Imaging bases, particularly photographic imaging bases, need to be able to resist liquid penetration in order to prevent print drying and print transport problems during photofinishing as well as subsequent print quality problems such as yellow edges. The use of `sizing` to induce some degree of liquid hold-out in paper is well known in the art. Sizing chemicals are typically added to papermaking fibers prior to the formation of the sheet (internal sizing) or added to the surface of a sheet that has been, at least partially, dried, (surface sizing). In the past, "sized" cellulosic paper bases alone have not been able to provide adequate liquid hold-out characteristics, limiting their application or necessitating additional processing steps. For example, fiber base sheets without special waterproofing layers, such as Black and White professional fiber base papers, are used in some photographic applications but require additional print drying times, making these papers non-functional in high speed commercial photofinishing equipment. Wetting of the cellulose fibers in the fiber sheet also results in weaker tensile strengths limiting the amount of tensile force that can be applied to transport the paper through the photofinishing equipment. Liquid holdout is also important for inkjet papers since penetration of ink into traditional cellulose papers causes a distortion of the paper surface known as cockle. In addition to the cockle manifesting as an unsightly surface, the non-planar distortion can also cause headstrikes in wide format ink jet printers thus resulting in density streaks.
In the prior art, adequate liquid hold-out for imaging base applications is typically provided through the use of polymeric protective layers. An imaging base such as a photographic paper typically comprises a cellulosic paper base that is extrusion-coated on both sides with a polyolefin resin liquid hold-out layer. An alternative imaging base proposed in U.S. Pat. No. 5,866,282 (Bourdelais et al), is made of cellulosic paper base laminated with biaxially oriented polyolefin sheets which provide the necessary liquid hold-out. The use of polymeric protective layers as described above is, however, perceived as a disadvantage by consumers. In voice of customer surveys, fiber base papers exhibit advantages over resin coated papers in that the feel of the fiber base paper is preferred over "plastic" coated papers, which many consumers and professional photographers perceive as "imitation paper".
Salsman U.S. Pat. No. 5,958,601 describes stearic acid esters. Salsman U.S. Pat. No. 5,281,630 describes water-dispersible polyester polymers.
Support coatings hardened with electron beams are disclosed in U.S Pat. No. 4,645,736 (Anthonsen et al) using a radiation hardened varnish and barrier layer. The barrier layers in the latter case, both extrusion and aqueous coated, are aimed at providing a fog barrier rather than a liquid holdout layer with the subsequent coatings that are treated with electron beams providing the waterproofness. The need for multiple operations to coat both sides of the paper make this method less than optimum since water must be removed from the back side of the paper during the hardening process on the side eventually receiving the light sensitive layer.
As typical for many high-value imaging bases, there are other properties that are also important. The underlying support should be stiff, to resist bending forces, yet smooth. A base paper with high tensile modulus and stiffness resists undesirable deformation and curl that result when exposed to certain handling and environmental conditions. For example, many photographic papers exhibit increased curl on being exposed to low humidity conditions. Stiffer papers, that lie flat within the same plane regardless of handling and environmental conditions, are perceived by the end-user to be of higher value due to a more substantial `feel`.
Surface appearance is also important to the consumer. Surfaces with "orange peel" or low frequency roughness are visually dissatisfying while high frequency roughness is known to relate to poor final product gloss. Papers with lower surface roughness have been shown to enhance the gloss of a photographic print as measured using specular reflection measurement techniques. In order to achieve the properties listed above, imaging paper bases typically utilize bleached hardwood chemical kraft pulp as it provides acceptable base surface and good formation while maintaining strength and stiffness. To further smooth the paper base surface, it is common to machine calender the paper and then coat it with mineral pigments such as baryta, kaolin clay, calcium carbonate etc., or extrusion coat with polyolefins, or laminate with biaxially oriented films.