Abstract:
The invention is directed to a process for treating a photographic support in the form of a web, said process comprising providing a first grounded drum shaped electrically conductive electrode and at least one electrically conductive wire electrode of which the diameter varies between 60 and 1500 μm facing said drum shaped electrode, establishing an AC voltage with a frequency range between 100 Hz to 300 kHz over said electrodes, moving the web at atmospheric pressure along said drum shaped electrode, thereby exposing it to atmospheric pressure glow discharge established between the said drum shaped and wire electrode.

Description:
FIELD OF THE INVENTION 
     The invention is directed to a process for treating polymeric supports by an atmospheric pressure glow discharge (APGD) process for improving the surface properties thereof, more in particular for improving the adhesion of photographic emulsions on polymer coated photographic base papers or on polymer film supports, such that the critical coating line speed can be increased. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,138,971 discloses a web charging apparatus, i.e. an apparatus that deposit electrostatic charges (ESC) on a moving web, preceding a coating apparatus so as to improve the affinity and adhesion of a coating solution on the web by depositing unipolar electrostatic charges on the surface of the web before the coating solution is applied on the surface. In the web charging apparatus, corona discharge is established with a DC power supply between wire electrodes and the grounded roller, while the wire electrodes are extended perpendicular to the direction of the moving web. The web is supported on a grounded roller or drum shaped electrode, functioning as a grounded electrode relative to the wire electrodes, so that the unipolar electrostatic charges can be deposited on the web. The apparatus according to this patent has the advantage that the coating solution can be easily applied on the web at the start of the coating and it is also possible to prevent the coating solution from being applied too thick. A corona treatment, however, offers several difficulties. One of the important difficulties thereof, associated with the manufacture of photosensitive materials is the occurrence of non-uniformity in photographic coating layer resulting from an uneven distribution of the electrical charge. This originates from non-uniform discharges. Since the photographic layer is a relatively thin layer of a few dozens of microns in thickness, the non-uniform coating severely affects the photographic characteristics as the density unevenness is recognised at the developed sample, such that the photographic products are markedly deteriorated in quality (“blur”). 
     The uneven distribution of the electrical charge can be improved by including a second grounded back-up electrode behind the wire electrodes in the web charging apparatus. It is also known to use a so-called plasma glow discharge process at atmospheric pressure conditions to improve the adhesion of various layers on a substrate. Examples of the use of this process are the treatment of photographic base papers or polymer film supports prior to applying the various layers containing the photographic sensitive emulsions. For example EP-A 0 467 639, U.S. Pat. No. 5,403,453 and EP-A 0 821 273 are directed to this type of process. All these processes have in common that it applies an AC power supply while a noble gas is present between the web and the electrodes. Only the U.S. Pat. No. 5,403,453 also mentions the use of atmospheric air. 
     The stability of the atmospheric pressure glow discharge with parallel electrode plates (see EP-A 0 467 639 and U.S. Pat. No. 5,403,453) can be realised with expensive gas atmospheres containing mainly helium gas. In case the gas impurities become too high or the power source is outside a specific frequency range the glow discharge may become unstable and a less preferred discharge is obtained like the filamentary discharge. Accordingly, the prior art processes are only functioning within relatively narrow sets of conditions. 
     EP-A 0 821 273 discloses a method to obtain a plasma glow discharge at atmospheric pressure conditions (mainly in helium gas) between a series of thick Corona electrodes and a grounded drum shaped electrode. The adhesion improvement of the lowest emulsion layer at a polymer film support which is treated with plasma was shown at relatively low linespeeds of 10 m/min. 
     SUMMARY OF THE INVENTION 
     The present invention is based on the unexpected phenomenon that a stable glow discharge may be created at atmospheric conditions, using a cheap gas, nitrogen or air, in case electrodes are used having a small diameter (such as between 60 and 1500 μm), more in particular wire electrodes and a specific frequency range for the AC power supply. Under these conditions it has been found that not only a stable glow discharge is obtained, but also that the web may be moved at speeds far above the speeds disclosed in the prior art, without any problems occurring with the subsequent coating methods. 
     The invention is accordingly directed to a process for treating a photographic support in the form of a web, said process comprising providing a first drum shaped electrically conductive electrode and at least one electrically conductive wire electrode facing said drum shaped electrode, establishing an AC voltage with a frequency range between 100 Hz to 300 kHz over said electrodes, moving the web at atmospheric pressure along said drum shaped electrode, thereby exposing it to atmospheric pressure glow discharge established between the said drum shaped and wire electrode. 
