Tech Articles
UV COATING PROPERTIES ENGINEERED FOR IN-MOLD LABEL APPLICATIONS
by Jim Wittig - Managing Director
Introduction and Topics of Discussion:
- Major Advantages of UV Curing Process
- Types of Chemistry Used to Formulate UV Coatings
- Advantages and Disadvantages of the Two UV Chemistries
- UV Coating Properties Required for In-Mold Label Applications
- Factors That Affect the Gloss and Performance of UV Coatings
- Review of Important Properties of UV Coatings
- Summary
1. Major Advantages of UV Curing Process
Most UV inks and coatings are 100% solids with no volatile or flammable solvents. The wet film thickness deposited on the substrate does not decrease significantly during UV curing from evaporation of ingredients. The chemical components used in UV curing react very rapidly upon exposure to high intensity UV light, which allows high production outputs. The chemical reaction creates a crosslinked polymer network in the properly cured UV ink or coating. This provides excellent chemical or solvent resistance and excellent abrasion resistance properties. The high solids allow very high gloss to be achieved under proper conditions. The process is much more energy efficient than conventional drying processes. UV curing requires much less energy than electric or gas drying since the UV light can be easily focused where it is needed at the substrate surface. The chemical components used in UV inks and coatings contain stored energy in the molecules, which is released when the coating is irradiated with UV light. This enables the UV process to be designed to handle heat sensitive substrates. Water and solvent inks and coatings require more energy to evaporate the carrier during drying. The curing equipment is relatively compact and can be mounted in tight spaces on printing and coating equipment. Since UV products don’t evaporate, they do not dry on press and are easy to clean up.
2. Types of Chemistry Used to Formulate UV Coatings
There are two major types of chemistries used in UV curing, free radical chemistry and cationic chemistry. Acrylate monomers (reactive diluents) and oligomers (reactive resins) are the primary components of the free radical based formulations, giving the cured coating most of its physical characteristics. Acrylate monomers are low viscosity components which are the major component of low viscosity formulations used in flexo and gravure applications. Acrylate oligomers are the major component in UV rotary letterpress and offset coatings. Photoinitiators are required to absorb the UV light energy, destabilize to form free radicals, which attack the acrylate group C= C double bond and initiate polymerization. Additives are used to control coating slip, gloss, stability, rub resistance, antistatic, and other physical properties. Typical additives often include silicones, waxes, and inhibitors.
Cationic chemistry utilizes cycloaliphatic epoxy resins and vinyl ether monomers as the primary components. Photoinitiators absorb the UV light to form a Lewis acid, which attacks the epoxy ring initiating polymerization. Similar to acrylate chemistry, additives are used to improve coating performance.
3.Advantages and Disadvantages of the Two UV Chemistries
Acrylate chemistry is the most widely used because of its versatility. More raw materials are available to achieve a complete range of properties. Some acrylate raw materials are made in high volumes and are priced very competitively; this enables formulation of lower cost UV coatings. However, specialty acrylate resins which yield much better performance in demanding ink and coating applications, are often much higher in cost. This accounts for the large spread in UV product cost and performance. Better solvent and abrasion resistance can be achieved with acrylates. They have better compatability with water and solvent borne inks. Acrylate coatings are inhibited by oxygen at the coating surface. This has a slowing affect on the coating cure, especially in very thin coatings, less than 1 micron. Nitrogen inerting is sometimes necessary to achieve optimum properties, especially abrasion resistance. Since most UV presses do not have nitrogen inerting capabilities, formulations must be designed to overcome this inhibition and still yield excellent performance.
Cationic formulations are generally lower in shrinkage, which can result in improved adhesion. They are also slower to develop full cure, and post cure is a major factor. Baking can be used to achieve excellent solvent resistance on heat stable products. These coatings generally have excellent barrier properties to water vapor and gases such as oxygen and carbon dioxide. The raw materials are generally lower in toxicity, possessing lower skin irritation potential. They also can be formulated to provide low odor and low extractables. Because they are acid catalyzed upon UV exposure, any amines will inhibit curing completely. These coatings can not be used over waterborne inks, which contain ammonia or amines. Solvent borne polyamide inks are also a problem, as are basic papers and UV products that contain amines. This limits use in many printing applications where incompatible materials are used extensively.
