Vacuum Web Coating

PET film haze

Is there any relation between Pet Film Haze with orientation of film or crystallinity or amorphousity?  Can we correlate HAZE with anyone or with all?

Answers

The answer is yes things such as crystallinity and haze are linked. The draw ratio of can affect the amount of crystallinity and in general the greater the draw ratio the clearer the film (lower haze) but this can then be worsened by the thermal setting process. The thermal setting allows the film to relax and helps reduce the problem of shrinkage but the longer the time or higher the temperature the more the film will relax and some clarity will be lost. (Bear in mind these may be quite small differences). If the draw is not equal in both orientations the crystalline regions within the film can be oriented and so there can be optical differences with orientation. This is most easily measured by plotting the refractive index with orientation. 

Similarly the polymerisation process, which affects things like the molecular weight can also have an effect on tensile performance which then affects the draw and so can affect crystallinity and haze.

Crystalline regions within the polymer are denser and harder and the amorphous material flows around these crystallites. It can be noticeable that cast film may be very clear but after the initial forward draw the film has more haze but after the sideways draw the haze reduces again.  This may be more noticeable with filled film. This is thought to be from crystallites or filler begin slower to re-orientate and so protruding through the surface and roughening the surface and increasing the haze. With the sideways draw this helps flatten then back into the film and so the haze reduces but possibly not to the low level of the cast film.   

I hope this helps.

Additional answer from Dilwyn Jones (AIMCAL Instructor in web handling)

Most thin gauge PET film (including that for metallizing) has a small amount (<0.5%) of inorganic filler, such as calcium carbonate, silica, china clay and glass bead, added to improve the handling behaviour both during manufacture and subsequent processing.  Without it, reels would telescope if wound at realistic speeds, and block during storage.  The composition, particle size and volume fraction of the filler are the main factors influencing haze.  Haze has contributions from both the bulk and the surface, as the filler particles near the surface increase the surface roughness.  There may also be particles that are so large they give individual optical effects, such as Newton's Rings, or visible marks in the metal layer.  Finally, voids form around some particles during the stretching steps of film manufacture, increasing the haze.

Stretching ratios, temperatures during crystallisation, and PET molecule factors such as glycol and IV have a smaller effect.

Without inorganic filler, there is still catalyst residue and internal contamination to scatter light and cause haze.  This is a mechanism in clear, thicker film.

A lot of the detailed knowledge on this is of course proprietary to the major manufacturers.  However, there may be examples in the patent literature especially, and the open scientific literature also.

Question re. Static.

Will the presence of static charge on the face material affect its surface energy? Since static charge is generated through friction, this becomes an unavoidable problem. But will this have an effect on the printability of the face material? Will the presence of dust particles cause the material to lose surface energy thereby causing poor ink adhesion? If so, is there a relationship between surface energy and static?

Answer

Static charge can be caused by friction, separation or induction. 

Of these, separation is the one that most affects winding webs.  As the two dissimilar materials are brought together the surface electrons are brought into close proximity to each other and it is possible for electrons to cross from on surface to the other and on separation for the electrons to stay with one surface rather than the other. In this way the polymer, which is negative on the triboelectric series can collect charge on leaving each roller. The larger the difference between materials in the Triboelectric Series, the greater the static charge that can be created on the material surface. In addition the faster the winding speed the greater the charge that can be built up.  The charge can be cumulative so that over a series of rolls the charge will increase after each one.  If winding in air the humidity will have an effect as water in the atmosphere affects the conductivity of the air. Higher humidity increases the conductivity and so will speed up the decay of any charge on a surface. It will also allow arcing to occur at lower charge levels.  Conversely a dry atmosphere is less conducting and the polymer will reach a higher charge before it can arc to discharge the surface charge.

When you wind film an electrostatic charge is already building as the film approaches each roller.  When the film winds around a roller it does not, in theory, move against the roller and so friction is less relevant.  As most rollers in your system will be metal and hence conducting they will be able to dissipate any static charge to earth and thus you only find the charge on the polymer film. 

