Vacuum Web Coating

Metallized film – lamination problem

Questions - Please tell me more detail about interfacial movement between 1) PET & LDPE and 2) BOPP & LDPE.
What is mechanism of swelling and Different thermal expansion of PET, BOPP and LDPE ?
How can I solve or at least minimise the problem of interfacial movement?
I think interfacial movement is the main problem because demetallization is mainly happened in the sealing part of the laminate pouch & at the point where small hole is created to remove air from pouch , hence I need your valuable suggestions for preventing interfacial movement of films in the laminate.

Answers

1.         Generally thin polymeric webs are produced by melting and extruding the polymer as a thin strip that is then stretched, both forwards and sideways, that increases the area but reduces the thickness and results in the thin film.  The stretching is used to increase some of the film properties such as tensile strength.

Where different polymers are laminated together there may be problems of differential stress because of a number of factors.  The two different polymers may have different coefficients of thermal expansion.  This means that for the same temperature rise they will expand by different amounts.  If they are stuck together this may show up as the laminated film curling.

A second factor may also play a part. When the films are manufactured and stretched the temperature is raised to make the stretching process more easily achieved. Once the film has been stretched the temperature is reduced and the film is locked into the new shape and size.  There will be some residual stress within the films. This can mean that if the temperature of the web is raised but without the web having any tensile load applied the web will shrink a little.  This shrinkage will be permanent where as the coefficient of thermal expansion is temporary and the expansion reversed as the temperature is lowered.   The forwards and sideways stretching may be balanced (equal stretching in both directions) or not. If the web has been stretched a different amount in the forwards direction to the sideways direction the shrinkage will most likely be different in each direction.  This ratio of stretching may be different for the different polymers and so may highlight the different shrinkage forces.

In addition to this the stretching affects the web differently at the edges of the web than in the centre of the web. The centre of the web sees the most uniform stress pattern and so sometimes customers of the web manufacturers request rolls slit only from the centre of the web but may expect to pay a premium for these rolls.

Thus laminating rolls of material slit from the edges of the mill roll may have different angles of stress to each other that compound the shrinkage differences rather than cancel out some of the differences. Thus increasing the interfacial stress.

 

2.         Swelling may or may not be a problem.  Polymers often contain some moisture & this is removed from the surface during the metallization process. Once out of the metallizer these polymers may, over time, re-absorb some moisture.

            Polymers may or may not be stable to solvents. If coatings are deposited onto the polymers, such as adhesives, they may contain a solvent. The solvent may, or may not, migrate through the polymer. There will most likely be some residual solvent in the coating and if the adhesive is used for lamination it may adversely affect the polymer that it is laminated onto. 

3.         Of the materials mentioned the BOPP and LDPE often contain additives such as slip additives (more so than PET that generally contain fewer additives) that are used to improve the polymer handling characteristics by reducing the coefficient of friction.  These additives are detrimental to the adhesion. Often any surface treatment such as flame, corona or plasma is done to remove these slip additives.  If the surface is treated it ought to be done on both surfaces as the slip additives will have migrated to all surfaces equally and so if only one surface is treated it may be re-contaminated from the second almost immediately.

Lifetime of flame treatment.

Question. - How long is the shelf-life of the activated surface on a flame treated part? I would like to understand how rapidly the surface activation decays before re-treatment would have to be carried out (assuming it can be re-treated?)

Answer. – It is almost impossible to give a definitive answer to the question. Firstly there is the flame treatment process that can vary in several ways that can affect the treatment.  There is the gas mixture which determines the chemistry of the flame. Coupled to this is the position the substrate surface is to the flame. Depending on the position the surface can see anything from an oxidizing to a reducing atmosphere. A final variable is the residence time of the surface within the flame.

For webs a short, well-defined flame can be used but for other products such as bottles or automobile parts it is common to use larger flames in order to give a wider tolerance to the flame chemistry. 

Assuming the flame has been optimised, the substrate surface properly positioned within the flame and the residence time optimised the other major variable is the composition of the material to be treated.  Polymers are frequently full of additives &/or fillers.  The fillers can be used to add strength, improve the handling, modify the colour, etc. Additives can be used to improve the processing, change the surface coefficient of friction, UV stability, heat reflectivity, flame retardation, etc.

For example in oriented polypropylene webs it is common to have slip additives added. These additives work by migrating to the surface and reducing the coefficient of friction. When the surface is treated the additive at the surface is either removed, converted or crosslinked into the polymer bulk thus stabilising the surface and improving any subsequent coating adhesion by eliminating the weak boundary layer of slip coating.  However this improvement is not permanent as the bulk polymer still contains the slip additive.  With time more will migrate to the surface.  There are several variables that control the rate of re-contamination.  The level and type of the additive within the bulk polymer, the type and degree of polymerisation and/or crystallinity of the polymer, temperature and the type of surface treatment all affect the re-contamination rate.  If the surface treatment has only removed the contaminants but not done any crosslinking to seal the surface the migration will be quicker. Crosslinking at the surface, a greater degree of polymerisation and/or a greater level of crystallinity will all slow down the rate of migration.   

