Working on a project of metallized poly, I had few observations and wanted your opinion.
Q1) Poly film shows strange behaviour in terms of treatment,before metallization the film had 44+ treatment, after metallizing in some cases it dropped below 34 while in some cases it retained 44+. What could be the reason?
A1) It can depend on what sort of treatment and the time between treatment and the measurement. Corona treatment may be time dependent, Additive such as slip agent can migrate to the surface and lower the surface energy, similarly unpolymerised monomer, oligomer, can also migrate to the surface lowering the surface energy. The amount migrating to the surface will depend on the time and temperature of storage.
Q2) For the same film (40 mic) metallized poly, once I achieved WVTR as low as 0.1-0.3 gm/m2/day at 2.2-2.6 OD. But didn't get such result thereafter. Which parameter could have played the trick?
A2) Barrier coatings are defect limited. No defects will give a perfect barrier. Statistically you will get the occasional very good and very bad performance and most will lie somewhere between the two.
Q3) For the same OD can there be any drastic difference in WVTR value? Coz my observations were like that.
A3) Optical density only tells how much metal is between the light and detector. It gives no measure of the nucleation density, crystal size, roughness or porosity or defect density. Barrier measurements give an indication of level of defects bt not of the coating thickness.
Q4) What role the chilled drum plays as far as deposition is concerned, I mean should there be any change in deposition/metal adhesion or WVTR if i keep the chiller once at -2 deg C & other time at -20 deg C.
A4) The temperature of the deposition drum can have a small effect of crystal size but more importantly can affect the peak temperature during deposition. The peak temperature can be a source of film relaxation which can result in a dimensional change. These dimensional changes, or not, can affect the defect size or cracking associated with defects and so can affect the barrier performance.
Q5) I increased OD to 5 & compared with another sample with OD 2.5, both gave almost same WVTR value? What should I conclude?
A5) The barrier performance is controlled by the defect density. Adding more metal does not change the number of defects and so the barrier performance can stay almost the same for very different thickness coatings.