Pigments and dispersions for coatings and graphic arts applications: part 1.
The desire for a new color is a reason many consumers choose to paint. Color affects the mood we wish to create and provides a fresh backdrop for the overall style in the spaces around us. However, selecting the perfect color can be challenging, as demonstrated by the indecisive chameleons who struggle to select their preferred color in a current advertisement. Yet the technology behind colorants does much more than deliver color: pigment contributes to a coating's durability, lightfastness, and opacity. as well as other properties.
For formulators, understanding the differences between pigment families is important when selecting the pigments suitable for paints. transparent stains, specialty coatings, energy-cure coatings, inks, or digital printing coatings. Equally important is delivering consistent color to the end-use application every time, which will be the focus of Part ll in this series, to be published in an upcoming issue.
WHY PIGMENTS HAVE COLOR
Pigments have color because they absorb light at some wavelengths more than at others. When a pigment absorbs light, it excites electrons from lower to higher energy orbitals. The unabsorbed light is reflected back to the eye, and the change in the reflected spectrum is perceived as color. Colorless molecules do not absorb visible light because the energy difference between the lower and higher molecular orbital is larger than the energy of light at visible wavelengths (Figure 1).
Pigments fall into two classes: inorganic (transition metal-based) and organic (containing carbon-carbon bonds and usually derived from petroleum). Generally, organic pigments have a more vibrant hue and higher tint strength than inorganic pigments. Inorganic pigments are known for their outstanding lightfastness and their insolubility, which prevents bleed and migration in coatings and inks. Inorganics are also typically less expensive, which accounts for much of their popularity. Within these classes are compound families with similar structures (Table 1). For paint applications. both organic and inorganic pigments are used. For stains, inorganic pigments are primarily used. For specialty applications and inkjet printing, organic pigments are typically used, as organics can create more stable dispersions due to their lower density.
Table 1--Pigment Families by Class and Their Related Structures and Colors ORGANIC PIGMENT FAMILIES Type Structure Typical colors Phthalocyanine Macrocyclic structures with varying types Blues, and degrees of halogenation, few greens carbon-hydrogen bonds Quinacridone Linear polycyclic heterocycles Magenta, reds, maroon, violet, gold Azo (Mono and Contains at least one nitrogen Orange, Disazo) double-bonded to another nitrogen in the violet, structure yellows, reds Other Non-Azo organics Perylenes, Polycyclic DPP Red INORGANIC PIGMENT FAMILIES Type Structure Typical colors Iron Oxides Derived from ores or synthetically Black, produced brown, sienna, red, yellow, gold, blue Cadmium Cadmium sulfides/cadmium selenide Yellow, compounds orange, red Other Single Titanates, aluminates, chromates, Violet, and Mixed Metal molybdate compositions (nickel, copper, green, Oxides manganese, chromium, lead, etc.) blue, yellow, orange
PERFORMANCE PARAMETERS: TRANSPARENCY, CHROMA, AND LIGHT-STABILITY
Lightfastness is determined by the pigment's molecu-lar/crystal structure and particle size. Fading will lower an item's perceived value, causing consumers to view it as old and out-of-date rather than new and appealing. This perception is particularly important for durable items such as flooring and cabinets. For architectural paints, minimizing color fade is critical for exterior use, but is also important for interior.
In UV applications for coatings and graphic arts, balance is crucial because the UV light must penetrate the coating to cure the film, but the final film must resist fading and substrate degradation caused by ambient UV light. Although both transparent and opaque pigments can be used for UV-curable applications, opaque pigments such as iron oxides make curing difficult because they absorb UV light. A new line of Trans-Oxide.' dispersions bridges the gap between high lightfastness and proper UV cure.
Non-UV graphic arts applications have their own challenges. Applying color onto a host of substrates is a growing application for inkjet, creating demand for higher performance colorants. The first generation of inkjet printers utilized dye-based inks to accommodate the shortcomings of the hardware then available. These inks had obvious deficiencies--for example, bleed and fading from light and ozone. Today, inkjet printers increasingly use dispersions of pigment colorants to address these issues, an area under active development.
A pigment's transparency or opacity largely depends on its physical structure. Larger, irregular surfaces scatter light in more directions and appear more opaque. As the pigment's particle size approaches and drops below the wavelength of light, there is less scattering, and it appears more transparent. The particle shape is also critical. For example, transparent iron oxides are chemically identical to their opaque counterparts, but the crystallization process creates either needle-like shapes, which are transparent, or shapes like snowflakes, which are opaque (Figure 2).
Particle size also impacts lightfastness in inks and coatings. Pigments with larger particle sizes--common in industrial and architectural coatings and certain inks--exhibit increased lightfastness. However, reducing the pigment's particle size can adversely impact light-fastness and contribute to fading. For this reason, it is essential in applications that require smaller particle size dispersions, such as inkjet, automotive coatings, and transparent stains, to select a pigment that balances lightfastness with other critical performance properties tailored for the end use (Figure 3).
In addition to lasting power and durability, a pigment's vibrancy is essential to deliver the color payoff that consumers desire. In their reflectance spectrum, pigments that appear "clean," bright, and chromatic have narrower, sharper absorbance peaks in their spectra caused by a narrower energy range for absorbed light. Conversely, pigments that absorb more wavelengths will appear muddier.
