SK82499A3 - Lumen loading of mineral filler into cellulose fibers for papermaking - Google Patents

Abstract

A method of introducing and retaining particles of mineral filler inside the lumens of wood pulp fibers is disclosed. The fillers are mixed with a high-consistency pulp slurry so that a portion of the total filler diffuses inside the lumens. The loaded fiber is treated with a chemical flocculating agent that exhibits a higher chemical affinity for the filler than for the fiber. The flocculated filler particles are prevented from diffusing outside the lumens. At equal filler loading levels, paper formed according to the lumen loading process of this invention exhibits superior mechanical strength than paper prepared in a conventional manner.

Description

The present invention relates to a paper filled with a mineral filler and a process for preparing the same, wherein the filler is dispersed in the pulp fibers, wherein the chemical flocculant is added to the furnish to bind the filler within the lumens. By means of the process of the present invention, the filler is kept inside the lumens even under the conditions of dilution and rapid feed during paper production.

BACKGROUND OF THE INVENTION

Any fibrous raw material such as wood, straw, bamboo, hemp, molasses, sisal (Mexican hemp), flax, cotton, jute and ramia (Chinese grass) can be used in the papermaking process. The separation of fibers of such materials is called spinning. Separate fibers that are used to make paper are called pulp. As wood is abundant and readily available, the paper mainly consists of wood pulp fibers and insoluble particulate mineral fillers. Most fillers are considerably cheaper than fibers and are therefore added during paper production to reduce the overall cost of paper production. Fillers are also added to the pulp to impart certain mechanical properties to the paper, such as bulk density and stiffness, as well as optical properties, such as whiteness and opacity. Mineral fillers in paper manufacture are typically kaolin clays, chalk, talc, titanium dioxide, calcium carbonates including precipitated calcium carbonate (PCC) and ground calcium carbonate (GCC).

During the last decade, precipitated calcium carbonate (PCC) has been extensively used as a mineral paper filler. One of the main drivers of its success is the versatility of PCC. For example, PCC may be prepared in the form of particles of various sizes and crystal shapes or morphologies. In general, PCC exhibits high whiteness resulting in high whiteness of the sheet of paper. Some of the morphological types of PCCs are particularly effective in increasing the opacity of paper that

236 / B is desirable when printed on both sides of the paper. Other morphological types of PCCs are effective in controlling the thickness (thickness in thousandths of an inch) of paper that is related to the stiffness of the paper. PCC may also affect the porosity of the paper, which may affect the ability of certain pneumatic devices to physically handle the paper during paper manufacture. There are also other advantages of using PCC in paper making.

In general, the material cost of producing paper decreases directly in proportion to the increasing amount of mineral filler in the filled paper. Maximizing the mineral content of paper is therefore a major goal for paper manufacturers. As the filler content increases, there is also a tendency to improve the optical properties of the filled paper, such as whiteness and opacity. However, these advantages are overlapped by a reduction in the mechanical strength that generally occurs with increasing filler levels in the paper. The mechanical strength of the paper is derived from the bond between adjacent pulp fibers. Fillers that are dispersed within the pulp fibers disrupt the close contact that allows binding between adjacent pulp fibers. The greater the amount of filler in the paper, the greater the tendency of fiber to break in the fiber bonds, resulting in lower paper strength.

Several methods are available to solve the problem of reduced strength due to increased filler content. Some of these processes involve chemically modifying the surface of the filler with agents typically used as retention aids. These agents are generally polyelectron synthetic polymers which function mainly by the flocculation mechanism. The most common are polyamines and polyacrylamides.

Supplementary binders are also well known as means of improving the strength of filler-containing papers. Such binders may be natural or synthetic polymeric substances which are added in the wet portion of the paper machine to improve the mechanical strength of the dried paper. These materials are usually starches obtained from various plants such as corn, tapioca, potato and wheat, or gums derived from the seeds of acacia or guar (Cyamopsis tetragonolobus). Before such gums and starches are used, they are "cooked" or otherwise hydrated to promote swelling. Gum and starch are usually also used in the manufacture

236 / B paper as surface coatings to improve paper size (i.e., water penetration resistance).

