Abstract:
A photographic printing paper support wherein one side of the raw paper is covered by a composition containing titanium oxide dispersed in polyethylene terephthalate resin or a mixture of polyethylene terephthalate and another resin, and the other side is covered by a composition containing calcium carbonate dispersed in polyethylene terephthalate resin or a mixture of polyethylene terephthalate with another resin, which can provide an excellent photographic paper having good water resistance, whightness and gloss is disclosed.

Description:
FIELD OF THE INVENTION 
     This invention relates to a photographic printing paper support, and in particular to a photographic printing paper support which does not tend to curl with time. 
     BACKGROUND OF THE INVENTION 
     In photographic printing papers, the surface of the raw paper to be emulsion-coated is generally covered by a polyethylene or other polyolefin resin layer in which titanium oxide has been dispersed by kneading so as to improve water resistance and light reflectance. The titanium oxide in particular improves the whiteness of the paper, but it also tends to adversely affect the flatness of the paper. 
     In high gloss printing papers which are at present most highly valued, it is desired to improve the smoothness and gloss of the paper surface in order to improve the appearance of the image. Good results can be obtained by replacing the polyolefin by polyethylene terephthalate, but the paper suffered from the disadvantage that it curled with the passage of time. 
     The Inventors, after detailed studies, found that this tendency to curl is reduced by covering the surface of the raw paper to be emulsion-coated with a composition comprising a dispersion of titanium oxide in polyethylene terephthalate or in a resin having polyethylene terephthalate as its principal constituent. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a photographic printing paper support having excellent water resistance, whiteness and surface smoothness which does not tend to curl with time. 
     The aforesaid object of the invention is attained by a photographic printing paper support wherein one side of the raw paper is covered by a composition containing titanium oxide dispersed in polyethylene terephthalate resin or a mixture of polyethylene terephthalate and another resin, and the other side is covered by a composition containing calcium carbonate dispersed in polyethylene terephthalate resin or a mixture of polyethylene terephthalate with another resin. 
     A photographic printing paper having excellent water resistance, whiteness and gloss may be obtained by applying, using any of the known methods, a layer of photographic emulsion to the layer of composition containing titanium oxide on a support of the present inventions 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The other resin added to polyethylene terephthalate (referred to as the blend resin) may be suitably chosen from among resins which can be extruded at 270°-350° C. e.g. polyolefin resins such as polyethylene or polypropylene, polyether resins such as polyethylene glycol, polyoxymethylene and polyoxypropylene, urethane resins such as polyester polyurethanes and polyether polyurethanes, and polycarbonates or polystyrenes. 
     Two or more of the aforesaid blend resins may also be used in conjunction. 
     The blending ratio of the polyethylene terephthalate and the blend resin is suitably determined according to the polymerization degree and type of blend resin, but in the case of polyolefin resins, this ratio in terms of weight preferably lies in the range 100:0-80:20. If the blending ratio of the blend resin is greater than 20 weight %, the toughness of the photographic printing paper support decreases, and its surface smoothness is unsatisfactory. 
     If a resin other than a polyolefin resin is used, the aforesaid blending ratio may lie in the range 100:0-40:60. If the blending ratio of the blend resin exceeds 60 weight %, the curl of the support with time cannot be adequately prevented. 
     The titanium oxide used in this invention may be in the anatase or rutile form, and its average particle size should lie in the range 0.1-0.8,μm. If this particle size is less than 0.1 μm, it is difficult to disperse it evenly throughout the resin layer. If on the other hand it is greater than 0.8 μm, flatness of the photographic printing paper support surface cannot be obtained, and the whiteness is unsatisfactory. It is further desirable that the amount of titanium oxide lies in the range 2-20 weight %. 
     It is preferable that the average particle size of calcium carbonate used in this invention lies in the range 0.01-1.5 μm. If this particle size is less than 0.01 μm, dispersion is difficult as in the case of the aforesaid titanium oxide, while if it is greater than 1.51 μm, flatness of the photographic printing paper support surface cannot be obtained. It is further desirable that the amount of calcium carbonate lies in the range 1-30 weight %. 
     The titanium oxide or calcium carbonate may be easily mixed with the aforesaid polyethylene terephthalate by any of the usual methods. 
     In order to manufacture the photographic printing paper support of this invention, one surface of the raw paper is covered by a composition comprising polyethylene terephthalate resin or polyethylene terephthalate resin mixed with a blend resin and titanium oxide so as to form the surface to be emulsion-coated, and the other surface of the raw paper is coated with a composition comprising polyethylene terephthalate resin or polyethylene terephthalate resin mixed with a blend resin and calcium carbonate. 
     It is preferable that the thickness of the resin layer covering each side of the raw paper lies in the range 5-50 μm and particularly preferable that it lies in the range 15-35 μm. If the thickness is greater than 50 μm, the covering layer easily breaks, while if it is less than 5 μm, the water resistance of the photographic printing paper support is insufficient and its toughness declines. Further, from the viewpoint of preventing curl, it is preferable that the thickness of the covering on each surface is approximately the same. 
     In order to apply the aforesaid composition to the raw paper, the raw paper surface may first be prepared by corona discharge treatment or by priming with an undercoat. 
     In the support of this invention, the raw paper surface is covered by a resin containing polyethylene terephthalate as its principal constituent which is known to give better surface smoothness than polyolefin. The printing paper therefore, while retaining the same water resistance and whiteness of conventional papers, has improved surface smoothness and excellent gloss. Further, one surface of the raw paper is covered by a composition containing titanium oxide mixed with a resin having polyethylene terephthalate as its principal constituent, and the other surface is covered by a composition containing calcium carbonate mixed with a resin having polyethylene terephthalate as its principal constituent. The paper therefore does not curl with time. 
     EXAMPLES 
     This invention will now be described in more detail by specific examples, but it should be understood that the invention is by no means limited to these examples. 
     EXAMPLES 1-4 and COMPARATIVE EXAMPLE 1 
     A composition containing titanium oxide shown in Table 1 was applied to one surface of a raw paper of thickness 180 μm, and a composition containing calcium carbonate shown in Table 2 was applied to the other side of the paper. Both compositions were hot extruded at 300° C. so as to form laminated layers of thickness 25-30 μm. The curl of the support obtained was evaluated, and the results are shown in Table 2. 
     