Dust particles will be attracted to the surface because of the static charge on the film.

For those of you who disbelieve this have a look at any TV screen and wipe your finger across the surface and you will usually find a layer of dust on the surface that has been attracted to the vertical surface by the static charge on the screen.  Dust does not affect the surface charge directly. Dust cannot provide a leakage path to earth to dissipate the charge. It is possible that dust can form a slight electrical charge concentrator and so if the surface is going to arc to atmosphere it could be initiated at a dust particle rather than from the flat surface of the film.

Static charge and surface energy are separate factors.  The same static charge can be built up on the same polymer irrespective of whether the surface has been plasma treated to raise the surface energy or not.  However the presence of a static charge can affect ink adhesion and wetting.  If you look at ink jet printing the ink droplets are often electrostatically charged as part of the print control process. Also if you look at many modern painting processes these too are electrostatically charged, to improve the surface coverage. Both of these show that liquids can be affected by static charges. It depends on the liquid chemistry as to precisely what effect the charge will have. Liquids may be conducting, or not, each of which will be affected differently.

Thus the ideal would be to use static eliminators to neutralise the film immediately before printing but to have also treated the surface to increase the surface energy to improve the wetting and adhesion of the ink.

If you use a corona treatment before printing this will serve both purposes as the corona plasma has both electrons and ions and so will naturally neutralise the surface whilst also increasing the surface energy.

If the corona treatment is positioned on the machine well before the printing such that the film winds around rollers between the corona and printing stations then there will still some charge on the film at the point of printing. 

Alternatively static neutralizers can be used just prior to printing to reduce the effects of any built up charge.

I hope this helps.

CAB

Just to give you a flavour of the triboelectric series here is a list of materials starting with the electropositive materials and moving down to neutral and then to electronegative.

The farther apart the two materials (film and roller) are the greater the charge that can be produced.

TRIBOELECTRIC SERIES

Glass                                                  Electro positive

Nylon

Wool

Silk

Aluminium

Paper

Cotton

Steel         ------------------------------- Electro neutral

Wood

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Acetate Fibre (rayon)

Polyester

Cling Film

Polythene

PVC

Silicon

Teflon                                                 Electro negative

 

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.    

Thermal capacity.

I wanted to check if there is any information available on comparison of Thermal Properties of Aluminium Foil vs any Metallized films like Metallized PET film or Metallized BOPP film or Metallized CPP film, etc.

The properties would be something like Thermal retention, Thermal conductivity, etc.

Answer.

I have the following details.

Specific Heat      

Al   938 J/kg/deg K

PP 1700 - 1900 J/kg/deg K

PET 1000  J/kg/deg K

Thermal Conductivity

Al  237 W/m.deg K

PP 0.1- 0.22 W/m.deg K

PET  0.24 W/m.deg K

Things like thermal retention will depend on many factors such as the shape, which affects the surface area to volume ratio, the surface condition which can affect the emissivity, the environment as in are there convection currents, or hot or cold surfaces close by, also the heat transfer coefficient to any contacting surface and the thermal conductivity of that material.

Material will lose heat more rapidly if it is contacting a cold surface or radiating to a cold surface than it would if the surrounding surfaces are hot. Similarly if the material is in a constant draft if will lose heat more rapidly than if the air conditions are static.

So if, for instance, you were comparing a food tray made from aluminium foil and one from either PET or PP you might first start with the quantity of material used in each case. The polymer tray is likely to be thicker to provide a similar stiffness to the aluminium. If you then calculate the specific heat for both you might then find that the higher values for the polymer but lower mass keeps the values comparable.  If you then move to the heat retention it is likely that you will find that the aluminium loses this heat more rapidly than the polymer because of the difference in emissivity and because as the aluminium radiates heat from the surface it is faster to conduct heat from the core of the foil to the surface compared to the polymer.

Note, this is just a speculation on my part and you would need to put in the numbers as accurately as possible to find the true differences.

I hope this help in your work.

