Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-07-02
2002-12-17
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S181700, C162S175000, C162S158000, C162S181100, C162S181500, C428S331000
Reexamination Certificate
active
06494991
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns composite products, particularly paper products, comprising fillers flocculated with starch granules prior to combining the flocculated filler particles with cellulosic material and/or polymerized mineral networks and methods for their manufacture.
BACKGROUND OF THE INVENTION
Paper products are composite products in which cellulosic fibers, particularly wood fiber, are the primary component. In addition to fiber, many other materials and chemicals are added to form the desired product. Paper products often include mineral additives, referred to as fillers, such as clay, calcium carbonate, talc, kaolin, calcium sulphate and titanium dioxide. Paper primarily comprises a web of cellulosic fibers and minor amounts of mineral and/or organic fillers. Fillers are used, as the name implies, to fill spaces bounded by the cellulosic fibers of the web. Fillers also improve certain paper properties including opacity, brightness and printability. Other additives also can be used to form paper having desirable end-product properties, such as pigments, dyes, starch, sizing agents and strength-enhancing polymers.
Cellulosic products can be made using conventional fillers more economically than products made without such fillers, primarily because of the cost of cellulosic material. Traditionally, minerals like Kaolin clay (hydrous aluminum silicate), chalk, ground limestone or marble, (calcium carbonate), talc (hydrous magnesium silicate), gypsum (calcium sulfate) and diatomaceous earth (silicon dioxide) have been used as fillers. Most other fillers are inorganic materials produced synthetically from minerals (e.g., titanium dioxide, synthetic silica, barium sulfate), or by regeneration after purification (e.g., limestone-to-lime-to-precipitated calcium carbonate).
Fillers used to form paper products using methods developed prior to the present invention reduce the strength properties, such as breaking length in kilometers of the product (tensile strength divided by the basis weight times 102 by TAPPI Method T494), as the percent of filler used to make such products increases.
FIG. 1
illustrates this effect and shows that as the filler content is raised above about 20 % the strength of the paper decreases considerably. Experience also has shown that other undesirable changes occur as the amount of filler used with conventional paper-making processes increases.
Numerous approaches have been proposed and investigated for raising the amount of fillers that can be used to make paper products. A common approach involves bonding filler particles to the fibrous materials. This approach has met with limited success, primarily because (1) the fibrous materials and the fillers are chemically dissimilar, and (2) the cost of the materials used to make the products using this approach.
Chemicals, chemical compositions, and methods directed to solving problems associated with increasing filler and reducing cellulosic material in paper products are known. For example, retention chemicals and compositions are commercially available. These materials have proved satisfactory in solving the problems associated with flocculating and retaining filler particles in the sheet. These prior inventions, however, do not improve the paper strength and therefore do not solve the problems associated with decreasing paper strength with increasing filler content. One possible explanation for this is that while the retention chemicals and compositions are good at flocculating filler particles, they also flocculate filler particles onto the fibers themselves. Paper strength is considered to arise primarily as a result of fiber-to-fiber bonding between adjacent fibers. This fiber-to-fiber bonding occurs at overlapping fiber surfaces. Filler particles flocculated onto the fibers reduce the surface area of the fibers available for this interfiber bonding, and perhaps intrafiber bonding as well, thereby reducing the strength of the paper product.
I. Silica and Silicates as Fillers
Silica and silicates are common materials, and have been used previously as fillers, retention aids, buffers, chelating agents, and coating components for making paper products. In fact, World Minerals Inc. manufactures a line of products, including calcium silicates, to increase bulk, control stickies, increase printability, etc. Thus, when silica or silicate materials are used to make filler or pigment materials, the particle size of the materials is reduced to the classical range (0.1 to 10 micron) for filler particles even if initially the particles are produced in larger sizes. For example, U.S. Pat. No. 4,790,486 describes a process for preparing paper that involves making hydrous silicic acid fillers by wet pulverizing a slurry to 1-30 microns from large particles. U.S. Pat. No. 5,030,284 discloses a spray drying technique of gelled alumina-silica-sulfate compositions to generate 1-10 micron particles.
Silica and silicate have been used as retention/drainage aids for the production of paper. For example, U.S. Pat. No. 5,127,994 describes using silica-based colloids. The colloidal particles have a particle size of 4 to 7 nanometers. U.S. Pat. No. 4,643,801 describes an improved binder system containing three ingredients, a cationic starch, anionic polymer, and dispersed silica having a particle size of 1-50 nanometers (0.001 to 0.05 microns). This system requires the three components and utilizes small dispersed silica. This seems to be a variation of U.S. Pat. Nos. 4,385,961 and 4,388,150, which have a binder system of colloidal silicic acid and cationic starch.
Other variations of these retention/drainage aids include the formation of microgels, which are three-dimensional chain networks formed of particles having a diameter of 1-2 nanometers. These small colloidal particles are stabilized to prevent further growth or gellation. See, for example, U.S. Pat. Nos. 4,954,220, 5,279,807 and 5,312,595. Neutralization of alkali silicate solutions forms polysilicic acid (from polymeric anions), which polymerizes to form microgels comprising three dimensional aggregates of very small particles of polysilicic acid. The formation of polysilicate microgels is initiated by the addition of an acidic material (aluminum sulfate, sodium stannate, sodium orthoborate decahydrate, acid ion exchange resins, sodium aluminate, etc.). The “initiator” starts the gelation (polymerization) process, which is stopped before total gelation of the solution. This process is done independent of the papermaking process. It is then added to the system as any other retention/drainage additive. See,
FIG. 2
, which shows the polymerization behavior of silica. Polysilicate microgel has been found to constitute a good retention and drainage aid when combined with a water-soluble cationic polymer.
Kaliski's U.S. Pat. No. 5,240,561 discloses the use of microgels formed similarly from alkali silicate solutions and a second aqueous solution of sodium aluminate or sodium zincate. Kaliski's patent concerns a process for making paper and teaches the formation of microgels in situ (in the furnish). But his microgels, described as transient, chemically reactive, subcolloidal sodium-silico-aluminate or similar microgels, must be crosslinked with bivalent and/or multivalent inorganic salts (like calcium chloride) prior to the formation of colloidal particles (Tyndall effect). This crosslinking immediately stops the gelation process (polymerization) and precipitates the calcium crosslinked microgel. Kaliski demonstrates the use of this procedure for papermaking as a new flocculation mechanism where precipitation of the microgel coagulates and flocculates all particulates present in the papermaking furnish. Thus, Kaliski's patent is a variation of the previous patents for retention and drainage aid but is done in situ. Kaliski's patent requires a specific order of addition (1st-sodium silicate, 2nd-sodium aluminate, 3rd-calcium chloride) with a termination of the polymerization at the subcolloidal stage of growth. Kaliski
Atha Barbara R.
Johnson James S.
Juang Mike S. D.
Lee David T.
Malcom Larry L.
Boise Cascade Corporation
Fortuna Jose
Klarquist & Sparkman, LLP
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