By Anthony Rohrer, Product Specialist, Effect Pigments; and Michael T. Venturini, Marketing Director, Coatings; Sun Chemical, Cincinnati, Ohio.
Pearlescent pigments are widely used in a variety of applications, from cosmetics to plastics and inks to automotive topcoats. Thegoal of pearlescent pigments is to mimic the inherent gleam and luster of natural pearls. This isachieved by utilizing the light reflection characteristicsof microscopic platelets and the optimization of theirdiffraction and diffusion properties. There are severaltypes of pearl pigments commercially available, includingnatural pearl essence, bismuth oxychloride crystalsand oxide-coated micas. There are also several othertypes of substrates that can be oxide coated, such asglass flake and platy alumina.
The most commonly used type is the oxide-coated micas;this also includes synthetic mica or fluorphlogopite. Theyare plate-like flakes of mica that are coated with thin layersof titanium dioxide (TIO2) and/or iron oxide, and potentiallyother metal oxides. The broad face of the platelets can rangefrom 4 microns to 1,000 microns across and are approximately0.5 micron thick, although synthetic micas canachieve thicknesses of less than 0.25 micron. The plateletsof coated mica are very smooth, so they are light reflectivewhile maintaining a high degree of transparency, whichmeans only a portion of the light is reflected. The portion thatis not reflected is transmitted through the platelet to the nextlayer where it can be further reflected (Figure 1A).
The results are multiple reflections from many layers.The eye cannot focus on any one layer, and a sense of depthis established, as seen in Figure 1B. Thus, a particular kindof shimmery luster called pearlescence is observed.
In addition to providing a pearlescent or lustrouslook, certain titanium dioxide-coated micas are capableof showing colors through the optical effect of interference.The large difference in the refractive index oftitanium dioxide (n ≈2.7) and mica (n ≈1.5) allows fora large degree of not only reflection of the light butbending and phase shift of the light waves as well.The color develops when certain wavelengths of lightreflected from one layer interfere or cancel out the samewavelength from another layer. The result is that onlya narrow band of wavelengths is reflected and a singlecolor is seen. This is much the same as colors seen infilms of oil on water or soap bubbles. Each interferencecolor is determined by the thickness of the titaniumdioxide layer on the mica. Increasing the titanium dioxidethickness results in the beginnings of interference,changing the reflection color to yellow. Increasing itfurther shifts the color to red, followed by blue and thengreen (Figure 2).
White pearl reflection color is produced from very thin,uneven titanium dioxide coatings, resulting in a mixtureof colors being reflected. The reflective light is additive andcombines back to form white light.
It is important to note that because these types of pigmentsdo not absorb light, only redirect (reflect, refractand transmit), they will behave in accordance to theadditive color scheme. This is different than the typicalsubtractive color scheme traditional pigments use. Withadditive color, the primary colors of reflected light (red,green and blue) can “add up” to white light (Figure 3).
Traditionally, blue and yellow may be mixed to creategreen, but if a blue interference and yellow interferencepearlescent pigment are mixed, a white reflection is created.For this reason, it is not advised to mix interferencecolors other than for mild shading.
The iron oxide coatings exhibit the same interferenceproperties as the titanium dioxide coatings, except theiron oxide contributes color of its own due to absorption.Bronze colors correspond to the combination of a yellowabsorption color due to the Fe2O3 and a yellow interferencecolor due to the thickness of the Fe2O3 layer. Copper andrusset colors are produced by similar effects (Figure 4).
Due to their inherent transparency, the color of pearlescentpigments can be affected by the color of the materialover which they are applied. When it is on a white background,the transmission color can be seen at all angles,and the interference color is not obvious. When it is on ablack or dark substrate, the transmission color is adsorbedby the substrate, and the interference color is strong andbright at the specular angle. At the specular angle, theinterference color is observed. At diffuse angles, the complementarycolor of that interference color is seen becausethe light is transmitted through the film.
For example, when an interference red pigment ispresent in a coating film, a red color is seen at thespecular angle and a green at the diffuse angle. Whitepearls are white at specular and will transmit eitherthe substrate or tinting color at diffuse angles. It is thesense of depth and color changes at different viewingangles that make a pearlescent pigment different froma metallic flake pigment.
