Pigment materials and their use in coating compositions

Abstract

A pigment material for use in a coating composition suitable for coating a sheet material to be printed by an electrophotographic printer which pigment material comprises a blend of Components A and B as follows: Component A: a fine pigment material suitable for gloss coating of a sheet material the pigment material comprising particles at least 80% by weight of which have an equivalent spherical diameter (“esd”) of less than 2 μm and having a particle size distribution (“psd”) such that its d 50 value, namely the particle esd value less than which 50% of the particles have an esd, is less than 1 μm; and Component B: a coarse pigment material having a psd such that its d 50 value is from 2 μm to 10 μm, desirably 2 μm to 5 μm and such that not more than 2% by weight of the particles of the coarse pigment material have an esd greater than 15 μm, desirably not more than 2% have an esd greater than 10 μm; wherein the weight ratio of Component A to Component B is at least 4:1.

We claim:  
     
         1 . A pigment material for use in a coating composition suitable for coating a sheet material to be printed by an electrophotographic printer which pigment material comprises a blend of Components A and B as follows: 
 Component A: a fine pigment material suitable for gloss coating of a sheet material the pigment material comprising particles at least 80% by weight of which have an equivalent spherical diameter (“esd”) of less than 2 μm and having a particle size distribution (“psd”) such that its d 50  value, namely the particle esd value less than which 50% of the particles have an esd, is less than 1 μm; and    Component B: a coarse pigment material having a psd such that its d 50  value is from 2 μm to 10 μm and such that not more than 2% by weight of the particles of the coarse pigment material have an esd greater than 15 μm;    wherein the weight ratio of Component A to Component B is at least 4:1.    
     
     
         2 . A pigment material as claimed in  claim 1  and wherein the weight ratio of Component A to Component B is in the range 4:1 to 100:1.  
     
     
         3 . A pigment material as claimed in  claim 1  and wherein Component A has a d 50  value of from 0.4 μm to 0.7 μm.  
     
     
         4 . A pigment material as claimed in  claim 1  and wherein not more than 2% by weight of the particles of Component A have an esd of 5 μm or more.  
     
     
         5 . A pigment material as claimed in  claim 1  and wherein Component A has on a log-normal particle size increment histogram, a histogram peak which at half peak maximum height has a width of from 1.0 to 1.2 along the esd logarithmic scale.  
     
     
         6 . A pigment material as claimed in  claim 1  and wherein Component A and Component B each independently comprises one or more of calcium carbonate, calcined kaolin, hydrous kaolin, talc, mica, dolomite, silica, zeolite, gypsum, satin white, titania, calcium sulphate and plastic pigment.  
     
     
         7 . An aqueous coating composition suitable for coating a sheet material to be printed by an electrophotographic printer which comprises a pigment material according to  claim 1  together with a hydrophilic adhesive and optionally other ingredients.  
     
     
         8 . A method of coating a paper sheet optionally which carries one or more previously applied coating layers which includes the step of coating the paper sheet with a coating composition according to  claim 7 .  
     
     
         9 . A method of printing paper sheets by an electrophotograhic printing process which includes the step of carrying out the printing on coated sheets of paper wherein the sheets have been coated by the method according to claim  8 .