Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1997-12-22
2001-01-23
Fiorilla, Christopher A. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S198000
Reexamination Certificate
active
06176974
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to control of paper formation properties of papermaking furnish. More particularly, it relates to an apparatus and method for measuring and adjusting the electrokinetic charge of papermaking furnish to aid in achieving optimal, stable paper formation properties and machine runnability.
BACKGROUND
During papermaking operations, an aqueous slurry of cellulosic fibers (furnish or pulp) is produced by subjecting wood to various mechanical and/or chemical processes with the aim of liberating and individualizing fibers from the wood stock. For some grades of pulp, the chemical treatment is geared toward removal of lignin from the fibers. The pulp is often subjected to additional chemical treatments such as bleaching where the fibers are further liberated and decolored to lighten the fibers for production of white paper or board product. Bleaching also tends to reduce fiber strength and viscosity, as the chemicals will attack the cellulose to some degree.
Once a pulp of the desired degree of liberation and whiteness if obtained, it is then diluted in water to make what is known as the furnish. The furnish is then sent to a headbox and released from a wide, slit-like opening known as a “slice” and the jet is deposited onto a continuously moving screen or “wire” (also sometimes referred to as a forming fabric) where the furnish is dewatered. As the papermaking furnish is dewatered, fibers and other particulates become trapped by the forming fabric, forming a paper web which is further processed into the end product. To maintain an acceptable rate of production on a commercial paper or paperboard machine, it is essential that the furnish permit sufficiently rapid drainage of the water from the forming mat of fibers.
The papermaker will often tailor the make-up of the furnish using various additives to produce a paper or board sheet with desired formation properties. Many of these additives are supplied as substantially water-insoluble particulate solids in a finely divided state. Exemplary solid particulate additives include clay, calcium carbonate, titanium dioxide, pigment dyes, rosin, and other materials. To achieve the desired effect, it is essential that these additives be efficiently retained in the web forming on the wire.
The furnish used by most papermakers is composed mainly of cellulosic fibers suspended in water such as groundwood pulp, chemithermomechanical pulp (CTMP), kraft pulp, or sulfite pulp. It is the nature of these pulp varieties to have, at the surfaces of fibers and other suspended particles, a net electrical charge. Charges on the surfaces of solids suspended in papermaking furnish arise for several reasons including (a) dissociation of ionizable groups such as carboxylic acids, sulfonic acids, amines, and hydrous oxides, (b) adsorption of ions from solution, and (c) substitution of atoms having differing valence into the lattices of inorganic crystals.
Charges at the surfaces of suspended particles attract ions of opposite charge (counter-ions), resulting in the formation of an ionic double-layer. Some of the counter-ions in the double layer are present immediately adjacent to the charged surface (Stern layer). Thermal energy causes some of the counter-ions to diffuse away from the surface so that they are present as a diffuse cloud of gradually decreasing density of net charge.
The net electrical charge of suspended particles within the furnish can have a profound effect on retention, drainage during web formation, and paper properties. For example, if a new batch of pulp contains a higher level of dissolved anionic polymers and colloidal anionic material (disco), then a decrease in the percentage of fine material retained during a single pass over the forming fabric (first-pass retention) is generally observed. An inherent difficulty, however, is that the colloidal charge of a papermaking furnish is subject to sudden changes. Sources of such disturbances include variations between different batches of pulp, transitions between different types of species of wood in pulping, the use of mechanical pulps of variable quality and variations in pulp washing efficiency, variations in the ratio between different types of pulp, in the amount of broke, in the concentrations of latex dispersants, carry-over of de-inking chemicals, in the level of hemicellulose and byproducts of oxidative bleaching, in micro-biological activity and in the quality of fresh water. Further disturbances in charge may result from changes in the levels of additives such as dyes, wet-strength additives, dry-strength resins, recycled size-press starch,-biocides, colloidal silica, bentonite, and other colloidally charged additives.
As a further source of colloidal charge disturbances, papermakers must deal with the consequences of intermittent interruptions of parts of the system. For example, when the web of paper breaks it must be rethreaded. Such interruptions have the tendency to move the system away from an optimum balance between the chemical additives having opposite colloidal charges, thereby making it more difficult for papermakers to maintain a uniform product.
Therefore, stability and control of the electrokinetic properties of the papermaking furnish is a necessary requirement for the efficient production of a uniform paper product.
Several approaches to monitoring and controlling the electrokinetic properties of a papermaking furnish have previously been attempted. These approaches include laboratory-based tests where a sample is withdrawn from the furnish or from the whitewater which drains from the furnish during web formation. One such lab-based test involves a process known as micro-electrophoresis, in which a microscope is used to observe the motion of a particle in the size range of about 0.2-500 microns as it migrates under the influence of an imposed electric field. The quotient of the particle velocity and the field strength defines the electrophoretic mobility. When the viscosity of the fluid and the conductivity are known, it is possible to calculate the “zeta potential” from this approach, which is the electrical potential at the plane of shear that divides the counter-ions, which tend to stay with the surface, from those ions which tend to move with aqueous fluid flowing past the surface.
Among the difficulties of micro-electrophoresis as applied to the study and control of papermaking operations is the requirement that the observed particles be much smaller than the diameter of the capillary cell in which the tests are conducted. This size requirement usually precludes direct microelectrophoretic analysis of the kind of cellulose fibers used in papermaking. Accordingly, the conventional approach is to screen the fibrous slurry and test the fine suspended solids remaining in the filtrate, or white-water. The capillaries used in micro-electrophoresis measurements are subject to contamination by the many polymeric and particulate ingredients in paper. Also, the focusing of the microscope tends to be too delicate for continuous operation in the industrial environment of a paper mill. An incorrectly focused micro-electrophoresis apparatus will give misleading data that is not representative of the actual zeta potential. A further problem with micro-electrophoresis is that the information obtained about the electrophoretic mobility, or zeta potential, is usually not linearly proportional with any process or additive which is under the direct control of the papermakers.
Another approach to monitoring the electrokinetic properties of papermaking furnish is to employ a device that measures “streaming current” of a sample of furnish. A typical streaming current device consists of a poly-tetrafluoroethylene (PTFE) plunger which is reciprocated within a PTFE cylinder containing the furnish sample. The reciprocating motion causes fluid to move rapidly in the annular space between the plunger and cylinder wall. The motion of the fluid induces a streaming current which is detected by connection of a suitable ammeter to electrodes
Fiorilla Christopher A.
International Paper Company
Luedeka Neely & Graham P.C.
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