 

Polymer webs have an additional source of re-contamination.  The slip additives will migrate to all surfaces equally. Thus if only one surface is treated then as soon as the web is re-wound it may immediately start to become re-contaminated from the slip additive on the opposite untreated surface. 

Thus it is possible to start to lose the performance almost immediately in some circumstances.  For other materials will fewer additives, that are stored at low temperature and humidity and without contacting any other surface the treatment can last several weeks to several months.

Thus, based on the above information, I can leave it to you to judge what is the most likely lifetime for your flame treated product. 

In general if the surface has not been over-treated during the first flame treatment there is no problem in treating the surface a second time (or more if required).

Pattern Metallization

Pattern metallization is the title given to the process where the aluminium deposition is masked in some way so that only part of the polymer web surface is metallized.  There are several ways that this can be achieved.

Either the polymer surface can be printed with a pattern before it is put into the vacuum coater. This printed pattern is of a soluble material so that when the whole surface is metallised and removed from the vacuum system it can then be wound through a solvent and the printed pattern will dissolve away taking with it the metal deposited onto the printed pattern.

A similar process is to metallize first and then print a protective pattern onto the metal in the areas where the metal is to remain and then pass the roll through a sodium hydroxide solution which will remove the aluminium in the areas not protected.

There has been a desire to reduce the number of process steps by bringing the patterning process into t he vacuum system so that the patterning and metallization are done at the same time. This was done for simple striping by rotating a metal mask that came into contact with the polymer whilst it passed through the deposition zone.  This process could be prone to generating dust as the accumulation of aluminium on the mask flaked off and also there was a limit to the fineness of the lines that could be produced.

The newer process is to print on the polymer web an oil. This oil is printed onto the areas where the aluminium is not required. The process is designed to print just enough oil such that the polymer is virtually dry of oil once the polymer has passed through the deposition zone. What happens is that the aluminium as it deposits onto the oil does not stick and it re-evaporates. As it re-evaporates it will also take off some of the oil and so eventually all the oil will be removed. If too little oil is printed on there will be some aluminium deposition, if too much oil is deposited there will be residual oil and this will contaminate the roll as it gets re-wound.

Question re. Corrosion of metallized film.

1)      I have analyzed some of products available in the market which are packed in the metallized packs mostly having structure PET/MET-PET/LDPE. When these products are stored under hot and humid conditions demetallization of the pack starts( which can be easily noticed when observed with torch or any light source) e.g. spices, pickles, chilli powder, coriander powder, etc.
what is the reason for such demetallization? how can we prevent that? if prevention is not possible how can we extent the period of demetallization? Is there any need to improve method of metallization or any equipment available which can produce such metallize film which can solve the problem of demetallization?

2)      What is the optical density required for corrosive or aggressive products to pack in the metallized PET laminate ?

3)      Is it possible to carry out metallization with aluminium alloy? if yes what type of equipment used for this

4)      Please tell me the comparison between process of metallization and comparison between properties of the MET-Paper, MET- LDPE, MET-PET, and MET-BOPP.

1.            Demetallization will only occur if the adhesion is not good enough. 

There are various things that can trigger demetallisation. Interfacial stress is a common cause.  Such stresses may be caused by differential thermal expansion between materials such as between the LDPE & PET or differential expansion due to swelling of one of the polymers by chemicals and/or moisture.

If the aluminium is not well adhered to the polymer then materials/chemicals can migrate to the interface changing the surface energy and encouraging the onset of delamination.

Getting the best possible metal adhesion using plasma treatment is essential.

2.         Optical density should not affect demetallisation but only corrosion resistance. In reality it does not prevent corrosion but only makes whatever corrosion takes place less noticeable. Generally corrosion of aluminium takes place at the following rate;

Monolayer             =            immediately

1nm                  =            1 hour

2nm                  =              1 week

3nm                  =             1 month

5nm                  =            1 year

This oxidation rate is only a rough guideline. The true rate will be affected by temperature, humidity, as well as crystal structure and material purity.  Thus it is necessary to assess the product the metallized film is to be used for. If it is to package a liquid at high temperature for a very long period of time the thickness would be greater than if it were to be used to package something that is to be frozen and used within a couple of months.

3.         It is possible to deposit other materials than pure aluminium.  Gold coatings can be deposited using an alloy wire. It does depend on the metallurgy of the alloy. If the alloy wire melts into a pool that evaporates with the same composition and the temperature is similar, or less, than pure aluminium then there is not reason not to use the same evaporation boats as in standard aluminium metallizers.

4.         There are several differences between metallizing paper, PET, OPP & LDPE.

Paper contains a considerable amount of water that needs to be pumped away otherwise the aluminium will become oxidised during deposition. However if the paper is dried out too much it loses all strength and will then tear very easily.

