Polyesters improve pigment wetting of flexographic inks.

The key to achieving high-quality UV flexographic inks lies in pigment wetting, ink rheology and the reactivity of the formulated system. Rheological behaviour is directly related to pigment wetting and accounts for many of the required properties of the formulated ink. Of equal importance are

The aim of printing is to place an image on a substrate by applying the thinnest possible ink film that will give the desired colour strength. Developments in equipment and in the production of better quality UV inks has contributed to the expansion of the UV flexographic process as a suitable alternative printing method to conventional flexo, gravure and offset printing. The major advantage of the flexographic process is its ease of use and versatility in comparison with the other methods used in the printing sector.

High-quality flexographic inks rely largely on the final rheology of the system, with the right amounts of carefully chosen pigments. Rheology is directly related to pigment wetting and, together, these endow the formulated ink with many different properties such as good dispersion, good flow, good transfer to the printing equipment and to the substrates, correct viscosity and print colour strength. Other important criteria are reactivity, adhesion and odour after cure of the formulated ink.

Promising systems require compromise

Flexographic printing being the most promising growth market in the UV ink industry, most ink manufacturers are promoting and pushing for the development of systems that are better adapted to this process. However, because the properties referred to above are all inter-related, formulators are obliged to make compromises, such as formulating UV pigmented systems of low viscosity with high reactivity and low odour after cure.

Flexographic inks are prepared in two stages. The first stage involves the preparation of the concentrated pigment paste, the grinding stage, from several types of resins, additives and pigments. In the second stage the pigment paste is mixed with a letdown vehicle to obtain a liquid ink in the dilution stage, using a number of types of resins, diluting acrylates, photoinitiator systems and additives.

The ink is evaluated on the basis of the final ink rheology and print quality. Rheological properties are tested using flow-plate and cone-plate viscometers. Gloss, light diffusion, transparency, shade and colour purity evaluations are performed on transparent PP prints using an RK Print (flexo K printing proofer). To be considered of good quality, UV flexo ink must flow, be transparent and glossy, with high colour strength and shade purity, odour and adhesion properties are also considererd.

Letting down design vehicles

The challenge for ink manufacturers is to produce an ink that is fluid and transparent using pigments, additives and resins that will give the correct pigment wetting properties in the manufacture of the pigment paste and the right diluting oligomers, additives and PI systems for the preparation of the letdown vehicle. This is where finding the right compromise comes in.

Pigment wetting can be described as an interaction between the chemical structure of the binder and of the pigment that helps prevent the flocculation process and colour development from the pigment as well as the gloss of the prints. Production of good ink of any type relies primarily on overcoming the difficulty of wetting dry organic pigments in the ink vehicle. The principal benefits of a well-wetted system are good flow rheology -- as Newtonian as possible in the case of flexography -- and good final printing quality.

Today, polyester acrylates and epoxy acrylate resins are the main constituents of UV radical inks. Several approaches to successful pigment wetting have been tried and polyester acrylates have proved to be among the best pigment wetting resins by the use of sophisticated polyols and/or fatty acid modifications. The principal advantages of epoxy acrylates lie in their capacity for increasing the cure response, chemical resistance and hardness of the final prints.

Print quality

The choice of the starting resins determines the print quality. Polyester acrylates give better pigment dispersions than standard epoxy acrylates, which lead to poor print properties. The flocculation process can be observed by microscopy (Figure 1) or by printing highly diluted inks.

[Figure 1 ILLUSTRATION OMITTED]

The most important criteria relating to pigment optimisation are final rheology and transparency of the prints for the dedicated application. Formulators have to optimise their use of pigments to achieve the best compromise between flow and transparency. Opaque pigments have mostly large pigment crystals and are either only slightly surface or not at all, which gives good fluid inks. Unfortunately, they cannot be used in four-colour printing applications because they exhibit colour shifts when overprinted (usually encountered with yellow pigments). Transparency is mainly a function of the pigment's chemical nature, crystal size and surface treatment during pigment and ink manufacturing.

Because epoxy acrylates are used in so many ink applications, we developed new materials especially adapted for pigment wetting properties. These materials are conventional bisphenol-A based epoxy acrylates chemically modified (without additives or any other artefacts) to achieve much better ink characteristics.

A substantial improvement is obtained in this way, permitting the use of larger quantites of pigment without any excessive effect on the rheological behaviour of the paste. Comparison with standard epoxy acrylates shows the difference -- the old systems producing strongly thixotropic behaviour, and new materials exhibiting better Newtonian behaviour at the same pigment concentrations.

Spectacular improvement in pigments

While the improvement is already of considerable interest in the case of organic pigments, it is positively spectacular in the case of inorganic pigments, such as Ti[O.sup.2] and carbon back.

