Products for Printing

Specialty Silica

Pigment Blacks

Quality criteria for black printing ink
The printer, as the end-user of printing inks and colors, specifically looks at the following criteria for determining black color quality:

• High color density (color depth)
• Blue tint
• Brilliance – insofar as the print medium permits
• Yield (mileage)
• Correct rheology

The requirements listed, however, represent a compromise as high color density implies fine-particulate small particle size pigment blacks that aren’t particularly prone to a blue tint in printing inks. The color tone of a pigment black depends mostly on the particle size (medium particle size) and the dispersion rate achieved. The finer the particles of the pigment black, the deeper the color and the browner the pigment will appear. The larger the particles, the lower the color depth and the more blue the pigment will appear. Brilliance or gloss is not greatly affected by particle size. Structure, on the other hand, plays an important role in achievable gloss. Color density, tone and gloss are also a function of the degree of coverage and dispersion achieved.

Effects
A number of different “effects” can be achieved by using pigment blacks in printing inks :

• Coloring
• Tinting
• Conductivity
• UV application

Coloring and tinting
Pigment blacks are used almost to 100% to color black printing inks and colors. They are also used to tint or tone gray and other colors (brown, olive green etc.), though this portion of applications is relatively small when measured against the overall pigment black consumption.

Coloring, Thickening
If one takes into account the desired effects of pigment blacks in printing inks, we distinguish between:

• the coloring component
• the rheological component

The coloring component comprises the elements of color density (color depth), tone and brilliance, while the rheological component focuses on the parameters of viscosity, flow characteristics and printing ink stability.

Color Density, Tone 
The coloring component is essentially dependent on the size of the pigment black’s primary particles. The finer the particles, the higher the color density of the pigmented ink – and vice versa. This is what we refer to when we speak of the “black value” in our brochures and bulletins. The higher this black value, the greater the color depth of the pigment black.

We can therefore infer:

• Decreasing particle size: increasing color depth, brown tint
• Increasing particle size: blue tint

Rheology 
The rheological component is also dependent on particle size and the resulting BET surface, in addition to structure and surface chemistry. Structure and surface chemistry are described in greater detail within our publication TI 1183 (Basics of Pigment Blacks).

Fine pigment blacks have a relatively high BET surface, which requires a higher binding agents demand and results in a marked thickening effect. As particle size increases, the BET surface gets smaller and so does the binding agent demand. The thickening effect is reduced as well. This means:

• Decreasing particle size: greater binding agent requirement, high viscosity
• Increasing particle size: lower binding agent requirement, low viscosity

Structure
Pigment black structure is an important criterion in printing ink manufacturing. A number of different quality factors depend, among other things, on structure:

• Dispersion properties
• Viscosity
• Flow characteristics
• Color density
• Brilliance
• Permanence
• Conductivity

The measure for the structure of pigment furnace blacks is DBP (Dibutylphthalate) absorption. If DBP absorption is high, we’re dealing with a high-structured pigment black; if it is low, the pigment black is of the low-structured kind. Printing ink blacks are in the 40 to 125 ml / 100 g DBP absorption range. Pigment blacks that have a DBP value lower than 90 ml / 100 g fall into the category “low structure”. A higher structure is an advantage when it comes to dispersion, as these kinds of pigment blacks can be much more easily dispersed in the ink to be pigmented. At the same time, the higher DBP absorption also means significant thickening and a high yield value – while the lower DPB absorption has a less pronounced thickening effect and results in better flow characteristics for the printing ink. Also, holdout is better in high-structure pigment blacks. Low-structure blacks have a tendency to bleed through the print medium, too. Since the high-structure pigment blacks tend to stay at the surface of the print medium, they feature less gloss and rub resistance. Printing results are smoother with low-structure pigment blacks because the smaller pigment clusters penetrate deeper into the print medium, resulting in better gloss and rub resistance.
To summarize:

High Structure

• High flow threshold
• High viscosity
• Good holdout
• Less brilliance
• Low resistance to smudging

Low Structure

• Not as easily dispersed or harder to disperse
• Good flow characteristics
• Decreased holdout
• More brilliance
• Good resistance to smudging

When gas blacks are used, the criterion is no longer DBP absorption but oil requirement, which also gives a good indication of thickening properties. Gas blacks essentially come with a high structure with good flow characteristics.

Electrical Conductivity
Depending on their structure, pigment blacks are more or less conductive. The higher the structure, the greater the conductivity. Conductive printing inks are made from high-structure pigment blacks. This special feature also gives conductive printing inks a relatively high viscosity. Good conductivity can be achieved by increasing the pigment black concentration and the ink quantity applied during the printing process.

UV Application 
UV-cured inks are becoming more and more commonly used. Black UV inks, however, can be challenging as pigment black effectively absorbs light both in the visible and in the ultraviolet band. The rule of thumb: the blacker the printing ink, the longer the drying time. Degussa developed a special black for this type of application to offset this disadvantage.

