Abstract:
A method of enhancing brightness of paper made from mechanical pulps or recycled pulps that comprise: adding an effective amount of one or more chelants and one or more surfactants to a bleaching stage of a papermaking process that involves the addition of hydrosulfite is disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]     This disclosure pertains to a method of enhancing brightness of paper produced from mechanical or recycled pulps.  
       BACKGROUND OF THE INVENTION  
       [0002]     Bleaching of mechanical and recycled pulps during a papermaking process enhances the brightness of paper produced from these pulps. Mechanical pulps include groundwood (GW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), chemimechanical pulp (CMP), variations thereof (e.g., stone GW, pressurized GW, thermo-RMP, pressure RMP, pressure TMP, chemi-RMP, long fiber CMP, thermomechanical chemi pulp); recycled pulp; and compositions containing mechanical, chemical and recycled pulps. Recycled pulps include any pulp obtained from recycled paper, for example newspapers.  
         [0003]     Some common bleaching chemicals utilized during a bleaching stage of a papermaking process include: sodium hydrosulfite; ammonium hydrosulfite; magnesium hydrosulfite; calcium hydrosulfite; zinc hydrosulfite; chlorine; chlorine dioxide; hydrogen peroxide; oxygen; and calcium hypochlorite.  
         [0004]     The efficacy, however, of these bleaching chemicals is retarded by transitional metal ions in the pulp that react with the bleaching chemicals. More specifically, these reactions cause decomposition of the bleaching chemicals. Therefore, a need exists to prevent this decomposition so that an optimum brightness of paper can be achieved.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides for a method of enhancing brightness of paper made from mechanical pulps that comprises the step of: adding an effective amount of one or more chelants and one or more surfactants to a bleaching stage in a papermaking process that involves the addition of hydrosulfite.  
         [0006]     The present invention also provides for a method of enhancing brightness of paper made from recycled pulps that comprises the step of: adding an effective amount of one or more chelants and one or more surfactants to a bleaching stage of a papermaking process that involves the addition of hydrosulfite. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0007]     “Bleaching” means a process of purifying and whitening pulp by chemical treatment to remove or change existing coloring material so that the pulp takes on a higher brightness characteristic.  
         [0008]     “DTMPA” means diethylenetriaminepentaacetic acid.  
         [0009]     “DTMPA” means diethylenetriaminepentakis(methylphosphonic acid).  
         [0010]     “EDTA” means ethylendiaminetetraacetic acid.  
         [0011]     “AMP” means aminotris(methylphosphonic acid).  
         [0012]     “o.d.” means oven dry.  
         [0013]     Conventional methods for bleaching pulp and making paper from pulps, whether that be virgin mechanical or recycled pulps, are well known in the art. Bleaching may be one stage or multistage. In multi-stage bleaching, the pulp is sequentially exposed to different types of bleaching chemicals, for example, hydrogen peroxide first, and hydrosulfite second.  
         [0014]     As stated above, brightness in both mechanical and recycled pulps is achieved by adding an effective amount of both chelants and surfactants to a bleaching stage in a papermaking process that involves the addition of hydrosulfite. In one embodiment, the chelants and surfactants are added as a mixture or sequentially. The chelants may be added to the bleaching stage at a dose of 0.01 to 10 lbs/ton of o.d. pulp, preferably 1 to 4 lbs/ton of o.d. pulp. The surfactants may be added at a dose of 0.01 to 10 lbs/ton of o.d. pulp, preferably 0.5 to 2 lb/ton of o.d. pulp.  
         [0015]     In another embodiment, the chelants are selected from the group consisting of: aminophosphonates, aminophosphates, aminocarboxylates, DTMPA, AMP, DTMPA-polyacrylate blends, EDTA, and DTPA.  
