Corona Treater Output vs. Increase in Dyne Level

Question: Can you offer any general guidelines on the relationship between corona treater power output and dyne level increase?

Answer: The most basic measurement used to address this question is called watt density (Wd). It is measured in kW per ft2 (or m2) per minute. The equation is

(1) Wd = PS/(EW x LS x NST),

where Wd = Watt density; PS = power supply output in kilowatts; EW = electrode width in feet or meters; LS = line speed in feet or meters per second; and NST = number of sides treated.

Other things equal, higher watt densities result in greater increases in the substrate’s surface energy (dyne level). However, the relationship is neither linear nor simple — watt density alone cannot predict dyne level. A myriad of other factors will have an impact on results.

The type of plastic (of the outer layer on coextruded or coated films) is probably the single most important consideration. Whereas some materials, such as polyester, accept treatment readily, others are less susceptible. For example, polyethylene tends to be moderately treatable, whereas polypropylene will require a considerably higher watt density to achieve the same improvement in surface energy.

Film gage will likely have an effect, especially if the substrate includes slip agents, anti-stat additives, or other constituents which tend to bloom to the surface during and after corona treatment. These all tend to decrease the effectiveness of the treatment, especially over time. Film age — especially if it was treated at extrusion — will therefore obviously also have an effect. Films which were corona treated when extruded (a very good practice, as polymer surfaces are more easily modified at higher temperatures, and prior to “setting” their molecular structure), and being re-treated (“bump-treated”) in line for printing, coating, laminating, etc., have a stronger dyne level increase at a given watt density than will films that have not been pre-treated.

During the primary treatment, at extrusion, there will be differences in efficacy between cast and blown films, as well as between films that are oriented or biaxially stretched vs. those that forego these processes. These variations are due to molecular structure and orientation, film temperature, and the proximity of the treater to the extrusion die — closer is better! For example, with cast film, the treatment may be on the cold side, which has been exposed directly to the chiller roll, or on the the hot side. The quench gap and quench tank temperature will have an effect, as both these factors influence molecular structure.

When treating a single side of a film, keep in mind that any back treatment will sap energy from the treater, resulting in a lower dyne level per Wd relationship. Along with back treatment’s potential to cause blocking, this is a good reason to routinely test for this unwanted phenomenon.

Finally, electrode type and gap, humidity, and possibly other effects such as static buildup downline from the treater and the film’s exposure to idler rolls may also have an effect on the relationship between dyne level increase and watt density applied to the surface.

Under any set of conditions, expect the relationship to be non-linear; the shape of the curve relating the two variables will be based on a combination of all factors discussed above.

Having put all these caveats on the table, we can still draw some very general conclusions as to appropriate watt densities for various processes, as follows:

For treatment at extrusion with cast PE film, treat at Wd 2.0 kW/ft2/min cold side; 1.8 kW/ft2/min warm side (no orientation); 2.2 kW/ft2/min for oriented film. With blown PE film, treat at Wd 1.6 kW/ft2/min at top of tower; 2.0 kW/ft2/min halfway down tower; 2.0 kW/ft2/min at winder.(1)

For coating and laminating pre-treated PE film, bump treat at Wd 1.2 – 1.4 kW/ft2/min for solvent coatings; 1.3 – 3.3 kW/ft2/min for water based adhesives; 2.0 – 3.0 kW/ft2/min for UV coatings; 1.0 – 1.5 kW/ft2/min for 100% solids adhesives.(2)

The following data, from Enercon Industries, show typical Wd values, in kW/ft2/min, for printing, coating, and laminating, as well as suggested watt densities to achieve appropriate dyne levels for several materials.(3)

Typical Watt Densities for Printing, Coating, Laminating
Solvent Water UV Solventless
Pretreated LDPE 1.5 – 2.0 2.0 – 2.5 2.0 – 2.5 1.0 – 1.3
Pretreated LLDPE 1.5 – 2.0 2.0 – 2.5 2.0 – 2.5 1.0 – 1.3
PET 1.0 – 1.5 1.0 – 1.5 1.0 – 1.5 1.0 – 1.3
Pretreated BOPP 2.0 – 2.5 2.5 – 3.0 2.5 – 3.0 1.0 – 1.3
Note: Variations in resin blend, additives or process will affect values.

