Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-11-02
2002-05-28
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S181600
Reexamination Certificate
active
06395134
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 for 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 WO-A-9829604 a process of making paper is described in which a cationic polymeric retention aid is added to a cellulosic suspension to form flocs, mechanically degrading the flocs and then reflocculating the suspension by adding a solution of a second anionic polymeric retention aid. The anionic polymeric retention aid is a branched polymer which is characterised by having a rheological oscillation value of tan delta at 0.005 Hz of above 0.7 or by having a deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding polymer made in the absence of branching agent. The process provided significant improvements in the combination of retention and formation by comparison to the earlier prior art processes.
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 description of the high molecular weight polymers indicates that they are linear polymers.
However, there still exists a need to further enhance paper making processes by further improving drainage, retention and formation. Furthermore there also exists the need for providing a more effective flocculation system for making highly filled paper.
According to 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, characterised in that the suspension is flocculated using a flocculation system comprising a siliceous material and an anionic branched water soluble polymer that has been formed from water soluble ethylenically unsaturated anionic monomer or monomer blend and branching agent and wherein the polymer has
(a) intrinsic viscosity above 1.5 dl/g and/or saline Brookfield viscosity of above about 2.0 mPa.s and
(b) rheological oscillation value of tan delta at 0.005 Hz of above 0.7 and/or
(c) deionised SLV viscosity number which is at least three times the salted SLV viscosity number of the corresponding unbranched polymer made in the absence of branching agent.
It has surprisingly been found that flocculating the cellulosic suspension using a flocculation system that comprises a siliceous material and anionic branched water soluble polymer with the special rheological characteristics provides improvements in retention, drainage and formation by comparison to using the anionic branched polymer in the absence of the siliceous material or the siliceous material in the absence of the anionic branched polymer.
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, aluminosilicates, polyaluminosilicates, borosilicates, polyborosilicates and zeolites. This siliceous material may be in the form of an anionic microparticulate material. Alternatively the siliceous material may be a cationic silica.
Desirably the siliceous material may be selected from silicas and polysilicates. The silica may be for example any colloidal silica, for instance as described in WO-A-8600100. The polysilicate may be a colloidal silicic acid as described in U.S. Pat. No. 4,388,150.
The polysilicates of the invention may be prepared by acidifying an aqueous solution of an alkali metal silicate. For instance polysilicic microgels otherwise known as active silica may be prepared by partial acidification of alkali metal silicate to about pH 8-9 by use of mineral acids or acid exchange resins, acid salts and acid gases. It may be desired to age the freshly formed polysilicic acid in order to allow sufficient three dimensional network structure to form. Generally the time of ageing is insufficient for the polysilicic acid to gel. Particularly preferred siliceous material include polyalumino-silicates. The polyaluminosilicates may be for instance aluminated polysilicic acid, made by first forming polysilicic acid microparticles and then post treating with aluminium salts, for instance as described in U.S. Pat. No. 5,176,891. Such polyaluminosilicates consist of silicic microparticles with the aluminium located preferentially at the surface.
Alternatively the polyaluminosilicates may be polyparticulate polysicilic microgels of surface area in excess of 1000 m
2
/g formed by reacting an alkali metal silicate with acid and water soluble aluminium salts, for instance as described in U.S. Pat. No. 5,482,693. Typically the polyaluminosilicates may have a mole ratio of alumina:silica of between 1:10 and 1:1500.
Polyaluminosilicates may be formed by acidifying an aqueous solution of alkali metal silicate to pH 9 or 10 using concentrated sulphuric acid containing 1.5 to 2.0% by weight of a water soluble aluminium salt, for instance aluminium sulphate. The aqueous solution may be aged sufficiently for the three dimensional microgel to form. Typically the polyaluminosilicate is aged for up to about two and a half hours before diluting the aqueous polysilicate to 0.5 weight % of silica.
The siliceous material may be a colloidal borosilicate, for instance as described in WO-A-9916708. The colloidal borosilicate may be prepared by contacting a dilute aqueous solution of an alkali metal silicate with a cation exchange resin to produce a silicic acid and then forming a heel by mixing together a dilut
Chen Gordon Cheng I
Richardson Gary Peter
Chin Peter
Ciba Specialty Chemicals Water Treatments Ltd.
Crichton David R.
Hug Eric
LandOfFree
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