In metallic flake pigments, such as Al, the aluminumflakes are opaque to light; no light is transmitted. Allthe light is reflected from one layer as in a mirror, andone cannot get a sense of depth as with pearlescentpigments. When absorption colorants are added topearlescent pigments, light penetrates through thetransparent platelets to the colorants and then reflectsback. Thus, the colors from the absorption pigments arebrightened and enhanced.
If colorants are added to metallic flake pigments, themetallic flakes being opaque do not allow the light topenetrate, and the colors are subdued. In the case of alu-minum, the aluminum itself has a grayish color, and thecolors from absorption colorants are actually muddied.
Metallic pigments, similar to pearlescent pigments intheir size and shape, reflect light in a specular fashion.That is, light coming in at a 45° angle will exit at a 45°angle. As the viewing angle is increased, the reflectiondrops off and the color appears dark. When one looks atdifferent viewing angles of a coating containing metallicpigment, only the lightness changes, not the color as seenwith pearlescent pigments.
There are several factors influencing the luster andappearance of a finished coating. Flake orientation, particlesize, pigment concentration and coating film transparencyare among the most important ones.
Pearlescent pigments are offered in several highly controlledparticle size distributions. The size will influenceluster, coverage and the appearance of coatings. Table 1features a list of common distributions and their apparenteffect.
The luster will generally increase as the particle sizeincreases, however, the optimum distribution is 10 to 60microns (Figure 5).
This particle size distribution is tight enough to ensurea uniform oxide coating across all the particles, helping tofurther boost the color intensity. It also creates a D50 thatis optimal for light reflection with minimal edge scattering.An example of this type of product would be SunMICAHigh Chroma Blue, 281-1257. The larger distributions willappear sparkly or star-like and provide the least coverage,while the small distributions will appear with a satineffect and have more coverage.
Particle size can be measured by several methods,including visual observation using a microscope, sievingand laser light scattering, which is the most commonapproach. In this method, a pigment sample is dispersedin a liquid while a laser is trained on the liquid sample sothat the platelets scatter the beam. Accurately positioneddetectors measure the scattered light. Large particles scatterto a narrow angle and small particles to a large angle.A computer calculates a size distribution based on theintensity of scattered light at the different angles.
Laser light instruments provide several pieces of information.They can provide differential or cumulative sizedistributions, percent below a specified size, size below aspecified percent (ex: D50), and a distribution mode (themost populous size interval). Each piece describes a differentattribute. No single piece is capable of completely describing the size distribution or the effect it will producein a paint. Therefore, it is often necessary to use severaldata points for this purpose.
An important point about light scattering instrumentsis that they assume the particle being measured is asphere; not a platelet. Therefore, the size distribution isrelative and not an absolute size measurement. In addition,different sizing instruments give different results.Large particles are emphasized because it takes thousandsof small particles to equal the weight of one large particle.
Maximum reflection and color travel are only achievedwhen the platelets are uniformly oriented parallel to thesubstrate. This is achieved primarily by film shrinkageand leveling of the coating while it is drying. It can also beaffected by the size of the particles.
Film shrinkage is directly related to the total solidsof the coating. Low-solids coatings shrink more thanhigh-solids coatings, thereby improving the orientationand color travel. This is one of the primary reasonswhy high-solids coatings containing mica are a differentcolor versus low-solids waterbornes, even though thepigmentation is the same. Better pigment dispersion, slowevaporating solvents used in the coating formulation,longer flashing time, or a ladder baking schedule will alsoimprove orientation of the pigments and achieve maximumluster. Figure 6 demonstrates this influence of twodifferent solids formulations.
In addition to the degree of orientation, orientationuniformity is also a factor affecting appearance. Poor orientationuniformity is referred to as mottling. The resultis light and dark patches of color and is usually visible onlarge flat areas. The dark areas are a result of platelets lying partially on edge. This problem is common to allflake pigments, including metallics. There can be severalcauses for this, including pigment flooding or floating,and convection currents created by solvent evaporation.Pigment flooding can be solved through proper dispersionand it will be discussed later.
