EP0678784A1

EP0678784A1 - Method and device for the rejuvenating of a polyester film base and method of drying a processed photographic material

Info

Publication number: EP0678784A1
Application number: EP94202570A
Authority: EP
Inventors: Jean Burtin; August Marien
Original assignee: Agfa Gevaert NV
Current assignee: Agfa Gevaert NV
Priority date: 1994-01-24
Filing date: 1994-09-07
Publication date: 1995-10-25

Abstract

A device for rejuvenation a motion picture film, which comprises coating means for coating a rejuvenation layer of an aqueous film-forming polymer dispersion to the rear side of the film (12), and drying means (14) for drying such coated layer, which drying means comprises a wave pipe (16) and a source of microwave energy (32) coupled at one end thereto, film base inlet (23) and outlet (24) ports in said wave pipe and film conveying rollers (17) between said openings, air inlet and outlet (31) openings axially spaced along said wave pipe, blower means (29) for producing a current of forced air through said inlet openings, and heater means for heating said forced air.

Description

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates to a method and a device for the rejuvenation of a polyester film base, in particular for use in the motion picture field.

Description of the prior art

Motion picture film on a polyester base has a number of advantages over conventional cellulose triacetate base, such as superior mechanical strength, toughness, tear-resistance and lower brittleness, which will eliminate film breaks, extend perforation life and reduce scratching. Better dimensional stability and flexibility ensure a smoother run through projection systems. The result is improved screen quality throughout the extended lifetime of a print in the cinema circuit.
Although polyester film base is more resistant to abrasions than collulose triacetate base, the risk for scratching still exists.
Contrary to cellulose triacetate base scratches on polyester materials cannot be treated by the traditional system using organic solvents such as acetone or a mixture of acetone and methylene chloride for polishing an acetate film base. A suitable composition, for the rejuvenation of a polyester base has been disclosed in EP-B1-0 078559 assigned to the same assignee as that of the present invention and in the "Proceedings of the International Symposium : The Stability and Conservation of Photographic Images : Chemical, Electronic and Mechanical" at the Siam Inter-continental Hotel, Bangkok, Thailand, from 3-5 Nov. 1986.
The disclosed rejuvenation composition is an aqueous film-forming polymer dispersion. We have found that this composition requires a specific drying technology, which is not always easy to achieve.
As a matter of fact, drying means the evacuation of moisture from the freshly coated layer, and as moisture is removed, the distance between polymer particles becomes smaller. During this "approaching phase" of the particles their temperature must become higher than their minimum film forming temperature (usually from 30 to 60 °C) so that at the moment of contact coalescence occurs. At that moment the formation of the rejuvenation layer is started, and it will be understood that the temperature had to be maintained for some time at the film in order to obtain a uniform smooth layer.
However, in usual drying conditions the surface temperature of the wet layer is limited to the wet bulb temperature which, in common circumstances, is too low (20 to 30 °C) to get film forming. Furthermore, the surface can also dry out and form a skin of the dried coating which severely retards drying, thus producing a falling-rate type drying-rate curve.
It is possible to increase the wet bulb temperature by increasing the air pressure in the drying space, which is difficult to realise, or to increase the relative humidity in this space, which has the disadvantage that it will slow down the drying process.
On the other hand the temperature of the polyethylene terephthalate support must be kept below 65°C in order to prevent curling of the film.
It is also known to use microwave energy for drying wet layers. The advantage thereof is that the energy is absorbed in the mass of the wet layer so that drying proceeds much faster than via the exposed surface of the layer. However, as drying proceeds, the amount of absorbed microwave energy diminishes so that it becomes difficult to maintain a good control of the layer temperature for obtaining satisfactory coalescence.
Unsatisfactory coalescence produces a rejuvenation layer having an uneven or reticulated surface.

SUMMARY OF THE INVENTION

Object of the invention

It is the object of the present invention to provide a method and a device for rejuvenating polyester film base by coating a rejuvenation layer of an aqueous film-forming polymer dispersion to the rear side of the film and next drying said layer, which allows improved control of the drying of said layer to obtain coalescence of the polymer particles under optimal conditions.

