Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
2000-02-09
2002-11-05
Fortuna, Jose (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S158000, C162S168100, C162S168300, C162S175000, C162S178000, C162S181200, C162S181300, C162S181800
Reexamination Certificate
active
06475341
ABSTRACT:
This invention relates to processes for making paper (by which we include paper board), and in particular processes of making paper which is strengthened by starch.
It is standard practice to make paper by a process comprising flocculating a cellulosic suspension by the addition of a high molecular weight, polymeric, retention aid, draining the flocculated suspension through a wire to form a wet sheet, and drying the sheet.
One particular class of paper-making processes are microparticulate processes in which the flocculation with polymeric retention aid is followed by degrading the flocs by agitation and then reflocculating by the addition of a microparticulate material, such as bentonite.
It is well known to include low molecular weight cationic polymer in the suspension, either by addition at the thick stock stage or subsequently, in various paper-making processes for various purposes. It is also well known to include inorganic coagulants such as poly aluminium chloride or alum for various purposes. Reference is made to, for instance, U.S. Pat. No. 4,913,775 for a description of various processes and, in particular, a microparticulate process sold under the trade name Hydrocol.
It is known to add cationic starch to the cellulosic suspension in papermaking processes as a strengthening aid, and in some processes it also contributes to retention. Processes have also been described which comprise addition of raw, untreated starch to the cellulosic suspension. Processes in which starch is added to the cellulosic suspension generally tend to have the disadvantage that particular care must be taken to ensure good retention of starch so that there are not significant levels of dissolved or undissolved starch in the whitewater draining through the wire. See for instance WO95/33096.
Processes are described in GB 2,292,394 in which anionic starch, carboxy methyl cellulose or other polymeric binder capable of hydrogen bonding to cellulose are added to the thin stock with a cationic polymer which has a molecular weight above 150,000, preferably 1 million or more and which insolubilises the anionic binder. Cationic starch can also be added.
Processes are described in WO93/01353 in which an anionic retention aid based on starch, a cellulosic derivative or guar gum free of cationic groups and an aluminium compound are added to the suspension. Another disclosure of processes in which an anionic compound and a low molecular weight cationic polymer are added to the suspension is in JP-A-03193996.
Although various processes which are known can be optimised to give useful strength in the dry sheet and can be optimised to give satisfactory short drainage times and/or good retention of the fibres and/or the binder, it would be desirable to be able to provide a process which gives optimum utilisation of the binder in the sheet (and thus optimum strength) together with good retention of the binder, the fibres and the fines in the cellulosic suspension, and good drainage properties.
It might have been thought that these objectives could be achieved by modifying the process described in GB 2,292,394 by adding a high molecular weight cationic polymeric retention aid to the suspension, but we have found that this does not give any significant or useful improvement.
According to the invention, a process for making paper (including paper board) comprises providing a thin stock suspension of cellulosic fibres, mixing into this suspension (a) a water soluble anionic or non-ionic polymeric binder and (b) a water soluble cationic material selected from water soluble organic polymeric coagulants having intrinsic viscosity not more than 3 dl/g and inorganic coagulants,
then flocculating the suspension by mixing into the suspension an anionic retention aid (which may be a microparticulate anionic retention aid),
draining the flocculated suspension to form a wet sheet, and
drying the wet sheet.
We have surprisingly found that the addition of the anionic retention aid, instead of traditional cationic polymeric flocculants, after addition of the binder and cationic polymeric coagulant, gives good flocculation of the suspension and subsequently a marked improvement in the drainage rate and good retention of fibre and fines. Further, it does not lead to any significant deterioration in the retention of binder and so gives good retention of the binder.
The cellulosic suspension may be any conventional thin stock formed from any conventional cellulosic feed, including recycled feed material. The thin stock may be substantially unfilled (i.e., without the deliberate addition of significant amounts of filler) or it may be filled.
The binder is a water soluble material capable of substantial hydrogen bonding with cellulose. That is, it is capable of binding with the cellulose fibres in the paper stock, for instance to levels of at least 1 or 2% (dry binder based on dry stock), often with a binder retention of at least about 60 or 70 or even 80%. In practice the binder needs to be non-ionic or anionic, since if it is cationic then the binding of the binder to the cellulosic fibres will predominantly be due to the cationic groups rather than due to hydrogen bonding. In order that hydrogen bonding predominates, the non-ionic or anionic binder will normally be a polyhydroxy material. In order that it acts as a binder in the final sheet, thereby increasing the strength of the sheet, it must be polymeric and of high molecular weight. Thus the molecular weight will normally be in excess of 5,000, and often in excess of 50,000 and generally above 100,000.
In practice, the polymeric binder is usually a cellulosic compound, a natural gum or a starch, but it can be a synthetic polymer such as polyvinyl alcohol. Natural and modified natural polymers include cellulosics, gums and starches, for instance carboxymethyl cellulose, xanthan gum, guar gum, mannogalactans and, preferably, anionic starch. The binder preferably has a pendant ionisable group which is generally sulphate, carboxylate or phosphate. Suitable starches include oxidised starch, phosphate starch and carboxy methylated starch.
The amount of binder is normally at least about 1% (dry weight binder based on dry weight suspension) and can be up to, for instance, 10%. Generally it is 1 to 8%, preferably around 3%, for instance 3 to 5% (i.e., 30 to 50 kg/t).
The cationic material is preferably a cationic polymeric coagulant which has IV not more than 3 dl/g. In this specification IV is intrinsic viscosity measured by suspended level viscometer at 25° C. in 1N sodium chloride buffered to pH 7. Preferably IV is not more than 2 dl/g, for instance 1.5 dl/g or below. Normally it is at least 0.1 or 0.5 dl/g. Preferred cationic polymeric coagulants have high charge density, for instance above 3 meq/g and usually above 4 meq/g.
Inorganic coagulant such as aluminium compounds, for instance poly aluminium chloride, can be used alone as the water soluble cationic material, or in combination with the polymeric coagulant.
The preferred cationic polymeric coagulants are materials such as polyethylene imines or polyamines (both preferably being fully quaternised), dicyandiamide condensation polymers (usually being substantially fully quaternised or in salt form) and polymers of water soluble ethylenically unsaturated monomer or monomer blend which is formed of 50 to 100 mole percent cationic monomer and 0 to 50 mole percent other monomer. The amount of cationic monomer is usually at least 80 to 90 mole percent, and homopolymers are often preferred. Ethylenically unsaturated cationic monomers that can be used include dialkylaminoalkyl (meth)-acrylates and -acrylamides (usually in quaternary or other salt form) and diallyl dialkyl ammonium chloride, for instance diallyl dimethyl ammonium chloride (DADMAC). Particularly preferred polymers are DADMAC homopolymers and copolymers.
When the polymer is a copolymer, the comonomer is usually acrylamide, or other water soluble non-ionic ethylenically unsaturated monomer.
The cationic polymeric coagulant may be a linear polymer. Alternatively it may b
Collett Lesley
Johnston Howard
Ciba Specialty Chemicals Water Treatments Ltd.
Fortuna Jose
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