Frequently Asked Questions: UV (From Cork's Tech Talk April, 2010)

It doesn't matter that the UV product curing industry is now at least into its fourth decade, certain questions related to use are going to be asked.  Not surprisingly, this is natural for a continuously expanding market defined by new users, new applications, and ever growing usage volume.Below are questions frequently asked about UV curing.  If you do not find your answer here, please call us for a consultation!

When is a UV product adequately cured?

The development of target performance properties generally define proper cure.  This is the result of the extent to which a UV products functional chemicals have been reacted, cross-linked, and tied up.  Cure is said to be inversely related to the amount of extractable monomer and oligomer levels remaining, or the conversion of these to cross linked polymer chains.  Performance may be tested by a variety of means including felt tipped marker wipe, surface energy fluid wipe, MEK rub, pencil hardness scratch, finger nail or coin scratch, computerized cure analysis, abrasion rub, slide angle, adhesion tape peel, and adhesion tape cross hatched peel testing.

Any of these may prove to be adequate for a particular UV product user.  Each user must decide which tests need to be performed to guarantee that product requirements will be consistently met. Once performance testing determines proper cure, the results must be used to establish production cure/ speed parameters.

How many lamps are required for curing?
The primary factor to be considered is: Will the selected curing unit’s capacity be adequate to cure UV products at an acceptable production/performance achieving speed. The end result has to be end products that are salable, able to meet customer’s expectations, and are profitable to the producer. Variables represented by the UV curable products themselves, and the substrates to be printed, or coated must be taken into consideration when determining the number of lamps that are required for curing.

Why are some UV products faster curing than others?

UV curable products are formulated to be specific to substrates and other specifications defined by end application requirements. Considering UV curing products such as UV inks, coatings and adhesives, they can be formulated to be faster curing or slower curing. There are trade-offs, for example: Generally the faster curing a UV curable is, the less flexible it is, and conversely the slower curing, the more flexible. This property can be important when considering applications where substrate bending, folding, die-cutting or embossing is to be accomplished without UV product cracking.

Can UV products be over-cured?

Yes, and this can present a problem. Over-cured UV curable products can be less flexible and become an embrittlement problem. For example: When curing a second down UV Ink, it may exhibit an adhesion problem if a first down ink beneath it is over cured. Over curing, in effect, (over cross-linking), causes a UV curable to become hard with the chemical bonds on the products’ surface broken down. Fewer bond sites on a surface leads to more adhesion issues.

Will plasticizers in plastic substrates affect a UV curable products adhesion?
Plasticizers are added to plastics so that the finished product is flexible and soft. Plasticizers are prone to migrate to a substrates’ surface as time goes on, so that when applying a UV curable product, adhesion is adversely affected. This is characterized by surface contamination resulting from the plasticizer migration and a subsequent lower surface tension.

What is surface tension?
All liquids have a surface tension and all solids have a surface energy, both of which are measurements of force per square centimeter, expressed as dynes per sq. cm. Nature will always move to cover a high surface energy surface with a lower energy surface. All printing and coating processes are affected by surface energy. Optimized wetting occurs when the surface tension of an ink or coating is equal to or less than the surface energy of the substrate.

What is dyne level?
Dynes per sq. cm. are the unit of measurement that is used to quantify the amount of force created on a surface, whether it is surface energy or surface tension. Many of the substrates used in the printing industry are based on plastics, which characteristically have very low dyne levels, i.e. polypropylene at 29 dynes/cm. Low dyne substrates can only be wet out or printed/coated after they are treated to raise their surface energy. Once treated, time becomes a factor as aging, handling; storing, humidity; dust and other contamination contribute to lower the dyne level again. Printers/coaters typically find it necessary to treat plastic substrates immediately before use to insure good results.

What do we mean by treatment?
Another way to describe plastics is that they are by nature non-polar with inert surfaces, and low surface energy. Several methods are available to treat these surfaces to effectively raise surface energy. Corona is one method where a substrate is passed through an ionized air space, which causes molecular bonds to fracture on a substrates surface, raising surface energy. Another method is flame treatment, which eliminates surface contamination and raises surface energy. The flame oxidizes the substrates surface, creates bond sites, changes polarity of the surface, and raises surface energy, improving wet ability. Yet another method is plasma treating. Plasma is the ionization of a gas by the application of electrical energy, like corona and gas corona, except at a much lower temperature. Like corona treatment, atmospheric plasma uses ionized air to treat a non-porous substrates surface to increase surface energy. The surface is converted from a non-polar state to a polar state.

How is a UV products shelf life controlled?
Oxygen permeable plastic containers, with an oxygen cure inhibiting headspace, are favored for packaging UV curable products. Ambient air storage (ideally 72° F) is required as high temperatures initiate premature curing reactions, which will compromise the performance of a UV curable.

LOOK TO CORK!

Our business at Cork Industries is the development and formulation of aqueous, energy curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings, adhesives and UV inks. Cork thrives on its ability to formulate novel, useful specialty products that offer the graphic arts industry printer/coater a competitive advantage.