Convection currents are caused by solvent evaporationproducing cooling at the surface of the coating. As additionalsolvent and heat rise from within the coating, theycreate a current that can tilt the plates on edge. Mottlingcan be reduced by using pattern control agents. Celluloseacetate butyrate (CAB), NADs (non-aqueous dispersions)and waxes are commonly used.
These are high-molecular-weight additives thatincrease the viscosity of the paint in a Newtonian fashionduring flash off and baking. The Newtonian increase inviscosity prevents the flakes from being tilted by the weakconvection currents but allows the flakes to orient as thecoating film shrinks. Thixotropic additives are usuallynot effective because they cause the viscosity to increasetoo quickly after spraying. They tend to lock or freeze theflakes in place in random orientation, not allowing themto flow out and level.
While a certain amount of pigment is needed to create acolor effect and reasonable hiding, excessive amounts ofpigment will quickly degrade the appearance. When toomuch pigment is added, the pigment particles contacteach other and prevent the free orientation of platelets,resulting in reduced luster. Another factor is edge scattering.Edge scattering is caused by light being scattered offthe edge of the platelet, increasing the diffuse reflection asopposed to the bright, specular reflection.
The optimum loading is generally 10-20% pigmentbased on total solids of paint. A lower amount can be usedto create subtle, soft effects. Sometimes, it may be moreadvantageous to use less pigment (1-1.5%) in order toobtain a pearlescent effect over a colored base coat.
It is also important to consider the particle size of thepigment when choosing a pigment loading. Larger-particlepigments are best used at lower loadings to avoid overcrowding.This results in a higher sparkle effect, as theindividual platelets are evenly spaced and the pinpointsof reflected light can more easily be seen by the eye. Pigmentsin the >200 micron range can be used at loadingsas low as 0.1%.
Given the optics of pearlescent pigments, it is importantthat light enters and exits the film unobstructed.Therefore, any pigment or additive that provides opacityor decreases light transmission (i.e., light scattering)through the film will reduce or eliminate the luster. Someexamples are pigmentary titanium dioxide, calcium carbonateand talc.
The disadvantage of the pearlescent pigment’s transparencyis the lack of coverage. It is possible, however, toformulate colors with a fair degree of coverage with dry film thickness of 15-25 microns. There are several ways toimprove the hiding during formulation.。
1. Smaller particle size of pearlescent pigment can be usedto increase hiding, but it will produce more satin andless brilliant luster. With the least appearance change,a portion of the large pearl pigment (not more than20%) can be replaced by a smaller-size pearl pigment.
2. Because of the opaque character, metallic pigments,such as aluminum or bronze flakes can be added intocoatings to improve the hiding. The amount is usually5-10% of the pearl pigment.
3. When a dark color is formulated, a small amount ofcarbon black will provide good hiding. The addition ofblack will increase the pearl’s interference color.
4. For a light color, TiO2 is a good choice to improve hiding,although the brilliant luster of the pearlescent pigmentwill be reduced. A relative large amount of pearl pigmentis suggested to use, so that the interference coloris stronger at a specular angle, and minimum amountof TiO2 is needed for hiding.
Keep in mind, the brilliance of a pearlescent pigmentis generally sacrificed when any opaque pigments or tintsare added to improve hiding as described above.
Pearlescent pigments can be dispersed with general mixing,similar to that used for metallics. A key benefit ofeffect pigments, both pearlescent and metallic, is thatthere is no need to grind or mill the pigments to fullydevelop their color/effect. Nevertheless, a proper dispersionis still key to maximizing the appearance.
A good technique is to make a pre-mix of 25-35% pigmentand 65-75% vehicle. Add increments of pigment tothe stirred vehicle using a low-shear, axial flow mixer.The pigment will simply stir in quickly because of therelatively large particle size and be wetted out by thevehicle. Additional portions of pigment may be added assoon as the previous addition appears completely wetted.Another 15 minutes of mixing should ensure a gooddispersion of pigment.
