Vacuum Web Coating

Problem of blocking & pinholes

We are metallized CPP producer and having these two problems:

1. Our VMCPP is having blocking problem on both side but not on the centre. If we see it on light table, the sides are having thinner coating. Our VMCPP OD is 2.4 - 2.6. The problem shows after we slit the jumbo roll into smaller roll, usually one day after the CPP is metallized. Please advise the possible cause of this problem.

2. We are having pinholes problem. Can we reduce this by using fewer antiblock on treated layer or using round shape antiblock?

Answers

1.         Blocking.

Blocking occurs for a number of possible reasons.  Essentially when the web is wound onto a roll the layers are not able to move relative to each other and so the winding becomes uneven and peculiar shaped rolls can result.  The reasons that the layers are unable to slide across each other can vary. If the surface is too smooth and the coefficient of friction (CoF) high could be the problem. It is common for additives to be added to reduce this problem. One is to add fillers, which protrude from the surface of the web preventing the whole surface from being in complete contact. This reduction in contact area reduces the CoF. This is often not a sufficient reduction in CoF to make the handling acceptable and additives, slip agents, that reduce the surface energy and CoF may be added to further reduce the CoF.

The slip agents added to the bulk of the polymer migrate through the bulk to the web surface. These slip agents may also have the effect of reducing the metal adhesion. To improve the adhesion it is usual to treat the surface of the film to remove the additives and so increase the surface energy. Treating one side of the web may not be sufficient to bring the adhesion up to the highest value. As the slip agent migrates to both sides of the web it is preferable to treat both side of the web to remove the slip agent form both sides.   If the web were only to be treated on one side then the low molecular weight material from the untreated side could be transferred to the higher surface energy side that has been treated. 

When the web is metallized the web sees a high temperature and this rise in temperature can speed up the migration process. 

One common problem of blocking with metallized films occurs if the web is re-wound at too high a temperature. If the rolls are coming out of the metallization process hot, where hot may be anything more than 30 Deg C but the higher the temperature the greater the chance of a blocking problem.   What occurs is that the roll, as it cools, shrinks and the pressure in the roll increases and this high pressure can exacerbate the blocking problem.

If you are having a problem of blocking at the edges of a roll but not in the centre I would start to investigate the uniformity of the temperature of the rolls exiting the metallizer. Measure the temperature across the roll and see if the temperature is uniform or if the temperature at the centre of the roll is greater than at the edges or vice versa.  If there is a variation then this would suggest that there is a non-uniformity of cooling within the vacuum system.  I would investigate the method of cooling the deposition drum. It may be that the coolant flow contributes to non-uniformity on the cooling of the deposition drum, which then leads to the difference in the temperature of the web between the centre and the edges.

I would also check the temperature rise of the coolant. There are several possible problems that can occur. Firstly the design of the deposition drum may not be ideal. I may be serviceable for most applications but may not work so well for high temperature loads, which may show up the design limitations.  Alternatively the design may be fine but there may be a blockage in one of the internal pipes/channels not allowing uniform cooling of the drum.  Alternatively there may be insufficient cooling capacity and what you are seeing is the progressive rise in the coolant temperature affecting the overall drum temperature.   If the drum heat sup the edges of the drum will tend to be cooler as the ends of the drum will radiate and cool the ends faster than the centre is cooled and thus the web will be hotter at the centre than the edges.  This may also show up as rolls having better coating uniformity and less problems blocking at the start of the rolls than towards the end of the metallization. This also might be might be measured as a progressive increase in the coolant supply temperature for a recirculating coolant flow or a progressive increase in temperature of the exhaust coolant temperature for a continuous flow system. 

If there is a temperature variation it may be that there may be differences in the rate of migration of any additives to the surface. If this is the case it may also be possible to detect the differences by measuring the surface energy of the web at the edges and centre and looking for differences.  If there is a different rate of migration it might be expected that the centre of the web would have a lower surface energy due to the greater migration of slip agents and that the edges would have a higher surface energy due to less migration because the edges are at a lower temperature.

In terms of the coating thickness it is common to have some thickness variations across the web. The coating thickness will always fall off towards the edges of the roll because there is no deposition contribution from beyond the width of the web. In the centre of the web there is the evaporation of material from directly below the web and from each side the boat below the web.  However once you reach the edge there is the boat directly below but only the boats towards the centre of the web, with nothing outside the width of the web. This can be compensate for by driving the boats towards the outside edge of the web harder than those in the centre but this then makes the lifetime of the boats at the edges less than for those in the centre.

Alternatively a shaped shield can be used to compensate by blocking off more of the deposition in the centre than towards the outside edge.

If this did not used to occur but has only recently started to happen I would look at ‘what has changed’ between the good rolls and the bad rolls.  For example has the system been idle and possibly built up some corrosion product in the cooling pipes, or has the coolant supply changed or has a filter change been missed and so the flow is reduced because of a blocked filter, etc.  If there has been a system change it is always worth checking out what precisely was done in case this has some adverse effect on the process. Then I would move on to the rest of the checks such as flows, temperatures, drum and roll temperature uniformity, etc.

