WO2007089146A1

WO2007089146A1 - Method for surface treatment by plasma and surface treatment apparatus

Info

Publication number: WO2007089146A1
Application number: PCT/NL2007/050040
Authority: WO
Inventors: Hindrik Willem De Vries
Original assignee: Fujifilm Manufacturing Europe B.V.
Priority date: 2006-02-02
Filing date: 2007-02-02
Publication date: 2007-08-09
Also published as: EP1979400A1; JP2009525381A

Abstract

Method for surface treatment of a triacetyl cellulose film (10) and a surface treatment apparatus. The surface treatment apparatus has a first electrode (16) and a second electrode (17) for generating an atmospheric pressure glow discharge plasma in a treatment space (15) between the first and second electrode (16, 17). The electrodes (16, 17) comprise a dielectric barrier on a surface directed towards the treatment space (15). The surface treatment apparatus is arranged to generate the atmospheric pressure glow discharge plasma in a substantially oxygen free atmosphere in the treatment space (15).

Description

Method for surface treatment by plasma and surface treatment apparatus

Field of the invention

The present invention relates to a method for surface treatment of a material, in particular of a triacetyl cellulose film. In a further aspect, the present invention relates to a surface treatment apparatus.

Prior art

A method and apparatus for surface treatment of a material are disclosed in American patent US6,512,562, which discloses a protective film for a polarizing plate and a method and apparatus for producing such a film. In one exemplary embodiment, an atmospheric pressure glow discharge plasma is being used to treat the protective film surface.

For producing polarising films, which may be used in the production of e.g. liquid crystal displays, a polarising film such as polyvinyl alcohol (PVA) may be laminated with two triacetyl cellulose (TAC) films. This e.g. described in American patent US2005/0063057.

In order to properly attach the TAC film to the PVA film, the TAC film surface may be pre treated with alkali (saponification). This allows formation of H-bridges with the polymer for proper bonding of the two materials. A disadvantage of this process is the use of wet material with the associated handling and waste problems. In an alternative process, which is described in Japanese patent application JP2004272078, the PVA film is pretreated before being laminated with two TAC films.

A further alternative for combining the TAC films to the PVA film is disclosed in Japanese patent application JP2000214329, in which polyol-containing adhesive layers are being used. This, however, requires additional material and processing steps.

Japanese patent application JP2002155371 discloses a method and system for manufacturing a semiconductor device. A transparent conductive film is deposited on a semiconductor substrate using a plasma generated in an atmosphere comprising argon, helium, neon, and/or xenon. Summary of the invention

The present invention seeks to provide a treatment method and apparatus for triacetyl cellulose (TAC) film in particular, which does not have the disadvantages of the known methods and systems described above. A particular disadvantage is the formation of a white deposit upon APG treatment of an TAC film in the presence of oxygen.

According to the present invention, a method for surface treatment of a triacetyl cellulose film is provided, comprising generating an atmospheric pressure glow discharge plasma in a treatment space, and exposing the surface of the triacetyl cellulose film to the atmospheric pressure glow discharge plasma in the treatment space for a predetermined amount of time, in which the atmospheric pressure glow discharge plasma is generated in a substantially oxygen free atmosphere in the treatment space comprising a mixture of a noble gas, such as argon, and an inert gas, e.g. nitrogen. The atmosphere may comprise at least 2% of nitrogen, e.g. 20% of nitrogen. Although the American patent US6,512,562 seems to disclose various compositions of the atmosphere in which the atmospheric plasma glow discharge is generated, it has been surprisingly found that using the features of the present invention, it is possible to reduce the water contact angle of the substrate to a large extent, without any form of white deposit formation. Such a treatment results in a significant decrease of the water contact angle of the triacetyl cellulose film, which makes it suitable for effectively laminating it to a polymer film, such as a polyvinyl alcohol film, for providing a polarising plate film. Treatment in an argon atmosphere already reduces the water contact angle of the surface of the triacetyl cellulose film, but adding nitrogen enhances this effect. Tests have shown that a 20% nitrogen atmosphere provides a better result than a 2% nitrogen atmosphere. From American patent

US6,512,562, the skilled person will learn not to use any nitrogen (see e.g. Table 2) as in these cases, the water contact angle remains higher even after long treatments.

