Abstract:
The present invention relates to surface sizing compositions, methods for making such compositions, processes for sizing paper products using such compositions, and paper products which have been sized with such compositions. In particular, the invention relates to novel surface sizing compositions comprising rosin-based component and styrene-carboxylic copolymer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Paper is typically treated with sizing composition to impart resistance to penetration and wetting of ink and aqueous liquid. The term “paper” in the present invention refers to all forms of paper, paperboard and related cellulosic products. There are two main categories of paper sizes: internal sizes and surface sizes. Internal sizes are added to the cellulosic or other fiber stock from which paper is later made, while surface sizes are applied to the surface of paper after the paper has been formed. Paper may be sized to a variety of degrees and for a variety of purposes. Writing paper is sized to prevent the spread of ink, while milk carton stock is sized to retain the strength of the carton and to prevent any fluid flow through the carton walls and edges. 
         [0002]    Surface sizing has several advantages over internal sizing. Surface sizing provides substantial savings in sizing cost, since almost all of the sizing composition is retained on the surface of the treated paper. Furthermore, surface sizing imparts enhanced paper quality such as printability, in addition to resistance to liquid penetration. However, surface sizing has some drawbacks. Surface sizes are typically added in a size press, which is continuously operated at a high speed and under a high pressure. In such an operation, surface sizes may suffer from heat and mechanical shock, leading to gradually reduction of solubility and consequently formation of scum. The presence of scum reduces sizing efficiency, and the deposition of scum on the surface of paper adversely impacts the paper appearance. 
         [0003]    The most widely used surface size is starch. However, starch sizing alone has not been effective in providing liquid resistance to paper and paperboard products. Various kinds of natural and synthetic resins have been used as surface sizing compositions in combination with starch. Some typical surface sizes are modified rosins, modified petroleum resins, polyurethane dispersion, and copolymers of styrene and vinylic monomers such as maleic anhydride, (meth)acrylic acid and its alkyl ester, and (meth)acrylamide. In particular, styrene-maleic anhydride copolymer resins are commonly used for surface sizing. However, polymeric surface sizing compositions are less effective in sizing than reactive sizes such as alkylketene dimers (AKD) and alkenylsuccinic anhydrides (ASA). U.S. Pat. No. 6,162,328 teaches that surface sizing efficiency of polymeric surface size is enhanced when it is used in combination with reactive sizes. U.S. Pat. No. 5,138,004 teaches the use of styrene-acrylic copolymers as surface sizes with improved sizing efficiency for alkaline paper. To address the scum problem resulting from heat and mechanical shock of surface size during sizing application, U.S. Pat. No. 4,030,970 discloses a surface sizing composition comprises a copolymer of (meth)acrylic ester, alkali metal salts of (meth)acrylic acid, and (meth)acrylic acid or its ammonium or lower alkyl amine salts thereof. This surface sizing compositions has high heat and mechanical stabilities and produces substantially no scum during sizing operation even after 8 hours. 
         [0004]    Rosin-based compounds have been used for surface sizing. U.S. Pat. No. 6,048,439 discloses a modified rosin surface sizing emulsion produced by mixing emulsified rosin with a water soluble salt of an alkylene-acrylic acid copolymer. The modified rosin surface sizing emulsion has improved compatibility with an anionic starch solution, and significantly increased emulsion stability. Furthermore, the modified rosin surface sizing emulsion enhances runnability of the paper machine and reduces undesirable scum deposits on machine parts such as the dryer cans and calendar rolls. U.S. patent application No. 6,074,468 teaches a surface sizing composition comprising a thermoplastic resin, starch, and surfactant that enhances process runnability and provides sized paper with improved print properties. 
         [0005]    Notwithstanding many surface sizing compositions are known, there is constantly a need for improved surface sizing compositions capable of improving print characteristics and enhancing process runnability. Adhesion of inks, such as toner inks, to the sized paper is one critical performance required to improve print quality. Styrene maleic copolymers and styrene acrylic copolymers are widely used surface sizes. However, they commonly generate high foam level during papermaking process that often results in troublesome processing and undesired paper appearance. 
         [0006]    Furthermore, there is a need for surface size that is less costly, yet as efficient as the commonly used styrene-maleic anhydride copolymer surface size. 
         [0007]    Therefore, it is an object of the present invention to provide a surface sizing composition having increased sizing efficiency, higher heat and mechanical stability, improved process runnability, and enhanced print characteristics. In particular, the present invention is to provide surface sizing composition that is less costly than styrene-maleic anhydride copolymer, yet having equal, if not improved, sizing efficiency. 
         [0008]    Another object of the present invention is to provide a method for producing paper surface sized with the invention sizing composition. 
         [0009]    A further object of the present invention is to provide paper sized with the invention surface sizing composition. 
         [0010]    Other objects and advantages of the present invention will become apparent from the following detailed description. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention relates to surface sizing compositions, methods for making such compositions, processes for sizing paper products using such compositions, and paper products which have been sized with such compositions. In particular, the invention relates to novel surface sizing compositions comprising rosin-based component and styrene-carboxylic copolymer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A surface sizing composition of the present invention comprises at least one rosin or rosin derivative and at least one styrene-carboxylic polymer. 
         [0013]    In one embodiment of the present invention, a surface sizing composition comprises:
       (a) about 50%-90% weight of rosin or rosin derivative; and   (b) about 10%-50% weight of styrene-carboxylic polymer.       
 
