Dyne levels are a great indicator of your chance for adhesion success. But dyne level does not guarantee adhesion. A few factors affect adhesion results: coating and material compatibility; surface energy; process variables; material variables and ultimately bonding requirements such as entanglement.

Coating and material compatibility is the first requirement for achieving adhesion success. Coating suppliers do an excellent job of identifying compatibility of their formulations with various surfaces so this information is readily available. Along with material compatibility will come recommendations for target surface energy for the surface that will receive the coating. Usually surface dyne levels are recommended to be approximately ten dynes greater than the coating’s surface energy.

Most engineered plastics have inert, non-porous surfaces with low surface energy. Atmospheric plasma and flame surface treaters are used to increase the surface energy of these plastics. Each of these technologies has its own treatment signature. In general, they remove organic contaminates, oxidize the surface, and create bonding sites to promote adhesion. It is a common practice to measure dyne level before and after surface treatment. This is a good idea as it will help you verify your starting dyne level and the effect of the surface treater on the dyne level. However there are a few inherent problems with dyne level readings.

Dyne pens and dyne solutions are used to help provide insight as to the current state of a surface in terms of surface energy and wettability. Dyne pens offer an easy and efficient tool to measure dyne level, but they can become contaminated from contact with surfaces. It is advised that dyne pens be used more as a pass or fail test than relied upon for a very specific reading. Testing techniques using dyne solutions rather than pens can provide more accurate results.

Dyne level gives some indication that you have a chance at success. But it does not guarantee it. It is not unheard of to see different adhesion results from materials that display the same dyne levels.  So how can this occur? Some common reasons are variables in the process and equipment used to apply the ink, coating or adhesive. Operating variables such as material temperature can play a major impact on adhesion results. Surface migration of additives in the material can play a role as well.

Ultimately good adhesion will require entanglement at a molecular level between the material surface and the coating. Entanglement is dependent upon the variables discussed above; including material and coating compatibility, surface wettability, the availability of functional bonding sites, and control of process parameters.

Corona treatment is a high frequency discharge that increases the adhesion of a plastic surface.  Whether a liquid wets a material good or poorly depends primarily on the chemical nature of both the liquid and the substrate. Wetting is defined as the ratio between the surface energies of the liquid and substrate.  In general the following rule is true: “A material will be wetted, if it’s surface energy (dyne/cm) is higher than the surface energy of the liquid. If not, there will be an adhesion problem.

How to avoid adhesion problems

The pre-treatment offered by Corona treatment is necessary to obtain sufficient wetting and adhesion on plastic films or metallic foils before the printing, laminating or coating. A Corona discharge unit is the solution to optimize the wetting and adhesion. This Corona technique has proved to be both highly effective, cost-effective and can take place in-line.

What is corona treatment?

Plastic is a man-made synthetic material, which contains long homogeneous molecular chains that form a strong and uniform product. The chains of molecules are normally joined end to end forming even longer chains, leaving only a few open chain ends, thus providing only a small amount of bonding points at the surface. The small amount of bonding points cause the low adhesion and wettability, which is a problem in converting processes.

During corona discharge treatment, electrons are accelerated into the surface of the plastic causing the long chains to rupture, producing a multiplicity of open ends and free valences are formed.

The ozone from the electrical discharge creates an oxygenation, which in turn forms new carbonyl groups with a higher surface energy. The result is an improvement of the chemical connection (dyne/cm) between the molecules in the plastic and the applied media/liquid. This surface treatment will not reduce or change the strength. Neither will it change the appearance of the material. The Corona only changes the top molecule chains, which is 0.00001 micron thick.

Determination of treatment level

The effectiveness of the Corona treatment depends on the specific material being used.  Different materials have different characteristics and different amounts of slip and additives, which will determine the effect of the Corona treatment. There are no limits with regard to the materials that can be corona treated.

Durability

Over time the obtained dyne-level will decrease and it can be necessary to corona treat the material again just before use. Storage conditions and temperatures can affect the decay of the corona treatment.

A non-porous substrate is any paper, metallized paper or plastic that will not absorb any ink oil that is present in a conventional ink. Because of this, special high solids inks are required.  Additionally, non-porous substrates do not absorb water and thus the press tends to run wetter which affects the inks.

Inks

Inks manufactured for non-porous substrates contain 100% solid vehicles and large amounts of drying oils. These inks dry by oxidization instead of penetration and evaporation as do conventional printing inks. This means that oxygen must be present in sufficient amounts to allow the ink to dry. They also contain other ingredients to allow them to perform well on the press, transfer well to the substrate and still set fast enough to minimize the amount of anti-offset spray powder that is used. Oxidizing inks will still not set as fast as a conventional ink would on a porous substrate like gloss coated but the slow setting and fast drying insures that the inks will adhere to the stock.

Fountain Solution

When running non-porous substrates the biggest concern is ensuring the inks will dry. When running conventional inks and conventional paper the excess water can often be absorbed by the substrate. When running non-porous substrates the substrate cannot carry it away and so that fountain solution is absorbed by the ink. In order to make sure that the ink can still dry with this extra water some care must be taken.

Start with fresh fountain solution. As the fountain solution gets used certain chemicals leach out of the papers and inks to make them more readily absorbed by the ink. This is bad because the inks will already be carrying more water when running the non-porous substrate.

When printing on non-porous substrates, most ink manufacturers prefer the use of isopropyl alcohol for two reasons. The first is it makes the water wetter, which translates into being able to run less water and the second is they evaporate readily out of the ink. The alternative to isopropyl alcohol is an alcohol replacement. These chemicals make the water wetter but remain in the ink while it is drying. The most important thing from the ink drying point of view is to run the minimum amount of fountain solution to get the job to print.

Additives

Ink driers can be mixed into the ink which is then emulsified on press. This releases oxygen into the ink film. This can reduce drying time by as much as 50%. Another useful additive when printing non-porous substrates is an ink drying stimulator containing water soluble cobalt – the most active drier available. By adding it to the water, it can also help the ink dry faster. This is preferred to adding the cobalt to the ink because it does not affect the ink on the rollers and it increases ink concentration as the ink absorbs more water and needs more help drying.

Spray Powder

Spray Powder is an often overlooked component of successful printing on plastic. The right size powder will give separation of the sheets. This affect is two-fold. First when the sheet initially delivers, it is less likely to slap together and cause ink transfer. Second, by giving separation of the sheets, more oxygen is able to flow into the pile and help dry that job. The size of powderrequired varies by print-job because non-porous substrates vary greatly in weight.

Further Points to Consider

There are many things that can affect any print job:

• Static eliminators help the sheets feed well and not slap together in the delivery.
• Stay-open sprays can entrap oxygen but without oxygen, oxidizing inks cannot dry.
• Low pH can affect ink drying but so can running a higher pH by not adding enough etch.  Allow the minimum amount of fountain solution to be run.
• Aqueous coating is a common printing practice. On non-porous substrates they allow the printer to run high piles and possibly back up jobs more quickly. However, oxidizing inks require oxygen to dry and an aqueous coating seals that ink away from the air. This may prevent the job from being further processed in a timely fashion.