Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2001-02-09
2001-08-07
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S181800, C162S183000
Reexamination Certificate
active
06270626
ABSTRACT:
This application is an application under 35 U.S.C. Section 371 of International Application Number PCT/FR99/00969 filed on April 23, 1999.
The present invention relates to a novel process for making paper based on cellulose fibre in sheet form, in which a novel retention system comprising bentonite and a cationic galactomannan is used to improve in particular the retention of the mineral fillers incorporated. The present invention also relates to a process for making paper with the use of a retention system which substantially improves the draining, i.e. the speed with which water flows from the fibre suspension.
In addition, the mechanical properties of the paper obtained according to the process of the invention are improved, for example the rigidity and the tear strength, as well as other properties such as the whiteness. Furthermore, the retention system according to the invention can have advantages as regards the quality and recyclability of the white waters derived from the papermaking process, as well as of the papers broken during the manufacturing process.
Papermaking poses several problems. One of the overriding single concerns is to reduce the cost of paper by reducing the amount of cellulose fibres in the paper pulp composition. Another approach consists in reducing the concentration of waste water on account of the increasingly strict environmental constraints.
Papermakers have proposed various means to reduce the cost of papers and to try to improve their properties. One of the approaches used consists in adding inexpensive mineral fillers to the papermaking process to replace the fibre. Moreover, certain mineral fillers are specifically used to improve certain properties of paper. Thus, for example, titanium oxide is used in its anatase and/or rutile forms to improve the opacity of papers, in particular in the case of laminated papers.
Unfortunately, the addition of mineral fillers which are micrometre-sized particles comes up against the problem of retention: during the formation of the sheet on the paper machine wire, the mineral particles have a tendency to pass through this wire, which gives rise to charged white water circuits. This poses problems as regards the treatment of the broke as well as the sheet quality.
At the present time, the prior art proposes the use of retention agents to reduce the problem of the lack of retention. For example, EP 490 425 A1 proposes a twin system based on anionic inorganic particles and on a cationic carbohydrate polymer modified with aluminium, the cationic polymer being either a cationic starch or a cationic galactomannan.
However, many solutions proposed to date are not economically viable to allow their use for the preparation of any type of paper. The reason for this is that certain retention agents or retention systems, such as those containing a carbohydrate polymer modified with aluminium, are complex and expensive products, which thus does not allow them to be used for products of ordinary quality.
The Applicant has now developed a novel papermaking process using a novel retention system which considerably increases the retention of mineral fillers, fibres and other materials in the sheet of paper.
Another object of the invention is to propose a retention system and a papermaking process in which the properties of the paper obtained, including, for example, the opacity yield of the mineral fillers, the tear strength, the whiteness and other necessary properties, are improved, optimizing the use of mineral fillers. Needless to say, the optimization takes place as a function of the type of filler used.
Another object of the invention is to propose a paper with a high concentration of mineral fillers, which has an acceptable tear strength and other acceptable characteristics.
Another object of the invention is to propose an economically viable alternative retention system which does not require the use of complex and expensive products.
Other objects and advantages of the invention will emerge on reading the description below and in particular in the tests, tables and figures illustrating various characteristics of the invention.
The present invention is based on the development of a retention system and the papermaking process using it, which markedly improves the retention of mineral fillers and of other characteristics of paper and which optimises the action of the mineral fillers present in the paper pulp.
The increase in the retention of the mineral filler and fines in the context of our papermaking process attenuates the problems of contamination of the white waters.
The present invention thus relates to a process for making paper by forming and drying an aqueous paper pulp containing cellulose pulp and mineral fillers, in which a retention system comprising a cationic galactomannan containing at least two vicinal groups and a bentonite suspension is incorporated into the stock pulp before formation of the sheet.
The amount of solids in the retention system is generally from 0.02% to 5% by weight, preferably from 0.1 to 1% by weight, relative to the weight of the paper pulp or stock pulp.
The bentonite/galactomannan ratio should be between 1 and 10 by weight, and this ratio is preferably between 2 and 6, depending in particular on the degree of substitution of the galactomannan.
As regards the bentonite suspension, this is understood as being a bentonite suspension consisting of any type of commercial product referred to as bentonite or as bentonite-type clay, i.e. anionic swelling clays such as sepialite, attapulgite or, preferably, montmorillinite. By way of example, the bentonites described in U.S. Pat. No. 4,305,781 are suitable for use in the context of the invention.
The montmorillonite clays that are suitable include Wyoming bentonites and soapy earths. The clays may or may not be chemically modified, for example by alkaline treatment to exchange the calcium of bentonite for an alkali metal.
The swelling clays are usually metal silicates comprising a metal chosen from aluminium and magnesium, and optionally other metals, and the ratio of silicon atoms to metal atoms at the surface of the clay particles, and generally within their structure, is from 5/1 to 1/1. For most montmorillonites, the ratio is relatively low, the metal being essentially or totally aluminium, but with a small amount of magnesium and occasionally with, for example, a small amount of iron. However, in other swelling clays, all or some of the aluminium is replaced with magnesium and the ratio may be very low, for example about 1.5 for sepialite. The use of silicates in which some of the aluminium has been replaced with iron appears to be particularly desirable.
The aqueous suspension is generally prepared by dispersing the bentonite powder in water. The amount of bentonite contained in the said suspension is chosen such that the final weight percentage of bentonite relative to the weight of the paper pulp will be between about 0.1% and 5%. The viscosity of the bentonite suspension is generally less than 500 mPa.s (measured using a Brookfield viscometer at 100 rpm).
The size of the bentonite particles is preferably such that at least 90% are less than 100 microns, and preferably at least 60% are less than 50 microns (size of the dry particles). The surface area of the bentonite before swelling is preferably at least 30 m
2
/g and generally at least 50 m
2
/g, typically 60 to 90 m
2
/g, and the surface area after swelling is preferably from 400 to 800 m
2
/g. The bentonite advantageously swells by at least 15 to 20-fold. The size of at least 90% of the particles after swelling is preferably less than 2 microns.
Commercial products which will be mentioned as non-limiting examples include the products Opazil AF and Opazil ADV from the company Sudchemie.
The cationic galactomannan according to the invention does not need to be modified with aluminium; it is preferably selected from galactomannans comprising at least two vicinal hydroxyl groups, in particular cationic guars. As regards the guars, it has been noted that their reactive centr
Chin Peter
Rhodia Chimie
Seugnet Jean-Louis
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