4. UV Coating Properties Required for In-Mold Label Applications.
The In-Mold label process involves insertion of the die cut and stacked label into the bottle or container mold cavity just prior to blowing or injecting the molten polymer into the mold. The label insertion equipment can jam if the label does not have the proper characteristics. The label must lay flat so that it feeds smoothly and then nests properly inside the mold cavity. The tendency for the label to curl can be increased significantly by shrinkage of the UV coating. Therefore, the formulation must be designed to minimize shrinkage stress on the label. Factors that affect shrinkage include type of formulation, UV coating thickness, label stock film thickness and density, UV ink and coating coverage, and size of the label. The label stack feed system also requires that the labels have excellent lubricity. The UV coating must have excellent slip qualities in order for the top label to release from the label below without jamming. The UV coating must also have antistatic properties for film In-Mold label stocks. Static electrical charges will cause labels to stick to each other causing jamming.
Once the label is applied to the container, protective and cosmetic properties are required of the UV coating. Most blow-molded containers have a satin or lower gloss finish. The UV coating can enhance label graphics by having high gloss qualities that stand out from the rest of the bottle- - - or, if the customer prefers, can match the gloss of the bottle providing a no label look. Label protection is one of the primary functions of any UV coating. In-Mold applications are usually very high volume applications. The labeled containers can be subject to severe punishment during automatic filling, boxing and shipping. It is essential that the label UV varnish have excellent abrasion resistance properties. The UV varnish must also provide good product resistance properties. Solvent resistance and or water resistance are often required. The type of label ink used will have a significant effect on final performance. UV flexo or rotary letterpress inks generally have better product resistance. Water and solvent inks can have a retarding affect on the UV varnish cure. Volatiles that are still in the ink can be absorbed by the UV coating and interfere with proper curing. Ambient humidity, type of stock, and percentage of ink coverage can all affect quality of cure.
In mold films usually are polyethylene based which results in a tough flexible film. This requires that the UV coating have excellent adhesion and toughness. The UV coating must stretch with the film at the edge of the label in the die cutting process. If the UV coating does not have enough elongation, it will crack and chip at the label edges. Adhesion of the UV coating is a function of several parameters. First a print treated receptive film surface is required to obtain good wetting and adhesion of the UV coating. The UV coating must have low shrinkage to minimize stress at the film/UV-coating interface. The resin and monomer components used in the formulation must have a good affinity to the particular film substrate. The UV coating must be completely cured at the interface, any unreacted material will weaken adhesion. UV coatings are often loaded with a fluorescent tracer for machine readability. The intensity of the fluorescence is greatly affected by the background color of the printed label. White is the best background since it reflects the light back at the label reading equipment. Dark colored inks will absorb light and decrease the fluorescent intensity significantly.
5. Factors That Affect the Gloss and Performance of UV Coatings
Proper flowout is very important for achieving high gloss in UV coatings. Formulation additives such as silicones are used to improve the smoothness and gloss of the coating. Wax additives usually have a detrimental effect on gloss and are sometimes used to produce a satin finish. Silica flattening agents are used when a very low gloss is required. Viscosity is an important feature of any UV coating and is usually adjusted to the particular application equipment. On films generally lower viscosity will provide better flowout. Viscosity is greatly affected by temperature. Lowering the ambient temperature will cause a rapid increase in viscosity and reduce flowout.
Surface energy of the substrate is very important and is usually measured in dynes. Higher dyne levels indicate a better ink and coating receptive surface. An approximate rating system for UV coatings is as follows: above 44 is excellent, 40-43 is good, 37-39 is fair, 34 - 36 is marginal or a potential problem, below 33 is trouble. This is not by any means a universal test, many films, inks and coatings behave differently. UV coatings have different abilities to wet surfaces which is referred to as surface tension. The higher the surface tension the poorer the coating will wet and flowout. Therefore, the surface tension of the UV coating must be lower than the film’s surface energy in order to achieve good wetting.