     Advantages over the prior art technology are inter alia: 
     a cheap gas medium of air or nitrogen (if preferred with some oxygen or other gases) can be applied at atmospheric conditions, thus dispensing with the requirement of using noble gases, which are expensive. This provides a significant cost advantage. 
     The breakdown of the electric discharge voltage will be reduced by the usage of thin wire electrodes instead of the parallel plate electrodes or the series of corona thick electrodes. 
     The capital investment is low. 
     The coatability performance of a subsequent coating process will be improved, such that higher line speeds may be realised without detriment to the homogeneity of the coating over the whole width of the web. It is to be noted that in European patent application 0 821 273 only web speeds of 10 m/min are disclosed for polymer film supports, whereas with the present invention much higher linespeeds can be realised. 
     The invention can be applied for surface treatment of photographic paper supports (like polyethylene-laminated paper, polyethylene terephthalate-laminated paper, polypropylene-series synthetic paper) and of polymeric film supports (like polyethylene terephthalate, polyethylene naphthalate, polycyclohexanedimethanol terephthalate, triacetyl cellulose, cellulose nitrate, polyamide film, polycarbonate film, polystyrene film etc.) just prior to photographic emulsion coating. 
     Preferably a second grounded electrically conductive back-up electrode is arranged behind said wire electrodes relative to said first grounded drum shaped electrode, resulting in a more even distribution of the electric plasma charge. 
     According to another preferred embodiment of the invention a dielectric coating may be present on the back-up electrode. This has the advantage that the stability of the glow discharge is improved. It is to be noted that both the stability and the even distribution of the glow discharge may have a positive effect on the coating performance at higher line speeds. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be elucidated on the basis of the attached FIGS. 1 and 2, wherein 
     FIG. 1 shows schematically a web treating apparatus for carrying out a preferred embodiment of the APGD process, and 
     FIG. 2 shows schematically a combination of a web treating apparatus and a web coating apparatus for carrying out a preferred embodiment of the APGD process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1 the web treating apparatus  10  for treating the photographic support by APGD comprises a first drum shaped electrode or a roller  16  that is grounded, discharge electrodes or wire electrodes  18 , which are parallel to the axis of the roller  16  and a second grounded back-up electrode or a grounded plate  24  is arranged behind the wire electrodes  18  relative to the roller  16 . The roller  16  serves both as a counter electrode and a supporting roller for a moving web  14 . 
     Atmospheric nitrogen, optionally mixed with oxygen or noble gases like helium, or air will be supplied from a duct  11  through the holes of the back-up electrode  24  and the gas flows further between the wire electrodes  18  to the web  14  preferably at a high gas flow rate. From the view point of the reactivity at the surface of the photographic supports nitrogen containing gas (nitrogen content of over 75 vol. %) mixed with some oxygen gas is preferable because these gases are more chemically reactive than the noble gases helium or argon. 
     The back-up electrode  24  is covered with a dielectric coating, which is beneficial for the stability of the APGD. Various dielectric coating materials can be applied to coat the back-up electrode  24  like for instance ceramics, such as Alumina Al 2 O 3 , Aluminium Silicate Al 2 SiO 5 , Forsterite Mg 2 SiO 4 , Corderite Mg 2 Al 4 Si 5 O 18 , Titanates: CaTiO 3 , SrTiO 3 , BaTiO 3 , and PbTiO 3 . These dielectric materials can be used in the form of a powder or porcelain. Other dielectrics are ZrO 2 , TiO 2 , SiO 2  that can be also used for this purpose. Preferably, Al 2 O 3  is used. Also mixtures of dielectric coating materials may be used. The layer of coating can be applied with flame spraying, though not limited by the deposition technique. A suitable thickness for the dielectric layer for the back-up electrode is between 0.3 mm and 5 mm. 
     Also the wire electrodes  18  can preferably be coated with the dielectric ceramic materials which are used also for the back-up electrode  24 . Basically a thin dielectric coating layer will improve the plasma stability. 
     A plurality of (e.g. four) wire electrodes  18  is arranged in parallel to each other at regular intervals on a circle concentric with the roller  16 , in other words, the wire electrodes  18  are arranged along the path of the web  14 . The wire electrodes  18  are made from a conductive material, e.g. tungsten, molybdenum, platinum, and carbon fibre, and preferably tungsten. It has a diameter of between 60 and 1500 μm. Although stable APGD-plasma can be obtained with wire electrodes having a diameter less than 100 μm, this is however not preferable because the wire material can not be used for long duration periods. 
     The APGD-process of the invention can be carried out with wire electrodes which can have different outer shapes (like a circular or a triangular or a rectangular shape). By proper selection of the outer shape and the thickness of the wire electrode as well as the APGD-process conditions the chances for the less preferred Corona discharges can be minimised and the stability of plasma positively influenced. The diameter of the wire electrode is determined as the width of the projection of the wire electrode on the web, in a direction normal to the web. This diameter corresponds accordingly to the width of the wire, that is directed to the web. 