LDPE can handle less heat than OPP which can handle less heat than PET.  Thus it is generally easier to metallize PET than OPP than LDPE.  The optical quality of the metallized surfaces generally is the same as the substrates it is deposited onto. Hence of the aluminium is deposited onto optical grade PET it will have a very high specular reflectance. If the aluminium is deposited onto a highly filled grade of polymer film the specular reflectance will be lower because the surface roughness will create much more diffuse reflectance.

                        

Aluminium wire used for metallizing

The question was as follows.

Hi, I just want to know the type and specification of aluminium used in Flexible Packaging? Is there any book I can read in order to find it out?

Answer.

Aluminium wire used for evaporation has usually got two separate specifications, one for purity and the second for the state of temper.

Typically the aluminium purity is 99% pure or better.  The code used for specifying aluminium is usually 4 digits long.  The first generally is a ‘1’ & specifies the 99% purity, the second digit is used where there is specific control of one particular impurity and the final two digits are the same as the two digits to the right of the decimal point indicating the minimum aluminium content.

i.e.   1199  would indicate a 99.99% purity aluminium wire, see Table below

            The wire can be supplied in different states of hardness. If the wire is fully annealed or soft tempered it is graded ’O’ & if it is fully hardened by strain hardening to 75% it is graded H18. In between these two extremes are quarter hard H12, half hard H14 & three quarters hard H16.  The temper determines how easily the wire can be bent from the drum of wire round to the boat.  As the wire approaches the boat it will be heated & may sag & so some temper may be advantageous particularly if the distance between guide and boat is large.

Alloy/Impurity		1100		1350		1188		1199
Silicon (Si)		0.95 Si+Fe		0.1		0.06		0.006
Iron (Fe)				0.4		0.06		0.006
Copper (Cu)		0.05-0.20		0.05		0.005		0.006
Manganese (Mn)		0.05		0.01		0.01		0.002
Magnesium (Mg)						0.01		0.006
Chromium (Cr)				0.01
Zinc (Zn)		0.10		0.05		0.02		0.006
Titanium (Ti)						0.01		0.002
Others		0.15		0.1		0.01		0.003
Aluminum (Al)		99.00		99.5		99.88		99.99

Alloy 1100 = 1st Digit means minimum 99% Aluminium

                    = 2nd Digit refers to any special contant control

                    = 3rd & 4th Digits refer to the % purity after the decimal point.

Wire Temper
Designation
O		Soft temper - Annealed after working to final dimension
H12		Strain hardened to a point 1/4 tensile strength between 'O' & H18
H14		Strain hardened to a point 1/2 tensile strength between 'O' & H18
H16		Strain hardened to a point 3/4 tensile strength between 'O' & H18
H18		Hard temper - full hard - strain hardening ~ 75%

It is worth noting that other metals and alloys can also be obtained in wire form and deposited in the same way. In particular there is an alloy that is used for golden coloured metallization from the same intermetallic boats.

The table shown is typical of what is available, however the different aluminium wire suppliers may have slightly different designations or present the information slightly differently. Any of the wire suppliers should be able to provide you with similar information.

More details re. Plasma treatments.

Question

In general, which of the following processes in low pressure plasma treatment are accomplished by inert gases, reactive gases, or electrons.

Ablation (Etching)

Deposition

Cross-linking

Functionalisation (Surface Activation)

Answer

(my apologies to Sam for the slow response to this question)

The specific processes involved with any plasma treatment will depend upon the power, pressure and type of plasma that is being used.  At very low powers there may be very little that happens at all.  There has to be sufficient energy to break bonds, after which chemistry can start to occur. 

            The mechanism for cleaning when using a plasma is that ion and electrons physically bombard the surface.  Under this bombardment the weakly bonded atoms can be sputtered from the surface or they are converted into easily volatilised species that do not polymerise or re-deposit onto the web/foil. In addition to cleaning the plasma will chemically change the surface usually this is done specifically to allow the depositing material to be better bonded to the surface. It can also be done specifically to produce non-wetting surfaces but this is much less common.

            Oxygen, air, water or N₂O can all be used to remove organics by oxidation.  These gases also can leave oxygen bonded into polymer surfaces that can act as a tie layer.    Adding oxygen to the process can increase the effectiveness by increasing the production of ozone and atomic oxygen.  This process can be particularly good at removing hydrocarbons.

            Hydrogen by itself or in mixtures may be used in some cases where the contaminant is sensitive to oxidation.  The contaminants are converted to low molecular weight volatile species that do not polymerise and hence are more easily evaporated or sputtered from the surface into the plasma and thence pumped away.

            Noble gases usually means that argon gas is used to give a more physical etch.  The problem with noble gases is that there is no mechanism to convert the fragments into permanently volatile compounds and hence they tend to redeposit on the surface or be polymerised.  The physical bombardment by heavier noble gases either causes more bond breaking to leave active sites or increases the amount of crosslinking both of which can lead to a more stable and higher adhesion interface.

            Noble gases are also used as carrier or diluent gases.  The noble gases increase the vacuum ultra-violet (VUV) output of the plasma significantly. This can aid the dissociation of the other gas increasing reactivity and thus speeding up the process.