To prevent excessive fragmentation of the pigmentplatelets, or stripping of metal oxides off the mica substrates,high-shear equipment, such as ball or pebble mills, should be avoided. In general, pearlescent pigmentsshould be added to a formulation at the end of mixingcycle, after all other ingredients have been thoroughlymixed. It is possible to use a radial flow, “saw tooth” high-speed disperser (Cowles dissolver), providing the pigmentloading is below 25% and the tip speed of the blade isbelow 2,000 ft/minute. The height of the blade should bechanged during the mixing cycle to distribute the materialin the dead spot directly below the blade.
Incomplete dispersion of pigment can contribute to lowtinting strength, poor rheological properties, appearancedifferences and even poor performance. The dispersion ofpigment in coating can be verified visually using a microscopeor by a relative technique using the settling rate andsediment volume of the pearlescent pigment.
In the microscopic technique, a visual determinationis made to see if the platelets are agglomerated, as seenin Figure 7.
Precautions should be taken when using this techniquebecause normally the pigment concentration is too highin a finished coating to accurately determine the dispersion.It is often necessary to reduce the pigment concentrationin the coating with additional vehicle or solvent.This changes the system and may cause the pigment toflocculate or separate, leading to erroneous conclusions.
The settling technique uses the actual finished coatingor a modified coating containing no rheology modifiersto determine the degree of dispersion. After several daysof standing undisturbed, slow or incomplete settling indicatesa good dispersion, as does the minimum sedimentvolume. The softness of packing is also considered for theevaluation because it is related to the re-dispersion. Due tothe high density of pearlescent pigments, they will inevitablysettle faster than most pigments, but re-dispersionshould be simple if hard-packing is avoided. This is mostdifficult in waterborne systems.
Pearlescent pigments, like most inorganic pigments, havea specific gravity significantly higher than the vehicles inwhich they are dispersed, causing them to settle and accumulatein the bottom of a container. During storage, thepearl platelets will orient parallel to each other and compactclose together, squeezing the vehicle from betweenthem. The smooth surfaces of the platelets form a seal,resulting in hard packing, making redistribution difficult.
The occurrence of this hard packing layer can be mitigatednot only by proper dispersion and suspension techniques,but also by the physical and chemical properties ofthe coating. Since this sedimentation occurs due to electrostaticinteractions, the following actions can be taken topromote a softer sediment layer and allow for easy redispersion:adding non-polar solvents, increasing salt concentrations,decreasing the large aspect ratio particle sizes, andchanging the high isoelectric point of the finished coating.
Suspension in Vehicles
Good suspension means the pigment particles are heldwell in the vehicles, providing more resistance to thesettling tendency of pigment. Increasing viscosity ofcoatings is undoubtedly the simplest way to preventsettling by providing more resistance to pigment movement.However, it is limited by the application of coatings,and little can be done to change the viscosity. Furthermore,increasing viscosity will do little to preventhard packing settling during long-term storage.
An alternative is to build a three-dimensional networkstructure in the vehicle by adding some special additives(suspending agent or thixotrope) like fumed silica ororganophilic clay. These function by forming hydrogenbonds among themselves and with the vehicles, and evenpigments. As a result, the association between pigmentand vehicles creates a network that “traps” the pigmentparticles, and settling tendency is reduced. In addition,a clay or silica particle will also act as a spacer betweenthe platelets if they do settle, preventing a tight seal fromforming. The pigment can then quickly and easily be reincorporatedinto the vehicle.
The efficiency of these additives is dependent on thetype of solvent and resins. A variety of additives withdifferent surface chemistry is available to fit different systems.Usually these additives should be incorporated duringthe wetting-dispersion step to achieve the best effect.However, some clays or silica require high shear and/orheat to be fully activated effectively. This should be doneas a pre-grind prior to dispersing the pearl pigment. Careshould be taken to follow the manufacturer’s guidelines.
Pearlescent pigments are a versatile group of effect pigmentsthat are widely used in many coatings applications.Their color is a result of light interference based on theirphysical structure and composition. Their semi-transparentnature and specular reflection work together to producemother-of-pearl effects and interesting color travelcombinations. Pearlescent pigments are available in awide range of sizes and colors that are easy to formulateand apply following a few simple rules.