2.         Pinholes.

Pinholes are mostly caused by residual debris on the surface of the web.  This starts with things like residual monomer that emanates from the extruder and web and condenses on the surrounding surfaces and eventually falls back down to the web. As the polymer web is wound the process of winding causes a triboelectric charge to be generated that is seen as a static charge on the web. This electrostatic charge attracts dust from the atmosphere. The web will be slit usually by knives, which produce slitting dust. The slitting dust is generated next to the web that is electrostatically charged and so some of the dust will be found on the web surface.  There will be more of the slitting dust towards the slit edges of the web than in the centre of the web. The more the web has been wound and slit the more debris will be present on the web.

It is possible to remove this dust by use of tacky rolls that have a stick roll that rolls on the surface and picks off anything that is loose.  Some film suppliers’ use this cleaning technique before they do the final re-wind, other do not.

Other sources of pinholes are spitting from the evaporation boats and the one you are thinking of, which is the pick-off caused by the high-pressure that is specifically seen by any protruding fillers.  There are a couple of options for modifying the filler, one is to modify the shape to a more rounded shape and the other is to change the size and distribution of the fillers. The aim is to change the number and area of the contact points to reduce the pressure per unit area and reduce the probability of pick-off.

Also reducing the re-wind tension in the metallizer could be of benefit.

Another aspect of the pick-off problem is that if the metal adhesion is poor then pick-off is more likely. Thus it is always worth checking that the adhesion has been optimised before looking to change the film specification and request changes to the filler type or size and distribution. 

It is worth bearing in mind that there are some aspects of these two problems that are in conflict.  Treating the web to optimise the adhesion will most likely increase the surface energy.  The treatment would ideally be done on both sides of the web to make sure any slip agents from the reverse side are not transferred from the back surface to the front surface.  This treatment of both sides of the web does not make the handling of the web any easier and it can increase the problems of blocking. Thus the treatment of the front and back surfaces may have to be different with a lower treatment on the back surface.    

Problem of delamination

Recently we observe delamination in liquor sachets.  We use high performance adhesive for this purpose. When the liquor is filled in the laminate there is no delamination.  But, even if one sachet leaked it delaminates all the other sachets. What could be the reason for it & how we can avoid it?

Answer

There are a couple of things that might be worth thinking about. One is the adhesion is only as good as the weakest part of the lamination interface. You have two (probably different materials) that have a surface that can affect the adhesion as well as the adhesive. Although you have chosen a high performance adhesive it can still be affected by a poor surface on either of the two surfaces it is trying to laminate together. The surfaces may be contaminated or have a low surface energy. This can mean that the adhesive does not wet the surface well or does not stick to the surface well. It may still stick better than a lower performance adhesive but still not as good as it might.

Contamination in this case would be things such as the additives used to control the coefficient of friction as used in materials such as OPP, which lowers not only the coefficient of friction but also the surface energy. If the surface energy is low the adhesive may not wet the surface fully and so there will be gaps between the adhesive and film where liquid could migrate easily. Also the area in direct contact is less and so the adhesion strength falls.

The chances are that the inner material is well matched to the liquor you are enclosing however the outer material may have a lower performance. One possible way that additional stress can be put on the laminate is that the liquor is taken up by one of the materials and it swells putting an additional stress on the lamination adhesive. If the adhesion is not good enough this stress can then cause delamination. In the same way if the outer material allows the liquor to migrate through to the lamination adhesive and the adhesive is affected by the liquor then the adhesive may swell or lose some of its adhesive properties and again the swelling can put too much load on the joint allowing delamination.

I would check out how the liquor interacts with each material, checking to see if it swells the film or adhesive and how if affects the adhesive tack strength.   I do not know what the liquor is but does it include a solvent and check to see if the materials are compatible to that solvent.

I hope this gives you a couple of things to look into.

Coating and Conversion - Issue 7 Jan/Feb 2008

This journal has the usual mixture of snippets of news and articles. Of which the following two articles were, for me, the highlight of this edition.

In this issue I was delighted to find an article on web cleaning entitled ‘Wiping the slate clean’.  This article is more of an advertorial for a number of company products than an independent review of competing technologies.   The article does describe the general problem of web contamination along with some of the basics of the different cleaning techniques.  The article is also very light when it comes to detailed information on the cleanliness of the film. The two specific bits of information that were included was that the tacky roll process from Teknek could remove debris as small as 1 micron and that the ultrasonic air knife and vacuum extract from Shinko could remove particles of 1.6 microns and above.

In the past both of these techniques have been used to remove particles down to 0.3 microns although this is much more difficult and it may be that they cannot guarantee this level of quality and so are using values that are more easily achievable.

In a similar type of advertorial there is an article entitled ‘No need to get wound up’ that deals with winding issues.  Although this article is directed at the winding of webs at atmospheric pressure a number of the basics are equally applicable to winding webs in vacuum.  Also as we all take webs that have already been wound in atmospheric pressure we need to know what we are receiving and why the rolls are in the condition we see them in. 

If any of you wish to find out more about this Journal the web site is www.C2-europe.eu