• Andy Richards

New dispersing agent for PR57.1

A new dispersing agent for Pigment Red 57.1 in UV systems. One of the most difficult pigments to disperse and stabilise in UV inks are the Pigment Red 57.1 type. It is one of the standard pigments used in a 4-colour set in printing inks.

The basic structure of the PR57.1 is shown below and the one most used in UV printing inks is the calcium salt, which is also known as calcium 4B toners. They tend to have deep masstones and a distinct blue undertone.

PR 57.1

CAS No: 5281-04-9

The shade of the reds is partly determined by the nature of the metal cation, due to the electronic interaction with the organic part of the pigment. Some examples: strontium based ones have a blue tone but less than the calcium-based ones. On the other hand, barium based ones have a yellow shade.

The metals are divalent which are associated with the organic azo molecule to form a complex. However, the bonds between these are quite weak and chelating agents can abstract the metal from the pigment.

Also, these metal-based pigments are susceptible to react with some anionic species which could be present in the formulated ink. This is seen readily in water-based systems using acrylic resins. The divalent calcium cation can bond with the acrylic resin which can cause an increase in the molecular weight and an increase in viscosity is a consequence of this. As part of Lankem’s commitment to producing innovative products and solutions, an extensive programme of work has been going on for some time. As a result, several patented products have been produced, with the typical structure being shown below.

These high molecular weight dispersants have a pigment affinic anchor group and a tail section which is highly compatible with the dispersing medium. Also, these materials have a third capping group which further strengthens the steric hindrance.

A ladder series of experiments was carried out on a typical red pigment PR57.1 ( Sincol Rubine 3160 -D32), comparing a standard UV dispersant with Lansperse UV51, with the optimum level in the simple UV formulation shown below.

DPGDA – di propylene glycol diacrylate (UV monomer)

The UV monomer and dispersant were added first and mixed with a high-speed mixer with a saw type blade. Finally, the pigment was added and again mixed with the high-speed mixer. The mill-base was then fully processed using an Eiger Torrance bead mill, running at 2000rpm for 60 minutes.

Samples were then measured for viscosity using a Brookfield viscometer and then stored at room temperature for many days and then re-measured. Viscosity profile

The viscosity profile of the Lansperse UV51 and the industry standard are comparable, but we do see a lowering in viscosity and possibly improved stability in the Lansperse UV51. The mill-bases were then made into a simple UV ink as shown below and coated onto Leneta card using an Automatic TQC film applicator and cured with a laboratory GEW UV unit.

The cards were then assessed for gloss and colour analysed using a Konica Minolta CM-5 spectrophotometer.

The photo-initiator (PI) blend was a concentrate made up of the following:

Standard Lansperse UV51

Colour analysis

Gloss Readings

The results of this work in a simple UV system show that the Lansperse UV51 performs as standard and produces a mill-base which has a lower viscosity.

A more detailed study comparing various dispersing agents and a number of different PR57.1 pigments against the Lansperse UV51 will provide more information, but initial results are very encouraging. Another positive benefit of using the Lansperse UV51 is the cost which is very competitive to the market leaders.

Featured Product: Lansperse UV51

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