Statement of the invention

In accordance with the present invention, a method for rejuvenating polyester film base by coating a rejuvenation layer from an aqueous film-forming polymer dispersion to the rear side of the film and next drying said layer, is characterised thereby that drying of said layer occurs by heating the film with heated air and microwave energy simultaneously.
Both energies complement each other in an almost ideal way and have for effect that a rejuvenation layer can be desired in a short time, while yet offering optimal optical characteristics.
Suitable embodiments of the method according to the invention are as follows.
Heating the air occurs by means of IR radiation. This may offer the additional advantage that a portion of the IR energy directly impinges on the film, whereby in fact three distinct forms of energy are effective to dry the rejuvenation layer.
The film is twisted at least once about its longitudinal axis during its exposure to microwave energy. It has been shown that this may conduct to a notable reduction of the drying time or, conversely, allows higher drying speeds in a given installation.
The film is wet-cleaned prior to the application of the rejuvenation layer, and wet-cleaned and wet-rejuvenated film portions are dried simultaneously, whereby a considerable gain in space for carrying out both drying operations may be obtained.
According to a suitable embodiment of the method according to the invention the polymer of said film-forming polymer dispersion is transparent and has a good scratch resistance. The polymer preferably has a refractive index between 1.48 and 1.70 and a glass transition temperature of at least 50°C. Although not limited thereto, the rejuvenation polymer is preferably a copolyester containing recurring ester groups derived from alkylene diols and a mixture of aromatic dicarboxylic acids and an aromatic sulphodicarboxylic acid, whose sulpho group is in salt form.
The copolyester may be slightly crosslinked as mentioned in EP-B1-0078559 and may also comprise non-reacted functional groups like hydroxy- and carboxylic acid groups allowing the copolyester to be cured during or after the rejuvenation step with polyepoxydes, polyisocyanates, polyalkoxysilanes etc. After such curing further improved scratch resistance is obtained.
Examples of aromatic dicarboxylic acids are terephthalic acid, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acids, 4,4'-diphenylenedicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-diphenylmethanedicarboxylic acid and 4,4'-diphenylsulphonedicarboxylic acid.
Examples of alkylene diols are ethylene glycol, diethylene glycol, 1,3-propane diol, 1,4-butane diol, neopentyl glycol and 1,4-cyclohexane dimethanol.
5-Sulpho-isophthalic acid, whose sulpho group is in form of sodium-, potassium- or lithium salt, is the most preferred sulphonated aromatic carboxylic acid to be incorporated in the copolyester chain, preferentially in amounts from 6 to 15 mol% of the total amount of acid.
Free hydroxyl- and carboxyl acid groups may be introduced in the polyester chain by adding glycerine or trimellitic anhydride to the monomer mixture before the polycondensation step. The preferred amount of glycerine or trimellitic anhydride lies between 1 and 10 mol% of the total amount of diols or carboxylic acids.
Other suitable polymers used in dispersed form (latex form) in the rejuvenation coating composition according to the present invention are homopolymers or copolymers made of polymerizable monomers selected from the group consisting of styrene, acrylamide, vinylidene chloride, vinylchloride and alkylmethacrylates preferably methylmethacrylate. Further are mentioned polyurethanes, polyamides, polysulfones and polyethers including mixtures thereof.
The present invention also includes a novel device for the rejuvenation of a film base.
In accordance with the invention, a device for rejuvenating polyester film base, which comprises coating means for coating a rejuvenation layer of an aqueous film-forming polymer dispersion to the rear side of the film, and drying means for drying such coated layer, is characterised thereby that said drying means comprises a wave pipe and a source of microwave energy coupled at one end thereto, a film base inlet and outlet port in a lateral wall of said pipe and film conveying rollers between said ports for the conveyance of said film base through the wave pipe in a direction parallel to the axial direction of the pipe, air inlet and outlet openings axially spaced along said wave pipe, blower means for producing a current of forced air through said air inlet openings, and heater means for heating said forced air.
According to a suitable embodiment of the invention, said heaters are formed by IR rodlike radiators extending parallel to the longitudinal direction of the wave pipe, and said wave pipe has perforated wall sections adjacent to said IR-radiators and forming said air inlet openings.
According to a further suitable embodiment of the device according to the invention, the device comprises a microwave absorber coupled to the other end of said wave pipe-and provided with a sensor for controlling the power delivered to the source of microwave.
Still further, the device comprises a wet cleaning station for cleaning the film prior to its rejuvenation, and the drying means is used for drying of the cleaned film prior to its rejuvenation as well as for drying the freshly coated rejuvenation layer.
The present invention has been developed in particular for improved screen image quality of 16-mm, 35-mm and 70-mm motion picture print film for the release market. However, it is clear that the method and device according to the present invention may be used also for improvement of other motion picture films, e.g. motion picture negative material that is not wet printed in the production of release copies. Finally, it should be understood that the invention is not limited to the rejuvenation of motion picture film, since in principle the invention can be used for drying any wet layer on a wet processed photographic film.
Therefore, the present invention also encompasses a method for drying a wet processed photographic film, which comprises heating said wet film by means of heated air and microwave energy simultaneously. The heating of said air can occur by means of IR radiation, and such IR radiation can be used to heat the film additionally by direct radiation.
Said method cna be used for drying motion picture film that does not require a rejuvenation layer, but that only needs a cleaning turn prior to its next release, for drying conventionally processed photographic film, e.g. cinefilm or microfilm, that has been developed, fixed, rinsed and next must be dried, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described hereinafter by way of example with reference to the accompanying drawings, wherein :