                                           TABLE 1__________________________________________________________________________  Composition covering one surface of raw paper          Weight    Weight % of                              Laminated          ratio of               Type of                    titanium oxide                              film          resin 1/               titanium                    with respect to                              thickness,Sample Resin 1      Resin 2          resin 2               oxide                    whole composition                              μm__________________________________________________________________________Example 1  R1  --  --   T1   10        25Example 2  R1  R2  90/10               T1   10        25Example 3  R1  --  --   T1   10        30Example 4  R1  RM  90/10               T1    8        30Comparative  R2  --  --   T1   10        30Example 1__________________________________________________________________________ The symbols in the Table have the following significance: R1: Polyethylene terephthalate R2: Polypropylene R3: Polyethylene RM: Composition having a weight ratio of R2 and R3 = 1:1 T1: Titanium oxide of average particle diameter 0.3 μm 
    
     
                                           TABLE 2__________________________________________________________________________  Composition covering other surface of raw paper          Weight    Weight % of calcium                               Laminated                                     Standard          ratio of  carbonate with                               film  deviation of          resin 3/               Type of                    respect to whole                               thickness,                                     height aboveSample Resin 3      Resin 4          resin 4               powder                    composition                               μm platform, mm__________________________________________________________________________Example 1  R1  R2  98/2 P1   7          25    0.2Example 2  R1  --  --   P1   7          30    0.2Example 3  R1  --  --   P1   7          30    0.2Example 4  R1  R2  98/2 P1   20         30    0.1Comparative  R3  --  --   --   0          30    0.7Example 1__________________________________________________________________________ The symbols in the Table have the following significance: R1: Polyethylene terephthalate R2: Polypropylene R3: Polyethylene P1: Calcium carbonate of average particle diameter 0.07 μm 
    
     Curl was evaluated for a plurality of samples. To evaluate curl, the aforesaid support was wound around a core of diameter 10 cm, and left at a temperature of 50 ° C. for 6 hours. A sample of dimensions 5 cm length×5 cm width was then mounted on a flat plate, the height of the sample above the flat plate in the vertical and horizontal directions was measured at 1 cm intervals (36 points including the 4 corners of the sample), and the standard deviation from this height was calculated to give the curl.