Question re. Static.

Will the presence of static charge on the face material affect its surface energy? Since static charge is generated through friction, this becomes an unavoidable problem. But will this have an effect on the printability of the face material? Will the presence of dust particles cause the material to lose surface energy thereby causing poor ink adhesion? If so, is there a relationship between surface energy and static?

Answer

Static charge can be caused by friction, separation or induction. 

Of these, separation is the one that most affects winding webs.  As the two dissimilar materials are brought together the surface electrons are brought into close proximity to each other and it is possible for electrons to cross from on surface to the other and on separation for the electrons to stay with one surface rather than the other. In this way the polymer, which is negative on the triboelectric series can collect charge on leaving each roller. The larger the difference between materials in the Triboelectric Series, the greater the static charge that can be created on the material surface. In addition the faster the winding speed the greater the charge that can be built up.  The charge can be cumulative so that over a series of rolls the charge will increase after each one.  If winding in air the humidity will have an effect as water in the atmosphere affects the conductivity of the air. Higher humidity increases the conductivity and so will speed up the decay of any charge on a surface. It will also allow arcing to occur at lower charge levels.  Conversely a dry atmosphere is less conducting and the polymer will reach a higher charge before it can arc to discharge the surface charge.

When you wind film an electrostatic charge is already building as the film approaches each roller.  When the film winds around a roller it does not, in theory, move against the roller and so friction is less relevant.  As most rollers in your system will be metal and hence conducting they will be able to dissipate any static charge to earth and thus you only find the charge on the polymer film. 

Dust particles will be attracted to the surface because of the static charge on the film.

For those of you who disbelieve this have a look at any TV screen and wipe your finger across the surface and you will usually find a layer of dust on the surface that has been attracted to the vertical surface by the static charge on the screen.  Dust does not affect the surface charge directly. Dust cannot provide a leakage path to earth to dissipate the charge. It is possible that dust can form a slight electrical charge concentrator and so if the surface is going to arc to atmosphere it could be initiated at a dust particle rather than from the flat surface of the film.

Static charge and surface energy are separate factors.  The same static charge can be built up on the same polymer irrespective of whether the surface has been plasma treated to raise the surface energy or not.  However the presence of a static charge can affect ink adhesion and wetting.  If you look at ink jet printing the ink droplets are often electrostatically charged as part of the print control process. Also if you look at many modern painting processes these too are electrostatically charged, to improve the surface coverage. Both of these show that liquids can be affected by static charges. It depends on the liquid chemistry as to precisely what effect the charge will have. Liquids may be conducting, or not, each of which will be affected differently.

Thus the ideal would be to use static eliminators to neutralise the film immediately before printing but to have also treated the surface to increase the surface energy to improve the wetting and adhesion of the ink.

If you use a corona treatment before printing this will serve both purposes as the corona plasma has both electrons and ions and so will naturally neutralise the surface whilst also increasing the surface energy.

If the corona treatment is positioned on the machine well before the printing such that the film winds around rollers between the corona and printing stations then there will still some charge on the film at the point of printing. 

Alternatively static neutralizers can be used just prior to printing to reduce the effects of any built up charge.

I hope this helps.

CAB

Just to give you a flavour of the triboelectric series here is a list of materials starting with the electropositive materials and moving down to neutral and then to electronegative.

The farther apart the two materials (film and roller) are the greater the charge that can be produced.

TRIBOELECTRIC SERIES

Glass                                                  Electro positive

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Steel         ------------------------------- Electro neutral

Wood

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Acetate Fibre (rayon)

Polyester

Cling Film

Polythene

PVC

Silicon

Teflon                                                 Electro negative

 

Comments

[George Fonseca]

There is easy reference to quantum of surface energy for BOPP films with regard to hot melt adhesive wetting and adhesion but for what is the permissible levels in KV for static charge for BOPP labels?

Thanks,

George Fonseca