Another means of improving the strength of the filled papers is a process called lumen filling. This procedure involves placing the fillers directly inside the hollow spaces (i.e., lumens) of the pulp fibers. Fillers that rest within the lumens of the pulp fibers do not interfere with the bonding between the fibers and therefore have a less negative impact on the mechanical strength of the filled papers. The lumen can be filled by simple mechanical mixing. However, as fillers pass into these lumens, they can also escape by diffusion from them. Using known processes, filling lumens and diffusion from lumens filled with filler during the paper making process continues to be a problem for paper manufacturers. Until now, a new product and method of filling the pulp fiber lumen has been considered novel and unexpected in the art, in which the lumens can be filled with a considerable amount of mineral filler while minimizing the reduction in paper strength.

It is therefore an object of the present invention to provide a process by which the mineral content of paper is increased. Another object of the present invention is to reduce the cost of the paper making process by reducing the amount of pulp necessary to obtain paper of the same quality by increasing the mineral filler content. Another object of the present invention is to increase the level of mineral paper filler so that substantially less filler is lost in the effluent, also known as "white water", during paper formation on a paper machine. The ultimate goal is to produce filled paper that has no drawbacks in its mechanical and / or optical properties.

Related areas

U.S. Pat. No. 5,223,990 discloses a method of filling cellulosic fibers with a calcium carbonate filler by reacting calcium hydroxide with carbon dioxide in the presence of cellulosic fibrous material.

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U.S. Pat. No. 5,096,539 discloses a process for preparing filled paper, wherein the filler comprises an insoluble precipitate that precipitates in situ within the fiber cell wall.

U.S. Pat. No. 4,510,020 discloses a process for preparing filled paper, wherein substantially all of the filler is present within the cellulosic fiber lumens.

None of the references from related areas discloses, individually or in combination, the present new product or the process for increasing the filler content of the paper.

SUMMARY OF THE INVENTION

The present invention provides a process for adding a mineral filler to wood pulp in the papermaking process, wherein a chemical additive is used to flocculate the filler particles within the fiber lumens. The filler is held within the fiber lumens under the conditions of dilution and rapid feed during paper production. The present invention further provides a product having an increased filler content and acceptable optical properties, while maintaining physical properties such as mechanical strength.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for producing filled paper by filling the pulp lumens with a mineral filler. The filler is added to the papermaking process at a point where the pulp concentration is usually from about 1.0 to 5.0 weight percent. After the subsequent addition of the chemical flocculant after the filler addition point, the filler is fixed and remains inside the lumens during paper production. Alternatively, additional filler may be added during the papermaking process at the next addition point to achieve fine control of the filler content in the resulting paper. By using the process of the present invention, the lumens of the pulp fibers are filled with the filler and the filler is protected from the drainage forces that normally cause the filler to be extruded during paper production. According to the process of the present invention, the cellulosic lumens are filled with a filler by mixing or shaking the cellulosic pulp, filler and flocculant at the production process point

236 / B paper in which the pulp concentration is from about 1.0 to 5.0 weight percent. To obtain the desired furnish density, which is typically from about 0.3 to about 1.0 percent, preferably about 0.5 percent, dilution of the filler / pulp mixture may be necessary before it enters the papermaking machine.

An inseparable aspect of the process of the present invention is the paper pulp structure. The strength of the paper is highly dependent on the pulp fibers used to make the paper and which become intensely bonded to each other during the manufacture of the paper. The most widely used fibers are wood-derived fibers, most of which appear to be long hollow tubes under microscope when pulled loose, uniform in size for most lengths, but tapered at each end. Along the length of the fiber, the fiber wall is perforated by small apertures (pores) that connect the central cavity (lumen) to the outer fiber. The process of placing a mineral filler in these lumens is called lumen filling and is applicable to a wide range of paper making fibers.

