Abstract:
A method for treating secondary fiber to achieve wet strength while retaining repulpability is provided. In an embodiment, the method involves supplying a first line of secondary fiber which represents a total supply of secondary fiber. The method has a further step of removing a portion of the total supply of secondary fiber in the first line and transporting the portion into a second line. The second line is treated with a cationic resin treatment. A third line supplies virgin fiber. The third line and the first line are combined. Next, the treated secondary fiber in the second line is combined with the mixed contents of the first line and third line. Separation of the virgin fiber line from the secondary fiber line at initial stages of the process lowers cationic demand for the system, and reduces an amount of resin required to treat the system.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to a method for treating fiber. More specifically, the method involves treating all or a portion of a secondary fiber line prior to combination with a virgin fiber line.  
       BACKGROUND OF THE INVENTION  
       [0002]     Paper mills through the country are presently using increasing amounts of secondary fiber in their products. This has in part resulted from more efficient collection of waste paper products, e.g., by businesses and by curbside recycling, and in part from improved technology-that has enabled acceptable primary products to be made from what were formerly waste products. An additional impetus has come following the realization that well over half of the volume of waste going into municipal landfills was paper-based. Many customers and consumers now demand paper products with a significant amount of post-consumer recycled fiber.  
         [0003]     Unfortunately, each time cellulosic fibers are recycled there is some loss in strength. This is in part due to fiber breakage and cutting during the repulping process and from subsequent refining. In part it is due to the inherent nature of the fiber itself. Fiber once dried from an aqueous system suffers a morphological change that affects subsequent fiber-to-fiber bonding. For any given paper type; e.g., papers of identical basis weight and additives, products made from recycled fiber of the same type will typically be approximately 30% lower in selected strength properties than the same product made from virgin fiber. Some mills may compensate by making products of higher basis weight, by using additives to increase lost strength, by increased refining, or by some combination of these methods. The result is a higher cost product that is often less competitive with a similar product made from primarily virgin fiber.  
         [0004]     Certain additives are commonly used to augment wet and dry strength. Cationic starches have long been used in linerboard to increase dry strength. Small quantities; e.g., 0.1-0.7%, of cationic polyamide-epichlorohydrin reaction products (PAE resins) are well known to increase both wet and dry strengths. They are routinely used in products such as facial tissues and paper towels. They are also used in a small percentage of the linerboard used for the manufacture of wet strength-type corrugated board products. Tissue and towels normally do not enter the recycle stream, although much of the wet strength corrugated board does. There it presents a problem because of less than desired repulpability. This is normally tolerable since typically not more than about 1% or 2% of the corrugated board produced for the marketplace has received this type of wet strength treatment. However, if significantly larger quantities of PAE treated products were in the recycle waste stream from screening, repulped fiber would increase substantially and production rates would be adversely affected. Thus, despite their known efficiency at increasing both dry and wet strength, PAE resins have been very selectively used only for specific products where their poor repulpability does not present a significant problem.  
         [0005]     In addition, it is generally known that virgin fiber, when exiting a digester or similar system, may have a high anionic charge. This is due in part to the byproducts that accompany the virgin fiber, such as, for example, sulfides, lignon fragments, methanol and other negatively charged anions. Considerable amounts of resin are therefore necessary to treat a line containing secondary fiber and virgin fiber because the cationic resin reacts with the anionic byproducts. Accordingly, a need exists for a method for treating secondary fiber to achieve wet strength while retaining repulpability. A further need exists for a method for treating secondary fiber which requires reduced levels of resin treatment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0007]      FIG. 1  is a diagram of a system for fiber treatment in an embodiment of the present invention;  
         [0008]      FIG. 2  is a chart of reject comparison for products manufactured via a conventional method and products manufactured via at least one of the methods of the present invention; and  
         [0009]      FIG. 3  is a diagram of a system for fiber treatment in an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     The present invention provides a method for treating fiber to achieve wet strength while retaining repulpability and/or recyclability. In an embodiment, a paper-making process is provided. The process has a first flow line which contains secondary fiber in the form of, for example, old corrugated containerboard (“OCC”). Secondary fiber may be defined as fiber which has been dried at least once. In an embodiment, a portion of this line is separated into a second line and is treated with cationic resin. A third, and separate, line contains virgin fiber. Virgin fiber may be defined as a predominance of cellulosic fiber which has never been dried after a pulping process. The virgin fiber line is combined with the untreated secondary fiber in the first flow line. The treated portion is then recombined with the mixed product of the first line and the virgin fiber line. Products made from the combined flow lines demonstrate wet strength as well as sufficient repulpability. Moreover, separation of the virgin fiber from the secondary fiber provides the system with less cationic demand. Accordingly, less resin is required to treat the secondary fiber.  
