Energy Curable Basics

Inks: "Drying" vs. "Curing"

In simplest terms, drying of an ink film occurs with conventional inks when the ink vehicle (solvent, oil) evaporates or is absorbed, leaving behind the solids (pigments, resins, waxes, driers, etc.) to form a film on the substrate surface.

In energy curing, on the other hand, all of the components in the ink or coating remain on the surface of the substrate, but are chemically transformed into a hard film through exposure to Ultra Violet lights (UV) or a concentrated beam of highly energised electrons (EB).

The difference lies in the chemistry of the materials in the inks and coatings, and in the pressroom equipment needed to "energise" the curing process.

Figure 1 is a graphic illustration of the relationships of the various ingredients in conventional, UV curable and EB curable printing inks.

 

Rudimentary Ink Chemistry

The materials used in energy curable ink formulas are considerably more "user friendly" today than ever before. Advancements in raw materials will continue to make energy curable inks and coatings more common place in the years ahead. The major components of UV and EB printing inks are:

• Reactive diluents (monomers)
• Fluid resins (oligomers)
• Photoinitiators (UV only)
• Additives
• Pigments

The jobs of each of these chemicals are as follows:

• Reactive Diluent (monomer): A reactive diluent monomer is a simple, lightweight chemical which is similar to a solvent in its ability to thin down the ink. Monomers determine the surface characteristics of the inks such things as gloss, hardness and flexibility. Monomers with high draize values are also the chemicals which give uncured UV and EB inks their most hazardous characteristics.
   
• Resin (oligomer): The resin in energy curable inks is actually called an 'oligomer'. As in conventional inks, the resin is the chemical backbone of the ink. Among others, it provides the body, wetting ability and binding properties of the ink.
   
• Photoinitiators: In UV inks, the photoinitiator is the chemical which becomes 'excited' and starts the curing reaction when exposed to ultraviolet light. The excited photoinitiator passes that energy to the other components. At that point, any component which becomes excited has the ability to attract other components to itself and transfer energy to the newly attracted component.
  
  Photoinitiators are not required in EB inks. The highly charged energy of the electron beam is sufficient to activate polymerisation.
   
• Additives: Additives include waxes, wetting agents, and rheology modifiers. They provide the added customising touches to the inks.
   

• Pigments: Pigment particle size and concentration can affect the curing rate of UV inks. Pigments are therefore selected not only for colour but for their wettability or oil dilution ratio, and for their receptivity to UV light. Among process colours, yellow and magenta are the easiest to cure, followed by cyan and black. Because black tends to simply absorb the wavelengths of UV light, more energy is required to obtain a satisfactory cure.

Pigment selection is not as critical in EB inks since the energy level of the Electron Beam is sufficient to cure even heavy ink films, regardless of colour.

Polymerisation
Any component that is excited or energised is called a 'free radical'. It is the free radicals that have the energy to keep the curing or "polymerisation" chain reaction going. Each chemical chain continues growing until one of two things happen:

• The excited chains use up all of the available components, or
• Some foreign substance, such as oxygen, 'quenches' or halts the reaction.

In the 1990s, improved print quality, together with growing economic and environmental advantages, created increased interest in energy curable inks and coatings. Today, UV and EB curing are used in nearly every printing process, from letterpress to flexography; on nearly every substrate from paper and paperboard to foil and film; and for both sheetfed and roll-to-roll applications.

Offset sheetfed and web printing are by far the most widely used applications. The high viscosity paste inks used for this type of printing are ideally suited for energy curable printing. Offset applications can be run both wet and dry trap depending on lamp configuration. Dry trap has a curing station after each printing unit, while wet trap has a curing unit only after the last printing unit. (See EQUIPMENT.) EB inks are run wet trap since the high energy levels achieved during curing do not require interstation curing.

EB curing is particularly well suited to aseptic packaging applications for fruit juices and wines since the full cure achieved eliminates concerns about set off as the web substrate is re-wound.

Letterpress printing is very well suited for rigid plastics applications such as cups, tubs and pails.

Screen printing is well suited to applications where high film thickness is required, e.g. posters.

Flexography is today's most rapidly expanding UV printing application, especially in narrow web labels. Wide web label manufacturers have recently entered into the UV flexo market and should enjoy the growth experienced in the narrow web market.

As flexo printing improves in quality and application, the need for specialised physical properties also continues to grow. Increased chemical or product resistance is the largest attraction to this process. The additional benefits of low energy cost, minimised downtime, and the reduction of VOC's will also continue to drive the market.

Coating applications can be performed either on or off-line depending on facilities and equipment capabilities. The most common application is roller coating.