         [0016]    Rosins that are suitable for use in the present invention include tall oil rosin, gum rosin, wood rosin, and mixture thereof. Tall oil rosin is preferred. Rosin derivatives that are suitable for use in the process of the invention include, but are not limited to, the following: hydrogenated rosins, disproportionated rosins, formaldehyde-treated rosins, dimerized rosins, polymerized rosin, fumarated rosins, maleated rosins, styrenated rosins, phenolic-modified rosins, acrylic-modified rosins, hydrocarbon-modified rosins, rosin-vinylic copolymers, rosin salts, hydrogenated rosin salts, disproportionated rosin salts, formaldehyde-treated rosin salts, dimerized rosin salts, polymerized rosin salts, fumarated rosin salts, maleated rosin salts, styrenated rosin salts, phenolic-modified rosin salts, acrylic-modified rosin salts, hydrocarbon-modified rosin salts, rosin-vinylic copolymer salts, rosin esters, hydrogenated rosin esters, disproportionated rosin esters, formaldehyde-treated rosin esters, dimerized rosin esters, polymerized rosin esters, fumarated rosin esters, maleated rosin esters, styrenated rosin esters, phenolic-modified rosin esters, acrylic-modified rosin esters, hydrocarbon-modified rosin esters, rosin-vinylic copolymer esters, rosin amides, hydrogenated rosin amides, disproportionated rosin amides, formaldehyde-treated rosin amides, dimerized rosin amides, polymerized rosin amides, fumarated rosin amides, maleated rosin amides, styrenated rosin amides, phenolic-modified rosin amides, acrylic-modified rosin amides, hydrocarbon-modified rosin amides, rosin-vinylic copolymer amides, and mixture thereof. Preferred rosin derivatives for the present invention are fumarated rosins, maleated rosins, rosin salts, rosin esters, and rosin amides. 
         [0017]    Styrene-carboxylic polymers suitable for the present invention include the polymerization product of: (i) at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, fumaric acid, crotonic acid, itaconic acid, maleic acid, maleic anhydride, carboxylic acrylics, and their corresponding esters thereof, and (ii) at least one styrenic monomer. Preferred styrene-carboxylic polymers are styrene-acrylic copolymer and styrene-maleic anhydride copolymer. 
         [0018]    Styrenic monomers suitable for the present invention include, but are not limited to, alpha-methyl styrene, styrene, vinyl toluene, tertiary butyl styrene, ortho-chlorostyrene and mixtures thereof. 
         [0019]    A process for producing surface sized paper of the present invention comprises:
       (a) providing an aqueous pulp suspension;   (b) sheeting and drying the aqueous pulp suspension to obtain paper;   (c) applying to at least one surface of the paper, a surface size composition comprising at least one rosin or rosin derivative and at least one styrene-carboxylic polymer; and   (d) drying the paper.       
 
         [0024]    In one embodiment of the present invention, a process for producing surface sized paper comprises:
       (a) providing an aqueous pulp suspension;   (b) sheeting and drying the aqueous pulp suspension to obtain paper;   (c) applying to at least one surface of the paper, a surface size composition comprising about 50%-90% weight of rosin or rosin derivative and about 10%-50% weight of styrene-carboxylic polymer; and   (d) drying the paper.       
 