         [0016]     In another embodiment, the surfactants are selected from the group consisting of: anionic; long-chain carboxylic acids; alkylarylsulfonates; alkylsulfates; alkylsulfonates; arylsulfonates; cationic; tertiary alkylammonium; alkylpyridinium salts; ampholitic species; alkylaminoacids; non-ionic; oxyalkylated alcohols; polyethyleneglycol esters of fatty acids; oxyalkylated; and alkylphenols. In a preferred embodiment, the surfactants are selected from the group consisting of: polyalkoxylated long-chain alcohols; ethoxylated alcohols; and propoxylated alcohols.  
         [0017]     In a preferred embodiment, the chelants and surfactants are added as a single product comprising: 15% pentasodium DTPA, 3% etoxylated, propoxylated hexadecanol and 17% sodium xylenesulfonate at pH about 7.  
         [0018]     In another embodiment, the chelants and surfactants are added as separate products, one chelant comprising: 29% pentasodium DTMPA and 14% sodium polyacrylate at pH about 6 and one surfactant comprising 16% ethoxylated, propoxylated hexadecanol at pH about 8.  
         [0019]     The present invention will be further described in the following examples and tables. The examples are not intended to limit the invention prescribed by the appended claims.  
       EXAMPLES  
       [0020]     The efficacy of the combination of chelants and surfactants were studied in a hydrosulfite bleaching experiment conducted according to the following protocol.  
         [0021]     The experiments must be conducted in an oxygen-free atmosphere to achieve reproducible results. Oxygen from air quickly oxidizes sodium hydrosulfite in the solution to sulfate and sulfite.  
         [0022]     The following procedure was followed for all examples: (1) weigh out a pulp sample (5 g dry pulp per sample, calculate actual weight based on the consistency) in a 250-ml glass bottle; (2) add water and additives when needed, mix well; (3) close the bottle and degas them the sample with a flow of nitrogen for 10 min; (4) degas deionized (“DI”) water for 10 min; (5) place desired amount of hydrosulfite in a 100-ml flask, close it, fill with nitrogen, then add required amount of degassed DI water through a syringe and mix. Recommended concentration of hydrosulfite provides for about 1% in the bleached sample after adding 5 ml stock solution; (6) add 5 ml of hydrosulfite solution to the pulp sample via syringe in a flow of nitrogen; (7) mix well quickly; (8) immerse in water bath for desired reaction time; (9) transfer the bleached pulp onto cheesecloth placed in a Büchner funnel and wash it with 2 L DI water under vacuum; and (10) prepare handsheets from a 0.5% pulp slurry (IL DI water) for brightness measurements.  
         [0023]     The handsheets were equilibrated at constant humidity 50% and 23° C. and brightness measured (standard TAPPI R457 brightness, E313 yellowness, Elrepho-3000 instrument, Datacolor International, Charlotte, N.C.).  
         [0024]     The doses in the following tables are calculated based on o.d. pulp, 40% active chelants and 80% active surfactants. For the interpretation of these tables, the following legend should be utilized: Br0—initial brightness, BrGain—gain in brightness vs. Control; Chelant 1—29% DTMPA/14% sodium polyacrylate, pH 5.8; Chelant 2—38% tetrasodium EDTA; Surfactant 1—79% C12-14 secondary alcohols, ethoxylated; Surfactant 2—70% ethoxylated oxoalcohols (“bottoms” from the oxo-process). TMP1 and TMP2 refer to TMP from different mills.  
                                                           TABLE 1                           TMP1, 4.6%, 70° C., 1 h, 0.9% sodium hydrosulfite                % Chelant 1   % Surfactant 1   Br   BrGain                            0   0   53.6   0.0           0.05   0   54.0   0.4           0.025   0.01   54.5   0.9           0.05   0.01   54.5   0.9           0.075   0.01   55.0   1.4           0.025   0.02   54.5   0.8           0.05   0.02   54.2   0.6           0.075   0.02   54.6   1.0           0.1   0.02   54.4   0.8           0   0.005   53.5   −0.1           0   0.01   53.8   0.2           0   0.02   54.2   0.5                      
 
