After reading the October 31, 2011 posting “Plasma treatment and treatment retention”, Yanti asks: Should the plasma setting be different for BOPP and BOPET? And if yes,
1. What is the setting difference (gas composition or electric power)?
2. And what is the best method for measuring the plasma effect at metalizing film?
Thanks for your answers; it would help us in optimize our plasma treatment.
Answer: In general the answer is yes, the settings for BOPP and BOPET should be different mostly because the chemical reactivity of the two polymers are different and they are oxidized (treated) by different chemical routes to give different chemical species on the film surfaces.
However, even more basic than that is what are you trying to accomplish with the treatment? I.e. what chemistry do you want to establish on the polymer surface. Are you trying to simply clean the surface to a native polymer surface free of additives or contamination or are you wanting to change the chemistry of the surface? For me I like plasma treatment because I think it can give a lower oxidation state for the film surface. This means alcohol groups instead of ketones or worse carboxylic acid groups, which will indicate chain scission for PP and likely PET as well.
The level of surface oxidation that you achieve on the film surface (the surface chemistry) will control the outcome of the treatment relative to the properties you develop. So you have to first state what you are attempting to achieve with the treatment and then determine experimentally if the Plasma treatment helps or hurts, and where to set the gas mixture and power levels. For me this is best done with a designed experiment sequence as it tells you if the treatment is significant, and then what settings are most important. Initial settings from suppliers are likely to be determined to improve adhesion and perhaps some barrier improvement, but suppliers will have little specific information about other final properties that you want. A major part of the study should include the type and level of surface treatment imparted to the film form the manufacturing line! Generally there will be an optimum combination of surface treatments from the orienter (type and level) and the metallization chamber plasma system.
In my experience I was always attempting to achieve the highest level of hydroxyl group (alcohol) introduction on to a coextruded surface of an OPP film. Generally the surface I was interested in was a coextruded layer of HDPE on the OPP which I was attempting to turn into EVOH by surface treatment. This could be accomplished by Flame treatment and to a certain extend by the Plasma treatment in the metallizer. However, in this instance for the HDPE skin the plasma did not add much to the flame treater results, and if done without flame treatment did not quite reach the same level as the flame. There may have been other benefits but not for barrier in this case. However, if you start with a different skin layer on the OPP there appears to be a benefit from a combination of on line and in chamber treatment on metallized barrier properties, I believe due to the enhancement of hydroxyls surface groups on the film surface.
This should also be possible to determine for BOPET films but in this case the surface energy is already high and adhesion and barrier are generally good if the surface is clean. So plasma for surface cleaning is generally what is seen. Here again the combination of in line and in chamber treatment will have an impact and that needs to be determined experimentally for each application and perhaps end user (do they retreat in converting?). Surface chemistry is a bit different for BOPET but here the addition of hydroxyl groups will also help. But first I think there needs to be a revolution in coextrusion for PET film production. For instance why not a Nylon skin layer on BOPET?
I will get to Point 2 above in the next posting.