Adhesion improvement using carbene

I heard this paper last week and thought that many of you would be interested in the process and so I have posted the text below. For those of you who wish to see the figures that accompany the text could you contact Mark Maloney (details given at the end) I was unable to transfer them (sorry about that). Functionalisation of Materials (Dr Mark Moloney, Dept Chemistry, University of Oxford)

The preparation of functional polymers can be achieved in two main ways, either by the preparation of the functional polymer by the polymerisation of the appropriate monomer (Approach 1) or the modification of an existing polymer (Approach 2).

Conventional dye technology uses reactive linking agents to attach suitable dyes to a given substrate, usually by an alkylation reaction. We are interested in the application of carbenes for a similar purpose; these highly reactive intermediates are capable of inserting into a wide range of bonds, notably C-H, O-H, N-H and of course alkenes, to form very stable bonds. Therefore, these reagents would be expected to be useful for the modification of a wide range of materials, and in particular those which would not normally be considered to be chemically reactive, e.g. low surface energy materials.

We have shown that highly reactive diarylmethylcarbene intermediates, easily generated under thermolytic or photolytic conditions, can be used for the modification of a wide variety of polymers, including polystyrene, polyethylene, polyethyleneterephthalate and nylon, as well as low surface energy materials such as polypropylene, to introduce a range of surface activity, including colour, biocidal function and adhesion onto an otherwise unreactive polymeric material.

The protocol uses a two step process (Scheme 1), involving firstly, pre-activation of the polymer by carbene insertion (Stage 1, Scheme 1) using the reagent 1b. The structure of this reagent has been carefully optimised, and consists of a diaryldiazo unit, a spacer unit and an electron rich aminoaryl unit, each of which play an important function in the process. The diazo compound 1b is easily available from ketone 1a in a simple two-step sequence (hydrazone formation followed by oxidation). Adsorption of diazo compound 1b onto a polymer substrate is followed by heating (ca. 150oC), which generates a carbene in situ, whereupon insertion to the polymer occurs, and the product 2a can be isolated free from any by-products by a simple washing procedure. The attachment of the carbene to the polymer is via insertion into a covalent bond, probably a C-H bond in most cases, in which case a new carbon-carbon bond is formed. When this occurs, the surface bound residue will therefore be hydrolysis insensitive, providing irreversible polymer surface modification. The minimum irreducible function required for this insertion step is the diaryldiazo unit of 1b, as demonstrated by the careful use of controls to understand the mechanism of the process. Treatment of the modified polymer 2a (Stage 2, Scheme 1) with any of a wide range of commercially available diazonium salts 3 in aqueous solution gives the highly coloured products 2b, which are isolated by a simple washing procedure. This second step depends crucially on the presence of the electron-rich aminophenyl side-chain in 2a, which after coupling with the diazo compounds 3, generates highly-coloured surface-modified polymers whose absorption characteristics depend on the structure of the

diazo compound which is used. In order to demonstrate the substrate flexibility of this process, we have been able to dye chemically diverse natural and synthetic polymers, and in the most successful cases to produce intense red, yellow, brown, orange and blue colours. This strategy makes for a highly general process, and will enable the insertion of a wide diversity of chemical functionality by suitable modification of the reagents used in the Stage 2. The methodology is intrinsically flexible, permitting the introduction of useful functionality on to what would otherwise be considered to be inert materials, by a chemically irreversible modification the polymer, as opposed to a physical dispersion. A modification of this approach, in which an activating amino residue is incorporated directly into the diarylmethyl residue, has been found to permit the colouring by diazo coupling of a wide variety of materials. Further, we have recently shown that surface modification of diamond is possible, at a density of about 1014 fluorescein groups cm-2, and this can be detected macroscopically as fluorescence.

Some of the results which we have achieved are shown in Figures 1, 2 and 3 below.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 1: Polymers modified according to the protocol shown in Scheme 2, Stage 2(b). Polymers (a) polypropylene sheet; (b) alumina; (c) nylon 6-10 beads; (d) polyethyleneterephthalate pellets; (e) cotton wool; (f) cotton; (g) Kevlar; (h) polystyrene XAD-4;

(i) polyethyleneterephthalate film.

