Surface treatment of polymers.

There have been a number of questions asked recently regarding surface treatment, specifically of BOPP, such as these below.

Can you kindly advise the different methods of surface treatment we can use for treating BOPP film (e.g.: flame treatment, plasma treatment in vacuum....)?
What are the advantages and disadvantages of each of the treatments? At the right conditions (can you pls also advise the conditions), what is the rate of improvement for different plasma powers, gas flow rates, and web speeds when we compare it with no
plasma treatment BOPP film

1)What do we try to achieve with plasma treatment (in vacuum) on BOPP film.

2)How is BOPP film affected when treated by plasma treatment in vacuum? In other words, how is the structure of the surface altered?

3)If there are any disadvantages of plasma treatment (in vacuum) what are they?

I will briefly run through the different surface treatments that have been used and comment on any advantages or disadvantages.  What I cannot do is give process details such as power & speed for a particular treatment.  This depends on so many factors that are system and substrate specific.

Firstly I will explain in more detail why I cannot give these details.

Read on for the complete answer.

In general the process for determining the optimum treatment for any film is to measure the surface energy, by measuring water droplet contact angle, of the film before treatment.  Then systematically measure the increasing surface energy as the treatment is increased.  The aim is to maximise the surface energy.  It is worth noting that once the surface has reached a maximum surface energy it is possible to overtreat the surface without seeing a decrease in the surface energy. What happens with overtreatment is that the surface degrades into shorter molecular fragments that act as a weak boundary layer thus reducing the metal adhesion. Thus the surface chemistry may appear the same but the adhesion has been lost so it is important to treat the surface only enough to achieve the maximum surface energy but no more.

Resist the temptation to think that because a little treatment is good more would be even better, it does not always s apply.

All manufacturers of polymer film have proprietary recipes for both the polymer film and the treatments they use to make the film handle better.  The bulk polymer will not have been completely polymerised. As the polymerisation takes place the mobility of the monomer decreases because of the increasing chain length of the polymer and any crystallinity within the polymer. Thus there will be some proportion of unpolymerised material at the end of the process.  This short chain material will be present on the surface and as it is not bound into the surface will act as weak boundary layer.  Further to this many manufacturers add a slip agent to reduce the coefficient of friction. These slip agents rely on being able to migrate to the surface with the aim of being present on the surface in quantities anywhere up to a monolayer. This being the case the surface could be pure polymer (unlikely), polymer + ‘mer’ units of unpolymerised material, polymer + ‘mer’ units + slip agents (or other additives), or even have no polymer present but solely consist of additives and low molecular weight ‘mer’ units.

Thus the level of treatment required will depend on the manufacturer and grade of film and changing either of these may require needing to re-optimise the treatment process.

Treatments.

The treatments range from the physical such as roughening the surface to provide a higher surface area, jigsaw type interlocking mechanism for increasing the adhesion. However this abrasion process also destroys the optical transparency and any specular reflectance of the metallisation and hence is little favoured.

Chemical treatments achieved by passing the web through a bath of some liquid have been done. Usually this treatment dissolves off any low molecular weight material and in some cases etches the surface. This etching is a finer version of the abrasion technique and increases the surface area but is not a gross as the mechanical abrasion. This too increases the haze of any metallisation. Generally this type of process is not preferred, as it requires several baths for the treatment and subsequent washing. Often the baths contain unpleasant chemicals and need to be run hot making the safety and environmental issues expensive and onerous.

Plasma treatments.

            Plasmas consist of ionised gas and include all of the next treatments.  The gas flame contains ionised gas as does the corona and so all are the following are technically plasma treatments. What becomes slightly confusing is that the term ‘plasma treatment’ is also used specifically for the generation of a plasma within a vacuum system.

            There are differences between the different plasmas. The regular flame is a ‘hot’ plasma whereas the vacuum generated plasma is a ‘cold’ plasma. This relates to the electron temperature of the gas.  In general the electrons only heat the surface whereas the ions do the work of chemically modifying the surface and so the vacuum plasma is more efficient than the atmospheric treatments.

Flame treatment process is as the name suggests. A gas/air flame is produced and the polymer web is passed through it. The gas/air ratio can be adjusted to change the flame chemistry and the web can pass through flames that are anywhere from oxidising to reducing.  Thus some care is required in setting up the flame treatment process to get the correct flame chemistry through gas ratios and throughput, the optimum web position within the flame as well as the correct residence time. 

