We are metallized film with plasma by applied a 3.0 to 4 kW power. We are
using 1200 sccm of oxygen & 400 sccm of argon combination.
With above combination we are facing problem of low metal bond strength in
metallization done on corona treated side film.
If we try to increased the power of plasma more then 5 or 6 kW, plasma get
tripped & unable to start for next 15-20 min.
I would like to understand that at what power range will give best metal
adhesion?
We are doing metallization on 12 mic with 2.65 optical density. Maximum
width of metallization is 2450mm.
Pl reply with possible cause of low metal bonds, cleaning frequency of
plasma, gas combination & rate of gas, power etc.
ANSWER
pre-treatments are one of the most frustrating parts of the process because it can be so variable, often varying with changes in the weather.
Corona treatment is aimed at increasing the surface energy of the polymer film. So too is the in-vacuum plasma treatment.
Using either or both of these processes there are three possible outcomes.
1. The treatment has little or no effect and the adhesion is little better than with no treatment.
2. The treatment delivers a higher adhesion.
3. The treatment has some effect but sometimes the adhesion is worse than using no treatment.
The pre-treatments are used to do a variety of different things to the. It can be used to help remove loosely bound material from the surface, also low molecular weight material that has migrated to the polymer surface can also be removed or carbonised or cross-linked into the polymer surface, also the polymer surface can be chemically modified to enhance both the wetting and adhesion.
What you are trying to achieve is a maximum value for the surface energy. However a simple measure of the surface energy can be misleading for the following reason. There will be a surface energy associated with the untreated polymer film. Now as the pre-treatment is done the surface energy should be higher than the untreated film. As you have a roll of material optimising this process should be done quite easily by winding material through and progressively increasing the power to the corona treater or in-vacuum plasma treater. It is then possible to measure the surface energy at each power level. What you should see is a progressive increase in the surface energy with power to a point where the surface energy levels off at some maximum value.
It is this maximum value that is misleading. If you also plot the adhesion of a coating or metallization it will follow a similar curve except that where the surface energy levels off and continues with higher powers (or longer treatment time) at an almost constant high value, the adhesion instead of levelling off it reaches a peak and then almost immediately falls off with increasing power or treatment time.
The reason for this fall off of the adhesion is that the surface of the polymer has been overtreated. The treatment is often a balance between chain scission that generates new bonding sites and aids adhesion and the scission that creates new short chain molecules that are short enough to be weakly bonded into the bulk polymer and hence form a new weak boundary layer. If the power is further increased these short chain molecules tend towards carbon molecules, thus the weak interface is made up of an excess of carbon even if high oxygen content plasma is used.
The use of corona and in-vacuum plasma can make it easier to overtreat the surface in some circumstances.
Often there is some time between the corona treatment and metallizing process. During this time it is common for low molecular weight materials to migrate back to the surface and re-contaminate the surface and so the in-vacuum plasma treatment is essential. However if the same material is both corona treated and vacuum plasma treated in the same day it may be that the surface is overtreated. Thus it becomes important not only to know what the treatments are but the time between the treatments can become equally important.
There are other variables that also need to be monitored and understood. The temperature and humidity can affect both the process and the polymer. The corona treatment at the same power can produce different results when the humidity is high compared to when the humidity is low.
Winding the polymer in high humidity will trap more water in the roll than winding in low humidity. This is released in vacuum and will also vary the gas content of the plasma treatment process.
I am slightly surprised that the oxygen flow is so much higher than the argon. Mostly I see argon being the larger flow and oxygen being somewhere in the 10% - 20% range. The argon provides the heavy ions for doing the chain scission and the oxygen provides the bonding to the carbon by-products making then volatile and capable of being pumped away and also the oxygen will bond onto the polymer where fresh chain ends have been created which can also improve the bonding to the aluminium.
So for your problem I would start by checking the optimisation of the process. Making sure that the surface energy has been maximised but also making sure that the polymer has not been overtreated and that the surface has not been carbonised.
I would also look at the variability in the process. What are the variations in the humidity and differences in time between corona treatment and metallizing? Look at the optimisation for a constant corona power but with highs and lows of humidity and time between processes. This should give you some idea how much the process can vary and what might be done with the vacuum plasma treatment to compensate for the variations.
I hope this gives you something to work with.
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