         [0011]     Referring now to the drawings wherein like numerals refer to like parts,  FIG. 1  illustrates a system  2  which may be used to produce a base sheet having a first line  4  into which is fed secondary fiber in the form of, for example, untreated OCC from a supply or furnish  6 . A flow rate extending from the furnish  6  may be in a range from 2500 gpm to 4500 gpm. Moreover, the secondary fiber supplied may represent 10-40% of the total fiber in the system. At point  8 , line  4  may be split into separate lines wherein the line  4  is untreated and wherein the line  12  is treated with a cationic resin treatment at a point  14 . The resin may be provided from a supply  15 . A flow rate for the line  12  may be in a range from 500 gpm to 3000 gpm. Examples of resins which may be utilized are cationic polyamide-epichlorohydrin (PAE) resins, as well as cationic urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) condensation products. In an embodiment, the OCC and/or other secondary fiber which has been drawn off from line  4  is treated with, for example, KYMENE®. A mix time for the cationic treatment may be in a range from 30 seconds to 90 seconds.  
         [0012]     The treated secondary fiber travels along line  16  to a blend chest pump  18  at a flow rate in a range from 1500 gpm to 2000 gpm. Approximately 20-30% of the total flow exiting the blend chest pump  18  consists of treated secondary fiber. More specifically, the total flow exiting the blend chest pump  18  may include untreated secondary fiber and/or treated secondary fiber and/or virgin fiber. Of this total flow, 10-40% may be treated secondary fiber; 5-50% may be untreated secondary fiber; and 60-90% may be virgin fiber.  
         [0013]     A virgin fiber furnish  20  provides a line  22  of virgin fiber to the blend chest  10  at a flow rate in a range from 5400 gpm to 7500 gpm. More specifically, the virgin fiber supplied may represent 60-90% of the total fiber in the system. At the blend chest  10 , the virgin fiber may be mixed with the untreated secondary fiber flowing from the line  4 . The mix time for the virgin fiber and the untreated secondary fiber is in a range from 5 minutes to 20 minutes. Next, the combined virgin fiber and untreated secondary fiber is mixed with the treated secondary fiber line  16  at the blend chest pump  18 . A mix time for the combination of the lines  4 ,  12  and  22  is in a range from 1 minute to 3 minutes. The entire mixture may then be transferred to a system  24  for drying and/or pressing and/or other finishing activities.  
         [0014]     In an embodiment, the line  12  of secondary fiber which is treated may be supplied by an independent stream rather than split from the line  4 . In an embodiment, a furnish used to supply the line  12  may be different than a furnish used to supply the secondary fiber in the line  4 . The independent line may be treated with cationic resin prior to combination with the secondary fiber line  4  and the virgin fiber line  22  in a manner similar to that described above. Flow rates may be adjusted to create the system parameters outlined above. For example, the flow rate of the independent line may be adjusted wherein the treated secondary fiber accounts for 20-30% of the total fiber exiting the blend chest pump  18 . In another embodiment, a single line of secondary fiber may be supplied. This line may be treated with a cationic resin treatment and combined with virgin fiber. In this embodiment, the virgin fiber line may be combined with only treated secondary fiber.  
         [0015]     EXAMPLE 1, illustrated in  FIG. 3 , describes an embodiment of the present invention in which fiber was treated to provide a product having wet strength and adequate repulpability. More specifically, in the example below, the objective was to produce paper with wet strength, and normal repulpablility. To achieve this, 15% to 25% of the furnish was treated with a strong dose of wet strength resin. The treated portion gave the sheet 50% to 70% of the strength found in a normal wet strength sheet. The sheet was considered repulpable because only 20% of the sheet was treated with wet strength resin. It should be understood that, although EXAMPLE 1 describes an embodiment in which all of the secondary fiber is treated, this should not be construed to limit any embodiments in which a portion of the total amount of secondary fiber used is untreated.  
       EXAMPLE 1  
       [0016]     In this embodiment, top sheet wet strength was added to a top tickler pressure relief line  40  using AMRES®. A tank  41  provides a supply of virgin fiber for the top ply of product. In a first step, the air was bled from the pressure relief line  40  at a point  42 . This was performed by opening a pressure control valve  44  to 50% output. This is the pressure relief line  40  from the top tickler outlet  46 . Next, isolation valves  45  on each side of an automatic pressure relief valve  44  were opened.  