(a) (b) (c) (d)

Figure 2: Polymers modified according to the protocol shown in Scheme 2, Stage 2(c) and metallised with silver. Polymers (a) polystyrene XAD-4; (b) polyethylene; (c) polypropylene;

(d) alumina.

Figure 3: Glass modified according to the protocol shown in Scheme 2, Stage 2(d). (a) unmodifed glass; (b) modified glass with increasingly hydrophobic groups (left to right)

Some relevant references to recent work:

* "The Preparation and Reactivity of Some Photoactivable Reactive Dyes", D. R. Braybrook, M. G. Moloney, H. M. I. Osborn, and W. J. Norris, J. Photochem. Photobiol. A: Chem., 1993, 70, 171.

* "A Method for the Functionalisation of Polymeric Substrates", K. Awenat, W. Ebenezer,

M.G. Moloney, GB9824023 D0 (1998-12-30); WO0026180 A1 (2000-05-11); EP1124791 A1 (2001-08-22); JP2002529542 T (2002-09-10); US6699527 B1 (2004-03-02);

“A Chemical Method for the Surface Functionalisation of Polymers”, K. Awenat, P.J. Davis, M.G. Moloney, Chem. Commun., 2005, 990 -992.                 “Functionalised Polymers by Surface Modification”, J.-P. Griffiths and M.G. Moloney, Fudan Xuebao, Ziran Kexueban (J. Fudan University Natural Science), 2005, 44(5), 772­

773.

“Biocidal Polymers”, J.-P. Griffiths and M.G. Moloney, UK patent application no. 0500895.8 filed on 18 Jan 2006.                 “Surface Adhesion using Arylcarbene Reactive Intermediates”, J.-P. Griffiths and M.G. Moloney, patent filed on 23th Aug 2006. (PCT/GB07/003194)                 “Direct Chemical Modification of the Surface of Diamond by Carbene Insertion”, R. G. Egdell, J. Foord, J.P. Griffiths, M.G. Moloney, and Hao Wang, Langmuir, 2008, in press.

Contact details:

Dr M. Moloney, Reader in Chemistry, Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA United Kingdom

Telephone: +44 (0)1865 275656 Fax: +44 (0)1865 285002 Email: mark.moloney@chem.ox.ac.uk

Marcelo Bravo, CEO, Oxford Advanced Surfaces Ltd, Centre for Innovation & Enterprise Begbroke Science Park, Sandy Lane, Yarnton OX5 1PF

Telephone: +44 (0)1865 854807/854808 Email: info@oxfordsurfaces.com

Differences between metallized PET & BOPP

1.         How do MET PET and MET BOPP differ from each other?

2.         Any specific difference in property?

3.         For Flexible packaging Applications - will MET PET or MET BOPP will be better?

Answer.

The basic differences in the mechanical properties will still be present following metallization. Thus PET generally has a higher tensile strength than BOPP and so where packages require this property PET would be preferred.

It used to be that BOPP was cheaper than PET and so a driving force to replacing PET with BOPP was cost and some compromise was made on the mechanical differences.  PET was optically slightly better with a lower haze.  Over the years BOPP has been improved and so the gap between properties has reduced but with process improvements I think the price differential has also been reduced and so the driving force to reduce cost by moving from PET to BOPP is not as great as it has been in the past. 

In the same way that for some products it was possible to move from PET to BOPP then in some applications it has also been possible to move from BOPP to PE, again as a cost saving process.

In terms of metallizing these same mechanical property differences can affect the metallizing process. PET has a higher yield point and tensile performance and so it is possible to pull the web harder against the cooled deposition drum and so for the same thickness web it is possible to metallize the PET at either a higher speed fro the same thickness or, for the same speed, a greater thickness.

The higher specification PET often appears to have fewer process problems, a higher throughput with higher adhesion and better optical performance and so has many advantages but the costs are higher and so it has to be justified by requiring a superior application specification for which a premium price can be charged.

