Abstract:
Improved compositions for coating paper stock, methods of forming such compositions, and methods of coating paper stock with those compositions are provided. The compositions comprise an aqueous dispersion including therein from about 20-45% by weight filler (e.g., a starch such as wheat starch) and from about 6-18% by weight wheat gluten. Preferably, the gluten is initially modified with a reducing agent so that the average molecular weight of the gluten is less than about 1,000 kDa. The compositions have a viscosity of up to about 2000 cP and a solids content of from about 25-57% by weight, thus making them suitable for high speed coating of paper stock. The finished, coated stock may be conventionally printed to achieve high gloss end products. The compositions are preferably water soluble, thereby greatly facilitating repulping of the coated stock.

Description:
RELATED APPLICATION  
       [0001]    This is a divisional application of Ser. No. 09/754,469 filed Jan. 3, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention is concerned with aqueous gluten-containing compositions useful for coating of paper stock, allowing the coated stock to be printed to obtain high quality, high gloss end products. More particularly, the invention is concerned with such compositions as well as methods of preparation and use thereof, where preferred compositions include reduced wheat gluten and wheat starch dispersions having relatively low viscosities up to about 2000 cP; these compositions are water soluble, so that the compositions and printing thereon may be easily removed from the paper stock by soaking in water, thereby facilitating repulping of the stock.  
           [0004]    2. Description of the Prior Art  
           [0005]    The paper and board industries are constantly searching for better ways to make coated paper and board with improved quality at reduced cost. Thus, coatings are commonly applied to paper stock in an attempt to make the surfaces of the paper stock conducive to printing using high speed web fed printing equipment by improving the smoothness, gloss, ink printing sharpness, drum adhesion and pick resistance of the stock. Known paper coating formulations include latex or other synthetic resin materials. The generally lower viscosity and water binding capabilities of these coatings allows applications of high solids coating layers onto paper stock. While these formulations do improve the surface properties of the paper stock, but they are very expensive and difficult to use. Moreover, latex/synthetic resin coatings are hard to remove, making de-inking and de-waxing of the stock very difficult; this in turn prevents effective repulping of the stock at a reasonable cost.  
           [0006]    Gelatinized, hydrolyzed, and other modified starches have also been used in prior art paper coating compositions. However, the viscosities of these starches are so high that the solids content of the formulations must be limited. This results in compositions which do not adequately coat the paper. Furthermore, the unpredictable behavior of starches such as corn starch, wheat starch, and potato starch typically leads to inconsistent coating properties, particularly at high coating speeds.  
           [0007]    Producers of consumer packaging products or other paper stock items requiring high quality, high gloss printing have almost without exception been forced to use relatively expensive grades of paper stock. Kraft stock is readily available and is much less expensive than other types of paper. However, it has heretofore been virtually impossible to use Kraft where high quality printing is needed, owing to the tendency of printing inks to absorb into and smear on Kraft.  
           [0008]    There is accordingly a real and heretofore unresolved need for improved compositions which can be used to form smooth, high quality coatings on paper products On the one hand a successful coating composition must permit sharp printing of ink images at high printing speeds using conventional printing equipment. On the other hand, such a coating must be relatively low cost and not present undue application problems. Finally, given the increasing concern about recycling of paper products, an improved coating must not interfere with, and preferably should enhance, the ability to repulp the coated paper products after use thereof.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention overcomes the problems outlined above, and provides coating compositions which can be used to coat paper stock of virtually any kind (and particularly relatively inexpensive Kraft stock), so that the coated stock can be printed using conventional equipment to obtain a final printed product of high quality with heretofore unobtainable gloss values. Moreover, the preferred compositions are water soluble to facilitate repulping of the coated and printed stock.  
           [0010]    In more detail, the coating compositions of the invention are in the form of aqueous dispersions including therein respective quantities of filler and wheat gluten. The filler is normally present in the dispersion at a level of from about 20-45% by weight, and more preferably from about 25-35% by weight, based upon the total weight of the dispersion taken as 100% by weight. The filler can comprise a mineral filler, a starch, or mixtures thereof.  