When the triacetyl cellulose film is exposed to the atmospheric pressure glow discharge plasma for less than four seconds, e.g. 0.1 seconds, already a decrease in water contact angle can be measured. Depending on the various possible embodiments very small water contact angles (WCA) can be reached, depending on the treatment time and the gas composition used. So using only nitrogen a WCA of 12° can be reached after 4 seconds while under same conditions using Ar a WCA of 29° is obtained

In a further embodiment, the triacetyl cellulose film is exposed to an atmospheric pressure glow discharge plasma which is stabilized according to methods described in for example US6774569B, or EP-A-1383359.

In still a further embodiment the triacetyl cellulose film is exposed to an atmospheric pressure glow discharge plasma, where the plasma is e.g. stabilized by an LC matching circuit.

In a further embodiment, the triacetyl cellulose film is exposed to a pulsed atmospheric pressure glow discharge plasma. A pulsed discharge plasma is another method for the control of the plasma formation and uniformity and also provides for a method to control the plasma effectiveness in relation to for example the treatment time wanted or the line speed range which is available in the plasma generation set-up

The method may further comprise laminating the treated triacetyl cellulose film to a polarizing film, such as polyvinyl alcohol film, to efficiently provide a polarizing plate film, without the need for further treatment of the films. Other methods of combining the triacetyl cellulose film to the polarizing film, such as attaching, contacting, bonding, etc. may also be used.

In a further aspect, the present invention relates to a surface treatment apparatus, comprising a first electrode and a second electrode for generating an atmospheric pressure glow discharge plasma in a treatment space between the first and second electrode, the first and second electrode comprising a dielectric barrier on a surface directed towards the treatment space, in which the surface treatment apparatus is further arranged to generate the atmospheric pressure glow discharge plasma in a substantially oxygen free atmosphere in the treatment space. The electrodes can be provided with a dielectric barrier in various arrangements. In one arrangement the dielectric barrier of at least the second electrode is formed by a triacetyl cellulose film. In another arrangement both electrodes are provided with TAC film as a dielectric barrier. In still another arrangement the electrodes can be provided with a dielectric barrier. Such as polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polytetrafiuoroethylene (PTFE), polyethylene (PE), ceramic such as silica or alumina, or combinations of these, also microporous dielectric materials attached to the electrodes can be used. In case of triacetyl cellulose the film may be moveable over the associated electrode which might be a bare electrode or an electrode provided with a dielectricum, allowing for a continuous treatment process. The other electrode may also be provided with a stationary or moveable film, e.g. comprising polyethylene terephthalate (PET) or other polymers, as dielectric barrier. In a further embodiment, both electrodes use a triacetyl cellulose film as dielectric barrier, allowing a higher throughput.

In further embodiments, the surface treatment apparatus may be arranged to generate the atmospheric pressure glow discharge plasma in the atmospheres as defined in the above described method embodiments. The surface treatment apparatus may in a further embodiment comprise a transport device for transporting the triacetyl cellulose film over the electrode. Also, the transport device may comprise a tensioning mechanism for keeping the triacetyl cellulose film in close contact with the electrode.

In an even further embodiment, the surface treatment apparatus further comprises a laminating unit positioned downstream of the treatment space, in which the laminating unit is arranged to laminate the treated triacetyl cellulose film and a polarizing film, such as a polyvinyl alcohol film. This allows to produce a polarizing plate film very efficiently, which may be used e.g. in the production of liquid crystal displays. The present invention also relates to a surface treatment apparatus, comprising a first electrode and a second electrode for generating an atmospheric pressure glow discharge plasma in a treatment space between the first and second electrode, the first and second electrode comprising a dielectric barrier on a surface directed towards the treatment space, in which the surface treatment apparatus is further arranged to generate the atmospheric pressure glow discharge plasma in a substantially oxygen free atmosphere in the treatment space, in which the surface treatment apparatus is arranged to generate a stabilised atmospheric pressure glow discharge plasma in the treatment space. By having the combination of the use of a substantially oxygen free atmosphere in the treatment space and the generation of a stabilised atmospheric pressure glow discharge plasma, the water contact angle of the treated surface is reduced without any formation of a white deposit on the substrate.

