BACKGROUND OF THE INVENTION
1. Brief Description of the Invention
This invention is related generally to cellulosic products, and in particular to such products containing improved percentage of calcium carbonate fillers.
2. Related Art
Cellulose is an essential raw material to make paper. The major sources of cellulose can be grouped into wood, non-wood and recycled fibers. Wood provides about 93% of the world's virgin fiber requirements. One of the challenges during papermaking is to separate the cellulose from components detrimental to papermaking, such as lignin. The process of pulping and bleaching removes much of organic binding material, most of which are lignin, and exposes the fiber for further treatment.
In general, subsequent to pulping and bleaching the fibers are refined. During refining, the fibers are subjected to a mechanical action to develop their optimal papermaking properties with respect to the product made. Overall, during the process of chemical and mechanical separation, the cellulose fibers are modified and degraded. The properties of paper largely depend upon the structure of various fibers that compose the sheets. Also, a wide range of chemicals are known to be used in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. Chemicals for control purposes, such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides and corrosion inhibitors are added as required based on process and quality requirements.
Calcium carbonate is one of these chemicals added during alkaline papermaking and is included within the classification of “filler”. Calcium carbonate is added to fill out spaces between fibers, to improve optical and surface properties of paper, and also to reduce the manufacturing costs. This is because fillers are generally less expensive compared to fibers, with the exception of certain specialty pigments. Fiber morphology and calcium carbonate characteristics largely determine calcium carbonate deposition mechanism in the final product.
In discussing fiber morphology, it is essential to consider origination of fiber in wood and transformations to fiber that occur during papermaking. Wood is classified into hard and softwoods. The differences are the result of the development and growth of wood cells. The cells are elongated cells and most are oriented in the longitudinal direction of the stem. The wood substance in soft woods is composed of two different cells: tracheids, also know as fibers, (90-95%) and ray cells (5-10%). Hardwood contains tracheids (55%), vessel elements (30%) with the rest being ray and parenchyma cells. Principal functions of the cells are to a) perform the function of liquid transport; b) provide mechanical strength; and c) food storage. The principal function varies with cell type. The “cell pits” between adjacent wood fibers or cells accomplish the distribution of aqueous solutions and nutrients. J. Gullichsen, H. Paulapuro, Paper Science and Technology, Book 3, TAPPI, Ed.2000, page 22. The type of pit depends on the cell type in which the pits are found. The tree grows through the division of cells. The division of cells and growth of the tree are a continuous process, and the growth rate depends upon the species and conditions of the environment.
Regarding fiber morphology, the wall of a typical tracheid or “fiber” is composed of several layers. Tracheids are a long, tapering cell formed essentially in soft woods. When a cell divides, it first develops a cell plate. Each of the two new cells subsequently encloses itself within a thin, extensible, primary wall, consisting of cellulose, hemi-celluloses, pectin, and protein. In summary, the cellulose forms a skeleton, which is surrounded by hemi-cellulose and lignin. During the phase of differentiation, all fibers grow inward from their very thin outer primary wall, successively forming a thin outer boundary (S1) wall, then a thick inner secondary (S2) wall, which is the main body of the fiber, and finally a very thin tertiary (T) wall and a thinner, and usually warty membrane around the lumen.
The S1 and T walls are similar to each other, both being only a fraction of a micron thick. The elements in the S1 wall of a wood fiber are wound spirally in both directions at a very steep angle to the fiber axis, and those in the S2 wall usually have a Z winding at a small angle to the fiber access, which angle often increases as the center is approached.
Natural fibers can be split into fibrils that have a width upwards of 0.2 microns. These visible ones are called fibrils and smaller ones down to about 30 nanometers are called microfibrils, and still smaller ones from 30 to about 3 nanometers, it is proposed, should be called nanofibrils. These in turn are bundles of elementary fibrils which are about 35 Angstroms in diameter, each composed of perhaps 16 laterally joined chains of cellulose molecules. (See d'A Clark, James, Pulp Technology and Treatment for Paper, Miller Freeman Publications, Ed. 1985, page 6.11.)
The process of refining during papermaking effects the partial removal of primary wall. Removal of primary wall exposes the secondary wall for refining. Further refining of secondary wall induces fibrillation and promotes hydration. (See Smook, G. A., Handbook for Pulp and Paper Technologists, TAPPI, Ed. 1989, page 181). Fibrillation involves loosening of the fibrils that are already present and raising of the finer microfibrils on the surface of the fibers, resulting in very large increase in surface area for the refined fibers.