Fig. 1 is a diagrammatic illustration of one embodiment of a device according to the present invention,
Fig. 2 is an isometric view of the drying station of the device of Fig. 1,
Fig. 3 is a transverse section on line 3-3 of Fig. 2,
Fig. 4 is an enlarged detail 4 of Fig. 2, and
Fig. 5 is a diagrammatic illustration of a second embodiment of a device according to the present invention.

Detailed description of the invention

Fig. 1 shows a diagrammatic representation of one embodiment of a device according to the invention. A film 12 on a polyester base to be rejuvenated is unwound from a roll 10 and conveyed along a path, shown in broken lines, through a coating station 13, a drying station 14 and finally wound onto a roll 15.
Fig. 2 shows drying station 14 in detail. Film 12 is guided through a rectilinear wave pipe 16 and supported therein by a plurality of idler rollers 17 (see also Fig. 3). A cover 22 extends over the length of one side wall of the wave pipe and allows access to the interior of the pipe for threading the film over the rollers 17. A microwave generator 32, in the present case a magnetron, is coupled through adapter 18 and curved wave guide section 19 with the inlet of wave pipe 16 whereas the outlet of said pipe is coupled via curved section 20 with microwave absorber 21. The curved sections have rectangular film entry, resp. outlet ports 23 and 24 in a lateral wall of them. The cross section and the length of said ports are such that no loss of microwave energy occurs.
Top wall 25 of wave pipe 16 is provided with a great plurality of perforations 26 that are disposed in staggered rows. A hood 27 interconnects the outlet 28 of a tangential blower 29 with wave pipe 16. Inside of hood 27 is a rodlike IR-heater 30 running parallel to the longitudinal axis of pipe 16. Heater 30 may be formed by one single element but may as well be assembled from several axially aligned small elements. The same is true for the configuration of the blower. Wave pipe 16 has a plurality of air outlets 31 in one or both lateral walls.
The device described so far also has a water cooler, not illustrated, incorporated in generator 32, and a water cooler 34 fitted to absorber 21.
Each water cooler makes part of a cooling circuit comprising circulation pumps 35, 36 and heat exchangers 37, 38 respectively. Both heat exchangers are located in front of the inlet opening 39 of blower 29 and pre-heat the air that is blown over IR heaters 30.
The installation finally comprises a power sensor 40 measuring the absorbed power in 21 and controlling via controller 41 the electric power supplied to magnetron 32.
The operation of the described installation is as follows.
The coating installation applies a thin layer of a rejuvenation dispersion to the rear side of the film which is being unwound from roll 10. We have found that a suitable coater for the application of this layer is a so-called slot coater. Such coater has a coating head with an outlet orifice ending in two lips that are closely spaced from the surface to be coated. In operation, the coating composition completely wets the outer surface of at least one lip, and the distance between this lip and the film surface determines the wet coating thickness of the rejuvenation layer. The coated film is introduced through port 23 in drier station 14 and moves in a direction parallel to the axial direction of the wave pipe. Magnetron 32 emits microwave energy in wave pipe 16, whereas blower 29 blows air onto the upper side of the film which bears the rejuvenation layer. The air has been pre-heated by heat exchangers 37 and 38, and is then further heated by IR heater 30. This IR heater also heats the film to some extent by direct radiation through bores 26, as illustrated by the hatched cone 40 in Fig. 3. Drying of the rejuvenation layer by the three forms of drying energy described proceeds rapidly and produces a rejuvenation layer with a uniform thickness, showing a smooth and tight surface structure.
The described drive is self-regulating since the electric power supplied to the magnetron is a function of the power absorbed by absorber 21.

The following example illustrates the drier described hereinbefore, pre-heaters 37 and 38 being not operative.