In a further embodiment, the means for controlling the plasma comprise an LC matching network formed by a matching inductance and a system capacity formed by the two electrodes and the discharge space, and a pulse forming circuit in series with at least one of the electrodes. This provides for an even better stabilisation of the glow discharge plasma, further enhancing the efficient surface treatment of the surface and further suppressing the formation of a white deposit.

Short description of drawings

The present invention will be discussed in more detail below, using a number of exemplary embodiments, with reference to the attached drawings, in which

Fig. 1 shows a schematic diagram of a first embodiment of a surface treatment apparatus according to the present invention;

Fig. 2 shows a schematic diagram of a second embodiment of a surface treatment apparatus according to the present invention;

Fig. 3 shows a graph showing the results of water contact angle measurements for a number of tests using the surface treatment method according to the present invention;

Fig. 4 shows a graph showing the results of water contact angle measurements for a number of control tests using an air corona surface treatment method;

Fig. 5 shows a schematic diagram of an apparatus for producing a polarising plate using a treated film; and Fig. 6 shows a schematic view of a stabilising arrangement used in an embodiment of the present invention.

Detailed description of exemplary embodiments

In Fig. 1, a surface treatment system is shown according to a first embodiment of the present invention. The system comprises a parallel plate double dielectric barrier discharge geometry, having a first or upper electrode 16 and a second or lower electrode 17. The electrodes 16, 17 define a treatment space 15, which can be constructed according to specific requirement. Both the electrodes 16, 17 can have a width of for example 1, 2, 3, 4 cm and more while also various electrodes in series with each other can be used thereby increasing the possible treatment time. The length in this respect is defined as the length of the electrode 16, 17 perpendicular to the treatment direction of a web 10 to be treated, i.e. perpendicular to the drawing surface of Fig. 1. In order to have a good and uniform plasma, the electrodes 16, 17 have to be strictly parallel over the total length, without this a stable plasma cannot be obtained. As long as the electrodes are parallel and remain parallel during the usage there is no limitation in length. The length is mainly dependant on the width of the web 10 which should be treated, so any length from 10cm (test purpose) to as long as 1.5 m or even 2.0m and 2.5m and more can be used.

The electrodes 16, 17 may be covered with a dielectric discharge barrier like for example polyethyleneterephthalate (PET), polyethylenenaphthalate (PEN), polytetrafluoroethylene (PTFE), polyethylene (PE), ceramic such as silica or alumina, or combinations of these. The upper electrode 16 is provided with a dielectric barrier layer or foil on its surface directed towards the treatment space, while for the lower electrode 17, in addition to an optional dielectric layer fixed to electrode 17, the dielectric barrier layer is provided by a triacetyl cellulose (TAC) film or web 10 to be treated. The electrode gap between the dielectric foils of the electrodes 16, 17 is not limited to any particular distance but should be preferably below 5 mm, preferably below 2 mm or more preferably below 1.5 mm and can be typically 1 mm or even lower.

The TAC film 10 is transported using a transport device including a supply roll 20 and a take up roll 21, via further rollers 22...25. The further rollers 22...25 are provided in an s-wrap configuration on both sides of lower electrode 17, and form a tensioning mechanism which allows alignment adjustment of the TAC film 10 over the lower electrode 17.

In a further embodiment, which is shown schematically in Fig. 2, also the upper electrode 16 is configured to have a film 10' as dielectric barrier, which is transported from a supply roll 20' to a take up roll 21 '. In both configurations the electrodes 16, 17 are covered with a dielectric foil 10,

10' to sustain the plasma stability. The function of the foils 10, 10' is thus to control the plasma stability and to have those foils 10, 10' exposed to the plasma in treatment space 15.

