Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-01-04
2001-10-23
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S164300, C162S164600, C162S165000, C162S168100, C162S183000
Reexamination Certificate
active
06306256
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a retention agent, more specifically to a method of improving the retention when dewatering a cellulosic fibre suspension and to a retention agent therefor.
BACKGROUND ART
When dewatering cellulosic fibre suspensions, such as in papermaking, it is desirable that as much as possible of the fibres and the additives such as filler, e.g. calcium carbonate, clay, titanium dioxide and the like, be retained in the dewatered product, i.e. the sheet of paper. To this end, it is known to add to the cellulosic fibre suspension (stock) a retention agent which improves the retention, i.e. increases the amount of solid substance, such as filler in the finished paper compared with the case where no retention agent is added.
With a view to improving the retention in the dewatering of cellulosic fibre suspensions, it is disclosed in CA 1,044,782 to use a combination of a phenol-formaldehyde resin and a polyethylene oxide. Moreover, it is disclosed in SE 8604975-6 to improve the retention by adding a phenol-formaldehyde resin and a high-molecular polyethylene oxide in combination with a cationic starch derivative or a cationic cellulose derivative. Finally, it is also disclosed in U.S. Pat. No. 5,516,405 to improve the filler retention in the dewatering of a pulp slurry by adding a cationic fixative, a promoter and a polyethylene oxide. The cationic fixative is selected from the group consisting of polyethylene amides, polydiallyldimethyl ammonium chlorides and polyethylene imines. The promoter can be selected from the group consisting of phenol formaldehyde, naphthalene sulphonate and black liquor.
Even if the retention is improved by using a combination of phenol-formaldehyde resin and polyethylene oxide as retention agent, there is a need of retention agents and methods which result in additionally improved retention. The above-mentioned U.S. Pat. No. 5,516,405 is an example of an attempt at achieving this by using a further additive in the form of a cationic fixative in addition to phenol formaldehyde and polyethylene oxide. Such further additives, however, cause further costs and work, for instance, in determining the correct dosage, dosing occasion and dosing place.
The present invention aims at obviating or decreasing the drawbacks of the prior-art technique and providing a simple agent and method for improving the retention when dewatering a cellulosic fibre suspension.
SUMMARY OF THE INVENTION
According to the present invention, it has surprisingly been found that the retention when dewatering a cellulosic fibre suspension can be further improved, compared with the use of a combination of phenol-formaldehyde resin and polyethylene oxide as retention agent, by using a combination of a modified phenol-formaldehyde resin and polyethylene oxide as retention agent, the modified phenol-formaldehyde resin containing cationic groups.
The invention thus provides a method of improving the retention when dewatering a cellulosic fibre suspension by adding to the suspension a retention agent, which comprises a phenol-formaldehyde resin and a polyethylene oxide, characterised in that a modified phenolformaldehyde resin containing cationic groups is added to the suspension.
The invention also provides a retention agent which comprises a phenol-formaldehyde resin and a polyethylene oxide, characterised in that the phenol-formaldehyde resin is a modified phenol-formaldehyde resin containing cationic groups.
Further features and advantages of the invention will appear from the following specification and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
As mentioned above, the retention agent according to the invention comprises a modified phenol-formaldehyde resin containing cationic groups. Apart from the amount of cationic groups, the inventive phenol-formaldehyde resin corresponds to conventional phenol-formaldehyde resins as used in retention systems in the pulp and paper industry. Thus the resin consists of a condensation product of phenol and formaldehyde and is preferably a resin of the resol type, i.e. a condensation product from the reaction between a phenol compound and an excess of formaldehyde. However, novolaks may also be used. Preferably, the resin is in the form of an aqueous solution when added as a retention agent.
The cationic groups included in the phenol-formaldehyde resin can be introduced either in the production of the phenol resin or by being added to the finished phenol-formaldehyde resin. There is no restriction to a special type of cationic group, but the cationic group preferably consists of an amine group, which may be derived from ammonia or be a primary, secondary or tertiary, amine. The amine group can possibly be linked to the phenol-formaldehyde resin by means of a link group. Preferred compounds for forming the cationic group are, inter alia, ethanol amine, n-propyl amine, dibutyl amine, triethyl amine and diethyl ethanol amine.
The cationicity, i.e. the amount of cationic groups in the phenol-formaldehyde resin is related to the anionic groups (phenol groups) which the phenol-formaldehyde resin normally contains. A cationicity of 0% means that the phenol-formaldehyde resin does not contain any cationic groups, but only the normal anionic phenol groups, the concentration of which usually amounts to about 24 microequivalents/g dry resin. A cationicity of 20% means that cationic groups have been introduced into the resin to an extent corresponding to 20% of the anionic groups, i.e. corresponding to about 5 microequivalents/g dry resin. Correspondingly, a cationicity of 35% means that the amount of cationic groups corresponds to about 8 microequivalents/g dry resin. In the invention, the cationicity of the modified phenol-formaldehyde resin preferably is about 5-85%. It is particularly preferred for the cationicity of the resin to be about 10-60%.
Regarding the determination of the cationicity of the phenol-formaldehyde resin, reference is made to the article by Lydia Bley, “Charge Titration for Selection and Dosage of Flocculants”, March 1994, Mütek Application 1501-e, Mutek Analytic GmbH, Arzbergerstrasse 10, D-82211 Herrsching, Germany.
In addition to the above modification of the phenol-formaldehyde resin with cationic groups, the resin can also be modified with sulphur. Modification of phenol-formaldehyde resin with sulphur is per se known, and reference can be made to, for instance, A. Knop, L. A. Pilato; “Phenolic resins, Chemistry, Applications and Performance”, Springer-Verlag, pp 152-153. As is evident from this reference, modification with sulphur causes a linking-together of the phenol rings with sulphur bridges containing one or more sulphur atoms.
In addition to the modified phenol-formaldehyde resin described above, the inventive retention agent comprises a polyethylene oxide as well. The polyethylene oxide used in the invention is of conventional type and preferably has a molecular weight in the range of about 0.5-20×10
6
. Like the phenol-formaldehyde resin, the polyethylene oxide should be in the form of an aqueous solution when added as retention agent.
When used as a retention agent, the weight ratio of the modified phenol-formaldehyde resin to the polyethylene oxide can be varied within wide limits, but preferably the weight ratio of phenol-formaldehyde resin:polyethylene oxide is from about 1:5 to about 50:1. It is also preferred to use a greater part by weight of the modified phenol-formaldehyde resin than of the polyethylene oxide, i.e. it is particularly preferred that the weight ratio of phenol-formaldehyde resin:polyethylene oxide be in the range from about 1:1 to about 50:1. Most preferred the weight ratio is from about 2:1 to about 20:1.
The amount of retention agent which in the invention is added to the cellulosic fibre suspension is not critical, but is preferably such that the added polyethylene oxide content is about 10-1000 ppm, more preferred 25-200 ppm, most preferred 50-150 ppm, based on the dry content of the suspension. Contents below about 10 ppm are too low for a c
Gytel Ulla
Rosengren Bo
Birch & Stewart Kolasch & Birch, LLP
Chin Peter
Kemira Kemi AB
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