           [0011]    Preferred mineral fillers include clay (#1 and #2), calcium carbonate (ground or precipitated), talc, and mixtures thereof, while preferred starch fillers include wheat starch, corn starch, potato starch, rice starch, tapioca starch, modified versions of these starches (e.g., hydroxypropylated, acetylated, crosslinked, oxidized, cationized, acid-thinned starches), and mixtures thereof.  
           [0012]    The wheat gluten should be present in the dispersion at a level of from about 6-18% by weight, and more preferably from about 6-12% by weight, based upon the total weight of the dispersion taken as 100% by weight. The gluten may be derived from commercially available wheat glutens of varying grades.  
           [0013]    As used herein, “gluten” or “wheat gluten” refers to native and/or modified wheat glutens of various types. For example, wheat gluten may be modified by reducing agent(s) as hereafter described. However, other wheat gluten modifications, either in addition to or in lieu of reducing agent treatment can be used. Thus, wheat gluten may be oxidized, acylated, alkylated, deaminated or hydrolyzed (with a degree of protein hydrolysis usually less than 1%) or subjected to combined treatments.  
           [0014]    As indicated, the preferred wheat gluten is initially modified with a reducing agent so as to cleave at least some of the disulfide bonds therein (preferably at least about 5%, and more preferably from about 10-100% of the disulfide bonds) and reduce the average molecular weight of the gluten. Thus, the gluten utilized preferably has a weight average molecular weight of less than about 1,000 kDa, more preferably less than about 500 kDa, and most preferably from about 20-60 kDa.  
           [0015]    The gluten reducing agent is preferably added to the dispersion at a level of from about 0.05-2.0% by weight, and more preferably from about 0.1-1.0% by weight, based upon the total weight of the gluten taken as 100% by weight. Preferred reducing agents include alkali metal sulfites, alkali metal bisulfites, alkali metal metabisulfites, sulfur dioxide, mercaptan, and cysteine, with sodium metabisulfite being the most preferred reducing agent.  
           [0016]    The compositions should have a Brookfield viscosity (determined on an RVT model equipped with a #2 spindle; 100 rpm; 73-74° F.) of less than about 2000 cP, preferably less than about 500 cP, and more preferably from about 60-150 cP. Furthermore, the solids content of the dispersion is preferably from about 25-57% by weight, and more preferably from about 30-50% by weight, based upon the total weight of the dispersion taken as 100% by weight. Finally, the finished compositions should have a pH of from about 9-12, and more preferably from about 9.5-11.  
           [0017]    Preferably, the compositions have a weight ratio of filler:wheat gluten of from about 3:1 to about 5:1, and more preferably from about 3:1 to about 4:1. Normally, the preferred dispersions made up of wheat gluten and wheat starch are formulated using initially separate starch and gluten, i.e., they are not both derived from a single wheat flour or the like. In another embodiment, the preferred compositions consist essentially of aqueous dispersions including therein starch (and especially wheat starch), wheat gluten, a reducing agent, and a base (e.g., NaOH).  
           [0018]    The compositions may be formed by preparing an aqueous dispersion of water and gluten, and may also include from about 0.1-0.5% by weight of a defoamer (such as a silicone defoamer), based upon the total weight of all ingredients utilized taken as 100% by weight.  
           [0019]    Thereupon, a base such as NaOH is mixed with the dispersion in sufficient amounts to yield a dispersion pH of from about 10-12, and more preferably from about 11.5-11.7. The base is typically mixed with the dispersion at a level of from about 1-3% by weight, and preferably from about 1.5-2.5% by weight, based upon the total weight of the gluten utilized taken as 100% by weight. After base addition, the reducing agent is mixed with the dispersion so as to cleave disulfide bonds in the gluten.  
           [0020]    Next, a starch such as Midsol 50 wheat starch (available from Midwest Grain Products, Atchison, Kans.) is added to the gluten dispersion in the ratio of 3:1 to about 5:1 as a filler. Final pH of dispersion at this stage is in a range of 9.0-12.0, but preferable from 10.0 to 10.8.  