The thickness of the dielectric foils used in general and of the TAC foils more in particular is not limited to any specific value and is merely dictated by the application for which it is used. In general the foils have a thickness of below 150 micrometer and preferably the thickness is below 100 micrometer for example 90 or 80μm. The atmospheric pressure glow discharge plasma can be generated according to the methods known in the art. Preferred plasma's are those which are stabilised for example according to the principles set forth in for example US6774569B, or EP-A- 1383359. An example of a preferred embodiment of a plasma stabilisation and control arrangement is shown in Fig. 6. In this figure an impedance matching arrangement is provided in the plasma control arrangement, in order to reduce reflection of power from the electrodes 16, 17 back to the power supply (i.e. AC power supply means 35 and intermediate transformer stage 36). The impedance matching arrangement may be implemented using a known LC parallel or series matching network, e.g. using a coil with an inductance of L_matohm_g and the capacity of the rest of the arrangement (i.e. formed mainly by a parallel impedance 43 (e.g. a capacitor) and/or the capacitance of the discharge space 15 between the electrodes 16, 17). However, such an impedance matching arrangement cannot filter high frequency current oscillations, which may occur during plasma breakdown. The high frequency supply 35 is connected to the electrodes 16, 17 via intermediate transformer stage 36 and matching coil with inductance L_matchmg- Furthermore, a pulse forming circuit 40 is connected to the lower electrode 17. A further impedance 43 is connected in parallel to the series circuit of electrodes 16, 17 and pulse forming circuit 40. The pulse forming circuit 40 is arranged to obtain the desired pulse shaping in order to suppress (or enhance) instabilities, which may possibly form at the pulse breakdown (onset of plasma pulse) and also to suppress (or enhance) instabilities at the end of the plasma pulse (after the plasma pulse maximum. The main idea is to use the pulse forming circuit 40 in series with a resonant LC series circuit. In this way when the plasma pulse forms (having a duration much shorter than the half period of the sine of the AC applied voltage) the serial resonant circuit will be unbalanced by the need of large frequency current (due to the forcing of the power supply 35 to provide large currents) and the displacement current provided from the power supply will tend to drop. The simplest implementation of the pulse forming circuit 40 is a capacitor in series with the plasma electrodes 16, 17. In order to be efficient its capacity must be comparable with the plasma reactor capacity.

The pulse forming circuit 40 can be formed out of several components. So for example it can be composed out of a choke coil in parallel with a capacitor. The circuit 40 of the capacitor and choke are chosen to be resonant at the frequency of the power supply 35. In another embodiment, the circuit 40 is composed out of a capacitor in parallel with a series resonant LC circuit, comprising a choke and a further capacitor. Also in this case the characteristics of the components are chosen in such a way, that the circuit is resonant to the frequency of the power supply 35. In order to achieve a surface treatment of a substrate 10 in general and TAC film or foil in more particular a gas or a gas mixture has to be introduced in the electrode gap 15. In case of a mixture of gases, this mixture is mixed on beforehand and subsequently injected in between the electrode gap (treatment space) 15. The gases preferably used in this invention are noble gases like for example neon, argon, helium and inert gases like for example nitrogen. Also mixtures of gases can be used like for example mixtures of a noble gas and an inert gas. Surprisingly it was found, that the lowest water contact angle on TAC was obtainable by using nitrogen alone. Good results were also obtained when using a mixture of a noble gas and for example nitrogen. In these embodiments the WCA became lower upon increasing the amount of nitrogen, keeping the treatment time the same. To our further surprise, using mixtures of a noble gas with oxygen did not result in a lowering of the WCA; instead a white deposit was formed on the TAC.