         [0029]    In one embodiment of the present invention, the invention surface sizing components were applied to the paper at the range from about 0.01% to about 0.5% by weight of solids based on total dry weight of the paper. The invention sizing component may be applied to the paper in step (c) using a variety of known application means including, but are not limited to, size press and water-box. 
         [0030]    In one embodiment of the present invention, internal sizes such as rosin sizes, AKD and ASA may be added to the aqueous pulp suspension prior to step (b). Furthermore, other surface sizing compositions may be used for step (c) in combination with the invention surface sizing composition to enhance sizing efficiency. 
         [0031]    In one embodiment of the present invention, the invention surface sizing compositions may further comprise other components that assist the papermaking process or enhance properties of paper such as protective colloids, starch derivatives, cellulose derivatives, polymeric materials, and surfactants. Such components may be added in the papermaking process at the same stage as the surface sizing compositions, or added separately. 
         [0032]    The effectiveness of surface sizing was determined using Hercules Size Test (HST), which is an industry standard test for measuring degree of sizing as defined in the tappi method T530 pm-89. The HST value, in seconds, is the time taken for a color test solution to penetrate into and through the sized paper. The higher HST values, the higher paper resistance to liquid penetration, and thereby the more effective surface sizing. 
         [0033]    Furthermore, the effectiveness of surface sizing was determined using a 2 minute Cobb Test, which is an industry standard test as defined by tappi method T441. The Cobb value, in grams/square meter, is the mass of water absorbed by the sized paper over a 2 minute period. The lower the Cobb value, the lower level of water being absorbed on the sized paper, and thereby the more effective surface sizing. 
         [0034]    In addition, the invention surface size compositions were evaluated for toner adhesion and foam formation, the two properties that significantly impact print quality and processability of the sized paper. 
         [0035]    The following detailed description illustrates embodiments of the present invention; however, it is not intended to limit the scope of the appended claims in any manner. 
         [0036]    The invention surface sizing composition were evaluated and compared to other surface sizes commonly used in the industry: polyurethane colloidal dispersion JETSIZE AP15 available from Eka Chemicals, styrene maleic copolymer SCRIPSET 740 from Hercules, styrene acrylic copolymer JONREZ A-2331 from MeadWestvaco, and alcohol soluble maleic rosin ester JONREZ H-2735 from MeadWestvaco. Furthermore, sizing performance of the invention sized paper was compared to two controls: one was plain paper without surface sizing, and the other was paper surface sized with starch only. 
         [0037]    Prior to paper application, surface sizing compositions were formulated with ethylated starch PENFORD GUM 280 from Penford at 10:90 dry weight ratio of surface size to starch. The formulated surface sizing compositions were about 10% solids. 
         [0038]    Formulated surface sizing composition was applied to the internally sized paper having properties as listed in Table  1 , using a flooded nip size press. The sizing efficiencies of the invention surface size compositions were evaluated using HST values and Cobb values, and compared to those of known surface sizing compositions. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Basis Weight 
                 55 g/sm 
               
               
                   
                 Furnish 
                 85% hardwood, 15% softwood 
               
               
                   
                 Ash Content 
                 0% 
               
               
                   
                 Internal Size 
                 1 lb/ton of ASA internal size 
               
               
                   
                 Freeness 
                 350 ± 25 
               
               
                   
                 Pick up 
                 29 lbs/Ton 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    Paper surface sized with the invention surface sizing composition of Example 2 showed a HST value of 150, while the paper surface sized with starch had a HST value of only 28. The HST values of paper surface sized with styrene maleic copolymer SCRIPSET 740 and polyurethane colloidal dispersion JETSIZE AP15 were 148 and 110, respectively. (Table 2) The paper sized with the invention surface sizing composition showed high HST value approaching that of paper sized with styrene maleic copolymer SCRIPSET 740; therefore, the invention composition had high sizing efficiency that about the same level as styrene maleic copolymer. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Surface Size 
                 HST Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Starch 
                 28 
               
               
                   
                 Surface size of Example 2 
                 150 
               
               
                   
                 Styrene maleic copolymer 
                 148 
               
               
                   
                 Polyurethane colloidal dispersion 
                 110 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    Each sized paper was also evaluated for its sizing efficiencies based on Cobb values. (Table 3) 
         [0041]    Paper surface sized with the invention sizing composition of Example 2 showed a Cobb value of 23, which was the same as that of paper surface sized with styrene maleic copolymer SCRIPSET 740. The Cobb value measurement confirmed that the invention surface sizing composition had about the same sizing efficiency as styrene maleic copolymer surface size. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Surface Size 
                 Cobb Value 
               
               
                   
                   
               
             
             
               
                   
                 Starch 
                 44 
               
               
                   
                 Surface size of Example 2 
                 23 
               
               
                   
                 Styrene maleic anhydride copolymer 
                 23 
               
               
                   
                   
               
             
          
         
       
     
         [0042]    Furthermore, the invention surface sizing compositions were applied to the non-internally sized paper. 
         [0043]    The paper sized with the invention surface sizing composition of Example 2 showed a HST value of 18, while the plain paper and the paper surface sized with starch each showed a HST value of only 1. (Table 4) 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Surface Size 
                 HST Value 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 None 
                 1 
               
               
                   
                 Starch 
                 1 
               
               
                   
                 Surface size of Example 2 
                 18 
               
               
                   
                 Polyurethane colloidal dispersion 
                 14 
               
               
                   
                 Alcohol soluble maleic rosin ester 
                 13 
               
               
                   