Equipment design is a significant factor in achieving a high gloss or smooth coating. The distance between the application of the coating and the UV curing lamps affects the time the coating has to flowout. Because UV coatings are 100% solids, there is no solvent present to lower viscosity and aid in leveling. Increasing the distance between application and curing can significantly improve gloss on nonporous substrates such as films and foils. The wet film thickness also has a significant impact on gloss. Thicker coatings increase the mobility of the top surface of the UV coating which can smooth out faster providing a higher gloss. The application equipment controls the coating laydown pattern and thickness. Flat or rotary screen equipment is used to apply coatings from 4 microms to 100 microns. This is one of the best methods for appling a smooth heavy laydown of coating and generally provides very high gloss values of 85 to 95 at 60° when sufficient flowout time is provided, however, it usually limits production speeds. Gravure equipment is sometimes used for application of UV coatings. It requires a low viscosity of 100 cps or less to achieve good flowout. Gravure presses operate at fairly high speeds, 400- 800 FPM, and tend to apply a relatively thick and heavily patterned coating. Therefore the need for a very low viscosity and sufficient flowout time is imperative. Flexo application equipment is one of the best methods for achieving a smooth uniform coat weight between 2 - 12 microns. Very high gloss coatings can be achieved when adequate flowout time is provided. Viscosities range from 50 to 5000 cps with the average being between 100 - 400 cps depending on the application. Higher viscosity coatings tend to go on heavier and decrease mileage for a particular anilox. Unfortunately, most narrow web presses do not provide adequate flowout to achieve a very high gloss at speeds greater than 200 FPM. Letterpress equipment applies a very thin and uniform coat weight of about 1-2 microns. It requires a relatively high viscosity to allow proper transfer between the rollers. Typical viscosities range from 2000 to 20,000 cps. Letterpress paste inks are significantly higher in viscosity than UV clear coatings. UV offset coatings range from 70,000 to 120,000 cps and also apply a very thin coat weight of 1-2 microns. High gloss coatings can not be achieved with letterpress and offset application equipment..
Other factors that influence gloss and also coating performance include foam, surface bloom, paper or film porosity and smoothness and surface energy of the substrate. Excessive foaming in screen, gravure and flexo applications will create pits or voids in the coating which will decrease gloss and, if severe enough, product resistance. Surface bloom is when some of the unreacted components of the UV coating migrate to the coating surface causing a hazy or foggy surface. Something is generally wrong with the formulation or curing conditions resulting in too much unreacted material migrating to the coating surface. Inadequate drying of inks under the UV coating can slow the cure or leave unreacted components in the cured coating, which can cause fogging. Substrate porosity and smoothness has a major affect on coating gloss and product resistance. Porous papers will soak up low viscosity UV coatings preventing proper gloss development and product resistance. Absorbtion into the paper stock can be a major problem since uncured UV components that remain underneath the paper surface will create possible migration problems which can seriously degrade the label product resistance and can cause contamination of the labeled product. Indications of a porosity problem are stronger than normal odor from uncured components, poorer gloss and product resistance than is usually obtained with the UV varnish. Some matte films can also have a high porosity. These types of stocks should generally be avoided. However, if they must be used consult your coating and ink supplier for proper recommendations as these problems can be overcome by use of either primers or special varnishes. Surface energy of the substrate has a major effect on coating smoothness, since the coating will bead up on low energy surfaces. Corona treatment will enhance surface energy on many film substrates. It helps vaporize any surface contaminants, such as film additives or plasticizers, if present, and etches the film surface. Effectiveness of the treatment will last for different time periods depending on the particular type of film.
6. Review of Important Properties of UV Coatings
The primary purpose of UV coatings for In-Mold Label applications is to provide protection to the label. This includes product and abrasion resistance. Most In-Mold applications are polyethylene bottles for shampoos and detergents. Therefore excellent water and detergent resistance are more important than solvent resistance. Excellent solvent resistance does not necessarily mean good water or detergent resistance. Because these are heavy bottles when filled, abrasion resistance is very important so the label graphics do not disappear during shipping. In-Mold UV coatings must have good toughness and adhesion to polyethylene films. There can be no cracking or chipping at the label edges.
In-Mold label UV coatings must have minimal shrinkage. Shrinkage is a bigger problem on film labels than on paper. Increasing solvent resistance generally increases shrinkage. This can be compensated to a large extent by decreasing the UV coating deposit, since shrinkage increases as the coat weight increases. Excessive shrinkage can cause edge curl, adhesion loss, cracking and chipping. Flexibility improves as you decrease the coating thickness.
Oxygen inhibits the UV acrylate curing reaction so that thinner coatings are slower to cure. Amines inhibit the cationic UV curing mechanism and must be avoided if this chemistry is considered. High humidity will also have a retarding effect on cationic systems.
Blocking increases with smoother high gloss coatings on film. UV coatings must have good slip and anti-static properties for In-Mold applications.
UV coatings are used very successfully for In-Mold label applications. However, the product must be optimized for the specific properties required in your application. Convey all the necessary properties needed for your application to your ink and coatings supplier, and they will be able to provide their best product for your application.
Radcure Product Recomendations for In-Mold Label Applications
| Product | Description |
| RAD-KOTE 108AS-FB | Low shrink, detergent & some solvent resistance with blue tracer for film. (Gloss Finish) |
| RAD-KOTE 112S | Low shrink, low odor, and low extractables varnish for film. (Gloss Finish) |
| RAD-KOTE 112SM | (Semi-Matte Finish) |
November 06, 2008
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November 06, 2008
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