I hope this highlights the generic differences and where the different materials fit in the scheme of things.

High barrier performance Question.

Question.
I represent a converter who is trying to achieve good barrier properties (wvtr<0.2, otr<0.5) using metallized pet as part of the laminate. We have our own metallizer and the highest OD we can achieve on a single pass is 3.0. However, at this OD, we are unable to achieve the above noted barrier properties.  Are there other methodologies that are available to enable to hit these properties? We have heard of double side metallized films utilizing plasma and chemical treating? Are these possible solutions and have you had experience with these types of films. Secondly, we are also concerned with loss of properties with downstream processing like rewinding and slitting. Are there special processing requirements for finished metallized film in order not to degrade the barriers achieved during metallizing?

Answer

The biggest limitation to producing good barrier films is generally pinholes.

Pinholes are produced primarily by debris that remains on the film surface that is larger than the very thin metal coating you deposit so that after metallization if the debris is moved it leaves behind a pinhole.

A second source of pinholes is spitting from the boat. This can be caused by a combination of reasons such as a low purity aluminium wire or the wire having a thick oxide coating on as well as an unstable pool of molten metal that as the pool size changes encourages spitting from the ends of the collection of crud built up from the wire oxide and impurities.

Another source of pinholes is any pickoff. This is where any high spots on the reverse surface presses hard against the freshly deposited metal and in some cases it overcomes the adhesion and picks off the metal from one surface and it transfers to the second surface. Again leaving a pinhole.  Typically this is associated with large fillers and a hard wound roll.

If the incoming roll is cleaned to a high standard it is possible to reduce the number of pinholes and so improve the barrier performance.

This can be done by using techniques such as the tacky roll type method of removing debris.  All polymer film will be covered with debris. This is partly because the polymer film, as it is wound, generates an electrostatic charge that attracts airborne debris to the surface. This can include slitter dust. Thus if the roll is cleaned just before the final rewind before the roll then goes into the vacuum system this can help. If it is done earlier it can become recontaminated very easily. Ideally after cleaning the film should be in under a clean filtered air hood to limit recontamination. Also the film should be cleaned on both sides otherwise the debris from the back surface can become transferred onto the front surface as the film is rewound.

Similarly it may be possible to improve the wire purity and make sure the wire surface has a minimum of oxide present and may also be possible to improve the wire feed control to reduce some of the pool variations.  This will help reduce any spitting problems

Other possible things that can be looked at is to see if it is possible to wind the material between metallization and lamination with fewer or no front surface rolls in order to minimise the change of moving the debris and so limiting the number of pinholes that appear. Although it is better to clean the material and not have the debris present but this may not be easy to accomplish.

The use of adhesion promoting measures, such as plasma pre-treatment, if done well with the process carefully optimised, can improve the adhesion as well as the metal wetting. This has two advantages. The improved wetting means that the metal will spread out on the surface and so will produce a continuous coating at a reduced thickness and this can be seen as either the same OD at a thinner coating thickness or for the same thickness a higher OD. The second advantage is that the coating is less likely to have problems with pick off as a method of producing pinholes.

In addition there may be other changes that can show benefits. If the metallized film is laminated very soon after metallization it may lose more barrier performance compared to if any further processing were to be delayed by a day or more.  The aluminium coating is very soft and is more prone to damage if it is handled very soon after metallization. If the roll is stored for a short period of time the native aluminium oxide is allowed to build up on the surface of the aluminium and this is much stranger and any damage is likely to be less.

Where double side metallization wins is that there statistically it is unlikely that any of the pinholes on each of the two aluminium coatings lines up with each other and so the increased tortuous path reduces the gas or moisture transmission.

I hope this gives you some explanation about where the lack of barrier comes from and some possible routes forward.

Humidity and metallized film.

Sir, I need some information about Humidity factor for Metallized Film.