The present process can be carried out with fibers obtained from many types of wood, by any of the conventional pulping and bleaching processes. The fiber may be a hardwood, softwood or a mixture of both hardwood and softwood fibers. However, in order to maximize lumen loading with the filler, the pulp should enter the process of the present invention in a "never-dried" form. Drying of the pulp causes irreversible collapse of a large portion of the lumens, making them unavailable for the mineral filler. The reconstituted dried cellulose suspension has less susceptibility to take full advantage of this novel lumen filling process of the present invention, but can be used provided that this limitation is fully understood.

The concentration or density of the pulp suspension is preferably in the range of from about one percent to about five percent calculated based on the total weight of the suspension. In a preferred embodiment of the invention, the density of the pulp suspension is preferably in the range of about

236 / B two percent to about four percent calculated based on the total weight of the suspension.

As mineral fillers, kaolin clays, chalk, talc, titanium dioxide, alumina, silicon dioxide, precipitated calcium carbonate (PCC) and ground calcium carbonate (GCC) are commonly used in paper manufacture. The filler consists of fine particles of insoluble solid, usually of mineral origin. Due to the large surface area and sometimes high refractive index, paper fillers give improved optical properties such as opacity and whiteness. Improving the optical properties of the mineral filled paper is the primary goal of adding fillers to the paper furnish, although the paper can also gain other benefits such as improved smoothness, improved printability, and improved durability.

Although kaolin clays, chalk, talc, titanium dioxide, alumina, silica, PCC, GCC and the like may be used in the process of the present invention, the preferred filler is precipitated calcium carbonate (PCC). The PCC has an average particle size ranging from about 0.5 to about 2.0 microns (gm). In a preferred embodiment of the present invention, the average particle size ranges from about 0.7 to about 1.4 µm. PCC morphology may be orthorhombic, rombic or scalenohedral. Other fillers such as ultrafine ground limestone (UFGL) may also be used in the process of the present invention.

According to one part of the present invention, the aqueous mineral filler suspension is mixed, by any known mechanical means, into the aqueous wood pulp fiber suspension. To fill the lumens, the pulp may be in a state without prior drying to obtain the maximum filling benefits. The filler is added to the pulp in the form of an aqueous suspension such that the filler solids, when mixed with the pulp, represent from about 5 weight percent to about 80 weight percent, calculated on the basis of total solids. In a preferred embodiment of the present invention, the filler solids are from about 20 weight percent to about 40 weight percent, calculated on the basis of total solids.

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The filler and fiber components are mixed while shaking for a period of time that allows the maximum lumen loading. It was found that a minimum of five minute shaking was required to obtain maximum loading. Shorter mixing times do not provide the maximum benefits of lumen loading. In a preferred embodiment of the invention, the mixing or shaking period may range from about 10 minutes to about 30 minutes. There is no limit to the upper mixing time limit, except to the limitations of practicality and economy. During the period when the fiber and filler components are mixed, part of the filler is dispersed within the fiber lumens. This occurs through mechanical diffusion.

Certain factors significantly affect the amount of filler that can actually be loaded into the fiber lumens. One such factor is the average particle size of the mineral filler. Fillers having smaller average particle sizes will be easier to diffuse into lumens than fillers with larger particle sizes. In addition, the relative fiber and filler concentrations that are present at the time the two components are mixed will also determine the lumen loading range. Finally, the length of time during which the fiber and filler components are mixed will affect the extent to which the lumens in the paper furnish are filled.

According to the present invention, the amount of filler that diffuses into the lumens by diffusion is usually from about 0.5 percent to about 10 percent of the combined weight of pulp and filler. In a preferred embodiment of the invention, the filler present is from about 3 percent to about 6 percent of the total weight of the fiber and filler.

Another indivisible aspect of the lumen filling process of the present invention requires that the chemical flocculant be added to the pulp and filler mixture after the pulp and filler components have been sufficiently mixed. The use of such a flocculant will cause any filler that resides within the fiber lumens to form clusters of a size sufficient to prevent them from diffusing out of the lumens during papermaking. This also causes the filler located outside the lumens to also form clumps. Cluster formation reduces the tendency

The 236 / B interference of the filler particles with interfiber binding, fixes and holds the filler inside and outside the lumens, preventing the filler from being extruded during paper production.