                 Styrene acrylic copolymer 
                 6 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    The paper surface sized with polyurethane colloidal dispersion JETSIZE AP15 showed a HST value of 14. The other known surface sizes alcohol soluble maleic rosin ester JONREZ H-2735 and styrene acrylic copolymer JONREZ A-2331 provided paper with HST values of 13 and 6, respectively. The paper sized with the invention surface sizing composition showed higher HST value than those sized with polyurethane colloidal dispersion and styrene-acrylic ester emulsion; therefore, the invention composition had higher sizing efficiency than those of polyurethane colloidal dispersion and styrene-acrylic ester emulsion known in the industry. 
         [0045]    Toner adhesion property was tested for paper surface sized with the invention sizing composition of Example 2. (Table 5) 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Surface Size 
                 % Retained Optical Density 
               
               
                   
               
             
             
               
                 Starch 
                 94 
               
               
                 Surface size of Example 2 
                 90 
               
               
                 Styrene maleic anhydride copolymer 
                 84 
               
               
                 Polyurethane colloidal dispersion 
                 79 
               
               
                   
               
             
          
         
       
     
         [0046]    Toner ink was applied to the sized paper using a HP LaserJet 4200tn printer. After ink application, an initial optical density of the print was measured using an E-Rite 418. Adhesive tape was then applied onto the surface of the print, followed by a 90 degree pull. The optical density of the print was measured again, and the percentage retained optical density of toner was calculated. Paper with higher toner adhesion is less affected by a 90 degree adhesion tape pull, thereby showing a higher % retained optical density compared to the paper with lower toner adhesion. The control paper surface sized with only starch showed a retained optical density of 94%, while the paper sized with the invention sizing composition of Example 2 showed a retained optical density of 90%. The paper surface sized with styrene maleic copolymer SCRIPSET 740 and polyurethane colloidal dispersion JETSIZE AP15 showed % retained optical density of 84 and 79, respectively. The invention surface sizing composition provided superior toner ink adhesion to the commonly used surface sizes. 
         [0047]    Foaming of the invention surface sizing composition was measured and compared to those of the commonly used surface size styrene maleic copolymer SCRIPTSET 740. Each surface size was diluted to about 10% solids in a glass jars, and the resulting glass jar was shaken for 30 seconds. The level of foam in each jar (in millimeter height) was measured two minutes after shaking. Surface size with higher potential for foaming during papermaking process showed higher level of foam. The invention surface sizing composition of Example 3 showed only 9 mm of foam, compared to the commonly used styrene maleic copolymer surface size at 68 mm of foam. (Table 6). The surface sizing of the present invention showed at least 7 times reduction in foam formation compared to the known styrene maleic copolymer surface size. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                 Level of Foam 
               
               
                   
                 Surface Size 
                 (mm height) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Surface size of Example 3 
                 9 
               
               
                   
                 Styrene maleic anhydride copolymer 
                 68 
               
               
                   
                   
               
             
          
         
       
     
         [0048]    The invention surface sizing compositions showed superior toner ink adhesion and lower foaming compared to the known surface sizes, while maintaining and if not enhancing the sizing efficiency. 
         [0049]    The foregoing description relates to embodiments of the present invention are exemplary and explanatory only and are not restrictive of the invention, as claimed. Any changes and modifications may be made therein without departing from the scope of the invention as defined in the following claims. 
       EXAMPLES 
     Example 1 
     Rosin Modified Polyamide 
       [0050]    A mixture of rosin and tributylphosphite was heated to 180° C. under nitrogen gas and fumaric acid was added. The temperature was raised to 210° C. and held at that temperature for 90 minutes. A piperazine solution was gradually added to the obtained fumaric-rosin adduct over 2 hours, while the temperature was maintained at about 195-210° C. After an addition of piperazine, the temperature was increased to 220° C. Diethylene glycol was added the resin products until the acid number of resin reached 175 mg KOH/g resin. The rosin modified polyamide resin was then poured into aluminum pans and rapidly cooled. 
       Example 2 
     Rosin Amide/Styrene-Acrylic Copolymer Size 
       [0051]    To the aqueous ammonium hydroxide containing isopropyl alcohol, rosin amide resin of Example 1 and styrene-acrylic copolymer were added. The slurry was heated to 80° C. and vigorously agitated for 2 hours. When all the solid resins were completely dissolved, the solution was cooled to 25° C. The obtained solution of rosin amide/styrene-acrylic copolymer surface size had pH of 9.5 and % solids of about 30.5%. 
       Example 3 
     Rosin Amide/Styrene-Maleic Anhydride Copolymer Size 
       [0052]    To the aqueous ammonium hydroxide containing isopropyl alcohol, rosin amide resin of Example 1 and styrene-maleic anhydride copolymer were added. The slurry was heated to 80° C. and vigorously agitated for 2 hours. When all the solid resins were completely dissolved, the solution was cooled to 25° C. The obtained solution of rosin amide/styrene-acrylic copolymer surface size had pH of 9.0 and % solids of about 30%.