Is It Possible To Predict A Film’s Barrier Performance Before Vacuum Metallizing? Part 3 Of 3

By Dr. Charles A. Bishop 
Justifying ROI for better barrier    

This is not something typically done because of the cost of installing the equipment to scan and measure the film-surface quality and clean it is significant, particularly if it has to match the normal operating speed. Where the film supplier also takes the film downstream and metallizes their own film, the cost might be more easily absorbed. If they are able to reproducibly produce a cleaner film than anyone else, they will, in theory, have a justification to charge a premium for the film which could offset cost. In reality this may not be the case. The size and number of particulates is so high that even if these were reduced by a factor of 10 the improvement in the barrier performance would still be only marginal. It would only be if the reduction in the particulate-surface contamination were reduced a 1000-fold, or more, that significant barrier improvements would result. 

With this in mind it is not surprising that there is not a rush to install measuring and cleaning stations if all they can expect is a marginal improvement. 

As a low-cost alternative, unwinding a roll and cutting samples to measure and size the number of particles per unit area also could be correlated with the barrier performance. This, in theory, should give a similar result, but the error is likely to be greater as when the roll is unwound there will be an increase in the triboelectric charge on the surface which can attract more airborne particles to the surface. In addition, when the sample is cut out of the web, the cutting process will generate particles that may also increase the number of particles on the surface. This may make the samples unrepresentative of the bulk of the roll. This extra contamination may be somewhat reproducible and so may result in only an offset between this static measurement of surface contamination compared to the on-line dynamic measurement of contamination.  

As even this low-cost approach would take time, effort and may have big errors in the results, it is hardly surprising that even this method of film evaluation is not routinely done, and so films are coated first and somewhere downstream will then have the barrier performance measured.  

I will be interested to see if this “measure and predict” approach is adopted for the production of barrier coatings. No doubt it will be adopted if there is perceived to be sufficient cost benefit in making the change, although this looks to have the hurdle of someone’s cost for someone else’s benefit.