In addition to the development of improved pigment-wetting prepolymers, the use of dispersing aids such as surfactants and dispersants can be of benefit in wetting highly pigmented systems. The obvious benefit of good pigment wetting is the development of maximum colour strength from the pigment, often the most expensive component of the ink. Unfortunately, dispersants are by nature specific to particular types of pigments, as a result of which several different versions are needed to cover a complete range of colours.

There are at least two mechanisms by which pigment-wetting agents can help the dispersion process: one is the reduction in the surface tension of the dispersing medium, promoting better wetting of the pigment surface by the grinding liquid; the second is stabilisation of the dispersed pigment particles by steric stabilisation, which tends to prevent the pigment particles from flocculating.

Pigment wetting and reactivity are sometimes mutually incompatible properties, and the development of binders better suited to the flexographic process calls for products with pigment-wetting properties and high reactivity in equal measure. Materials such as acrylated soya bean oil are somewhat slowcuring and the spread of their use has been restricted.

Most of the materials used in the ink industry today are based on polyester acrylate chemistry and offer the best compromise in terms of reactivity by having a high functionality and much better pigment-wetting properties than is the case with materials of chemical origins. Moreover, polyester acrylate chemistry offers the possibility of preparing oligomers having a low final viscosity without pre-dilution, such as amino-modified polyester acrylates that are helping to get low final viscosity and higher reactivity in flexographic ink formulations.

To lower the viscosity of inks will inevitably require the use of diluting acrylate and, unfortunately, the best monomers with strong dilution properties are not always the most suitable in terms of final rheology. A good choice will result in a fluid Newtonian ink, whereas a bad choice can result in low viscosity but high yield values that can be a problem in the printing stage. Diluting acrylates of low molecular weight will penetrate into porous substrates, such as paper, and will give poor print gloss.

Ink profiles

To obtain a low viscosity and the right rheological profile without the problems of penetration into porous substrates, it is recommended that you use materials with a slightly higher molecular weight to decrease the viscosity of the systems. When these materials are combined with low-viscosity polyester and/or polyether acrylates, it is possible to prepare a mixture with properties that are more suitable for the preparation of the letdown vehicle.

Adhesion is also a limitation in the sense that flexographic inks are today mostly applied onto plastic substrates in the packaging or in the label industry, the most important substrates being PE, PP, PVC, PET and PA.

Highly reactive resins with a high degree of acrylation often result in extensive shrinkage compared to the traditional resins used in the radiation-curing technology. Because only a very thin ink layer is applied to the plastic substrate, it is possible to use highly reactive resins. The shrinkage problems being decreased by the low thicknesses applied. However, on materials such as polypropylene these highly reactive resins do not always give the desired result. This is the reason why chlorinated polyesters were developed in the past.

Currently, the market is calling for highly reactive resins with good adhesion characteristics to be applied to plastics and difficult substrates. An interesting way to achieve a satisfactory adhesion/reactivity/pigment-wetting compromise is through the use of flexible epoxy acrylates that are currently under development. These materials being elongated materials of higher molecular weight seem to offer excellent pigment-wetting properties, good chemical resistance and good adhesion to plastics when compared to standard EA.

On account of the need to overcome the problems of strong oxygen inhibition in thin layers ([+ or -] 1[micro]m) and the high reactivity requirements of the graphic arts market, the photoinitiator package often reaches 8-12% of the final flexo ink formulation.

Major advances have been made in the field of photoinitiators in order to achieve suitable odour levels for packaging applications, but today low odour is still only obtainable through compromises on the final reactivity of the inks. One possibility lies in decreasing final odour by the use of acrylated benzophenone type materials, but these systems are unfortunately not particularly well suited to the curing of inks.

Low odour, adhesion and reactivity can be obtained through the use of products capable of curing without photoinitiator systems. Chlorinated polyester chemistry having advantageous pigment-wetting properties offers the possibility of build materials having good cure response without a photoinitiator system and, accordingly offer low odour after cure and good adhesion characteristics to plastic substrates.

These materials work in a non-pigmented system but require small amounts of PI systems into ink formulations, because of the UV absorbtion by the pigments. By decreasing the photoinitiator content of the formulations it is possible to obtain final inks with a much lower odour after cure. Moreover, these materials are also efficient for excimer 308nm curing in combination with low amounts of carefully chosen photoinitiator systems, offering another option for low-odour applications.

Obviously, the formulation of UV flexographic inks calls for an unexpectedly high degree of skill, and the formulation of a UV pigmented system with low viscosity, high reactivity and low odour after cure constitutes a considerable challenge.

Flexography is not just a passing trend. It is the most promising technology in the radiation curing graphic arts market. Fortunately, equipment, raw material and photoinitiator manufacturers as well as printers are pushing radiation-curing flexography as a highly promising technique for the future of printing technology.

A question of compromises! Today, when used on the right equipment UV ink formulations are already giving results as good as those obtained by other conventional techniques.

The development of more suitable resins for this industry will help formulators to produce more efficient inks, better adapted to the market for which they are intended.

Author: P. De Micheli, UCB Chemicals, Belgium