Problem with low bond strength

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.

Comments

[Andrew Mitchell]

Can you provide details of the power supply you are using and the plasma treater system.
The power supply will generally only trip out if certain arc events are detected or if some other safety interlock is breached.

If for example you are using a Huttinger TIG RF generator there are a number of water temperature interlocks that can cause the power supply to switch off.

If the power supply trips when running higher power and will not restart for 15-20 minutes, it is possible that the cooling water for the power supply has exceeded the safety limit set for the power supply. The reason why the supply will not start for a further 15-20 minutes is due to the fact that the water remains above the cut out limit and the power supply will not restart until the cooling water has dropped below this point.
If your plasma treater system has a heat exchange unit that uses the factory water circuit to remove heat from power supply and plasma treater water circuit you will have issues removing heat if the factory water supply temperature exceeds 32oC. A standalone chiller would therefore be recommended to remove heat from the plasma treater and power supply circuit.

5Kw to 6Kw would seem to be quite a moderate power setting for a 2450mm plasma treater and i would be suprised if this was sufficient to trip out the power supply unless the treater electrode plates were severely pitted or contaminated or there was insulation breakdown between the electrodes and the treater housing.

I would suggest running Argon/Oxygen mix with Oxygen at 10%-20%. A power density of 5Kw/1000mm at 540m/s should be sufficient to obtain good adhesion on Polyester providing their are no losses throughout your plasma treater system.
I would recommend running the treater in pressure control rather than fixed gas control. A setpoint of 4.0 -5.0x 10-2 mbar would be a good starting point.

You may want to check the quality of the plasma treater water circuit, as if this is not maintained to acceptable limits, power losses can occur within the plasma system through the water circuit that can have an affect on adhesion. You should also ensure that the power supply and plasma treater circuit manifold and pipework are constructed from stainless steel to prevent corrosion that would lower water resistivity.

[Daryl Stahler]

Another possible reason for the generator to switch off is a load mis-match, caused by either improper initial set up, or impedance change of the treater.

The impedance change could be insulation break down, as power is increased, typically so is the voltage. Often this can be verified by checking the resistance between the cathodes and ground using a high voltage megohm meter (1000 VDC). Depending on the construction of the treater, aluminum dust can build up on and around the insulator(s) thus a potential for surface tracking.

Another cause of impedance change is pressure and secondarily process gas flow and type. As mentioned earlier, typically 60% to 80% Argon is combined with either Nitrogen or Oxygen. Before running higher than 20% Oxygen, you should check with the manufacturer of the metalizer to see if it is recommended . Most manufacturers recommend fomblinized system for running high levels of Oxygen. Changing the ratio or flow rate of the gasses can have an effect on the pressure, thus impedance.

Plasma Treater construction and materials of construction also plays a big role in the stability and achievable power levels. Some designs employ insulators that can easily be exposed to sputtering from the treater itself, or residual dust that is generated from the metalizing process. When the insulators become contaminated, the treater will arc. This can be very evident if power is increased (thus voltage increase), creating excessive arcs, ultimately shutting down the generator. The material of construction must be considered when choosing processes gasses. Some manufactures build the cathodes from Aluminum, which when exposed to oxygen, forms a heavy oxide layer, thus causing arcing. If the cathodes are aluminum, try cleaning them off (light sanding) and running Argon/Nitrogen 80/20.

Frequency of cleaning depends on the construction of the treater and the specific process. The best factor to determine when a treater should be cleaned is the number of arcs during a given run. Once the treater is running stable, proper gas, gas flow, etc., a deviation of arc in excess of 10% from the initial run, indicates the treater needs cleaned. If cathode oxidation (charging and arcing) is not an issue, then typically blowing out the treater with compressed air and wiping all accessibly surfaces with a cleaning agent is the only thing required. Once a year the treater should be removed, disassembled, and inspected for defective insulators, excessive wear of the cathodes, electrical connection, water connection etc.