Recommendations
Printing inks are adapted to the specific printing process and the print medium used. The medium is of course an important criterion in pigment black selection. Low-structure pigment blacks help achieve a higher quality in terms of color density and brilliance on coated papers, for instance. On the other hand, high-structure pigment blacks achieve greater color density on non-coated paper stock.

Offset inks (heatset inks, sheetfed offset inks) on coated stock
When coated stock is used, color depth and brilliance are usually expected. Depending on the desired color properties, low-structure pigment blacks with smaller primary particles and their oxidized versions are usually the first choice for high-quality printing inks.

Offset inks on non-coated stock

– Offset inks
Surface roughness of the printing medium and its absorbency determine color composition. The best inks are those with a high flow threshold that do not penetrate that deeply into the uncoated, very absorbent paper and therefore deliver greater color depth. Flow characteristics and thixotropy can be defined based on the pigment black structure. Inks with good flow characteristics improve the resistance to smudging when penetration drying is the process used, although because they penetrate deep into the stock they lose some of their color depth.

– Newspaper inks
Inks used in the newspaper printing industry are made with medium to high.-structure pigment blacks. One of the less desirable characteristics of conventional newspaper inks is their tendency to smudging. This is mainly due to the fact that the “drying” process essentially happens as the paper gets whipped around the presses, precluding the pigment black from really getting stuck to the paper fiber – which facilitates later recycling. The pigment black’s structure is a factor in determining the smudge resistance: a low structure improves resistance to smudging; a high structure makes smudging easier. That is why a combination of both pigment blacks is often used.

Nowadays, beaded blacks are usually the norm rather than powder blacks. The beaded version does not produce any dust and greatly facilitates handling in automated processes. Of the available versions (oil, dry or wet), the oil-beaded pigment blacks are becoming more and more popular on account of their relatively trouble-free dispersion.

-Magazine inks
Processes in this industry involve very high printing speeds, which puts a premium on low ink viscosity. At the same time, insufficient viscosity means a drop in print result quality. Low-structure pigment blacks with a medium particle size help to meet this challenge while achieving good coverage, color depth and brilliance.

-Decorative and packaging inks
A wide variety of stocks are used in the decorative paper and packaging printing industry. Here, stock and binding agent are carefully fine-tuned, with a wide range of binding agents to choose from. Gas blacks and low-structure furnace blacks in both oxidized and non-oxidized form are the staple pigment blacks.

Water-soluble inks are also gaining in importance, particularly in the packaging printing industry. Here, low-structure and oxidized pigment blacks yield better results when it comes to printing on coated stocks, cardboard or non-absorbent surfaces. Low-structure pigment blacks display better flow characteristics and allow for higher pigment concentrations.

-UV / EBC cured inks
UV-dried or radiation-cured printing inks require short radiation or drying times. Too much color depth means too much UV or electron radiation will be absorbed. That is also why the binding agents require pigment blacks with good coverage because the pigment might otherwise not be fully dispersed. Here, low-structure, oxidized furnace blacks have consistently proved to be the better option by supporting good coverage with low viscosity, good flow characteristics and high brilliance.

-Conductive printing inks
Special furnace blacks are available for manufacturing electrically conductive inks. This property calls for a higher DBP absorption and greater BET surface, which distinguishes these blacks from regular pigment blacks. Though these parameters conductivity is improved, significant thickening results and the appropriate pigment concentration is determined for ink formulations. Conductivity levels can be adjusted with highly conductive pigment black with a very high structure (high DBP absorption) and a high BET surface. In this case, only a relatively low concentration will achieve the desired results. Conversely, a high-structure, coarser conductive black with a lower BET surface can be used in a higher concentration.

-Metal printing inks
Binding agents involved in making ink stick to metal require a pigment black that is easily covered, i.e. one with a high structure and with good color depth. Oxidized gas blacks deliver the best results. This pigment class is characterized by high color depth and trouble-free compatibility.

-Special applications (small offset, steel engraving)
The many printing inks for the most varied requirements also include special applications, such as small run offset printing and steel engraving. Although both applications are fundamentally different from another, they both require a relatively high viscosity and color depth. In the case of steel engraving, in particular, the ink also has to have a “short” consistency. Here, gas blacks easily fit the bill.

Matting Agents

As the world’s largest supplier of precipitated silicas and silicates, Evonik Degussa GmbH markets Sipernat® and ACEMATT® for use in printing ink formulations. Custom products are available for the most diverse inks, including water or solvent-based rotogravure or flex print inks, offset inks or silkscreen inks. Depending on the application, Sipernat types can be used as extenders to improve flow and drying characteristics, ACEMATT® products will deliver excellent quality in matte effect and allow for optimum rheology adjustment. Evonik Degussa’s synthetically manufactured products are also characterized by good transparency compared to mineral products.