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Technical
Information
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| Theory
of Corona Treatment: |
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The
surface tension of plastic films is not sufficiently high to permit
good adhesion of print ink with plastic film . In case of lamination
, it prevents better bonding between two or more materials. Hence,
for better quality of printing, lamination and coating processes
, surface tension of the plastic films should be increased. The
Corona Treatment method is widely used nowadays to increase the
surface tension of the films.
The Corona Treatment of plastic film is done by passing the film
through the corona discharge created between electrode and dielectric
covered ( ususally Silicone Rubber Sleeve) earthed roller as shown
in Fig. 1. The air gap between electrode & roller is 2-3 mm.
The electrode is connected to electrical supply having high voltage
& high frequency. As film passes through the corona discharge,
molecules of surface gets oxidises which has more surface tension.
The corona discharge between electrode and roller is visible by
violet color flame bet'n them.
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| • TREATMENT
TERMS: |
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| » Specific
energy : |
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This
measurement identifies the corona dose to which a film is subjected.
Included in it are the working speed, width and applied power. |
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| » Power
density : |
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This
specifies the power per cm of discharge line. This information
is important in order to assess the saturation effect of the discharge
and, in my opinion, as an assessment criterion concerning the
degree of thermal shock applied to the foil by the corona. |
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| » Corona
Dose : |
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The
is the specific quantity of electrical energy applied to the web
material. Using this parameter, it is possible to calculate the
increase in the surface energy or to dimension a corona treatment
system for a particular application. |
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| » Power
Density : |
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This
is the specific power applied to a 10mm length of an individual
corona section. Using this parameter, the effectiveness of a discharge
can be assessed or a treatment station can be correctly dimensioned
for a specific treatment application. |
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| » Treatability
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Not
all film materials can be equally well treated. PP is very hard
to treat and requires more treatment power. PE is less difficult
to treat compared to PP. PVC and PET does not require treatment
for printing as their initial surface tension is high.
Unit of Surface Tension : Dynes/Cm.
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| » Generator
Power : |
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The
required generator power is determined by the following:
Treat width x line speed x # of sides x the watt density required
to increase the surface energy of the substrate to the desired
level. The watt density is determined by the type of substrate
and the amount of additives in the substrate. Having calculated
the correct kilowatt rating, it is important to determine the
correct discharge area (size and # of electrodes) in order to
match the impedance of the system properly and not over stress
the electrodes. Having a good match will optimize the efficiency
of the discharge area or electrode.
It must be kept in mind that if you have a segmented electrode,
it is most likely that not all segments will be engaged at the
same time. This is done by moving certain sections of the electrode
out of the treat position. This increases the amount of power
available to the remaining electrodes which are still engaged
for treatment.
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| » Segmented
Electrode : |
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There
are a number of factors determining the kind and style of electrode
that would best suit an application. In the Blown/Cast Film industry,
a typical application requires the material be strip or lane treated.
In this case a segmented electrode would be best on a covered
roll treater, which means the base roll would have an insulated
covering such as silicone sleeve, vulcanized or ceramic dielectric.
Segmented electrodes constructed of stainless steel or aluminum
are widely used. |
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| How
To Check Corona Treatment: |
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| 1. For
Plastic Film : |
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» Qualitative Method : |
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This is the simple
and widely used method to check the treatment on the
film. This method only gives indication of good or
weak treatment but it doesn’t give any figure
of dynes level achieved. In this method, small piece
of treated film is taken and thin layer of printing
ink is applied across the film. Allow time to dry
the ink. Stick cellotape over the ink area firmly
and then peel it off in one stroke. Observe for any
ink sticking on the cellotape and treatment can be
classified as follows: |
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a). GOOD
TREATMENT : If peeled out ink is not more than
5 % of ink
area over which cellotape is applied.
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b). WEAK
TREATMENT : If peeled out ink is between 5%
and 30% of ink
over which cellotape is applied.
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c). POOR
TREATMENT : If peeled out ink is more than
30% of ink over
which cellotape is applied.
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| » Quantative
Method: |
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This method
gives figure of treatment level in terms of dynes/cms.