     The web  14  moves over the roller  16  while being in contact with the outer surface of the roller  16 , which is grounded to function as the counter electrode. The distance between each wire electrode  18  and the web  14  supported on the roller  16  is between 1 and 6 mm. The wire electrodes  18  are connected to an AC power source  22  at a frequency range between 100 and 300000 Hz. APGD is established between the wire electrode  18  and the roller  16  via the web  14 . 
     The second grounded back-up electrode or a grounded plate  24  is arranged behind the wire electrodes  18  relative to the roller  16 . The grounded plate  24  can be like an arc, or a rectangle of which the width is substantially equal to the width of the roller  16 . The grounded plate  24  is made from a metal, e.g. aluminium, copper, iron, stainless steel, or a non-metallic conductor, and it may be covered with dielectric coating material, as discussed above. The back-up electrode has small holes where the gas flow is supplied to the web  14 . This ground plate  24  should not be necessarily a rectangular plate. It may be any shape that does not disturb the electrostatic field between the wire electrodes  18  and the web  14 . 
     A description will be given of the operation of the web charging apparatus  10 , which is constructed in the above-mentioned manner with reference to FIG. 2 showing the path of the web  14  in a coating system that includes the web charging apparatus  10 . 
     As shown in FIG. 2, the web  14  passes through the web charging apparatus  10 , and then it reaches a coating apparatus  26 . In the web charging apparatus  10 , the source  22  applies an AC voltage to wire electrodes  18  so that APGD is established between the wire electrodes  18  and the roller  16  via the web  14 . Then, the web  14  reaches the coating apparatus  26  via a pass roller  30 . A coating head  34  of the coating apparatus  26  applies a coating solution  36  to the surface of the web  14  supported on a back-up roller  32 . Thus, the web  14  is coated with the coating solution  36 . 
     During the coating operation, the web surface is treated in APGD, thereby improving the affinity and adhesion of the coating solution  36  to the web  14 . As a consequence the coating performance can be improved. 
     In the present process short exposure periods, for example between in 0.01 to 10 seconds suffice for getting a good result. In this respect it is to be noted that the prior art treatment times tend to vary between 40 sec. and 8 minutes. 
     WORKING EXAMPLES 
     The invention is now elucidated on the basis of the following examples, which are not intended to limit the invention. 
     Example 1 
     (1) Coating Test 
     Sample A (ESC Condition) 
     A description will be given of experiments with the coating system in FIG. 2 including the web charging apparatus  10 . 
     A web  14  was made of polyethylene-laminated paper (a white pigment (TiO 2 ) and a blue dye (ultramarine) were added to the polyethylene layer at the front side), 180 mm in width, and ordinarily used for a photographic color paper. The web  14  was transported at a certain speed. In the web charging apparatus  10 , four wires as the wire electrodes  18  were made of tungsten, 150 μm in diameter in which the wire has a circular shape and 200 mm in length. The wire electrodes  18  were arranged in parallel so that each distance to the web  14  was 1.5 mm. The pass roller  16  is connected to ground. The power supply  22  applied a direct voltage of 7000 V to the wire electrodes  18 , so that the corona discharge was established between the wire electrodes  18  and the web  14 . Thereby, the unipolar electrostatic charges were deposited on the surface of the web  14 . Then, the surface electrometer  28  measured the electrostatic potential on the charged surface of the web  14 . After the measurement, the coating apparatus  26  applied the coating solution  36  on the web  14 . 
     The discharge current in ESC is relatively low to prevent current breakdown, the low discharge current consequently causes homogeneous treatment of the support. The coating solution  36  is the same as those used in sample  201  in JA 09-146237 (JA means the unexamined published Japanese patent application.). By means of ESC, a charge (equivalent to 600 Volt) was deposited on the web. With the deposition of this charge the maximum coating speed can be raised until a normalised maximum speed v max normalized . Further increase of the coating speed with 1% results in air entrainment defects. The sample which was obtained at the coating speed v max normalized  is defined as Sample A. 
     Sample B (APGD Condition) 
     The AC-power supply is connected to the same circular Tungsten wire electrodes as for sample A and the pass roller is connected to ground. The atmospheric gas is supplied above the wire electrodes and flows preferably between the wire electrodes in the direction of the web. Atmospheric nitrogen was mixed with 3% oxygen and supplied to the web at a rate of 20 l/min. 
     With the APGD plasma process, similar coatability experiments were carried out. For the photographic application a frequency of 10 kHz was applied and the maximum coating speed was evaluated before air entrainment defects are formed. All the treatments were carried out in an atmospheric nitrogen/oxygen mixture (97% resp.3%) with an electrode distance of 1 mm(see Table 1). 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Experimental conditions of APGD 
               
