Paper making and fiber liberation – Processes and products – Non-fiber additive
Patent
1997-05-28
1999-03-02
Chin, Peter
Paper making and fiber liberation
Processes and products
Non-fiber additive
162135, 1621682, 1621683, 162175, 162178, 162179, 1621816, 1062872, 10628717, 10628723, 10628724, 10628725, D21H 2116
Patent
active
058765620
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to anionic sizing dispersions and more particularly to such dispersions which comprise an anionic dispersion of a cellulose-reactive sizing agent and colloidal anionic aluminum-modified silica particles.
Aqueous dispersions of cellulose-reactive sizing agents, like alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), are widely used for sizing at neutral or slightly alkaline pH levels in order to impart liquid repellent or hydrophobic properties to cellulose-based products. Anionically stabilized dispersions are usually used in stock sizing applications involving high levels of cationic charge, for example as a result of cationic papermaking aids added to the stock, where retention of cationically stabilized dispersions has proved problematic. Such highly cationic wet-end systems are used for instance in the production of liquid packaging board and for photographic paper.
Commercial anionic AKD dispersions are sufficiently stable to permit transportation, storage and usage. In order to maximize shelf life and sizing efficiency, the dispersions are kept at low temperatures, without being allowed to freeze, and the products usually have a shelf life of at least one month at room temperature. However, there have been examples of agglomeration and separation of storage-stable anionic AKD dispersions when subjected to high shear forces, in particular at pumping and mixing operations. The agglomeration and separation has resulted in deposition problems, an increased need of maintenance and difficulty in dosing the dispersion.
It is known from EP 564994 to incorporate alum in sizing compositions of cationic ketene dimers and cationic colloidal silica, alumina or zirconia in order to aid in stabilization.
The object of these compositions is to achieve the desired level of sizing while increasing the coefficient of friction of the paper surface.
EP 418 015 discloses sizing compositions comprising a ketene dimer and an anionic dispersing agent. In addition, an extender or modifyer such as an anionic polyacrylamide, an anionic starch or colloidal silica can be present to extend or modify the anionic charge density of the compositions.
According to the present invention it has been found that the stability of anionic dispersions of cellulose-reactive sizing agents can be improved by the incorporation of colloidal anionic aluminum-modified silica particles. The anionic sizing dispersions according to the invention have less tendency to agglomerate and form deposits when subjected to shear forces and show very good shelf life at storage temperatures usually employed. In comparison with anionic sizing dispersions containing colloidal silica particles, the present dispersions comprising colloidal anionic particles of aluminum-modified silica show improved stability. Introducing aluminum-modified silica particles into the anionic sizing dispersions also results in a very good sizing with low amounts of the sizing agent when used in stock systems where high levels of cationic charge are evident, for example resulting from cationic wet-strength resins, cationic starches, polyacrylamides and polyethyleneimines added to the stock, since a very good retention of the sizing agent is obtained.
The present invention thus relates to aqueous anionic sizing dispersions as further defined in the claims.
The cellulose-reactive sizing agent which is present in the dispersions according to the invention can be selected from any of the cellulose-reactive sizing agents known in the art. Suitably, the cellulose-reactive sizing agent is selected from the group consisting of hydrophobic ketene dimers, acid anhydrides, organic isocyanates, carbamoyl chlorides and mixtures thereof, preferably ketene dimers and acid anhydrides, most preferably ketene dimers.
Suitable ketene dimers have the formula (I) shown below, in which R.sup.1 and R.sup.2 represent hydrocarbon groups suitably having from 8 to 36 carbon atoms, usually being alkyl groups having 12 to 20 carbon atoms, such as hexadecyl and octadecyl group
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Chin Peter
Eka Chemicals AB
Mancini Ralph J.
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