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Inks for IML: Proper Choices for Premium Results
by Tom Hammer, Product Manager, North America, Flint Group Narrow Web

Technology Feature

How much do you really know about the inks you use in your in-mold label (IML) applications? Are they really the right choice? What do you need to know about inks to make the right choice for your diverse applications of IMLs? How do the changes in the marketplace affect the inks you are currently using? No two inks are alike; just like in-mold labels, each ink is specially designed with its intended use characteristics and role in the end application in mind. What attributes are formulated into the ink and how inks are supported are key considerations in the proper selection of ink for premium results for IML applications.

Ink Performance Specifications
In order for ink to perform well for IML applications, the ink must provide acceptable on-press performance, time after time, as well as provide desired end-use characteristics for its application of use. Additionally, from the printer to the molder to the end user, the ink must satisfy the customer and be cost-effective in its use (robust).

Today, since virtually any commercial print process is used for IMLs, this holds true for all types and chemistries of inks. Whether the ink is printed with offset, letterpress, flexographic, gravure, screen, digital or combinations thereof, or whether the ink is water-based, radiation-curable, solvent- or oil-based, the ink needs to be selected based on its ability to provide the customer with acceptable results both during and after the printing process. Inks need to enable the printer to maintain minimal dot gain, clean halftones with high contrast and resolution, smooth solid line work, and prevent color variation throughout the job to provide best value.

In-mold label applications also cover a seemingly limitless potential of diverse end-use applications, from health and beauty products, food packaging, household goods and petroleum products to toys, other durable applications, and on and on. Each of these applications requires slightly different end-use performance based on its processing, handling, and consumer use:

  • Certain color hue and strength, opacity or appearance
  • Various levels of gloss or matte finish
  • Lightfastness requirements
  • Resistance to rub, scuff, or various chemicals to which the label will be exposed
  • Controlled coefficient of friction and static electricity
  • Low shrinkage and curl
  • Limited heavy metals
  • High-quality graphics
  • Good adhesion to papers and non-corona treated films
  • Fast drying or curing
  • Press-ready and press-friendly to all styles of operators and presses
  • Blow molded or injection molded (this is critical)
  • Surface printed or subsurface printed

    Today, the importance of color and graphics design on a label or package is well understood for the success of products in being selected by the consumer. A package has an estimated five seconds to make an impression; thus, it is very important that the ink designed into the product packaging catches the consumer’s attention. Any failure with the ink on the label or package during the lifecycle of the product will be disastrous to the product’s success in the marketplace. The label cannot fade in a storefront window or on the container in the greenhouse; it cannot bleed off a shampoo bottle or beverage container; and the label or package must be safe for medical devices and prescription bottles, or for use on toys. It is the ink chemist’s job to use the raw materials and end-use application knowledge to formulate the ink that offers the right choice.

    Ink Composition
    Raw materials used to produce inks can be grouped into various categories such as resins, diluents, drying or curing components, additives, and colorants. Each of these materials must be formulated in combination to work as a system to achieve final performance as required of the ink.

    The resin, or vehicle, is really the backbone of the ink and has a great effect on the performance of the in-mold label. In radiation-curable systems these are referred to as the oligomer. There are many types of chemistries available such as acrylic, polyester, urethane, epoxy, etc., and each presents the formulator choices for such ink attributes as pigment wetting, rheology, tack, adhesion, chemical, heat and scuff resistance, flexibility and shrinkage/curl, cost, and more. The resin carries the colorant from the press to the substrate and binds the colorant to the substrate. The ink resin must allow the ink to remain stable and not dry prematurely on rollers, anilox, plates, or blankets. This reduces downtime, preventing substrate waste and press stoppages to clean press components, which increases productivity and profits.

    Diluent is in the ink as a thinner or reducer, which controls the final ink viscosity. It can be water for water-based inks, organic solvent for solvent inks, and monomer for radiation-curable inks. Diluent, especially in water-based and solvent inks, can make up a rather large portion of the ink. In any case, the diluent needs to be carefully selected as it affects surface wetting, drying/curing speeds, cost, and print quality. The diluent should not contribute to ink flammability, human skin sensitivity, or other health, safety, and environmental concerns.

    The drying or curing components are primarily in reference to amines for water-based systems and photoinitiators in radiation-curable systems. Amines in the water-based inks are used to solubilize and stabilize the water-reducible resins in the water medium. Maintaining the proper level or selection of the amine (determined by the pH of the water-based ink) will have a critical effect on the ink’s performance on press in terms of viscosity, stability, drying rate, and print quality, as well as having a crucial effect on the water sensitivity of the finished ink film on the in-mold label. It is important to know what types of amine are in the ink! Relative to radiation-curable inks, the photoinitiators are the chemicals that respond to UV radiation-forming radicals that cause liquid ink to polymerize forming solid ink. Proper selection of initiators is critical based on the colorant concentration and color, and the film thickness of the ink being applied at a given rate of press speed. If the drying/curing additive is not properly selected for the ink formulation, print-process variables, and the end application intended, there could be severe problems resulting in poor adhesion, ink bleed, and more.

