Products for Plastics

Specialty Silica

Pigment Blacks

Applications in the plastics industry involve pigment blacks not only for coloring purposes, but also to improve the durability of light and weather-sensitive plastics. They are also used to improve conductivity and antistatic properties of polymers.

Pigment Blacks for coloring
Normally, 0.5-1% pigment black is enough to color plastics in a full tone black. In those cases in which the plastic has a color of its own (e.g. acrylnitrile-butadien-styrol), quantities of pigment black may be necessary to achieve desired pigmentation.
Jetness, color strength, hue etc. are commonly used words when it comes to coloring plastics and explained below.

Jetness represents the blackness – the intensity of the coloration – achievable with a pigment black. The lower the degree of light refraction and light diffusion, the greater the color depth.
Color strength is the degree to which pigment blacks can darken or blacken other coloring constituents – in contrast to white pigments, which are used to lighten colors. If the idea is to only slightly darken a color (be it white or another color), we speak of toning or hueing.
Jetness and (theoretically) color strength tend to increase as the pigment’s primary particle size and structure decreases.

When coloring plastics with pigment blacks, the blackening process results in a certain hue that, depending on the type of pigment, can go either into blue or brown. Here, one must distinguish between surface appearance and transparency when plastics are blackened in a pure tone.

The surface appearance of a black pure tone coloring shows a blue hue that reinforces the sense of color depth, especially when a fine pigment black is used. The structure of the pigment black also influences jetness. With even, fine particles, a low structure will shift the hue towards blue. Coarser pigment blacks with a higher structure, on the other hand, tend to produce a brownish hue.
The effect is reversed when we look at transparency: in the case of a semi-transparent film, for instance, a layer with a pure tone coloring with a pigment black that has a small particle size and low structure will result in a brown hue; coarser pigments with a higher structure will result in a bluish hue.

In the case of white mixes (grayscale), coarse pigment blacks with a high structure lead to a gray with a slight bluish tint.

Good pigment dispersion is crucial to a pigment black’s optimum jetness and color strength. In other words, pigment black agglomerates must be completely dispersed for maximum coloring effect. It is important to note that achieving optimum dispersion becomes more difficult when a finer particle size and a lower structure pigment black is used.

The form (powder or bead) in which the pigment black is supplied also plays a role when it comes to dispersion. Though beaded blacks are easier to ship and handle (higher bulk density, less dust, more accurate dosage etc.), they do not disperse. For this reason, powdered pigment blacks are recommended for coloring plastics in processes that involve limited shearing forces (e.g. when coloring PVC-P, liquid polymer systems) or for toning.

Dispersion problems and dust build-up inherent to handling pigment blacks can be avoided by using Pigment Black Preparations that contain the pigment black in a wetted and well dispersed form.

The added cost of a preparation is more than offset not only by a cleaner handling process, but also by a higher yield (lower pigment dosage for same results) and lower mechanical dispersion requirements.

Pigment Blacks as UV stabilizers

UV stabilization is an important application for pigment blacks in the plastics industry. Their ability to absorb light effectively protects plastics that are exposed to direct sunlight against the photo-oxidative decomposition. Here, pigment blacks fulfill several functions:

• Shielding of plastic surface from radiation across a broad wavelength band
• Transformation of light energy into heat
• Stabilization by capturing radicals

The degree of UV protection provided by a pigment black depends on the type used and usually improves with increasing carbon black concentration.
Polyolefins in particular suffer damage from UV light. Trials using polyethylene have shown that a 2% concentration of a sufficiently fine pigment black will deliver adequate UV protection. Allowing for a safety margin, the upper concentration limit is, as a general rule, 2.5 %. This limit for adding pigment black is in line with most regulations governing the use of pigment blacks in goods used in the food industry. It also ensures that important plastic-specific properties, e.g. mechanical specifications, are not altered beyond measure.
Pigment blacks in the medium specific surface range and with a good dispersion rate are best suited as UV stabilizers. Here, high-structured Regular Color Furnace Blacks with medium-fine particles (19-25 nm) are the recommended solution as they maximize these two key properties. In practice, these pigment blacks are mainly used in the form of highly concentrated pigment black/polyethylene compounds.

Carbon Blacks for conductivity
Plastics are generally not conductive and are very effective insulators. Adding the amount pigment black necessary for coloring or achieving required UV resistance will have virtually no effect on the plastic’s electrical properties. However, once a relatively high concentration is exceeded (and this threshold is a function of plastic and pigment type), electrical resistance suddenly drops and gradually decreases with increasing pigment black concentrations.
High-structure and/or high-surface Color Furnace Blacks and Conductivity Blacks (and High Conductivity Blacks) are especially suited for achieving antistatic properties ort or modifying conductivity. The required amount of pigment black depends on pigment used, binding agent, process and desired specifications.
With homogenous distribution of the conductive black in the system, a pigment black content between 5 and 35 percentage by weight is usually the minimum requirement. Here, the method with which the black is added and the dispersion process play a central role. Differences in processing conditions may lead to wide variances in electrical properties, even if the conductive black concentration remains the same. This is due primarily to process-related orientation phenomena involving the pigment black particles within the matrix and also, to a lesser degree, to differences in gross volume. Also, the type of polymer system used will have an effect on the electrical resistance curve. Though increasing the pigment black concentration in polymer systems will deliver a lower electrical resistance, this does not come without a number of less desirable side-effects, e.g. increased viscosity, reduced thermal stability and disrupted curing or polymerization reactions. These side-effects, however, can mostly be contained to acceptable levels with appropriate additives or by modifying the formulation. Here, the end-user has the choice of a number of alternatives to pure conductive blacks that range from conductive black/polymer compounds to special preparations.
Applications for plastics with enhanced conductivity include packaging for sensitive electronics, semi-conducting cables, floor coverings, manufactured plastic goods in protected areas, conveyer belts, explosives breather tubes etc.

Matting Agents

In addition to ACEMATT® products based on precipitated silicas, Evonik Degussa GmbH also supplies ACEMATT® OP 278 for special applications.