Once an electron in the pigment has been raised into a higher energy orbital, the pigment molecule loses energy by any of several different pathways. The more light-stable the pigment is, the more likely this path simply returns the electron to the original lower energy or "ground" state. However, some paths may lead to a different destination with a new chemical structure and a changed spectrum. These paths may proceed through the formation of radical and triplet electronic states, oxidation of the excited state, etc. Pigment structures more likely to follow these paths are less light-stable [see Photochemistry and Photobiology, Vol. 31, pp. 627-629 (1980)].
HOW SOME EXCITED ELECTRONIC STATES AFFECT LIGHT STABILITY AND CHROMA
When it comes to photodegradation, a pigment's structure is only as stable as its weakest link. Most organic dyes and pigments absorb light because they have vr-electrons in adjacent (conjugated) bonds that spread some of their molecular orbitals over a larger part of their structure than just the space between two atoms. Spread-out molecular orbitals decrease the energy necessary to excite an electron from a lower to higher energy, allowing the pigment to absorb light at visible wavelengths. These delocalized n-electrons may be in aromatic rings or in double bonds between carbon or nitrogen atoms. Azo pigments containing the (-N=N-) structure are examples of some of the brightest, most chromatic yellow pigments. This makes them highly desirable as part of a CMY color set. Three families of organic pigments, common in coatings applications, have become important for inkjet use: phthalocyanines, quinacridones, and azos.
Phthalocyanine blues and greens contain a transition metal complexed by a conjugated aromatic macrocycle, making them a hybrid inorganic-organic pigment that exhibits features from both classes. Phthalocyanines such as pigment blue 15:3 are among the most light-stable organic pigments. The excellent light stability of copper phthalocyanines is attributed to spin orbital coupling between the phthalocyanine and copper atom. shortening the triplet excited states' lifetime [J. Chem. Soc., Faraday Trans. II 74. 1870-1879 (1978)]. It is interesting that although high-level molecular orbital calculations predict a single spectrum peak, the actual spectrum shows a second peak (Figure 4), which changes the pigment's shade and chroma (http://www.rzepa.net/blog/?p=3641,8/12/14). This may result from changes in orbital energies due to interactions between pigment molecules in the crystal structure. However. attempts to model this imperfectly reproduce the actual spectrum's features (http://www.ch.irnperial.ac.uk/rzepa/blog/?p=3736.8/12/14).
Beyond the molecular level, the crystal structure substantially impacts properties. The spectral and color properties of quinacridones in isolation are quite different from the crystalline pigment properties. Dilute solutions of quinacridone are pale yellow, but in the solid state, it is bright red [App. Phys. Lett., 101, 023305 (2012): J. Phys. Chem B. 110, 19154-19161 (2006)], indicating the color properties are dramatically affected by molecules interacting in the crystal. Hydrogen bonding and dipolar interactions between quinacridone molecules are strong and may promote quicker relaxation of reactive excited states along pathways that help preserve the quinacridone's chemical structure (Figure 5). For this reason, quinacri-dones are quite light-stable for organic pigments.
There is large variability in the light stability within azos. Unfortunately, the excited states of many azo pigments are particularly susceptible to reactions that change the pigment's structure, compromising light stability. For example, the strong, bright azo Pigment Yellow 74 (PY74) is perhaps the most commonly applied yellow pigment for coatings and digital printing, where lightfast-ness is critical. Like many pigments, PY74 is offered in a variety of particle sizes and crystal structures, which enables formulators to tailor end-performance and pigment shade. For example, larger-particle PY74 (red shade) used in coatings is known for good light stability; however, at the very small particle sizes required for inkjet, this property suffers. Yellow pigments are the weakest member in CMY sets for digital printing. More light-stable yellow pigments may be used for digital printing, but many lack the chroma, strength, and hue angle of PY74. Pigment Yellow 180 (PY180) is also an azo, but it is a member of the azo benzimidazalone subfamily. PY180 has color properties similar to PY74, with excellent shade, strength, and brightness. However, PY180 has very good light stability, even at miniscule particle sizes (Figure 6). Benzimidazolone azo pigments have very strong associations between molecules. These strong intermolecular interactions within the crystal may also promote quicker relaxation of reactive excited states, contributing to light stability. But this strong tendency to self-associate also makes colloidal dispersions less stable. A stable, self-dispersed, nanoparticle dispersion based on PY180 has recently become available.
Each application--whether architectural or interior coatings, UV or inkjet--comes with its own challenges related to lightfastness. stability, durability, and vibrancy. These color performance properties derive from the individual pigment molecule's structure. the interactions between molecules within the pigment crystal, and the pigment particle's size and shape. Small particle sizes needed for jettability in inkjet increase transparency. but also complicate achieving good lightfastness. Conversely, large particle sizes used in coatings result in higher opacity, lightfastness, and durability, but may complicate UV applications where it is essential for the UV light to penetrate during cure. Selecting the right pigment for a formulation can be challenging due to the wide range of pigments available today, but it also allows a high degree of tailoring to ensure that a pigment exhibits the ideal balance of properties for a given end use.
By Greg Duncan and Charles Lubbers, Emerald Performance Materials
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|Title Annotation:||FORMULATOR'S CORNER|
|Author:||Duncan, Greg; Lubbers, Charles|
|Date:||Oct 1, 2014|
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