Patent Publication Number: US-2010126683-A1

Title: Paper Dyeing Process

Description:
FIELD 
     This invention relates to the dyeing of paper materials, such as packaging grade paper materials. 
     BACKGROUND 
     The production of coloured paper for use in packaging can be achieved by printing the paper with a coloured ink followed by varnishing to prevent ink rub-off. While packaging products made from such paper perform acceptably, the process costs are high. 
     The dyeing of paper pulp to obtain fast colours using sulphur-based dyes has been practised for many years. However, the addition of the dye to the paper pulp in the pulper during stock preparation, in which the dye is fixed by the addition of alum, causes colouration of the process water unless pH is lowered to less than 5, which creates environmental as well as other process difficulties. In addition, in the case of paper mills with multi-ply paper making facilities but only one pulper, such dyeing processes mean that the addition of the dye to the pulper would result in dyeing of the whole sheet, even if colour is only required for the outer ply of paper. This results in a significant increase in the cost of dyeing. 
     SUMMARY 
     It is an object of the present invention to provide an alternative dyeing process which removes at least the colouration problem outlined above. 
     The invention provides a process for dyeing paper materials, comprising preparing pulp stock suitable for a paper making process, subjecting the pulp stock to a hot dispersion process, characterised by the steps of adding a sulphur dye to the pulp stock after the hot dispersion process, and acidifying the dyed pulp stock to fix the dye. 
     By addition of the dye to the pulp stock making process after the hot dispersion stage, the contamination of the process water is avoided. In addition, the dye can be added to the stock for producing a selected paper ply, such as the outer ply, rather than dyeing all plies of the paper. 
     The dye can be added at any suitable time immediately following hot dispersion and may be added just prior to or during the treatment of the pulp by disperser plates, or the like. 
     The pulp stock may be de-watered prior to the hot dispersion step, and is passed through disperser plates, or the like, following hot dispersion. When the pulp stock is passed through the disperser plates, or the like, the pulp stock is diluted to about 5% to 6% stock concentration and the sulphur dye is added at this stage at an addition rate of about 20% to 30%. The dyed pulp stock is then stored in a stock chest for papermaking, and while in the stock chest, the stock is acidified by the addition of a suitable acid, such as dilute sulphuric acid, to increase the pH level to about 5.5 to 6.0. This results in complete fixation of the dye in the fibres of the stock. 
     Since the addition of acid to the sulphur dye results in the formation of hydrogen sulphide gas, the stock chest is preferably covered with a lid and the gas extracted for treatment. 
     In one form, the gas is treated by passing it through another stock tank full of dyed stock prior to acidification. This results in over 90% adsorption of the hydrogen sulphide gas. The vent gas from the top of this tank may be passed through a bed of activated carbon, and this results in about 99% removal of the hydrogen sulphide gas. 
     The pulp from the pulper may be fractionated to produce long fibre stock for the top ply and short fibre stock for the back ply in a two-ply paper. The long fibre fraction may be de-watered by any suitable means, such as a screw press, to about 25% consistency and given hot dispersion by increasing the stock temperature to around 100° C. by the addition of steam, following which the hot stock is passed through disperser plates in a known manner. The pulp stock fraction is at this time diluted to about 5% stock concentration and the sulphur dye added at this stage following which the stock fraction is stored in a stock chest for paper making. The stock fraction is then acidified as described above to fix the dye and the hydrogen gas removed, such as described above. By operating in this way, the pulp fraction for the outer ply only is dyed thereby resulting in a significant saving in dye costs. 
     It has been found that the addition of the dye following hot dispersion and dilution to about 5% to 6,% stock concentration results in a better yield from dye in terms of colour, and fixing the dye at a higher pH using a stronger mineral acid, rather than alum, is more economical. 
     The process described above is suitable for use with any sulphur dye and has been found to be particularly suitable for use with sulphur black dye, such as Clariant Diresul black P-FT liquid (CI Sulphur Black 1). 
     The invention also provides a dyed paper product made by the process defined above. 
     In the present specification, the paper making process is not described in any detail since any paper making process known to persons skilled in the art may be adopted and modified by a dyeing process embodying the present invention. 
    
    
     EXAMPLE 
     Old corrugated cartons were slushed in a continuous pulper and the pulp was screened through a 3 mm screen to remove contaminants. The screened pulp was fractionated through a screen with 0.35 mm slots to get two streams of pulp. The rejects fraction of long fibre was used to feed the top wire of a paper machine whereas the accepts fraction (short fibre) was used to feed the bottom wire of the paper machine. The long fibre fraction was dewatered to about 25% consistency in a screw press and the thickened pulp was heated with steam in a hot dispersion unit to around 95° C. to melt wax, stickies and the like. The heated stock was mixed with water and sulphur black dye in the disperser at about 5% stock concentration. This process broke the waxes and stickies into very small particles and mixed the black dye thoroughly with pulp. The sulphur black dye used was Diresul Black P-FT liquid from Clariant. The characteristics of this dye are as follows: 
     Colour Index: Sulphur Black 1 
     Specific Gravity: 1.32 g/cc 
     Active Concentration: 35% 
     pH: 12 
     Various amounts of this black dye were added from 20%-30% on a dry fibre basis which equals 200 kg dye per ton of fibre to 300 kg dye per ton of fibre. 
     The pulp stock mixed with dye was highly alkaline (pH 10-10.5) and the dye was fixed on the fibre by addition of 50% dilute sulphuric acid to pH of 5.8 while the stock was held in a stock chest. The resulting hydrogen sulphide gas was extracted and neutralised as described above. 
     This pulp was fed to the top ply of two-ply fourdrienier machine running at 330 m/min with production rate of 11 t/hr. The black colour of the top side of the paper with varying dosages of black dye was as follows: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Sulphur Black dose 
                 L* 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                  0% 
                 67 
               
               
                   
                 20% 
                 23.5 
               
               
                   
                 25% 
                 22 
               
               
                   
                 30% 
                 21 
               
               
                   
                   
               
            
           
         
       
     
     The L* values go down as the black colour becomes darker. These results show there is very significant drop in L* by addition of 20% dye but further addition in steps of 5% reduce L* only marginally, although the dye is still fully fixed on the fibre. 
     The machine wire water did not have black colouration showing complete fixation of dye on fibre. This effect can sometimes be very helpful in grade change of paper on paper machine from one colour to another. 
     The invention has been described by way of non-limiting example only and many modifications and variations may be made thereto without departing from the spirit and scope of the invention. 
     Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.