Abstract:
A process for producing paper or paperboard from mechanical pulp, in which the pulp is treated with a phenol-oxidizing enzyme after mechanical refining of the pulp has been completed. The resulting paper exhibits an increased strength relative to paper produced from untreated pulp.

Description:
This application is a continuation of U.S. application Ser. No. 08/338,456, filed Nov. 10, 1994 now abandoned, which is a 371 national phase application of PCT/DK93/00168, filed May 18, 1993. 
    
    
     TECHNICAL FIELD 
     This invention relates to a process for production of paper or paperboard of increased strength from mechanical pulp. 
     BACKGROUND ART 
     Mechanical pulp, such as ground wood pulp, refined mechanical pulp, thermomechanical pulp, etc. is generally produced by grinding logs or chips in a grinder or refiner where the temperature increases to near or above boiling. The three main constituents of the pulp are cellulose, hemicellulose and lignin. 
     From these types of pulp, paper or paperboard is generally manufactured as follows: 
     The pulp is treated in a screening process, is mixed with paper or paperboard making additives in the stock preparation section in order to prepare the paper or paperboard furnish. A paper or paperboard is then made from the furnish on a paper machine. 
     The mechanical pulps have an advantage of providing high yield but on the other hand they have inferior strength properties compared to chemical pulps. 
     High paper strength is generally desirable. Conventional methods for increasing the paper strength include the use of wet strength additives and binders. 
     EP,A1,0 433 258 discloses a process for the production of mechanical pulp from a fibrous product wherein the fibrous product is subjected to an enzymatic treatment in which a binding agent is linked with the lignin in the fibrous product. The binding agent may be a hydrophilic substance, particularly a protein or a carbohydrate, e.g., oxidized, medium cationic starch. The enzymatic treatment may be done with a laccase. 
     U.S. Pat. No. 4,687,745 discloses a process for enhancing the strength properties of mechanical pulps. The process uses ligninolytic enzymes present in the extracellular growth medium of a fermentation of  Phanerochaete chrysosporium.    
     STATEMENT OF THE INVENTION 
     We have found that, surprisingly, the strength of the paper of paperboard can be increased by treating the pulp with a phenol-oxidizing enzyme system (e.g. laccase and oxygen) after the mechanical refining. It is believed that this strengthening is due to polymerization or cross-linking of the lignin present at the surface of the fibers. 
     Accordingly, the invention provides a process for increasing the strength of a mechanical pulp for paper or paperboard production, comprising treating the pulp with a phenol-oxidizing enzyme system, selected from the group consisting of: 
     a) a peroxidase together with hydrogen peroxide, and 
     b) a laccase or a catechol oxidase together with oxygen, provided that no grinding or refining occurs during this treatment, and that no binding agent is present during this treatment. 
     EP 429,422 discloses reduction of energy consumption in the refining stages by use of laccase during pulp preparation between the first and second refining stage; the document indicates that some increase of paper strength is also obtained. However, the enzyme treatment is done at 20° C., so this prior-art process requires an extra cooling stage after the first refining step. The process of this invention would obviously defeat the object of EP 429,422 as the enzyme treatment occurs after the last refining stage and therefore has no effect on energy consumption during refining. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Phenol Oxidizing Enzyme System 
     The enzyme system used in the invention consists of a suitable oxidase together with O 2  or a suitable peroxidase together with H 2 O 2 . Suitable enzymes are those which oxidize and polymerize aromatic compounds such as phenols and lignin. 
     Examples of suitable enzymes are catechol oxidase (EC 1.10.3.1), laccase (EC 1.10.3.2) and peroxidase (EC 1.11.1.7). Some preferred enzymes are peroxidase derived from a strain of Coprinus, e.g.  C. cinerius  or  C. macrorhizus,  peroxidase from Bacillus, e.g.  B. pumilus  and laccase from Trametes, e.g.  T. versicolor  (previously called Polyporus). It may be preferable to use two different phenol oxidizing enzymes together. 
     The amount of peroxidase should generally be in the range 10-10,000 PODU per g of dry substance (PODU unit of peroxidase activity defined below). The amount of laccase should generally be in the range 10-10,000 units per g of dry substance (unit of laccase activity defined below). 
     Molecular oxygen from the atmosphere will usually be present in sufficient quantity. A suitable amount of H 2 O 2  will usually be in the range 0.01-10 mM, particularly 1-10 mM. 
     Process Conditions 
     The enzyme treatment can be done at conventional consistency, e.g. 0.5-10% dry substance, at temperatures of 20-90° C. 
     Determination of Peroxidase Activity (PODU) 
     Peroxidase activity is determined from the oxidation of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) by hydrogen peroxide. The greenish-blue colour produced is photometered at 418 nm. The analytical conditions are 0.88 mM hydrogen peroxide, 1.67 mM ABTS, 0.1 M phosphate buffer, pH 7.0, 30° C., 3 minutes reaction. 
     1 peroxidase unit (PODU) is the amount of enzyme that catalyses the conversion of 1 μmol hydrogen peroxide per minute at these conditions. 
     Determination of Laccase Activity 
     Laccase activity was determined by a similar method without addition of hydrogen peroxide. 1 unit of laccase activity was defined as the amount of enzyme that catalyses the oxidation of 1 μmol ABTS per minute. 
    
    
     EXAMPLE 1 
     6.96 g of a ground wood pulp (GWP) was dissolved in 72 ml 0.1 M buffer (Britton-Robinson buffer consisting of boric acid, phosphoric acid, and acetic acid) at pH 5.5, corresponding to a dry solid content of 1.8 g (2.5%). A laccase from  Polyporus pinsitus  was added to a concentration of 528 laccase units/g dry pulp. The mixture was shaken in a water bath at 50° C. for 2 hours. Subsequently a paper sheet was made from the pulp in a laboratory hand sheet former. The sheet was subsequently pressed and dried in a rapid sheet dryer. 
     The tear index of the paper sheet was determined, and for comparison a similar experiment was done without any enzyme added. The results were as follows: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Invention 
                 Reference 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tear index 
                 4.18 
                 3.50 
               
               
                   
                 (mN*m 2 /kg) 
               
               
                   
                   
               
             
          
         
       
     
     EXAMPLE 2 
     249 g of a ground wood pulp having a dry solid content of 15%, was dissolved 2500 ml 0.1 M buffer similar to the one described in Example 1 at pH 5.5, corresponding to a dry solid content of 1.5%. The mixture was defibrated in a lab-pulper for 5 minutes and thereafter divided into 3 different parts. To one part laccase from  Polyporus pinsitus  was added in the same concentration as in Example 1, to another part inactivated laccase was added, and to the last part water was added. The mixtures were treated for 24 hours at 50° C. Subsequently paper sheets were made as in Example 1, and the tear index and the tensile index were determined. The results were as follows: 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 inact. 
                   
               
               
                   
                 Reference 
                 laccase 
                 laccase 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tensile index 
                 29.62 
                 29.52 
                 33.43 
               
               
                   
                 (kNm/kg) 
               
               
                   
                 Tear index 
                 3.29 
                 3.35 
                 3.57 
               
               
                   
                 (mN*m 2 /kg)