 

Typical Treat Levels & Watt Densities
Incoming Level Desired Level Watt Density
Treated BOPP 34 – 36 40 – 42 2.5 – 3.5
Treated BOPET 40 – 42 54 – 56 0.9 – 1.5
Treated LDPE, high slip 34 – 36 40 – 42 2.5 – 3.5
Cast PP, no slip 38 – 40 40 – 42 1.5 – 2.5
Untreated LDPE, low slip 30 – 31 no data no data
Note: Variations in resin blend, additives or process will affect values.

The following figure shows results published by Kasuga Denki.(4) Note that one square meter = 10.75 square feet, so this includes watt densities of as high as 11 kW/ft2/min for the 10% EVA. This is an unusually high — and probably in most cases unachievable — watt density, as most corona treating systems are sized for a maximum Wd of 4.0 kW/ft2/min or less. The higher watt density data points were probably produced at low line speeds.

corona_10.gif

References:

1) D.A. Markgraf, “Determining the size of a corona treating system,” TAPPI J.72, (Sep 1989), 173-178.

2) no author cited, “Position of corona treating station,” Faustel, http://www.faustel.com/position-of-corona-treating-station/.

3) T.J. Gilbertson, “Using watt density to predict dyne levels,” Enercon Industries, http://www.enerconind.com/treating/library/technical-articles/using-watt-density-to-predict-dyne-levels.aspx.

4) no author cited, “Wettability (wetting tension) and watt density, Kasuga Denki, http://www.ekasuga.co.jp/en/product/185/00235.shtml.

Additional reading:

T.J. Gilbertson, “Blame the corona treater  the truth about watt density, dyne levels, and adhesion,” Converting Quarterly, 4, (Quarter 2, 2014), 82-84.

no author cited, “Corona treating watt density,” Faustel, http://www.faustel.com/corona-treating-watt-density/.

no author cited, “Watt density: What is the formula to calculate watt density?,” Pillar Technologies, http://www.pillartech.com/Surface-Treatment/Service-info/Troubleshooting-Guides/Watt-Density.

Discrepant Results From One Test Marker Compared to Others at the Same Dyne Level

Question: We purchased several ACCU DYNE TESTTM Marker Pens from you recently, all at 30 dynes/cm. One test marker seems to be getting a significantly higher amount of “failure” results. Can you provide any insight on this?

Answer: The first thing to check is whether all the test markers have the same lot number, meaning they were all produced from the same master batch. It is extremely unlikely that any significant variation in actual surface tension from lot to lot would occur, but it is not impossible. If the suspect marker is the only one with a non-uniform lot number, we would want to know immediately.

Generally speaking, if one test marker reacts differently from others at the same dyne level, it is due to one of three causes: Either evaporation of test fluid from the pen’s barrel, contamination of the tip (typically by airborne silicone or residual oil from a previous processing stage), or absorption of water from extreme humidity or accidental immersion.

Evaporation will generally increase the surface tension of the test fluid, as 2-ethoxyethanol evaporates faster than formamide. However, the 30 dyne/cm formulation is 100% 2-ethoxyethanol, so any evaporation that does occur should not affect the surface tension of the test fluid.

Contaminants are usually of lower surface tension than the test fluids, but this may not be true at this low a dyne level — the suspect unit may have picked up a contaminant of higher surface tension, raising its dyne level and decreasing its wettability. This would cause false failures. Any absorption of water would also increase the surface tension of the test liquid, with the same result.

Another possibility is that the suspect unit is allowing a smaller amount of liquid to flow through its tip, which could result in a thinner film of liquid being applied. Thicker fluid films will wet somewhat more readily, due to gravitational spreading from the mass of the liquid. Taking care to saturate and then flush the tip in accord with the test procedure so that all test markers apply a similar volume of fluid on the final pass will help minimize this effect.

If you need a more concrete answer, the best thing to do is send us the suspect test marker and one that reads as expected, along with some of your material samples, and we will evaluate the issue in our test lab.