Typically, the calcium carbonate source is either ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC). GCC is made from grinding the natural product limestone, while PCC is made synthetically outside of the papermaking process. Various types of water-soluble polymers are typically added to retain fillers. Certain polymers are also added to improve water drainage from cellulosic material. In general, all these chemicals are added to improve the runability of the paper machine or the paper quality. Very often, however, excessive use of chemicals leads to problems causing chemical imbalances in the wet end of the paper machine. This negatively impacts runability and paper quality.
With such large variation in fiber morphology and function of these chemicals, it is important to control the quantity and point of addition of these chemicals. A preferred method to monitor and control the chemical imbalances is using charge measurements. Zeta potential is a tool used for charge measurements.
The current methods of direct addition of calcium carbonate to the pulp or cellulosic fiber have severe drawbacks. The current method is to add calcium carbonate to the cellulosic pulp as a slurry diluted with water either shipped to the mill or made on site. The drawbacks of this are:
(a) It is difficult to achieve filler content above 20-30% on the final product based on retention aids presently used.
(b) It results in low levels of single-pass retention. The single-pass retention is expressed as (amount retained in sheet)/(amount from headbox). Low levels of single-pass retention give rise to non-uniform distribution of filler particles in the cross-section of the sheets.
(c) It leads to variations in charge, either positive or negative, depending upon the system, causing charge imbalances.
(d) Industry demands increased paper machine speeds, minimized wash water usage, and minimized effluent discharge are all limited by the need to have an improved filler retention method.
In the last several years, several paper-producing companies have confirmed their interest in the in-situ production of “precipitated calcium carbonate” (PCC), and these companies have either run their own tests or have patented new technologies.
U.S. Pat. No. 4,510,020 describes paper having improved properties produced from pulp in which filler is selectively loaded within the fiber lumen by agitating a suspension of pulp and filler until the fiber lumens become loaded with filler, separating the residual suspended filler from the loaded fibers, and vigorously washing the pulp until substantially all the filler on the external surfaces of the fibers is removed.
U.S. Pat. Nos. 5,275,699 and 5,096,539 describe a process for the in-situ production of fillers from two salts. The salts are combined to precipitate calcium carbonate in the cell wall of fibers and within the fiber itself. The fibers used either can be dried fibers or wet fibers. The fibers can be further washed to remove the calcium carbonate from the exterior of the cell walls.
U.S. Pat. Nos. 5,158,646 and 5,122,230 describe fillers produced by adding a water-soluble inorganic compound to a fiber slurry. The fibers are then pressed so that only the water-soluble compound remains in the fiber wall. The high-consistency fibers are then subjected to a precipitant gas that renders the inorganic compound water-insoluble.
U.S. Pat. No. 5,223,090 describes a process for the production of in-situ calcium carbonate, wherein the calcium carbonate is produced by adding calcium oxide or calcium hydroxide to a fiber slurry and subjecting the slurry to high shear mixing while injecting carbon dioxide.
U.S. Pat. Nos. 5,731,080 and 5,824,364 describe processes for the production of in-situ calcium carbonate, wherein the processes consist of providing fibers with microfibrils. Calcium ions are then added to the suspension of fibers followed by carbonate ions. This procedure results in a product where most of the calcium carbonate is precipitated and grafted on the microfibrils. This patent claims to provide a product with more than 20% filler.
U.S. Pat. No. 5,679,220 describes a method of adding a solution of calcium oxide to a fiber stream and then adding carbon dioxide under light mixing conditions to produce in-situ calcium carbonate. The order of addition of carbon dioxide and calcium oxide can be changed to better control the pH of the slurry. This process is performed at low consistency (less than 5%).
U.S. Pat. No. 5,665,205 describes a process of adding an alkaline salt to recycled fibers and then adding a gas to precipitate a filler to improve brightness and cleanliness of the final product.
EP Patent No. 791,685 and World Patent Application W097/01670 describe a procedure for adding a filler to a pulp based on cellulose fibers, characterized in that the pulp is fluidized, whereupon calcium hydroxide is added into it and calcium carbonate is precipitated with carbon dioxide. The '670 document describes precipitation of PCC on fiber fines that are added as a filler.
World Patent Application W099/42657 describes a process of preparing one composition of calcium bicarbonate and one composition of calcium hydroxide, followed by mixing of these two compositions in the presence of fibers to produce in-situ calcium carbonate. Also disclosed is a product consisting of unitary crystals grafted on the fibers themselves. Finally, also disclosed is the use of carbon dioxide to dissolve calcium carbonate in waste papers and to remove calcium carbonate from the waste papers.
World Patent Application W001/12899 describes a process of using cellulosic fibers wherein bicarbonates are added, carbonates or silicates of alkaline and alkaline-earth metals (K, Ca, Na, Mg) followed by adding a hydroxide compound to precipitate the silicates or carbonates.