1- What value of Humidity is necessary for storage of Metallized Film?
2- Is Humidity also effects process of Metallization?
3- What possible defects can occur in Metallized Film?
4- What are the temperature requirements for the Process of Metallization and for Storage of Metallized Film?
5- What are the recommended conditions for storage of Metallizeable film after slitting process?

Sir kindly guide me if you have any useful information regarding Humidity.

ANSWER

As far as I am aware there are no standards or set levels for temperature or humidity levels for the storage of polymer films or metallized films.  (I am sure someone will write in and correct me if I am wrong)

The humidity affects some films more than others, i.e. PET can absorb moisture whereas OPP is not so affected.

Films that absorb moisture that are used for metallization will take longer to pump down and may have a slightly higher pressure during metallization.  This can lead to small differences in reflectivity and resistivity between films. Similarly if you have a 'wet' or 'dry' season there may be differences in films produced in each season. 

Humidity can affect the film winding. If the humidity is very low the electrostatic charging of the film will be greater and this can lead to winding difficulties. As the humidity increases the charge leakage is speeded up and so in high humidity conditions the static charge will be much lower and there will be no, or fewer, winding problems.

Following metallization high temperature and high humidity can accelerate the oxidation process of the aluminium. However this is still a self-limiting process.

Thus for storage the temperature and humidity should be low but for processing ambient temperature and slightly higher humidity will be necessary.   Also remember that in transferring rolls from one temperature area to another the rolls should be given enough time to equilibrate otherwise there is a very high risk of telescoping the rolls as the polymer changes dimensions.  Also as the layers of film slip over each other there is an increased risk of introducing micro-scratches into the surface.

I hope this helps even though there is not precise answer.

PVD coloured coatings

I am a graduate student in design at Stanford and I am currently making eyeglasses from stainless steel and am interested in PVD finishing. I understand that watch companies use PVD to finish watches but the colours seem to be limited. I'm hoping for more varied colours like red and was wondering if you know of cutting edge vacuum coating processes that accommodate this (ideally corrosion and scratch resistant). Any pointers to resources, experts of companies would be appreciated. Thanks.

ANSWER
generally all colours are available through the mechanism of mixing materials such as combined deposition of TiN and TiC or similar such as TaN or ZrN, etc.  Alternatively it is possible to grow oxides and get the more traditional interference colours using titania on titanium.  The whole rainbow of colours can be obtained; in fact some years ago there was a trend for titanium jewellery using this titania/titanium material.

In the UK there is a company called Bodycoat who already offer spectacle frames that are coloured using the very PVD technique you are asking about. I believe they have a website that shows examples of these frames.

There is a PVD Group at PVD_coatings@yahoogroups.com where people can ask
questions. I do not know if it is possible to access the archives, as there have been, in the past, some postings as to the change of alloy composition and the expected colour.

Mirror coatings onto Polycarbonates.

Currently we are trying to coat a PC film by PVD process.  We do RF plasma before coating.  But, still have delamination problem.  Currently the coating process is :-

1. RF plasma.

2. coated with pure Chromium. (ratio : 30%)

3. Then coated with pure aluminium.(ratio:70%)

The reason of choosing chromium and aluminium are due to the mirror appearance effect.

I would like to try below combination:-

1. 1st by aluminium then follow by chromium.

2. 1st by tin then follow by aluminium.

From your expert point of view, do you think which way should be the best way?

Among Cr, Al, Sn, which one have higher surface energy?  Does higher surface energy imply to better adhesion to PC film?

Looking forward for your kind answer.

ANSWER

With plasma treatment it is possible to treat the polymer and make the adhesion worse than without treating the surface if the plasma treatment is not done well.  When plasma treating the surface there are two main mechanisms that you are looking to achieve, one is to remove any loose, or low molecular weight material from the surface, secondly is to break some bonds and provide more anchor points for the coating to stick.  If you are using an argon plasma this can break bonds but cannot remove any carbon based contamination from the surface. It may sputter off some carbon but it will most likely end up back on the surface and still be a problem to good adhesion.  It is best to use an oxygen containing plasma. This causes the oxygen to combine with any carbon into a volatile species, which can then be pumped away. 