The chemical flocculants may be selected from the group of materials that are glucose polymers derived from plants and are called starches. Plants from which starches are mainly obtained are maize, tapioca, potato and wheat. Before being used as chemical flocculants in the present product, the starch is "cooked" in a manner consistent with the manufacturer's recommendations to promote its dissolution and swelling. This is an important aspect of the present invention since the formation of clumps, both inside and outside the lumens, results in maximum lumen filling.

The starches may be used in their natural state, or may be chemically modified by various techniques. Chemical modification can cause starch to become cationic, anionic or amphoteric. In the manufacture of this product and according to the process of the present invention, the starch is preferably an amphoteric waxy corn starch.

Other chemical agents may also be used to flocculate the lumen-filling filler. The flocculating agent is selected from those materials which exhibit a stronger chemical affinity for PCC than for wood pulp. Chemical affinity is measured by volume calorimetry. When a chemical agent comes into contact with a mineral surface, the chemical agent adsorbs onto the mineral surface only if there is sufficient affinity between the two materials. Adsorption is an exothermic process in which heat is released. Adsorption can also occur when the chemical agent comes into contact with wood pulp fibers. The extent of the interaction between the flocculating agent and the mineral filler is measured to determine which flocculating agent provides the maximum benefits. Separate measurements were performed to determine the extent of the interaction between the flocculating agent and microcrystalline cellulose, which is chemically similar to wood pulp.

In a preferred embodiment of the present invention, the interaction intensity between flocculant and mineral filler is expressed in calories, and is at least equal to the extent of interaction between flocculant and cellulose for the same

236 / B parts materials. More preferably, the interaction intensity between the flocculant and the mineral filler is at least 50 percent greater than the interaction between the flocculant and the cellulose. Most preferably, the interaction between flocculant and cellulose is zero (0). The present invention is more advantageous than other lumen filling methods in that it provides a method of preventing the diffusion of filler particles out of the fiber compounds without the need for a drying step. The result is a reduction in energy costs and an increase in the profit margin for the paper manufacturer.

In a further aspect of the invention, filler and chemical flocculant suspensions are formed which are first mixed, preferably from about 10 to about 30 minutes, and then the mixture is added to the aqueous wood pulp suspension.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a view of how this process of the present invention could be used in a typical paper making process. The addition of filler is distributed such that the filler can be added to the furnace in the mixed vat and again in the headbox. Between these two filler addition sites, but after the first addition site, a chemical flocculant is added to the already mixed fiber and filler components to fix and maintain the filler at the point where it rests inside and outside the fiber lumens.

DETAILED DESCRIPTION OF THE INVENTION

The following examples are presented to better illustrate this new process and novel product of the present invention. These examples should be regarded as non-limiting illustrations of the present invention and should in no way serve to limit the scope of the present invention, except as set forth in the appended claims.

The following examples describe one or more processes which include adding a mineral filler to a high density wood pulp suspension. After mixing the filler and pulp, a chemical flocculant is added to agglomerate the filler particles that have been diffused inside the fiber lumens and thereby prevent them from diffusing out of the lumens. The flocculating agent is selected from

236 / B those chemicals which exhibit greater chemical affinity for the filler than for cellulose fiber. Chemical affinity was quantified by volume calorimetry. In the latter case, the mineral filler and the chemical flocculant are mixed and then added to the high density wood pulp suspension. The resulting blend is formed into paper that exhibits good mechanical strength at high filler contents.

Example 1

A 4 liter glass reactor was charged with 200 milliliters (ml) of previously undried hardwood pulp at a density of 4% by weight (solids). The pulp was blended at 300 rpm using a large flat blade rotor to gently swirl. A scalenohedral calcium carbonate slurry containing 20 weight percent solids was added to the blending hardwood pulp in an amount such that the dry calcium carbonate content was 40 weight percent of the total solids of the composition. The aqueous suspension of calcium carbonate and pulp was stirred for an additional 30 minutes. To an aqueous suspension of calcium carbonate and pulp was added amphoteric waxy corn starch (CATO 225 from National Starch Corporation) containing 1 weight percent solids, which was boiled at 200 ° F for 30 minutes. This amount of starch corresponds to 4.0 weight percent calculated on the basis of dry calcium carbonate. The aqueous suspension of calcium carbonate, pulp and starch was stirred for 30 minutes. The mixture was made of paper on a stainless steel sieve using a Formax machine. The basis weight of the paper was approximately 170 grams per square meter (g / m ² ). The paper was compressed between felts at 25 psi and dried in a roller drier at 265 ° C. This dried paper represents a commercial dry hardwood pulp sheet that contains calcium carbonate and that has been combined with a dry folded softwood sheet in the desired proportions. The percentage of calcium carbonate in the folded sheet of hardwood pulp was measured to definitively determine the amount of dry hardwood present. Hardwood-containing papers were refined by mixing 360 grams of hardwood paper (folded sheets)