           [0021]    In alternate forms, a dry mixture of wheat gluten, starch and a reducing agent (e.g., sodium metabisulfite) can be provided. As needed, this dry mixture may be processed by the addition of water in a vacuum dissolver so as to simultaneously reduce the gluten and provide the necessary mixing to create the use dispersion. The latter may be used directly by application to paper stock, or can be stored for future use. In such an embodiment, the starch:gluten ratio should be from about 3:1 to 5:1. The gluten is normally present at a level of from about 16-25% by weight of the dry composition, the starch is present at a level of from about 75-85% by weight of the dry composition, and the reducing agent is present at a level of from about 0.05-1% by weight of the wheat gluten. In the mixing procedure, an appropriate amount of the dry composition is added to water to achieve the above-described amounts of ingredients in a flowable coating composition.  
           [0022]    The compositions hereof can be applied to paper stock (as used herein, paper stock is intended to include all forms, types and weights of paper such as Kraft stock). Conventional coating equipment can be employed for this purpose. Normally, in order to achieve the best quality coated stock, the compositions are applied as separate coatings or “bumps”, with intermediate partial or complete drying and curing of the compositions. The final coated stock can be printed using normal web-fed printing equipment and conventional inks. A particular feature of the coated stock of the invention is that the final printing can achieve very high gloss values on the order of from about 50-65 units. Furthermore, the coated stock will give a passing result when subjected to Testing Association for the Pulp and Paper Industry (TAPPI) Useful Method 557 as described herein (also known as the “3M” test).  
           [0023]    Another feature of the invention is that the coated stock may be readily repulped. That is, the compositions of the invention are preferably water soluble, so that when the coated/printed stock is placed in water the coating and ink layers come off as a skimmable layer, leaving the stock ready for conventional repulping. Thus, previously hard to repulp products such as polyether wax treated poultry boxes can be easily repulped if the paper stock is first coated with compositions in accordance with the invention prior to application of wax and printing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a schematic illustration of the preferred equipment for coating paper with the inventive coating composition;  
         [0025]    [0025]FIG. 2 is an end view of the coating storage and circulation systems depicted in FIG. 1; and  
         [0026]    [0026]FIG. 3 is a schematic illustration of a modified coating apparatus making use of a doctor blade. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Turning now to the drawing, a coating assembly  10  is depicted in FIG. 1. Assembly  10  includes first and second coating stations  12 ,  14 , a web handling assembly  16  and drying station  18 . The stations  12 ,  14  share a coating composition circulating assembly  20 .  
         [0028]    In more detail, the first coating station  12  includes an upper rubber coated cylinder  22  and an adjacent, lower engraved coating cylinder  24 ; as shown, the cylinders  22 ,  24  cooperatively present a coating nip  26 . The overall station further includes an elongated, triangular in cross-section trough or pan  28  for holding a recirculating supply of coating composition  30 . The cylinder  24  is situated within pan  28  below the normal level of composition  30  therein. The roll  24  is preferably provided with a helical surface groove (250 coils per inch of cylinder length and approximately 40 μm deep).  
         [0029]    As illustrated in FIG. 2, the pan  28  includes three spaced apart lower composition outlets  32   a,    32   b,    32   c  and four spaced apart upper inlets  34   a,    34   b,    34   c,    34   d.  Valve-controlled inlet lines  36   a,    36   b,    36   c,    36   d  are operatively connected to the respective inlets and to a supply  38  of the coating composition  30  via line  37 . A return line  40  extends from the outlets  32   a - 32   c  to supply  38 .  
         [0030]    The coating station  14  is very similar to station  12 , and includes rubber coated upper cylinder  42 , engraved lower cylinder  44 , and pan  46  adapted to hold the composition  30 . The cylinders  42 , 44  cooperatively define a coating nip  47  as shown. These components are identical with their counterparts in station  12 , and thus need not be further described. However, the station  14  includes a 10 mm diameter smooth coating rod  48  adjacent upper rubber cylinder  42  and defining therewith another nip  49 . It will also be seen that a return line  50  extends from supply  38  and is operatively coupled with the three lower outlets provided with pan  46 , and that a supply line  51  extends from the supply  38  to the pan inlets. Thus, the circulation assembly  20  is made up of the supply  38  and the respective supply and return lines leading to the pans  28 ,  46 . Although not shown, it will be appreciated that appropriate pump(s) are interposed within the supply lines for delivery of composition  30  to the individual pans.  
