Coated base paper for photographic printing paper

Details Coated base paper for photographic printing paper Coated base paper for photographic printing paper

1999-04-14

2002-11-19

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation sensitive product

Identified backing or protective layer containing

C430S533000, C430S536000, C430S538000

Reexamination Certificate

active

06482581

ABSTRACT:

The present invention is directed to a base paper for photographic printing paper, comprising a base paper sheet and a metaloxide filled polymer resin coating. Further the invention is directed to a photographic printing paper at least consisting of such a base paper and a photographic emulsion on one side of the paper.
Base paper for photographic printing paper is conventionally prepared from a paper basis that is coated with a resin layer, usually a titanium oxide filled polyethylene, polypropylene or a polymethyl-methacrylate resin. An important aspect of the polymer resin coated base paper is the resin surface appearance, which should be smooth and not uneven and/or grainy. Further it is important that the resin surface does not have a large number of crater defects after melt-extrusion coating of resin on the base paper.
Further an important aspect of photographic printing paper is the speed of production and the thickness of the polyethylene layers. Both aspects are important in view of the economy of the process of producing the photographic printing paper. The formation of crater defects, or pits, has turned out to be strongly dependent on those aspects.
It has been attempted to increase the smoothness of the base paper by calendering the paper at high pressures between metallic rolls. A limitation of this method is that calendering will also reduce the thickness of the base paper and results in a decrease of whiteness and stiffness. Such a calender treatment is accordingly not really effective and creates other disadvantages. Also the calender treatment reduces the adhesion between the paper and the polymer resin, which has to be compensated by the use of adhesion promoting additives in the paper, or in an additional paper coating, or by further treatment of the paper.
It has also been attempted to suppress the crater defects by increasing the thickness of the polymer resin coating layer. At high extrusion speeds, such as over 300 m/min this is not sufficiently effective. Furthermore, it has economical disadvantages as the polymer resin is an important cost factor in the production of photographic printing paper.
In U.S. Pat. No. 4,994,357 it has been indicated, that increase of extrusion speeds above 100 m/min and more in particular above 150 m/min results in an increase of crater defects. According to this patent the use of paper having a low average surface roughness of R
A
<1.0 &mgr;m for the outer PE layer and a specific linear pressure of the nip rol in combination with a surface roughness of the base paper sheet, results in decreased crater formation. According to EP-A 285,146 it is possible to decrease crater defects at high extrusion linespeeds such as 200 m/min by injecting a high permeability gas, such as carbon dioxide at the air entrainment spot of the cooling roll.
At extrusion coating speeds of the polymer resin of 300 m/min or more it has been found to be much more difficult to reduce the number of crater defects. The above conventional methods did not lead to sufficiently high smoothness to be commercially acceptable.
In JP-B 06-048365 it has been disclosed to coat the base paper sheet followed by gloss calendering prior to the melt extrusion coating of titanium oxide filled polymer resin. According to this patent the gloss of the photographic paper is related to the gloss of the base paper. This latter value is improved by providing a pigmented surface coating at the base paper followed by a gloss supercalender treatment, prior to laminating with the titanium oxide filled polymer resin. The required high gloss value was achievable by providing more than 50 wt. % of kaolin in the pigmented surface coating.
In the art of providing photographic base paper there is a need for high speed production of the paper, especially during the (co-)extrusion of the paper with-the polymer resin, whereby the amount of defects in the paper (pits or crater defects) remains at a sufficiently low value.
It is an object of the present invention to provide a pigment coated base paper that is suitable for preparing photographic printing paper with superior surface properties, i.e. a minimum amount of crater defects. It is a further object of the present invention to provide such a paper, that can be produced at very high production speed during melt extrusion-coating, without sacrifice to the surface properties.
It is yet another object of the present invention to produce a base paper that can be produced using a low thickness of polymer resin, expressed as weight of resin per surface area, more in particular produced at high extrusion speed, whereby good surface properties are maintained.
It is also an object of the invention to provide a photographic paper having a good whiteness and a good stiffness in combination with a good thickness of the base paper sheet.
The present invention is based on the surprising insight, that the paper web, having a surface roughness below a certain maximum level, can be melt-extrusion coated with a low amount (thin layer) of polymer resin at high speed, without detrimental effects to the properties of the final surface.
The present invention is directed to a paper support for photographic printing paper having a topside and a backside, at least said topside being provided with a pigmented coating based on clay and/or other pigments, said coating having a clay content of less than 3.3 g/m
2
, which topside of the pigmented coating has an average surface roughness R
A
of 1.0 &mgr;m or less, and is provided with at least one pigmented polymer resin layer.
Surprisingly it has been found that it is possible to fine-tune the properties of a photographic base paper by the careful selection of the properties of the base sheet. More in particular the provision of the minimal surface roughness, in combination with a limited content of clay in the pigmented surface coating, provides the required combination of properties, such as the possibility to melt coat the paper with polymer resin at high speed, without an undue amount of crater defects, a good whiteness, a good stiffness in combination with a useful thickness of the base paper.
With the invention it has thus become possible to provide on the one hand a paper that can be produced at high speeds, without unacceptable crater defects. Further the invention provides a paper that has a good whiteness, a good stiffness and a useful thickness.
The invention is based thereon that the inventors have discovered, that the amount of crater defects occurring at high extrusion speeds, when pigments are applied in the coating with a limited amount of clay, is related to the average surface roughness of the coated base paper surface. More in particular the invention is based thereon, that when a coated base paper, having an average surface roughness of 1.0 &mgr;m or less, is used, it is possible to laminate the paper using very high (co-)extrusion speeds, without running into problems with crater defects in the surface of the paper. Keeping the said speeds at lower, but even at constant levels, the invention provides the possibility to reduce the amount of polymer resin, without having the crater defect problems.
The base paper to be used as the support for the photographic printing paper of the present invention is selected from materials conventionally used in photographic printing paper. Generally it is based on natural wood pulp and if desired, a filler such as talc, calcium carbonate, TiO
2
, BaSO
4
, and the like. Generally the paper also contains internal sizing agents, such as starch, alkyl ketene dimer, higher fatty acids, paraffin wax, alkenylsuccinic acid and the like. Further the paper may contain a reinforcing agent such as polyacrylamide or starch. Further additives in the paper can be fixing agents, such as aluminum sulfate, starch, cationic polymer and the like. In order to obtain especially good photographic printing paper usually short fibers are used in the natural pulp.
The base paper is prepared from the above components in a conventional way using known machinery. After laying the pap

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