             
          
           
               
                   
                   
                   
                   
                 Wire 
                   
               
               
                 Exp. 
                   
                 Frequency 
                 Gap 
                 diameter 
                 Air flow 
               
               
                 No. 
                 Atmosphere 
                 (kHz) 
                 (mm) 
                 (μm) 
                 (l/min) 
               
               
                   
               
               
                 1 
                 nitrogen + 3% 
                 10 
                 1 
                 150 
                 20 
               
               
                   
                 oxygen 
               
               
                   
               
               
                 Gap: the distance between electrodes 18 and the web 14.  
               
             
          
         
       
     
     After the glow discharge was established, the coating experiments were carried out. The maximum coating speed can be increased with 4%, because the air entrainment started at 5% higher speed. So, in the case of a polyethylene-laminated surface, a higher maximum coating speed of 1.04 *v max normalized  can be achieved with the APGD-process than with the ESC-process. 
     The sample which was obtained at the maximum coating speed of 1.04 *v max normalized  is defined as Sample B. The exposure time of the web to the glow discharge was about 0.2 s. 
     (2) The Blur Evaluation 
     Blur means color and/or density unevenness which is detectable on the developed sample, and the cause is supposed to be the unevenness of surface charging on the PE-laminated paper. Samples A and B were exposed to uniform light source so as to get a gray uniform density. After developing these exposed samples, we evaluated the blur (unevenness of gray density). As for Sample A, blur can be found to some extent, but as for Sample B, we could find less blur. Blur evaluation shows that the APGD treated sample (Sample B) has less blur than the ESC treated sample (Sample A). 
     The electrostatic potential of Sample A was 600 V. If we made a sample whose electrostatic potential is more than 600 V, the blur became worse. 
     (3) Adhesion Test 
     Dry Adhesion Test 
     Using Samples A and B, making cuts on only coated emulsion layer (not in laminated paper) with knife, putting adhesion tape on them, and after  7  hours this tape was removed. In case of Sample A, some part of emulsion layer was removed, but in case of Sample B, we could find less emulsion removal. The adhesion properties of the APGD treated surface are better than the ESC treated surface. 
     Wet Adhesion Test 
     Using Samples A and B, putting them into warm water, rubbing the surface with finger, we evaluated the adhesion strength of emulsion layer in wet condition. In case of Sample A, some part of emulsion layer was removed easily, but in case of Sample B, we could not find any emulsion removal. The adhesion properties of the APGD treated surface are much better than the ESC treated surface even in wet condition. 
     The disadvantage of the web which is obtained by ESC treatment is that although during coating the adhesion can be improved, the static charge does not improve wet and dry adhesion properties after the coating because the web loses its charge. 
     Example 2 
     Sample C (ESC Condition) 
     The polyethylene-laminated paper (a white pigment (TiO 2 ) and a blue dye (ultramarine) were added to the polyethylene of the first layer side) which contains a thin gelatine coated layer on it was used as the web  14  in the FIG.  1 . This paper was manufactured by the same way as sample  101  described in JA 09-171240. Others are the same as FIG.  1 . 
     Coatability of this paper support was investigated under the application of ESC. Under the application of 300 Volt on the web, the maximum normalised coating speed can be increased with 4%, further increase of the web voltage to 600 V can raise the maximum normalised coating speed with 9%. Air entrainment starts if coating speed is further raised, maintaining the same web voltage. Further increase of the web voltage over 600 V caused the increase of blur. 
     Sample D (APGD Condition) 
     In the next experiment, the web was exposed to an APGD-process of which the conditions are summarised in Table 1. Under this condition, a stable atmospheric glow discharge can be obtained and the maximum coating speed can be increased with 16%. The exposure time of the web to the glow discharge was about 0.15 s. A further increase of the coating speed results in air entrainment at this power level. Evaluation of blur of both ESC and APGD sample reveals that the APGD treated surface shows less blur than the ESC treated sample. 
     Due to the strong increase in surface energy of the gelatine, the APGD plasma is very suitable to improve coating speed without the negative effect that ESC has on the blur quality. 
     Example 3 
     TAC Coatability Evaluation of ESC Treatment and APGD Plasma Treatment 
     Sample E (ESC Treatment) 
     TAC (triacetyl cellulose) was used as the web  14  in FIG.  1 . The process conditions for this experiment with ESC-process was comparable as for sample A. A specific maximum normalized coating speed (v max normalized ) was determined at which no entrainment defects happen. 
     Sample F (APGD Plasma Treatment) 
     TAC (triacetyl cellulose) was used as the web  14  in FIG.  1 . The process conditions for this experiment with APGD-process was comparable as for sample B. A 10% higher maximum coating speed (=1.1 *v max normalized ) was obtained with the APGD treatment (sample F) than for the ESC-process (sample E) 
     Example 4 
     In FIG. 1 various APGD-conditions were selected and compared with the plasma glow stability. The condition and results are summarized in Table 2. When the wire diameter is 50 μm, the wire can easy break which is not preferred. the view points of the plasma stability and also wire durability, the most preferable condition is that the wire diameter varies between 150 and 1000 μm. Nitrogen gas with oxygen is preferred. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Plasma glow stability 
               