         [0017]     A 1.5″ flush valve  48  was opened on the pressure relief line  40  just above an entry point in the machine chest pump suction  50 . This was performed for a duration sufficient to bleed the air from a pressure recirculation line  52 . The isolation valve  45  from the top tickler pressure relief valve  44  was opened at the top machine chest pump suction  50 . A 250 to 300 gpm difference was established between the top basis weight flow and the top tickler flow. The valve  56  on the wet strength resin addition point  58  was opened. A 2#/ton wet strength addition was then established. The top tickler power was minimized as shear may reduce wet strength resin efficiency. The wet strength addition set point was increased to 6#/ton at a point in the process which was 2 reels before starting the order. Wet strength addition was adjusted to control test. The virgin fiber in this process was delivered to a blend chest  47 .  
         [0018]     Base sheet wet strength resin was added before the OCC refiner  60 . To this end, the total OCC flow from a tank  61  was set at 20% of the base basis weight flow (1600 to 1900 gpm). The OCC flow controller (not shown) was set to manual because the wet strength resin may negatively influence the flow indication. The flow indicator (not shown) from the OCC refiner  60  can be used for control. As shown in the FIGURE, treated secondary fiber and virgin fiber are mixed in a blend chest  65 . The base blend chest level set point was reduced to meet the residence time requirement in the chest because excessive mix time may reduce wet strength resin efficiency. The valve  62  on the wet strength resin addition point was then opened. A 2#/ton wet strength addition was then established.  
         [0019]     The wet strength addition set point was increased to 6#/ton at a point  2  reels before starting the order. Wet strength addition was adjusted to control test. The system was then flushed. To this end, the wet strength addition rate was reduced to 2#/ton. The suction valve (not shown) on the wet strength supply tank (not shown) was then closed. Next, the flush water valve (not shown) was opened for sufficient time to flush the system of resin. The wet strength pump (not shown) was stopped after the flush was complete. The isolation valves (not shown) at the base and top addition points were closed when the flush was complete.  
         [0020]      FIG. 2  illustrates a chart of a comparison of product rejects based on conventional methods of paper manufacturing and methods of the present invention. In the embodiments of the present invention, a portion of secondary fiber is treated with cationic resin prior to combination with virgin fiber. In the FIGURE, the square-shaped symbols represent a percentage of rejects for a set of rolls which were produced. The diamond-shaped symbols represent an amount of resin used per ton to treat the system. Each diamond-shaped symbol corresponds to each square-shaped symbol, as they represent a trial collectively. From the FIGURE, it can be seen that those products in which a portion of secondary fiber was treated prior to combination with virgin fiber provided less rejects. Thus, these embodiments demonstrated greater repulpability on average. Moreover, the products of the present invention required less resin, on average, in comparison to conventional products. This is due to the separation of the virgin fiber line from the secondary fiber line. This separation may prevent any possible reaction between the anionic byproduct associated with the virgin fiber and any cationic resin added to the system to treat the secondary fiber. For example, in conventional systems, a line combining secondary fiber and virgin fiber may have a charge of 0.3-3.0 meq/L. However, in the present invention, a secondary fiber line, prior to combination with the virgin fiber, may have a charge in a range from 0.1-1.0 meq/L. Accordingly, less resin is necessary to treat the secondary fiber.  
         [0021]     Table 1 shows data in a comparison between products prepared using conventional methods (denoted “WS”) and products prepared using at least one of the methods of the present invention (denoted Reels 1, 2 and 3).  
                                                                 TABLE 1                                   Unit   Reel 1   Reel 2   Reel 3   WS                                    Basis Weight   Lbs/MSF   57.0   56.6   56.8   56.1       Caliper   Points   15.6   15.2   15.5   15.0       Density   kg/m 3     705.5   719.5   705.4   722.3       Mullen   Lbs/In 2     120.9   124.2   112.4   127.4       Mullen Wet   Lbs/In 2     39.7   40.7   41.4   39.4       Repulpability - Rejects   %   5.5   4.7   6.2   28.6       STFI - CD   Lbs/In   33.9   36.6   35.7   31.5                  
 
         [0022]     As can be seen in the table, the method of the present invention enables wet strength grade products. Moreover, the present invention allows for greater repulpability, as evidenced by the considerably fewer percentage of rejects.  
         [0023]     While the embodiments of the invention have been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the embodiments. Instead, the invention should be determined entirely by reference to the claims that follow.