Wave pipe 16 :	length	3500 mm
	width (internal)	40-55 mm
	height (internal)	80-110 mm
Magnetron 32 :	type NL10254
	nominal power	1500 Watt
	frequency	2455 Mhz
Absorber 21 :	adjusted to absorb	250 Watt
IR heaters :		1000 nm
IR heaters :	power	10-20 Watt/cm length
Blower 29 :	rate	7.5dm³.cm⁻¹.min⁻¹
Total surface of openings 26 per cm length of pipe 16 :		40%

The installation was used for drying a rejuvenation layer on 35-mm motion picture colour film.
Rejuvenation composition was coated at a rate of 25 cc.min⁻¹, and resulted in a rejuvenation layer with a dry thickness of 4 µm The film speed through coater and drier amounted to 30 m.min⁻¹.
Very good results were obtained with the following most preferred rejuvenation composition in a concentration of 12 percent by volume in water : a copolyester containing recurring ester groups derived from ethylene glycol and a mixture containing terephthalic acid or 2,6-naphthalene dicarboxylic acid, isophthalic acid and 5-sulpho-isophthalic acid whose sulpho group is in salt form (sodium-, potassium- or lithium salt) wherein said acid mixture consists essentially from 20 to 60 mol% of isophthalic acid, from 6 to 15 mol% of 5-sulpho-isophthalic acid, from 0.05 to 1 mol% of an aromatic polycarboxylic acid compound with at least three carboxylic acid groups, said carboxylic acid not being capable of forming intramolecularly an anhydride and including at least two carboxylated aromatic nuclei and the remainder in said acid mixture being terephthalic acid or 2,6-naphthalene dicarboxylic acid or a mixture thereof.
We refer to EP-B1-0078559 mentioned already, wherein more details can be found about aqueous copolyester dispersions suited for polyester film.
A second embodiment of the invention is as follows with reference to Fig. 5.
A film 12 to be rejuvenated is unwound from a roll 10 and first passed through a cleaning station 42. This station may comprise a lick roller or other coating system for the application of an aqueous cleaning liquid to the film. Alternatively, the film may be dipped into a body of cleaning liquid which is put in motion by means of an ultrasonic transducer. Cleaning of a film prior to its rejuvenation is useful, since it offers an improved adherence of the rejuvenation layer to the film, and at the same time it removes all particles and the like that otherwise would become overcoated by the rejuvenation layer and/or that could jeopardize the uniform application of the rejuvenation layer by getting captured in the coating gap of the coater.
The film is conveyed along a path 43 through wave pipe 14 and passed in a dry condition through coating station 13 for the application of a rejuvenation layer. This layer is dried during a second passage of the film through the wave pipe according to path 45 which is opposed to the first one. The rejuvenated film is wound onto roll 15.
The invention is not limited to the embodiments described hereinbefore.
The heating of the air for drying the rejuvenation layer may also occur by long-wave IR heaters rather than by the medium-wave rods 31 described hereinbefore.
The heaters may also be disposed such that they only heat the drying air but do not radiate the film.
The drying wave pipe may be folded and comprise several stretches that run parallel to each other and are interconnected by sections curved through 180 degrees. Suchlike construction offers an even more compact apparatus that may be useful for drying film at high speeds, requiring a corresponding drying length.
The film can be twisted about its longitudinal axis in the drier as mentioned already in the introduction of the specification. To that end, the position of the conveying roller may be changed accordingly, or the wave pipe, or pipes as the case may be, can be arranged vertically whereby control of the film path may become easier.
Finally, if scratches and/or other surface defects of a film are such that one rejuvenation overcoat, even in a large layer thickness, does not allow to obtain a satisfactory result, a second rejuvenation layer can be applied over the first one. Such second overcoat can occur in line with the first one, e.g. by locating one coating station at the place of the cleaning station in the Fig. 5 embodiment described hereinbefore.
Photographic processing of a motion picture as such does not belong to the subject of the present invention. Such wet processing, however, may exhibit some advantages, in particular a reduction of the time of processing, if carried out in the presence of ultrasonic energy.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic illustration of one embodiment of a device according to the present invention,
Fig. 2 is an isometric view of the drying station of the device of Fig. 1,
Fig. 3 is a transverse section on line 3-3 of Fig. 2,
Fig. 4 is en enlarged detail 4 of Fig. 2, and
Fig. 5 is a diagrammatic illustration of a second embodiment of a device according to the present invention,

10: is a roll,
12: is a film,
13: is a coating station,
14: is a drying station,
15: is a roll,
16: is a wave pipe,
17: are idler rollers,
18: is a magnetron to wave guide adapter,
19 and 20: are curved wave guide sections,
21: is an absorber,
22: is a cover,
23 and 24: are ports,
25: is a top wall,
26: are perforations,
27: is a hood,
28: is an outlet,
29: is a blower,
30: is an IR heater,
31: is an air outlet,
32: is a magnetron,
34: is a water cooler,
35 and 36: are pumps,
37 and 38: are heat exchangers,
39: is an inlet opening,
40: is a power sensor,
41: is a controller,
42: is a cleaning station, and
43 and 45: are film paths.