A continuous treatment of a web 10 is preferable and therefore the web to be treated preferably has a continuous speed over the electrode 16, 17. This speed is dependant on the electrode width and on the electrode arrangement and the treatment time one wants to achieve. So in case of more electrodes placed behind each other the line speed can be higher giving the same treatment time as with only one electrode. Treatment times as long as 4 seconds give very good results, but already significant results can be obtained with 1 second treatment time or less. In case of the double layer system of Fig 2, the line speed of the bottom web 10 and that of the upper web 10' may be the same or may be different. Line speeds are possible of more than lOcm/min to lm/min to over 30m/min and higher as stated above, dependent on the configuration used. Wrinkling of one of the foils 10, 10' is to be prevented because a condition to maintain a stable and uniform glow discharge is that the foil 10, 10' should have a uniform and intimate contact with the associated electrode 16, 17.

Another attention point in the double layer system of Fig 2 is the amount of volatiles evaporating from the web materials used. It was observed, that stable atmospheric glow plasma's could be obtained using industrial available web materials like PET, PEN, TAC and the like and no deterioration occurred by the presence of water vapor or solvents evaporating from the dielectric barriers (foils 10, lO'at the electrode 16, 17 surface). Upon changing the web material at the top electrode in the double layer structure of Fig 2 plasma stability need not to be deteriorated by substituting one PET or PEN foil 10' by a second TAC film or foil 10'. In such a case the dielectric capacity of the dielectric barrier discharge gap is somewhat changing and therefore the pulse network controlling has to be adjusted slightly.

The TAC film treated according one of the embodiments of this invention has a very low WCA. This low WCA cannot be reached using common available techniques. The low contact angle remains also after aging for several days, meaning, that the treated material can be stored until further processing.

One application in which this treated TAC film can be used preferably is in the manufacturing of polarizing films. A specific embodiment of such manufacturing is shown schematically in Fig 5. According to the embodiment in this figure, the treated TAC is combined with a polarising film 11 , as shown in the simplified schematic view of Fig. 5. The polarising film 11 can be for example polyvinyl alcohol (PVA) film. This film is provided from a source roll 31. The treated TAC films 10, 10'are provided from supply rolls 32, 33, respectively, or may be provided directly from the treatment apparatus as described above. The three films 10, 11, 10' are then laminated in a lamination unit 30, to produce a polarizing plate film 12. Because of the APG treatment of the TAC films 10, 10', adhesion to the PVA film 11 is achieved without further pre- treatment of the TAC films 10, 10' or PVA film 11. The lamination unit 30 may be of a known type, and may use pressure and/or heat to bond the three films 10, 11, 10' together.

Example 1

The set-up consists of a parallel plate double dielectric barrier discharge geometry (Fig 2) with a total electrode length of 15 cm and an electrode width of 4 cm (dimension in treatment direction). The electrode gap 15 between the dielectric foils is typically 1 mm. Both electrodes 16, 17 are covered with a dielectric foil to sustain the plasma stability, see the description above. The function of the foils is thus to control the plasma stability and to have those foils exposed to the plasma. The top electrode foil used comprises of a 90 um PET foil whereas the bottom foil is 80 um TAC film. The choice to mount the TAC film on the bottom roll to roll system is because of ergonomics.

In this dielectric barrier discharge (DBD) system the metal electrodes 16, 17 are powered by a high frequency power generator 35 comprising of an RFPP generator LFlOa connected to an impedance matching transformer 36. The output of the matching transformer 36 is connected to a series resonant network, similar to the structure described with reference to Fig. 6 above. The APG reactor 15, 16, 17 is connected parallel to the capacitor 43. The resonance frequency of the system is tuned at about 120 kHz. By tuning a displacement pulse network 40 connected in series with the APG branch the plasma stability is controlled such that the glow plasma mode is enhanced and filamentary discharge is suppressed. The displacement current control operates within each halve cycle of the electric field (within pulse train). The generator 35 is set to slave mode and driven by external pulse control unit (not shown). The external pulse control unit can either be computer controlled or consist of two function generators where one of these generators provides the triggering of pulse trains for the other generator. The pulse trains consist of a series of AC pulses defined as: T_0n followed by a period of no electric field T₀ff. In this case the pulse train duration can be set from 10 microseconds to 1 second. In this particular case the pulse on time was chosen in the millisecond range.

