Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-11-02
2002-06-18
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168300
Reexamination Certificate
active
06406593
ABSTRACT:
This invention relates to processes of making paper and paperboard from a cellulosic stock, employing novel water soluble polymers as flocculating agents and also to the novel polymers used therein.
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 the 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, whilst maintaining optimum retention and 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. 5882525 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. 5393381 i.e. by polymerising a mixture of acrylamide, cationic monomer, branching agent and chain transfer agent.
In WO 98/29604 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 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. Generally this branched anionic water soluble polymer is prepared by polymerising a water soluble anionic monomer or monomer blend in the presence of low levels of branching agent. The process provided significant improvements in 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 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.
However, there exists a need to further enhance paper making processes by improving the retention and retaining or improving formation.
Thus the first aspect of the present invention relates to a process of making paper comprising forming a cellulosic suspension, flocculating the suspension, mechanically shearing the suspension and optionally reflocculating the suspension, draining the suspension on a screen to form a sheet and then drying the sheet, wherein the suspension is flocculated and/or reflocculated by introducing a water-soluble polymer of intrinsic viscosity above 3 dl/g into the suspension, characterised in that the water soluble polymer exhibits a rheological oscillation value of tan delta at 0.005 Hz of above 1.1.
The tan delta at 0.005 Hz value is obtained using a Controlled Stress Rheometer in Oscillation mode on a 1.5% by weight aqueous solution of polymer in deionised water after tumbling for two hours. In the course of this work a Carrimed CSR 100 is used fitted with a 6 cm acrylic cone, with a 1°58′ cone angle and a 58 &mgr;m truncation value (Item ref 5664). A sample volume of approximately 2-3 cc is used. Temperature is controlled at 20.0° C.±0.1° C. using the Peltier Plate. An angular displacement of 5×10
−4
radians is employed over a frequency sweep from 0.005 Hz to 1 Hz in 12 stages on a logarithmic basis. G′ and G″ measurements are recorded and used to calculate tan delta (G″/G′) values.
The value of tan delta is the ratio of the loss (viscous) modulus G″ to storage (elastic) modulus G′ within the system.
At low frequencies (0.005 Hz) it is believed that the rate of deformation of the sample is sufficiently slow to enable linear or branched entangled chains to disentangle. Network or cross-linked systems have permanent entanglement of the chains and show low values of tan delta across a wide range of frequencies, Therefore low frequency (e.g. 0.005 Hz) measurements are used to characterise the polymer properties in the aqueous environment.
It has surprisingly been found that polymers which exhibit a rheological oscillation value of tan delta at 0.005 Hz of above 1.1 provide improved performance in terms of improved retention and yet still maintain good drainage and formation performance. We find that polymers of high tan delta flocculate the cellulosic fibres and other components of the cellulosic paper making stock more efficiently thus inducing improvements in retention.
In a preferred form the water soluble polymer exhibits a rheological oscillation value of tan delta at 0.005 Hz of above 1.2 or 1.3. More preferably the polymer has an intrinsic viscosity of above 4 dl/g and a tan delta at 0.005 Hz of above 1.4 or 1.5. In some instances the tan delta may be as high as 1.7 or 1.8 or even as high as 2.0 or higher. Thus the polymer exhibits a high tan delta.
The high tan delta water soluble polymer may be anionic, nonionic, amphoteric but is preferably cationic. The polymer is prepared by polymerisation of a water soluble monomer or water soluble monomer blend. By water soluble we mean that the water soluble monomer or water soluble monomer blend has a solubility in water of at least 5 g in 100 ml of water. The polymer may be prepared conveniently by any suitable known polymerisation process for instance by solution polymerisation to provide an aqueous gel which is cut, dried and ground to form a powder or by reverse phase polymerisation as defined by EP-A-150933, EP-A-102760 or EP-A-126528.
The high tan delta water soluble polymer may be formed from a water soluble monomer or monomer blend comprising at least one cationic monomer and at least 2 ppm by weight, preferably 5 to 200 ppm, in particular 10 to 50 ppm of a chain transfer agent.
In the process of making paper according to the invention the water soluble polymer may be added to the paper making stock as the sole treatment agent in the paper making process, although preferably the polymer may be added as part of a multi-component flocculant system in which the invention the cellulosic suspension is flocculated and then reflocculated.
In one aspect of the inven
Chen Gordon Cheng I
Heard Michael Barry
Chin Peter
Ciba Specialty Chemicals Water Treatments Ltd.
Crichton David R.
Hug Eric
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