             
          
           
               
                   
                 Wire 
                   
                   
                   
                 Gas 
                   
                   
                   
               
               
                   
                 dia- 
                 Fre- 
                 Vol- 
                   
                 flow 
                 Gap 
                 glow 
               
               
                 Sam- 
                 meter 
                 quency 
                 tage 
                   
                 (l/ 
                 dis- 
                 sta- 
                 special 
               
               
                 ple 
                 (μm) 
                 (Hz) 
                 (kV)) 
                 Gas 
                 min) 
                 tance 
                 bility 
                 effects 
               
               
                   
               
             
          
           
               
                 ESC 
                 150 
                 DC 
                 7 
                 air 
                 no 
                 4 
                 good 
                   
               
               
                 1 
                 50 
                 500 
                 3 
                 Ar 
                 20 
                 2 
                 good 
                 too brittle 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 material 
               
               
                 2 
                 150 
                 5000 
                 3 
                 Ar 
                 20 
                 2 
                 good 
               
               
                 3 
                 1500 
                 5000 
                 3 
                 Ar 
                 20 
                 2 
                 good 
               
               
                 4 
                 2000 
                 5000 
                 3 
                 Ar 
                 20 
                 2 
                 poor- 
                 strong 
               
               
                   
                   
                   
                   
                   
                   
                   
                 er 
                 filamentary 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 discharges 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 at edges 
               
               
                 4 
                 150 
                 10000 
                 4 
                 N2 
                 20 
                 1 
                 good 
               
               
                 5 
                 150 
                 10000 
                 4 
                 N2 
                 20 
                 3 
                 poor 
               
               
                 6 
                 150 
                 10000 
                 5 
                 N2 
                 20 
                 5 
                 very 
                 filamentary 
               
               
                   
                   
                   
                   
                   
                   
                   
                 poor 
                 discharges 
               
               
                 7 
                 150 
                 5000 
                 3 
                 N2 + 
                 20 
                 1.5 
                 good 
               
               
                   
                   
                   
                   
                 3% 
               
               
                   
                   
                   
                   
                 O2