Request to improve barrier, adhesion & optical density
Sir I would like know about
1) how can we improve Metal Bond Strength ?
However we are applying plasma in process.
2) Is metal bond strength have any technical relation with Optical Density, as per our experience High OD have low Bond Strength and low OD have comparatively high Bond Strength.
What is tech. reason for it?
3) How can we improve Barrier Properties of Metallize PET film and BOPP film.
Kindly suggest and oblige us.
Answer
Let me start to answer your questions with some comments on the bond strength.
The surface of the polymer web is rarely, if ever, the same chemical composition as the bulk polymer that makes up the web. This is because there will be present on the surface a variety of other materials. There are likely to be some monomer fragments or oligomers that are left over from the polymerisation process. These will be of low molecular weight and very mobile and if left in place will form a weak boundary layer with any coating applied. This weak boundary layer is where the adhesion is most likely to fail. In addition if the polymer has had any additives included in the bulk polymer then these too may migrate to the surface further altering the surface chemistry. In some cases this could include slip agents that are added to help reduce the surface energy of the polymer to reduce the coefficient of friction and so improve the polymer handling. These slip agents are also low molecular weight materials and are very mobile.
Surface treatment is often carried out on the polymer web in order to reduce the effects described above. The surface treatment can be by flame, corona, atmospheric plasma or vacuum plasma. These surface treatments are aimed at either crosslinking the contaminants to the bulk polymer or to convert the organic material into a volatile compound that can be removed more easily from the surface. If the pre-treatment is done outside the vacuum system then the time between the treatment and the metallization becomes important. As a number of the contaminants are contained within the bulk polymer then even if they have been removed from the surface they may return given time and temperature. Also as the contaminants are removed from the surface the surface energy of the polymer may be increased and if the web is re-wound then the clean from surface will contact the still contaminated back surface. As the surface energies will try to equilibrate some of the low surface energy material will transfer across from the back surface to the front surface, re-contaminating the surface.
With the vacuum plasma treatment there is no opportunity for the surface to become re-contaminated before metallization which will improve adhesion. Vacuum plasma also can include oxygen as one of the treatment gases which can substitute some of the carbon atoms with oxygen atoms. This increase in oxygen content of the surface increases the surface energy of the polymer surface and this helps adhesion in two ways. One is that the metal coating can bond directly to the oxygen which is a stronger bond that without the oxygen present. Also the higher surface energy means that the metal more easily wets the polymer surface.
These pre-treatments need to be used with care as it is just as possible to over treat the surface as under treat the surface. If the polymer surface is over treated the plasma can cause too much chain scission breaking the polymer chains up into ever smaller fragments. Breaking some bonds is good as it allow additional bonding sites to be created but if this goes too far the surface is fragmented so much that even if the coating bonds well there is too much coating bonded to short chain fragments which themselves are no longer well bonded into the bulk polymer.
Measuring the surface energy has to be done with care as it does not tell you if there has been over treatment. The surface energy will go up as the plasma treatment increases. This increase will continue with either increasing power or treatment time up to a maximum value. Once the maximum value is reached the surface energy will stay at the plateau value irrespective of any increase in treatment time or power. In comparison the adhesion will also increase with treatment time or power up to a maximum but once the maximum is reached any further increase in power or time will only result in a decrease in adhesion.
As different grades of polymer or different suppliers of polymer can contain different proportions or types of additives it means that the same plasma treatment cannot be used on every web. Sometimes the same treatment can be used but in other cases the treatment will have to be changed to optimise the adhesion.
The metal adhesion and the optical density are both affected by the plasma treatment and so there appears to be some connection between adhesion and optical density although it can be hard to figure out the connection. As the plasma treatment changes the surface energy of the polymer it also changes the wetting of the metal coating. If the surface energy is low the metal will not wet the surface and the metal will form islands on the surface which can be thought of as hemispheres. Compare this to if the surface energy is increased and the metal better wets the surface the islands will spread out and the diameter of the islands will be much larger and the height lower. Thus the higher energy surface will produce a continuous metal coating a lower thickness. Thus a coating with poor adhesion and a given optical density will usually be thicker than a coating with high adhesion where for the same optical density the coating will be thinner.
Barrier coatings will also be improved in the same way as the optical density by increasing the surface energy and adhesion. Although this has an effect on the barrier performance it is usually much smaller that the effect caused by the number and size of pinholes in the coating. The pinholes are primarily caused by debris on the polymer surface. This debris is present on all polymer films and includes polymer powder debris from volatilised unpolymerised monomer that condenses and fall onto the surface, powder debris from the slitting of the web as well as any airborne debris attracted to the surface from the triboelectric charge generated by the polymer web winding over various rollers. This debris gets metallized and if moved following metallization it leaves behind an unmetallized area known as a pinhole. If the debris rolls away the pinhole will be circular but if it slides across the surface the hole will have a tail to it which is a small scratch. Some other pinholes are generated by pick-off as any hard contact with the back surface when the roll is re-wound can, on unwinding, pick off any poorly adhered metal leaving behind a pinhole. So again good adhesion can help improve the coating properties, in this case reduce the amount of pick-off and so improve the barrier performance slightly. The amount of pick-off is generally much smaller than the number of pinholes caused by debris left on the surface.
Removing debris is not easy. Plasma treatment does not clean away the debris. The debris is held onto the surface by Van de Waals forces more than electrostatic charge and so the plasma although it eliminates any triboelectric charge does not release the debris from the surface as some people think. To reduce the levels of debris takes some positive action such as a pulsed ultrasonic air jet cleaning system or probably more easily using a tacky roll system. This is where an adhesive roll is brought into contact with the polymer surface and the debris sticks to the tacky roll leaving behind a clean polymer web.
It is only by reducing the number and size of these pinholes that the barrier performance can be significantly improved.


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