As explained in the above patents, fillers are typically added during papermaking to improve the quality of final product. The addition of calcium carbonate as a filler improves optical properties of the paper and also reduces the amount of expensive fiber required for making paper. A typical content of calcium carbonate in white coated (or) uncoated paper grades is in the range of 20-30%. The calcium carbonate is either obtained as ground calcium carbonate (GCC) or precipitated on-site, so-called precipitated calcium carbonate, or PCC. In order to contain the calcium carbonate in paper, polymeric compounds are frequently used as retention aids.
As explained above in reference to World Patent W099/42657, one alternate method to prepare calcium carbonate is by in-situ deposition on the fibers or in the fibers, where the in-situ PCC is prepared by mixing the reactants necessary to form calcium carbonate directly with the fiber pulp. One such method, as explained in the '657 patent document, is by using calcium carbonate as one of the reactants, along with lime and carbon dioxide.
It would be preferred if an improved method of in-situ calcium carbonate deposition could be conceived. Calcium carbonate present in known papermaking methods, including known in-situ precipitation methods, tends to be uniformly distributed inside and outside the fiber lumen. Furthermore, the trend is to avoid binders and retention aids.
In all of the above methods, there was a desire to increase calcium carbonate in the final sheet of paper greater than 20 weight percent and also to eliminate the use of retention aids. For example, in U.S. Pat. No. 5,824,364, the calcium carbonate crystals are disclosed as being directly grafted to microfibrils as granules without binders and retention aids present at the interface between the microfibrils and the granules. Also, the precipitated calcium carbonate is uniformly distributed when directly grafting to the microfibrils. The majority of protocols trap the microfibrils by reliable and non-labile mechanical binding. The surface area of the fiber as defined by the inventors is a function of the number of microfibrils. The surface area of the fibers of the composite is in the range of 3-200 square meters per gram. The calcium carbonate contained in the sheet is limited to the available microfibrils surface area, as specified by the inventors in the range of 3-200 square meters per gram, and which can also significantly vary depending on the fiber source.
In the above-mentioned U.S. Pat. No. 4,510,020, the calcium carbonate is prepared within the fiber lumens until the fiber lumens become loaded with fiber and further washing is used to remove substantially all of the filler on the external surfaces of the fibers, including microfibrils. In case of fibrillated pulps, considerable difficulty is experienced in removing the filler from the fibrillated external surfaces. The inventors also indicated that it is possible to produce fibrils by prolonged beating, described as “fine fuzz”. The inventors also disclosed that fibrillated pulps, such as highly-beaten chemical pulps, and most mechanical pulps, pose a problem to their invention. The fibrillated external surfaces tenaciously hold pigments throughout the washing treatment. Furthermore, as explained by the inventors, there can be instances when the lumen can be collapsed, limiting the accessibility for calcium carbonate precipitation due to severe pressing or drying. The fibrillated pulp can compete for calcium carbonate, thereby minimizing the availability inside the lumen.
U.S. Pat. No. 4,892,590 explains a method of using separately prepared calcium carbonate and a cationic starch for improving filler retention, opacity and strength during papermaking.
All of the above prior art methods achieve high filler retention by focusing on individual sections of the fiber, such as in the lumen, cell wall, or microfibrils. The overall calcium carbonate contained in the final sheet is limited to the availability and surface area of these individual sections. The calcium carbonate contained in the paper sheet using the prior art methods is prepared without using retention aids or binders. The methods require controlled conditions to achieve the presence of calcium carbonate in the targeted sites.
SUMMARY OF THE INVENTION
In accordance with the present invention, cellulosic products are prepared using methods significantly different from the prior art, resulting in cellulosic products that are different from the prior art. The cellulosic products of the present invention contain calcium carbonate in all available areas of a fiber, and not constrained to a single location as in the prior art. The formation of calcium carbonate is found to be random (non-uniform) in relation to size, shape, location and crystal morphology. During paper production, non-saleable paper is reused, which is generally known within the industry as “broke.” The non-saleable paper originates during papermaking before drying, or after drying but before reeling, or after drying and reeling. The amount of broke addition can vary, depending upon operating conditions and preference. Since the broke is already paper, it contains all the chemicals that are added during the papermaking process. The broke is an essential part of the papermaking process, which cannot be ignored. These “broke chemicals” are expected to interfere and interact in calcium carbonate formation to provide a chemical bonding between the fiber and the calcium carbonate. It's also valid in case of mills using recycled paper for example mixed office waste as a portion of the furnish. Similar to the broke the recycled paper contains chemicals and additives used during papermaking before being recycled.
