Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
1999-10-12
2001-08-07
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S165000
Reexamination Certificate
active
06270632
ABSTRACT:
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 &mgr;m), 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.
REFERENCES:
patent: 4940521 (1990-07-01), Dinter et al.
patent: 5138971 (1992-08-01), Nakajima et al.
patent: 5403453 (1995-04-01), Roth et al.
patent: 5558843 (1996-09-01), Glocker et al.
patent: 5888713 (1999-03-01), Arrington et al.
patent: 0 467 639 (1992-01-01), None
patent: 0 821 273 (1998-01-01), None
patent: 9-146237 (1997-06-01), None
patent: 9-171240 (1997-06-01), None
Fuji Photo Film B.V.
Mayekar Kishor
Merchant & Gould P.C.
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