         [0031]    The web handling assembly  16  is designed to guide and transfer a continuous web  52  into and through the coating stations  12 ,  14 , and ultimately through drying station  18  for downstream processing. The assembly  16  includes, adjacent station  12 , spaced apart guide rollers  54 ,  56  serving to guide the web  52  into and through nip  26 . Downstream of the station  12 , the assembly  16  includes a large heated roll  58  and spaced conveyor rollers  60 ,  62 . At the region of station  14 , the assembly  16  includes guide rolls  64 ,  66  serving to direct the web  52  through the coating nip  47 . Finally, the assembly  16  includes one or more downstream conveyor rolls  68  serving to guide the web  52  through secondary nip  49  and drying station  18 .  
         [0032]    The drying station  18  includes one or more fans  70  as well as downstream dryer drums (not shown) preferably heated to a temperature of from about 240-300° F. Of course other types of drying apparatus can be used in lieu of that illustrated, so long as the composition  30  applied to the web  52  is sufficiently dried and cured.  
         [0033]    Referring to FIG. 3, a preferred alternate coating embodiment is illustrated, making use of a doctor blade  72  in contact with coating cylinder  24 . In some instances, use of such a blade  72  provides improved coating performance. A doctor blade could be used in either or both of the coating stations  12 ,  14 . Moreover, the blade  72  may be placed in a leading relationship as shown in FIG. 3, or in a trailing relationship where the blade is oppositely oriented against the cylinder  24 .  
         [0034]    In use, a web  52  of paper stock is trained through the assembly  10  as illustrated in FIG. 1. Thus, the web passes in serial order through the coating nips  26  and  47 , and through secondary nip  49  during processing. As will be appreciated, the stations  12 ,  14  are operated during passage of the web  52  therethrough, with the cylinders  22 ,  24  and  42 ,  44  being continuously rotated. The helical grooves formed in the surfaces of the cylinders  24  and  44  “catch” portions of the coating composition  30  within the pans  28  and  46  and transfer such composition onto the surface of web  52 . Between the stations  12  and  14 , the composition  30  on the web  52  is at least partially cured and dried by passage around heated roller  58 .  
         [0035]    It will also be appreciated that the composition  30  is continuously circulated through the stations  12  and  14  by circulating assembly  20 . It is preferred that the composition  30  is flowing or moving at all times. Moreover, the flow of composition  30  is adjusted so that the amount entering each pan  28 ,  46  through the inlets  34   a - 34   d  is slightly larger than the quantity of the composition which exists through the outlets  32   a - 32   c.  Thus, there is an excess amount of composition  30  entering each pan, as compared to the amount exiting the pan, with that excess amount being approximately equal to the amount of composition  30  which is applied to the web  52  in the respective coating stations. It has also been found preferable that the outlets  32   a - 32   c  and inlets  34   a - 34   d  be substantially uniformly spaced along the length of the coating cylinders  24  and  44 , so as to provide a substantially even distribution of the composition  30  on the cylinders.  
         [0036]    A first “bump” of coating composition  30  is applied to the web  52  in the station  12 , and this “bump” is at least partially dried during passage around roller  58 . A second “bump” of composition  30  is applied in station  14  with the latter being smoothed during passage through nip  49  by coating rod  48 .  
         [0037]    The fully coated and cured web  52  can be immediately printed using standard web-fed printing equipment and conventional inks. This can be done in-line, i.e., the coated web  52  is fed directly to the printing apparatus. Alternately, the web  52  can be rolled up for storage and later use.  
       EXAMPLES  
       [0038]    The following examples set forth preferred methods in accordance with the invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.  
