well as mold temperate (about 75-100° F higher than normal).

  • It is advisable to use single gate molds and place gate at thicker part of the article.

  • Minimize the length of sprue and runner to keep the melt hot.

  • Keep part thickness as uniform as possible.

  • Use overflow vents to remove flow lines.

  • Use concentrates rather than powdered pigment for easier handling and better dispersion.

  • Dies and mandrels must be clean and free of defects. Scratches, burrs, and burnt deposits can tip platelets and create surface marks.

  • Screen packs should be properly sized. As a rule of thumb, mesh sizes of 40 to 60 give good pigment dispersion. Backpressure can become excessive if screens are too small or too many are used, especially with large sized pearls.

  • Tricks of Mixing Colours with Pearlescent Pigments

  • Combining pearlescent with other pigments is as much art as science, given the complexity of how their colors interact. The interference colors formed by pearlescent pigments add when mixed so that red and blue form magenta.

  • Blue and green form blue-green, and red and green form yellow.

  • White results when the three primary colors are mixed in the right proportion.

  • By contrast, absorption colors created by other pigments subtract when mixed so that yellow and blue form green, red and yellow form orange, and red and blue form violet. Black results when the three primaries are mixed in the right proportion.

(To be continues)

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