- February 01, 2002, David J. Bentley Jr., RBS Technologies
Printed paper is a key ingredient in many packages and labels. And whether the printing provides information, graphics, or both, it has to be protected.
One way to do this is by using a resistant ink, provided the paper has 100% ink coverage. Another possibility is to laminate a clear film over the inked paper. Using an overcoat is another approach. This last technique is the subject of this column.
Overcoats have existed for many years. Chemists have been formulating them using various raw materials ranging from nitrocellulose to PU. The overcoats contained formulating ingredients intended to provide specific properties necessary for the end use. Overcoats could be curing or noncuring. The curing variety generally offered improved properties.
Initially, overcoats were available as solutions of volatile organic solvents such as toluene, methyl ethyl ketone, ethyl acetate, and similar materials. When air pollution became a concern, converters could not freely exhaust large quantities of these solvents into the atmosphere. Chemists devised new overcoating systems supplied in aqueous form. Eventually, overcoats cured by UV or EB radiation also became available. Today, companies exist whose sole business is selling aqueous or high-energy-curing overcoating systems. These are available to converters to do almost any job.
The purpose of an overcoat is to provide protective characteristics to something such as the printed paper mentioned above. These characteristics obviously relate directly to the final application. A very common requirement is that an overcoat must impart gloss. This is generally for esthetic appeal. In other cases, an overcoat will provide a matte finish where this is the desired surface appearance.
Another frequent benefit an overcoat can provide is moisture resistance. An overcoating can protect both a substrate and its printing ink from deleterious attack by water. This would be important for something used in a very humid or wet environment. An example might be the label applied to a container holding shampoo. Bottles containing drinks that undergo immersion in an ice-water medium are also examples where resistance to water is necessary.
A more demanding requirement for an overcoat is chemical resistance. This is necessary in labels for containers that will hold cooking oils, motor oils, ketchup and other condiments, etc. As the properties for an overcoat become more demanding, using a curing system often may be necessary. This could involve a single- or a double-component system. The single-component system usually has a formulating ingredient that heating or some other component of the coating process activates. UV- and EB-curable overcoats are good examples of single-component systems that can cure to provide very high chemical resistance to many substances.
Heat resistance is, perhaps, on the same level as chemical resistance. Such overcoats also can be single- or double-component. UV- and EB-curing materials are excellent approaches to obtaining very high heat resistance for a product that may see exposure to heat sealing at high temperatures, for example.
A big advance to many overcoats that are commercially available to converters today is the combination of all these traits in a single material. One overcoat can, therefore, provide gloss as well as resistance to moisture, chemicals, and heat. This allows a converter to make one product that can have very broad use for many applications.
Now converters can find overlay coatings to perform in applications ranging from very simple to very demanding. These can be aqueous systems or nonsolvent systems that cure using UV or EB irradiation. Suppliers of commercial overcoatings can provide specific help for a converter to select the best product for an application. Converters choosing overcoatings for protection will be able to make a superior product that will withstand the application as well as it can with any other protective option.
David J. Bentley Jr. is a recognized industry expert in polymers, laminations, and coatings with more than 30 years of experience in R&D and technical service. Contact him at firstname.lastname@example.org.