The time of plasma treatment is important and needs to be optimised. Too little time or too much time will not produce the best adhesion.  To much time will cause too much chain scission and produce smaller and smaller polymer chains and this will tend towards a weak boundary layer which will be seen as a decrease in adhesion over the optimum.

The method of testing the optimum time is to test the surface energy of the polymer. As the time is increased the surface energy increases up to a point where the surface energy plateaus. If one plots the adhesion strength this too will increase as the surface energy does until the surface energy reaches the plateau where the adhesion only reaches a peak and then immediately starts to fall. Hence the aim is to just reach the peak of surface energy but not to add some extra time.

I am most familiar with roll-to-roll coating systems where the time available for plasma treatment is low. These systems use most often use a DC plasma and these may be magnetically enhanced to increase the plasma density so that the plasma treatment can be achieved within a second.

Increasing the substrate surface energy improves the wetting of the depositing metal. If you imagine the nuclei as growing as hemispheres on the untreated surface then as the surface energy increases the nuclei flatten, increasing in diameter and reducing in height. This means the coatings become continuous at a lower thickness and are more likely to be smoother and conformal.

The chromium is generally used as seed layer. Normally it is expected that the seed layer will nucleate more readily and increase the number of nucleation sites as well as being better anchored to the surface.  To do this seed layers are often only ~2nm in thickness. It is not expected that the coating will be produced as a continuous layer and the effectiveness of the seed layer can be less if the thickness is too high. 

The brightness of any mirror depends on two things, the perfection of the surface and the quality of the material that is deposited.  The substrate should be smooth and flat. The coating should be conformal and the crystal structure should be such that it remains equally as smooth on the surface.  The quality of the vacuum and the deposition rate play a part. The faster the deposition rate the smaller the crystal size and, more importantly, the cleaner the coating.  All vacuum systems have some background contamination, usually water vapour from outgassing of the system walls, and this will be bombarding the substrate. Typically at aluminium metallizing rates the contamination will arrive at the substrate at around 1 monolayer pre second and at typical deposition rates there will be 1-2% oxygen contamination from the residual water vapour in the system.  This will still appear as a bright mirror surface.  Where things can go wrong is that if the deposition rate is slowed down the contamination rate will increase. If the vacuum is poor and the rate is low this can result in a change in the coating growth and the inclusion of more oxygen can reduce the mirror performance. In the worst case this can result in the surface of the coating becoming milky, or dull in appearance, or even yellowish. This can be because he coating is more porous, or the crystal size has increased making the surface much more rough, or because the aluminium has been converted to aluminium oxide and there is an interference layer appearing giving the yellow colour.  The crystal size will increase with thickness too and so the aim should always be to produce the best coating at the minimum thickness.

Where the ultimate brightness mirrors are required it is common to deposit silver. As this can be corroded easily a silica layer often protects it. The silica is also used as an antireflection coating to further enhance the reflectance.  It would be possible to improve the aluminium reflectivity using a silica antireflection coating too.

If I were starting I would start from the beginning and try to optimise the plasma treatment and deposit only aluminium without any seed layer. Once the plasma treatment was optimised I would deposit the aluminium at the fastest possible speed to just above the minimum thickness that would achieve the desired reflectivity. In doing this I would also check on the performance of the vacuum system that the deposition conditions were consistent with delivering a high quality coating.

It would be only if I could not achieve the desired adhesion and/or reflectivity that I would look to using a seed layer.

If I needed to use a seed layer I would still aim for the optimised plasma treatment and aim for the minimum thickness of seed layer.

Offhand I do not know the respective energies of the metals Al, Sn, Cr. I would have to search through some books and papers to not only check the respective energies but also to check if they form alloys and look at the electrochemistry to see if they are likely to have an affinity to form a strong bond. Hence I cannot comment which system would be preferable. However I think that there you may not need to use a seed layer.

I hope this help even if it does not quite answer your question directly.