Home' FLEXO Magazine : September 2015 Contents SEPTEMBER 2015 | FLEXO 49
• It forms positive and negative sites by adding and deleting
• It rids the film of organic and inorganic contaminants that can
interfere with adhesion
• From SEM imaging techniques, we know corona treatment can
microscopically change and increase the surface area of films,
which increases the chances for successful adhesion
But since the corona treater doesn’t know what type of film is going
through it, it’s your responsibility to understand the dynamics associ-
ated with this process.
VARIABLES UNDER YOUR CONTROL
Assuming you have your corona treater properly set up with a con-
sistent air gap across the web, the primary variable under control is
the amount of treatment applied to the film. This is most effectively
measured as watt density.
Determining a watt density is not as simple as setting the kilowatt
indicator on your power supply. The watt density formula takes into
consideration the corona treater’s kilowatt output, the width of the
treatment area, the number of sides being treated and the speed of the
film being treated.
As a point of reference, most corona treating systems for converting
applications are sized between a watt density of 2 and 3 watts per
square foot per minute (w./sq. ft./m).
Of course, you can’t simply look at a film and determine if it has been
surface treated, let alone to what level. The best indicator of surface
treatment success is running your full process and evaluating your
adhesion results. A quick test to check for changes in the surface after
treatment is a dyne test.
Dynes are a unit of measurement that indicate the surface energy of
a film. It’s important to note three especially relevant considerations
when using dyne levels:
• The first is that the dyne results depend
on properly conducting the test and in-
terpreting the results; variations of plus
or minus 2 dynes are very common
• Secondly, your dyne measurements
prior to and after treating are only
testing a small part of the film
• Lastly, please understand that reaching
a target dyne level does not guarantee
adhesion. It is not uncommon to see
different adhesion results from film
with identical dyne levels
There is a great deal of information available in the industry on how
to properly conduct dyne tests. Dyne testing ranges from simple dyne
pen markers to more accurate Meyer Rod testing procedures. Which-
ever method your team chooses to adopt, be sure to properly train
your personnel in following proper test protocols.
In general, plastic films are chemically inert, have limited bonding sites
and are hydrophobic (see Figure 2). We’ll save surface contamination
and migrating additives for later in the discussion. Many corona treater
users expect that applying the same amount of treatment to different
films will produce the same dyne result. This is simply not true. There
are two variables to keep in mind: the film’s initial dyne level and the
film’s receptiveness to being affected by corona treatment.
In Figure 3, we see laboratory trial data comparing the surface
treatment results of pretreated polypropylene (PP), polyethylene
terephthalate (PET) and polystyrene (PS). All films were treated with
a watt density of 1.1. You can see that the PS was very receptive to
corona treatment and its dyne level increased from 36 to 60 dynes.
The PET was somewhat responsive and an increase to 46 dynes was
reached. The PP film, however, barely showed any increase.
If a target of 44 dynes was needed, a higher watt density would need
to be applied. If you recall the watt density formula, you will see you
can either increase the power output, reduce the speed or do both to
increase the watt density.
Since the corona treater is incapable of knowing what type of film is
running through it, it is imperative your operators understand that
each film will respond differently to applied treatment levels and
set the watt density accordingly. Word of caution: Increases in watt
density do not create proportional changes in dyne levels; therefore it
is important to test and develop recipes for the treatment levels that
work best for each of your applications.
CHANGING TREATMENT LEVELS
Generally speaking, higher watt densities produce higher dyne levels.
However, you cannot use watt density to predict dyne levels. Why is
that? A corona treating system is not an intelligent device. For exam-
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