In accordance with the present invention, cellulosic products containing improved percentages of calcium carbonate are described. The products of the invention are characterized by non-uniformly distributed calcium carbonate in and around the fiber structure, inside lumen, cell wall, microfibrils, deposited in pits, and outside of the lumen. In other words it is possible by using the techniques of the invention to produce cellulosic products having all available real estate available in a fiber for containing calcium carbonate, that is, the entire surface area. Furthermore, since the processes used to produce the cellulosic products of the invention are positioned within a papermaking machine wet end, the calcium carbonate and fibers will interact with chemicals typically used in paper making producing chemical bonding between calcium carbonate and cellulose chemical structure. By the techniques of the invention, the cellulosic products of the invention have improved calcium carbonate addition, the improved addition occurring during the paper making process.
As used herein “papermaking process” means conditions that exist within the fiber mass during formation of a sheet of paper. In commercial sheet forming paper machines, there are three elementary forming operations: dilution, turbulence, and oriented shear, all of which are applied in different degrees in an attempt to optimize the final sheet quality. (See G. A. Smook, Handbook for Pulp and Paper Technologists, TAPPI, Ed. 1989, page 219.) Substantial binding energy is expected between chemicals added to the fiber and between the fibers itself so that the fibers do not break loose due to these forces during basic sheet forming processes.
Cellulosic products of the invention comprising PCC prepared in-situ during papermaking are prepared by mixing a bicarbonate with lime and with diluted pulp containing fibers, wherein the precipitated calcium carbonate is positioned in the final product in a non-uniform manner on all physical locations of the fiber either as single crystals, agglomerates of crystals, combined agglomerates of crystals, amorphous powders, or combinations thereof. The PCC crystals are contained in the sheet with other various chemical residues used in papermaking, and wherein a majority of PCC is attached to the fiber surface based on chemical bonding, and the PCC comprises at least 15% by weight based on total solids of the cellulosic product, more preferably at least 40%.
Preferred products of the invention are those wherein the zeta potential during papermaking ranges from −5 to about +5, more preferably the zeta potential of the papermaking process ranges from about −2 to about +2, and more preferably is zero. Considerable deviations in zeta potential from zero affect the production and quality of cellulose products. The retention of calcium carbonate on pulp fibers is also affected (negatively) by deviations from zero zeta potential.
Also preferred for making paper products of the invention, the freeness of the diluted pulp ranges from about 300 csf to about 900 csf, more preferably ranging from about 400 csf to about 500 csf. In order to produce a pulp of certain freeness, the pulp is either beaten or refined using a special mechanical device. Canadian Standard Freeness (csf) is one of the measures used to control freeness. The terms “beating” and “refining” are used interchangeably in the art and herein. The process of refining increases surface area of the fiber and also the amount of water retained by pulp fibers. This also contributes to higher PCC loading of the final product. Extending the process of refining beyond a certain optimum decreases the product quality and paper production.
The products of the invention preferably have PCC crystals having size ranging from about 0.1 micrometer to about 10 micrometers, and the fibers are derived from material selected from the group consisting of wood, non-wood (for example bagasse, nut shells, corn cobs), recycled cellulosic materials, and mixtures thereof. It is also possible, in preferred embodiments, for the products of the invention to include other components, such as glass particles, organic or inorganic pigments, coatings and the like, although these are optional only. See for example d'A Clark, James, Pulp Technology and Treatment for Paper, Miller Freeman Publications, Ed.1985.
The products of the invention preferably employ chemical residues selected from the group consisting of retention aids, flocculants, drainage aids, resins, dispersants, chelants, scale inhibitors, corrosion inhibitors, and slimicides.
Other preferred products of the invention are those derived from diluted fiber aqueous suspensions having a consistency ranging from about 0.05 percent to about 50 percent, more preferably ranging from about 5 percent to about 10 percent.
Preferred cellulosic products of the invention include building products and paper products. Preferably the fibers in the inventive products are bleached fibers, preferably derived from a kraft process. The products of the invention preferably have PCC morphology selected from the group consisting of amorphous, crystalline, powder, and mixtures thereof. If crystalline, it has a crystal morphology selected from the group consisting of calcite, aragonite, vaterite, rhombohedral, sclenohedral, prismatic, and mixtures thereof.
Other referred products of the invention are those wherein the fiber has native calcium present in an amount ranging from about 1 ppb to about 1 ppm, and products wherein the PCC is derived from reaction of non-precipitated calcium carbonate with carbon dioxide. More preferred are products wherein the PCC is derived from reaction of a salt of a bicarbonate with carbon dioxide.
Other aspects and advantages of the invention will be understood upon review of the following brief description of the drawing figures and the description of preferred embodiments.