236 / B hardwood pulp) and 24 liters of water and mixing the mixture in a Valley beater grinder to a Canadian Standard freeness (CSF) 360 degree of grinding. The mixture was prepared by combining 360 grams of untreated softwood pulp and 24 liters of water. This mixture was refined by stirring in a Valley Beater grinder at 400 CSF. Refined hardwood and refined softwood were combined so that the resulting mixture contained 60 percent hardwood and 40 percent softwood. Water was added to the mixture to adjust the solids content to 0.3 weight percent calculated on a dry pulp basis.

Handmade papers (60 g / m ² ) were prepared using a Formax sheet former (Noble and Wood type, from Adirondack Machine Corp.) from a furnish consisting of 60 percent hardwood (containing lime calcium carbonate) and 40 percent bleached softwood pulp ground to 400 CFS at pH 7 in distilled water. The pulp density was 0.3 percent. A retention aid (high molecular weight cationic polyacrylamide) Percol® from Allied Colloids was added to the pulp furnish in an amount equal to 0.05 percent (1 pound / ton of paper). Synthetic sizing agent (alkyl ketene dimer) was added to the pulp furnace in an amount of 0.25 percent (5 pounds / ton of paper). Paper was prepared from the mixture on a stainless steel mesh using a Formax machine. The sheet was squeezed between felt at 25 psi and dried in a roller drier at 265 ° C. Prior to testing, the paper was conditioned to 50 percent relative humidity and 23 ° C.

The paper strength was determined by testing the paper length (TAPPI T-494 OM-88 test procedure) and Scott binding (TAPPI test method UM-403 test procedure). Paper brightness was measured according to TAPPI test method T452-OM-92 and paper opacity was measured according to TAPPI test method T452-OM-91 (corrected to a basis weight of 60.5 grams / m ² ).

The strength of the paper using the calcium carbonate present in the lumens in accordance with the present invention was greater than that of the control paper (which did not have lumen loaded but had an equivalent amount of filler that was added in a traditional manner).

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The present invention (Unfilled lumen) (Filled lumen)

Filler weight (percent) 24 24 Market length (meters) 2127 3128 Scott bond (pounds in thousands) 34.2 53.6 TAPPI whiteness (percentage) 89.3 89 TAPPI Opacity (Percentage) 87.6 87.6

Example 2

A mixture of 270 grams of hardwood pulp and 90 grams of softwood pulp (75 percent hardwood and 25 percent softwood) was combined with 24 liters of water and refined in a Valley Beater mill to 400 CSF. The pulp density was 1.5 percent. Water was removed by dewatering the pulp through a 100 mesh screen to increase the pulp density to 3.8 percent. The water containing the fines was set aside for later use. A 3.8 percent density was used to simulate pulp solids in a pulp and paper combination. In the pulp and paper production combination, the pulp itself is prepared, which is in contrast to obtaining pulp from the free market.

