Dendrimeric polymers for the production of paper and board

Details Dendrimeric polymers for the production of paper and board Dendrimeric polymers for the production of paper and board

2001-04-04

2002-10-22

Griffin, Steven P. (Department: 1731)

Paper making and fiber liberation

Processes and products

Non-fiber additive

C162S168200, C162S168300, C162S164600, C162S158000, C528S332000, C525S451000, C260S001000, C260S001000, C260S001000, C260S66500B

Reexamination Certificate

active

06468396

ABSTRACT:

TECHNICAL FIELD
The present invention is in the technical field of papermaking, and, more particularly, in the technical field of wet-end additives to the papermaking stock or furnish. In particular the invention relates to a papermaking stock, a method for increasing or enhancing the retention of components of a papermaking stock during the manufacture of paper, and a method of producing paper. In an especially important embodiment the methods are carried out in relatively “closed” mill water systems while simultaneously increasing drainage and decreasing the amount of deposits from colloidal hydrophobic particles often referred to as “stickies” or “pitch” on the paper machine.
BACKGROUND ART
The manufacture of paper is a complex process which can be broken down into a series of less involved processes. One of the more important processes occurs at the paper machine. At this location, an aqueous cellulosic suspension, stock, furnish or slurry is formed into a paper sheet. The cellulosic suspension is made by providing a thick stock, diluting the thick stock to form a thin stock, draining the thin stock on a forming fabric to form a sheet, and drying the sheet.
The cellulosic slurry is generally diluted to a known consistency (based on percent dry weight of solids in the slurry) of less than 2 percent. Ideally, the consistency is between 0.8 and 1.5 percent.
The cellulosic slurry is generally, but not necessarily, a mixture of chemical, mechanical and secondary (e.g., deinked) pulps. For example, this includes all paper and board furnishes based on mechanical pulp and, in part, semi-bleached kraft pulp, unbleached kraft pulp, and/or unbleached sulfite pulp. The mechanical pulps may be stone-groundwood, pressure groundwood, thermomechanical pulp, or semi-chemical mechanical pulp. Other pulps may include deinked pulps, reslushed newsprint or any secondary fiber source.
Cellulosic slurries of high quality pulps can also be used to produce fine paper grades (e.g., photocopying paper), tissue or toweling sheets. These slurries include highly bleached mechanical or chemical pulps.
It is common to include various inorganic materials, such as bentonite and alum, and/or organic materials, such as various natural, modified natural, or synthetic polymers in the thin or thick stock for the purpose of improving the drainage and retention processes.
Such materials can be added for diverse purposes such as, for example, pitch control, increased drainage and retention, improved formation, increased wet and dry strength, defoaming, facilitation of release from drying rolls, and decolorization of effluents.
In addition, many grades of paper include substantial levels of inorganic fillers such as, for example, kaolinite, calcium carbonate, and titanium dioxide. The percentage of mineral filler added to a papermaking slurry may vary between 0 and 35% by weight of dry paper depending on the type of sheet being formed.
In the papermaking process, much of the pulp is separated from the fibers, fillers, and pigments by filtration. The filtrate, which is called the white water, contains a large amount of unretained colloidal particles which may be fibre fragments, mineral fillers, deinking plant materials, or pigment particles. The poor retention of these is a consequence of the difficulty in the filtration of material characterized by colloidal or nearly colloidal dimensions. Poor fines retention is a serious problem because it results in the loss of valuable cellulosic material and the additional loading of water treatment facilities.
The least expensive and oldest dewatering method is simple gravity drainage. More expensive methods which are also used include vacuum, pressing, and evaporation. Drainage may be accomplished either horizontally or vertically, by one side of the forming sheet only or by both sides.
In practice, a combination of such methods is employed to dewater or dry the sheet to the desired water content. Since drainage is the first dewatering method and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other more costly methods such as drying. This will improve the overall efficiency of the process.
The papermaking fibers employed in papermaking are often of low grade and are predominantly of the mechanical type and include groundwood, thermomechanical pulp, deinked secondary fibers, semi-chemical pulps, and semi-bleached chemical kraft pulps. The cellulosic fibers thus produced are rarely very “clean” and are rarely completely separated from the residual process liquors which contain substantial levels of both organic and inorganic impurities. These impurities are derived from the pulping process and by-products which are naturally present in wood (Linhart F., Auhorn W. J., Degen H. J. and Lorz R., Tappi J. 70(10) 79-85 (1987), Sunberg K., Thornton J., Pettersson C., Holmbom B., and Ekman R., J. Pulp Paper Sci., 20(11), J317-321 (1994)). These are often referred to as detrimental substances because they interfere with the function of many additives.
Detrimental substances increase the cationic demand of the pulp slurry. The cationic demand is the number of equivalents of cationic charge that has to be added to the slurry to neutralize the excess anionic charge of the pulp slurry. The cationic demand is usually met using a low molecular weight (<500 000) highly charged synthetic cationic polyelectrolyte. These polymers are, for example, the following: polyethyleneimines, polyamines having a molecular weight of more than 50,000, polyamidoamines modified by grafting onto ethyleneimine, polyamidoamines, polyetheramines, polyvinylamines, modified polyvinylamines, polyalkylamines, polyvinylimidoazoles, polydiallydialkyl ammonium halides, in particular polydiallyldimethylammonium chloride. These polyelectrolytes are soluble in water and are used in the form of aqueous solutions.
The cationic demand of pulps used for making, for instance, newsprint is often above 1000 meq./mL of stock so that improvements only become significant with polymer weights of above 1000 grams dry polymer per tonne dry weight of paper. Such large amounts render treatment uneconomical.
Impurities in papermaking furnishes which need to be neutralized by the cationic polymer are present in solution as dispersed colloidal particles, and/or dissolved substances such as lignosulfonates and sulfites, kraft lignin, hemicelluloses, lignans, humic acids, dispersed wood resins, rosin acids and chemical by-products. These impurities impart a large negative charge on the surfaces of cellulose fibers and other materials when they are dispersed in water.
Recently, due to environmental legislation, the level of the aforementioned impurities in papermachine white-water systems has further increased. This increase is a consequence of the increased tendency for paper mill operations to “close up” the paper machine white water systems and recycle as much white water as much as possible.
A second problem often associated with the manufacture of paper is the accumulation of wood resin and synthetic hydrophobic materials on the surfaces of the process equipment. Wood resin is usually defined as the material in wood which is insoluble in water, but soluble in organic solvents (Mutton, D. B., “Wood Extractive and Their Significance to the Pulp and Paper Industries” Chap. 10, Wood Resins, Ed. W. E. Hills, Academic Press, New York (1962)). The weight of wood resin from all species of trees consists usually of 1-5% based on total weight. From the teachings of U.S. Pat. No. 5,468,396 it is seen that increased reuse of mill white water causes a build-up in the concentration of water-borne resins (Allen L. H. and Maine C. J., Pulp Paper Can., 79(4): pp. 83-90 (1978)) and exacerbates the tendency for pitch deposition (Allen L. H., Tappi J., 63(2), pp. 82-87, (1980)). Many chemicals used to combat foam in pulp and paper mills end up dispersed in the aqueous phase of a pulp suspension and co-deposit with wood resin (Dorris G. M., Douek M., and Allen L. H., J. Pulp Paper

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