The gas is mixed and subsequently injected from the left hand side in between the electrode gap.

Line speed of the bottom roll to roll (with TAC film) was varied, whereas the line speed of the top roll to roll was set to the lowest line speed being 10 mm/min. First the plasma was stabilized for a certain gas and subsequently the foil is exposed to the plasma with increasing line speed from 15 mm/min doubling the line speed in 7 steps up to 960 mm/min.

Four different runs are carried out in argon, argon with 2 and 20% nitrogen, argon with 2% oxygen, and nitrogen. The process parameters used for the surface activation are listed in the table below.

slm = standard liters/minute ms = milliseconds

% = duty cycle (pulse on time divided by the pulse off time together with the pulse on time

After each run in a specific gas (mixture), seven different treatments are obtained and subsequently analyzed by contact angle measurements. Water contact angles are measured using a contact angle meter as known to the skilled person. The TAC foil 10 is thus exposed to plasma as a function of treatment time (dose) and process gas. Fig. 3 shows the results of the water contact angle measurements for the various tests.

Reference example 1 :

The TAC films 10 were treated in a table corona unit equipped with six alumina electrodes, as known to the skilled person. The TAC film 10 was put on an alumina coated drum electrode rotating with a speed V. Further parameters are listed in the following table (No. rev giving the number of revolutions of the coated drum electrode resulting in the total treatment time in second(s)).

Again, the water contact angle of the surface was measured, and the results are shown in the graph of Fig. 4.

In the case of argon and argon/nitrogen treatments no visual degradation of the TAC foil 10, 10' is visible even in the case of the longest treatments. The foil 10, 10' remains (visually) untouched. However in the case of argon/oxygen mixture a clear white deposit is formed on the substrate which can be wiped off, for treatment times of more than about five seconds (as indicated in Fig. 3). This phenomenon is caused by very strong oxidation of the surface which results in etching of the TAC surface of the foil 10, 10'. The small organic compounds are subsequently oxidized and deposited back on the substrate as Low Molecular Weight Oxidized Material (= LMWOM). The LMWOM is also known from very heavy corona treatment of polyethylene. Normally this is undesirable because the LMWOM is covering the surface inhibiting further activation and in principle forming a weak boundary layer. Also in the case of treatment of a TAC foil 10 with corona discharges, LMWOM deposits occur, as indicated in the graph of Fig. 4 for treatment times larger than about five seconds. As can be seen in Fig. 3 for the argon and argon/nitrogen mixtures a strong decrease in water contact angle can be obtained. Note that the treatment times shown on the x-axis is the real plasma exposure time corrected for duty cycle.

No decrease in WCA was observed in the case of argon/oxygen treatments. Possibly the LMWOM is already formed on the shortest treatment times creating already a thin deposit. This may explain why the plasma treatment in oxygen has no effect on the WCA.

Because it is known that plasma chemistry (in air corona) in the presence of oxygen is dominated by oxygen a similar result was expected as in the case of the APG plasma treatment in argon oxygen mixture. Indeed no decrease in the WCA was observed using corona treatment, see Fig. 4. Only in the case of the strongest corona treatment a slight decrease in the WCA was obtained. This is probably due to very strongly oxidized LMWOM which is also clearly visible on the TAC film for treatment times of more than 5 seconds. With treatment time of less than four seconds, no such LMWOM depositions are observed. Example 2:

The treated TAC film 10, 10' obtained by one of the embodiments described above, was combined with a polarising film 11, as shown in the simplified schematic view of Fig. 5. The polarising film 11 was a polyvinyl alcohol (PVA) film 11, which is provided from a source roll 31. The treated TAC film 10, 10' is provided from supply rolls 32, 33, respectively,. The three films 10, 11, 10' were laminated in a lamination unit 30, to produce a polarizing plate film 12. Because of the APG treatment of the TAC films 10, 10', adhesion to the PVA film 11 was achieved without further pre- treatment of the TAC films 10, 10' or PVA film 11. The lamination unit 30 was a known type, using pressure and heat to bond the three films 10, 11, 10' together.