A 4 liter glass reactor was charged with 1000 ml of the previously described pulp at a density of 3.8 percent. The pulp was blended at 100 rpm using a large flat blade rotor to gently swirl. A scalenohedral calcium carbonate slurry containing 20 weight percent solids was added to the mixed hard and softwood pulp in an amount such that the dry calcium carbonate content was 15 weight percent total solids in the mixture. The aqueous suspension of calcium carbonate and pulp was stirred for 10 minutes. Cationic potato starch (STA-LOK® 400 from A.E. Staley Manufacturing Company) containing 1 weight percent solids was added to the aqueous suspension of calcium carbonate and pulp,

236 / B which was boiled at 200 ° F for 30 minutes. This amount of starch corresponds to 4.0 weight percent calculated on the basis of dry calcium carbonate. The aqueous suspension of calcium carbonate, pulp and starch was stirred for 10 minutes. The mixture was then diluted with 1622 ml of water containing a fine pulp content to return the entire fine fraction which was removed during the paper forming process on the screen. This mixture was further diluted with 9500 mL of water to adjust the final density to about 0.3 percent.

The handmade papers were prepared from the above described calcium carbonate-containing hard and soft wood pulp and calcium-carbonate-free hard and soft wood in the lumen. The handmade papers were prepared and tested as described in Example 1. The Hercules size test (HST) was used to measure fluid penetration through handmade sheets. This test was performed on a Hercules sizing tester, model KA or KC, and the test method is the TAPPI test method T-530 PM-89 (revised in 1989).

Paper strength, i. the tearing length and Scott bond of the paper containing the lumen filled with calcium carbonate in accordance with the present invention is greater than the strength of the control paper (which did not have the filled lumen). Paper containing lumen filled with calcium carbonate also showed improved sizing.

The present invention (Unfilled lumen) (Filled lumen)

Filler weight (percent) 16 16 Market length (meters) 2970 3350 Scott bond (pounds in thousands) 49.2 58.2 HST size (%) 23 103 TAPPI whiteness (percentage) 88.1 88.2 TAPPI Opacity (Percentage) 87.5 88

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Claims (17)

A method of producing mineral filled paper comprising mixing an aqueous mineral filler suspension with an aqueous wood pulp fiber suspension and adding a chemical flocculant to the papermaking furnish to produce paper having a substantial amount of filler within the cellulosic fiber lumens. The method of claim 1, wherein the furnish is substantially diluted to from about 0.3 weight percent to about 1.0 weight percent. The process according to claim 1 or 2, wherein the mineral filler is calcium carbonate. The method of claim 3, wherein the calcium carbonate is precipitated calcium carbonate. The method of any one of the preceding claims, wherein the concentration of wood pulp is from about 1.0 weight percent to about 5.0 weight percent calculated on the basis of the total weight of the suspension. The method of claim 5, wherein the concentration of wood pulp is from about 2.0 weight percent to about 4.0 weight percent, based on the total weight of the suspension. The method of any one of the preceding claims, wherein said mineral filler is a precipitated calcium carbonate having an average particle size of from about 0.5 micrometer to about 2.0 micrometer. The method of claim 7, wherein the average particle size of the precipitated calcium carbonate is from about 0.7 micrometer to about 1.4 micrometer. The method of any one of the preceding claims, wherein said mineral filler is calcium carbonate and said calcium carbonate represents from about 5 weight percent to about 80 weight percent of the total solids of the furnish. 31,236 / B The method of any one of the preceding claims, wherein said mineral filler is calcium carbonate and said calcium carbonate represents from about 20 weight percent to about 40 weight percent of the total solids of the furnish. The method of any one of the preceding claims, wherein the aqueous mineral filler suspension and the aqueous pulp suspension are mixed from about 10 to about 30 minutes. The method of any preceding claim, wherein the flocculating chemical is selected from the group of starches consisting of corn, tapioca, potato and wheat starch. The method of any one of the preceding claims, wherein the flocculating agent is a cationic, anionic or amphoteric starch. The method of claim 13, wherein the starch is an amphoteric waxy corn starch. The method of claim 1, wherein the aqueous mineral filler and chemical flocculant suspension is first combined and mixed before combining with the aqueous wood pulp fiber suspension in the paper making furnish. 16. Paper filled with a mineral filler, wherein the weight of the filler within the lumens is from about 0.5 weight percent to about 10 weight percent of the filler calculated on the basis of the total weight of the fiber and filler. 17. Paper filled with a mineral filler, wherein the weight of the filler within the lumens is from about 3.0 weight percent to about 6.0 weight percent filler calculated on the basis of the total weight of pulp and filler.