Paper product

Abstract

The invention relates to a supercalendered paper product comprising first cellu-losic fibres and a gypsum-fibre composite product as a filler pigment or coating pigment, wherein the gypsum in the gypsum-fibre composite product appears as crystals on the surface of the fibre, and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspension. The invention also relates to a process for producing a supercalendered paper product.

1 . A supercalendered paper product comprising first cellulosic fibres and a gypsum-fibre composite product as a filler pigment or coating pigment, wherein the gypsum in the gypsum-fibre composite product appears as crystals on the surface of the fibre, and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous second fibre suspension, and wherein the gypsum crystals have a size from 0.1 to 5.0 μm. 
     
     
         2 . The supercalendered paper product according to  claim 1 , wherein the first cellulosic fibres comprise chemical, mechanical, chemi-mechanical or deinked pulp fibres. 
     
     
         3 . The supercalendered paper product according to  claim 1 , wherein the second fibre of the gypsum-fibre composite product comprises a second cellulosic fibre such as a chemical, mechanical, chemi-mechanical or deinked pulp fibre or a synthetic fibre. 
     
     
         4 . The supercalendered paper product according to  claim 3 , wherein average length of the second fibre of the gypsum-fibre composite product is between 0.5 and 5 mm. 
     
     
         5 . The supercalendered paper product according to  claim 1 , wherein the weight ratio of gypsum to fibre in the gypsum-fibre composite product on dry basis is in the range from 95:5 to 50:50. 
     
     
         6 . The supercalendered paper product according  claim 1  additionally comprising one or more of following substances: a natural or synthetic polymer binder, an optical brightener, a rheology modifier and a sizing agent. 
     
     
         7 . The supercalendered paper product according to  claim 1 , wherein the amount of the gypsum-fibre composite product is from 10 to 60%, by weight on dry basis. 
     
     
         8 . The supercalendered paper product according to  claim 1 , wherein in the crystallization the calcium sulphate hemihydrate comprises α-calcium sulphate hemihydrate or β-calcium sulphate hemihydrate. 
     
     
         9 . The supercalendered paper product according to  claim 1 , wherein in the crystallization the weight ratio of calcium sulphate hemihydrate and/or calcium sulphate anhydrite to water is in the range from 0.03 to 0.6:1, preferably from 0.05 to 0.5:1. 
     
     
         10 . The supercalendered paper product according to  claim 1 , wherein in the crystallization the dry second fiber content is from 3 to 30% by weight. 
     
     
         11 . The supercalendered paper product according to  claim 1 , wherein in the crystallization the content of calcium sulphate hemihydrate and/or calcium sulphate anhydrite is from 5 to 57% by weight. 
     
     
         12 . A process for producing a supercalendered paper product as defined in  claim 1  comprising providing a stock where first cellulosic fibers are mixed with water and the gypsum-fibre composite product, the obtained slurry is delivered to the forming section of a paper machine, then water is drained in the draining section and the formed web is conducted to the press section where additional water is removed, then the web is conducted to the drying section where most of the remaining water is evaporated, and finally supercalendering the paper to improve the surface smoothness and gloss of the paper. 
     
     
         13 . The process of  claim 12 , wherein the gypsum-fibre composite product is introduced in the form of an aqueous product or in a dried form, optionally comminuted into the form of particles. 
     
     
         14 . Use of a gypsum-fibre composite product wherein the gypsum appears as crystals on the surface of the fibre and wherein the gypsum crystals are obtained by contacting calcium sulphate hemihydrate and/or calcium sulphate anhydrite and an aqueous fibre suspension, and wherein the gypsum crystals have a size from 0.1 to 5.0 μm, as a filler pigment or coating pigment in the production of a supercalendered paper product. 
     
     
         15 . Use according to  claim 14 , wherein the amount of the gypsum-fibre composite product is from 10 to 60%, by weight on dry basis of the supercalendered paper product.