Introduction to color

This article looks at the role of inks and coatings from multiple points of view, to build an understanding of how ink is manufactured, stored, specified, mixed, used on press and tested before final despatch of the finished label roll.

The label industry is unique as the only sector of the wider print industry to still use every printing technology: letterpress, flexography, lithography, flatbed and rotary screen, gravure and the various flavors of digital. All ink chemistries are used including solvent, water-based, oil based, UV, LED-UV and Electron Beam (EB).

The way inks are manufactured is directly related to the requirements of each print process. Printing inks are classified according to their viscosity into either liquid or paste inks.

Liquid inks are employed in gravure and flexo printing, while paste inks are used in letterpress and lithography.

Screen inks are intermediate between paste and liquid inks.

After delivery to the converter we follow the ink though the various steps onto the press, including quality control at materials – in, color mixing, proofing on the production substrate and delivery to the press. We look at how different ink chemistries key (adhere) to different materials and the different ways each ink is dried by penetration, oxidation, evaporation or ‘cured’ by interaction with ultra-violet light or EB energy.

Finally, we look at testing procedures for the dried ink film which ensure the printed label is fit for purpose for its intended end use. Labels will encounter a range of hostile conditions including high speed applicator and packaging lines, rubbing against other containers and attacks by UV light, moisture, chemicals, and extremes of heat and cold. The colors must remain vibrant, the text fully legible and the cosmetic appearance pristine throughout.  

Mention must also be made of the complex legislative environment faced by label converters and their supply chain. We look in some detail at how inks and coatings form a key part of compliance programs.

MEASURING COLOR

Before embarking on this journey, we need to consider the meaning of ‘color’ as it relates to ink selection, and how color is defined and measured.

The first thing to point out is that ‘color’, as such, does not exist. What appears to us as an object’s color is what is left after parts of the white light spectrum have been absorbed and reflected back. Thus, a sandstone rock appears red because it absorbs the wavelengths which define ‘green’ and ‘blue’ and reflects the remaining ‘red’ back to our optical system. The properties of the object surface – for example how porous it is – will affect how wavelengths are absorbed or reflected, and thus an object’s perceived color.

Colors can also appear to shift under different lighting conditions, a phenomenon known as metamerism. Technically, metamerism is the difference between the reflection curves of two colors which look the same under a given form of lighting, such as an electric bulb light.

Only colors which display the same remission curves will be perceived as being the same color under multiple light sources.

The appearance of a color will also look different depending on the substrate it is being applied to. The smoother the substrate surface, the more light will be reflected and the lighter the color will appear. A rougher or more penetrative surface can make the same ink color look darker, while an off-white substrate can also change the appearance of the color.

We have also to take account how the human optical system processes color. It is estimated that color blindness (or Color Vision deficiency, CVD) affects approximately one in 12 men (eight percent) and one in 200 women in the world. In the UK, for example, this means that there are approximately 2.7 million ‘color blind’ people – about 4.5 percent of the entire population – most of whom are male.

Given the difficulty of defining and describing a color by observation alone, researchers have built models of human color perception which allow color to be described according to agreed and measurable criteria.

COLOR MODELS

A key breakthrough was made by Thomas Young and Hermann von Helmholtz as far back as 1805 with their theory of three-color vision. The researchers observed that although white light is composed of light from a continuous spectrum of wavelengths, humans only perceive three bands – blue, green and red. Any other color can be created from a rough mixture of these three (see Figure 1.1). 

Thus, red and blue light together produce magenta, blue and green produce cyan, and red and green produce yellow. These colors are known as subtractive because they are produced by ‘subtracting’ one of the three primary colors from white light (see Figure 1.2). 

This insight led ultimately to the development of the 4-color printing system, which uses Cyan, Magenta and Yellow, plus Black, to generate a wide range of color shades described by different percentages of each color.

Black is known as the Key color.

Although C+M+Y should print black, in reality it is closer to a shade of brown.

To reproduce a color image, the file is separated into the four different colors. During separation, screen tints comprised of small dots are applied at different angles to each of the four colors. The screened separations are then transferred to four different printing plates, one for each color, and run on a printing press with one color overprinting the next. The composite image fools the naked eye with the illusion of continuous tone (see Figure 1.3).

The alternative to separating a color into screened elements is to mix a single color, known as a Spot, or Solid color. These inks are pre-mixed to a set color value and printed without screens or dots. They are typically used for brand colors or for the colors outside the gamut of 4-color process inks.

The CMYK model worked well for describing how colors are built, but did not describe the different ways humans perceive color. This led to the development of the Hue, Saturation & Brightness/Lightness (HSB/L) model, which describes three fundamental characteristics of color, imagined along a color ‘wheel’ (see Figure 1.4).

• Hue represents the color reflected from, or transmitted through, an object. It is expressed as a degree between 0° and 360° on the color wheel. Hue is usually identified by the name of the color, such as red, orange, or green.

• Saturation represents the strength or purity of the color (sometimes called chroma). Saturation represents the amount of gray in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge.

• Brightness is the relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).

Another widely adopted model based on the human perception of color is L*a*b, developed by the Commission Internationale d’Eclairage (CIE), an organization dedicated to creating standards for all aspects of light (see Figure 1.5).

