Thursday, July 25, 2013 4:44 PM
The first question would suggest that you are seeing differences between the levels of adhesion that you are seeing between your own coatings. If this is so I would throw the question back to you. What is different between the processes you use when you deposit the different thickness coatings?
You will have to make some change if you are to change the thickness of the coating. This could be more power to the boats and so a higher temperature and higher wire feed rate for the same winding speed or the same power and wire feed but a slower winding speed. Whatever you do you will have changed the conditions and so will change the response of several things. If you increase the power you will increase the boat temperature and so you will increase the heating of the whole system and hence increase the outgassing rate. The sudden rise in substrate temperature will be steeper and to a higher temperature which will initially change the outgassing rate and oligomer migration rate and so you change the initial nucleation and growth of the coating. These may only be fine changes but nevertheless they are changes.
Is the PET plasma treated before metallization? If so, if you changed the winding speed to change the coating thickness did you also changed the plasma treatment to compensate for the change in residence time during plasma treatment. If the treatment time is changed the adhesion could also be expected to change. If the process were optimised for the slower process then it could be expected that there would something less than an optimised treatment for the faster winding speed.
The other thing to consider is the adhesion test and the failure mechanism. I would check in finer detail where the plane of failure is. It is always the first assumption that the failure is at the interface but often the metal may be well adhered to the surface it deposits onto but the surface it deposits onto is not well adhered to the bulk polymer and so the true plane of failure is a cohesive failure of a weak boundary layer in the surface of the polymer.
So I would contest that the adhesion does not have to be different. Differences are most usually due to differences, however small, in the process that are ignored, or not monitored or controlled well enough.
There are several reasons why you would not would not deposit aluminium at atmospheric pressure.
The first reason would be safety. Aluminium metal has a great affinity to oxidisers. It is often used in explosives/pyrotechnics. As a powder it can have an autoignition temperature around 760 Deg C. If you have a powder the aluminium as it oxidises will heat the powder because the oxidation reaction is an exothermic reaction. If the powder has a small enough mass compared to the surface area the temperature will rise above the autoignition temperature and the aluminium particle will burn. For solid aluminium this becomes less predictable as the oxide on the surface can restrict the rate of oxidation and if the oxide remains intact the autoignition temperature will relate to the melting of the aluminium oxide which is closer to 2000 deg C. However it the oxide is disrupted and the rapid oxidation of aluminium occurs this will relate to the autoignition of the metal at the lower 760 Deg C. Hence evaporating aluminium from boats at atmospheric pressure could be an exciting process but without managing to put a coating onto the substrate.
The second reason for a vacuum when metalizing aluminium is that aluminium oxidises extremely rapidly. Within the vacuum system it is already oxidising before it has been wound up into the roll. It obtains the oxygen from the partial pressure of water that is present in all vacuum systems as a result of outgassing from all the surfaces.
The third reason would be gas scattering. In a vacuum system the metal is evaporated from the boats and if the vacuum is good the mean free path between collisions will be longer than the distance between the evaporation source and the winding substrate and so the atoms will land on the substrate without collision. If the pressure is increased the atoms will be scattered and more material will be scattered away from the substrate and so the deposition rate would go down. At atmospheric pressure the scattering would be enormous and so the substrate would need to be moved closer to the sources ad so the heat load from radiation ant at atmospheric pressure from convection would increase but the heat of condensation would fall and so you would get a greater heating of the substrate but with less coating being deposited and the coating would be transparent because of oxidation, assuming it did not all catch fire.
The fourth reason again relates to the increase in pressure. The rate of arrival of the depositing coating is reduced and the background gas arrival is massively increased and so the deposited coating would be very porous and the included gas content would be larger, assuming you got any coating at all.
I will stop here. There are other reasons why it would not be and easy or good process.
If you want to deposit materials at atmospheric pressure you can change the process from evaporation to something like atomic layer deposition (ALD) where you build up a coating atom by atom which optimises a chemical approach to deposition. In ALD atmospheric pressure or something very close to atmospheric pressure can be used but the rate can be slow unless larger pumps to rapidly move different gases into and out of the deposition zone are used as many tens of cycles are needed to build up a coating thickness of a few nanometres. To build up a thickness of 30nm would (I think) require something between 50 and 100 cycles (each cycle requires 4 steps, (1st gas, purge, 2nd gas, purge) and either the deposition zone needs to be long or multiple deposition heads are required to work very quickly to build up sufficient thickness to compete with vacuum evaporation. So far vacuum deposition of aluminium is still the cheapest process and most of the process limitations can be managed sufficient to be able to produce acceptable products.
I hope this answers your questions.