       Example 1  
     Preparation of a Coating Composition  
       [0039]    First, 740 parts by weight of H 2 O (ambient temperature and a pH of 8.3) and 0.1% by weight of a silicone defoamer (based upon the total weight of all ingredients used taken as 100% by weight; sold under the name PI-135 and obtained from INX International Ink Company) were placed in a Cowles dissolver having a peripheral speed of around 600 rpm. Mixing was commenced, and 100 parts by weight of FP3000 (a wheat gluten product containing approximately 90% protein and available from Midwest Grain Products, Inc., Atchison, Kans.) was gently and incrementally added to the mixer, verifying that all of the protein was wet with the water/defoamer solution. Mixing was carried out for about 5 minutes. The pH of the mixture at this point was about 5.  
         [0040]    Next, 17 parts NaOH (10% solution) was slowly added to the mixture while mixing to give a pH of about 11.6 Mixing was continued for 5 minutes.  
         [0041]    The mixture was quite thin at this stage. It was allowed to degas for about 30 minutes. Then, sodium metabisulfite (0.6% by weight, based upon the total weight of the FP3000 taken as 100% by weight) was added dry to the mixture under low rpm mixing for about 30 minutes. After mixing, 375 parts by weight of Midsol 50 (a wheat starch available from Midwest Grain Products) was added slowly over the course of about 7-8 minutes.  
         [0042]    The final coating composition had a viscosity of 80 cP on a #2 spindle at 100 rpm and 73-74° F. The composition was bright tan in color, had a pH of 10.3, and a solids content of about 35% by weight.  
       Example 2  
     Properties of Coating  
       [0043]    The composition prepared in Example 1 was applied to the finished side of 42 Kraft liner following the process illustrated in FIGS. 1 and 2 and described above. Coated samples were printed upon with black, red, blue, green and yellow ink according to conventional printing methods. The glosses of the samples were then determined using an Horiba IG-320 gloss checker with a 60° optical path. The gloss values ranged from 50 (for blacks) to as high as 60 and 65 (for reds and yellows, respectively).  
         [0044]    Testing Association for the Pulp and Paper Industry (TAPPI) Useful Method 557, (also known as the 3M test) was used to determine the quality of the coating. In this test, solutions from various “kits” (see Table 1) were applied to the coating and allowed to remain thereon for as described in Useful Method 557. The penetration of the solution into the coating and paper was observed. If no penetration occurred for kits 1-5, then the coating is considered to have passed. Each of the samples coated with the inventive coating passed this test.  
                                                         TABLE 1                                   Kit #   Castor Oil (mL)   Toluene (mL)   Heptane (mL)                                        1   200   0   0           2   180   10   10           3   160   20   20           4   140   30   30                5 a     120   40   40           6   100   50   50           7   80   60   60           8   60   70   70           9   20   80   80           10    20   90   90           11    0   100   100           12    0   90   110                                  
 
       Example 3  
       [0045]    One hundred parts of FP 3000 (a wheat gluten supplied by Midwest Grain Products, Inc. Atchison, Kans.), 375 parts Midsol 50 (a granular wheat starch supplied by Midwest Grain Products, Inc., Atchison, Kans.) and 0.5 parts sodium metabisulfite are mixed together in a batch mixer. The mixture is transferred to a Vacuum dispersion unit (VacuShear manufactured by Admix) equipped with a Rotosolver mixing head and additional mixing blades under 20 in” Hg vacuum. The VacuShear unit contains 700 parts water and 0.1% by weight of a silicone defoamer (based upon the total weight of all ingredients used taken as 100% by weight; sold under the name PI-135 and obtained from INX International Ink Company). The mixing speed is set at 1750 rpm. The transferring process takes about 3 minutes. The vacuum is allowed to increase to near full vacuum and about 17 parts of 10% NaOH solution is introduced quickly through a tube immersed under the dispersion media at a mixing speed of 2500 rpm. The dispersion goes through a gel phase and breaks loose to form a flowable dispersion. The whole process takes about 8-10 minutes. The temperature of the dispersion is around 76° F. The dispersion has a Brookfield viscosity of 180 cp (#2 spindle at 100 rpm) at a solids content of about 37% and a pH of 10.4. The dispersion is ready for use after release of the vacuum.