In this model, imagined as a 3D sphere, ‘L’ stands for ‘Luminosity’, ‘a’ for the Red/Green axis and ‘b’ for Blue/Yellow. The numeric L*a*b values describe how a color looks, not the colorants or constituents which make it up. L*a*b is therefore considered a ‘device-independent’ color model which can be used by color management systems to map a color predictably from one ‘space’ to another.

One common example is to map the millions of colors achievable on a computer monitor to the limited range of colors achievable with the CMYK ink system. This allows a designer to predict how the colors she sees on her monitor will reproduce when printed on a 4-color press.

EXTENDED GAMUT COLOR

Up to quite recently CMYK was the province of offset printers. Flexographers have mainly mixed their own inks on a job by job basis to hit the desired color on press. Mixing is performed in an ink ‘kitchen’ either manually by highly skilled operators, or on automated mixing systems (see article Automating ink logistics).

At least one reason flexo took this route was the dot gain observed on early flexo plates, combined with the variables of manual impression setting and gear lash on shaft-driven narrow web flexo presses. These factors made it difficult to hold the precise dot percentages demanded by the 4-color process. In addition, the flexo industry lacked the rigorous standardization of the offset inks sector.

This began to change with the development of servo-driven web transport and print units, which made much more precise control of web tension and registration possible. At the top end of the flexo press spectrum we are now seeing presses with fully automated control of web tension, print pressure and register.

At the same time, ink manufacturers and pre-press specialists including Kodak and Esko started promoting extended gamut color (EGC) systems which add Orange, Violet and Green (O,G.V) inks to the standard CMYK set, allowing printers to hit a much wider range of vibrant colors without having to mix separate colors for each job.

For the converter, this means that the same anilox and inks can be left in the press between jobs, eliminating the need to clean down the print units, so greatly reducing press downtime. ECG also places more emphasis on getting the job right in pre-press through scientific measurement and rigorous standardization rather than chasing color on the press.

Many brands still insist on using a solid (spot color) ink for key brand colors. But the impact of digital printing, where spot colors have not been available, has been to make flexographic ECG systems more acceptable to label buyers, providing the printer has the process under control.

PIGMENTS AND COLOR

Inks gain their color properties from colorants. Colorants are grouped into pigments and dyes, with the difference between the two the degree of solubility: pigments are insoluble, and dyes fully soluble.

DYES

Dye inks consist of small particles (1.5–4 nm), which have a larger color gamut and more vibrant colors than can be achieved by pigments, but are very prone to fade in the presence of light and ozone. Because Dyestuffs are fully soluble in the vehicle, they require additives to make them more resistant to water.

Dyes are used mainly in the manufacture of water-based flexo inks and some specialist inks for heat transfer printing, in ‘invisible’ (fluorescent) inks and some security inks.

PIGMENTS

Pigments provide the coloring matter that goes into an ink to give it, say, a red, blue or green color (see Figure 1.6). Although pigments may appear the same color when first printed, they can perform quite differently after they are processed . for instance, when they have been exposed to strong sunlight, used in a chemical environment, or passed through a heated shrink sleeve tunnel. In these examples the pigments may change color or ‘bleed’ to differing degrees. An understanding of pigment chemistries and the end-user requirements can therefore be an important factor in ink sourcing.

The robustness of pigments makes these inks the main choice in the label printing industry. This makes it worth looking at pigment technology in more detail.

Pigments consist of larger particles than dyes (50–200 nm), which are tightly packed together and are held in a suspension. They have a narrower color range (or gamut) than dyes, but they are more resistant to fading from light and ozone.

Pigments are divided into two main classes – inorganic and organic. 

• Inorganic pigments are used to formulate achromatic (literally ‘without color’) inks. The most important are titanium dioxide for White inks, and carbon black, the only pigment used in the manufacturer of Black ink.

• Organic pigments are used in the manufacture of colored inks. For green and blue shades, phthalocyanine pigments are commonly employed, and for red and yellow inks azo pigments are most commonly used. 

Pigments are identified, globally, by a Colour Index (CI) number that defines the particular chemistry of the pigment, enabling it to be classified.

In addition, there are specialist pigment types:

• Metallic pigments consist of small metal flakes which act as tiny mirrors. Silver pigments are made of aluminum and gold pigments of brass or dyed aluminum. The pigments are orders of magnitude larger than ordinary pigments.

• Pearlescent pigments are coated with very thin layers which include titanium dioxide and silicate. The thickness of these layers is only a fraction of the wavelength of visible light, so parts of the reflected light are extinguished by interference.

• Florescent pigments deliver brilliant inks by absorbing light in the visible or UV range and emitting it at another, longer wavelength. This means most fluorescent pigments are red or yellow. They are most effective when illuminated in the dark with UV light.

INK SPECIFICATION

It is not just color fidelity and consistency that we are seeking when specifying inks. We are equally interested in the conditions to which the print will be subjected after it leaves the converter’s despatch bay, and this may affect the choice of ink system used.

For example, achieving the required resistance properties suitable for after-processing may well limit the pigment choice.

This means that converters should always obtain a full specification of the job before any planning decisions are taken. This requires:

• The substrate to be printed, with a sample if possible

• The target press and print process

• Any further in-line processing steps

• The end use of the print, including any special lighting, temperature, atmospheric, chemical, storage or product conditions to which the label will be subject.

We will return to the subject of ink specification in more detail within further articles.