Pigments vs Dyes: Understanding the Differences Between Dyes and Pigments
Pigments and dyes represent the two principal types of colorants. A basic level of understanding of the differences between these classes of colorants will provide a helpful foundation of knowledge for anyone tasked with selecting and identifying the right colorant for their product. These colorants are present in almost every product imaginable. The major difference between pigments and dyes is that dyes/dyestuffs exhibit their color by dissolving the dye in a solvent matrix or by attaching the dye molecule to a substrate; and pigments exhibit their color without any required chemical interaction. Not including dyed textiles, a properly dissolved dye will provide a translucent coloring effect (mouthwash, windshield wash fluid are good examples) while a pigment will provide a coloring effect with significant opacity (examples include paint, lipstick).
What is a Dye?
Dyes are organic (carbon containing) molecular compounds with a structural portion known as a chromophore which is attached to the auxochrome functional group of the dye molecule. The chromophore group absorbs visible light and allows the compound to exhibit color through selective absorption. Other structural components of the dye molecule contribute to its ability to dissolve in a particular solvent, or attach to a particular substrate. Dyes are categorized by their chemical and physical properties. Examples of some common dye categories and types of dyes are Acid Dyes, Basic Dyes, Solvent Dyes, Direct Dyes, Mordant Dyes, Natural Dyes, and Reactive Dyes. Different classifications of dyes will take advantage of varying dyeing processes to maximize their performance, some of which rely on binding agents and mordants.
Dyes, which are typically synthetic dyes in most modern applications, are used in a truly vast array of industries and applications which include cleaning products, foods, drugs, cosmetics, , the paper industries, household products, industrial products, , hair dye and hair coloring products, leak tracing products, candles and waxes, plastics and resins, adhesives, lubricants, pH indicators, textile dyeing, inks, etc.
Some products require a consistent unchanging color effect. Some products require a colorant that changes its appearance between specific chemical environments such as changes in pH levels. Some products are designed to provide a stable stain on a substrate which may rely on the use of a binding agent or mordant through the colorant use and application process. Some dye products are designed to exhibit color without staining. Certain industries may have specific requirements for colorants. Foods, drugs, and cosmetics sold in the U.S. have very strict requirements for colorants which are governed by United States Food and Drug Administration (FDA). Most other countries also have regulations governing the use of colorants in various consumer products.
Requirements of a True Dye
- Exhibits a visible color
- Soluble in the desired solvent, or adheres strongly to a desired substrate
- Consistent coloring effects within a chosen environment
What is a Pigment?
Pigments are solid organic or inorganic compounds that exhibit color in their solid form; i.e. they do not need to interact with their chemical environment in order to exhibit color. Organic pigments are commonly used in the colorant industry. Organic pigments contain carbon in a crystallized and organized structure, and can sometimes be obtained naturally. Some examples of natural organic pigments include, natural indigo pigment which is obtained from the woad plant and chlorophyll which imparts a green color and is often derived from alfalfa and similar plants. Inorganic pigments are chemical compounds that are usually derived from metallic salts and which do not include carbon in their molecular structure.
Pigments are typically ground or milled into powder for use. The powdered pigment can be suspended in a viscous liquid product, a paste, or a solid. Pigments are often the colorant of choice for paints, lipsticks, plastics, and rubber for example. Pigments do not dissolve in their chemical environment. Instead, pigments are suspended, or dispersed in their environment. The dispersion is sometimes enhanced with a dispersing agent suitable for the specific pigment and the host matrix. The suspended pigment particles will typically provide an opaque, or “cloudy” coloring effect. Pigments are generally more resistant than dyes to the effects of heat and oxidizers. Some pigments may exhibit much greater lightfastness than many dyes do.
Examples of common synthetic inorganic pigments are titanium dioxide, iron oxides, and ultramarine blue and other ultramarines, cobalt blue, and even some versions of carbon black. Synthetic pigments are used in applications like automotive products, cleaning agents, paints, coatings, stains, drugs, cosmetics, cement applications, plastics and resins, and a large number of other products. Organic pigments are commonly used in paints, coatings, polishes, stains, resins, cleaning agents, adhesives, foods, drugs, cosmetics, and many more applications.
Requirements of a Pigment
- Insoluble solid
- Consistent color in its pure form
Similarities Between Dyes and Pigments
Pigments and dyes are used in a wide variety of products to impart color. These colorants can be natural or synthetic. They are used in textile dyeing, foods, cosmetics, paints, inks, pharmaceuticals, candles, cleaning products, wood stains, and countless other items. The tinting strength of dyes and pigments both depend on their concentration, chemistry, and chemical compatibility in the solution or environment in which they are used.
Differences Between Dyes and Pigments
The main difference between dyes and pigments is that dyes provide their color by attaching to a substrate, or by dissolving in a solvent through which the dye becomes part of the material chemical solution; whereas pigments provide color without any required interaction with another substance. A dye works to exhibit color at the molecular level, and each dye molecule is individualized within the chosen matrix. Even though the pigment particle size may appear very small to the naked eye; pigment particles are often thousands of times larger than the individual dye molecules.
There are numerous examples of both dyes and pigments. Some dyes are readily available, while some are difficult to procure. The same is true for pigments. The costs of dyes and pigments vary widely and are generally related to the complexity of producing the colorant, quantity being purchased, availability, purity, and evaluation (purity testing) requirements.
Dyes and pigments are ubiquitous in industrial and consumer products. They impart color through different methods. Pigments will provide color on their own and can be mixed with liquids, pastes, and solids to provide a coloring effect that is usually opaque. Pigments may require a dispersing agent to maintain a homogeneous distribution in some less viscous liquids. Dyes show their coloring effect through interaction with a solvent, or through attachment to a substrate. These molecular level interactions contribute to the observed color, solubility, physical properties, and chemical properties of the dye.