In this method solution is prepared by mixing Ethyl
Cellosolve and Formamide by volume percentage as
shown in table 1. The victoria blue dye (in powder
form) is added to make the solution visible on the
surface of the plastic film. To start with take
a solution of 45 Dynes/cm. Take a cotton bud and
dip it in the solution . Apply it over the plastic
film whose treatment level is to be checked. If
solution film breaks down into droplets within two
seconds , then go for lower dynes solution. The
correct dyne will be the solution for which solution
film does breakdown into droplets in two seconds. |
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Formamide volume
(%) |
Ethyl cellosolve
(%) |
Wetting tension
:
(Dynes/cm)
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| 54.0 |
46.0 |
38 |
| 59.0 |
41.0 |
39 |
| 63.5 |
36.5 |
40 |
| 67.5 |
32.5 |
41 |
| 71.5 |
28.5 |
42 |
| 74.7 |
25.3 |
43 |
| 78.0 |
22.0 |
44 |
| 80.3 |
19.7 |
45 |
| 83.0 |
17.0 |
46 |
| 87.0 |
13.0 |
48 |
| 90.7 |
9.3 |
50 |
| 93.7 |
6.3 |
52 |
| 96.5 |
3.5 |
54 |
| 99.0 |
1.0 |
56 |
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| 2. Paper
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There is no method
to check treatment on paper. Corona Treatment of paper
burns fine tissues on paper surface which may hinder to
LD Extrusion/Coating on paper. |
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| 3. Conductive
Film. Al. Foil : |
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Basically corona treatment
cleans and activates the surface of conductive film by nothing
but Atmospheric Plasma Cleaning. |
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| Corona Treatment v/s. Time
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Surface
Tension (i.e Level of Treatment ) gradually reduces with time.
This is due to intermigration of polymers at Surface of the film.
As time passes treated polymer molecules goes inside the film
and untreated polymer molecules comes to the surface. The rate
of decay of treatment level depends upon initial treatment level.
i.e 48 dynes/cm comes to 44 dynes/cm within 2 days while it takes
15 days from 44 dynes/cm to 40 dynes/cm.
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| Static
Electricity & Problems |
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| • STATIC
ELECTRICITY |
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Denoting
or pertaining to electricity at rest. How simple and inadequate
this definition is of a phenomenon that creates problems which
cost industry millions of dollars per year.
Static electricity is generated by unbalancing
the molecular construction of relatively non-conductive insulators
such as plastics and paper. All matter is composed of atoms.
A balanced atom contains positive charges that are present in
the nucleus of the atom. An equal amount of negative charges
orbits this nucleus in the form of electrons. Both charges are
equal and, therefore, the overall charge of a balanced atom
is zero. However, should this configuration be disturbed and
several electrons removed from this atom, we end up with a greater
positive charge in the nucleus and a deficiency of electrons,
which gives you an overall charge in the positive direction.
Conversely, should we add a few extra electrons, we have an
overall charge of negative, due to the fact that we now have
an excess of electrons and the net charge is now in the negative
direction.
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| • Static
Electricity Problems? |
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Static
Electricity can be defined as an electrical charge on a material
due to a surplus or deficit of electrons. The problems caused
by static electricity break down into five main areas |
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| • Production
problems and slow downs: |
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A
static charge generates an electrical field which acts like a
magnet-repelling similar charges and attracting opposite or neutral
charges. This accounts for the attraction between charged materials
and machine frames or rollers causing machine jams. In many cases
the machine operator will be forced to run at slow speeds to avoid
the problems caused by static. |
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| • Dust
Attraction: |
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Many
products, such as plastic molding materials and film, develop
high static charges on their surfaces. This high charged surface
will attract airborne dust, sometimes from over 3 feet away, and
hold it tightly to the surface. Subsequent operations (such as
molding, printing or laminating) and their end products can be
seriously affected by such difficult-to-remove
contamination.
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| • Shocks
To Personnel: |
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When
handling highly-charged materials, people receive unpleasant shocks
either directly from material, or indirectly. An example of indirect
shock is when a person slides out of an automobile seat and then
touches the door handle. |
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| • FIRES
AND EXPLOSIONS: |
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Static
is usually found on non-conductive materials where high resistivity
prevents the movement of the charge. There are at least two situations
where the static charge can move quickly and be dangerous in a
combustible atmosphere. The first is where a grounded object intensifies
the static field until it overcomes the dielectric strength of
the air and allows
current to flow in the form of a spark. The second case is where
the charge is on a floating conductor such as an isolated metal
plate. Here the charge is very mobile and will flash to a proximity
ground at the first opportunity.
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| • Damage
And Interference In Electronic Components: |
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Strong
static fields and static discharges can cause interference with
electronic equipment. However, even greater problems are caused
by static electricity in the electronics manufacturing industry.
E.O.S . (Electrical Overstress) and E.S.D. (Electro-static Discharge)
have become areas of vital concern in controlling the quality
of static sensitive MOS and FET devices where it is believed that
50 volts or less can cause component failure. |
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