Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2002-08-02
2003-09-09
Griffin, Steven P. (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300, C162S181600, C162S181800
Reexamination Certificate
active
06616806
ABSTRACT:
This invention relates to processes of making paper and paperboard from a cellulosic stock, employing a novel flocculating system.
During the manufacture of paper and paper board a cellulosic thin stock is drained on a moving screen (often referred to as a machine wire) to form a sheet which is then dried. It is well known to apply water soluble polymers to the cellulosic suspension in order to effect flocculation of the cellulosic solids and enhance drainage on the moving screen.
In order to increase output of paper many modern paper making machines operate at higher speeds. As a consequence of increased machine speeds a great deal of emphasis has been placed on drainage and retention systems that provide increased drainage. However, it is known that increasing the molecular weight of a polymeric retention aid which is added immediately prior to drainage will tend to increase the rate of drainage but damage formation. It is difficult to obtain the optimum balance of retention, drainage, drying and formation by adding a single polymeric retention aid and it is therefore common practice to add two separate materials in sequence.
EP-A-235893 provides a process wherein a water soluble substantially linear cationic polymer is applied to the paper making stock prior to a shear stage and then reflocculating by introducing bentonite after that shear stage. This process provides enhanced drainage and also good formation and retention. This process which is commercialised by Ciba Specialty Chemicals under the Hydrocol® trade mark has proved successful to more than a decade.
More recently there have been various attempts to provide variations on this theme by making minor modifications to one or more of the components.
U.S. Pat. No. 5,393,381 describes a process in which a process of making paper or board by adding a water soluble branched cationic polyacrylamide and a bentonite to the fibrous suspension of pulp. The branched cationic polyacrylamide is prepared by polymerising a mixture of acrylamide, cationic monomer, branching agent and chain transfer agent by solution polymerisation.
U.S. Pat. No. 5,882,525 describes a process in which a cationic branched water soluble polymer with a solubility quotient greater than about 30% is applied to a dispersion of suspended solids, e.g. a paper making stock, in order to release water. The cationic branched water soluble polymer is prepared from similar ingredients to U.S. Pat. No. 5,393,381 i.e. by polymerising a mixture of acrylamide, cationic monomer, branching agent and chain transfer agent.
In EP-A-17353 a relatively crude pulp, having high cationic demand, is treated with bentonite followed by substantially non-ionic polymeric retention aid. Although the suspension in this process is a substantially unfilled suspension, in AU-A-63977/86 a modification is described in which the suspension can be filled and in which bentonite is added to thickstock, the thickstock is then diluted to form thinstock, a relatively low molecular weight cationic polyelectrolyte is added to the thinstock, and a high molecular weight non-ionic retention aid is then added. Thus in this process, coagulant polymer is used, and it is added to the thinstock after the bentonite.
Processes such as those in EP 17353 and AU 63977/86 are satisfactory as regards the manufacture of paper from a suspension that has relatively high cationic demand and relatively low filler content, but tend to be rather unsatisfactory as regards filler retention when the suspension contains significant amounts of filler.
EP-A-608986 describes a process for making filled paper by adding a cationic coagulant to the feed suspension to flocculate a relatively concentrated suspension of fibre and filler adding bentonite or other anionic particulate material to the cellulosic thinstock or thickstock and subsequently adding polymeric retention aid to the thinstock before draining the thinstock to form a sheet. Fibre and filler retention are said to be improved by the presence of the coagulant in the concentrated suspension of the fibre and filler.
EP-A-308752 describes a method of making paper in which a low molecular weight cationic organic polymer is added to the furnish and then a colloidal silica and a high molecular weight charged acrylamide copolymer of molecular weight at least 500,000. The disclosure appears to indicate that the broadest range of molecular weights afforded to the low molecular weight cationic polymer added first to the furnish is 1,000 to 500,000. Such low molecular weight polymers would be expected to exhibit intrinsic viscosities up to about 2 dl/g.
TM Gallager 1990 TAPPI Press, Atlanta p141 Short Course entitled Neutral/Alkaline Paper making describes an allegedly commercial available silica microparticle system using a cationic coagulant polymer, a high molecular weight anionic polyacrylamide and a 5-nm colloidal silica sol. Such coagulant polymers would have low molecular weights and high charge density. It is stated that although there is a potential for high retention, formation is still an issue with high doses of anionic polyacrylamide. A lower addition of silica (less than 0.10%) is commonly used in this system.
However, there still exists a need to further enhance paper making processes by further improving drainage and retention without impairing formation. Furthermore there also exists the need for providing a more effective flocculation system for making highly filled paper.
According to a first aspect of the present invention a process is provided for making paper or paper board comprising forming a cellulosic suspension, flocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, wherein the cellulosic suspension is flocculated by addition of a substantially water soluble polymer selected from,
a) a polysaccharide or
b) a synthetic polymer of intrinsic viscosity at least 4 dl/g
and then reflocculated by a subsequent addition of a reflocculating system, wherein the reflocculating system comprises i) a siliceous material and ii) a substantially water soluble polymer,
characterised in that either, the siliceous material and water soluble polymer are added to the suspension simultaneously or the siliceous material before the addition of the water soluble polymer.
According to a second aspect of the present invention a process is provided for making paper or paper board comprising forming a cellulosic suspension, flocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, wherein the cellulosic suspension is flocculated by addition of a substantially water soluble polymer selected from,
a) a polysaccharide or
b) a synthetic polymer of intrinsic viscosity at least 4 dl/g
and then reflocculated by a subsequent addition of a reflocculating system, wherein the reflocculating system comprises i) a siliceous material and ii) a substantially water soluble anionic polymer,
characterised in that the water soluble anionic polymer is added to the cellulosic suspension before the addition of the siliceous material.
It has surprisingly been found that flocculating the cellulosic suspension using a flocculation system that comprises applying to the cellulosic suspension a multicomponent system comprising a water soluble polymer of intrinsic viscosity above 4 dl/g which is followed by the refluctuation system of the invention provides improvements in retention and drainage without any significant impairment of formation in comparison to other known processes.
The siliceous material may be any of the materials selected from the group consisting of silica based particles, silica microgels, colloidal silica, silica sols, silica gels, polysilicates, cationic silica, alumino silicates, polyaluminosilicates, borosilicates, polyborosilicates, zeolites and swelling clays. This siliceous material may be in the form of an anionic microparticulate material. When the siliceous material is a swelling clay it may typically a bentonite type clay. The preferred clays are swellable in water and includ
Ciba Specialty Chemicals Water Treatments Limited
Crichton David R.
Griffin Steven P.
Hug Eric
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