Vacuum Web Coating

On-line measurement of metallized film for barrier use.

This topic relates to the following question.

‘What equipment or process is available to test the metallized film for barrier as it is running’?

I am not aware of any direct measurement process that will measure the barrier of a metallized film as it is moving.  Generally the time constants are too long compared to the film residence time to allow for any direct measurement of gas or moisture diffusion.

The primary source of lack of barrier in metallized films is gross defects in the metallized film.  Mostly these are pinholes or pin-windows that may also be a prime source of scratches too.

The polymer webs all have debris on the surface that can be metallized and if then moved will leave an uncoated area that is seen as a pinhole.  The number & size of the pinholes will depend on the quality of the manufacturing process & the attention to detail on key areas such as slitting.  The polymer web will pick up a static charge during any winding process and this static charge will attract dust/debris to the surface. This can include the likes of slitting debris or in certain seasons pollen & other airborne contaminants.

Some of this debris can be removed by various cleaning techniques.  Hence the same film from different suppliers can have different amounts of debris on the surface and so produce metallized film with different numbers & sizes of pinholes.

A further source of pinholes can be pick-off where any fillers protruding from the back surface can press hard enough onto the soft newly metallized surface & can, on unwinding, pick off some metal leaving behind a pinhole.  Rolls wound in vacuum will always be hard wound even if with little tension applied because there is no air entrainment because of being wound in vacuum.

As the metallized web is wound and debris that is moved can be moved in a number of different ways.  Debris can simply be rolled over the surface, transferred to another roller or the back surface of the web or it can slide over the surface changing the pinhole into a scratch. 

If the scratches are all in the machine direction it is likely that the roll is simply slipping. If the scratches have some transverse direction component then there will also be a web alignment problem too.

If one looks at the work done by Bernard Henry at Oxford University and others who have looked extensively at barrier materials these pinholes dominate the barrier performance.  It is only once these gross defects have been eliminated that the other factors such as crystal size, grain boundaries, nano-scale defects, etc. come into play.

The pinholes are related to the debris size as it is the debris that shadows the web & leaves behind the uncoated area.  Typically the size & quantity of the dust can be related to a number of different process factors.  One is the atmospheric contamination which can contain pollen, smoke particulates, etc. Many of these can be quite large up to a few tens on microns. These very large particles will be fewer in number compared to those of less than 5 microns.

So the question is what can be done to improve the situation?

There are two approaches, one is to try to improve the incoming metallized material & the second is to try to measure & sort whatever incoming material you receive.

In an ideal world the first option would be preferred in the real world this could take some time & the second option may well have to be taken at least as an interim step.

Assuming you have to monitor the web for pinholes then it would require some kind of inline inspection.  One company that has specialised in this type of equipment is Dark Field Technologies Inc, Sheldon, CT. contact Tim Potts.

There are the inevitable trade-offs that will need to be considered. The faster the web winds the lower the resolution of the system & so only the largest of pinholes will be found.  This can be improved by using multiple scanners/detectors but this significantly adds to the costs and also a speed reduction may also be required to achieve the quality of detection to meet your product needs.

The number of defects will typically rise at the beginning & end of rolls.  This relates to an increase in debris reaching the surface as the winding speed changes from zero to full winding speed and also can relate to an increase in local debris from dusty cardboard cores.

It may be worth investigating some rolls by the slower method of cutting out sheets & counting the number of pinholes of a given area using a back-light to highlight the pinholes.  If you have more than one supplier it would be worth comparing the source of rejected rolls to see if the quality is different from different suppliers. This could also be checked using the light box & counting method.

It may be that if you talk to your metallized film supplier they may be able to run the web over a tacky roll prior to metallization to minimise the surface contamination & bring the roll quality up.  The cost of adding this pre-metallization step may well be more cost effective than installing a costly & possibly speed limiting monitoring system.

Hopefully this answer will have given you some ideas for a way forward.

ZnS deposition for embossed holographic film

There was a question relating to the deposition of Zinc Sulphide onto rolls of 12 micron embossed polyester.

The Zinc Sulphide is one of those materials that can be successfully evaporated from a slot source at very high rate.  Typically the source will be a radiant heated single crucible of the full web width that can be located close to the web and so has high deposition efficiency.

All of the major system manufacturers can offer this type of source in their machines Applied Films, Galileo, General Vacuum Equipment, Sidrabe, etc.   As it is a slot source they may also be able to offer a source that would fit an existing machine as a retrofit unit.

If you are looking for something that is specifically aimed at the narrow web holographic market then General Vacuum Equipment makes a machine called the ‘Holosec’.  This machine has a 2m 3m footprint & will take a 450mm dia. roll of up to 400mm width and can deposit any of Chromium, Aluminium, Copper or Zinc Sulphide onto embossed material. 

If you are buying a new machine it is worth also looking at the pattern printing options that some of the suppliers offer.  This can also enhance the value of some of the holographic labelling materials.

If you have access to the Society of Vacuum Coaters and the AIMCAL Fall Annual Technical Conference Proceedings in the past few years there have been a number of papers relating to the deposition of ZnS for this use.

Residual Gas Analyser - Leak Tester

In response to the question - 'Can you help me understand how a RGA leak tester works. and if this is a outdated system?'

First let me say that Residual Gas Analysers (RGA) are good to have & are certainly not outdated.

The group of instruments that include Mass Spectrometers, Residual Gas Analysers & Helium Leak Detectors are broadly based on the same technology. 

Basically all are mass spectrometers.  On all the system gas is sampled, ionised & the ionised gas separated by mass, detected & amplified. 

Mass spectrometers can range in sophistication & can be found on ultra-high vacuum systems, surface analytical machines and research systems.  They tend to have a larger mass range, better mass discrimination & are more expensive.

Residual gas analysers are usually more limited in mass range but are more suitable for installing on production vacuum coating systems to monitor the process & system as well as used for troubleshooting.   As the residual gas analyser will detect helium it can also be used for leak detection. 

The helium leak detectors are designed only to detect helium. Everything has been optimised for this purpose & so it is much more sensitive than the residual gas analyser to helium The systems usually have other features to help make it easier to use such as automatic range changes and audible output.  Leak detectors are often made to be portable to allow one unit to be used on many systems.   

My Preference.

            Given the option I would always have an RGA included on any vacuum system.  Not only can it be used to monitor the day to day operation of the system and so give an early indication of leaks or poor cleaning but in more complex processes can be used to troubleshoot loss of product quality due to small process changes/drifts.

            Often RGAs and even helium leak detectors are regarded by accountants as a luxury.  However it is common that the first time a system goes wrong or a product fails to meet specification that the loss of production is higher than the cost of any RGA.or leak detector. 

            If you have an RGA, think yourself lucky & use it for all its worth it will pay dividends. 

Brief General Update

First my apologies for lack of up-dates recently I was taken ill following my trip to the AIMCAL Fall Conference & ended up in hospital & my convalescence took a little longer than hoped for.  Hence I have been neglecting my duties.

Back to business.   One of the items that was being publicised at the AIMCAL Fall Conference was the launch of a new resistance heated boat with a front surface coating on to promote faster & better wetting on the first use.  Rather than expand on this more what I have done is asked the supplier to write an article & post it on the Blog.  So watch this space. 

This conference was also an opportunity to ask a number of other prospective authors to increase the number of postings.  Hopefully these will start to feed through in the New Year.