    Additives cover a wide variety of functions and chemistries and are used in all combinations, but sparingly. This category includes the following:

    • Waxes to impart coefficient of friction, slip, scuff, and rub properties
    • Dispersants for optimum pigment wetting and stability
    • Surfactants to lower surface energy of the ink to flow and level
    • Antifoam and defoamer to minimize foam in the manufacture of the ink or in the use of ink on press
    • Matting agents to control gloss levels
    • Inert fillers to affect tack and rheology
    • Anti-static additives so labels diecut, stack, and sort easily into molder
    • Various others additives

    Lastly, the colorant portion of the ink is comprised of a pigment or a dye. In our market application of in-mold labels and packages, the inks are predominately formulated with pigments. Common exceptions to this are fluorescent shades, which are primarily dye-based colorants. Pigments are selected over dyes because they are more physically and chemically stable in all types of ink chemistries. Pigments are not soluble in diluents (such as solvent, water, or monomer used in inks) and thus, must be dispersed into a resin or surfactant in order for the colorant to develop its color and to remain stable in a liquid ink form.

    The chemical structure of any pigment is denoted by a number, called the Color Index Number (C.I. #). This C.I. # is used globally to identify the chemical structure of the colorant. It also is a good indicator of the colorant’s properties such as hue, lightfastness, chemical resistance (solvent, alkali, oil), and cost. In selecting inks for your applications, you should know what C.I. # of pigment is used in your inks. Why?

    Because pigments come in a multitude of colors, and within each color spectrum, there are different qualities of pigment. For example, within a green shade yellow spectrum there is a Yellow 12, Yellow 14, Yellow 74, and Yellow 109. Each of these pigments has a different chemical makeup and thus will perform differently in varying conditions of application use, even though each can provide a similar color hue. The lightfastness rating of any pigment, called its Blue Wool (BW) Rating, is generally a good indication of the pigment’s quality. Typically, the higher the BW Rating, the higher the cost, and the better the pigment will perform under conditions of sunlight, chemical, and heat exposure.

    Another important attribute of pigment and defined by its C.I. # is heavy metal content. The primary heavy metals that are regulated in our industry are barium, mercury, hexavalent chromium, lead, cadmium, antimony, arsenic, and selenium. Legislation such as the Resource Conservation and Recovery Act (RCRA), Coalition of North Eastern Governors Model Toxins Legislation (CONEG), and Toy Safety Regulations such as ASTM F963 and EN-71 are good examples of this. Pigments are the primary contributors of these heavy metals in the inks and thus careful selection of the pigments will help to keep levels very low.

    It is just as important for the printer, as it is for the ink chemist, to know and understand the end application of the label or package and any intermediate processes it will undergo in the lifecycle of the product. With this knowledge, the proper selection of ink can be made, which will provide a label or package with performance attributes needed by the consumer at a reasonable cost. Just like the ink chemist, the converter has a multitude of variables to consider when selecting components for the in-mold label. Any changes to one of the components of the in-mold label may require the printer to ‘re-qualify’ the ink selected for that label construction. So remember, next time you need a certain color, you are not just qualifying the color but instead, are selecting a whole gamut of characteristics that is formulated into the ink, which you will need to consider. Discuss the requirements and options with your suppliers as, most likely, your supplier can offer you choices and advice on the best selection to give you premium results.

    Trends with UV Inks
    Today there is a trend toward UV inks for many label applications including IML for various reasons. Some of these include solvent-free, no emission; low maintenance/very good press stability; high printing speed; higher color strength; combination printing; less dot gain/dot spread and sharper text/lines; higher gloss; consistency in print quality throughout the run; better water resistance and chemical resistance; better adhesion and scratch resistance to filmic substrates; low odor available; and overall better value with UV inks.

    However with that comes some challenges with using UV inks. Heat generation from UV lamps will distort films, especially thin films if there is not heat management techniques on press (‘cool UV’ systems, chill rollers, etc.) Proper curing can be a challenge for both surface cure and through cure (which will affect adhesion) if applied with too coarse of anilox rollers or at too fast of press speeds. Shrinkage or label curl can occur as a result of UV cure chemistry, which requires proper ink formulation (oligomer and photoinitiator selection, proper anilox roller application, and screenprinting plates are important). UV inks generally impart more odor to labels and packaging and produce a tendency to have materials migrate toward product, which in turn requires special ink technology. Finally, some substrates may require corona treatment for adhesion but this imparts static (which requires static-elimination equipment).

    All of these challenges can be addressed easily! Selection of the coating is often very critical for IML applications, especially as it relates to static or slip/co-efficient of friction. This also is very dependent on whether the molder is using blow or injection techniques. No one coating can meet all requirements so it’s important to discuss with suppliers and understand the molding process and end application.

    Inks and coatings for IML as a labeling process require a different science to conventional inks and coatings. However, some similar functional requirements do exist. Due to processing and application processes, the IML label ink and coating must meet demanding requirements that are more specific. These include specific COF for accurate dispensing; certain anti-static properties for diecutting, dispensing, and overall processing; high scuff and abrasion resistance to prevent marring in handling; and balanced cross-linking to prevent shrinking and curl.

    IML inks and coatings have the ability to meet these functional requirements and match the finish of the molded package to achieve the seamless ‘no-label’ look that packaging customers desire. Work with your consortium of suppliers as a group to identify your goals, your problems, and ultimately, your solutions. To be most successful, look at the challenges from an overall process and not just from one component of the process. Working with your suppliers will allow you to make proper choices for premium results!

    Tom Hammer is product manager for North America at Flint Group Narrow Web, part of the Flint Group. He has worked in the printing ink industry for 23 years, mostly in a technical or marketing role, holding positions in product research and development, technical management, and product management. With Flint Group for 20+ years, Hammer has been instrumental in developing solutions to many print technologies and applications. For more information, he can be contacted at (763) 398-2725.