         [0025]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 TMP2, 3.5%, 50 min, 65° C., 1.0% sodium hydrosulfite 
               
             
          
           
               
                   
                 Chemicals 
                 Br 
               
               
                   
                   
               
               
                   
                 Control 
                 66.90 
               
               
                   
                 0.2% Chelant 2 
                 67.54 
               
               
                   
                 0.05% Chelant 1 
                 67.59 
               
               
                   
                 0.05% Chelant 1 + 0.01% Surfactant 1 
                 67.72 
               
               
                   
                 0.05% Chelant 1 + 0.01% Surfactant 2 
                 67.86 
               
               
                   
                   
               
             
          
         
       
     
         [0026]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 TMP - Deinked Pulp (Recycling), 3.9%, 30 min, 
               
               
                 70° C., 0.7% sodium hydrosulfite 
               
             
          
           
               
                   
                 Surfactant 
                 Chelant 1 
                 Chelant 2 
                 Br 
               
               
                   
                   
               
             
          
           
               
                   
                 Control 
                 0.0% 
                 0.0% 
                 62.70 
               
               
                   
                 N/A 
                 0.0% 
                 0.05% 
                 63.42 
               
               
                   
                 0.025% Surfactant 1 
                 0.0% 
                 0.05% 
                 63.65 
               
               
                   
                 0.025% Surfactant 2 
                 0.0% 
                 0.05% 
                 63.79 
               
               
                   
                 N/A 
                 0.025% 
                 0.025% 
                 63.65 
               
               
                   
                 0.01% Surfactant 2 
                 0.025% 
                 0.025% 
                 63.90 
               
               
                   
                   
               
             
          
         
       
     
         [0027]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
               
               
                 TMP - Deinked Pulp (Recycling), 3.9%, 30 min, 
               
               
                 70° C., 0.7% sodium hydrosulfite 
               
             
          
           
               
                   
                 Chemicals 
                 Br 
               
               
                   
                   
               
               
                   
                 Control 
                 62.67 
               
               
                   
                 0.025% Surfactant 1 
                 63.16 
               
               
                   
                 0.01% Surfactant 1 
                 62.93 
               
               
                   
                 0.05% Chelant 1 + 0.025% Surfactant 1 
                 63.95 
               
               
                   
                 0.05% Chelant 1 + 0.01% Surfactant 1 
                 63.84 
               
               
                   
                 0.1% Chelant 1 + 0.01% Surfactant 1 
                 64.00 
               
               
                   
                 0.05% Chelant 1 + 0.025% Surfactant 2 
                 64.21 
               
               
                   
                   
               
             
          
         
       
     
         [0028]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                   
               
               
                 TMP - Deinked Pulp (Recycling), 3.9%, 30 min, 
               
               
                 70° C., 0.7% sodium hydrosulfite 
               
             
          
           
               
                   
                 Chemicals 
                 Br 
               
               
                   
                   
               
               
                   
                 Control 
                 62.18 
               
               
                   
                 0.2% Chelant 2 
                 63.31 
               
               
                   
                 0.05% Chelant 1 + 0.05% Surfactant 2 
                 63.66 
               
               
                   
                 0.1% Chelant 1 + 0.025% Surfactant 2 
                 63.89 
               
               
                   
                 Control 
                 62.68 
               
               
                   
                 0.2% Chelant 2 
                 63.90 
               
               
                   
                 0.05% Chelant 1 + 0.05% Surfactant 2 
                 63.73 
               
               
                   
                 0.05% Chelant 1 + 0.025% Surfactant 2 
                 63.78 
               
               
                   
                 0.1% Chelant 1 + 0.025% Surfactant 2 
                 64.00 
               
               
                   
                   
               
             
          
         
       
     
         [0029]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                   
               
               
                 TMP - Deinked Pulp (Recycling), 3.9%, 30 min, 
               
               
                 70° C., 0.7% sodium hydrosulfite 
               
             
          
           
               
                   
                 Bleach 
                 Br 
               
               
                   
                   
               
               
                   
                 Control 
                 62.09 
               
               
                   
                 0.05% Surfactant 2 
                 62.18 
               
               
                   
                 0.01% Surfactant 2 
                 62.12 
               
               
                   
                 0.1% Chelant 1 
                 62.64 
               
               
                   
                 0.05% Chelant 1 
                 62.44 
               
               
                   
                 0.05% Chelant 1 + 0.01% Surfactant 2 
                 62.86 
               
               
                   
                 0.05% Chelant 1 + 0.05% Surfactant 1 
                 63.45 
               
               
                   
                 0.2% Chelant 2 
                 63.32 
               
               
                   
                   
               
             
          
         
       
     
         [0030]     As shown in Table 1, the addition of a phosphonate chelant and a surfactant to a mechanical type pulp has a synergistic effect on brightness. Table 2 also shows improved performance of a chelant when a surfactant was added.  
         [0031]     With respect to recycled pulps, tables 3 through 6 demonstrate that the addition both a chelant and a surfactant have a synergistic effect on brightness.