Polyvinylchloride Coloration - Processing Properties

Dispersion

With dispersion the degree of which pigment agglomerates are broken down into primary particles and aggregates is meant. The more finely the pigment is dispersed (smaller average particle size), the larger the total surface area of the pigment particles and the greater its colour strength is. The hue (shade) and the opacity of the coloured material are also affected by the particle size distribution of the dispersed pigment. As a general rule to disperse agglomerates of transparent organic pigments higher shear forces are needed than when dispersing agglomerates of opaque organic pigments. The performance of a dispersion depends furthermore on the particle size distribution especially the size and amount of larger particles. A dispersion suitable for injection moulding applications may not always suitable for film extrusion. Oversized particles may have a dramatic influence at processing: for example: filter blockage or undesired splitting of a film. Also the physical and mechanical properties of the end article may be affected in a negative way. Due to this Ciba develops pigments specific to these application areas, with controlled pigment particle size distribution; and also a number of preparations designed for particular polymers.

Calendering Effect

This is an effect observed particularly in PVC. It refers to the development of colour strength at high levels of shear during calendering. It is dependent upon crystal forms, particle size and how strong the primary particles of the pigment adhere together as agglomerates. Calendering effect can be characterised by % of colour strength developed at high shear (during cold milling of PVC according to DIN 53775) compared with low shear ( warm milling) conditions. Transparent organic pigments, because of their very small particles, show much less calendering effect and less development of coloristic properties by fast high shear mixing than more opaque pigments.

Compaction

This is the re-agglomeration by mechanical means of the pigment to form hard, virtually impossibly to disperse particles. This can take place in mixers, feeders, and compounding machinery. Compaction occurs where pigment and polymer is exposed in the solid state, to increasing pressure and friction. Certain pigments can be more prone to this phenomena than others. The sensitivity of pigments towards compaction depends on the physical properties of the specific pigment. By handling, storing and generally treating the colorant more carefully this phenomena can be avoided.

Pigment Preparations

Pigments in a prepared form such as Ciba® MICROLEN® MC - for PO and selected engineering plastics and Ciba® MICRANYL® NQ-F for PVC, possess significant advantages when used in production processes over their powder pigment variants. They exhibit no calendering effect, and thus offer greater shade consistency and reproducibility. They are insensitive to agglomeration by high shear fast speed mixers during dry-blending, and have higher colour strength (at representative comparable concentrations), purity and transparency. Other key features of these products lie in their ease of metering and thus incorporation; better value in use at similar strength ratios; and significantly better correlation from laboratory to production scale up. They are also much cleaner to handle and thus offer more hygiene.

For further information, please contact your local Ciba representative.