Claims

Method for rejuvenating polyester film base by coating a rejuvenation layer from an aqueous film-forming polymer dispersion to the rear side of the film and next drying said layer, characterised in that the drying of said layer occurs by heating the film by heated air and microwave energy simultaneously.
Method according to claim 1, wherein the heating of said air occurs by means of IR radiation.
Method according to claim 2, comprising using said IR radiation to heat the film by direct radiation.
Method according to any of claims 1 to 3, wherein the wet bulb temperature of the film is kept above the minimum film forming temperature of the said polymer during the drying of the rejuvenating layer.
Method according to any of claims 1 to 4, comprising wet cleaning said film prior to the application of the rejuvenation layer, and drying wet cleaned and wet rejuvenated film portions simultaneously.
Method according to any of claims 1 to 5, comprising conveying said film through a wave pipe, and subjecting the film to heated air which is blown normally onto the film through appropriate openings in the wave pipe.
Method according to claim 6, wherein said wave pipe is coupled at one of its two ends to a microwave generator, and at its other end to a microwave absorber.
Method according to claim 7, comprising using microwave energy absorbed by said microwave absorber to control the electric power applied to the microwave generator.
Method according to claim 7, comprising using the heat dissipation of the heat absorber and/or of the microwave generator to additionally heat the air for drying the film.
Method according to any of claims 1 to 9, wherein the polymer of said film-forming polymer dispersion is transparent, has a refractive index between 1.48 and 1.70 and has a glass transition temperature of at least 50°C.
Method according to any of claims 1 to 10, wherein the polymer of said film forming polymer dispersion essentially consists of a copolyester containing recurring ester groups derived from ethylene glycol and an acid mixture containing terephthalic acid or 2,6-naphthalene dicarboxylic acid, isophthalic acid and 5-sulpho-isophthalic acid whose sulpho group is in salt form.
Method for drying a wet processed photographic film, which comprises heating said film by means of heated air and microwave energy simultaneously.
Method according to claim 12, which comprises heating said air by means of IR radiation.
Method according to claim 13, which comprises using said IR radiation to heat the film additionally by direct radiation.
A device for rejuvenating a motion picture film on a polyester film base, which comprises coating means (13) for coating a rejuvenation layer of an aqueous film-forming polymer dispersion to the rear side of the film (12), and drying means for drying such coating layer, characterised in that said drying means comprises a wave pipe (14) and a source of microwave energy (18) coupled at one end thereto, a film base inlet (23) and outlet (24) opening in a lateral wall of said wave-pipe and film-conveying rollers (17) between said openings for the conveyance of said film base through the wave pipe in a direction parallel to the axial direction of the pipe, air inlet (26) and outlet (31) openings axially spaced along said wave pipe, blower means (29) for producing a current of forced air through said air inlet openings, and heater means for heating said forced air.
A device according to claim 15, wherein said heater means are formed by IR rodlike radiators (30) extending parallel to the longitudinal direction of the wave pipe.
A device according to claim 16, wherein said wave pipe has perforated wall sections (26) adjacent to said IR radiators and forming said air inlet openings.
A device according to any of claims 15 to 17, which comprises a microwave absorber (21) coupled to the other end of said wave pipe and provided with a sensor for controlling the power delivered to the source of microwave.
A device according to any of claims 15 to 18, wherein the film is twisted along its longitudinal direction during its passage through the wave pipe.
A device according to any of claims 15 to 19, comprising a cleaning station (40) for cleaning the film prior to its rejuvenation.
A device according to claim 20, wherein said cleaning station is a wet cleaning station, and wherein the wave pipe (14) is used for drying the cleaned film prior to its rejuvenation, as well as for drying the freshly coated rejuvenation layer.
A device according to claim 20, wherein a wet, cleaned film section and a dried, cleaned film section bearing a freshly coated rejuvenation layer are conveyed in opposite directions through the wave pipe.