Claims

1. Method for surface treatment of a triacetyl cellulose film (10), comprising generating an atmospheric pressure glow discharge plasma in a treatment space (15), and exposing the surface of the triacetyl cellulose film (10) to the atmospheric pressure glow discharge plasma in the treatment space (15) for a predetermined amount of time, in which the atmospheric pressure glow discharge plasma is generated in a substantially oxygen free atmosphere in the treatment space (15) comprising a mixture of a noble gas and an inert gas, in which the atmosphere comprises at least 2% or more of nitrogen, e.g. 20% of nitrogen.

2. Method according to claim 1, in which the triacetyl cellulose film (10) is exposed to the atmospheric pressure glow discharge plasma for more than 0.1 second and less than 4 seconds.

3 Method according to any one of claims 1 or 2, in which the triacetyl cellulose film (10) is exposed to a stabilised atmospheric pressure glow discharge plasma.

4. Method according to any one of claims 1 through 3, further comprising laminating the treated triacetyl cellulose film (10) to a polarizing film (11), such as polyvinyl alcohol film.

5. Surface treatment apparatus, comprising a first electrode (16) and a second electrode (17) for generating an atmospheric pressure glow discharge plasma in a treatment space (15) between the first and second electrode (16, 17), the first and second electrode (16, 17) comprising a dielectric barrier on a surface directed towards the treatment space (15), in which the surface treatment apparatus is further arranged to generate the atmospheric pressure glow discharge plasma in a substantially oxygen free atmosphere in the treatment space (15) comprising a mixture of an inert gas and a noble gas, in which the atmosphere comprises at least 2% of nitrogen or more, e.g. 20% or more of nitrogen.

6. Surface treatment apparatus according to claim 5, in which the dielectric barrier of at least the second electrode (17) comprises a triacetyl cellulose film.

7. Surface treatment apparatus according to any one of claims 5 or 6, further comprising a transport device (20...25), for transporting the triacetyl cellulose film (10) over the electrode (16; 17).

8. Surface treatment apparatus according to claim 7, in which the transport device comprises a tensioning mechanism (22...25) for keeping the triacetyl cellulose film (10) in close contact with the electrode (16; 17).

9. Surface treatment apparatus according to any one of claims 5 through 8, in which the surface treatment apparatus is further arranged to expose the triacetyl cellulose film (10) to the atmospheric pressure glow discharge plasma for more than 0.1 second and less than 4 seconds.

10. Surface treatment apparatus according to any one of claims 5 through 9, in which the surface treatment apparatus is arranged to generate a stabilised atmospheric pressure glow discharge plasma in the treatment space (15).

11. Surface treatment apparatus according to any one of claims 5 through 10, further comprising a laminating unit (30) positioned downstream of the treatment space (15), in which the laminating unit (30) is arranged to laminate the treated triacetyl cellulose film (10) and a polarizing film (11), such as a polyvinyl alcohol film.

12. Surface treatment apparatus, comprising a first electrode (16) and a second electrode (17) for generating an atmospheric pressure glow discharge plasma in a treatment space (15) between the first and second electrode (16, 17), the first and second electrode (16, 17) comprising a dielectric barrier on a surface directed towards the treatment space (15), in which the surface treatment apparatus is further arranged to generate the atmospheric pressure glow discharge plasma in a substantially oxygen free atmosphere in the treatment space (15), in which the surface treatment apparatus comprises means for controlling the plasma for generating a stabilised atmospheric pressure glow discharge plasma in the treatment space (15).

13. Surface treatment apparatus according to claim 12, in which the means for controlling the plasma comprise an LC matching network formed by a matching inductance (L_matohm_g) and a system capacity formed by the two electrodes (16, 17) and the discharge space (11), and a pulse forming circuit (40) in series with at least one of the electrodes (16, 17).