Corona treatment is where an electric circuit is formed with an air gap. At a suitable frequency and gap the air will breakdown and a plasma will be formed allowing a current to flow. The web is passed through the gap and will be bombarded. To prevent the surface charging up and stopping the process the power supply is an alternating current high frequency type. 

There are some different variations for this process. Generally the web is kept in constant position by passing the web over a roll that is also one of the plasma electrodes.  This roll can be metal or sleeved with an insulating material, the latter being to increase the voltage of operation.  The disadvantage of this process is the occasional arcs that can occur during the process. These arcs condense all the power into one spot and this concentrated energy can pierce the web.  This corona in breaking down the air does produce ozone. This is an aggressive chemical that does help with the surface treatment but does require the area is enclosed in a box and the ozone extracted safely.

Atmospheric plasma is a newer process that is able to operate in atmosphere at a lower voltage than corona and so does not suffer from the problems of arcing nor ozone formation.  However to obtain the plasma stability it does require a significant use of Helium.  The process has been described as being between flame and corona.

In-vacuum plasma treatment is achieved by passing the web through any plasma generated within the vacuum. This can be and RF, DC, microwave or magnetically enhanced versions of the same.  The advantage of treating the web in vacuum is that immediately after the treatment the coating is applied and so the surface reactivity is at a maximum and so the bonding of the coating will be as good as it can be.  If the metallizer does not already contain a plasma treatment stage it can be hard to retrofit.

Vacuum plasma treatments can be customised to change the chemistry much more than the other treatments.  Where there is thought to be organic contamination the use of oxygen in the plasma allows the organics to be converted and volatilised thus cleaning as well as functionalising the remaining surface.

Surface treatment process.

In general if we look at the chemistry of polymer surfaces (assuming pure polymer) they are mostly carbon.  This results in a fairly low surface energy.  Changing some of the carbon atoms to oxygen atoms and the surface energy increases. Thus measuring surface energy can be an indication of the changes the oxygen present on the surface.

In general terms the treatments are all breaking bonds forming free radicals which can bond to oxygen thus changing the chemical nature of the surface.  The different processes will make this conversion through slightly different routes but all aim for a similar end result that of reduced carbon and increased oxygen content.  The depositing aluminium can then bond to the oxygen as aluminium oxide thus becoming bonded to the polymer via the oxygen.  The more the metal coating is bonded to the polymer the better the adhesion and the more stable the coating. 

	Untreated	Treated
Solids	Surface	Surface
Polymers	free energy	free energy
	Dyne/cm	Dyne/cm
	(N/mm)	(N/mm)
Nylon	38--46
Polyester	41--44	48 -- 52
Cellulosics	42
PVC, acrylic	39
PMMA	30--36
polystyrene	38
polystyrene (low ionomer)	33
L/HDPE	30--31	38 -- 40
Polypropylene	29--31	39 -- 41
PTFE	19--20

	Surface	Number
	Tension	& type
		of
	Dyne/cm	significant
	(N/mm)	atoms
CF3	5	FFF
CF2	18	FF
CF2--CFH	22
CH3	24	HHH
CF2--CH2	25
CH2	31	HH
CHOH--CH2	35	O
CClH--CH2	38	Cl
CCl2--CH	40	Cl.Cl
CCl2	43
CO2--CH2	43	OO

A further advantage of plasma treating in vacuum is that the surface does not have the chance to become re-contaminated.  On all of the other processes a consideration needs to be given to treating both sides. Any additives such as slip coatings will also be present on the reverse surface. Thus following treatment when the roll is rewound there is the possibility of any contamination on the reverse side being transferred across to the front surface.  This contamination would be reduced if the reverse surface were treated too.

Timing is also important.  If the treatment is done a long time before the roll is put into the vacuum it is possible for more material to migrate to the surface to re-contaminate the surface.

This is not to say that treating the reverse surface is unnecessary for the in-vacuum plasma treatment.  Once the metallization has taken place and the roll rewound anything on the back surface will be transferred to the front surface. Thus any downstream process can be affected by the low molecular weight material reducing adhesion to the metal.

I hope this suitably answers the question.